StarDate

StarDate

  • Crowded Skies

    The western evening sky features several bright stars and planets in late May. This view, on May 22, is roughly an hour after sunset, and includes the crescent Moon. The Moon will move up and to the left on succeeding nights as the crescent grows fatter.

    Text ©2015 The University of Texas at Austin McDonald Observatory

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Moon and Companions

    The crescent Moon stands at the middle of a beautiful celestial grouping this evening. That grouping includes the planets Venus and Jupiter — the second- and third-brightest objects in the night sky — and several bright stars.

    Venus is to the lower right of the Moon. It’s the dazzling “evening star,” so there’s no way to miss it. In fact, it’s so bright that you might mistake it for an approaching airplane with its landing lights turned on. Keep an eye on it for a bit, though, and you’ll see that it doesn’t move — it maintains its position relative to the other astronomical objects around it.

    Jupiter is about the same distant to the upper left of the Moon. It, too, is extremely bright — brighter than any true star in the night sky. That’s because Jupiter is the biggest planet in the solar system, and because it’s covered by clouds that reflect a lot of sunlight. It’s only about a tenth as bright as Venus, though, because it’s hundreds of millions of miles farther.

    A few stars are part of the evening tableau as well. Procyon, the brightest star of Canis Minor, the little dog, stands below the Moon. And Pollux and Castor, the twins of Gemini, are off to the right of the Moon.

    So enjoy this beautiful grouping throughout the evening. It’s especially pretty as the last of the twilight adds a bit of color to the view.

    And since that’s a lot of stuff to keep track of, we’ve posted a chart on our web site.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Moon and Venus

    Venus is slowing down.

    Our closest planetary neighbor takes about 243 Earth days to make a single turn on its axis — far longer than any other planet in the solar system. But observations by Venus-orbiting spacecraft showed that from 1986 to 2012, that time increased by about six-and-a-half minutes.

    We can’t see the surface of Venus from Earth because the planet is blanketed by clouds. In the 1980s, though, the Magellan spacecraft used radar to peer through the clouds. It mapped thousands of surface features. Scientists used the motions of those features to measure Venus’s rotation rate to within a few seconds.

    A few years ago, another craft, Venus Express, found that those features weren’t where they were supposed to be. The features hadn’t actually moved, though. Instead, the time of a single rotation had gotten longer — by six-and-a-half minutes.

    Scientists are still trying to understand the reason for the change. Some of it is probably caused by the planet’s atmosphere. It’s about 90 times denser than Earth’s air, so as Venus turns, the atmosphere exerts a drag on the planet itself. And strong weather systems may magnify the effect.

    But the atmosphere can’t explain the entire difference, so scientists are still pondering why Venus is slowing down.

    And Venus teams up with the Moon to put on a grand showing this evening. Venus is the brilliant “evening star” to the right of the Moon as night falls. More tomorrow.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Ringfest

    A massive ring system surrounds a giant planet in the star system J1407, as depicted in this artist's concept. The rings are hundreds of times wider than those of Saturn. A moon that's somewhere between the size of Earth and Mars appears to be embedded in the rings. Most of the ring system may disappear of the next few million years. [Ron Miller]

    Text ©2015 The University of Texas at Austin McDonald Observatory

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Saturn at Opposition III

    The planet Saturn is best known for its amazing rings. They span about two-thirds of the distance from Earth to the Moon, and seen from the right angle they can double Saturn’s brightness. If you have a telescope, you can see them yourself right now. Saturn is low in the southeast at nightfall, and looks like a bright golden star.

    Yet Saturn’s rings are puny compared to those that encircle a planet in the constellation Centaurus. Those rings are hundreds of times wider than Saturn’s — wide enough to span the distance from Earth to the Sun.

    The system is more than 400 light-years away. It’s known by a catalog number — J1407. A few years ago, automated telescopes recorded a series of eclipses of the star. Its light dimmed by up to 95 percent, sometimes in as little as a few hours.

    Earlier this year, astronomers from the U.S. and England produced a detailed profile of what was going on. They found that the eclipses were caused by a giant planet with a giant ring system passing in front of the star. There are at least 37 rings. One big gap in the rings probably was cleared out by a moon that’s between the size of Mars and Earth.

