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  1. #941
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    The enduring mystique of Barnardís Star
    Larry Sessions in ASTRONOMY ESSENTIALS | November 14, 2018

    Sometimes called Barnardís Runaway Star, itís one of the best known stars in the history of astronomy and in popular culture.


    Our sunís closest neighbors among the stars, including Barnardís Star. Image via NASA PhotoJournal.

    Perhaps you know that, over the scale of our human lifespans, the stars appear fixed relative to one another. But Barnardís Star Ė sometimes called Barnardís Runaway Star Ė holds a speed record of sorts as the fastest-moving star in Earthís skies. It moves fast with respect to other stars because itís relatively close, only about 6 light-years away. What does its fast motion mean? It means Barnardís Star is nearby, and also that itís not moving with the general stream of stars around the Milky Wayís center. Instead, Barnardís Star is merely passing through our neighborhood of space. Relative to other stars, Barnardís Star moves 10.3 arcseconds per year, or about the width of a full moon in 174 years. This might not seem like much. But Ė to astronomers Ė Barnardís Star is virtually zipping across the sky.

    But, of course, thatís not the only reason this star is famous!

    Barnardís Star in history and popular culture. Yerkes Observatory astronomer E. E. Barnard discovered the large proper motion of Barnardís Star Ė that is, motion across our line of sight Ė in 1916.

    He noticed it while comparing photographs of the same part of the sky taken in 1894 and again in 1916. The star appeared in significantly different positions, betraying its rapid motion.

    Later, Harvard astronomer Edward Pickering found the star on photographic plates taken in 1888.

    Barnardís star came to our attention barely 100 years ago and canít be seen with the human eye, so the ancients didnít know about it. It doesnít figure into the lore of any constellation or cultural tradition. But that doesnít mean that it doesnít a have certain mystique about it that extends beyond the known facts.

    For example, even as long ago as the 1960s and í70s Ė long before successful planet-hunters like the Kepler spacecraft Ė there were suggestions that Barnardís Star might have a family of planets. At that time, reported discrepancies in the motion of the star led to a claim that at least one Jupiter-size planet, and possibly several planets, orbit it. Although the evidence was disputed and the claim now largely discredited, there has remained a chance of planetary discoveries. And, indeed, in November 2018 an international team of astronomers announced it was ď99 percent confidentĒ that a planet for Barnardís Star has now been found.

    The long-standing rumor of planets for Barnardís Star secured this starís place in science fiction. Itís featured in, for example, ďThe Hitchhikerís Guide to the GalaxyĒ by Douglas Adams; ďThe Garden of RamaĒ by Arthur C. Clarke and Gentry Lee; and several novels of physicist Robert L. Forward. In these works, the fictional planets of Barnardís Star are locations for early colonization or way-stations for exploration further into the cosmos.

    Barnardís Star also was the hypothetical target of Project Daedalus, a design study by members of the British Interplanetary Society, in which they envisioned an interstellar craft that could reach its destination within a human lifetime.

    And Barnardís Star has been featured in online games.

    Clearly, Barnardís Star captures peoplesí imaginations!

    How to see Barnardís Star. Barnardís Star is faint; its visual magnitude is only about 9.5. Thus this star canít be seen with the eye alone.

    Whatís more, its motion Ė though large in astronomical terms Ė is still too slow to be noticed in a single night or even easily across a human lifetime.

    Since Barnardís Star canít be seen without powerful binoculars or a telescope, finding it requires both experience and perseverance. It is located in the direction of the constellation Ophiuchus the Serpent Bearer, which is well placed for viewing on June, July and August evenings.

    Because Barnardís Star is a telescopic object, details on how to observe it are beyond the scope of this article, but there are procedures available online here.

    The science of Barnardís Star. The fame of Barnardís Star is in its novelty, the fact that it moves fastest through Earthís skies. But its real importance to astronomy lies in the fact that being so close, it is one of the best sources for studying red dwarfs, the most abundant stars in the universe.

    With only about 14 percent of the solar mass and less than 20 percent of the radius, it would take roughly seven Barnardís Stars to match the mass of our sun, and 133 to match our sunís volume.

