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  • richardmitnick 12:35 pm on November 30, 2017 Permalink | Reply
    Tags: , , , , , ESA Gaia, Red clump (RC) stars   

    From astrobites: “Gaia and the Red Clump” 

    Astrobites bloc

    astrobites

    Nov 30, 2017
    Philipp Plewa

    Title: Red clump stars and Gaia: Calibration of the standard candle using a hierarchical probabilistic model
    Authors: K. Hawkins, B. Leistedt, J. Bovy, D. W. Hogg
    First Author’s Institution: Department of Astronomy, Columbia University, New York
    1
    Status: Published in MNRAS, open access

    Red clump (RC) stars are common stars, once similar to the sun, that have evolved into red giants now supported by helium fusion in their cores. Independent of their exact age or composition, all RC stars end up having about the same absolute luminosity. This is why they tend to “clump” in a particular spot in a color-magnitude or Hertzsprung–Russell diagram, and what makes them standard candles: The apparent brightness of RC stars is directly related to their distance.

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    Red clump stars and Gaia, MNRAS
    3
    Hertzsprung-Russell diagram. A plot of luminosity (absolute magnitude) against the colour of the stars ranging from the high-temperature blue-white stars on the left side of the diagram to the low temperature red stars on the right side. “This diagram below is a plot of 22000 stars from the Hipparcos Catalogue together with 1000 low-luminosity stars (red and white dwarfs) from the Gliese Catalogue of Nearby Stars. The ordinary hydrogen-burning dwarf stars like the Sun are found in a band running from top-left to bottom-right called the Main Sequence. Giant stars form their own clump on the upper-right side of the diagram. Above them lie the much rarer bright giants and supergiants. At the lower-left is the band of white dwarfs – these are the dead cores of old stars which have no internal energy source and over billions of years slowly cool down towards the bottom-right of the diagram.”

    Next year’s second Gaia data release is expected to provide a significantly larger sample of RC reference stars.

    ESA/GAIA satellite

    In particular, this will allow a closer study of the impact of such effects as stellar age and metallicity, helium abundance, or binarity on the RC distance calibration, and thus make RC stars even better standard candles.

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

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  • richardmitnick 5:09 pm on November 27, 2017 Permalink | Reply
    Tags: , , , , ESA Gaia, Hubble and Gaia team up to measure 3D stellar motion with record-breaking precision,   

    From ESA GAIA and NASA/ESA Hubble: “Hubble and Gaia team up to measure 3D stellar motion with record-breaking precision” 

    ESA Space For Europe Banner

    European Space Agency

    NASA Hubble Banner

    NASA/ESA Hubble Telescope

    NASA/ESA Hubble Telescope

    ESA/GAIA satellite

    27 November 2017

    Davide Massari
    Kapteyn Astronomical Institute
    University of Groningen
    The Netherlands
    Email: massari@rug.astro.nl

    Amina Helmi
    Kapteyn Astronomical Institute
    University of Groningen
    The Netherlands
    Email: ahelmi@rug.astro.nl

    Anthony Brown
    Leiden Observatory
    University of Leiden
    The Netherlands
    Email: brown@strw.leidenuniv.nl

    Timo Prusti
    Gaia Project Scientist
    European Space Agency
    Email: timo.prusti@esa.int

    Mathias Jäger
    ESA/Hubble, Public Information Officer
    Garching bei München, Germany
    Cell: +49 176 62397500
    Email: mjaeger@partner.eso.org

    Stellar motions in nearby galaxy hint at underlying dark matter.

    By pinning down, for the first time, the three-dimensional motions of individual stars in the nearby Sculptor dwarf galaxy, astronomers have shed new light on the distribution of invisible dark matter that pervades the galaxy. This study combined the positions of stars measured by ESA’s Gaia mission with observations from the NASA/ESA Hubble Space Telescope taken twelve years earlier.

    1
    This image shows a small part of the Sculptor Dwarf Galaxy, a satellite galaxy of the Milky Way, situated only about 300 000 light-years away from Earth. This is one of two different pointings of the telescope that were used in a study combining data from the NASA/ESA Hubble Space Telescope and ESA’s Gaia satellite to measure the 3D motion of stars in this galaxy. Credit: NASA/ESA HUBBLE

    A team of astronomers used data from both the NASA/ESA Hubble Space Telescope and ESA’s Gaia satellite to directly measure the 3D motions of individual stars in a nearby galaxy. The achieved accuracy is better than anything previously measured for a galaxy beyond the Milky Way. The motions provide a field test of the currently-accepted cosmological model and also measure the trajectory of the galaxy through space. The results are published in Nature Astronomy.

    Our Milky Way galaxy, home to hundreds of billions of stars, is orbited by more than a dozen smaller satellite galaxies. The best known in this swarm, the Large and Small Magellanic Clouds, are visible with the naked eye from middle and southern latitudes, while the others are much fainter and can only be observed with the aid of telescopes.

    These satellite galaxies are classed as dwarfs, since they contain only a small fraction of the stars hosted in regular galaxies. They are all engaged in gravitational interactions with the Milky Way, and these dynamical processes might leave telltale traces in their shape.

    Astronomers have been studying stars in dwarf galaxies for decades, striving to reconstruct their origin and to uncover the past history of our Galaxy and its environment.

    A new study has now brought one of these galaxies, the dwarf Sculptor, into unprecedented focus.

    “For the first time, we’ve been able to determine how individual stars are moving through a dwarf satellite of our Milky Way,” says Davide Massari from Kapteyn Astronomical Institute in Groningen, The Netherlands, who is lead author of the paper published today in Nature Astronomy.

    “This was possible by combining the exceptionally accurate measurements of stellar positions from Gaia’s first data release with equally outstanding observations taken over twelve years earlier by the Hubble Space Telescope.”

    As expected, the shifts in the star positions as observed on the plane of the sky – called proper motions – are tiny, even over the long time baseline between 2002, when the Hubble observations were performed, and the first set of publicly released Gaia data, gathered between 2014 and 2015.

    “We couldn’t wait to see whether the Gaia observations could be used to measure individual proper motions in another galaxy, so we were thrilled to see this was indeed possible,” says Amina Helmi, a co-author of the study, also based at Kapteyn Astronomical Institute in Groningen.

    Massari, Helmi and colleagues measured the proper motions of roughly a hundred stars in the Sculptor dwarf galaxy. For a smaller subset of ten stars, chosen among those with the smallest errors, the astronomers could also retrieve from the literature an estimate of the radial velocity, which quantifies the stellar motion along the line of sight.

    Using the proper motion and radial velocity measurements, they were able to reconstruct how these stars move in three dimensions – the first time this was done for a dwarf galaxy.

    Previous studies of stellar positions in this and other dwarf galaxies had enabled astronomers to determine the overall motion of these satellites around the Milky Way.

    “Our study went beyond: joining these two exceptional data sets, we could for the first time extract information about both the dwarf galaxy’s motion and about the motions of stars within it,” explains Massari.

