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  • A blob of gas spreads out and heats up as it circles the supermassive black hole at the center of the Milky Way galaxy. Known as Sagittarius A-star, the black hole is about 4.2 million times as massive as the Sun and is about 27,000 light-years away. It is normally very "quiet," but it occasionally flares up as material funnels toward it. [ESO/APEX/2MASS/A. Eckart et al./L. Calçada] Text ©2014 The University of Texas at Austin McDonald ObservatoryFor more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The heaviest single object in the entire galaxy is also one of the most difficult to see because it produces no energy at all. But it reveals its presence through its effects on the stars and gas around it. The object is Sagittarius A-star, a black hole that’s four million times as massive as the Sun. Gravity squeezes it so tightly that nothing can escape it — including light. The black hole sits at the center of the galaxy, where it’s orbited by stars and gas clouds. Stars move much faster when they’re close to the black hole than when they’re farther away. From that, astronomers calculate the mass of the object the stars are orbiting. Although the black hole is completely dark, matter around it produces a faint X-ray glow — wisps of gas that are heated as they spiral toward the black hole. And that zone occasionally flares up when an asteroid or other chunk of matter falls into the black hole. Eventually, astronomers hope to see the black hole itself as a dark outline against the background of stars and gas. Until then, they’ll have to settle for looking at its powerful effects on the center of the galaxy. And the galaxy’s center is in Sagittarius, which scoots low across the south on summer nights. We can’t see the center with our eyes because it’s hidden behind clouds of dust. It takes special instruments to peer through the dust and behold the wonders in the heart of the Milky Way. More about those wonders tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The center of the Milky Way is already crowded with stars. But in a few million years, tens of thousands more could flare to life in a region known as the Brick. It’s a vast cloud of cold, dark gas and dust that’s shaped like a brick. The Brick may be the biggest future star cluster in the entire galaxy. It’s more than a hundred thousand times the mass of the Sun. And recent observations with a giant new radio telescope show that it contains at least 50 dense blobs of material — embryos that could be giving birth to stars even now. Stars are born when such blobs collapse under their own gravitational pull. As a blob collapses, it gets hotter. If it gets hot enough, nuclear fusion ignites at its center — giving birth to a star. There are indications that a few small stars have already formed in the Brick. But such giant clouds are also where big, heavy stars are born — stars that shine tens of thousands of times brighter than the Sun. It’s not certain that the Brick will give birth to many stars at all, though. While it has the right ingredients, it’s in the busiest, most crowded region of the galaxy. The gravity of the stars and clouds around it could keep it too stirred up to form stars. Powerful magnetic fields could hinder star formation as well. Astronomers will study the Brick in more detail in the coming years to determine whether it’s the site of future fireworks — or a dark galactic dud. More about the heart of the Milky Way tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Big clouds of gas and dust appear to be taking a roller-coaster ride around the center of the Milky Way galaxy. The oval track is about 600 light-years long by 400 light-years wide, with a big hump on each side of the track. The track was discovered a few years ago in images from the Herschel space telescope, which looked at the infrared glow of the galaxy’s heart. Cool clouds of gas and dust shine brightest in the infrared. The observations revealed several big clouds that outline the ring-like structure. The clouds are moving along the track at a couple of hundred thousand miles per hour. In all, they contain enough gas and dust to make about 30 million stars as massive as the Sun. In fact, new stars are already being born at the ends of the track. In those locations, the outer edges of the gas clouds may interact with clouds that orbit a little farther from the galactic center. That may create shock waves that ripple through the clouds. The shock waves can squeeze knots of gas within the clouds, causing them to collapse to make new stars. One of the biggest clouds is still dark — it’s given birth to only a few stars at most. And in the turbulent center of the galaxy, it may remain dark. But it could also turn into one of the most impressive stellar nurseries in the entire Milky Way — a cluster of thousands of stars lighting up the busy galactic center. We’ll have more about that tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • When a typical star cluster is born, it’s a tightly packed family — hundreds of stars that