StarDate Podcast

[SFX: Fireworks] Lots of bright lights will punctuate tonight’s sky — part of the coast-to-coast celebrations of Independence Day. But the biggest, brightest skylight of them all — the Sun — is actually a little smaller and fainter than usual. That’s because we’re farthest from the Sun for the entire year — a point in Earth’s orbit around the Sun called aphelion. The distance to the Sun changes because Earth’s orbit is an ellipse, which looks like a circle that’s been stretched and flattened. The Sun is a bit off the center of the ellipse. Today, this elliptical orbit has carried us to a distance of more than 94 million miles from the Sun — about three million miles farther than we were back in January. The change in distance affects how much light and heat we receive from the Sun — about six percent more in January than in July. But Earth’s oceans and atmosphere are quite efficient at storing and distributing heat, so they keep the planet’s overall temperature pretty much the same year ’round. One other effect of the elliptical orbit is that Earth’s orbital speed changes over the year. We move fastest when we’re closest to the Sun, and slowest when we’re farthest, as we are now. That means that summer in the northern hemisphere lasts about five and a half days longer than in the southern hemisphere. So enjoy the extra days of the summer season — days that are made possible by our planet’s lopsided orbit. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Messier 104 is one of the most photogenic galaxies in the nearby universe. A lane of dark dust outlines its edge like the brim of a hat, giving M104 a distinctive nickname: the Sombrero. Because of that broad rim, astronomers have long classified the Sombrero as a spiral galaxy, similar to our own Milky Way. But a recent study says that’s not quite right — the galaxy is really a sort of hybrid — a cross between a spiral and another type of galaxy known as an elliptical. The study was headed by Dimitri Gadotti of the European Southern Observatory. It showed that the bulk of the Sombrero closely resembles an elliptical — a shape like a fat, fuzzy football. Such galaxies consist mainly of old, faint stars. They give birth to almost no new stars because they don’t have much gas and dust — the raw materials for making stars. And the Sombrero is surrounded by about 2,000 clusters of especially old stars — a number usually associated with ellipticals, not spirals. Yet the wide brim that gives the Sombrero its name has the look of a spiral — a broad, flat disk with lots of gas and dust to spawn new stars. The study suggests that the galaxy is basically a big elliptical. Several billion years ago, though, it pulled in vast amounts of gas and dust. That material encircled the galaxy’s center, forming a wide, thin disk. That would make the Sombrero two galaxies in one — a vigorous young spiral sitting inside an old, fat elliptical. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Two bright planets are passing through the constellation Virgo this summer. Orange Mars is in the west-southwest at nightfall, with golden Saturn well to its upper left, near Spica, Virgo’s brightest star. Virgo is best known not for its stars, but for its galaxies. The Virgo cluster, which contains thousands of galaxies, spans much of the constellation. The most photogenic of them all is M104, the Sombrero galaxy. Through a small telescope, it looks like a tall “crown” of stars surrounded by a broad, thin disk. A dark lane of dust in the disk makes it look like the brim of a sombrero. American astronomer Vesto Slipher made an important discovery about the galaxy 100 years ago. Working at Lowell Observatory in Arizona, he was studying many of the “nebulae” that sprinkle the universe. At the time, most astronomers thought they were motes of matter inside the Milky Way galaxy, which was thought to be the full extent of the universe. Slipher found that many of these objects were moving away from Earth at high speed — so fast that there’s no way they could have been contained inside the Milky Way. The Sombrero was the fastest of all. The finding suggested that the nebulae were really separate galaxies of stars far outside the Milky Way. But it took a couple of more decades for most astronomers to accept that finding. The Sombrero, by the way, is about 32 million light-years away. We’ll have more about the Sombrero tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

As Earth moves around the Sun, our star slowly treks through the background of distant stars — a journey that takes precisely one solar year to complete. The path the Sun follows for this journey is the ecliptic. And early this evening, that path is staked out by an array of bright objects that show us where the Sun will travel over the next few months. The first marker is low in the west shortly after sunset, before the sky gets dark: the planet Mercury. It looks like a fairly bright star, but it’s so low in the sky that it’s tough to see. Those in the southern states will have a better view of the little planet than those in the north. Well to the upper left of Mercury, look for Regulus, the brightest star of Leo. The Sun will scoot past Regulus in late August, then spend several more weeks transiting the rest of the constellation. Even higher along the Mercury-to-Regulus line, look for the orange planet Mars. It’s much fainter than it was just a few months ago, but it’s still a pretty sight, and easy to pick out as well. Well to the left of Mars, and a little higher in the sky, are two objects that flank the ecliptic — the planet Saturn, and the star Spica below it. The ecliptic runs directly between them. Finally, arc down to the southeast for the gibbous Moon, which is passing through Ophiuchus, the serpent bearer. Because the constellation lies along the ecliptic, the Sun will pass through its borders as well — in early December. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