    Stellar motions are especially useful because these can lead astronomers to something they cannot see: dark matter, an invisible component that is vastly more abundant than ordinary matter across the cosmos.

    Still elusive to direct detection, the presence of dark matter can be inferred by its gravitational effect on the motion of other objects, like stars or galaxies.

    “Dwarf spheroidal galaxies like Sculptor are some of the most dark matter dominated objects we know of in the Universe,” says Helmi. “It’s in these places that we can really ‘see’ this mysterious component at play.”

    The new data indicate that stars in the Sculptor dwarf galaxy move preferentially on elongated radial orbits. This is consistent with the distribution of dark matter in the galaxy being ‘cuspy’, meaning that its density increases towards the centre instead of flattening out.

    “Cuspy profiles are predicted by simulations of the large-scale distribution of dark matter within the standard cosmological scenario, and our new results are definitely in agreement with that,” adds Helmi.

    While the findings seem promising, the data sample used in this study is not very large, and more data are needed to place stronger constraints on the dark matter distribution in this galaxy. Even though Gaia has measured thousands of stars in the field of Sculptor, in this study the astronomers could only determine the proper motions for those that had also been observed by Hubble, which looked at two small patches within the broader galaxy field.

    Gaia, on the other hand, has been scanning the entire sky. As its operations are continuing, the astronomers are looking forward to the larger, more precise data sets that will be released in future years.

    The second data release, based on data collected between 2014 and 2016 and planned for April 2018, will not only contain exquisitely accurate positions of more than one billion stars, but also proper motions. This release will not provide proper motion measurements for stars in dwarf galaxies like Sculptor with sufficient precision to investigate the motions of individual stars, but the measurements will be precise enough to study the bulk motion of the galaxy.

    “The second Gaia data release can be used to accurately pin down the overall motion of Sculptor around the Milky Way, and measure the motion of other dwarf galaxies, too, to unprecedented precision,” says Anthony Brown from Leiden University in The Netherlands, chair of the Gaia Data Processing and Analysis Consortium (DPAC) and a co-author of the paper.

    “But it’s the final release, based on several years of Gaia observations, that will reveal the individual proper motions of thousands of stars in the satellites of our Milky Way.”

    Last week, ESA approved an extension of Gaia operations for an additional 18 months, so it will keep scanning the sky until at least 2020. One of the key science drivers for the extension was the study of proper motions of stars in dwarf galaxies, which requires observations taken over as long a time baseline as possible.

    “With Gaia, we are looking at our Galaxy and its neighbours ‘in action’, delving into the motions of stars, and unveiling their secrets,” says Timo Prusti, Gaia project scientist at ESA.

    “This study provides us with an amazing preview of the exciting science that can be done with the full Gaia catalogue.”

    The international team of astronomers in this study consists of D. Massari (University of Groningen, The Netherlands), M. A. Breddels (University of Groningen, The Netherlands), A. Helmi (University of Groningen, The Netherlands), L. Posti (University of Groningen, The Netherlands), A. G. A. Brown (Leiden University, The Netherlands) and E. Tolstoy (University of Groningen, The Netherlands).

    Notes

    The team measured the proper motions of roughly a hundred stars in the Sculptor Dwarf Galaxy. For a smaller subset of ten stars, chosen among those with the smallest errors, the astronomers could also retrieve from the literature an estimate of the radial velocity, which quantifies the stellar motion along the line of sight. Using the proper motion and radial velocity measurements, they were able to reconstruct how these stars move in three dimensions.

    The data used contain images taken with Hubble’s Advanced Camera for Surveys in 2002.

    NASA/ESA Hubble ACS

    Newer positions of individual stars were taken from the Gaia, which was observed between 2014 and 2015.

    See the full ESA article here.
    See the full Hubble article here.

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    Gaia is an ESA mission to survey one billion stars in our Galaxy and local galactic neighbourhood in order to build the most precise 3D map of the Milky Way and answer questions about its structure, origin and evolution. A large pan-European team of expert scientists and software developers, the Data Processing and Analysis Consortium (DPAC), located in and funded by many ESA member states, is responsible for the processing and validation of Gaia’s data, with the final objective of producing the Gaia Catalogue.

    The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI), is a free-standing science center, located on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy (AURA) for NASA, conducts Hubble science operations.

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    NASA image

    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 7:48 am on October 27, 2017 Permalink | Reply
    Tags: , , , Cloudy with a chance of protons, , ESA Gaia,   

    From ESA: “Cloudy with a chance of protons” 

    ESA Space For Europe Banner

    European Space Agency

    26/10/2017
    No writer credit

    1

    ESA’s Gaia mission, in orbit since December 2013, is surveying more than a thousand million stars in our Galaxy, monitoring each target star about 70 times over a five-year period and precisely charting their positions, distances, movements and brightness.

    ESA/GAIA satellite

    Although Gaia is not equipped with a dedicated radiation monitor, it can provide information about the space weather – and the solar particles and radiation – that it encounters at its unique orbital position, 1.5 million km from Earth towards the Sun.

    In September, Gaia unexpectedly detected a large quantity of protons – subatomic particles – emitted by a solar flare.

    In this image, captured by Gaia’s Wave Front Sensor – a sort of ‘camera within a camera’ in its main star-sensing instrument – the streaks of ‘snow’ are trails of individual protons. During normal space weather conditions, the image would only include one or two proton trails. The long trail running horizontally across the image indicates a particularly energetic proton.

    This proton storm was also reported by NASA’s GOES weather satellite, which is equipped with a particle-sensing instrument.

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    GOES-R | NASA

    The solar flare that gave rise to these protons took place on 10 September 2017, and the peak flow of protons streaming past Gaia occurred at about midnight on 11 September.

    “Gaia is designed to withstand these space weather storms and it was able to continue as normal throughout the period of increased radiation,” says spacecraft operations engineer Ed Serpell.

    “During the days of the increased radiation, the amount of ground contact with the ESA deep-space network was increased to provide more realtime information about the spacecraft performance. This additional visibility confirmed how well Gaia was performing and no intervention was necessary.”

    The storm had some minor, temporary effect on Gaia’s attitude and orbit control system. The excess protons also caused the main science instrument to generate much more data than usual, which had to be offloaded from the onboard memory.

    More information

    Gaia

    Space Situational Awareness

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    See the full article here .

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    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 3:01 pm on April 18, 2017 Permalink | Reply
    Tags: , , , , Can you imagine the sky in five million years?, , ESA Gaia   

    From Astronomy: “Can you imagine the sky in five million years?” 

    Astronomy magazine

    Astronomy Magazine

    April 13, 2017
    Alison Klesman

    Now you don’t have to — Gaia has the answer.

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    In five million years, the sky will look a little different. The constellations will be unrecognizable, and many of the stars we can see today will have moved significantly. ESA/Gaia/DPAC

    Have you ever wondered what it would be like to stare up at the sky millions of years from today? Would things look exactly the same, or would the sky be totally unrecognizable? Wonder no longer — the European Space Agency (ESA) has just released a video answering that exact question.

