Tagged: Type 1a supernovas Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 12:42 pm on August 6, 2019 Permalink | Reply
    Tags: "Ghosts of Ancient Explosions Live on in Stars Today", , , , , , Type 1a supernovas, ,   

    From Caltech: “Ghosts of Ancient Explosions Live on in Stars Today” 

    Caltech Logo

    From Caltech

    August 05, 2019

    Contact
    Lori Dajose
    (626) 395‑1217
    ldajose@caltech.edu

    The chemical composition of certain stars gives clues about their predecessors, stars that have long since exploded and faded.

    1
    Image of a Type Ia supernova. Credit: Zwicky Transient Facility

    Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

    Edwin Hubble at Caltech Palomar Samuel Oschin 48 inch Telescope, (credit: Emilio Segre Visual Archives/AIP/SPL)

    Caltech Palomar Intermediate Palomar Transient Factory telescope at the Samuel Oschin Telescope at Palomar Observatory,located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

    Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

    When small, dense stars called white dwarfs explode, they produce bright, short-lived flares called Type Ia supernovae. These supernovae are informative cosmological markers for astronomers—for example, they were used to prove that the universe is accelerating in its expansion.

    White dwarfs are not all the same, ranging from half of the mass of our sun to almost 50 percent more massive than our sun. Some explode in Type Ia supernovae; others simply die quietly. Now, by studying the “fossils” of long-exploded white dwarfs, Caltech astronomers have found that early on in the universe, white dwarfs often exploded at lower masses than they do today. This discovery indicates that a white dwarf could explode from a variety of causes, and does not necessarily have to reach a critical mass before exploding.

    A paper about the research, led by Evan Kirby, assistant professor of astronomy, appears in The Astrophysical Journal.

    Near the end of their lives, a majority of stars like our sun dwindle down into dim, dense white dwarfs, with all their mass packed into a space about the size of Earth. Sometimes, white dwarfs explode in what’s called a Type Ia (pronounced one-A) supernova.

    It is uncertain why some white dwarfs explode while others do not. In the early 1900s, an astrophysicist named Subrahmanyan Chandrasekhar calculated that if a white dwarf had more than 1.4 times the mass of our sun, it would explode in a Type Ia supernova. This mass was dubbed the Chandrasekhar mass. Though Chandrasekhar’s calculations gave one explanation for why some more massive white dwarfs explode, it did not explain why other white dwarfs less than 1.4 solar masses also explode.

    Studying Type Ia supernovae is a time-sensitive process; they flare into existence and fade back into darkness all within a few months. To study long-gone supernovae and the white dwarfs that produced them, Kirby and his team use a technique colloquially called galactic archaeology.

    Galactic archaeology is the process of looking for chemical signatures of long-past explosions in other stars. When a white dwarf explodes in a Type Ia supernova, it pollutes its galactic environment with elements forged in the explosion—heavy elements like nickel and iron. The more massive a star is when it explodes, the more heavy elements will be formed in the supernova. Then, those elements become incorporated into any newly forming stars in that region. Just as fossils today give clues about animals that have long ceased to exist, the amounts of nickel in stars illustrates how massive their long-exploded predecessors must have been.

    Using the Keck II telescope, Kirby and his team first looked at certain ancient galaxies, those that ran out of material to form stars in the first billion years of the universe’s life.

    Keck 2 telescope Maunakea Hawaii USA, 4,207 m (13,802 ft)

    Most of the stars in these galaxies, the team found, had relatively low nickel content. This meant that the exploded white dwarfs that gave them that nickel must have been relatively low mass—about as massive as the sun, lower than the Chandrasekhar mass.

    Yet, the researchers found that the nickel content was higher in more recently formed galaxies, meaning that as more time went by since the Big Bang, white dwarfs had begun to explode at higher masses.

    “We found that, in the early universe, white dwarfs were exploding at lower masses than later in the universe’s lifetime,” says Kirby.”It’s still unclear what has driven this change.”

    Understanding the processes that result in Type Ia supernovae is important because the explosions themselves are useful tools for making measurements of the universe. Regardless of how they exploded, most Type Ia supernovae follow a well-characterized relationship between their luminosity and the time it takes for them to fade.

    “We call Type Ia supernovae ‘standardizable candles.’

    Standard Candles to measure age and distance of the universe from supernovae NASA

    If you look at a candle at a distance, it will look dimmer than when it’s up close. If you know how bright it is supposed to be up close, and you measure how bright it is at a distance, you can calculate that distance,” says Kirby. “Type Ia supernovae have been very useful in calculating things like the rate of expansion of the universe. We use them all the time in cosmology. So, it’s important to understand where they come from and characterize the white dwarfs that generate these explosions.”

