StarDate

The just-past-full Moon has an almost-full companion tonight: the planet Saturn. They’re low in the southeast at nightfall, with Saturn close to the right or lower right of the Moon. The solar system’s second-largest planet looks like a bright star, so it’s hard to miss.

The Moon is full when it lines up opposite the Sun in our sky. At that angle, the Sun illuminates the entire lunar hemisphere that faces Earth — creating a full Moon.

Since the Moon is on the opposite side of Earth from the Sun, that configuration is known as opposition. And the planets experience opposition, too. In fact, Saturn will be at opposition next week. So like the full Moon, it’ll rise around sunset, remain in the sky all night, and shine at its brightest.

Unlike the Moon, though, Saturn doesn’t go through a cycle of phases. It always stays outside Earth’s orbit around the Sun, so we never see a “new” Saturn, or a crescent one. Instead, it’s always almost fully illuminated as seen from Earth, so when it’s not full, it’s almost full.

Saturn shines brightest at opposition for a couple of reasons. For one thing, it’s closest to Earth for the year at opposition — about 800 million miles. And for another, it’s a more effective mirror at opposition, so more of the sunlight that strikes its cloudtops is reflected in our direction.

So keep an eye on Saturn over the next few nights, as it puts in its best appearance of the year. We’ll have more about Saturn next week.

Script by Damond Benningfield

A bright triangle highlights the southern sky tonight: the Moon, the star Antares, and the planet Saturn. As night falls, Antares stands to the lower right of the Moon, with brighter Saturn about the same distance to the lower left of the Moon.

Antares is one of the most impressive stars in the galaxy. For that and other reasons, it’s unlikely that it has any planets of its own.

One of those reasons is the star’s age — only about 12 million years. Most models say that it takes almost that long to make planets.

And when Antares was born, it probably was much hotter than it is today. It would have produced an extraordinary amount of radiation, especially at ultraviolet wavelengths. That energy would have vaporized most of the planet-building materials close to Antares and blown them out into interstellar space.

Even if planets did form, they probably would have been incinerated in the last million years or so. That’s because Antares has puffed up to gigantic proportions — almost a thousand times the diameter of the Sun. So anything that formed close in would have been engulfed by the growing star, and anything farther out would have been vaporized as the surface of Antares grew closer.

And if, by some miracle, planets formed and survived, they’re doomed anyway. Sometime in the next hundred thousand years or so, Antares will explode as a supernova — an event that’s bad news for nearby planets.

More about the Moon and Saturn tomorrow.

Script by Damond Benningfield

Astronomers learn about the universe by studying not just what’s there, but what’s not there as well. A case in point involves a space telescope that was launched 25 years ago today.

EUVE — the Extreme Ultraviolet Explorer — cataloged and studied more than 1200 hot stars and other objects. But the fact that it could see this many targets at all told astronomers a great deal about the material between the stars.

When EUVE was launched, astronomers weren’t sure just how much it would see. Extreme ultraviolet energy is produced by some of the hottest objects in the universe, such as the stellar corpses known as white dwarfs, and the disks of hot gas around supermassive black holes. But gas and dust between the stars absorbs these wavelengths. So some thought that EUVE might see only a few dozen stars through the interstellar “fog.”

Instead, the fog was pretty thin. And it wasn’t evenly distributed — the craft found big bubbles and gaps in the material between the stars, giving the galaxy the texture of Swiss cheese.

That allowed EUVE to study halos of hot gas around hundreds of stars; to watch powerful outbursts of energy from some of the smallest stars; and to map a “doughnut” of hot gas along the orbit of Io, one of the moons of Jupiter.

EUVE was shut down in January of 2001. It burned up when it plunged into the atmosphere a year later — ending a mission that told us a lot about what isn’t between the stars.

Script by Damond Benningfield

Scorpius is one the most prominent constellations around. Antares, its leading light, is one of the brightest and reddest stars in the night sky. And Scorpius has many other fairly bright stars, which form the easy-to-see outline of a scorpion.

If we could see Scorpius at ultraviolet wavelengths, though, it would look even brighter. That’s because the constellation is home to many stars that are young and hot — tens of thousands of degrees hotter than the Sun.