    Since July 2014, ESA’s Gaia mission has been charting the positions of stars in the Milky Way with higher accuracy than ever before. Its goal is to create a three-dimensional map of our galaxy, which is uniquely challenging because we’re trying to make a map from inside the galaxy, rather than being able to take a step back and view it from outside.

    ESA/GAIA satellite

    With such precise stellar positions, however, comes something else: stellar motions. The stars seem perpetually fixed in the sky — sure, they rise and set, and change throughout the year as we go around the Sun, but they always form the same patterns. A significant percentage of the constellations most of us know are those derived, after all, by the Greeks just a little under 2,000 years ago. So, of course, it’s natural to assume that the stars just don’t move, because they’ve looked pretty much the same for thousands of years.

    But thousands of years is but an eyeblink in the lifetime of a galaxy, and the notion that the stars don’t change positions is false. The stars do move, largely in bulk as they rotate around the center of the Milky Way, but sometimes they zip off in random directions dictated by the conditions of their formation or past interactions. This latter effect is exacerbated by perspective — the closer a star is to us, the more it will appear to move. This perspective effect is also essentially how Gaia measures stellar positions so accurately, using a technique called parallax that causes nearby stars to shift against the background as Earth orbits the Sun.

    But largely, from our perspective, the stars are just so far away that even though they’re moving at hundreds of kilometers per second, they seem pretty fixed to the casual observer. Now, though, ESA has released a video containing 2,057,050 stars that have been measured well enough to predict where they are and where they’re going in the future. The overall motion of a star from our point of view against the background of extremely far away stars is called proper motion, and that’s the basis for the stellar motions in this video. Using the projected proper motions of the stars in the Gaia catalog, the result is a fast-forwarded trip through time that ends with the sky as it would appear from Earth in 5 million years. Each frame in the video represents the passage of 750 years.


    Copyright: ESA/Gaia/DPAC

    There are a few key takeaway points from this video. At first, very little appears to happen, but that’s an illusion. Consider the constellations. They’re two-dimensional projections on a three-dimensional sky, which means that although the stars form a picture, virtually none of the stars in a given constellation are at the same distance from us. In the video, you can spot Orion on the far right, just below the bright plane of the galaxy. The Big Dipper (technically an asterism, not a constellation) appears in the upper left, high above the galaxy’s plane.

    When you hit play, the constellations are quickly distorted beyond all recognition within about 100,000 years — literally just a few frames into the video. That’s because the stars nearer to us appear to move significantly, while those farther away don’t appear to move as much. None of the stars move in tandem, so the patterns are torn apart. So while our current constellations may last for thousands more years, between ten and a hundred thousand years from now, astronomers will need to come up with some new patterns.

    Next, keep in mind that this video has some limitations. The first frame seems to show intricate structure and even stripes in the plane of the Milky Way, but those are actually just artificial data artifacts in areas where Gaia hasn’t measured the positions of stars (or hasn’t measured them accurately enough to predict a believable proper motion). Those artifacts are washed out pretty quickly, though, as closer stars move into the areas where less data exists.

    Some dark areas, though, are clouds of interstellar dust, which block the light from stars sitting behind them. Those stars aren’t visible today, so the video doesn’t include the “new” stars we might see popping out from behind the clouds as the millennia scroll by. The clouds themselves can also move, which similarly isn’t taken into account here. The motion of these clouds over time would basically change the detailed structure of the Milky Way we see when we look up in the sky, like subtly shifting the stripes on a tiger’s back.

    Finally, the stars themselves aren’t eternal. Because it takes time for light to travel across space, you may wonder if the stars you’re seeing in the sky are even really there. For the most part, the answer is yes, simply because stars live so long compared to humans (or human civilization), that the chances of one disappearing in a human lifetime are pretty small. But, if you wait a few hundred thousand years, that won’t be true — that’s when the bright red star in Orion’s shoulder, Betelgeuse, is expected to go supernova. We’ll notice about 600 years later (give or take a few hundred years, because the distance to Betelgeuse is fairly hard to pin down), when the star grows very bright, then dims away past naked-eye visibility. Orion’s really going to start falling apart then, because the bright star that marks one of his knees, Rigel, will go out similarly relatively soon after.

    So keep in mind that although this video carries the stars through the next five million years, not all of the stars you see will make it that long. Some will disappear, and new ones might become visible. Regardless of the changes that occur, the sky is changing — but with the data we currently possess, we can now take a peek at what the far future holds in store.

    See the full article here .

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  • richardmitnick 9:01 am on April 13, 2017 Permalink | Reply
    Tags: , , , , ESA Gaia   

    From ESA: “Two million stars on the move” 

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    European Space Agency

    12 April 2017

    The changing face of our Galaxy is revealed in a new video from ESA’s Gaia mission. The motion of two million stars is traced 5 million years into the future using data from the Tycho-Gaia Astrometric Solution, one of the products of the first Gaia data release. This provides a preview of the stellar motions that will be revealed in Gaia’s future data releases, which will enable scientists to investigate the formation history of our Galaxy.

    ESA/GAIA satellite


    Two million stars in our Galaxy, with their motions traced five million years into the future. Credit: ESA/Gaia/DPAC.

    Stars move through our Galaxy, the Milky Way, although the changes in their positions on the sky are too small and slow to be appreciated with the naked eye over human timescales. These changes were first discovered in the eighteenth century by Edmond Halley, who compared stellar catalogues from his time to a catalogue compiled by the astronomer Hipparchus some two thousand years before. Nowadays, stellar motions can be detected with a few years’ worth of high-precision astrometric observations, and ESA’s Gaia satellite is currently leading the effort to pin them down at unprecedented accuracy.

    A star’s velocity through space is described by the proper motion, which can be measured by monitoring the movement of a star across the sky, and the radial velocity, which quantifies the star’s motion towards or away from us. The latter can be inferred from the shift towards blue or red wavelengths of certain features – absorption lines – in the star’s spectrum.

    Launched in 2013, Gaia started scientific operations in July 2014, scanning the sky repeatedly to obtain the most detailed 3D map of our Galaxy ever made. The first data release [1], published in September 2016, was based on data collected during Gaia’s first 14 months of observations and comprised a list of 2D positions – on the plane of the sky – for more than one billion stars, as well as distances and proper motions for a subset of more than two million stars in the combined Tycho–Gaia Astrometric Solution, or TGAS.

    2
    The first results from the Gaia satellite, which is completing an unprecedented census of more than one billion stars in the Milky Way, are being released to astronomers and the public.

    The TGAS dataset consists of stars in common between Gaia’s first year and the earlier Hipparcos and Tycho-2 Catalogues, both derived from ESA’s Hipparcos mission, which charted the sky more than two decades ago.

    This video shows the 2 057 050 stars from the TGAS sample, with the addition of 24 320 bright stars from the Hipparcos Catalogue that are not included in Gaia’s first data release. The stars are plotted in Galactic coordinates and using a rectangular projection: in this, the plane of the Milky Way stands out as the horizontal band with greater density of stars. Brighter stars are shown as larger circles, and an indication of the true colour of each star is also provided; information about brightness and colour is based on the Tycho-2 catalogue from the Hipparcos mission.

