StarDate Podcast

To understand a star, you need to know how far away it. Its distance reveals how big and bright it is, for example, which helps reveal what’s happening inside the star. But measuring astronomical distances is tough. You can’t drive out to a star and measure the miles as they click by. Instead, astronomers have developed a “stepladder” approach. Each rung on the ladder helps measure distances to farther objects. Texas astronomer Fritz Benedict and his colleagues have helped solidify one of the first steps on that ladder. Using Hubble Space Telescope, they’ve measured the distances to several stars that pulse like beating hearts. RR Lyrae stars get bigger and smaller over a period of half a day or so. There’s a relationship between the length of each “beat” and the star’s true brightness. That means you can compute the star’s distance by comparing the length of its pulses to how bright the star appears in the sky. For the technique to work, though, astronomers first must use another technique, called parallax, to find the distances to a few RR Lyrae stars. Benedict’s team did that for five stars, using Hubble to measure the position of each star when Earth was on opposite sides of the Sun, providing a slightly different viewing angle. Before this project, astronomers had a good measurement for only one RR Lyrae star — the prototype, RR Lyrae itself. The measurements of more stars help make that rung on the distance ladder a good bit sturdier. Script by Damond Benningfield, Copyright 2012 This program was made possible in part by a grant from NASA. For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Most stars are pretty steady — their size and brightness seldom change by more than a tiny bit. But a star in the constellation Lyra undergoes major changes every few hours. Its diameter changes by hundreds of thousands of miles. And that causes its brightness to change dramatically, too — at its brightest, the star is twice as bright as when it’s faintest. RR Lyrae changes because it’s nearing the end of its life. Nuclear reactions have created a shell of helium around the star’s core. That traps the core’s heat like a lid atop a pot of boiling water. But the energy is still trying to get out, so it pushes the helium outward, causing the star’s outer layers to expand as well. As they do, the energy escapes into space, the helium cools, and the outer layers fall back inward. Each of these in-and-out “beats” takes less than 14 hours. There are many other stars like RR Lyrae. And it turns out there’s a relationship between the length of each beat and the star’s true brightness. Astronomers can use that relationship to measure how far away these stars are. But first, they have to use other techniques to get the distances to a few of them, including RR Lyrae itself. And astronomers at the University of Texas have done just that. More about that tomorrow. In the meantime, look for Lyra in the west this evening, marked by its brightest star, Vega. RR Lyrae stands above it, but you need binoculars or a small telescope to spot it. Script by Damond Benningfield, Copyright 2012 This program was made possible in part by a grant from NASA. For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Milky Way arches high overhead on December evenings. This faint band passes from the Northern Cross, which is in the west, to W-shaped Cassiopeia high overhead, to near the face of Taurus, the bull, in the east. The Milky Way glows a little more softly at this time of year than it does in summer. That’s because we’re looking away from the galaxy’s crowded center and toward the thinly settled edge of its disk. And beyond that edge is the halo — an extended volume of space that contains the galaxy’s oldest stars. Astronomers classify the stars in the halo as Population II, while most of those in the disk are Population I. The difference isn’t just where the stars reside — it’s what they’re made of. All stars consist mainly of hydrogen and helium, which were created in the Big Bang. But they also have a smattering of heavier elements. These elements were created in the hearts of other stars, then hurled into space as the stars died, where they could be incorporated into new stars. Population II stars formed when there were almost no heavier elements around, so they have only tiny amounts of them. But Population I stars — stars like the Sun — are younger, so they have higher proportions of heavy elements. It’s still not much, but it’s enough to set these stars apart from their older kin. One class of Population II star is in a stage of life that helps astronomers measure the size of the universe. More about that tomorrow. Script by Damond Benningfield, Copyright 2012 This program was made possible in part by a grant from NASA. For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A small but brilliant orb arcs high across the sky tonight: Jupiter, the planet named for the king of the gods of ancient Rome. It’s quite low in the east-northeast as night falls, but soon ascends into good view. And it remains in view all night, so there’s plenty of time to look for it. You don’t have to look hard, though — for most of the night, it’s the brightest pinpoint of light in the sky. Jupiter’s so bright because it’s the largest planet in the solar system — about 11 times wider than Earth. And it’s blanketed by clouds that reflect a lot of the sunlight that strikes them. But Jupiter lords over the night because of its location. In astronomical parlance, it’s a superior planet — not because of its great size, but because its orbit is outside the orbit of our own Earth. In fact, Jupiter is about five times farther from the Sun than Earth is. Right now, Jupiter’s orbit is carrying it “behind” us — directly opposite the Sun in our sky — so the planet is highest in the sky around midnight. And this is a particularly good time of year for that configuration. The nights are nice and long, so there’s a lot of time to enjoy the view. And Jupiter follows the same path across the sky as the Sun. At this time of year, that path scoots low across the south during the day as seen from mid-northern latitudes. But at night, the path climbs high overhead, so Jupiter does as well, crowning the night sky with its brilliance. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

