StarDate Podcast

The celestial water bearer pours a streamer of faint stars across the southern sky on October nights. Aquarius is low in the southeast as the sky gets good and dark, and stands due south before midnight. The constellation was important to the cultures of the ancient Middle East. In Babylon, it represented the god Ea, who ruled the Sun’s path for a quarter of the year. And in Egypt, it represented the god of the Nile because it was associated with the river’s annual life-giving floods — the floods came when the water bearer dipped his jug into the Nile. Aquarius’s brightest star is Sadalsuud — an Arabic name that translates as “best luck” or “luckiest of the lucky.” The name is related to the rains that accompanied it in the spring, when it first appeared in the morning sky. Sadalsuud is many times bigger and heavier than the Sun, and more than 2,000 times brighter. When it was born it was smaller but much hotter than it is today. But it’s nearing the end of its life, so changes in its core have caused its outer layers to puff up like a balloon. As it puffed up its surface got cooler. But the surface also got a lot bigger, making the star much brighter. We’ll talk about another star system in Aquarius tomorrow. In the meantime, look for Aquarius in the southeast in early evening. Sadalsuud is at its top right corner. It stands high above Fomalhaut, the brightest star in that part of the sky, which can help you pick it out. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

A bright, skinny triangle climbs high across the sky late tonight — the Moon, the planet Jupiter, and the star Aldebaran, the “eye” of Taurus, the bull. They’re in good view in the east by around 10 or 11 o’clock, with Aldebaran close to the lower right of the Moon and brilliant Jupiter farther to the lower left. Jupiter is the largest planet in the solar system, and it outweighs all the other planets and moons combined. Thanks to that great mass, it shepherds an entourage of more than 60 known moons, plus several thin, dark rings. Jupiter’s gravitational influence doesn’t end there, though — it extends throughout the solar system. For one thing, Jupiter’s gravity traps two clumps of asteroids. They share Jupiter’s orbit around the Sun, but they’re well ahead of and behind the giant planet. For another, Jupiter can deflect comets onto new paths. This role is important for Earth — Jupiter prevents many comets from reaching the inner solar system, where they could threaten our planet. And in the distant past, Jupiter affected the layout and even the population of the solar system. Gravitational interactions with the other three giant planets pushed all of the giants into their current orbits. And Jupiter kept a ring of material that’s between Jupiter and Mars from coalescing to form a planet. Today, the remnants of that failed planet form the asteroid belt — a band of rubble courtesy of giant Jupiter. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Fifty years ago today, a Nigerian farmer near the village of Zagami was trying to shoo some cows from his corn field when he heard a loud explosion and was rocked by a blast wave. Seconds later, he saw something fall from the sky and slam into the ground just 10 feet away. When he investigated, the farmer found a black rock, weighing about 40 pounds, buried in the ground. This was no ordinary rock, though — it was a piece of Mars. At first, scientists thought the rock was just another meteorite — a piece of debris from an asteroid. But a couple of decades later, they analyzed tiny pockets of gas inside the Zagami meteorite. The amounts of certain forms of argon, nitrogen, and xenon matched those found in the atmosphere of Mars. Later analysis showed that the volcanic rock formed about a billion years ago. It was blasted into space less than three million years ago, when a good-sized asteroid slammed into the Martian surface. After that, the rock looped around the Sun until it ran into Earth — startling a farmer 50 years ago today. But there’s a footnote to the story. In 1997, the Mars Global Surveyor spacecraft entered orbit around Mars. It carried a tiny grain of the Zagami meteorite contributed by one of the mission scientists. Today, the spacecraft is dead and orbiting high above Mars. Eventually, though, its orbit will decay and it’ll hit Mars — returning the speck of the Zagami meteorite to its birthplace. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

