StarDate

Jupiter is a solar-system vacuum cleaner. The giant planet hoovers up comets and asteroids like dust bunnies. They don’t disappear quietly, though. Instead, they can create fireballs that are bright enough to see from Earth.

Two amateur astronomers caught the most recent of these impacts back in March. They saw a short, bright burst of light from the planet’s edge. It probably was triggered by an asteroid the size of a small office building slamming into Jupiter’s atmosphere. It was moving so fast that it created an explosion equal to many hydrogen bombs.

The leader of a world-wide network of amateurs recently reported that such impacts probably happen every couple of months, on average. That estimate was based on what the amateurs have seen, and what they haven’t seen.

The best-known impact came in July of 1994, when Jupiter was hit by the shattered remains of Comet Shoemaker-Levy 9. The string of impacts left dark scars in Jupiter’s atmosphere for months.

Jupiter gets hit so often in part because it’s big — 11 times Earth’s diameter. The key factor, though, is that its gravity is much stronger than Earth’s, so it pulls in comets and asteroids from a greater range. And when they hit, they’re moving much faster than objects that hit Earth, so they create a much bigger bang.

Look for Jupiter early this evening. It looks like a bright star just above the Moon. They set not long after nightfall, so there’s not much time to enjoy the view.
 

Script by Damond Benningfield

One of the most famous paintings of the Renaissance is “The Birth of Venus” by Sandro Botticelli. It depicts the naked goddess standing atop a shell as she rises from the sea.

The planet Venus is rising right now, and it’s also a thing of beauty. The planet is the brightest object in the night sky after the Moon, so it’s long been associated with the goddess of love and beauty.

Venus has been out of sight for months, hidden in the glare as it passed behind the Sun. Now, though, it’s beginning to climb away from the Sun, and into view in the evening sky. But it’s a gradual process. The planet won’t be in good view for several more weeks.

This evening, for example, it’s just a few degrees above the western horizon about 20 or 30 minutes after sunset, when there’s still a lot of light in the sky. Venus is so bright, though, that if you have a clear horizon, you might still be able to pick it out.

And a couple of other bright objects help point the way. The crescent Moon stands to the upper left of Venus, also quite low in the sky. If you have binoculars, look close to the Moon for the planet Mercury, which looks like a moderately bright star.

And the planet Jupiter is farther to the upper left of the Moon. It also looks like a brilliant star, so you can’t miss it. And before long, you won’t be able to miss Venus, either — an astronomical beauty rising into the starry night sky.

We’ll have more about the Moon and Jupiter tomorrow.
 

Script by Damond Benningfield

More than three billion years ago, two tsunamis may have raced across a vast ocean. One lifted giant boulders and carved channels, while the other deposited big blocks of ice. Traces of these powerful events still exist — outlining a possible ocean on the planet Mars.

There’s been a lot of discussion about a Martian ocean in recent years. The elevation of most of the planet’s northern hemisphere, for example, is much lower than the southern hemisphere, suggesting that the north was once a water-filled basin. Networks of dry riverbeds seem to flow toward that basin. And studies of the water on modern Mars suggest there was much more of it on the planet in the distant past.

More support for a global ocean came from a study released a few months ago. Researchers found evidence of two ancient tsunamis. One carried boulders as big as houses far inland, and gouged deep gorges as the water retreated. And the other, which came a few million years later, after the ocean had gotten smaller and colder, dropped slabs of ice inside canyons.

The researchers say that each tsunami was created when an asteroid slammed into the ocean. The impacts generated waves that were hundreds of feet high and raced hundreds of miles inland — leaving more evidence that an ocean once covered the northern half of Mars.

Look for Mars low in the southern sky this evening, blazing like an orange star. The planet Saturn and the star Antares are off to its left.

Script by Damond Benningfield

The Sun is pretty big as stars go — bigger than perhaps 80 or 90 percent of the stars in our entire galaxy. To get an idea of just how big, consider this: At the speed of a jet airliner, it would take about seven months to circle all the way around the Sun’s equator.

But if you really want to rack up the frequent-flier miles, try circling around the star at the tail of Cygnus, the swan.