    The video starts from the positions of stars as measured by Gaia between 2014 and 2015, and shows how these positions are expected to evolve in the future, based on the proper motions from TGAS [2]. The frames in the video are separated by 750 years, and the overall sequence covers 5 million years. The stripes visible in the early frames reflect the way Gaia scans the sky and the preliminary nature of the first data release; these artefacts are gradually washed out in the video as stars move across the sky.

    1
    The location of the Orion constellation (right) and of two stellar clusters (left) in the first frame of the video. Credit: ESA/Gaia/DPAC

    The shape of the Orion constellation can be spotted towards the right edge of the frame, just below the Galactic Plane, at the beginning of the video. As the sequence proceeds, the familiar shape of this constellation (and others) evolves into a new pattern. Two stellar clusters – groups of stars that were born together and consequently move together – can be seen towards the left edge of the frame: these are the alpha Persei (Per OB3) and Pleiades open clusters.

    Orion Nebula M. Robberto NASA ESA Space Telescope Science Institute Hubble

    Stars seem to move with a wide range of velocities in this video, with stars in the Galactic Plane moving quite slow and faster ones appearing over the entire frame. This is a perspective effect: most of the stars we see in the plane are much farther from us, and thus seem to be moving slower than the nearby stars, which are visible across the entire sky.

    Some of the stars appear to dart across the sky with very high velocities: for some stars, this is an effect of their close passage to the Sun – for example, in about 1.35 million years, the star Gliese 710 will pass within about 13 500 au (10 trillion kilometres) from the Sun. Other stars seem to trace arcs from one side of the sky to the other, passing close to the galactic poles, accelerating and decelerating in the process: in fact, this acceleration and deceleration are spurious effects since these stars move with a constant velocity through space.

    Stars located in the Milky Way’s halo, a roughly spherical structure in which the Galactic Plane is embedded, also appear to move quite fast because stellar motions in the video are calculated with respect to the moving Sun, which is located in the Galactic Plane; however, halo stars move very slowly with respect to the centre of the Galaxy.

    Although this visualisation displays only the motion of stars, there is an indication in the first frame of interstellar clouds of gas and dust that block our view of more distant stars. The subsequent sequence of stellar motions shows where each star is expected to be at a given time in the future, but does not track the motion of interstellar clouds. The fact that dark clouds seem to disappear over time is a spurious effect. Similarly, the video does not predict the future positions of stars that are currently hidden by interstellar material and hence have not been observed by Gaia.

    After a few million years, the plane of the Milky Way appears to have shifted towards the right: this is mainly the consequence of the motion of the Sun with respect to that of other, nearby stars in the Milky Way. However, the regions that are depleted of stars in the video will not appear as such to future observers looking at the sky from Earth: instead, they will be replenished by stars that are not part of the TGAS sample and therefore not present in this view. The Large and Small Magellanic Clouds, whose stars are not well sampled in the TGAS data, are not visible in this view.

    Compiled as a taster to the much larger and more precise catalogue that will be published with Gaia’s second data release, TGAS is twice as precise and contains almost 20 times as many stars as the previous definitive reference for astrometry, the Hipparcos Catalogue. As such, it represents a major advance in terms of high precision parallaxes and proper motions.

    Scientists across the world have been combining TGAS data with other stellar catalogues assembled using ground-based observations, to obtain larger samples of stars for which positions, distances and proper motions are available. Thus far, three such catalogues have been compiled: the HSOY (“Hot Stuff for One Year”) catalogue, which contains the proper motions for 580 million stars, the US Naval Observatory CCD Astrograph Catalog 5 (UCAC 5), listing 100 million proper motions, and the Gaia-PS1-SDSS (GPS1) proper motion catalogue, which includes 350 million proper motions.

    Gaia’s second data release, in April 2018, will include not only the positions, but also distances and proper motions for over one billion stars, as well as radial velocities for a small subset of them. This will mark a new era in the field of astrometry, enabling scientists to study the past positions of stars – to explore the formation history of our Galaxy – and to predict their future positions to a level of accuracy that was never achieved before.
    Notes

    [1] Gaia’s first data release (Gaia DR1) was published on 14 September 2016. This comprised a catalogue of the positions on the sky and the brightness of more than a billion stars – the largest all-sky survey of celestial objects to date – as well as the Tycho-Gaia Astrometric Solution (TGAS), containing the distances and motions for the two million stars in common between the Gaia dataset and the Hipparcos and Tycho-2 catalogues. The TGAS dataset is twice as precise and contains almost 20 times as many stars as the previous definitive reference for astrometry, the Hipparcos Catalogue.

    [2] To calculate the future positions of stars, the astrometric measurements from the TGAS dataset were combined with a sample of 235,966 radial velocity measurements from the RAVE, GALAH, and APOGEE catalogues. The calculation is based on a linear extrapolation of the measured velocities of stars, which is a reasonable first-order approximation to study stellar motions on short timescales of millions of years, such as the ones shown in the video; to investigate longer timescales, scientists make use of N-body simulations, a numerical procedure that takes into account the gravity actually experienced by the stars at any time in the past or future.

    See the full article here .

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    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 9:55 am on March 23, 2017 Permalink | Reply
    Tags: , , , , ESA Gaia, Gaia 1 and 2, Omega Centauri   

    From astrobites: “From One Satellite to Another: Finding Clusters with Gaia” 

    Astrobites bloc

    Astrobites

    Mar 23, 2017
    Matthew Green

    Title: Gaia 1 and 2. A pair of new satellites of the Galaxy
    Authors: S. E. Koposov, V. Belokuro, G. Torrealba
    First Author’s Institution: Institute of Astronomy, University of Cambridge, UK
    1
    Status: Submitted to MNRAS, open access

    The Milky Way does not travel alone through the void. It has a gaggle of followers. Most obvious from the Earth are some of the dwarf galaxies, such as the Magellanic Clouds, which orbit the Milky Way. Smaller but much more numerous are the globular clusters, of which the Milky Way has around 150. A globular cluster is a very tight group of old, red stars, and generally orbits the Milky Way within its halo (they are also found around other galaxies, but for today we are focussing on satellites of our own galaxy). A famous example is Omega Centauri (Figure 1).

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    Figure 1: Omega Centauri, one of the better known globular clusters. Credit: European Space Agency (ESO), released under creative commons.

    With every generation of telescopes, we can see fainter stars and resolve their positions more precisely. We can detect and explore more of these satellites now than at any time in the past.

    Understanding dwarf galaxies is important for understanding not just their evolution but that of our own galaxy, as there is clearly a large amount of interaction between the Milky Way and its smaller neighbours. It seems that globular clusters are part of the puzzle as well: we know that globular clusters can be stolen from one galaxy by its neighbour as they pass close to each other, and there is a suggestion that globular clusters may be formed from the hearts of dwarf galaxies that have had many of their stars stripped away.