    Like all stars, Barnardís Star shines via thermonuclear fusion, changing light elements (hydrogen) into more massive elements (helium), while releasing enormous amounts of energy. Even so, the lower mass of Barnardís Star makes it about 2,500 times less powerful than our sun.

    In other words, Barnardís Star is much dimmer and cooler than our sun. If it replaced the sun in our solar system, it would shine only about four ten-thousandths as brightly as our sun. At the same time, it would be about 100 times brighter than a full moon. No life on Earth would be possible if we orbited Barnardís Star instead of our sun, however. The much-decreased stellar heat would plunge Earthís global temperatures to hundreds of degrees below zero.

    Although very common, red dwarfs like Barnardís Star are typically dim. Thus they are notoriously faint and hard to study. In fact, not a single red dwarf can be seen with the unaided human eye. But because Barnardís Star is relatively close and bright, it has become a go-to model for all things red dwarf.

    At nearly six light-yearsĎ distance, Barnardís Star is often cited as the second-closest star to our sun (and Earth). This is true only if you consider the triple star system Alpha Centauri as one star.

    Proxima Centauri, the smallest and faintest of Alpha Centauriís three components, is the closest known star to the sun at just 4.24 light years away. It, too, is a red dwarf. So Barnardís Star is only the second-closest red dwarf star. It is perhaps more important for astronomical purposes, though, because Proxima is four times fainter and thus harder to study.

    Special thanks to David J. Darling and Jack Schmidling for their help with this article.
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  2. #942
    Moderator at Work ilan's Avatar
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    Gaia spots enormous ghost galaxy on Milky Wayís outskirts
    EarthSky in SPACE | November 16, 2018

    Gaia satellite data revealed the galaxy, which has avoided detection until now, thanks to its extremely low density and hiding place behind the shroud of the Milky Wayís disk.


    L-R: Large Magellanic Cloud, the Milky Way, Antlia 2.
    Image via V. Belokurov based on the images by Marcus and Gail Davies and Robert Gendler

    The Gaia satellite has spotted an enormous ďghostĒ galaxy lurking on the outskirts of the Milky Way. An international team of astronomers discovered the massive object when trawling through data from the European Space Agencyís (ESA) Gaia satellite. The object, named Antlia 2 (or Ant 2), has avoided detection until now thanks to its extremely low density as well as a perfectly-chosen hiding place, behind the shroud of the Milky Wayís disk. The researchers published the study results online November 9, 2018.

    Ant 2 is known as a dwarf galaxy. As structures emerged in the early universe, dwarfs were the first galaxies to form, and so most of their stars are old, low-mass and metal-poor. But compared to most other known dwarf satellites of our Milky Way galaxy, Ant 2 is immense: it is as big as the Large Magellanic Cloud, and a third the size of the Milky Way itself.

    The researchers say that Ant 2 never comes too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years) away. What makes Ant 2 even more unusual is how little light it gives out. Compared to the Large Magellanic Cloud, which is another satellite of the Milky Way, Ant 2 is 10,000 times fainter. In other words, it is either far too large for its luminosity or far too dim for its size.

    Gabriel Torrealba of University of Cambridge is the paperís lead author. Torrealba said in a statement:

    This is a ghost of a galaxy. Objects as diffuse as Ant 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data.

    The Gaia data allowed the researchers to obtain the galaxyís mass, which was much lower than expected for an object of its size. Study co-author Sergey Koposov from Carnegie Mellon University said:

    The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the galactic tides of the Milky Way. What remains unexplained, however, is the objectís giant size. Normally, as galaxies lose mass to the Milky Wayís tides, they shrink, not grow.

    Study co-author Matthew Walker, also from Carnegie Mellon University, said:

    Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball. We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one.Read more about how the astronomers discovered Ant 2 here.

    The ESAís Gaia mission has produced the richest star catalogue to date, including high-precision measurements of nearly 1.7 billion stars, and revealed previously unseen details of our Milky Way galaxy. Earlier this year, Gaiaís second data release made new details of stars in the Milky Way available to scientists worldwide.
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