    The rings probably won’t last long, though. The star system is only about 16 million years old, so things are still taking shape. Over the next few million years, much of the ring material may coalesce to make more moons — depriving the young planet of its glorious rings.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Saturn at Opposition II

    Only a handful of worlds in the solar systems are considered to be realistic homes for life. Two of those are moons of Saturn: Enceladus and Titan.

    Geysers of water and ice shoot into space from cracks around the south pole of Enceladus. The geysers appear to come from an ocean of liquid water that’s far below the moon’s icy crust. The geysers also carry minerals that likely come from the bottom of that ocean. That gives Enceladus all the basic ingredients for life as we know it: water, a source of energy to keep it from freezing, and the right mineral content.

    Titan is the largest of Saturn’s moons. It may have an even bigger ocean below the surface than Enceladus does, giving it the right ingredients for life as well.

    Its surface is less hospitable. Titan has a cold atmosphere that’s rich in hydrocarbons, and lakes of liquid hydrocarbons. Those conditions aren’t especially friendly to life like that on Earth.

    But it could be friendly to life that’s not as we know it.

    Scientists at Cornell found that compounds in Titan’s atmosphere and lakes could form cell membranes similar to the water-based membranes that are important for life on Earth. That suggests that life might be possible on the surface of this amazing moon.

    And Saturn itself is putting in an amazing appearance right now. It looks like a bright golden star in the southeast as night falls, and arcs across the south during the night.

    More about Saturn tomorrow.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Saturn at Opposition

    The second-largest planet in the solar system is putting in a first-rate performance right now. Saturn will be at opposition on Friday, lining up opposite the Sun in our sky, which is the best time for watching the planet all year. It’s in view as night falls, remains in the sky all night, and shines brightest for the year, outshining all but a handful of planets and stars.

    Saturn is second only to Jupiter in size and mass. It’s almost 10 times the diameter of Earth. It probably has a dense core of rock and metal, but most of its great bulk is made of hydrogen and helium, the two lightest chemical elements.

    That indicates that Saturn and Earth were born in different regions of the solar system. Earth was born close to the Sun. The Sun’s heat vaporized ice and other materials with a low boiling point. The solar wind then blew away those materials, along with the leftover hydrogen and helium gas. So all that remained were chunks of rock and metal, many of which merged to form Earth and the other inner planets.

    Saturn formed much farther from the Sun, where conditions were colder and calmer. The cores of Saturn and the other giant planets were built not only of rock and metal, but also of ices. As the cores grew, their gravity swept up vast amounts of gas, forming truly giant worlds.

    And giant Saturn is low in the southeast as night falls, near the head of Scorpius, the scorpion. It looks like a bright golden star. More about Saturn tomorrow.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Sailing On

    The constellation Corvus, the crow, glows to the lower right of the brilliant star Spica on May evenings. Its brightest stars form a pattern that resembles a sail. Two names are shown for each of the crow's major stars. The top name is its designation by Johann Bayer, who used letters of the Greek alphabet to label the stars in every constellation. "Alpha" usually designates a constellation's brightest star, but in this case it is applied to a relatively faint star. The bottom name is the star's proper name, which generally pre-dates the Bayer name by centuries. [background image: NASA/ESA/Z. Levay (STScI)/A. Fujii]

    Text ©2015 The University of Texas at Austin McDonald Observatory

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The Crow

    You can organize a constellation’s stars in many ways — by brightness, color, position, and so on. But the system that astronomers have been using for centuries can’t quite make up its mind. In most cases, it goes by brightness. In others, it goes by position. And in some, there doesn’t seem to be a guiding principle at all.

    An example of the latter is Corvus, the crow. It’s due south at nightfall, to the lower right of bright Spica, the leading light of Virgo.

    Four stars form an angled box that looks like a sail. A fainter star is off the lower right point of the sail. Yet it receives top billing.

    The naming system was devised more than four centuries ago by German astronomer Johann Bayer. He labeled the bright stars in each constellation with the letters of the Greek alphabet. “Alpha” usually designated the brightest star. In some instances, though, the title went to a star that was the starting point for an easily defined pattern.

