StarDate

A century ago, astronomers had cataloged a thousand asteroids — chunks of rock, metal, and ice that orbit the Sun. Today, they’ve cataloged more than a million. And the numbers are growing fast.

There are a couple of reasons for the bonanza. One is improved technology — modern instruments can detect objects that are smaller and fainter than ever before. And the other is that astronomers are putting more effort into looking for asteroids.

Most asteroids are found in a wide belt between Mars and Jupiter. They’re left over from the formation of the solar system. Jupiter’s gravity prevented them from coalescing to form a planet. The largest member of the belt, Ceres, is only about 600 miles across — roughly a quarter the size of the Moon.

As long as asteroids stay in the belt, they’re fine. Sometimes, though, an asteroid escapes the belt. It can move into our region of the solar system, where it could hit us, causing a global catastrophe.

And that’s why there’s so much effort to find asteroids. Plotting their orbits tells us whether they might threaten Earth. With enough lead time, we might be able to deflect a potential killer.

So astronomers are operating several automated searches. Telescopes photograph the skies every night, looking for objects that move against the background of stars. That reveals asteroids close to Earth. But it also reveals asteroids throughout the solar system — adding to the census of these cosmic rocks.
 

Script by Damond Benningfield

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Most of the dust in your home has an earthly origin: bits of dirt and pollen from the yard, skin cells from you and your pets, and even dust mites. But some of it probably comes from outer space — tiny bits of debris from comets and asteroids. In fact, a recent study says that almost 6,000 tons of that debris falls to Earth every year.

As comets orbit the Sun, they shed tiny grains of rock and dirt. And asteroids ram together, spraying similar grains into space. The particles spread out. And some of them enter Earth’s atmosphere and settle on the surface.

Estimates of how much reaches the surface vary by quite a bit. The numbers have been based on how much cosmic dust is found in sediments, how much is captured by high-altitude research aircraft, or how much is detected by spacecraft.

The recent study examined layers of ice in Antarctica. Over the last 20 years, researchers dug near Concordia Station, a European research base a few hundred miles from the south pole. They counted the number of cosmic dust grains found in different layers. When they projected the same rate across the entire planet, they came up with an annual fall of about 5700 tons.

They also compared the types of grains to models of how dust is scattered through the solar system. That told them that about 80 percent of the grains probably come from comets, with the rest from asteroids — extra sources of dust for your kitchen cabinets.
 

Script by Damond Benningfield

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Pegasus, the flying horse, is climbing across the night sky. It’s in the east as night falls. It’s preceded by another horse, which is to the upper right of Pegasus.

Equuleus is far smaller and less prominent than its equine cousin. In fact, of all the constellations passed down from the ancient world, it’s by far the smallest — you can cover it with your palm held at arm’s length.

Unlike most ancient constellations, there’s not much of a story associated with Equuleus. And as it’s drawn in the sky, it’s not even a full horse — only a head, which is outlined by a lopsided rectangle of four meager stars.

The brightest of the four is Alpha Equulei, which actually is a pair of stars locked in a tight orbit around each other. Both stars are about twice as massive as the Sun, with one slightly heavier than the other.

The difference in heft has made a big difference in the lives of the two stars. The heavier one has already ended its “normal” lifetime, and is entering one of its final stages. That’s caused the star to puff up like a balloon.

The smaller star is still in the prime of life. Yet as the bigger star puffs up, the surfaces of the two stars may get so close that the stars will swap some of their gases — altering the evolution of both stars.

Equuleus is in the southeast at nightfall. It’s below Delphinus, the dolphin — another small constellation, but one that’s much easier to pick out.
 

Script by Damond Benningfield

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Although the Moon looks bright, its surface is actually one of the darkest of any major body in the solar system. On average, it reflects only about a tenth of the sunlight that strikes it. That makes the Moon almost as dark as charcoal.

There is one prominent bright spot on the Moon, though — a crater named Aristarchus. Although it’s tiny as seen from Earth, it’s such a contrast to the surrounding landscape that it’s visible to the unaided eye. It’s near the left edge of the Moon as the Moon rises late this evening, above the bright star Aldebaran, the “eye” of the bull.