To truly understand a star, you have to know how far away it is. Yet measuring astronomical distances is tough — even for stars that are fairly close. As an example, consider Antares, the brightest star of Scorpius, which is to the lower left of the Moon this evening. Antares shines reddish orange. Stars of that color are either really big and bright, or small and faint — too faint to see with the eye alone. So the fact that it’s visible at all means it’s a giant. But the type of giant depends on its distance. If it’s within a hundred light-years or so, then it would be a normal giant — the final stage of life for a star like the Sun. But if it’s much farther than that, then it’s a supergiant. Astronomers measure a star’s distance by plotting its position relative to other stars when Earth is on opposite sides of the Sun. Viewed from these different angles, the star moves back and forth a tiny bit — like holding out your finger and looking at it with first one eye, then the other. The size of the shift reveals the star’s distance. For Antares, the best measurement was made by an orbiting satellite. But it took years for astronomers to analyze and then re-analyze the data. Based on the latest analysis, the distance to Antares is about 550 light-years — one of the most-distant stars that’s visible to the eye alone. So Antares is not just a giant, but a supergiant — one of the biggest and brightest stars in the entire galaxy. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Moon just slips past the claws of the scorpion tonight. They’re represented by two fairly bright stars that are above the Moon at nightfall. Today, the stars are part of Libra, the balance scales. The name dates to a time when the Sun appeared against those stars at the autumnal equinox, when day and night are of equal length — a time of “balance” in the heavens. Yet the stars bear names that pre-date Libra by thousands of years: Zubenelgenubi and Zubeneschamali — the southern and northern claws. They were originally part of Scorpius, the scorpion, which is next door to Libra. Zubenelgenubi stands a little to the upper right of the Moon this evening. Look carefully and you’ll see two stars, not one. The stars probably are bound together by their mutual gravitational pull, even though they’re a long way apart — about 200 times the distance from the Sun to the solar system’s most distant planet. Zubeneschamali is farther to the upper left of the Moon. It shows only one star, but it’s an impressive one. It’s bigger and brighter than the Sun, and thousands of degrees hotter. Such hot stars shine white or blue-white. But many skywatchers say the star looks green. There’s no way for a star to shine that color, though, so it’s probably just an optical illusion — a trick played by the stars. The Moon will slide down close to the heart of the modern-day scorpion tomorrow night — and we’ll have 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.

Shooting through space on planet Earth is the astronomical equivalent of a carnival ride. Our planet zips around the Sun at about 67,000 miles an hour. And the entire solar system zips around the center of the galaxy at close to a half-million miles an hour. But if Earth is the equivalent of a carnival ride, then some other planets in the galaxy may be the equivalent of NASCAR, racing through space at breakneck speed. Astronomers from Dartmouth and Harvard recently calculated what might happen to planets in binary systems that passed close to the supermassive black hole at the center of the galaxy. And none of the results were good news for inhabitants of such planets. At the right distance, the binary system would be split apart. One of the star systems could be ingested by the black hole or settle into orbit around it, with the other kicked away from the black hole. Even for the survivor, though, the outlook is bleak. The planet could be hurled into its star or ripped apart by the black hole’s gravity. The planet also could be stripped from the star and sent hurtling into space on its own. And even in the best-case scenario, the star and planet would stay together, but they’d be slung away at tens of millions of miles per hour — a few percent of the speed of light. That snap acceleration would be highly unpleasant for anything that had managed to survive in the deadly environment around a black hole. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