    In the case of Corvus, though, there doesn’t seem to be any pattern at all to Bayer’s naming system. The stars aren’t named from top to bottom, left to right, or in a circle. They don’t form any logical order within the figure of the crow, either.

    And the Alpha star is only the fifth-brightest in the constellation — a faint pinpoint that’s not even visible under light-polluted skies.

    So no one is quite sure what Bayer was thinking when he named the stars of this pretty little constellation.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Rising Serpent

    The head of the serpent is slithering into the evening sky this month, with its tail twisting along a bit later.

    Serpens is the only constellation that’s split apart. The two halves are separated by Ophiuchus, the serpent bearer. The snake’s head is to the west of Ophiuchus, so it rises first. It’s low in the eastern sky as night falls, marked by a serpentine trail of faint stars. The tail, which is to the east of Ophiuchus, follows a couple of hours later.

    The brightest of the snake’s stars is Unukalhai — an Arabic name that means “the serpent’s neck.” It’s also known as Alpha Serpentis, indicating its ranking as the constellation’s leading light.

    The star is in the final stages of life. It converted the hydrogen fuel in its core to helium, causing the core to shrink and get hotter. That triggered the next round of nuclear reactions, with the helium being converted to carbon and oxygen.

    The changes in the core have also caused the thick layers of gas around the core to puff up like a balloon, making the star about 15 times wider than the Sun. They’ve also made the star’s surface much cooler, so it shines yellow-orange.

    In time, all the reactions in the star’s core will stop, and the outer layers will puff out into space. For a while, that expanding cloud will form a colorful bubble. As the bubble cools and dissipates, though, only the star’s dead core will remain — depriving the serpent of its leading light.

     

    Script by Damond Benningfield, Copyright 2012, 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Starry Spokes

    Like the spokes on a wheel, two prominent star patterns circle around the North Star night after night. They’re on opposite sides of the hub, though, so when one is high in the sky, the other is low.

    That’s how they start the night at this time of year. W-shaped Cassiopeia, the queen, is quite low in the northern sky as night falls. At the same time, the Big Dipper is high in the north.

    As the hours roll by, though, Cassiopeia rotates up and around the celestial hub, so it’s well up in the northeast at first light.

    The Big Dipper rotates on the opposite side of the hub, so it is in the northwest at the first blush of twilight.

    As seen from the northern hemisphere, all of the constellations appear to rotate around the North Star. But most of them are far enough away from the North Star that we don’t see them making that circle — we see them setting in the west, disappearing for a while, then rising in the east. So for most nights of the year, they’re out of sight for at least part of the night.

    But from much of the U.S., that’s not the case for Cassiopeia and the Big Dipper. They’re close enough to the celestial pole that they remain in view all night, every night.

    From far-southern latitudes, some or all of both star patterns briefly dips below the northern horizon. Even from those latitudes, though, Cassiopeia and the Big Dipper are in view almost all night, every night of the year, circling the hub of the sky — the North Star.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Blue Skies

    The Curiosity rover recorded this Martian sunset on April 15. Particles of dust in the atmosphere absorb red wavelengths of light, allowing the bluer wavelengths to shine through. The effect is particularly prevalent at sunrise and sunset, when the sunlight must pass through a thicker layer of air. The Sun is setting behind the rim of Gale Crater, which Curiosity has been exploring for more than two years. Click on the image to see an animated version. [ NASA/JPL/Caltech/MSSS/Texas A&M]

    Text ©2015 The University of Texas at Austin McDonald Observatory

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Hercules Cluster

    Stars pass by each other without much effect. That’s because the average distance between stars is millions or billions of times the size of the stars themselves.

    But it’s a different story with galaxies. These collections of billions of stars are so big that there’s not a lot of space between them. As a result, they frequently sideswipe one another, or even collide head-on.

    Many examples of interacting galaxies are found in the Hercules Cluster, a collection of a couple of hundred galaxies that’s about 500 million light-years away. The cluster is in the constellation Hercules, which is in the eastern sky at nightfall.

    The cluster is a bit unusual.

    Most clusters in the nearby universe contain many big, dull galaxies known as ellipticals. These galaxies are dominated by old, faint stars. And they have little gas and dust for giving birth to new stars, so not much happens there.