Deneb is classified as a white supergiant. The “white” designation means that its surface is thousands of degrees hotter than the Sun’s. And the “supergiant” designation means that it’s one of the biggest stars in the galaxy. In fact, some recent measurements say it could be 200 times the diameter of the Sun. So even at the quick pace of a typical airliner, it would take more than a century to circle Deneb’s equator.

One problem you might have is deciding just where the star’s “surface” is. Supergiants are so puffed up that their outermost layers of gas are quite thin. And Deneb is blowing a strong “wind” of material out into space, which makes it even harder to tell where the star ends and space begins.

But from our distance of perhaps 2500 light-years or so, that’s not a problem — Deneb looks like a sharp little point of light. It’s high in the east-northeast at nightfall, at the left point of the bright Summer Triangle. The rest of the swan stretches to its right, with the body parallel to the horizon and the wings spread above and below.

Script by Damond Benningfield

The modern calendar owes its existence to the most famous of all Roman leaders — Julius Caesar. In 46 B.C., he ordered the creation of a new, highly organized calendar to replace the messy system that was in use at the time. With a few minor corrections, we still use that same calendar today.

One correction is the names of the months of July and August. Under the old calendar, the year began with March. The two hot summer months were called Quintilis and Sextilis — names that mean the fifth and sixth months. But soon after the new calendar was introduced, the Roman Senate honored its benefactor by changing the name of Quintilis to July.

Then, in the year 8 B.C., the emperor Augustus Caesar decided that he should have his own month, so Sextilis was changed to August.

Scholars disagree on whether other changes were made when August was named. Some say that Sextilis originally contained only 30 days, but Augustus added a day so that his month would be as long as that of Julius Caesar.

Incidentally, August 1st is the date of an ancient English festival called Lammas. It’s a “cross-quarter” day — a day that’s roughly half-way between a solstice and an equinox. In some cultures, the cross-quarter days marked the beginnings of the seasons, not their mid-points as they do today. Lammas was a day for celebrating the first harvest — marking an important date on the summer calendar.

Tomorrow: Circling around a giant star.

Script by Damond Benningfield

The Sun is passing through the middle of Cancer right now. We can’t see the constellation’s stars through the sunlight, of course, but we can see where the Sun is headed over the next few months. All you need to do is connect a few dots in the evening sky.

All of these dots lie near the ecliptic, which is the Sun’s path across the sky. The dots outline where the Sun will appear along that path into late fall.

Before the last blush of twilight drains from the evening sky, look due west for the brightest of all the dots — the planet Jupiter. It’s quite low in the sky, so you need a clear horizon to spot it. It’s so bright, though, that it’s easy to pick out.

Next, look well to the left of Jupiter, and a little higher in the sky, for Spica, the leading light of Virgo. The Sun will move into Virgo in mid-September, and pass closest to Spica in mid-October.

After that, it’ll move through the adjoining constellation Libra, then into Scorpius around Thanksgiving. And that’s where you can find three more bright dots tonight: the scorpion’s brightest star, Antares, and the planets Mars and Saturn. Mars is the brightest of the three, with Antares to its lower left, and Saturn a bit farther to the upper left. The ecliptic passes just below Saturn, and above Mars and Antares.

And by Christmas, the Sun will be standing near the “teapot” of stars that outlines Sagittarius. They’ll be invisible then, so look for them now instead, passing low across the southern sky.

Script by Damond Benningfield

A break-up can be tough — especially if you’re an asteroid. A collision can break a big asteroid into thousands of little ones.

One of the likely offspring of such a collision was discovered 100 years ago today. It gained fame 75 years later as the first asteroid ever to be studied from close range.

Russian astronomer Grigory Neujmin discovered the asteroid in 1916. He named it Gaspra for a resort in Crimea.

The asteroid orbits near the inner edge of the asteroid belt — a band of rocky debris between the orbits of Mars and Jupiter. Because of its orbit, astronomers classified it as a member of a family of asteroids named for its largest member, Flora. A large impact may have shattered Flora, sending chunks of rock and ice flying off on their own.

Gaspra is so small and far away, though, that not much else was known about it until 1991, when the Galileo spacecraft flew just a thousand miles from it on the way to Jupiter.