Such stars are extremely bright at visible wavelengths. But because they’re so hot, they produce much more light in the ultraviolet.

The stars are so hot because they’re much heavier than the Sun. Their gravity squeezes their cores tightly, revving up their nuclear reactions. Radiation from the hot cores heats the stars’ outer layers, making them shine blue-white.

One of the few bright stars in Scorpius that wouldn’t look brighter in the ultraviolet is Antares, which is far to the lower left of the Moon tonight, and closer below the Moon tomorrow night.

Antares consists of two stars. The heavier of the two has undergone big changes in its core. That’s caused the star to puff up, cooling its outer layers. So it produces most of its energy in the infrared.

Antares wouldn’t disappear in the ultraviolet, though, because the smaller companion is much hotter. So like many of the bright stars around Antares, it glows brightly in the ultraviolet.

More about ultraviolet astronomy tomorrow.

Script by Damond Benningfield

When we look at the night sky, it’s like being stuck watching just one TV channel or visiting just one web page. There’s a lot more to see, but our eyes can’t tune to the other channels.

One of those missing channels is the UV channel. Ultraviolet energy is produced by some of the hottest and brightest objects in the universe. That includes massive young stars, which are tens of thousands of degrees hotter than the Sun. They can emit dozens of times more ultraviolet light than visible light; more about that tomorrow.

Other UV targets include the remnants of exploded stars, and the disks of superhot gas around black holes. Studying these objects in the ultraviolet reveals details about their temperature, chemical composition, and other characteristics that we can’t see in other wavelengths.

Fortunately, we can build instruments that allow us to see the ultraviolet glow of these objects. But the ozone in Earth’s upper atmosphere blocks most ultraviolet wavelengths, so they never reach the ground. To see the ultraviolet, then, the instruments have to be launched into space.

Over the decades, more than a dozen UV telescopes have been launched. They’ve detected and studied thousands of objects. Their observations have helped astronomers find and map the birthplaces of new stars, peel back the layers of exploding stars, and study how hot gas funnels into black holes — switching on an exciting new channel in our study of the universe.

Script by Damond Benningfield

Spica, the leading light of Virgo, is one of the brighter stars in the night sky. It’s also one of the more thoroughly studied. The file compiled by astronomers shows that it consists of two stars that are only a few million miles apart. They’re so close, in fact, that their gravity distorts both stars, making them look a bit like eggs. And both stars are bigger, heavier, and brighter than the Sun.

Astronomers amassed much of that file with spectroscopy, a technique that breaks Spica’s light into its individual wavelengths, creating a rainbow of colors. Each element in the stellar atmospheres imprints patterns of dark lines on the rainbow, revealing the stars’ compositions.

Since there are two stars in the system, there are two sets of lines. In fact, that’s how astronomers discovered the system’s binary nature. As the stars orbit each other, the sets of lines shift back and forth. The size of the shift reveals the stars’ orbital speed, while the timing of the shift reveals their orbital period — about four days.

The spectrum also reveals the temperatures of the two stars, as well as how fast each of them is turning on its axis. And it shows that the heavier star is pulsing in and out like a beating heart — one more entry in the file on this bright system.

Spica stands close to the lower right of the Moon as night falls this evening. The brilliant planet Jupiter is to their upper right, completing a beautiful triangle in the night sky.

Script by Damond Benningfield

The best-known storm in the solar system is Jupiter’s Great Red Spot. It’s big enough to swallow Earth, and it may have been whirling for four centuries.

But many other big storms also spin through Jupiter’s atmosphere. There’s a “string of pearls” to the south of the red spot — eight white ovals that are thousands of miles across. And another storm inhabits the latitudes between the red spot and the white ovals. It started as a set of white ovals, but they merged to form another red spot.

The string of pearls was first observed in 1986. The number of pearls has varied over time — from as few as six to as many as nine. They’re spun up by powerful jet streams to their north and south that blow in opposite directions.

Three white ovals were born to the north of the pearls in 1939. They lasted for decades, separated by smaller storms that spun in the opposite direction. But two of the ovals merged in 1998, and the third joined them in 2000.