were born from the same big cloud of gas and dust, all jammed into a fairly small volume of space. As the cluster orbits the center of the galaxy, though, the stars tend to head their separate ways. The cluster is pulled apart by the gravity of the rest of the galaxy, and pushed apart by interactions between members of the cluster itself. So within a billion years or so, most clusters fall apart. A rare exception is Ruprecht 147, a cluster in Sagittarius. The constellation is in the south this evening, with its brightest stars forming a teapot. Ruprecht 147 is to the upper left of the teapot’s handle, although you need a telescope to see it. The cluster was discovered in 1830, but until recently it was largely ignored. A few years ago, though, astronomers began studying the cluster in detail. To their surprise, they found that its stars are roughly two-and-a-half billion years old — much older than the typical cluster. That’s important because clusters are great stellar laboratories. Since the stars in a cluster were all born at the same time, from the same mixture of ingredients, studying how they look today reveals important details about how all stars evolve. So Ruprecht 147 is a rare cluster that provides a look at stars that are well into middle age — a special insight into the lives of the stars. More about Sagittarius tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • One of the first items on almost any amateur astronomer’s “to-do” list is completing the Messier marathon — looking at all 110 objects cataloged by Charles Messier. Messier himself began the first marathon 250 years ago. Charles Messier ca. 1770Messier was most interested in finding comets. Indeed, the French astronomer discovered one in January of 1764. Not long afterward, though, he discovered something that looked like a comet but wasn’t. Today, we know that it’s a globular cluster — a tight grouping of ancient stars. All Messier knew, though, was that it was in the way of his comet hunting. So he set out to compile a catalog of similar comet-like objects. He’d already logged a couple, although he found that they’d already been discovered by others. But the globular cluster had never been recorded by anyone — it was Messier’s first discovery of a deep-sky object. During the summer of 1764, he scanned the skies in and around Sagittarius. The constellation is in the southeast at nightfall, with its brightest stars forming the outline of a teapot. Among Messier’s discoveries in Sagittarius are a pair of young star clusters known as M21 and M25, and another globular cluster, M22. All three stand above the teapot, and are visible through binoculars. By the end of 1764, Messier’s catalog listed 40 objects — putting him more than a third of the way through the first Messier marathon. We’ll talk about another Sagittarius cluster tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The Milky Way forms a glowing arch that passes high overhead on summer nights, outlining the flat disk of our home galaxy. This view, toward the center of the galaxy, shows vast fields of stars, dark dust lanes, glowing clouds of gas and dust that are giving birth to new stars, and much more. The colorful wisps at right include the nebula Rho Ophiuchi and the region around Antares, the bright orange star at the heart of Scorpius, the scorpion. Two nebulae, the Lagoon the Trifid, are at upper right. [ESO/S. Guisard] Text ©2014 The University of Texas at Austin McDonald ObservatoryFor more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • A big, steaming teapot floats across the southern horizon on summer evenings: the constellation Sagittarius, the archer. To modern eyes, its brightest stars form a teapot, with the handle to the left and the spout to the right. The pot looks like it’s steaming because it’s immersed in the Milky Way — the combined glow of millions of stars in the disk of our Milky Way galaxy. You need dark skies to see it; city lights overpower its subtle glow. But if your skies are dark enough for you to see the Milky Way, then you can look for some of the most prominent star clusters and nebulae in all the night sky. That’s because Sagittarius marks the heart of the galaxy. When we look that way, we’re looking into the most heavily populated region of the galaxy, so there are lots of beautiful sights. A few of these objects are visible to the unaided eye. But they’re faint and spread out, so they look like small puffs of cloud or smoke. Binoculars enhance the view, while small telescopes show much more detail. One of these objects is a big cluster of geriatric stars. But other objects are at the opposite end of the stellar life cycle. They’re stellar nurseries — regions where clouds of gas and dust are collapsing to give birth to new stars. Look for all of these beautiful objects floating above the teapot, which is low in the southeast in early evening, and due south around midnight. We’ll have more about Sagittarius tomorrow.   