    The next steps are to study elements other than nickel, in particular, manganese. Manganese production is very sensitive to the mass of the supernova that produces it, and therefore gives a precise way to validate the conclusions drawn by the nickel content.

    The paper is titled Evidence for Sub-Chandrasekhar Type Ia Supernovae from Stellar Abundances in Dwarf Galaxies. In addition to Kirby, co-authors are Justin L. Xie and Rachel Guo of Harvard University, Caltech graduate student Mithi A. C. de los Reyes, Maria Bergemann and Mikhail Kovalev of the Max Planck Institute for Astronomy, Ken J. Shen of University of California Berkeley, and Anthony L. Piro and Andrew McWilliam of the Observatories of the Carnegie Institution for Science. Funding was provided by the National Science Foundation, a Cottrell Scholar award from the Research Corporation for Science Advancement, and Caltech.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.


    Stem Education Coalition

    The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

    Caltech campus

     
  • richardmitnick 7:59 am on September 27, 2017 Permalink | Reply
    Tags: , , , , , , Dark energy may not exist, , Type 1a supernovas   

    From COSMOS: “Dark energy may not exist” 

    Cosmos Magazine bloc

    COSMOS Magazine

    27 September 2017
    Stuart Gary

    1
    A model of the universe that takes into account the irregular distribution of galaxies may make dark energy disappear. NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA

    The accelerating expansion of the universe due to a mysterious quantity called “dark energy” may not be real, according to research claiming it might simply be an artefact caused by the physical structure of the cosmos.

    The findings, reported in the Monthly Notices of the Royal Astronomical Society, claims the fit of Type Ia supernovae to a model universe with no dark energy appears to be slightly better than the fit using the standard dark energy model.

    The study’s lead author David Wiltshire, from the University of Canterbury in New Zealand, says existing dark energy models are based on a homogenous universe in which matter is evenly distributed.

    CMB per ESA/Planck

    ESA/Planck

    “The real universe has a far more complicated structure, comprising galaxies, galaxy clusters, and superclusters arranged in a cosmic web of giant sheets and filaments surrounding vast near-empty voids”, says Wiltshire.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

    Current models of the universe require dark energy to explain the observed acceleration in the rate at which the universe is expanding.

    Scientists base this conclusion on measurements of the distances to Type 1a supernovae in distant galaxies, which appear to be farther away than they would be if the universe’s expansion was not accelerating.

    Type 1a supernovae are powerful explosions bright enough to briefly outshine an entire galaxy. They’re caused by the thermonuclear destruction of a type of star known as a white dwarf – the stellar corpse of a Sun-like star.

    All Type 1a supernovae are thought to explode at around the same mass – a figure known in astrophysics as the Chandrasekhar limit – which equates to about 1.44 times the mass of the Sun.

    Because they all explode at about the same mass, they also explode with about the same level of luminosity.

    This allows astronomers to use them as standard candles to measure cosmic distances across the universe – in the same way you can determine how far away a row of street lights is along a road by how bright each one appears from where you’re standing.

    2
    Standard candles. https://www.extremetech.com

    On a galactic scale, gravity appears to be stronger than scientists can account for, using the normal matter of the universe, the material in the standard model of particle physics, which makes up all the stars, planets, buildings, and people.

    To explain their observations, scientists invented “dark matter”, a mysterious substance which seems to only interact gravitationally with normal matter.

    To explain science’s observations of how galaxies move, there must be about five times as much dark matter as normal matter.

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    It’s called dark because whatever it is, it cannot emit light. Scientists can only see its effects gravitationally on normal matter.

    On the even larger cosmic scales of an expanding universe, gravity appears to be weaker than expected in a universe containing only normal matter and dark matter.

    And so, scientists invented a new force, called “dark energy”, a sort of anti-gravitational force causing an acceleration in the expansion of the universe out from the big bang 13.8 billion years ago.

    Dark energy isn’t noticeable on small scales, but becomes the dominating force of the universe on the largest cosmic scales: almost four times greater than the gravity of normal and dark matter combined.

    The idea of dark energy isn’t new. Albert Einstein first came up with it to explain a problem he was having when he applied his famous 1915 equations of general relativity theory to the whole universe.

    Like other scientists at the time, Einstein believed the universe was in a steady unchanging state. Yet, when applied to cosmology, his equations showed the universe wanted to expand or contract as matter interacts with the fabric of spacetime: matter tells spacetime how to curve, and spacetime tells matter how to move.

    To resolve the problem, Einstein introduced a dark energy force in 1917 which he called the “cosmological constant”.