Just five years later, the storm turned red — as red as the Great Red Spot. It might have dredged up compounds from deep within Jupiter that are red, or that turn red when exposed to sunlight. The storm continues to blow today — keeping things stirred up in Jupiter’s turbulent atmosphere.

And Jupiter is in great view tonight. The planet is just a couple of degrees from the Moon. For the hours they’re in view, nothing outshines Jupiter except the Moon.

More about the Moon and another companion tomorrow.

Script by Damond Benningfield

The lovely planet Venus has been sneaking farther from the Sun in the early morning sky over the last few months. And tomorrow, it’s at its farthest. At this time of year, though, its path across the sky tilts low above the horizon, so the planet doesn’t climb very high. Yet Venus is still easy to pick out because it’s the brilliant “morning star” — the brightest object in the night sky other than the Moon.

Venus orbits closer to the Sun than Earth does, so it never moves far from the Sun as seen from Earth. As a result, it’s visible for only a short time before sunrise or after sunset. Right now, despite its apparent distance from the Sun, it rises only a couple of hours before the Sun does.

Like the solar system’s other major planets, Venus stays close to the ecliptic, which is the Sun’s path across the sky. Over the course of a year, the angle of the ecliptic at any given time of day changes. At this time of year, the ecliptic is quite low in the early morning sky. So as Venus rises, it doesn’t climb very high. Instead, it hugs the horizon, scooting more to the right than up. So the planet is still quite low in the sky at first light.

The angle varies a bit depending on your location. So skywatchers in the southern United States will see Venus climb a bit higher than those at more northerly latitudes.

No matter where you are, though, beautiful Venus is well worth a look — the dazzling morning star shining in the east at dawn.

Script by Damond Benningfield

As you look out into the night sky, you might think that you’re seeing a representative sampling of the stars — in other words, a little of everything. That’s not the case, though. In fact, most of what you see is the Milky Way’s equivalent of the one percenters — stars that are among the galaxy’s brightest.

An example is in the constellation Centaurus, which wheels quite low across the south on June nights. It’s so low, in fact, that much of it stays below the horizon for those of us in the United States. That includes its brightest star, Alpha Centauri.

The brightest star most of us can see in Centaurus is Menkent, a name that means “shoulder of the centaur.” It’s low in the south at nightfall.

Like many of the other stars that are visible to the unaided eye, Menkent looks so bright because it’s nearing the end of its life. It converted the original hydrogen fuel in its core to helium. That made the core get smaller and hotter, which in turn made the star’s outer layers puff up. Now, Menkent is more than 10 times the Sun’s diameter, and about 60 times the Sun’s brightness.

Dying stars like Menkent are rare — most of the galaxy’s stars are still in the prime of life, so they’re not as bright as they’ll become later on. But because rare stars like Menkent are so big and bright, they’re greatly over-represented in the stars we see — stellar giants shining brightly in the night sky.

Tomorrow: a nearby planet that shines brightly in the morning sky.

Script by Damond Benningfield

Mighty Hercules stands high in the east as night falls at this time of year. His most prominent feature is the Keystone, a lopsided square of modest stars that represents the strongman’s body. But the constellation’s brightest star isn’t part of the Keystone. Instead, it represents the entire strongman: Its name, Kornephoros, comes from a Greek word that means “the club bearer” — Hercules himself.

Like most of the stars in the galaxy, there’s more to Kornephoros than meets the eye: It consists of two stars, not one. One star is smaller and fainter than the Sun, so it’s not visible to the eye alone.

The visible star, on the other hand, is about three times the mass of the Sun, almost 20 times the Sun’s diameter, and 150 times its brightness. So the star is an easy target even though it’s about 140 light-years away.

The star is nearing the end of its life. It’s probably consumed the hydrogen fuel in its core, converting it to helium. That’s caused the core to get smaller and hotter. The extra energy pushes on the star’s outer layers, causing them to puff up to giant proportions.

Today, Kornephoros is fusing the helium to make carbon and oxygen. Eventually, though, that process will end. The star will lose its outer layers, leaving only its dead core — and the “club bearer” will vanish from sight.

For now, though, look for it due east as night falls, halfway up the sky — the first modestly bright star to the right of the Keystone.

Script by Damond Benningfield