StarDate Podcast

After the Sun goes down, a dome of light covers every city and town — the combined glow of the fixtures that illuminate streets, businesses, and our own yards. The light reflects off molecules and particles in the atmosphere, creating a glare that overpowers the natural lights in the night sky — the stars. In many big cities, the sky glow is so bright that only a handful of stars shines through it. Like the light from the stars themselves, the sky glow is a blend of many colors. And thanks to changes in outdoor lighting technology, the color is starting to shift a bit to the blue end of the spectrum. Blue wavelengths are just the right size to scatter in the atmosphere — which is why the daytime sky is blue. So blue wavelengths intensify the sky glow at night. And new LED lighting tends to be much bluer than some older types of outdoor lighting, such as the yellowish high-pressure sodium fixtures that line many streets today. There’s some evidence that bluer light can affect a person’s natural circadian rhythm, and can cause problems for wildlife as well. And blue light makes it even harder to see the faint stars and galaxies sprinkled through the universe — a problem for astronomers, and for anyone else who wants to appreciate the beauty of a dark night sky. There are still a few places in the U.S. where the sky remains nice and dark, and we’ll talk about a couple of those tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Most of the life of a star like the Sun is pretty dull. It spends billions of years steadily “fusing” the hydrogen its core to make helium, releasing energy as it does so. When the hydrogen runs out, though, the star undergoes a series of big changes that play out quickly on the astronomical timescale. A star known as R Trianguli, for example, is going through a phase that may last as little as a few centuries. The star is puffing in and out like a beating heart — the result of the constant tug of war between radiation and gravity. The star is burning the hydrogen in a thin layer around its core. Radiation from that process heats the star’s outer layers and pushes them outward. As they expand, though, some of the radiation escapes into space, so the outer layers cool and fall back toward the core. In R Trianguli, each of these “pulses” takes about nine months, and the results are spectacular. At its brightest, the star is more than 600 times brighter than at its faintest. R Trianguli is losing a lot of its gas to space. Eventually, its outer layers will puff away, leaving only the star’s hot, dense core — a white dwarf. The same fate awaits the Sun — in several billion years. R Trianguli is in the eastern sky at nightfall, to the lower left of wedge-shaped Triangulum’s brightest star. At its brightest, the star is just visible to the unaided eye. Right now, though, it’s in the “fading” part of its cycle, so you’ll need a telescope to find it. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A thin wedge of stars climbs the eastern sky on November evenings. It fills an otherwise dark space between the well-known constellations Aries, Perseus, and Andromeda. None of its stars is all that bright, but the distinctive triangular pattern will help you pick it out. Triangulum is just slightly north of due east as darkness falls, lining up roughly parallel to the horizon. Its brightest star, Beta Trianguli, is at the top left corner of the triangle, with the second-brightest star well to the right, forming the tip of the wedge. Both of those stars are actually double — two stars locked in tight orbits. The primary star of each system is nearing the end of its life. The stars are using up the last of the hydrogen fuel in their cores, “fusing” the hydrogen atoms together to make helium. As they finish that process, their outer layers are puffing up. So each star is several times wider than the Sun, and quite a bit brighter. Over millions of years, they’ll start to burn the helium, so they’ll get even bigger and brighter. And as they get close to the ends of their lives, they’ll become unstable, so they’ll puff in and out like a beating heart. In fact, another star in Triangulum is already doing just that. Known as R Trianguli, it’s to the lower left of Beta. Don’t bother looking for it, though, because it’s not bright enough to see with the unaided eye. It will be just bright enough to see in a few months, though; more about that tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A white dwarf would seem to be one of the most innocuous objects in the universe. It’s the small dead core of a once-normal star like the Sun. It no longer produces energy through nuclear reactions, but it continues to shine because it’s extremely hot. But give a white dwarf a companion star and things change in a hurry. Interactions between the two can trigger explosions — big ones that blast away the white dwarf’s outer layers, or gigantic ones that blast the entire star to cosmic dust. And sometimes, a single white dwarf can do both. A research team led by Ben Dilday of UC-Santa Barbara found evidence of a double-blaster in a galaxy 600 million light-years away. A supernova flared to life there, briefly outshining its entire home galaxy. The supernova was the complete destruction of a white dwarf. But observations revealed expanding shells of gas and dust around the supernova. The shells were produced by earlier nova explosions. The two types of explosions are related. Both occur when gas piles up on the surface of a white dwarf. A nova is an explosion of the shell of extra gas, while a supernova is the disruption of the entire star. To complicate matters even more, that’s only one of the ways in which a white dwarf can become a supernova. The other is when two white dwarfs ram together. Astronomers have seen evidence of a few of these collision-powered supernovae — the violent ends of otherwise quiet stars. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Cirrus clouds are a common sight at this time of year — high, thin streamers of ice. Clouds are common sights on other worlds, too. But many of those clouds aren’t made of water. The clouds on Venus, for example, are made of sulfuric acid, while those on Titan, the largest moon of Saturn, are made of methane and ethane. And on Jupiter, the solar system’s largest planet, the clouds at the top of the atmosphere are made of ammonia. Jupiter is basically a big ball of gas that consists mainly of hydrogen and helium, the lightest chemical elements, with a smattering of carbon, oxygen, nitrogen, and other elements. At the top of Jupiter’s atmosphere, the hydrogen and nitrogen come together to make ammonia, which condenses to form a layer of clouds that’s about 30 miles thick. A bit of sulfur in the atmosphere mixes with other elements to color the clouds orange, tan, and brown. Sometimes, a gap opens in the clouds, allowing us to see the layer below them. That layer contains clouds of liquid water, which look white. The clouds produce big thunderstorms, with lightning a thousand times more powerful than anything here on Earth. Jupiter’s clouds reflect a lot of sunlight, which helps make the giant planet one of the brightest objects in the night sky. Right now, it rises a couple of hours after sunset, and looks like a brilliant cream-colored star. And tonight, it rises just above the Moon — a beautiful sight for a cloud-free night. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

