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  • richardmitnick 12:36 pm on May 30, 2017 Permalink | Reply
    Tags: , , , , , Do Stars Fall Quietly into Black Holes, Harvard Astronomy, or Crash into Something Utterly Unknown?,   

    From CfA: “Do Stars Fall Quietly into Black Holes, or Crash into Something Utterly Unknown?” 

    Harvard Smithsonian Center for Astrophysics


    Center For Astrophysics

    May 30, 2017
    Rebecca Johnson
    UT Austin Astronomy Program
    512-475-6763
    rjohnson@astro.as.utexas.edu

    Peter Edmonds
    Harvard-Smithsonian Center for Astrophysics
    +1 617-571-7279
    pedmonds@cfa.harvard.edu

    1

    Astronomers at The University of Texas at Austin and Harvard University have put a basic principle of black holes to the test, showing that matter completely vanishes when pulled in. Their results constitute another successful test for Albert Einstein’s General Theory of Relativity.

    Most scientists agree that black holes, cosmic entities of such great gravity that nothing can escape their grip, are surrounded by a so-called event horizon. Once matter or energy gets close enough to the black hole, it cannot escape — it will be pulled in. Though widely believed, the existence of event horizons has not been proved.

    “Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” said Pawan Kumar, a professor of astrophysics at The University of Texas at Austin.

    Supermassive black holes are thought to lie at the heart of almost all galaxies. But some theorists suggest that there’s something else there instead — not a black hole, but an even stranger supermassive object that has somehow managed to avoid gravitational collapse to a singularity surrounded by an event horizon. The idea is based on modified theories of General Relativity, Einstein’s theory of gravity.

    While a singularity has no surface area, the noncollapsed object would have a hard surface. So material being pulled closer — a star, for instance — would not actually fall into a black hole, but hit this hard surface and be destroyed.

    Kumar, his graduate student Wenbin Lu, and Ramesh Narayan, a theorist from the Harvard-Smithsonian Center for Astrophysics, have come up with a test to determine which idea is correct.

    “Our motive is not so much to establish that there is a hard surface,” Kumar said, “but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes.”

    The team figured out what a telescope would see when a star hit the hard surface of a supermassive object at the center of a nearby galaxy: The star’s gas would envelope the object, shining for months, perhaps even years.

    Once they knew what to look for, the team figured out how often this should be seen in the nearby universe, if the hard-surface theory is true.

    “We estimated the rate of stars falling onto supermassive black holes,” Lu said. “Nearly every galaxy has one. We only considered the most massive ones, which weigh about 100 million solar masses or more. There are about a million of them within a few billion light-years of Earth.”

    They then searched a recent archive of telescope observations. Pan-STARRS, a 1.8-meter telescope in Hawaii, recently completed a project to survey half of the northern hemisphere sky.

    Pan-STARRS1 located on Haleakala, Maui, HI, USA

    The telescope scanned the area repeatedly during a period of 3.5 years, looking for “transients” — things that glow for a while and then fade. Their goal was to find transients with the expected light signature of a star falling toward a supermassive object and hitting a hard surface.

    “Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true,” Lu said.

    They did not find any.

    “Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades,” Narayan said. “General Relativity has passed another critical test.”

    Now the team is proposing to improve the test with an even larger telescope: the 8.4-meter Large Synoptic Survey Telescope (LSST, now under construction in Chile). Like Pan-STARRS, LSST will make repeated surveys of the sky over time, revealing transients — but with much greater sensitivity.

    This research has been published in the June issue of the journal Monthly Notices of the Royal Astronomical Society.

    See the full article here .

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    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

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  • richardmitnick 12:50 pm on March 25, 2014 Permalink | Reply
    Tags: , , , , Harvard Astronomy, Science@NASA   

    From Harvard Astronomy: “Milky Way Has 4 Billion Years to Live — But Our Sun Will Survive” 

    Harvard Astronomy Banner
    Harvard Astronomy

    March 24, 2014
    No Writer Credit

    “Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda.

    The galaxies as we know them will not survive.

    In fact, our solar system is going to outlive our galaxy. At that point, the sun will not yet be a red giant star – but it will have grown bright enough to roast Earth’s surface. Any life forms still there, though, will be treated to some pretty spectacular cosmic choreography.”

    graph

    A video from Science@NASA

    See the full article here.