    Previous astrobiters have written about Gaia, the space telescope that will measure the position and velocity of over a billion stars.


    ESA/Gaia

    At the moment, only the first set of Gaia data has been released: it contains the brightness and positions of most of the targets, but not yet their distances, colours or velocities.

    The team behind today’s paper have been using just these stellar position measurements to hunt for new galactic satellites. Towards this end, they search for small patches of the sky that have an unusually high number of stars when compared to nearby regions. Figure 2 shows the density they find. The bright yellow spots are regions of unusually high density such as clusters or dwarf galaxies. The team were surprised at how effective their algorithm was: it highlighted several incredibly faint dwarf galaxies that were only discovered in the past two years. It also found two new objects, now named Gaia 1 and Gaia 2, which they analyse in today’s paper.

    2
    Figure 2: The density of stars on the sky when compared with surround regions. Bright yellow points are small, dense groups of stars such as clusters and dwarf galaxies. You can also see the disc of the Milky Way (across the centre of the image) and the 2 Magellanic Clouds (towards the lower right). The sweeping web of blue-green lines throughout the image follow the paths Gaia has observed along. They represent areas that have been observed more frequently and hence where fainter stars have been detected. This is Figure 1 in today’s paper.

    Gaia 1 and 2

    The larger of the two clusters was hiding close to Sirius, the brightest star in the night sky. Images taken close to stars this bright are often hard to use: the reflection and refraction of its light creates blurs and smudges on the surrounding image, as you can see in Figure 3. Because Gaia takes numerous images of each region of sky, these effects average out in their data allowing the nearby cluster to be identified for the first time.

    The cluster appears to be dominated by old stars, with a particularly prominent number of red giants, as would be expected from a globular cluster. An estimate of the cluster’s age from its Hertzprung-Russell diagram puts it at 4.6 billion years. To put that in perspective, it’s comparable to the sun’s age, and fairly typical for a globular cluster — as opposed to the other main class, open clusters, which are less tightly bound by gravity and tend to drift apart on a timescale in the millions of years. The team measured a distance to the cluster of 4600 parsecs (well within the halo of the Milky Way) and a size of 9 parsecs (also typical of globular clusters; for comparison, a parsec is roughly the distance between us and our closest star, Proxima Centauri). They estimate the total mass of the cluster as 14,000 times the mass of our sun — so clearly these stars are densely packed!

    The second cluster, Gaia 2, is smaller than Gaia 1 at 3 parsecs across, and even older at around 10 billion years. It has a distance of 5500 parsecs. Given this size, age, and appearance, they believe that this is another globular cluster.

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    Figure 3: Detection of Gaia 1. Top row: The density of stars according to Gaia (first two images) and a previous survey, 2MASS (third image). In all images Gaia 1 is clear as a denser region. Bottom row: infrared image of Gaia 1 and the surrounding sky. In the first panel the bright star on the right is Sirius. In the second panel Sirius has been subtracted, though a lot of scattered light remains in the image. The third panel is the same image with all Gaia-detected stars over-plotted as red points. This is Figure 3 in today’s paper.

    The Future

    With better images, it will be possible to find out more about these clusters and confirm some of their less certain details. Beyond this, the discovery of these two new objects highlights the revolutionary potential Gaia has in this field. With the release of future sets of Gaia data, which will include colours, distances, and movements for each star it targets, it is clear that many more galactic satellites will be revealed. The next data release is scheduled for early next year, so until then, watch this space!

    See the full article here .

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 8:30 am on January 25, 2017 Permalink | Reply
    Tags: , , Atira asteroids, , ESA Gaia, Gaia-606, Initial Data Processing (IDT) software   

    From ESA: “Gaia turns its eyes to asteroid hunting” 

    ESA Space For Europe Banner

    European Space Agency

    24 January 2017
    Paolo Tanga
    Observatoire de la Côte d’Azur, France
    Email: Paolo.Tanga@oca.eu

    Benoit Carry
    Observatoire de la Côte d’Azur, France
    Email: benoit.carry@oca.eu

    William Thuillot
    Observatoire de Paris, France
    Email: William.Thuillot@obspm.fr

    Timo Prusti
    Gaia Project Scientist
    Directorate of Science
    European Space Agency
    Email: timo.prusti@esa.int

    Whilst best known for its surveys of the stars and mapping the Milky Way in three dimensions, ESA’s Gaia has many more strings to its bow. Among them, its contribution to our understanding of the asteroids that litter the Solar System. Now, for the first time, Gaia is not only providing information crucial to understanding known asteroids, it has also started to look for new ones, previously unknown to astronomers.

    ESA/GAIA satellite
    ESA/GAIA satellite

    1
    Asteroid Gaia-606 on 26 October 2016. Credit: Observatoire de Haute-Provence & IMCCE

    Since it began scientific operations in 2014, Gaia has played an important role in understanding Solar System objects. This was never the main goal of Gaia – which is mapping about a billion stars, roughly 1% of the stellar population of our Galaxy – but it is a valuable side effect of its work. Gaia’s observations of known asteroids have already provided data used to characterise the orbits and physical properties of these rocky bodies more precisely than ever before.

    “All of the asteroids we studied up until now were already known to the astronomy community,” explains Paolo Tanga, Planetary Scientist at Observatoire de la Côte d’Azur, France, responsible for the processing of Solar System observations.

    These asteroids were identified as spots in the Gaia data that were present in one image and gone in one taken a short time later, suggesting they were in fact objects moving against the more distant stars.

    2
    Gaia’s asteroid detections. ESA/Gaia/DPAC/CU4, L. Galluccio, F. Mignard, P. Tanga (Observatoire de la Côte d’Azur)

    Once identified, moving objects found in the Gaia data are matched against known asteroid orbits to tell us which asteroid we are looking at. “Now,” continues Tanga, “for the first time, we are finding moving objects that can’t be matched to any catalogued star or asteroid.”

    The process of identifying asteroids in the Gaia data begins with a piece of code known as the Initial Data Processing (IDT) software – which was largely developed at the University of Barcelona and runs at the Data Processing Centre at the European Space Astronomy Centre (ESAC), ESA’s establishment in Spain.

    This software compares multiple measurements taken of the same area and singles out objects that are observed but cannot be found in previous observations of the area. These are likely not to be stars but, instead, Solar System objects moving across Gaia’s field of view. Once found, the outliers are processed by a software pipeline at the Centre National d’Etudes Spatiales (CNES) data centre in Toulouse, France, which is dedicated to Solar System objects. Here, the source is cross matched with all known minor bodies in the Solar System and if no match is found, then the source is either an entirely new asteroid, or one that has only been glimpsed before and has never had its orbit accurately characterised.

    Although tests have shown Gaia is very good at identifying asteroids, there have so far been significant barriers to discovering new ones. There are areas of the sky so crowded that it makes the IDT’s job of matching observations of the same star very difficult. When it fails to do so, large numbers of mismatches end up in the Solar System objects pipeline, contaminating the data with false asteroids and making it very difficult to discover new ones.