Aristarchus is about 25 miles across, and a couple of miles deep. A mountain peak rises from its floor like a skyscraper. The crater is surrounded by bright “rays” of debris that were blasted out by the impact that made it.

Observations by spacecraft in lunar orbit show that the crater floor contains a lot of minerals that are rich in titanium. They probably came from far below the surface, and were brought to the surface by the impact.

There’s some evidence that the area around Aristarchus is volcanically active. Observers have reported seeing flashes of light or odd glows in the area, including in the crater itself. And instruments have revealed traces of a radioactive gas. That could mean that gas sometimes erupts from below the surface — briefly adding to the luster of this bright lunar feature.

Tomorrow: drawing up a little horse.
 

Script by Damond Benningfield

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Fast radio bursts are among the most powerful objects in the universe. Many of them are hundreds of millions of light-years away. Despite their great power, though, there’s no consensus on what they are. But a whole passel of new discoveries may help.

Fast radio bursts were first seen in 2007. But they last only a tiny fraction of a second, which makes them hard to track down.

By earlier this year, astronomers had seen about 140 of them. But a few months ago, they got 535 more to study. The new ones were discovered by CHIME — a radio telescope in British Columbia that scans the entire northern sky.

Most of the objects in the new catalog have produced a single outburst. But a few have repeated themselves. Their outbursts last a little bit longer than the others, and they have a tighter range of frequencies. That suggests there are two classes of “bursting” objects.

The class that repeats may be produced by magnetars — the super-dense and ultra-magnetic corpses of once-mighty stars. They spin up to hundreds of times per second, beaming energy into space with each turn.

In fact, CHIME and other telescopes detected a radio outburst from a magnetar in our own galaxy. So repeating bursters could be powered by one mechanism, while those that are seen only once are powered by something different. Detailed study of all the new observations — as well as future outbursts — will help astronomers figure it out.
 

Script by Damond Benningfield

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The flow of the seasons has taken on a different feel over the past year and a half. Children spent months at home instead of in the classroom, so spring break and summer vacation felt no different from any other time. Adults didn’t leave home for days or weeks at a time, so conditions outdoors made little difference to the routine. And busy downtowns closed up, so they looked deserted regardless of the time of year.

We mention all of that because today is the first full day of autumn in the northern hemisphere. It’s the time of the equinox, as the Sun crosses the equator from north to south. Over the next three months, it’ll move even farther south, so we’ll see shorter, cooler days.

The equinoxes and solstices mark the starting points of the astronomical seasons. They’re convenient points for marking the seasons because they’re precise and easy to predict.

But there are many other ways to mark the changing seasons. The meteorological season, for example, began on September 1st. And in centuries past, some cultures used the equinoxes and solstices as the mid-points of the seasons, not the beginnings. These cultures picked various dates for these markers, so there was no common way to mark the seasons.

And in modern times, we’ve marked the seasons by school and work calendars, vacation schedules, and other everyday events. COVID scrambled those cycles — making it a bit harder to follow the rhythms of the seasons.

Tomorrow: radio outbursts by the hundreds.
 

Script by Damond Benningfield

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A thousand years ago, a Sun priest might have watched a hillside on the outskirts of present-day Tucson. In particular, he would have been looking for a dagger of sunlight to pass across a spiral carved into the rocks — an indication that autumn was arriving.

Researchers at LSU studied a site known as Picture Rocks. It contains hundreds of petroglyphs — pictures carved into the rocks. Most of them show people or animals. The pictures probably were created by the Hohokam culture, around a thousand years ago.

One of the glyphs is a spiral. A tall triangle of light, known as a Sun dagger, slices across it around the equinoxes, with another around the summer solstice. The daggers are formed by sunlight passing through notches carved in the rocks near the spiral. That indicates that the light show is intentional, and not a chance alignment.

The researchers say the alignments probably weren’t designed to mark specific dates. Instead, they marked important periods of the year. Priests would have watched a dagger cross the spiral, perhaps chanting prayers and making offerings.