An astronomical demon creeps up the northeastern sky this evening — a close pair of stars in which one is cannibalizing the other. The system is known as Algol. It’s in Perseus, the hero, which is in the northeast at nightfall and directly overhead in the wee hours of the morning. “Algol” comes from an Arabic name that means the “demon.” It’s a shorter version of “head of the demon,” because the star represented the head of Medusa — a snake-headed monster who was decapitated by Perseus. Algol consists of three stars, two of which are separated from each other by just a few million miles. One of those stars is several times as massive as the Sun, while the other is less massive than the Sun. The less-massive star is in the final stages of life, while the more-massive star is still in the prime of life. And that’s a problem, because heavier stars age more quickly than their lighter-weight counterparts. The solution is simple: stellar cannibalism. Algol’s less-massive star actually started out heavier than its companion. As it neared the end of its life, it puffed up to giant proportions. That loosened its grip on its outer layers, so gas in those layers began pouring onto the other star. Over millions of years, the two stars swapped enough gas that the lighter star became the heavier one. So while Algol may not be a demon, it is a bit ghoulish — a perfect star to follow across the sky on a Halloween night. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

To fans of the Batman movies and comic books, the name Ra’s al Ghul is well known. The character is a supervillain who’s intent on saving the planet through rather nasty means. But the name didn’t originate with Batman. Instead, it was first given to a star in the constellation Perseus. The name means “the demon’s head.” It’s been shortened in modern times to just Algol — the demon — a name that seems appropriate for the Halloween time of year. The star represents the head of Medusa, a snake-headed monster who was decapitated by Perseus. It may have gained that fearsome rep because for a few hours every three days or so, Algol’s brightness drops by more than two-thirds. For astute skywatchers, that big of a change is easy to see with the eye alone. There’s nothing supernatural about Algol’s changing brightness, though. It’s all a matter of geometry. Algol is a system of three stars. Two of them are separated from each other by just a few million miles. One of the stars is quite bright, while the other is much fainter. About every three days, the fainter star passes in front of the brighter one as seen from Earth, blocking much of the brighter star’s light, so the system fades dramatically. A few hours later, though, the eclipse ends and Algol returns to its usual bright glow. But there is something a bit creepy going on with Algol — one star is consuming the other. We’ll save that part of the story for tomorrow — Halloween. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Perseus, the celestial hero, strides boldly across the northeastern sky on autumn evenings. The constellation stands close to the horizon as night falls, and high overhead in the hours after midnight. The figure is outlined by three streamers of stars that intersect at Perseus’s bellybutton — the constellation’s brightest star, Mirfak. It doesn’t just look bright in our sky, though. Mirfak is one of the brightest stars in our region of the galaxy — thousands of times brighter than the Sun. That great brilliance makes the star easily visible across more than 550 light-years of space. If you look around Mirfak, especially with binoculars, you’ll see a good sprinkling of fainter stars. Most of those stars are siblings of Mirfak — they’re all members of a cluster of about 150 to 200 stars. All of the stars in the cluster were born about 60 million years ago, from a single giant cloud of gas and dust. One way astronomers deduce the birthday of the cluster’s stars is by looking at Mirfak, which is the cluster’s brightest and heaviest member. Such stars live relatively short lives, and Mirfak is getting close to the end of that lifespan. So the fact that it’s still around puts an upper limit on the age of the entire cluster. While Mirfak is the brightest star in Perseus, it’s not the most famous. That honor goes to a star known as the demon — from a name that it shares with a villain from Batman. 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.

To the eye alone, the stars that sparkle in the night sky are mere pinpricks of light in the dark cosmic background. And many of them are so small and so far away that they remain pinpricks even when you view them through a good-sized telescope. One exception is Hamal, the brightest star of Aries, the ram. The constellation is in the east this evening. Aries isn’t much to look at, although Hamal is fairly bright, and in a fairly barren region of the sky. And tonight it’s due left of the Moon. Hamal is close enough that astronomers can get an accurate measurement of its distance — about 66 light-years. That makes it a close neighbor. And the star is big enough to measure its angular size. It’s an incredibly tiny angle, though — the equivalent of a dime seen from a distance of about 30 miles. Combining the star’s distance and its angular size in the sky allows astronomers to determine its true size — about 15 times wider than the Sun. Hamal is so big because it’s nearing the end of its life. Changes in the nuclear reactions taking place in its core have caused its outer layers to puff up like a big balloon. That makes the star’s surface much cooler, so Hamal shines yellow-orange. Millions of years from now, another set of changes in its core will cause Hamal to puff up even more — to perhaps dozens of times the diameter of the Sun. That’ll make it even easier for future astronomers to take the measure of this puffy star. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

You can’t feel it, but Earth is wobbling on its axis like a spinning gyroscope that’s running down. It’s a very slow motion though — it takes about 26,000 years to complete a single wobble. We can see the effects of the wobble in the changing tableau of the night sky — an effect called precession. It’s not visible from night to night, but it adds up. It causes Earth’s axis to point in different directions, giving us different “pole stars.” It also causes the Sun to appear against different sets of stars at the same time of year. An example of the latter effect is seen in Aries, the ram. More than 2,000 years ago, the Sun passed across the constellation at the time of the March equinox, which marked the start of the year. So Aries took on special significance — it was “first” among the constellations. The point of the equinox was known as the First Point of Aries. And it marked the “zero” point for the celestial coordinate system — a system of measuring positions in the sky that’s the equivalent of longitude and latitude here on Earth. But thanks to precession, over the centuries the equinox has moved westward, into the adjoining constellation Pisces. Yet it’s still known as the First Point of Aries — the central marker for mapping the entire sky. Aries is in the east this evening, well to the lower left of the Moon. It’s not all that much to look at, although its brightest star does stand out. More about that tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