If you’re feeling a bit jovial today, perhaps it’s related to the sky. Jupiter, the largest planet in the solar system, is putting in its best appearance of the year. It rises around sunset, scoots high across the sky during the night, and sets around sunrise. And you won’t have any trouble finding it; until Venus peeks into view shortly before dawn, Jupiter will be the brightest point of light in the entire sky. To ancient skywatchers, the planets were more than just bright, moving points of light in the night — they were the embodiments of the gods. Jupiter was the king of the Roman gods. He was also known as Jove, and that’s the source of an old exclamation — “By Jove!” Each planet was associated with a long list of objects or characteristics, such as colors, metals, and precious gems. Jupiter was associated with the color blue, the metal tin, and with sapphires. Each planet was also associated with human characteristics or behaviors. This association was part of the attempt to relate the orderly motions of the heavens to the cycles of life on Earth. Jupiter was thought to make the people born under its influence feel jolly or joyful — or using Jupiter’s other name, Jove, it made people “jovial.” So spend a little time looking at Jupiter as it lords over the sky the next few nights, by Jove, and you’ll almost certainly come away feeling jovial yourself. We’ll have more about Jupiter tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

In 1969, with the goal of landing men on the Moon achieved, the architect of the giant Moon rockets had one more giant plan. Wernher von Braun proposed a mission to Mars. It would consist of two nuclear-powered rockets, each carrying six astronauts. Each would have a lander for the crew, plus a small fleet of robotic landers. The expedition could set sail in 1981. Neither Congress nor the public had much interest in more big space projects, though, so plans for Mars missions were dropped — along with the plans for nuclear-powered rockets. Yet engineers continue to study nuclear-powered rockets. That’s because nuclear rockets would produce at least twice as much “kick” as conventional rockets. That could cut the travel time for a Mars mission by weeks or months, make it possible to launch a heavier spaceship, or both. Nuclear engines would fire only in space, not inside Earth’s atmosphere. And by cutting the travel time for a Mars mission, they would reduce the total radiation exposure for the crew. NASA and the Atomic Energy Commission worked on nuclear rockets from the late 1950s to the early ’70s. They developed prototypes that were tested on the ground, and were about ready to be tested in space. Today, engineers are giving those designs another look. There are plenty of challenges — challenges of safety, cost, and technology. Yet it may be that missions of the future will incorporate this futuristic technology from the past. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

In the heady days of the early space program, the Moon seemed like only a meager first step in the human exploration of space. Years before Neil Armstrong and Buzz Aldrin first walked on the Moon, in fact, NASA was already planning for missions to Mars. And one of the keys to those plans was nuclear-powered rocket engines. In fact, a test of an early prototype took place 50 years ago tomorrow. The engine almost blew itself apart, but analysis of the problems led to better designs. The rockets pumped liquid hydrogen through the nuclear reactor. The uranium fuel was heated to thousands of degrees, so as the hydrogen passed through, it quickly heated and expanded, then blasted out of a rocket nozzle. The nuclear rocket could deliver twice as much “kick” as a conventional one. Also in 1962, NASA commissioned a series of studies to take advantage of those engines to send astronauts to Mars. Two of the studies produced missions in which astronauts would simply fly past Mars, while a third envisioned them entering orbit. Each ship would have been enormous — a million pounds or more at launch — the equivalent of today’s International Space Station. In the less-heady days after the Apollo missions, though, NASA’s grand plans lost support. The nuclear rocket was ready to fly, but it had no place to go, and the project was canceled. Yet the idea of a nuclear-powered trip to Mars isn’t completely dead. We’ll have more about that tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

