StarDate

The surface of the Moon seems eternal. There’s no air or water to reshape it, and no plate tectonics to jiggle things around. But the Moon does change – sometimes in a hurry. Scientists recently discovered an impact crater that’s almost 750 feet in diameter. It had been gouged during the spring of 2024.

The crater was found in images snapped by L-R-O – Lunar Reconnaissance Orbiter. The craft’s been mapping the lunar surface for years. It’s photographed almost all of the surface many times. Scientists compare images taken at different times to see how things have changed.

The new impact crater was the biggest change seen so far. The crater is three times wider than any other crater gouged during LRO’s time in orbit. It’s about 150 feet deep, and its rim rises more than 25 feet above the surrounding landscape. The impact blasted out boulders as big as houses. And bright “rays” of smaller bits of debris were splashed far from the crater’s edge. Such a powerful impact on Earth could level a city, and kill everything within miles of the impact site.

Some good-sized pieces of debris from the crater’s formation could have traveled dozens of miles – something for designers of lunar habitats to keep in mind.

Script by Damond Benningfield

The second-largest asteroid in the solar system is a puzzler. Most studies say it has the same layers as a planet, but that it’s not massive enough to pull itself into a fully rounded shape like a planet. But one study found that it’s not layered at all.

Vesta accounts for almost a tenth of all the mass in the asteroid belt. Its average diameter is about 325 miles. But it’s flattened and a bit stretched out.

The surface of Vesta is covered with volcanic rock. Combined with other observations, that suggests that Vesta is layered, with a distinct core, mantle, and crust. If so, then it formed very early in solar-system history, allowing it to incorporate a lot of radioactive material. Heat from radioactive decay partially melted it, allowing heavier materials to sink to the center and lighter ones to float to the surface.

But a study last year contradicted that. Using delicate measurements of Vesta’s gravity field, scientists determined that the asteroid isn’t fully layered. It might have started to develop layers, but never got hot enough to finish the job. Or it might have formed from debris blasted off a larger body by a giant impact – creating a massive space rock that’s all jumbled up.

Vesta lines up opposite the Sun in our sky tonight. It’s brightest for the year, and it’s in view all night. Even so, you need binoculars to spot it. It’s not far below the bright planet Saturn, which is low in the east at nightfall.

Script by Damond Benningfield

The stars in a binary system can be quite intimate. One star can pull gas off the surface of its companion, for example. And in extreme cases, the stars can merge to form a single star. Both types of interactions can make the stars look a lot younger than they really are.

Many examples of this are found in Messier 10. It’s a globular cluster – hundreds of thousands of stars all packed into a ball a few dozen light-years across.

The cluster includes an especially large number of blue stragglers – stars that look a lot younger and bluer than the cluster should contain. M10 itself is billions of years older than the Sun. It probably was born with many hot, blue stars. But such stars burn out quickly. So there’s no way that any of the cluster’s original blue stars could still be around.

Instead, the blue stragglers probably were rejuvenated. Some appear to have stolen gas from a companion. The extra material “revs up” the nuclear reactions in the recipient, making it look much younger.

Other blue stragglers may be the results of mergers – the stars spiral together. That would increase the rate of nuclear reactions as well – turning two old stars into what looks like a brand new one.

Messier 10 is in Ophiuchus, the serpent bearer. The cluster is in the southwest as night falls, to the upper right of the teapot of Sagittarius. Through binoculars, the cluster looks like a hazy smudge of light.

Script by Damond Benningfield

A pair of big, young star clusters is one of the skywatching highlights of autumn evenings. Each cluster contains thousands of stars. And hundreds of those stars are especially impressive – supergiants that are many times the size and mass of the Sun, and many thousands of times brighter.

The clusters are NGC 869 and 884. Together, they’re known as the Double Cluster. They’re in the northeast at nightfall, in the constellation Perseus the hero. Under dark skies, they’re visible to the eye alone as a single smudge of light. Binoculars split them apart, revealing two sparkling collections of stars.

The clusters are more than 7,000 light-years away. They form the core of the Perseus OB1 Association – a huge complex of stars and clouds of gas and dust. The region has given birth to perhaps tens of thousands of stars.

All of the stars were born in the recent past, as measured on the astronomical time scale. The Double Cluster is only about 14 million years old, compared to four-and-a-half billion years for the Sun. One way astronomers determine that age is through the types of stars found in the clusters. The most-massive stars burn out in a hurry. But the clusters still contain many such stars.

Some of those monsters won’t last much longer, though – they’ll blast themselves to bits as supernovas – brilliant light shows in the Double Cluster.

Script by Damond Benningfield

Cassiopeia is an easy constellation to pick out. The stars of the queen form a prominent letter M or W, which is visible even through moderate light pollution. And under dark skies, binoculars reveal several star clusters around that pattern, all immersed in the glow of the Milky Way.

One of the highlights is NGC 7789. It was discovered in 1783 by Caroline Herschel – the first woman to be paid for her work as an astronomer. Through a telescope, the contours of the cluster and nearby clouds of dust resemble a rose. So NGC 7789 also is known as Caroline’s Rose.

The cluster is about 6800 light-years away, in the opposite direction from the center of the galaxy. And it’s packed with stars. It has 1300 confirmed members, with an estimated total of 3,000 stars or more.