    It was a mathematical invention, a fudge factor designed to solve the discrepancies between general relativity theory and the best observational evidence of the day, thus bringing the universe back into a steady state.

    Years later, when astronomer Edwin Hubble discovered that galaxies appeared to be moving away from each other, and the rate at which they were moving was proportional to their distance, Einstein realised his mistake, describing the cosmological constant as the biggest blunder of his life.

    However, the idea has never really gone away, and keeps reappearing to explain strange observations.

    In the mid 1990s two teams of scientists, one led by Brian Schmidt and Adam Riess, and the other by Saul Perlmutter, independently measured distances to Type 1a supernovae in the distant universe, finding that they appeared to be further way than they should be if the universe’s rate of expansion was constant.

    The observations led to the hypothesis that some kind of dark energy anti-gravitational force has caused the expansion of the universe to accelerate over the past six billion years.

    Wiltshire and his colleagues now challenge that reasoning.

    “But these observations are based on an old model of expansion that has not changed since the 1920s”, he says.

    In 1922, Russian physicist Alexander Friedmann used Einstein’s field equations to develop a physical cosmology governing the expansion of space in homogeneous and isotropic models of the universe.

    “Friedmann’s equation assumes an expansion identical to that of a featureless soup, with no complicating structure”, says Wiltshire.

    This has become the basis of the standard Lambda Cold Dark Matter cosmology used to describe the universe.

    “In reality, today’s universe is not homogeneous”, says Wiltshire.

    The earliest snapshot of the universe – called cosmic microwave background radiation – displays only slight temperature variations caused by differences in densities present 370,000 years after the Big Bang.

    However, gravitational instabilities led those tiny density variations to evolve into the stars, galaxies, and clusters of galaxies, which made up the large scale structure of the universe today.

    “The universe has become a vast cosmic web dominated in volume by empty voids, surrounded by sheets of galaxies and threaded by wispy filaments”, says Wiltshire.

    Rather than comparing the supernova observations to the standard Lambda Cold Dark Matter cosmological model, Wiltshire and colleagues used a different model, called ‘timescape cosmology’.

    Timescape cosmology has no dark energy. Instead, it includes variations in the effects of gravity caused by the lumpiness in the structure in the universe.

    Clocks carried by observers in galaxies differ from the clock that best describes average expansion once variations within the universe (known as “inhomogeneity” in the trade) becomes significant.

    Whether or not one infers accelerating expansion then depends crucially on the clock used.

    “Timescape cosmology gives a slightly better fit to the largest supernova data catalogue than Lambda Cold Dark Matter cosmology,” says Wiltshire.

    He admits the statistical evidence is not yet strong enough to definitively rule in favour of one model over the other, and adds that future missions such as the European Space Agency’s Euclid spacecraft will have the power to distinguish between differing cosmology models.

    ESA/Euclid spacecraft

    Another problem involves science’s understanding of Type 1a supernovae. They are not actually perfect standard candles, despite being treated as such in calculations.

    Since timescape cosmology uses a different equation for average expansion, it gives scientists a new way to test for changes in the properties of supernovae over distance.

    Regardless of which model ultimately fits better, better understanding of this will increase the confidence with which scientists can use them as precise distance indicators.

    Answering questions like these will help scientists determine whether dark energy is real or not – an important step in determining the ultimate fate of the universe.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 9:35 pm on March 3, 2017 Permalink | Reply
    Tags: , , , Type 1a supernovas   

    From NC State via phys.org: “Calculations show close Ia supernova should be neutrino detectable offering possibility of identifying explosion type” 

    NC State bloc

    North Carolina State University

    phys.org

    phys.org

    1
    Density contour plots including deflagration (white) and detonation (green) surfaces. Credit: arXiv:1609.07403 [astro-ph.HE]

    A team of researchers at North Carolina State University has found that current and future neutrino detectors placed around the world should be capable of detecting neutrinos emitted from a relatively close supernova. They also suggest that measuring such neutrinos would allow them to explain what goes on inside of a star during such an explosion—if the measurements match one of two models that the team has built to describe the inner workings of a supernova.

    Supernovae have been classified into different types depending on what causes them to occur—one type, called a la supernova, occurs when a white dwarf pulls in enough material from a companion, eventually triggering carbon fusion, which leads to a massive explosion. Researchers here on Earth can see evidence of a supernova by the light that is emitted. But astrophysicists would really like to know more about the companion and the actual process that occurs inside the white dwarf leading up to the explosion—and they believe that might be possible by studying the neutrinos that are emitted.