It’s been quite a year for the planet Venus. Back in June, it passed across the face of the Sun — the last “transit” for more than a century. And it continues the close encounters before dawn tomorrow, when it slips just a lion’s-whisker away from Regulus, the brightest star of Leo. Venus is the dazzling “morning star,” so you can’t miss it. Venus passes close to Regulus fairly often — once or twice a year. And this year’s encounter is especially close. At first light tomorrow, Venus will stand only a fraction of a degree below the bright star. At times, though, the encounter can be even closer. In fact, Venus can sometimes pass directly between Earth and Regulus, briefly hiding the star from view. Such passages are rare, though. For one thing, Venus’s path across the sky slides back and forth a little bit with respect to Regulus’s position, so the circumstances of each encounter are a little different. For another, while Regulus climbs all the way across the sky, Venus doesn’t. So there are only certain times of year when it can appear close to Regulus — when the star is in the east shortly before dawn, or in the west shortly after sunset. And for yet another, Venus is tiny as seen from Earth, so there’s not much to cover Regulus up — the encounter has to be a bullseye. Even so, it does happen. The last of these occultations took place in July 1959, but the next won’t happen until October of 2044. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Like a train barreling down the track, Venus is sliding eastward along the ecliptic — the Sun’s path across the sky. And that trip will carry the brilliant planet past a bright “station” the next few mornings: Regulus, the leading light of Leo, the lion. The ecliptic also is a close approximation of the paths of the planets. That means that if the Sun were sitting in the middle of a big table, the planets would roll around it on the same tabletop. Since the ecliptic is viewed from Earth, Earth would fit precisely on the tabletop. But the orbits of the other planets are all tilted a tiny amount compared to Earth’s orbit. Venus is tilted by a little more than three degrees, which means the planet can appear up to that distance away from the ecliptic. Right now, though, it’s almost directly atop the ecliptic, which is why it’s passing so close to Regulus. The star always maintains the same position relative to the ecliptic. So as the planets pass by it they snuggle close, but just how close depends on where they are in their orbits. At their closest, they can even pass directly in front of Regulus; we’ll have more about that tomorrow. Look for Regulus just to the lower left of Venus, the brilliant “morning star,” in the hours before dawn tomorrow. They’ll be even closer the next day, with Venus a fraction of a degree below Regulus. After that, Venus will keep on rollin’ down the ecliptic — leaving Regulus behind. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

You don’t have to knock down all the bowling pins with a single roll — you can always try to pick up the spare. And the same thing may apply to Uranus, the solar system’s third-largest planet. Instead of standing more or less upright, as the other planets do, Uranus lies on its side, so its axis aligns roughly with the plane of its orbit around the Sun. The consensus has been that Uranus was knocked over by a single large impact — a collision with another planet-sized body. But one recent study suggests the planet’s orientation is the result of a few smaller impacts, not a single giant one. The study was conducted by a team of European scientists. They found that if Uranus had suffered a single impact early in its history, then its moons would all orbit backwards — in the opposite direction from Uranus’s rotation — but they don’t. When the scientists used two or more smaller impacts, though, their simulations matched what we see today. That suggests that Uranus is the equivalent of a spare, not a strike — a planet that was knocked over by more than one planetary bowling ball. Uranus is putting its best showing of the year now — it rises around sunset and is in the sky all night. It’s brightest for the year, too. But you need dark skies and binoculars to find it. Tonight, it’s well to the upper right of the Moon. But it’ll be easier to see in the nights ahead, as the Moon leaves the planet behind. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

In the 14th century, the people of China were ready to launch a revolt against the Yuan Dynasty. Legend says they coordinated the start of the revolution by passing messages inside mooncakes — small pastries that are part of the celebration of mid-autumn. The messages said the rebellion would begin on the 15th day of the eighth month in the Chinese calendar — the equivalent of the Harvest Moon in the western calendar. The legend may or may not be true, but the Mid-Autumn Festival is still one of the most important events in the Chinese calendar — and it’s still tied to the Moon. And the festival is getting underway now, because tonight is the night of the Harvest Moon — the full Moon closest to the autumnal equinox. There are many legends associated with the Mid-Autumn Festival. Most are tied to the moon goddess Chang-E. One version says she swallowed a pill of immortality that was intended for her husband. The angry husband chased her across the sky, but she escaped to the Moon, where she still lives today. Mooncakes have been part of the celebration for centuries. They’re small pastries with a thick filling — traditionally a paste made from beans or lotus seeds, although modern versions are more varied. Some include an egg yolk in the center to symbolize the Moon. They’re often decorated with the Chinese character for harmony, along with lunar symbols. They celebrate the Moon of mid-autumn — the Harvest Moon. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

One of the giant worlds of the solar system is putting on its best showing of the year right now. It rises at sunset, remains in the sky all night long, and shines brightest for the year, too. And for the next couple of nights it appears close to the Moon. Unfortunately, though, the planet Uranus is so far and faint that you still need binoculars and a starchart to find it. Uranus is the third-largest planet in the solar system — about four times the diameter of Earth. On average, though, it’s about 1.8 billion miles away, so it’s a mere speck in the night sky. In fact, under especially dark skies it’s just bright enough to see with the unaided eye. Yet no one realized that tiny speck of light was a planet until 1781, when William Herschel observed it through a telescope for several nights. It turns out that others did see it before then, and even charted it. But none of them watched it long enough to see that it moves against the background of stars, so they thought it was a star. Even then, it’s certain that many people saw Uranus well before the invention of the telescope. Yet it’s so inconspicuous that no one paid any attention to it — not realizing they were looking at one of the worlds of the solar system. Uranus rises well to the lower left of the Moon tonight, and quite close to the lower right of the Moon tomorrow night — the night of the Harvest Moon, by the way. 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.