    The Hercules Cluster, though, is dominated by spiral galaxies, which are similar to our own Milky Way. These galaxies have many hot, young stars, plus the raw materials for many new stars. As a result, they’re bright and vigorous.

    Many of the cluster’s galaxies are interacting. Groups of two or three galaxies are passing so close together that their gravity rips out big streamers of stars, and causes clouds of gas and dust to collapse to create new stars. So even though the Hercules Cluster isn’t all that big, it’s one of the busiest clusters around.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • M92

    A globular cluster is an impressive sight — a tightly packed ball of hundreds of thousands of stars, held together by their mutual gravitational pull.

    The finest globular in northern skies is M13, in the constellation Hercules. It’s a favorite target of amateur astronomers. But professional astronomers are just as likely to point their telescopes at another cluster in Hercules. That’s because M92 may well be the oldest star cluster in the entire galaxy.

    One way astronomers know that is because the stars in a globular have much less iron relative to hydrogen than the Sun does. Hydrogen was formed in the Big Bang, so it’s the most abundant element in the universe. But iron and other heavy elements were forged inside stars, then expelled into space when the stars died, enriching the clouds that gave birth to new stars. So any stars with low abundances of iron must have formed quite early, before there was much iron for them to incorporate.

    Even for a globular cluster, though, M92 is extreme — the amount of iron in its stars is tiny. From that, astronomers estimate that the cluster is 12-and-a-half billion years old, which means it formed less than a billion-and-a-half years after the Big Bang. M92 therefore preserves a record of the conditions that prevailed in the early universe. That makes it an excellent target for those who want to study the birth of our home galaxy.

     

    Script by Ken Croswell, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Old Sparklers

    Thousands of ancient stars sparkle in the heart of M13, one of the largest star clusters in the Milky Way galaxy. It contains perhaps 500,000 stars packed into a ball that's only a few dozen light-years across. The cluster's stars are all 10 billion years old or older. Under dark skies, M13 is just visible to the unaided eye in the constellation Hercules. [NASA/ESA/STCsI]

    Text ©2015 The University of Texas at Austin McDonald Observatory

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • M13

    The star cluster known as M13 is impressive. It contains perhaps a half-million stars, all packed into a ball that’s 150 light-years across. And most of those stars reside near the center of the cluster, filling the night skies of any planets that are there with a brilliant display.

    Yet most of the individual stars in the cluster aren’t all that impressive. Almost all of them are less massive than the Sun, which means they’re also fainter than the Sun.

    There’s a direct relationship between a star’s mass, its brightness, and how quickly it evolves. Heavy stars burn through their nuclear fuel faster than lightweight stars, so they shine brighter but die faster.

    A star as massive as the Sun will live a “normal” lifetime of about 10 billion years. M13 was born about 13 billion years ago, so any stars that were at least as heavy as the Sun have already expired. All that remain are less-massive stars, which live longer but shine less brightly.

    Occasionally, a star in the cluster “steals” gas from a companion, increasing its mass and its brilliance. And when any star nears the end of its life, it gets bigger and brighter for a while. Otherwise, though, M13 is left with old, faint stars that aren’t much to shout about.

    Even so, there are enough of those stars to make M13 visible to the unaided eye. Under dark skies, it looks like a faint, fuzzy star in Hercules, which is in view all night.

    We’ll talk about another cluster in Hercules tomorrow.

     

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Hercules

    Much of what we know about ancient astronomy was passed down to us in a single book, known as the Almagest.

    It was written about 1900 years ago by Claudius Ptolemy, a Greek astronomer, mathematician, and poet. It recorded the cosmology of Aristotle, which taught that Earth was at the center of a perfectly spherical universe. It explained the motions of the Sun, Moon, and planets. And it introduced a scale for measuring the relative brightness of stars that’s still in use today.

    The Almagest also passed along 48 constellations from ancient Babylonia and Greece. Those, too, are still around. In fact, without Ptolemy, many of those star pictures might have disappeared.

    The list includes Hercules, the strongman. It’s low in the east-northeast at nightfall, and soars high across the sky later on. It’s marked by a lopsided square of stars known as the Keystone. None of the stars of Hercules is all that bright, though, so the strongman is best viewed on Moon-less evenings — like tonight.