Script by Damond Benningfield, Copyright 2007, 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • There’s nothing like a trip to the mountains to escape the summer heat — whether you’re here on Earth or on Venus, our nearest planetary neighbor. The coolest place on the entire planet is its tallest mountain range, Maxwell Montes. Of course, cool is a relative term — temperatures on the mountain peaks probably top 700 degrees Fahrenheit. The Maxwell Montes range is more than 500 miles long, and its highest peaks extend almost seven miles above the surrounding volcanic plains. The mountains may have formed as portions of the crust pushed together, forcing the rock between them upward. The average surface temperature on Venus is more than 850 degrees, and atmospheric pressure is 90 times greater than at the surface of Earth. On the mountain peaks, though, the pressure is only half that, and temperatures are appreciably cooler — perhaps cool enough for snow. An orbiting spacecraft revealed a highly reflective coating on the mountaintops that looks like snow. Instead of frozen water, though, it’s probably made of a frozen metal. The material forms a gas at lower, hotter altitudes. But as it rises through the atmosphere it cools and condenses — eventually forming a layer of snow atop the “cool” mountains of Venus. And Venus is in good view at first light tomorrow. The planet shines as the brilliant “morning star,” close to the left of the beautiful crescent Moon. Tomorrow: a “steaming” teapot in the southern sky.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The star that anchors the Big Dipper’s handle is a bit of an outcast. The five stars that make up the dipper’s middle are members of the same family — the Ursa Major Moving Group. They were born from the same cloud of gas and dust, and they move through space together. The stars at the ends are different. Tens of thousands of years from now, they will have moved a good distance from the others as seen from here on Earth, pulling the “dipper” apart. Alkaid is the end of the dipper’s handle. It’s farther away than the members of the moving group — 100 light-years. And it’s only about 10 million years old, which is far younger than the members of the moving group. Ten million years isn’t long on the astronomical time scale — the Sun is four-and-a-half billion years old. Even so, Alkaid is getting well along in life because it’s more than six times as massive as the Sun. The cores of heavy stars are much hotter than those of lighter stars, which revs up their nuclear reactions. They consume their nuclear fuel at a much faster rate, so they burn out much more quickly. Alkaid, for example, will live a “normal” lifetime of less than a hundred million years, versus 10 billion years for the Sun — a short life for a stellar outcast. Look for Alkaid in the north as night falls, with the rest of the dipper hanging below it. The dipper rotates down toward the northern horizon during the night, ready to scoop up a dipperful of stars.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The star Aldebaran, which represents the eye of Taurus, the bull, is nearing the end of its life, so it has puffed up to dozens of times the size of the Sun. The Sun will undergo this same process in several billion years. [Wiki] Text ©2014 The University of Texas at Austin McDonald ObservatoryFor more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • A star is the product of a delicate balancing act between gravity and radiation. They remain in balance for most of a star’s life. But as the star ages, the balance changes, causing big changes in the star itself. An example of that process is visible at dawn tomorrow. Aldebaran, the bright orange eye of Taurus, stands just below the Moon. Gravity squeezes a star tightly, smashing its gas together into a compact ball. At the middle of the ball, the gas is heated to millions of degrees — hot enough to ignite nuclear fusion. The fusion produces radiation, which pushes outward, keeping the star in balance. As the star ages, though, it uses up the hydrogen fuel in its core, leaving an “ash” of helium. The helium doesn’t fuse at the same temperature as hydrogen, though. Without nuclear fusion, there’s no radiation to push outward, so gravity squeezes the core even tighter. As the core shrinks, it gets hotter. Eventually, it gets hot enough to ignite fusion in the helium. That achieves a new balance — one with a smaller, hotter core. The radiation from the hotter core exerts a stronger outward pressure, causing the surrounding layers of gas to puff outward like a giant balloon. And that’s what’s happened to Aldebaran. The star has used up its hydrogen, so it’s now beginning to burn its helium. So Aldebaran has puffed up to many times the size of the Sun, causing its outer layers to cool — giving the bull a glowing orange eye.