When you switch on a lamp, there’s no lag between the “click” and the light hitting your eyes. Or at least it seems that way. There’s really a lag of a few billionths of a second — an interval that’s far too short for human senses to perceive. But for some light sources, the lag is a lot longer. In fact, for three bright objects that huddle close together in this evening’s sky, the lag is measured in seconds, minutes, and years. The Moon, the star Spica, and the planet Saturn are well up in the sky at nightfall. Spica is close to the upper left of the Moon, with Saturn above Spica. We see these objects not as they look now, but as they looked in the past, because light doesn’t travel across space instantaneously. Light moves through the vacuum of space at 186,000 miles every second — nothing in the universe is faster. Yet the universe is so vast that even at that pace, it takes a while for light to get around. The Moon, for example, is only about a quarter-million miles away, so its light takes a bit more than a second to reach us. In other words, when you look at the Moon, you’re seeing it as it looked about a second ago. Saturn is a good bit farther, so its light takes about 78 minutes to reach Earth. And Spica is farther still. Its light takes 250 years to reach us. So as you look at the star tonight, you’re actually seeing it as it looked in the year 1762 — at the start of the reign of Russia’s Catherine the Great. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The Moon, two stars, and two planets all look a bit chummy tonight. As night falls, Mars stands to the upper right of the Moon, and looks like a fairly bright orange star. The planet Saturn and the star Spica are a little farther to the upper left of the Moon, with Saturn higher in the sky. The other star is pretty faint, but under a fairly dark sky it’s easy to pick out because it’s especially chummy with Mars. Tonight, it’s separated from the planet by a bit more than the width of the Moon, and it’ll be even closer tomorrow night. The star is Beta Virginis, one of the brighter stars of Virgo. Spica is Virgo’s brightest star, so there’s quite a drop-off between it and the constellation’s other leading lights. Beta Virginis is also called Zavijava, from an ancient Arabic name that meant “the turn.” It referred to a bend in a string of stars. The name originally applied to a different star, but somehow migrated to Beta Virginis. The star is about 36 light-years away, and it’s quite similar to the Sun. It’s a bit bigger and heavier, which makes the star about three times brighter than the Sun. It’s roughly the same age as the Sun, which means it’s in the same stage of life — it’s steadily “burning” the hydrogen in its core to make helium. Because it’s a bit more massive than the Sun, though, Beta Virginis will use up its hydrogen more quickly — ending its life at a much younger age than the Sun will. More about the Moon and its companions tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

[SFX: dust devil] On the surface of Mars, this might be a common sound — the sound of a dust devil whirling past. These twisting columns of air and dust are common on the desert planet. Heated by the Sun, a column of air rises and spins. It picks up sand and carries it into the sky. An orbiting spacecraft recently photographed a dust devil that was 12 miles high — many times taller than the ones here on Earth. No one has actually heard a Martian dust devil. But a team from the University of Southampton in England recently created some Mars sounds. Team members used knowledge of the Martian atmosphere and of the way sound moves to simulate what you’d hear on the Red Planet. Mars’s atmosphere is less than one percent as dense as Earth’s. It’s much colder than Earth’s, and it’s made of a different mixture of gases. In that environment, sounds would be thinner and appear more distant than those here on Earth. Here, for example, is a comparison of a clap of thunder, first on Earth, then Mars: [SFX: thunder] And here’s what a voice would sound like — again, first as it sounds on Earth, then Mars: [SFX: voice] We won’t hear actual sounds from Mars for a long time. But we can appreciate the view of the planet. Tonight, it’s to the upper left of the Moon as darkness falls, and it sets by around 1 a.m. Mars looks like a bright orange star. More about the Moon, Mars, and some other companions tomorrow. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Most constellations bear little resemblance to their namesakes. You need a great imagination to see a king in the stars of Cepheus, for example, or a “sea goat” in Capricornus. One constellation that does resemble its namesake is the dolphin, Delphinus. It’s a small grouping of stars that’s low in the east at nightfall, and swims high across the south later on. And that little group of stars really does look like a dolphin jumping through the edge of the starry Milky Way. In Greek mythology, Delphinus carried a poet to safety when he jumped overboard to escape some nasty sailors. The poet and the dolphin were commemorated in several coins that were issued around 500 B.C. And the dolphin gained longer-lasting recognition through its own constellation. None of the individual stars of Delphinus is particularly interesting. The brightest is Beta Delphini. It’s near the center of the constellation, and marks the end of the dolphin’s torso. The dolphin’s body stretches to the left as it climbs higher in the sky, and its tail to the right. Beta Delphini is actually a binary. The two stars were born from the same cloud of gas and dust, and they remain bound to each other by their mutual gravitational pull. They orbit each other once every 27 years. Look for the beautiful dolphin swimming into view in the east as darkness falls, and climbing high across the south during the night. Tomorrow: listening in on the Red Planet. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