It turns out that you can’t go much of anywhere in the solar system without splashing into some water. There’s frozen water on Mars, in comets and in many asteroids, and at the surfaces of many moons. And there’s liquid water in the atmospheres of the giant planets, and beneath the icy crusts of some of their moons. Water is especially prevalent at Jupiter, the largest planet. On Jupiter itself, it forms towering clouds that produce some of the solar system’s most powerful thunderstorms. Liquid water is also abundant on perhaps three of Jupiter’s big moons. There’s little doubt that a global ocean sits beneath the icy crust of Europa. It contains two or three times as much water as Earth’s oceans. And there may well be “chimneys” that belch hot, mineral-rich fluids into the bottom of the ocean, providing possible habitats for life. There’s also evidence for an ocean on Callisto. It’s buried far deeper than the ocean on Europa. And there would be less energy in the ocean environment, making it a less likely home for life. The third possible water world is Jupiter’s largest moon, Ganymede. Evidence includes oddities in its magnetic field that are best explained by a salty ocean, cracks in the crust like those on Europa, and minerals on the surface that likely were borne by a salty ocean. Ganymede’s ocean would be the deepest of the three, though, so there’s little chance of splashing into this water world anytime soon. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

There’s no other world quite like Triton, the largest moon of the planet Neptune. For one thing, it orbits in the opposite direction from Neptune’s rotation. That means it was captured by Neptune instead of forming with the giant planet. It also means that Triton will eventually be pulverized as it spirals inward. Triton’s surface is different, too. Much of it looks like the skin of a cantaloupe. And some areas are marked by long dark streaks — the plumes of large, icy geysers. But in one way, Triton may be similar to some of the solar system’s other large moons: It may have an ocean of liquid water far below the surface. Triton is one of the coldest worlds in the solar system. Its surface is made of water that’s frozen as hard as granite. But recent research suggests that its interior could have been heated enough in the past to melt some of the ice below the surface. Some of the heat came from radioactive elements in Triton’s core. But Triton originally followed a much more elongated orbit than it does today. As its distance from Neptune changed, Neptune’s gravity continually stretched and squeezed Triton, heating its interior. Triton’s orbit is more circular today, but it may still generate enough heat to maintain an ocean. The water may be mixed with ammonia, though. That would keep the ocean liquid even at temperatures that are cold enough to freeze water alone. We’ll talk about another water world tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The deepest spot on Earth is the Challenger Deep — a part of the Mariana Trench that lies seven miles below the surface of the Pacific Ocean. But that’s just barely a wading pool compared to a possible ocean on Titan, the largest moon of Saturn. It could be up to 150 miles deep, and hold far more water than Earth’s oceans. Titan is enveloped in a cold, thick atmosphere that’s topped with an organic haze — a sort of smog. Lakes of liquid methane and ethane dot its surface. But recent observations suggest there’s far more liquid beneath Titan’s crust, which is made largely of frozen water. Over the last few years, scientists have used the Cassini spacecraft to measure how much the surface of Titan flexes — the result of tides caused by the gravitational pull of Saturn. They found that the tides are about 30 feet high — much higher than if Titan were made just of rock. So instead, it must be made of about a 60-40 combination of rock and water. Some of the water makes up the frozen crust, but much of it must still be liquid — warmed by the energy of the tides and the decay of radioactive elements inside Titan. The scientists haven’t measured the size of the ocean. But they suspect it begins perhaps 50 miles down, and could be 150 miles deep. If so, then it contains about four billion cubic miles of water — more than a dozen times more water than all of Earth’s oceans combined. We’ll talk about another water world tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Water, water everywhere/Nor any drop to drink. In “The Rime of the Ancient Mariner,” Samuel Taylor Coleridge’s character was talking about being surrounded by a calm ocean on a sailing ship. But the verse would work just as well on several other worlds of the solar system. Some may host oceans of liquid water that are far more massive than those on Earth. Others may have smaller pools of water. But all of them are hidden beneath crusts of rock or ice. These water worlds are farther from the Sun than Earth is. At that distance, the raw materials for planets and moons included a good bit of frozen water. These bits of ice came together with other ingredients to form some of the moons of the giant outer planets. Denser materials, like metals, sank to the centers of these