Sun daggers are found all across the Southwest. Most of them probably served similar purposes — helping native cultures mark the changing seasons.

And the season is changing today. It’s the autumnal equinox. The Sun crosses the equator from north to south, beginning a season that will last until the solstice, in December.

More about the equinox tomorrow.
 

Script by Damond Benningfield

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Our home galaxy, the Milky Way, is like a leaky faucet. It gives birth to new stars a few drips at a time — the equivalent of no more than a few Suns per year. By contrast, a galaxy seen when the universe was young is like a fire hose: It’s giving birth to thousands of stars a year.

SPT0346-52 is so far away that we see it when the universe was just a billion years old. The galaxies of that era were stuffed with hydrogen and helium — the raw materials for making stars.

Even for that era, though, SPT0346-52 is a standout. The galaxy is only a few thousand light-years across. That means its total mass and volume are much less than one percent of the Milky Way’s. Yet it appears to be giving birth to the equivalent of as many as 4500 Suns every year. That makes it one of the most prolific star factories ever discovered — star-for-star, it’s hundreds of thousands of times busier than the Milky Way.

Astronomers are still trying to figure out why it’s so busy. It could be two galaxies that are merging. That process would slam together their clouds of gas, squeezing them and triggering a massive starburst — a torrent of new stars from a busy galaxy.

The galaxy is much too far and faint to see without a major telescope. But it’s in the constellation Horologium, the pendulum clock. From the southern half of the United States, it scoots low across the southern horizon before dawn.
 

Script by Damond Benningfield

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If you asked people to name one full Moon, the top vote-getter probably would be the one that lights up the sky tonight: the Harvest Moon — the full Moon that’s closest to the start of autumn.

For most of the modern world, the Harvest Moon is little more than a bit of folklore. But in the days before automation and electric lights, it was a big help to farmers, who could work their fields well into the night.

And that’s not just because of the Moon’s brightness. It’s also because the Moon rises close to the same time for a few nights around the Harvest Moon — around sunset.

The effect is more pronounced as you go farther north. From states like Minnesota and Washington, the Moon will rise only about 15 or 20 minutes later tomorrow night than it will tonight. That’s compared to an average of almost 50 minutes later over the course of a year. At such high latitudes, where the frost comes early, every hour of harvesting could make a difference.

Perhaps because of its big reputation, many people think the Harvest Moon looks bigger than other full Moons. But that’s not the case. The Moon’s apparent size depends on its distance, which can vary by about 10 percent. The distance isn’t related to the Moon’s phases. So sometimes the Harvest Moon occurs when the Moon is closer than average, but other times it comes when the Moon is farther than average. Either way, it’s always fun to stroll beneath a beautiful Harvest Moon.
 

Script by Damond Benningfield

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There’s nothing quite like the Double Cluster — two bright star clusters that stand side by side. They’re easily visible to the unaided eye — especially under dark skies, away from the troublesome glow of city lights. The clusters are in the northeast at nightfall, below the “W” outlined by Cassiopeia. They wheel over to the northwest by first light.

The clusters are known individually as NGC 869 and 884. They’re about 7500 light-years away. Combined, they’re about ten thousand times the mass of the Sun. Yet they contain more than 10,000 stars. The Sun is a fairly heavy star, so there are a lot more stars that are “skinnier.” So the clusters have a lot of stars that are small and feeble.

Fortunately for skywatchers, the clusters are young — only about 13 million years old. At that age, they still have quite a few stars that are big, bright, and massive. Those stars are the reason the Double Cluster is easily visible. The stars will burn out and expire fairly soon, though, so the clusters will fade by quite a bit.

Astronomers have recently discovered a wide “halo” of stars around the clusters. It forms long filaments that extend hundreds of light-years into space. Because the clusters are so young, the stars in the halo weren’t pulled out of the clusters. Instead, they probably formed along with them. And they plow through space together — extending the size of the beautiful Double Cluster.
 

Script by Damond Benningfield

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