Modern America isn’t the first culture to expect a lot of flash from its stars. The astronomers of ages past, for example, linked the brightest stars in the night sky to make pictures. They depicted beasts, such as the lion; people, such as the strongman and the hunter; half-beast-half-people, such as the centaur; and even beastly people, such as Cassiopeia. But the astronomers neglected the faint stars between these patterns. In fact, many stars weren’t given a constellation home until just a few centuries ago. An example is the stars of Leo Minor, the little lion. It’s in the west this evening, between Leo, the big lion, and the Big Dipper. It’s quite faint, though, so you need dark skies to find it. The constellation was created 325 years ago by Johannes Hevelius. He included 18 stars in this new bit of celestial geography, but he didn’t give them names. Those were added later by other astronomers. But the list has a glaring omission. In most constellations, the brightest star is designated with the Greek letter “alpha.” But Leo Minor has no alpha star at all. In fact, only one star has any Greek designation — Beta Leonis Minoris. And it’s not even the constellation’s brightest star. That honor goes to 46 Leonis Minoris, which isn’t much to look at. It should have gotten the “alpha” designation, but the first person to label the stars simply forgot. Then, as now, it seems a star had to have some flash to get anyone to notice it. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

For a good part of late winter and early spring, the planets Venus and Jupiter staged a spectacular encounter in the evening sky. And now they’re ready for an encore — this time in the morning sky. You can’t miss these two worlds because they’re the brightest objects in the night sky other than the Moon, with Venus clearly outranking Jupiter. As twilight paints the early dawn sky tomorrow, Venus is quite low in the eastern sky, with Jupiter a few degrees to its upper right. Over the next few weeks, both will climb into view earlier, providing more time to enjoy the pairing. The two planets will also move just slightly closer to each other. Venus won’t be able to overtake its fainter sibling, though, because of their positions in the solar system. Venus is the second-closest planet to the Sun, so from our perspective it has a limited range away from the Sun. As Venus moves into the dawn sky, it scoots away from the Sun in a hurry. Within weeks, though, its pace slows as our viewing angle to the planet begins to change. Eventually, Venus appears to stop and then reverse direction, moving back toward the Sun. Jupiter, on the other hand, is outside Earth’s orbit, so it loops all the way across the sky. So as Venus starts to slow down, Jupiter will leave it behind. That won’t happen for a few weeks, though, so there’s plenty of time to enjoy the spectacular pairing of the two brightest points of light in the night sky. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.

No probe has ever drilled into the core of Mercury. In fact, no probe has ever even landed on the planet. Even so, scientists have looked all the way to Mercury’s center. The view is still a bit blurry, but it’s revealed the likely layout of the solar system’s smallest planet. Scientists have pieced together the picture of Mercury’s interior from several sources. They’ve looked at small “wobbles” in the planet’s rotation recorded by radio telescopes. And they’ve measured the planet’s gravitational and magnetic fields with two spacecraft — Mariner 10 in the 1970s, and Messenger, which is orbiting Mercury now. These readings show that Mercury’s core accounts for about 85 percent of the planet’s diameter — almost twice the size of Earth’s core. It probably consists of three layers. The outer layer is solid, and is made of iron and compounds of sulfur. The middle layer is probably molten iron and nickel. And the inner layer is probably solid iron and nickel. As the planet rotates, the layers spin at different rates, generating a weak magnetic field around the planet — and providing a peek into Mercury’s heavy heart. And Mercury is peeking into view in the early evening. It’s quite low in the west-northwest as the sky begins to darken. It looks like a bright star, but it’s so low that it’s tough to find. Luckily, there’s a bright signpost to point the way tonight: the crescent Moon. Mercury is a little to the Moon’s upper right. 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 summer solstice is just another day on the calendar. Although it’s the longest day of the year, we generally don’t plan anything special to commemorate it. That wasn’t the case in centuries past, though. In many cultures, the summer solstice was one of the most important days of the year — a time to come together in celebration. These cultures often set up Sun-watching stations to let them know when the solstice was at hand. A site in Egypt, for example, dates to before the time of the pharaohs. And one in northern New Mexico required the Sun-watcher to climb to a small niche in a mountaintop. Another Sun-watching site may have been discovered in England. It’s not far from the city of Manchester, and it’s part of a complex known as Gardom’s Edge. The site was inhabited as early as 4,000 years ago. It consists of cultivated fields, a few houses, and an enclosure where clans may have come together. It also includes a seven-foot triangular rock pillar that may have been aligned to the solstice. The pillar appears to have been carefully prepared and aligned. It’s angled so that the northern side receives full sunlight only around the solstice. Researchers say that may indicate the marker was erected to give the gathering place extra significance — a symbol of the long, prosperous days of summer. And this summer begins today, at 6:09 p.m. Central Time — just another day on the modern calendar. Script by Damond Benningfield, Copyright 2012 For more skywatching tips, astronomy news, and much more, read StarDate magazine.