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The Pleiades star cluster is climbing higher into the pre-dawn sky each day. Tomorrow, it’s well up in the east at first light, not far to the left or upper left of a thick crescent Moon. In western skylore, the Pleiades represent seven sisters — the daughters of Atlas. To Chinese skywatchers, though, they represented Mao — a spear or a fuzzy head of hair. The earliest confirmed reference to Mao comes from a box found in the well-preserved burial place of Zeng Hou Yi. He was a governor who lived more than 2400 years ago, when seven feudal kingdoms were fighting for supremacy. The box is decorated with an astronomical map that shows the Pleiades with a Chinese character for spear, known as Mao. When a historian wrote a famous book two centuries later, though, he used another character for Mao. It has almost the same pronunciation, but it means “hairy head.” Some speculate that’s because the historian thought the Pleiades looked like the fuzzy fringe of a spearhead. Whether spear or fuzzy fringe, the Chinese version of the small dipper-shaped cluster is composed of seven stars, just like the Pleiades. They’re commonly referred to with the word Mao followed by the Chinese word for the numbers one through seven. Again, look for the Pleiades about a third of the way up the eastern sky at dawn tomorrow, near the Moon. The cluster will rise a few minutes earlier each night, eventually moving into the evening sky of autumn.   Script by Robert Tindol and Paris Liu, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Aquila, the eagle, soars across the southern sky on summer nights. It’s in the east as night falls and high in the south after midnight. It’s marked by Altair, one of the brightest stars in the summer sky. But almost a century ago, Altair was dwarfed by a “new” star that popped into view near by. Known today as V603 Aquilae, it was a nova — the explosion of hot gas on the surface of a white dwarf. It briefly shined as brightly as any star in the night sky. Over the decades, astronomers have used various techniques to try to measure the power of nova explosions. These observations seemed to show that all novae were basically alike — they exploded and faded from view in the same way. Recently, though, a team of astronomers used Hubble Space Telescope to get the most accurate distances yet to V603 Aquilae and three other old novae. The distance of a nova reveals its maximum brightness, providing clues about what’s happening in the explosion. McDonald Observatory astronomer Fritz Benedict, a member of the Hubble team, explains that the observations showed that nova explosions aren’t all alike: BENEDICT: The big piece of information is that these things are very individualistic. The assumption that the same energetics are always involved is wrong. We thought we knew what was going on, and now we know we don’t know what’s going on. You look for the “Aha!” moment, but you also have an aha moment when you say, “Ha, we were dead nuts wrong about that, we need to do it some other way.” So there’s a lot more work to be done to explain the explosions of these “new” stars.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • At Christmas of 1934, a bright “new” star exploded to life in Hercules. For a few days, it was one of the brightest stars in the night sky. It made the front pages of newspapers, and astronomers tracked it for months as it slowly faded from sight. The star is known as DQ Herculis, and astronomers are still tracking it today. In fact, a team of astronomers has used Hubble Space Telescope to study DQ Herculis and three similar stars. Their work has demonstrated that some of the ideas about this class of stars are wrong. Each of these stars erupted as a nova — a powerful explosion on the surface of a dead star known as a white dwarf. The white dwarf stole gas from a nearby companion star. When enough gas piled up on the star’s surface, it triggered a runaway nuclear explosion. But when the four target stars first appeared, in the early 20th century, the cause of these outbursts was still unknown. All that astronomers knew was that a bright star suddenly appeared where no star had been seen before. Yet they observed these enigmatic objects in detail. Using their eyes and their knowledge of the night sky, they tracked how bright each nova looked as it faded from view over the weeks or months. In the decades since, other astronomers have used those observations and those of similar stars to try to draw some conclusions about all novae. But the recent Hubble observations demonstrated that some of those conclusions were wrong. More about that tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • A white dwarf sounds harmless enough. It’s the small, dead core of a once-normal star like the Sun. But to a close companion star, a white dwarf is anything but harmless. It’s the living dead. Although a typical white dwarf is only about as big as Earth, it’s half as massive as the Sun or heavier, so its surface gravity is quite strong. So if a companion is close enough, the white dwarf can pull hot gas off its surface — like a vampire drinking the blood of the living. Over time — anywhere from a few decades to a few centuries — a layer of gas many miles thick can build up atop the white dwarf. This gas gets hotter and hotter until it triggers a nuclear explosion, like a giant hydrogen bomb. For a while, the white dwarf shines thousands of times brighter than normal. As seen from Earth, a “new” star suddenly appears where none was seen before, so such an outburst is known as a nova, which is the Latin word for “new.” In a few weeks or months, though, the nova once again fades from sight as the exploded material dissipates. So the white dwarf “dies” once again. This cycle of death, rebirth, and re-death has led some astronomers to give novae another name: zombie stars. And the cycle can repeat. After an explosion, the white dwarf can once again begin sucking the gas from its companion. Over the decades, that can lead to another nova explosion — once again reviving a stellar zombie. We’ll have more about novae tomorrow.