    “At the beginning, we were disappointed when we saw how cluttered the data were with mismatches,” explains Benoit Carry, Observatoire de la Côte d’Azur, France, who is in charge of selecting Gaia alert candidates. “But we have come up with ways to filter out these mismatches and they are working! Gaia has now found an asteroid barely observed before.”

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    Asteroid Gaia-606 on 26 October 2016. Credit: Observatoire de Haute-Provence & IMCCE

    The asteroid in question, nicknamed Gaia-606, was found in October 2016 when Gaia data showed a faint, moving source. Astronomers immediately got to work and were able to predict the new asteroid’s position as seen from the ground over a period of a few days. Then, at the Observatoire de Haute Provence (southern France), William Thuillot and his colleagues Vincent Robert and Nicolas Thouvenin (Observatoire de Paris/IMCCE) were able to point a telescope at the positions predicted and show this was indeed an asteroid that did not match the orbit of any previously catalogued Solar System object.

    However, despite not being present in any catalogue, a more detailed mapping of the new orbit has shown that some sparse observations of the object do already exist. This is not uncommon with new discoveries where, as with Gaia-606 (now renamed 2016 UV56), objects that first appear entirely new transpire to be re-sightings of objects whose previous observations were not sufficient to map their orbits.

    “This really was an asteroid not present in any catalogue, and that is an exciting find!” explains Thuillot. “So whilst we can’t claim this is the first true asteroid discovery from Gaia, it is clearly very close and shows how near we are to finding a never-before-seen Solar System object with Gaia.”

    4
    During the course of its five-year nominal mission Gaia is expected to observe several hundred thousand asteroids. Many of these will be in the main asteroid belt, located between Mars and Jupiter.

    One of the strengths of Gaia is that it will also observe regions that are not extensively observed by existing ground-based surveys – this gives it the potential to find asteroids in areas where others would not, or could not, look. Ground-based observations are made during the night when the angle between a source and the Sun is fairly large. Gaia can make observations at any time and hence observes objects much closer to the Sun. In particular, Gaia is ideally situated to probe the region between the Sun and Earth. This is where the Atira asteroids are found, orbiting inside Earth’s orbit. To date, only 16 of these asteroids have been discovered.

    The dashed lines indicate regions of the sky that are unobservable by Gaia. All other regions are accessible to Gaia, including swathes within Earth’s orbit.

    Gaia-606 was found in the main asteroid belt, which is not surprising given how many asteroids exist there. However, Gaia also provides data from swathes of the sky not extensively observed by existing ground-based surveys giving it the potential to find asteroids in areas where others would not look. One such area is a region close to the Sun as seen from Earth. Observations are made from the Earth during the night when the angle between any source and the Sun is fairly large, whilst Gaia can make observations at any time and so observe objects much closer to the Sun. This gives Gaia the exciting potential to observe asteroids that orbit within Earth’s orbit – these are known as Atira asteroids and only sixteen are currently known.

    Gaia also has the potential to make discoveries at high ecliptic latitudes. Not because ground-based surveys of Solar System objects cannot observe there, but because they tend not to. The vast majority of asteroids exist in the ecliptic plane and, as a result, it is here that most surveys concentrate their efforts. Gaia has no such prejudices and scans the entire sky, giving it the potential to discover new asteroids in the less crowded areas missed by other surveys.

    “Whilst Gaia’s primary role in Solar System science remains its ability to characterise the movement and physical properties of known asteroids, it has now shown that it can also play a role in finding new ones, adding to its ever expanding catalogue of Solar System objects,” concludes Tanga.

    About Gaia

    Gaia is an ESA mission to survey one billion stars in our Galaxy and local galactic neighbourhood in order to build the most precise 3D map of the Milky Way and answer questions about its origin and evolution.

    The mission’s primary scientific product will be a catalogue with the positions, motions, brightnesses, and colours of the more than a billion surveyed stars. The first intermediate catalogue was released in September 2016. In the meantime, Gaia’s observing strategy, with repeated scans of the entire sky, is allowing the discovery and measurement of many transient events across the sky: among these are the detection of candidate asteroids which are subsequently observed by astronomers in the Gaia Follow-Up-Network. During the five-year nominal mission, Gaia is expected to observe about 350 000 asteroids of which a few thousand will be previously unknown.

    5
    Gaia Follow-Up Network for Solar System Objects. Credit: Google Earth

    The nature of the Gaia mission leads to the acquisition of an enormous quantity of complex, extremely precise data, and the data-processing challenge is a huge task in terms of expertise, effort and dedicated computing power. A large pan-European team of expert scientists and software developers, the Data Processing and Analysis Consortium (DPAC), located in and funded by many ESA member states, and with contributions from ESA, is responsible for the processing and validation of Gaia’s data, with the final objective of producing the Gaia Catalogue. Scientific exploitation of the data only takes place once the data are openly released to the community.

    See the full article here .

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    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 2:43 pm on October 25, 2016 Permalink | Reply
    Tags: , , ESA Gaia, Four luminous blue variables found to be much closer than previously assumed,   

    From phys.org: “Four luminous blue variables found to be much closer than previously assumed” 

    physdotorg
    phys.org

    1
    AG Carinae (AG Car) – an example of a Luminous Blue Variable (LBV) star. Credit: Judy Schmidt/Hubble Space Telescope.

    A new study based on the first Gaia data release (DR1) reveals more accurate measurements of the distance of four canonical luminous blue variables (LBVs) in the Milky Way galaxy.

    ESA/GAIA satellite
    ESA/GAIA satellite

    According to a research paper published Oct. 20 on the arXiv server, they are much closer to Earth than previously thought.

    Published on Sept. 14, 2016, DR1 contains a catalog of over 1 billion stars with precise measurements of their brightness and positions in the sky. These data were obtained by ESA’s Gaia satellite, which is completing the first-ever “galactic census”—the most detailed three-dimensional map of the Milky Way ever made. The release of DR1 offers the scientific community an excellent opportunity to improve knowledge of our stellar environment and to redefine some previous calculations.

    Combing through the data obtained by Gaia, Nathan Smith of the Steward Observatory in Arizona and Keivan Stassun of Vanderbilt University in Nashville, Tennessee, have searched for LBVs and LBV candidates. These massive evolved stars showcase unpredictable and sometimes dramatic variations in both their spectra and their brightness. Their strong mass loss is believed to play a critical role in the evolution of massive stars, however the exact role LBVs play and the physics of their instability are still uncertain.

    Four canonical LBVs in the Milky Way were of special interest for Smith and Stassun, namely: AG Car, HR Car, HD 168607 and Hen 3-519 (an LBV candidate).

    “Here, we report direct distances and space motions of four canonical Milky Way LBVs—AG Car, HR Car, HD 168607, and (the LBV candidate) Hen 3-519 – whose parallaxes and proper motions have been provided by the Gaia first data release,” the researchers wrote in the paper.

    The most important findings were made regarding the distance of these LBVs. The clue to understanding their peculiar instability is their high observed luminosity, which, in the case of those stars, often depends on an uncertain distance calculations.