    In this new effort, a team led by Warren Wright calculated that neutrinos from a relatively nearby supernova should be detectable by current sensors already installed and working around the planet and by those that are in the works. Wright also headed two teams that have each written a paper describing one of two types of models that the team has built to describe the process that occurs in the white dwarf leading up to the explosion—both teams have published their work in the journal Physical Review Letters.

    The first model is called the deflagration-to-detonation transition; the second, the gravitationally confined detonation. Both are based on theory regarding interactions inside of the star and differ mostly in how spherically symmetric they are. The two types would also emit different kinds and amounts of neutrinos, which is why the team is hoping that the detectors capable of measuring them will begin to do so. That would allow the teams to compare their models against real measurable data, and in so doing, perhaps finally offer some real evidence of what occurs when stars explode.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    NC State campus

    NC State was founded with a purpose: to create economic, societal and intellectual prosperity for the people of North Carolina and the country. We began as a land-grant institution teaching the agricultural and mechanical arts. Today, we’re a pre-eminent research enterprise that excels in science, technology, engineering, math, design, the humanities and social sciences, textiles and veterinary medicine.

    NC State students, faculty and staff take problems in hand and work with industry, government and nonprofit partners to solve them. Our 34,000-plus high-performing students apply what they learn in the real world by conducting research, working in internships and co-ops, and performing acts of world-changing service. That experiential education ensures they leave here ready to lead the workforce, confident in the knowledge that NC State consistently rates as one of the best values in higher education.

     
  • richardmitnick 2:34 pm on January 5, 2017 Permalink | Reply
    Tags: , , , Type 1a supernovas,   

    From U Chicago: “Research reinforces role of supernovae in clocking the universe” 

    U Chicago bloc

    University of Chicago

    January 3, 2017
    Greg Borzo

    1
    New research confirms the role Type Ia supernovae, like G299 pictured above, play in measuring universe expansion. Courtesy of NASA.

    How much light does a supernova shed on the history of universe?

    New research by cosmologists at the University of Chicago and Wayne State University confirms the accuracy of Type Ia supernovae in measuring the pace at which the universe expands. The findings support a widely held theory that the expansion of the universe is accelerating and such acceleration is attributable to a mysterious force known as dark energy. The findings counter recent headlines that Type Ia supernova cannot be relied upon to measure the expansion of the universe.

    Using light from an exploding star as bright as entire galaxies to determine cosmic distances led to the 2011 Nobel Prize in physics. The method relies on the assumption that, like lightbulbs of a known wattage, all Type Ia supernovae are thought to have nearly the same maximum brightness when they explode. Such consistency allows them to be used as beacons to measure the heavens. The weaker the light, the farther away the star. But the method has been challenged in recent years because of findings the light given off by Type Ia supernovae appears more inconsistent than expected.

    “The data that we examined are indeed holding up against these claims of the demise of Type Ia supernovae as a tool for measuring the universe,” said Daniel Scolnic, a postdoctoral scholar at UChicago’s Kavli Institute for Cosmological Physics and co-author of the new research published in Monthly Notices of the Royal Astronomical Society. “We should not be persuaded by these other claims just because they got a lot of attention, though it is important to continue to question and strengthen our fundamental assumptions.”

    One of the latest criticisms of Type Ia supernovae for measurement concluded the brightness of these supernovae seems to be in two different subclasses, which could lead to problems when trying to measure distances. In the new research led by David Cinabro, a professor at Wayne State, Scolnic, Rick Kessler, a senior researcher at the Kavli Institute, and others, they did not find evidence of two subclasses of Type Ia supernovae in data examined from the Sloan Digital Sky Survey Supernovae Search and Supernova Legacy Survey.

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    The recent papers challenging the effectiveness of Type Ia supernovae for measurement used different data sets.

    A secondary criticism has focused on the way Type Ia supernovae are analyzed. When scientists found that distant Type Ia supernovae were fainter than expected, they concluded the universe is expanding at an accelerating rate. That acceleration is explained through dark energy, which scientists estimate makes up 70 percent of the universe. The enigmatic force pulls matter apart, keeping gravity from slowing down the expansion of the universe.

    Yet a substance that makes up 70 percent of the universe but remains unknown is frustrating to a number of cosmologists. The result was a reevaluation of the mathematical tools used to analyze supernovae that gained attention in 2015 by arguing that Type Ia supernovae don’t even show dark energy exists in the first place.

    Scolnic and colleague Adam Riess, who won the 2011 Nobel Prices for the discovery of the accelerating universe, wrote an article for Scientific American Oct. 26, 2016, refuting the claims. They showed that even if the mathematical tools to analyze Type Ia supernovae are used “incorrectly,” there is still a 99.7 percent chance the universe is accelerating.