Galileo found that the asteroid is shaped a bit like a spearhead, and it’s about 12 miles long. The craft’s pictures revealed lots of small impact craters on Gaspra, but not many big ones. That suggests that the surface is no more than a few hundred million years old — supporting the idea that it’s a chip off a larger body.

That scenario is also supported by long, wide grooves on the surface — possible fractures caused by the impact that may have broken up a big asteroid and created a whole family of little ones.

Script by Damond Benningfield

The glowing band of the Milky Way arches high across the sky on these mid-summer nights. At nightfall, it stretches from almost due north, high across the east, to almost due south. And it stands high overhead by midnight. But you need to get away from the glow of city lights to see it.

The Milky Way is the combined glow of millions of stars in the disk of our home galaxy. The galaxy’s busy core is in the teapot-shaped constellation Sagittarius, which is low in the southern sky at nightfall. We can’t see anything in the core because it’s hidden behind clouds of dust, which form dark lanes within the Milky Way.

Astronomers use special instruments to peer through this dust. These instruments reveal some remarkable sights, including clusters of some of the hottest, brightest stars in the entire galaxy.

Two of these clusters are known as the Arches and the Quintuplet. They probably formed just a few million years ago, when big clouds of gas and dust rammed together, triggering an intense bout of starbirth.

Many of the stars in these clusters are among the most massive in the galaxy. The heaviest is called the Pistol Star, and it’s probably more than a hundred times the mass of the Sun.

These massive stars are also extremely hot, so they burn through their nuclear fuel in a hurry. Within just a few million years, they’ll blast themselves to bits in titanic explosions. Even so, they’ll likely remain hidden from human eyes behind the cosmic haze.

Script by Damond Benningfield

The crescent Moon cozies up to the star Aldebaran before dawn tomorrow. In fact, for those in parts of the United States, the Moon will get especially cozy. It’ll “occult” the star, passing directly in front of it and blocking it from view.

Aldebaran is the bright orange eye of Taurus, the bull. It’s a big, puffy star that’s nearing the end of its life. That puffiness makes it shine hundreds of times brighter than the Sun, which is why it looks so prominent.

Aldebaran lies just a few degrees from the ecliptic — the Sun’s path across the sky. So the Sun passes quite close to Aldebaran each year, at the end of May.

The Moon’s path stays within a few degrees of the ecliptic as well, so the Moon passes close to Aldebaran about once a month. And this month, it gets especially close.

As seen from the United States, Aldebaran will be no more than a degree or two from the Moon as they rise around 2:30 or 3 tomorrow morning. And for a swath of the country from about Illinois down to Texas, Aldebaran will disappear behind the top portion of the Moon’s visible crescent. It’ll remain hidden for anywhere from a few minutes to about an hour, depending on the viewing location. It’ll then reappear from behind the dark portion of the lunar disk.

For the rest of the country, the Moon will just miss Aldebaran, or the occultation will take place after sunrise. Even so, the Moon and Aldebaran will still form a beautiful pairing in the pre-dawn sky.

Script by Damond Benningfield

There’s one star in the night sky that you should never need help to find. In fact, it can help find you. It’s Polaris, the North Star. It always stands due north — every night of the year. And it’s always at the same altitude in the sky — an altitude that tells you your latitude in the northern hemisphere. If you’re at 40 degrees, for example, Polaris is 40 degrees above the horizon.

Polaris won’t always be the North Star, though. In a couple of thousand years, that honor will go to a fainter star, in the constellation Cepheus, the king.

Gamma Cephei is about 45 light-years away. It’s a good bit bigger and brighter than the Sun, and about half-again as massive. It has a small, faint companion star, plus a giant planet.

Gamma Cephei will take over “North Star” honors because Earth wobbles on its axis like a spinning top. As it does so, the north pole aims at different stars. Right now, it aims at Polaris. But around the year 4100, it’ll aim at Gamma Cephei. And around the year 7500, it’ll aim at Alpha Cephei, the constellation’s brightest star. It’ll return to Polaris as Earth nears the end of one full wobble — in about 23,000 years.

Cepheus is to the right of Polaris as darkness falls. Its brightest stars form a pattern that looks like a child’s drawing of a house, with Gamma Cephei at the peak. At nightfall, the house is turned on its side, so Gamma Cephei is the first star you come to you as you head away from Polaris.

Script by Damond Benningfield