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The first fragment of Comet Shoemaker-Levy 9 strikes the planet Jupiter while the others (including the rocky objects in the foreground) line up for their own collisions in this artist's concept. The impact took place on July 16, 1994, with the others following over the next few days. Jupiter's gravity had shattered the comet into about 20 large chunks plus countless smaller bits of debris. They hit Jupiter with the energy of thousands of hydrogen bombs, leaving dark scars in the planet's upper atmosphere that persisted for weeks. [Don Davis/NASA] Text ©2014 The University of Texas at Austin McDonald ObservatoryFor more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Our solar system can be a dangerous place. The most striking evidence of that came 20 years ago today, when the first fragment of a shattered comet blasted the planet Jupiter. Another 20 pieces of the comet followed over the next week, creating dark blemishes that lasted for months. Comet Shoemaker-Levy 9 was discovered in 1993. By then, Jupiter’s gravity had already pulled the comet apart, creating what was described as a string of pearls — almost two dozen bright chunks of rock and ice. The first of the pearls hit Jupiter on July 16th, 1994. It hit on the back side of the planet as seen from Earth, so the impact itself wasn’t visible. But a spacecraft en route to Jupiter did see the flash, and measured the temperature of the fireball at about 45,000 degrees. The fireball was about 2,000 miles tall — tall enough for telescopes on Earth to see before the impact site rotated into view. The largest fragment of the comet hit a few days later, exploding with the force of millions of nuclear bombs. It and the other impacts created dark scars in Jupiter’s clouds that were as big as Earth. In the two decades since, a couple of other large objects have plowed into Jupiter, creating similar dark scars. And a small asteroid exploded in the skies above Siberia last year. These and other events, along with a lot of close calls, show that impacts between planets and smaller bodies are common — an ever-present danger in our own solar system.   Script by Damond Benningfield, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • Our home galaxy, the Milky Way, is a giant, encompassing every individual star the unaided eye can see. But the Milky Way hasn’t always been so huge. Instead, it grew over time, as smaller galaxies smashed together, squeezing their gas clouds and triggering the birth of new stars. Astronomers have recently seen this process in action near the edge of the galaxy’s disk. The Milky Way is so large that many lesser galaxies go around it the way moons orbit a planet. The two brightest of these satellite galaxies are the Magellanic Clouds. They’re named for Portuguese explorer Ferdinand Magellan, who described them as clouds in the southern sky. The clouds abound with gas, which is the raw material for making new stars. The two galaxies are shedding a stream of gas that wraps around much of the Milky Way. Try though they might, though, astronomers couldn’t find any stars in this ribbon of gas until recently. A study examined the stream and detected six bright, young stars within it. The stars likely were born as the gas from the Magellanic Clouds collided with gas at the edge of the Milky Way. That squeezed the gas from the Magellanic clouds, causing it to collapse and give birth to new stars. The newborn stars add even more luster to the Milky Way. And they illustrate how our home galaxy has grown into a giant that’s home to hundreds of billions of stars. We’ll talk about a collision right here in the solar system tomorrow.   Script by Ken Croswell, Copyright 2014 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

  • The large and small Magellanic clouds, which are companion galaxies to the Milky Way, glow above the antennas of ALMA, a radio telescope in Chile. Each galaxy contains a few billion stars, and lie within about 200,000 light-years of the Milky Way. The Magellanic clouds are visible the unaided eye, but only from the southern hemisphere and the lowest latitudes of the northern hemisphere. [C.Malin/ESO] Text ©2014 The University of Texas at Austin McDonald ObservatoryFor more skywatching tips, astronomy news, and much more, read StarDate magazine.

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