    The new findings are reassuring for researchers who use Type Ia supernovae to gain an increasingly precise understanding of dark energy, said Joshua A. Frieman, senior staff member at the Fermi National Accelerator Laboratory [FNAL] who was not involved in the research.

    “The impact of this work will be to strengthen our confidence in using Type Ia supernovae as cosmological probes,” he said.

    Citation: “Search for Type Ia Supernova NUV-Optical Subclasses,” by David Cinabro and Jake Miller (Wayne State University); and Daniel Scolnic and Ashley Li (Kavli Institute for Cosmological Physics at the University of Chicago); and Richard Kessler (Kavli Institute for Cosmological Physics at University of Chicago and the Department of Astronomy and Astrophysics at the University of Chicago). Monthly Notices of the Royal Astronomical Society, November 2016. DOI: 10.1093/mnras/stw3109

    Funding: Kavli Institute for Cosmological Physics at the University of Chicago, Kavli Foundation, Fred Kavli, Space Telescope Science Institute, and National Aeronautics and Space Administration.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    U Chicago Campus

    An intellectual destination

    One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

     
  • richardmitnick 9:16 pm on January 20, 2016 Permalink | Reply
    Tags: , , , Type 1a supernovas   

    From CAASTRO: “Simulated supernovae match many observations but lack diversity” 

    CAASTRO bloc

    CAASTRO ARC Centre of Excellence for All Sky Astrophysics

    21 January 2016
    No writer credit found

    Temp 3

    Supernovae are dramatic explosions that occur at the end of the lives of certain types of star. These events are intensely energetic, ejecting debris with speeds of tens-of-thousands of kilometers per second and radiating, albeit briefly, with luminosities billions of times greater than our Sun. The energy and material ejected by supernovae have a key role in the evolution and chemical history of galaxies, and the brightness of supernovae means that it is feasible to observe them at great distances and use them to probe the expansion history of our Universe. However, we still lack a complete understanding of the nature of supernovae: which stars explode, and why? In an ongoing attempt to answer such questions, astronomers within CAASTRO, and around the world, are studying supernovae using a combination of observational campaigns, such as the SkyMapper supernova survey, and theoretical simulations of explosion physics.

    ANU Skymapper telescope
    ANU Skymapper telescope interior
    SkyMapper telescope

    As part of an international research team, Dr Stuart Sim (CAASTRO Associate Investigator, Queen’s University Belfast) and Dr Ivo Seitenzahl (CAASTRO Associate Investigator, ANU) led two studies that help make the link between theoretical ideas and observed properties of supernovae. Their work focuses on so-called “Type Ia” supernovae. These supernovae are thought to result from the explosion of white dwarf stars, but the means by which the explosion is triggered is widely debated. One of the leading models for Type Ia supernovae is the “Chandrasekhar mass delayed-detonation” scenario in which explosion occurs as a result of a white dwarf star increasing in mass due to transfer of material from a main-sequence (or giant) companion star in a binary system. In their publications, the team (also including CAASTRO Associate Investigator Dr Ashley Ruiter, ANU) presents the first set of theoretical predictions for light curves and spectra from a set of full three-dimensional hydrodynamical simulations of the delayed-detonation model. These synthetic observables can be compared to real data to judge the success of the theoretical models.

    Temp 1

    The researchers found that the full explosion models are remarkably successful in explaining many observed properties: although imperfect, the match to many observed features in the optical spectra of real supernovae is remarkable, and variations due to the orientation from which the supernova is observed can explain some of the observed differences between supernovae. However, the set of models considered do not yet account for the full range of observed properties of Type Ia supernovae. Building on this work, the team is now focused on extending their studies to alternative theoretical models and identifying the best way to use modern observations to distinguish between models.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Astronomy is entering a golden age, in which we seek to understand the complete evolution of the Universe and its constituents. But the key unsolved questions in astronomy demand entirely new approaches that require enormous data sets covering the entire sky.

    In the last few years, Australia has invested more than $400 million both in innovative wide-field telescopes and in the powerful computers needed to process the resulting torrents of data. Using these new tools, Australia now has the chance to establish itself at the vanguard of the upcoming information revolution centred on all-sky astrophysics.

    CAASTRO has assembled the world-class team who will now lead the flagship scientific experiments on these new wide-field facilities. We will deliver transformational new science by bringing together unique expertise in radio astronomy, optical astronomy, theoretical astrophysics and computation and by coupling all these capabilities to the powerful technology in which Australia has recently invested.

    PARTNER LINKS

    The University of Sydney
    The University of Western Australia
    The University of Melbourne
    Swinburne University of Technology
    The Australian National University
    Curtin University
    University of Queensland

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: