StarDate Podcast

Oh, be a fine guy — kiss me! No, that’s not flirtation, it’s science — a way to remember how astronomers classify stars — with the letters O, B, A, F, G, K, and M. A star at the “me” end of the scale is in good view this evening. Antares, the heart of Scorpius, is to the lower left of the Moon at nightfall. The classification system was devised about 150 years ago. It grouped the stars based on features in their spectra — the rainbow of colors that’s produced when you break starlight into its individual wavelengths. Later, the system was revised to classify the stars by letters of the alphabet, starting with “A” and working up. In the early 20th century, Annie Jump Cannon reworked the system, also based on spectra. She dropped most of the letters, and rearranged the ones that were left. The new arrangement also classified stars by their temperature. “O” stars are the hottest, with surface temperatures of more than 60,000 degrees Fahrenheit, so the stars are blue-white. “M” stars are the coolest, with temperatures as low as about 5,000 degrees, so they shine orange or red. Most M stars are of a type that’s far cooler and fainter than the Sun, so not a single one is visible to the unaided eye. But some M stars are much brighter giants or supergiants. Antares fits into the “super” category. It’s one of the biggest, heaviest stars in the galaxy — an impressive feat for a star at the bottom of the scale. More about Antares tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

According to the old cliché, the three rules of real estate are location, location, location. And if the human race ever settles on other worlds, that rule will apply there as well. Two examples huddle in the southwest early this evening — the Moon and the planet Mars. Mars looks like an orange star a little to the right of the Moon. The best locations on the Moon are at the poles, where some craters appear to offer abundant resources. The rims of the craters are in perpetual sunlight, so they’d offer a limitless supply of solar energy. And the bottoms of the craters are in perpetual darkness, so they preserve large amounts of frozen water. The ice could provide not just drinking water, but its hydrogen and oxygen could be separated to make rocket fuel. On Mars, though, you’d want to stay away from the poles. They do offer huge amounts of frozen water, but in wintertime the water is topped by frozen carbon dioxide. In spring, the CO2 vaporizes, rushing into the atmosphere and stirring up giant dust storms. Instead, the best places on Mars are close to the equator. They’re easier to reach, and temperatures don’t get as cold. And there’s plenty of ice mixed with the soil across much of the planet, providing a good resource for colonists. Neither Mars nor the Moon is going to be settled for decades, and perhaps even centuries. Still, if and when people do head there, at least they’ll know where to look for prime real estate. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Building a planetary system is messy. It starts with small grains of rock and ice that come together to form larger and larger blocks. Some of those big blocks may eventually merge to make planets. But while some of the impacts are slow enough for the blocks to stick together, others are high-speed collisions that rip them apart, scattering debris back into space. That process appears to be taking place around Fomalhaut, the brightest star of Piscis Austrinus, the southern fish. It’s low in the southeast by around 9 o’clock, and due south around midnight. There are no other bright stars around it, so it’s hard to miss. A broad disk of dust encircles the star, which is quite young. The disk appears to have given birth to at least one planet. It’s about three times as massive as Jupiter, the giant of our own solar system, and it’s far, far away from Fomalhaut itself. Observations by space telescopes gave conflicting readings on the size of the dust grains in the disk. One said they were tiny, another said they were larger. Earlier this year, a team of European astronomers reported a possible solution. It said the grains must be large but fluffy — like the debris from comets in our own solar system. But there’s so much of this material that the supply must be constantly renewed by collisions. And the collisions wouldn’t be rare — up to hundreds of them every day — keeping things messy in a planetary building zone. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Autumn has few stars to call its own. A few lingering stars of summer are in view during the evening, with the bright stars of winter climbing into view in the wee hours of the morning. But there’s really only one bright star that puts in its best showing during the nights of autumn: Fomalhaut, the leading light of Piscis Austrinus, the southern fish. Fomalhaut itself is young and vigorous. It’s only a few hundred million years old, and it’s bigger and heavier than the Sun. That makes the star hotter and brighter than the Sun — one reason that Fomalhaut shines so brightly in our night sky. The other reason is that it’s a close neighbor — just 25 light-years away. A broad disk of dust encircles the young star. The disk seems to be bounded by a giant planet that’s about 10 billion miles from Fomalhaut — more than a hundred times farther than Earth is from the Sun. Hubble Space Telescope has snapped pictures of the possible planet, although it’s so small and far away that it looks like nothing more than a tiny dot. The supply of dust in the disk may be renewed by constant collisions between comets; more about that tomorrow. In the meantime, look for Fomalhaut low in the southeast not long after night falls, and due south around midnight. From the northern states, it never climbs very high above the horizon, but the view is a little better from the south — a view of the only true contender for the title of “the Autumn Star.” Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Several brilliant stars are in good view as night falls this evening. Yellow-orange Arcturus is in the west, with reddish-orange Antares, the heart of the scorpion, low in the southwest. And the stars of the Summer Triangle — Vega, Deneb, and Altair — stand high overhead. Many other stars are in good view as well, thanks to the Moon — or to be more specific, the Moon’s absence. The Moon was “new” on Saturday night, when it crossed the line between Earth and Sun. So as night falls this evening, the Moon is roughly 24 hours old — 24 hours past new. It’s too close to the Sun to find without optical aid. And even if you could find it, it’s nighttime across almost the entire lunar hemisphere that faces our way — only a tiny edge of it is bathed in sunlight. Moonless nights are some of the best for skywatching. As the Moon grows fatter, it casts more light into the sky, overpowering the glow of the stars. It also overpowers meteors, which are some of the night sky’s most ooh-inspiring sights. To fully appreciate those sights, you have to get away from an even peskier glow — city lights. They fill the sky with such a haze that the stars all but disappear. If you’re a fan of the Moon, though, you won’t have long to wait for it to return to view. It’ll be quite low in the southwest as darkness begins to fall tomorrow night — a thin crescent that’s easy to see, but that’s not yet exerting its dominance over the night sky. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

51 Pegasi is a pretty ordinary star. But in 1995, it became one of the best-known stars in the galaxy. That’s because it was the first “normal” star with a confirmed planet. The newfound world was a giant, like Jupiter, the largest planet in our own solar system. But it was orbiting much closer in than Mercury is to the Sun. So astronomers described the new world as a “hot Jupiter.” Since then, astronomers have discovered scores of hot Jupiters orbiting other stars. And they’ve recently studied 63 of them that were discovered by the planet-hunting Kepler spacecraft. The scientists looked for small deviations in the orbits of these planets caused by the gravitational tug of other planets in the same star systems. But they found that not a single hot Jupiter has a close planetary neighbor. The result may shed light on just how hot Jupiters form. The planets probably are born far from their stars, just as Jupiter was, and are kicked inward by the gravity of another giant planet. As the hot Jupiter moves toward its star, its gravity hurls away any other planets that lie closer to the star. Some of these planets fall into their star, while others are ejected into the deep cold of interstellar space. 51 Pegasi is about a third of the way up the eastern sky at nightfall. It’s just above the top side of the Great Square of Pegasus, which is in fine view. 51 Pegasi is faint, so you need dark skies to see it — a star system with a hot but lonely planet. Script by Ken Croswell, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Lacerta, the lizard, is a small constellation that’s lodged between Cygnus and Andromeda. The hazy band of the Milky Way runs through it, so you might expect its best-known objects to reside inside our galaxy. In fact, though, none of Lacerta’s stars is particularly bright, so its best-known object is a strange galaxy nearly a billion light-years beyond its stars. For decades, astronomers mistook that object for a mere star in the Milky Way. But in 1929, they discovered that its light varied from night to night. They christened the object BL Lacertae. In the 1960s, though, astronomers realized that BL Lacertae was no star. Instead, they detected the fuzzy glow of a galaxy around it. What’s more, the object emitted lots of radio waves — something that no “normal” star does. Other distant galaxies resemble BL Lacertae — so much so that BL Lacertae serves as the prototype for an entire class of these objects. All of them shine profusely at all wavelengths, from radio to visible light to gamma rays. Each galaxy is powered by a large black hole at its center. Material falling into the black hole gets heated to extreme temperatures, so it emits huge amounts of radiation. Some of this material shoots away from the black hole as high-speed jets. In BL Lacertae, the jet happens to be aimed right at us, so even slight changes in its angle can cause the variability that first caught astronomers’ attention nearly a century ago. Script by Ken Croswell, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Andromeda galaxy is the best-known galaxy beyond our own, and for good reason: It’s just two-and-a-half million light-years away — the closest giant spiral galaxy. Recent observations with Hubble Space Telescope have revealed that Andromeda owes a tiny part of its beauty to an encounter with another galaxy. Astronomers examined the outskirts of Andromeda, 85,000 light-years from its center. The observations indicate that many new stars were born in that region about 1.5 billion to 3 billion years ago. The reason for Andromeda’s starburst isn’t hard to find. A spiral galaxy in a neighboring constellation is likely responsible. M33 is smaller than Andromeda, but still a substantial galaxy. In fact, it’s the third-largest galaxy in the Local Group, the gathering of several dozen galaxies that includes our own. Only Andromeda and the Milky Way are larger. About three billion years ago, M33 swung by Andromeda and stirred up the other galaxy’s gas, which then collapsed to form new stars. Andromeda’s gravity did the same to M33, whose outskirts have stars of a similar age, so each galaxy likely enhanced the beauty of the other. Both galaxies are in the sky tonight. Under dark skies, Andromeda is visible to the unaided eye as a faint smudge of light, about a third of the way up the east-northeastern sky at nightfall. M33 is well below it, and it’s fainter, so you need binoculars or a telescope to pick it out. Script by Ken Croswell, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Our home galaxy, the Milky Way, is on a collision course with another spiral galaxy. They’ll slam into each other in about four billion years, and eventually stir together to form a rather dull place containing perhaps a trillion stars. Our partner in this galactic pile-up is M31, the Andromeda galaxy. It’s in the eastern sky on September evenings, and to the unaided eye looks like a small, faint, fuzzy blob of light. M31 is moving toward us at about 250,000 miles an hour. It’s about two-and-a-half million light-years away, though, so the motion is imperceptible. Yet over the eons, M31 will grow ever larger in Milky Way skies as the two galaxies are pulled together by their powerful gravity. As they merge, giant clouds of gas and dust will ram together and give birth to millions of new stars. The existing stars will be thrown into new orbits that buzz about the cores of the two galaxies like angry bees, while some stars are thrown into intergalactic space. The stars are all so far apart, though, that it’s unlikely that any of them will collide with each other. After a couple of billion years, the combined galaxies will settle into a new galactic form — a fat, fuzzy football known as an elliptical. With most of its gas and dust gone, the combined galaxy will give birth to few new stars. Instead, its vast population of stars will grow old and faint — and over billions of years more, the giant galaxy will slowly fade away. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

One of the highlights of the autumn sky is M31, the Andromeda galaxy. It’s the most distant object that’s readily visible to the unaided eye — about two-and-a-half million light-years away. M31 and our own galaxy, the Milky Way, are near twins. Both are spiral galaxies. They’re shaped like wide, thin disks. Their hottest, brightest stars outline spiral arms, so from afar, the galaxies look like pinwheels. The Milky Way spans a hundred thousand light-years, while Andromeda is a bit bigger. Both galaxies are also at least a trillion times as massive as the Sun. Astronomers used to think that Andromeda was the heavier of the two, but more recent research seems to give the nod to the Milky Way. Each galaxy has several hundred billion stars, but most of their mass is in the form of dark matter — matter that produces no detectable energy, but that exerts a gravitational pull on the stars and gas clouds around it. The dark matter is concentrated in vast “halos” that surround the galaxies — ball-shaped volumes of space that contain only a few stars. Andromeda’s halo may extend a half-million light-years from the galaxy’s heart — a long way toward our own galactic home. Over the eons, though, the two galaxies will get even closer; more about that tomorrow. M31 is well up in the east-northeast by the time it gets good and dark. It looks like a small, fuzzy patch of light, so you need to get away from city lights to see it. Script by Damond Benningfield, Copyright 2007, 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

One of the most elaborate stories in the night sky offers love, vanity, a big dose of adventure, and a Hollywood-style rescue. One of the characters in the story is Andromeda, the princess. Her constellation is in the east and northeast as the sky gets good and dark on September nights. It’s not all that bright, but you can find it by looking to the lower left of the Great Square of Pegasus. The myth says that Andromeda was the daughter of Cepheus and Cassiopeia, the king and queen of Ethiopia. Cassiopeia was beautiful but vain — she boasted that she was more beautiful than the sea nymphs. So they went to Poseidon, the god of the sea, in search of vengeance. He sent the monster Cetus to attack Ethiopia. To appease the gods, Cepheus ordered Andromeda chained at the ocean’s edge as a sacrifice. At the last minute, though, Perseus saved the day. He flashed the head of Medusa at the monster, turning it to stone. The original story doesn’t include Pegasus, but modern versions do. They say that Perseus rode the winged horse when he fought Cetus — adding another player to a grand story in the stars. Perseus is to the lower left of Andromeda during the evening. W-shaped Cassiopeia is to her left, with Cepheus farther to the left. Cetus is a couple of constellations to the lower right and rises a little later — a safe distance from the princess. One of the treasures of Andromeda is the spiral galaxy M31; more about that tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Moon closes in on Venus, the “morning star,” the next few days. It’s a good distance away from Venus tomorrow, but will stand side by side with the planet on Wednesday. Venus’s surface is hidden from view by clouds that cover the entire planet. In fact, they’re one reason that Venus looks so bright. In decades past, many scientists thought the clouds were hiding a steamy world covered with vast oceans of water. It turns out, though, that Venus is far too hot for water — surface temperatures average more than 850 degrees Fahrenheit. Yet Venus may have been covered with water early in its history — perhaps more water than on present-day Earth. Observations by orbiting spacecraft suggest that Venus lost its water through interactions with the Sun. Solar radiation split apart water molecules in the atmosphere. The lightweight hydrogen atoms escaped into space, while the heavier oxygen atoms combined with rocks at the surface. More water then evaporated into the atmosphere, where it, too, was destroyed by sunlight. That process left less water to bind with carbon dioxide to form rocks, so the ratio of carbon dioxide in the air went up. CO2 traps heat, so it caused Venus’s surface to get ever hotter. Over time, this cycle destroyed the Venusian oceans, leaving the planet hot and dry — just as it is today. Look for Venus far below the Moon at first light tomorrow, a little closer on Tuesday, and standing next to it on Wednesday. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The surfaces of the solar system’s big, rocky bodies all look a lot alike — they’re rugged and barren. But below the surface, they can be quite different. Two examples play a game of “peek-a-boo” in the wee hours of tomorrow morning — the Moon and Ceres, the largest asteroid. The Moon will cover up Ceres for about 45 minutes or so, with the exact timing depending on your location. Ceres is quite faint, so you need a telescope to see it before and after it disappears behind the Moon. The Moon’s surface sits atop layers of rock. But the surface of Ceres appears to sit atop a layer of water — some of it frozen, but perhaps some of it liquid. Christopher Russell is the lead scientist for Dawn, a mission that’s supposed to enter orbit around Ceres in 2015. RUSSELL: We think there’s subsurface liquid water. We think that, maybe, even some of that water gets out to the surface. That’s one of the things we’ll be looking for is evidence that, perhaps in the equatorial regions, where it’s a little warmer, as to whether water is leaking out on the surface. Ceres is only about a quarter the size of the Moon, and only about one percent as massive. That means its density is quite low, indicating that it may have a substantial amount of water — a layer perhaps 50 or 60 miles thick. And much of its surface is made of clay, which is formed in a wet environment. Dawn will help refine that picture of Ceres, telling us what’s beneath the skin of the largest asteroid. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

More than 70 percent of Earth’s surface is covered by water — more than 300 million cubic miles of it. Yet that’s just a drop in the ocean compared to the water on one of the moons of Jupiter. It may have two or even three times as much water as Earth — most of it contained in a salty ocean that’s hidden beneath the moon’s crust. Europa is slightly smaller than our own Moon. Unlike our dry, dusty Moon, though, Europa has water to spare. Its surface is a fairly thin crust of ice sitting atop what appears to be an ocean that enwraps the entire moon. This ocean could be 50 miles deep. Volcanic vents at its bottom may pump out hot water and a rich mixture of minerals — all of the raw ingredients for life. A recent study by geologists at the University of Texas found that there may also be liquid water inside the crust — pools of it as big as the Great Lakes. These pools may sometimes squirt water onto the surface, while getting more water from the ocean below — an ocean that dwarfs those here on Earth. Jupiter is just to the lower left of the Moon as they rise around midnight tonight, and looks like a brilliant star. The true star Aldebaran is a little farther to the right of the Moon. Through binoculars, Europa and three of Jupiter’s other big moons look like tiny stars quite near the giant planet. We’ll talk about another body that may have liquid water below its surface — the asteroid Ceres — tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Moon has some impressive companions this weekend — a brilliant planet, a bright star, and a famous star cluster. The whole party is in good view by about one o’clock in the morning. Tonight, the member of the party that’s closest to the Moon is the Pleiades star cluster, a little above or to the upper left of the Moon as they rise. Its brightest stars form the outline of a tiny dipper. The Pleiades is more than 400 light-years away. Yet for centuries, the cluster has provided weather forecasts right here on Earth. In ancient Greece, for example, sailors often studied the Pleiades before heading out to sea. If the cluster looked bright and clear, it was a harbinger of good weather. If it was obscured by high, thin clouds, then it was a sign of stormy weather — and a good time to stay home. Villagers in the Andes Mountains of South America used the Pleiades to decide when to plant crops. Good visibility at the start of the planting season meant early and abundant rains, so villagers planted early; poor visibility meant poor rains, so planting started later. Recent studies have shown that the Pleiades was especially good at forecasting El Niño, which had a big impact on the rains. The Moon’s other prominent companions are the star Aldebaran, which is below the Moon, and the dazzling planet Jupiter, farther to the Moon’s lower left. The Moon will snuggle even closer to them tomorrow night; more about that on our next program. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.