The cluster’s age has been tough to pin down. A recent study says it’s one-and-a-quarter billion years old. Previous estimates are a little higher. Regardless of the exact age, the cluster has made several orbits around the center of the galaxy. By the time many other clusters have made that many orbits, they’ve been pulled apart by the gravity of the rest of the galaxy. The stars survive, but they head their separate ways.

Script by Damond Benningfield

A quiet monster lurks in the heart of M31, the Andromeda Galaxy. Known as M31-star, it’s a black hole a hundred million times the mass of the Sun, and 25 times heavier than the black hole at the heart of the Milky Way.

Every large galaxy probably has a supermassive black hole. And many of them are quite bright.

The black hole itself is completely black – it swallows up all the matter and energy that falls into it, and nothing can come back out.

But the black hole’s gravity pulls in huge amounts of gas and dust, along with an occasional asteroid, star, or other large object. This debris forms a disk around the black hole. As material in the disk spirals closer to the black hole it gets extremely hot, so it shines brightly.

But M31-star is one of the quietest of all supermassive black holes – its disk produces very little energy.

That tells us that the black hole is not actively “feeding” – not much debris is falling into it. A distant ring of gas and dust encircles the black hole, with filaments of material spiraling into the disk. But the “feeding” rate is low, so M31-star remains quiet – a dark monster in the heart of M31.

Under dark skies, the Andromeda Galaxy is visible to the naked eye. It looks like a faint smudge of light that’s wider than the Moon. Right now, it’s about a third of the way up in the east-northeast at nightfall, and climbs high overhead later on.

Script by Damond Benningfield

Two-and-a-half million years ago, Earth was going through some big changes. It was the start of the Pleistocene epoch – a period of major ice ages. And the ancestors of modern humans were just appearing.

That timescale puts a little perspective on the distance to M31, the Andromeda Galaxy – the closest major galaxy to the Milky Way. It’s two-and-a-half million light-years away. So when we look at the galaxy, we’re seeing it as it looked two-and-a-half million years ago.

Andromeda and the Milky Way are much alike. Both of them are barred spirals – they have a long bar of stars in the center, with spiral arms wrapping off the ends of the bar. They’re about the same mass – roughly one trillion times the mass of the Sun. Andromeda appears to be wider, although it’s hard to measure the size of the Milky Way from our location inside it.

Andromeda isn’t giving birth to as many stars as the Milky Way is. That may be because it collided with a smaller galaxy in the distant past. The collision triggered a huge wave of starbirth that used up most of the star-making material – limiting the future population of the Andromeda Galaxy.

M31 is well up in the east-northeast at nightfall. Under fairly dark skies, it looks like a faint smudge of light wider than the Moon – the most-distant object that’s easily visible to the eye alone.

We’ll have more about the Andromeda Galaxy tomorrow.

Script by Damond Benningfield

The Moon has had a busy social calendar this week. It’s had meet-and-greets with the twins of Gemini and the planets Mars and Jupiter. And it has one more engagement tomorrow. Regulus, the heart of the lion, will stand above the Moon at dawn.

Script by Damond Benningfield

For skywatchers across the eastern United States, the planet Jupiter will vanish early tomorrow – hidden behind the crescent Moon. The rest of the country will see an impressively close encounter between the two bodies.

The vanishing act is an occultation – from a Latin word that means “to hide.” Over the centuries, astronomers used these events to learn more about the Jovian system. Precise timing of occultations helped astronomers understand the planet’s orbit, which helped them zero in on Jupiter’s distance. And it takes a few seconds for the planet to vanish and reappear. The exact timing, combined with Jupiter’s distance, helped refine its diameter.

Occultation studies also have included radio waves. Jupiter’s magnetic field sculpts a teardrop-shaped “bubble” around the planet. It also creates powerful radiation belts. Watching Jupiter vanish behind the Moon revealed the extent of both.

Occultations are less important to scientists than they once were – but they’re still fun to watch.

This one plays out before or during dawn. Jupiter looks like a brilliant star. From the eastern U.S., it’ll wink out before dawn, with Jupiter and the Moon quite low in the sky. From a little farther west, the occultation will be in progress as they rise, with Jupiter popping into view shortly thereafter. Those in the western states will have to settle for an especially close encounter between Jupiter and the Moon.

Script by Damond Benningfield

To borrow from “Alice in Wonderland,” the case for life on Mars keeps getting curiouser and curiouser. Two rovers have discovered organic compounds there – the chemical building blocks of life – in locations that are thousands of miles apart. And some recent looks at results from the Viking landers of the 1970s suggests they, too, might have found organics. So it’s possible that microbes might have inhabited Mars in the past – and might do so even today.

Some of the best evidence comes from the Perseverance rover. In 2024, it studied a formation known as Bright Angel – layers of sediment deposited at the bottom of a river perhaps three and a half billion years ago.

A rock named Cheyava Falls contained a lot of organics. And a study earlier this year confirmed the finding. The compounds could have been produced by microscopic life. But there are several other possible explanations, including chemical reactions in the rocks themselves.

Perseverance stored a sample of Cheyava Falls for return to Earth. Laboratory analysis could tell us for sure how the organics were formed. But NASA has nixed a planned sample-return mission. So we’re a long way from satisfying our curiosity about life on Mars.

Look for Mars quite near the Moon early tomorrow. It looks like a bright star. They’re especially close as they climb into good view, around 2:30 or 3.

Script by Damond Benningfield