    Hercules and the other constellations of the Almagest were simple connect-the-dots pictures. Almost a century ago, though, professional astronomers expanded them. So today, each constellation is a well-defined plot on the sky, and encompasses everything within its borders — not just stars, but star clusters, galaxies, and everything else.

    In Hercules, one of those objects is a cluster of hundreds of thousands of stars. More about that tomorrow.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Bear’s Nose

    Ursa Major, the great bear, may be the most famous constellation that isn’t well known. That’s because most of us know it for a pattern formed by only some of its stars: the Big Dipper. But few know that the dipper’s bowl forms the body of a bear, while its handle represents the bear’s tail.

    The Dipper is high in the north as darkness falls right now, with the bowl hanging upside down. But there’s so much more to the bear — his long legs and the rest of his body extend well above and to the left of the dipper.

    The star that represents the bear’s nose is far to the lower left of the bowl — by about twice the width of a fist held at arm’s length. It’s known as Muscida — a Latin name that means “the muzzle.” And it’s about to undergo a big change.

    The star is classified as a giant. As you might expect, that means it’s much bigger and brighter than the Sun — so bright that it’s easily visible even though it’s about 185 light-years away.

    Right now, the star’s heart is quiet. It’s converted its original hydrogen fuel to helium. It hasn’t yet moved on to the next step — but it’s about to. The core is getting smaller and hotter, which soon will trigger the next round of nuclear reactions — converting the helium to carbon and oxygen. As that happens, the star’s outer layers will puff up even more. That’ll make the star brighter and redder. For a while, the bear’s nose will resemble that of a certain famous reindeer — bright, shiny, and red.

     

    Script by Damond Benningfield, Copyright 2012, 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • North Poles

    The Big Dipper stands high overhead early this evening, with the bowl upside down. As you take in the view, link the two stars at the outer edge of the bowl. If you follow that line down toward the horizon, the first moderately bright star you come to is Polaris. Earth’s north pole aims directly at the star, so it’s also known as the North Star or the Pole Star — it marks true north in the sky.

    Another north pole stands much higher in the sky at that hour. It’s in Coma Berenices, a faint constellation that’s well above Arcturus, a bright yellow-orange star that’s high in the east.

    That spot marks the north galactic pole — the projection of the north pole of the Milky Way galaxy.

    The galaxy is shaped like a disk. It’s about a hundred thousand light-years wide, and about ten thousand light-years thick. If you draw a line through the center of the galaxy, at a right angle to the disk itself, and project it above the disk, it aims toward Coma Berenices.

    So when we look in that direction, we’re looking through a thin section of the Milky Way, so there aren’t all that many brilliant stars in that part of the sky. Instead, we’re looking into intergalactic space. And in fact, a telescope reveals many other galaxies in that direction. More than a thousand of them form a giant cluster, known as the Coma Cluster — a beautiful collection of galaxies that stands due north.

    We’ll have more about the Big Dipper tomorrow.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Dark Danger

    As our solar system orbits the center of the Milky Way galaxy, it bobs up and down like a pony on a merry-go-round. It’s not all a joyride, though. Each passage through the plane of the galaxy’s disk might trigger destruction here on Earth.

    Some researchers have found evidence that nasty things happen to our planet every 30 million years or so. That includes bombardment by comets, and huge volcanic eruptions, which trigger mass extinctions.

    30 million years is also the gap between the solar system’s passages through the densest part of the galaxy’s disk. To some, that suggests there could be a relationship between these passages and the cycles of destruction.

    Perhaps the gravity of stars and clouds of gas and dust disturbs the orbits of comets far from the Sun. Some of the comets might fall toward the inner solar system, where they could hit Earth.

    Some recent studies say the comets might also be given a shove by dark matter, which produces no energy but exerts a gravitational pull on the visible matter around it. There could be more dark matter in the plane of the galaxy’s disk, so it would exert a stronger pull when the solar system passes through that plane.

    And one study says that dark matter could accumulate in the center of Earth itself. The dark matter particles might annihilate each other, heating our planet and triggering massive volcanic outbursts — one possible hazard in our merry-go-round ride around the center of the galaxy.

     

    Script by Damond Benningfield, Copyright 2015

    For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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