    According to the paper, the distance of HD 168607 was re-calculated from about 7,000 to 3,800 light years. Similar correction was made in the case of HR Car, as its distance from Earth, previously thought to be approximately 16,000 light years, also turned out to be only half of the value – about 7,500 light years.

    The astronomers found more surprising results regarding AG Car and Hen 3-519. DR1 data reveal that AG Car is located just 6,400 light years away, replacing earlier calculations indicating a distance over a three times larger, approximately 21,500 light years.

    Finally, the study finds that the distance of Hen 3-519 shows the biggest discrepancy between the previous and latest estimates. The new measurements reveal that its distance is 5,200 light years from the Earth, while the previously adopted distance was 26,000 light years.

    “The distances to all four objects are closer than traditionally assumed in the literature, lowering their intrinsic luminosities,” the paper reads.

    The researchers noted that the lower luminosities suggest that AG Car and Hen 3-519 passed through a previous red supergiant phase. They also imply that binary evolution could explain their peculiar properties. Moreover, the scientists concluded that the new results may initiate a re-evaluation of our current understanding of LBVs.

    “More precise values of the parallax for a larger number of Galactic LBVs will be available soon; the results reported here may signal upheaval in our understanding of massive star evolution, even if they are regarded as preliminary,” the astronomers wrote.

    See the full article here .

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page. set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • richardmitnick 10:06 am on October 20, 2016 Permalink | Reply
    Tags: , , , ESA Gaia, , , How far away are the stars?, The parallax method   

    From Ethan Siegel: “How far away are the stars?” 

    10.20.16
    Ethan Siegel

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    This is the Milky Way from Concordia Camp, in Pakistan’s Karakoram Range. To the right is Mitre Peak, and to the far left is the beginning of Broad Peak. Photograph by Anne Dirkse, of http://www.annedirkse.com under a c.c.-by-s.a.-4.0 license.

    Scientists still don’t know, but the answer could hold the key to the expanding, accelerating Universe.

    “Scratch a cynic and you’ll find a disappointed idealist.” -Jon F. Merz

    When you look up at the night sky and see the glittering stars overhead, your first thought might be to wonder what, exactly, they are. Once you know they’re very distant suns, however, with different masses, brightnesses, temperatures and colors, your next thought might be to wonder just how far away they are. It might surprise you to learn that despite centuries of advancement in astronomy and astrophysics, from telescopes to cameras to CCDs to observatories in space, we still don’t have a satisfying answer. When you consider that much of our understanding of the Universe today — how it was born, how it came to be the way it is and what it’s made of — is based on the distances to the stars, it highlights just how important this problem is.

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    Stars that appear to be at the same distance, like the ones in the constellation of Orion, may in fact be many hundreds or even thousands of light years more-or-less distant than one another. Image credit: La bitacora de Galileo, via http://www.bitacoradegalileo.com/2010/02/07/orion-la-catedral-del-cielo/.

    If you want to know how fast the Universe is expanding at any point in time, you need to know how fast the distant galaxies are moving away from us and how far away they are. Measuring a galaxy’s recession speed is straightforward — just measure its redshift and you’re done — but distances are a tricky thing. There needs to be some type of relationship between a quantity you can measure, like observed brightness, angular size, periodicity of a particular signal, etc., and something that will tell you an object’s intrinsic brightness or size. You can then calculate its distance. That’s how we figure out a whole slew of properties about the Universe, including:

    how fast it’s expanding today,
    how the expansion rate has changed over time,
    and what makes up the Universe, including matter, radiation and dark energy.

    3
    The construction of the cosmic distance ladder involves going from our Solar System to the stars to nearby galaxies to distant ones. Each “step” carries along its own uncertainties. Image credit: NASA,ESA, A. Feild (STScI), and A. Riess (STScI/JHU).

    But all of that knowledge requires a starting point for measuring cosmic distances. All of our measurement methods are dependent on knowing how these objects we’re measuring operate nearby: they all require an understanding of the closer star or galaxy types that we also find at great distances. No matter how you go about it, there’s one key step we need to begin with, and that’s an assumption-free method to measure the distances to the nearest stars. We only know of one, and we’ve known of it since before the time of Galileo.

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    The parallax method, employed since the 1800s, involves noting the apparent change in position of a nearby star relative to the more distant, background ones. Image credit: ESA/ATG medialab.

    It’s the idea of parallax, which is a purely geometrical way to measure the distances to the stars. Regardless of what type of star you have, what its brightness is or how it’s moving through space, measuring parallax is exactly the same.

    Measure the star you’re trying to observe today from your location, at its current position relative to the other objects in the sky.
    Measure the star from a different position in space, and note how the star’s apparent position appears to change relative to the other points of light you can identify.
    Use simple geometry — knowing the difference in your position from those first two measurements and the apparent change in angle — to determine the distance to the star.

    We’ve been using this method since the mid-1800s to measure the distances to the nearest stars, including Alpha Centauri, Vega and 61 Cygni, which has the distinction of being the first star to ever have its parallax measured back in 1838.

    5
    61 Cygni was the first star to have its parallax measured, but also is a difficult case due to its large proper motion. These two images, stacked in red and blue and taken almost exactly one year apart, show this binary star system’s fantastic speed. Image credit: Lorenzo2 of the forums at http://forum.astrofili.org/viewtopic.php?f=4&t=27548.

    But as straightforward as this method is, it comes along with its own inherent flaws. For starters, these angles are always very small: about 1 arcsecond (or 1/3600th of a degree) for a star that’s 3.26 light years away. For comparison, our nearest star, Proxima Centauri, is 4.24 light years away and has a parallax of just 0.77 arcsec. Stars more distant than perhaps one or two hundred light years can’t have their parallaxes measured from the ground at all, since the atmospheric turbulence contributes too greatly to uncertainties. In 1989, the European Space Agency attempted to overcome all of these difficulties by launching the Hipparcos satellite, which — from space — could measure precisions down to an accuracy of just 0.001 arcsec.

    12
    ESA/Hipparcos satellite

    6
    Testing the Hipparcos satellite in the Large Solar Simulator, ESTEC, February 1988. Image credit: Michael Perryman.

    Ideally, this would have meant that we could get accurate parallaxes for stars up to 1,600 light years away: about 100,000 stars total. The brightest and closest stars would be able to have their distances measured to better than 1% precision, which would then mean we’d be able to measure things like the expansion of the Universe throughout its history to that precision level as well. But a number of difficulties prevented that.

    The Earth doesn’t just move throughout the year; the Sun moves through the galaxy as well.
    Because parallax measurements aren’t simultaneous, other stars move relative to the Earth-Sun system as well.
    The more distant stars are not “fixed” in the sky, but exhibit relative motions as well. All stars have their own parallax, dependent on their distance.
    And the influence of gravitational bodies in our Solar System and throughout the galaxy can cause small deflections in starlight due to General Relativity.

    When you take all of these uncertainties into account, we wound up with uncertainties in positions that were much greater than 1%. In fact, if you expected a known nearby, bright star to simply have its position change the same way your thumb’s position, held at arm’s length, changed when you switched which eye you looked at it with, the actual data would be a rude awakening to you.