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  • richardmitnick 12:34 pm on November 19, 2013 Permalink | Reply
    Tags: , , , , , Harvard Astronomy,   

    From Harvard Astronomy: “Shep Doelman: Imaging Black Holes with The Event Horizon Telescope” 

    Harvard Astronomy Banner
    Harvard Astronomy

    November 19, 2013

    Shep Doelman, sdoeleman@cfa.harvard.edu

    Recent technical advances and observations have now demonstrated that the goal of making an image of a black hole is within reach. Using the technique of Very Long Baseline Interferometry (VLBI), in which widely separated radio dishes are linked together to form an Earth-sized array, our group has succeeded in confirming event horizon scale structures in two super massive black holes: Sagittarius A*, the 4 million solar mass black hole at the center of the Milky Way (Nature, 455, 78, ’08), and M87, a 6 billion solar mass black hole in the giant elliptical galaxy Virgo A (Science, 338, 355, ’12). This has been accomplished by extending the VLBI technique to the highest observing frequencies and bandwidths, which has provided the required angular resolution and sensitivity.

    bh

    To achieve true imaging capability, an international collaboration is developing next-generation VLBI instrumentation for deployment on a Global array of mm and submm wavelength facilities. This will extend the current 1.3mm VLBI array to Earth-diameter baselines for which the angular resolution obtained is well matched to the SgrA* and M87 event horizons. Efforts are also aimed at shorter wavelength observations at 0.87mm, where Global baselines can achieve <20 micro arcsecond resolution. This new array is called the Event Horizon Telescope (EHT).

    EHT observations will target modeling and imaging of strong General Relativistic signatures that should become evident hear the black hole. Foremost among these is the black hole ‘shadow’, a consequence of light bending in the black hole’s strong gravity, leading to an annular brightening of the last photon orbit. The size and shape of this shadow is a prediction of Einstein’s GR. Non-imaging analyses of EHT data will be very sensitive to asymmetries caused by orbiting ‘hot-spots’ or Magnetohydrodynamic turbulence in the accretion flow. Observations of M87 will lead to direct imaging of emission at the base of a relativistic AGN jet. The overall goal is to spatially resolve a region of space-time where gravity is dominant, with an aim to test GR and models of black hole accretion and jet formation on Schwarzschild radius scales.

    See the full article here.


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  • richardmitnick 9:07 am on August 18, 2013 Permalink | Reply
    Tags: , , , , Harvard Astronomy   

    From Harvard Astronomy: “Gamma-ray burst illuminates invisible galaxy in the ‘dark ages’ “ 

    Harvard Astronomy Banner
    Harvard Astronomy

    August 7, 2013
    David A. Aguilar
    Director of Public Affairs
    Harvard-Smithsonian Center for Astrophysics
    617-495-7462
    daguilar@cfa.harvard.edu

    Christine Pulliam
    Public Affairs Specialist
    Harvard-Smithsonian Center for Astrophysics
    617-495-7463
    cpulliam@cfa.harvard.edu

    “More than 12 billion years ago a star exploded, ripping itself apart and blasting its remains outward in twin jets at nearly the speed of light. At its death it glowed so brightly that it outshone its entire galaxy by a million times. This brilliant flash traveled across space for 12.7 billion years to a planet that hadn’t even existed at the time of the explosion – our Earth. By analyzing this light, astronomers learned about a galaxy that was otherwise too small, faint and far away for even the Hubble Space Telescope to see.

    blast

    ‘This star lived at a very interesting time, the so-called dark ages just a billion years after the Big Bang,” says lead author Ryan Chornock of the Harvard-Smithsonian Center for Astrophysics (CfA).

    ‘In a sense, we’re forensic scientists investigating the death of a star and the life of a galaxy in the earliest phases of cosmic time,’ he adds.

    The star announced its death with a flash of gamma rays, an event known as a gamma-ray burst (GRB). GRB 130606A was classified as a long GRB since the burst lasted for more than four minutes. It was detected by NASA’s Swift spacecraft on June 6th. Chornock and his team quickly organized follow-up observations by the MMT Telescope in Arizona and the Gemini North telescope in Hawaii.

    We were able to get right on target in a matter of hours,’ Chornock says. ‘That speed was crucial in detecting and studying the afterglow.'”

    See the full article here.


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