For most of the modern world, the Moon is little more than a bright, pretty bauble in the night sky. In ages past, though, the Moon was an important part of everyday life. It served as a calendar marker for many cultures, helping regulate everything from planting seasons to religious festivals. So it’s not surprising that some of those cultures gave names to the brightest Moons of all — the full Moons. To some of the tribes of North America, for example, the full Moon of mid to late autumn was known as the Frost Moon — a time to prepare for the coming cold of winter — or as the Beaver Moon — a time to collect beaver pelts for winter clothing. In fact, the Frost Moon is on good display tonight. It has two prominent companions — the brilliant planet Jupiter, which is close to the upper left of the Moon as they climb skyward in early evening, and the fainter star Aldebaran, the “eye” of Taurus, the bull, to the right of the Moon. This year’s Frost Moon could also be called the Little Moon. That’s because the Moon is not only full today, but it’s also at its farthest point from Earth in its slightly elongated orbit. That makes the Moon look a little smaller than average. The effect isn’t going to jump out at you, though — you’d need to keep a really close eye on the Moon to notice the difference. For most of us, the Moon won’t look any different from any other full Moon — a beautiful bauble, lighting up a “frosty” autumn night. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A feeble eclipse will slightly darken the Moon before dawn tomorrow across much of the United States. The eclipse will be underway as the Moon sets around the crack of dawn. The eclipse is so subtle, though, that it’s tough to see. Lunar eclipses can take place only at full Moon, when the Moon lines up opposite the Sun in our sky. When the alignment of Sun, Earth, and Moon is just right, the Moon passes through Earth’s shadow. The Moon’s orbit is tilted a bit with respect to the Sun, though, so most months the Moon misses the shadow entirely. And even when it does pass through the shadow, the eclipse isn’t always a good one. That’s because Earth’s shadow has two parts — a dark inner part, which cloaks the lunar surface in darkness, and a hazy outer part, in which a lot of sunlight filters through. And it’s that part of the shadow — the penumbra — that’ll cover the Moon early tomorrow. At the peak of the eclipse, a little after 8:30 Central Time, the penumbra will cover more than 90 percent of the Moon’s diameter. If the Moon is still up at your location then, you may notice that one edge of the Moon appears a little darker than the opposite edge. The darker edge is the one that’s closest to Earth’s inner shadow. Except for the time around the peak, though, that shadow will do little to dim the brilliance of the full Moon — which is known, by the way, as the Frost Moon. We’ll have more about that tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Late November brings an abundance of treats, including Thanksgiving, college football’s biggest rivalry games, and cool nights. And the sky offers a treat of its own to enjoy during those chilly evenings: the return of Orion. The hunter is in full view in the east by 8:30 or 9. Look for his “belt” of three stars pointing almost straight up from the horizon, with a bright orange star to the left and a blue one to the right. The Belt makes Orion one of the easiest constellations to find. From top to bottom as Orion climbs into view, its stars are Mintaka, Alnilam, and Alnitak. The three stars don’t just look impressive, they really are some of the more impressive stars in our region of galaxy. Mintaka actually consists of two brilliant stars, while Alnilam and Alnitak are single. All four stars are much bigger, brighter, and heavier than the Sun. They’re hundreds of light-years away, so they’re among the most distant of all the bright stars in the sky. And all are destined to shine even brighter — but only for a while. They’ll end their lives with titanic explosions known as supernovae. For a few weeks, each of them will outshine everything else in the night sky except the Moon. For now, watch Orion climb skyward during the evening and soar high across the south during the night. The hunter will rise a few minutes earlier each night, giving us even more time to appreciate this astronomical treat as we head toward Christmas. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

When people started paying attention to those little points of light in the night sky, it didn’t take long to notice that there’s something a little different about a few of them. Most of those points maintain the same relative positions from night to night — they don’t move with respect to each other. Those came to be known as the “fixed” stars, and the brightest of them formed the classical outlines of the constellations. But a few of them did change position — they “wandered” through the background of fixed stars from month to month, or even night to night. So these objects were given special designations. The Greeks called them planetes — the wanderers. And today, we still know them as “planets” — objects that spice up the night sky with their roving ways. In fact, two planets are wandering past each other in the morning sky — Venus and Saturn. Venus is the brilliant “morning star,” low in the eastern sky at first light. Much-fainter Saturn is close to its left or lower left tomorrow, but will stand above Venus on Tuesday. It made sense to early skywatchers that the planets were closer to Earth than the “unmoving” stars. But it took millennia to work out just how much closer. While the visible planets are all within about a billion miles of Earth, the visible stars are typically millions of times farther — so remote that they provide a stationary background for the motions of the wandering planets. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Learning about the Big Bang isn’t easy. Cosmologists have to rely on obscure clues to probe conditions during that moment of creation almost 14 billion years ago. One of the most important is something that’s in your water: a form of hydrogen called deuterium. Most hydrogen atoms have one proton and no neutrons. But deuterium is a heavy form of hydrogen, with both a proton and a neutron. Deuterium can occur in molecules just as normal hydrogen does. In fact, a glass of water contains not just molecules of H2O, but also trillions of molecules of H-D-O, in which one of the hydrogen atoms is deuterium. Deuterium is valuable to scientists who study the universe’s origin because it’s one of the few elements forged in the Big Bang. And nuclear reactions in stars destroy deuterium, so the universe’s supply has dwindled over time. By looking at the space between the stars, though, astronomers can detect it and estimate how abundant it is. This abundance reveals the conditions that prevailed just moments after the Big Bang. In particular, the more deuterium the universe has, the less dense the universe must be. That’s because in a denser universe, more nuclear reactions would have destroyed deuterium just after the Big Bang. But it turns out that a fair amount of deuterium is still around. That’s one indication that the universe will expand forever — there’s not enough matter packed tightly enough to cause the universe to pull itself back together. Script by Ken Croswell, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