There’s no limit on how big a black hole can get — keep funneling in stars and gas, and it keeps getting bigger. In fact, the largest black holes yet discovered are close to 20 billion times the mass of the Sun. But there is a lower limit on the size of a black hole — about three times the Sun’s mass. And a black hole of about that size lurks in the constellation Scorpius. It’s part of a binary system that consists of the black hole plus a puffy companion star. The black hole steals gas from the companion. The gas forms a rapidly spinning disk around the black hole. And some of the gas is funneled into “jets” that squirt away at close to the speed of light. As the gas spirals closer to the black hole, it gets so hot that it emits X-rays. The X-rays push away the infalling gas, which shuts down the jet and causes the X-rays to drop. But the inner part of the disk quickly heats up again, then plunges toward the black hole, restarting the jet. The whole process takes less than a minute, and produces “pulses” of energy like a beating heart. The length of the beats reveals the mass of the black hole — about three times the mass of the Sun. Such black holes form when the core of a massive star collapses. Any core that’s less than about three times the mass of the Sun will form not a black hole, but a neutron star. So the black hole in Scorpius is about as small as one can get — the smallest black hole yet discovered. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The universe is filled with life-and-death struggles. Comets pass too close to their stars and are vaporized. Dying stars shatter nearby planets. And black holes gobble up anything that passes within their sphere of influence — accompanied by spectacular displays of fireworks. An example of that was discovered just last year. Orbiting X-ray telescopes detected bright pulses of energy from a galaxy that’s almost four billion light-years away. At first, astronomers thought the outburst was a powerful explosion known as a gamma-ray burst. But it didn’t fade out in the same way as gamma-ray bursts normally do. Instead, the space telescopes saw pulses of X-rays lasting about three-and-a-half minutes. Analysis showed that the most likely explanation was the death of a star that was passing close to a black hole. As the star approached the black hole, it was ripped apart by the black hole’s increasingly powerful gravitational pull. As it spiraled into the black hole, the star’s gas formed a superhot disk that produced a lot of X-rays. As the gas in the innermost part of the disk whipped around the black hole, it produced the pulses of X-rays. And some of the gas was funneled into “jets” that squirted away from the black hole’s poles. The pulses of energy didn’t last long, though. They faded away in weeks, as material in the disk poured into the black hole — and the star lost its life-and-death struggle. More about black holes tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Gravitational waves are so weak that no one has yet discovered a single one. Yet if you gang up enough of them, their effect can be profound — strong enough to kick a supermassive black hole out of its home galaxy, for example. Gravitational waves are produced by the motion of any object with mass — from fireflies to fiery stars. They’re so extremely weak, however, that it takes the motion of an extremely massive object to produce any detectable effect at all. Astronomers have detected the signature of gravitational waves in the changing orbits of pairs of dense stellar corpses known as neutron stars, for example. They’ve also detected the likely result of gravitational waves in the heart of a galaxy that’s about four billion light-years away. Observations by ground- and space-based telescopes revealed two large, compact objects near the center of the galaxy that are moving away from each other at about three million miles an hour. Follow-up observations suggest that one is a supermassive black hole, while the other is a star cluster. Astronomers suspect that the galaxy formed from the merger of two smaller galaxies. The black holes at the centers of these galaxies also merged, forming an even bigger black hole. But the way the black holes merged produced a torrent of gravitational waves — enough to send the combined black hole skittering out of the galaxy at millions of miles an hour. More about black holes tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Yellow supergiant stars are the same color and temperature as the Sun -- but that's about all they have in common. These stars pour out thousands of times more light than the Sun, and they're much younger. What's more, they're quite rare. Yet one constellation features two of them shining side by side. The constellation is Aquarius, the water bearer. Its two brightest stars -- Alpha and Beta Aquarii -- are yellow supergiants that look as bright as the faintest star of the Big Dipper. Yellow supergiants are rare for the same reason that a yellow traffic light is rare. A yellow traffic light represents a brief transition from green to red. Likewise, a yellow supergiant is a brief transition from blue to red in the life of a massive star. Both Alpha and Beta Aquarii were born with more mass than the Sun, so they once shined hot and blue. They generated energy in their cores by converting hydrogen to helium. But their cores ran out of hydrogen, so the stars expanded and cooled, turning from blue to yellow. Eventually, they'll puff up even more, becoming red supergiants. For now, each of the stars shines about 2,000 times brighter than the Sun, so in a single night each star emits as much light as the Sun will during the next three years. Alpha and Beta Aquarii are hundreds of light-years from Earth. They move through space together, which means they were born together -- explaining why a single constellation has two stars of such a rare type. Script by Ken Croswell, Copyright 2009, 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

If you've ever wanted to thank your lucky stars, this is a good time to try. The constellation Aquarius sails across the southern sky on autumn evenings, and several of its stars have names that signify luck or good fortune. The most prominent examples are the constellation's two brightest stars. Their proper names are Sadalmelik and Sadalsuud. The names are from ancient Arabic, and mean "lucky one of the king" and "luckiest of the lucky." The names may refer to the stars' positions. Back when the constellations were named, the Sun passed through this region of the sky in early spring, when the long nights of winter had passed and the new season brought gentle, life-giving rains. In fact, Aquarius is one of several "watery" constellations in that part of the sky. As luck would have it, both of the stars are members of a rare class of stars known as yellow supergiants. Supergiants of any kind are rare enough. But most supergiants are either hot and blue or cool and red. Yellow ones are the rarest of all. Sadalmelik is the larger and brighter of the pair. It's a few dozen times the diameter of the Sun, and several thousand times brighter. It's 600 light-years away. These "lucky stars" -- both moderately bright -- are in the southeast at nightfall, and pass fairly high across the south later on. We'll have more about them tomorrow. Script by Damond Benningfield, Copyright 2009, 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.