worlds, while lighter materials, like water, bubbled to the top. Exposed to the cold and vacuum of space, the water at the surface of such a moon remained frozen. But some of the ice below the surface melted from the heat of the moon’s formation, the decay of radioactive elements, or tides caused by the gravitational pull of other bodies. That’s created global oceans on some of these moons, as well as a possible ocean on the largest asteroid. It’s also created a smaller ocean on another moon of Saturn. So there’s plenty of water scattered throughout the solar system — but not much way to tap into it. We’ll talk about one of these water worlds tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The start of the 1340s was an an unpleasant time. England and France were, as usual, at war. And the Black Death was poised to sweep across Europe, eventually killing millions — including up to a third of the population of England. But on the star that marks the nose of Pegasus, the flying horse, things appear to have been pretty quiet, as the star began the transition from middle age to old age. We know about the events here on Earth by reading the history books. But we know about what was happening on Enif because we can see those events playing out right now. The star is about 670 light-years away, so the light we see from it tonight actually left the star around the start of the 1340s. Enif is well up in the east at nightfall, to the upper right of the Great Square of Pegasus. In fact, it’s the brightest star in the whole constellation, so it’s easily visible to the unaided eye. That we can see the star at all tells us that it’s extremely big and bright — about 150 times wider than the Sun, and thousands of times brighter. That makes it a supergiant. Enif is nearing the end of its life, so it’s undergoing a transition that eventually will make it even bigger and brighter. But that impressive phase won’t last for long. Within a few million years, the star will either blast itself to bits as a supernova, or eject its outer layers in a more gentle process. Whatever happens, we’ll see it here on Earth — centuries later. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The first night of autumn seems like a great time for a little skywatching. And in fact, an international group of astronomy enthusiasts has set up a network of events to help others do just that. And they have a specific target in mind: the Moon. The project is called International Observe the Moon Night. It’s designed to help others become more aware of the night sky by focusing on its most prominent resident. Observatories, schools, planetariums, and others are hosting Moon-watching events all across the country and the world. And there’s plenty to look at. The Moon is at first quarter today, so sunlight illuminates half of the lunar hemisphere that faces our way. And the Sun angle is pretty low, so mountains and craters produce long shadows, helping them stand out when you look at the Moon through binoculars or a telescope. And the Moon stands above a prominent summer constellation — teapot-shaped Sagittarius. Of course, you don’t need any help at all to appreciate the beauty of the Moon or the rest of the night sky — just a safe, comfortable skywatching spot and a clear view of the heavens — not just tonight, but every night of the year. And as we mentioned earlier, today is the start of autumn in the northern hemisphere. The Sun crosses the equator from north to south, bringing the promise of shorter days and longer, cooler nights ahead — good conditions for enjoying the beauty of the night sky. Script by Damond Benningfield, Copyright 2012 Moon Night logo For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Our home planet is made almost entirely of recycled materials. The iron in the core, the silicon and magnesium in the mantle, the nitrogen and oxygen in the air were all made in the hearts of stars, then expelled into space when the stars died. Many of these elements came from the heart of a supernova — a massive star that blasted most of itself to bits in a titanic explosion. A future stellar recycling center is in good view this evening. Antares, the brightest star of Scorpius, is to the lower right of the Moon at nightfall and sets a couple of hours later. Antares is at least 15 times as massive as the Sun. The pressure in the centers of such stars makes them extremely hot. The heat allows chemical elements to “fuse” together to make heavier ones. So by the end of its life, Antares will have built up an iron core, surrounded by layers of oxygen, carbon, silicon, and other elements. And when Antares explodes, the extreme heat of the supernova will produce elements that are even heavier than iron — elements like lead, uranium, and gold. The explosion will blast these elements outward at a few percent of the speed of light. Eventually, some of the debris may ram into clouds of cold gas and dust. The impact will squeeze the clouds, causing them to collapse and give birth to new stars and planets. These newborn objects will be seeded with the elements created by the supernova — elements recycled from an exploding star. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.