One of the main ingredients in the atmosphere of Titan, the largest moon of Saturn, is methane. The problem is, methane doesn’t hang around for long — it’s ripped apart by radiation from the Sun. That means that something must be replenishing the supply. There’s evidence that at least some of the methane could come from volcanoes. Not the kind we have on Earth, with towering columns of scalding ash and rivers of molten rock, but ice volcanoes. Titan’s surface is made mainly of water that’s frozen as hard as granite. But Saturn’s gravitational tug heats Titan’s interior, perhaps creating pools of slushy water mixed with ammonia and methane far below the surface. This mixture may force its way to the surface through cracks or weak spots in the ice, building cone-shaped mountains and perhaps releasing fresh methane into the atmosphere. One of the most likely ice volcanoes on Titan is known as Sotra Facula. It consists of two mountain peaks, the tallest of which is about a mile high. A frozen puddle on one side of the peaks looks like lava floes here on Earth. And the feature has a different chemical composition from the surface around it. Another region, Hotei Arcus, has shown significant changes over the last few years. While some researchers say the changes are most likely caused by blowing dunes, others interpret them as evidence of ongoing eruptions of an ice volcano — a source of fresh methane for Titan’s cold, thick atmosphere. Script by Damond Benningfield, Copyright 2012 This program was made possible in part by a grant from the NASA Science Mission Directorate For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Sahara Desert is one of the driest regions on Earth. Even so, small oases dot the landscape — regions where water bubbles up from below the surface to fill ponds or small lakes. An oasis may also highlight the “desert” regions of Titan, the largest moon of Saturn. But this oasis is as big as Utah’s Great Salt Lake. Titan is blanketed by a cold, dense atmosphere. A layer of orange haze tops the atmosphere, so we can’t see Titan’s surface directly. But the Cassini spacecraft has used radar to peer through the haze and map a large fraction of the surface. Those observations have revealed hundreds of possible lakes close to the moon’s poles. Titan is so cold, though, that water at the surface is frozen as hard as granite. So instead, the lakes are filled with liquid methane and ethane. Planetary scientists recently reported the discovery of a big lake at Titan’s equator, too. Models of Titan’s atmosphere suggest that any liquid in that region would quickly evaporate and be carried toward the poles, so the equator should stay dry. The lake may be fed by springs. If so, then the discovery may help solve a puzzle. Titan’s atmosphere contains a lot of methane. But methane is quickly destroyed by solar energy. That means there must be a source of fresh methane to replenish the atmosphere. And underground springs would be a good source — helping fill not only desert oases, but the lakes at higher latitudes as well. More about Titan tomorrow. Script by Damond Benningfield, Copyright 2012 This program was made possible in part by a grant from the NASA Science Mission Directorate. For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Like an actress concealing her age with cosmetics and plastic surgery, the largest moon of Saturn is older than it looks. Like Saturn and the other planets and moons of the solar system, Titan is four-and-a-half billion years old. Yet its surface is only a few tens of millions of years old — rejuvenated by Titan’s own actions. Scientists judge the age of the surface of a moon or planet by looking at impact craters. Every body in the solar system undergoes a constant pounding by space rocks large and small. The big rocks create large craters, like the ones that cover the face of our own moon. The Moon itself keeps the impact craters because there’s nothing to erase them — no air, no rivers, and little or no volcanic activity. On active worlds like Earth, though, the craters are soon washed away or covered up. And that’s what’s happening on Titan. It’s blanketed by a frigid atmosphere that’s thicker than Earth’s. Clouds of liquid methane and ethane in the atmosphere have produced rain that eroded some of the craters, and created rivers that erased more. Winds add to the constant process of resculpting the surface. Some of the changes to Titan’s surface may come from below. Volcanoes may belch frozen or even liquid water onto the surface, where it paves over the existing terrain. All of that activity has erased all but a few large craters from Titan’s surface — giving the ancient moon a youthful appearance. More about Titan tomorrow. Script by Damond Benningfield, Copyright 2012 This program was made possible in part by a grant from the NASA Science Mission Directorate. For more skywatching tips, astronomy news, and much more, read StarDate magazine.