    7
    The “real” motion of Vega, just 26 light years away, as made from three years of Hipparcos data. Image credit: Michael Richmond of RIT, under a creative commons license, via http://spiff.rit.edu/classes/phys301/lectures/parallax/parallax.html.

    Over a period of three years, Hipparcos taught us a great deal about the motion of stars in our Milky Way, which is a combination of parallax and a series of true proper motions. The way to overcome these constraints is to take continuous measurements of stars as the Earth moves around the Sun and the Sun moves through space, with clearly identified, bright, distant “reference stars” which won’t show any discernible parallax. If you heard about the ESA’s Gaia mission, this is exactly what it’s attempting to do.

    ESA/GAIA satellite
    ESA/GAIA satellite

    With much greater accuracy and precision than Hipparcos, Gaia is undertaking an all-sky survey of the galaxy to measure the positions and motions of approximately 1 billion stars within the Milky Way.

    8
    A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, large and small Magellanic Clouds, and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. Image credit: ESA/GAIA.

    Parallaxes should be available for hundreds of millions of these stars, with a precision of just 10 µas (0.00001 arcsec) at maximum. We should be able to get significantly better than 1% precision for all of the Hipparcos stars, and — at last — should get outstanding parallax measurements for the closest Cepheid variable stars: Polaris and Delta Cephei. If we can understand the distances to this type of variable star within our own galaxy, we should be able to much better constrain our measurements of the cosmic distance ladder, and therefore, better understand how the Universe has expanded over its history and what makes it up.

    9
    Image credit: NASA/JPL-Caltech, of the (symbolic) cosmic distance ladder.

    It’s a bold, ambitious plan, and after hundreds of years of uncertainty in the distances to the stars, we’ll finally have the answer. By the year 2020, when Gaia’s data catalog is complete, we should know whether our various methods of measuring extragalactic distances have flaws or tensions, or whether all the pieces fall into place. We might not know exactly how far away the stars are today, but thanks to our greatest space observatories, we’re finally about to find out!

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:40 am on October 14, 2016 Permalink | Reply
    Tags: Astrometry, , , ESA Gaia, , , Parallaxes   

    From nationalgeographics.com: “New Milky Way Map Is a Spectacular Billion-Star Atlas” 

    National Geographic

    National Geographics

    September 14, 2016 [This just now appeared in social media.]
    Michael Greshko

    1
    The Milky Way, Scorpius, Sagittarius, Lagoon Nebula, Sagittarius Star Cloud, and Antares. From Atacama Desert, Chile.
    Photograph by Babak Tafreshi, National Geographic Creative

    After more than two years spent gluing its eyes to the heavens, an advanced celestial mapmaker has released its first results—wowing astronomers with the most accurate view of our Milky Way ever assembled.

    The announcement marks the first release of data from Gaia, a spacecraft operated by the European Space Agency (ESA).

    ESA/GAIA satellite
    ESA/GAIA satellite

    Launched in December 2013, the spacecraft currently sits a million miles away from Earth in the gravitational parking spot known as L2.

    LaGrange Points map. NASA
    LaGrange Points map. NASA

    From this unique vantage point, the craft has been cataloging stars and looking for shifts in their apparent positions caused by the spacecraft’s orbital motion around the sun.

    Measuring these shifts, or parallaxes, lets astronomers calculate the stars’ actual positions and movements through the galaxy with great precision, a field of study called astrometry that Gaia and its predecessor Hipparcos have revitalized.

    For thousands of years, astronomers from the ancient Babylonians to Tycho Brahe had preoccupied themselves with noting the stars’ precise locations, a crucial foundation to understanding the cosmos. But the field sputtered in the 1960s, when scientists reached the smallest parallaxes that Earth-based telescopes could measure, stymied by interference from our rippling atmosphere.

    It wasn’t until the 1980s and 1990s that the ESA satellite Hipparcos took astrometry to space, where it ultimately measured the precise distances of more than 100,000 stars. Gaia is even better: Hipparcos’s gaze reached only as far as 1,600 light-years away, barely leaving our celestial backyard, but Gaia is able to spy on stars up to 30,000 light-years away.

    2
    This all-sky view of the stars in our galaxy and its neighbors is based on the first year of observations from ESA’s Gaia satellite, taken from July 2014 to September 2015. Map by ESA

    Revolutionary Mapping

    With the new data release, Gaia has tracked the positions and motions of the brightest two million stars in the Milky Way, smashing the 100,000-star mark set by Hipparcos.

    The release, the first of five planned through 2022, also contains an atlas detailing the positions and brightnesses of some 1.1 billion stars in the Milky Way, based on 14 months of observations starting in July 2014. Over 400 million of these stars have never been seen before. (Also see “How Much Does the Milky Way Weigh?”)

    “It’s the largest-ever map made [of the Milky Way] from a single survey, and it’s also the most accurate map ever made,” says Anthony Brown of Leiden University, a member of the Gaia Data Processing and Analysis Consortium. He notes that while the map will improve dramatically in the coming years, it already pinpoints a star’s location to within 10 milliarcseconds—equivalent to determining an object’s position to within a hair’s breadth from more than a mile away.

    “Gaia is at the forefront of astrometry, charting the sky at precisions that have never been achieved before,” Alvaro Giménez, ESA’s director of science, says in a statement. “Today’s release gives us a first impression of the extraordinary data that await us and that will revolutionize our understanding of how stars are distributed and move across our galaxy.”

    What’s more, the data release adds hundreds of variable stars to astronomers’ toolkits. These stars regularly dim and brighten in predictable ways, allowing astronomers to measure vast cosmological distances. Having more of them to work with is akin to getting a larger, more precise yardstick.

    And Gaia is just getting started. By the end of its scheduled observation run, the spacecraft will have tracked the accurate positions and motions of roughly a billion stars, or one percent of the Milky Way’s estimated stellar population.

    “[Gaia] is going to give you this sort of three-dimensional movie of the galaxy, which is absolutely unprecedented,” says astronomer Michael Perryman of University College Dublin, who worked on Hipparcos before becoming Gaia’s lead project scientist from 1993 to 2007.

    “It’s very unusual, very revolutionary, and very spectacular—and it’s going to keep thousands of scientists busy for years.”

    And Gaia, which sports a billion-pixel camera and optics that rival those aboard the Hubble Space Telescope, isn’t just for tracking stars. The final survey will contain 250,000 new solar system objects, a million distant galaxies, 500,000 quasars—and about 20,000 exoplanets, says Gaia lead project scientist Timo Prusti.

    It’s no small wonder, then, that astronomers are agog. Columbia University astronomer Kathryn Johnston calls Gaia “the data set for galactic science for my generation of astronomers.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    The National Geographic Society has been inspiring people to care about the planet since 1888. It is one of the largest nonprofit scientific and educational institutions in the world. Its interests include geography, archaeology and natural science, and the promotion of environmental and historical conservation.

     
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