The surface of Ariel looks like a sheet of paper that’s been loosely crumpled. It’s covered with ridges, wrinkles, and gashes. That may be telling us that Ariel once had a deep ocean of liquid water.

Ariel is one of the larger moons of the planet Uranus. It’s about 720 miles in diameter – a third the size of our moon. It orbits just a hundred thousand miles from the planet – much closer than the Moon is to Earth. It’s roughly a 50-50 mix of ice and rock.

Our only good look at Ariel came in 1986. Voyager 2 flew past it and photographed about a third of its surface. The pictures revealed a complex face. It has a mixture of old and young craters, deep ridges, and smooth plains that might have been paved by water gurgling up from inside the moon.

A recent study modeled the orbit of Ariel over the ages. It found that the orbit was once much more lopsided than it is today. As Ariel moved in and out, the gravity of Uranus stretched and squeezed the little moon. That could have melted some of the ice inside it, creating an ocean a hundred miles deep, topped by a thin crust of ice. The stress of all the stretching and squeezing could have cracked the ice, creating the wrinkly surface we see today.

Uranus is high overhead at nightfall. It’s below the Pleiades star cluster, and farther to the right of the bright orange star Aldebaran. Through good binoculars, the planet looks like a faint star.

Script by Damond Benningfield

Ursa Major III is doomed. It’s falling apart, and may vanish completely in a couple of billion years.

There’s not much to it even now. It’s so faint that it wasn’t discovered until 2023. It contains about 60 stars – all of them ancient, and all much smaller and fainter than the Sun. They add up to only about 16 times the Sun’s mass. They’re packed into a loose ball about 20 light-years wide.

But the total mass is about 2,000 times greater than the mass of the visible stars. That’s led to some confusion about its nature.

One idea is that it’s a small galaxy that’s orbiting the Milky Way. Most of its mass would consist of dark matter – matter that produces no energy, but that reveals its presence through its gravitational pull on the visible matter around it.

A study last year suggested a different nature – a star cluster held together by a clump of black holes. The cluster might have been born with a hundred thousand stars or more. When some of the stars died, they formed black holes, which congregated near the cluster’s middle.

The gravity of the Milky Way pulled away many of the cluster’s stars. Encounters with the black holes kicked out many more. And the study says the cluster will fall apart completely in about two billion years.

The cluster – or galaxy – is about 30,000 light-years away, in the great bear. But Ursa Major III is far too faint to see, even with a telescope.

Script by Damond Benningfield

Stars are born from huge clouds of gas and dust. Many of the stars remain close together, forming clusters. But as a cluster moves through the Milky Way, it gets pulled apart. The gravity of the rest of the galaxy tugs away the stars on the outskirts of the cluster. It also loosens the rest of the cluster, making it easier to pull away more stars.

A recent study looked at how that’s played out in the region around the Pleiades cluster. Using telescopes in space and on the ground, researchers measured how fast the stars in the region are spinning. That provides a rough measure of their age – the younger the star, the faster it spins. They also measured the motions of the stars through the galaxy, allowing them to trace the paths of the stars far into the past. And they compared the compositions of the stars; stars that were born together are made of the same mixture of elements.

From that, they found that the Pleiades and several smaller groups were close together tens of millions of years ago. That suggests they were born together before heading their separate ways. The study also found hundreds of stars between the groups that had belonged to one of the groups in the past. Today, the groups and loners are spread across 2,000 light-years of space – the Giant Pleiades Complex.

Look for the Pleiades high overhead at nightfall. It looks like a tiny dipper – the heart of a once larger family of stars.

Script by Damond Benningfield

Antares is a rare star. It’s one of the few named for what it’s not. The name is Greek. It combines “anti,” which means “against” or “opposed to” – with Ares – the Greek version of Mars, the god of war. So the name means “not Mars” or “rival of Mars.”

It was given the name because its color is similar to that of Mars – bright orange. The color indicates that the surface of Antares is thousands of degrees cooler than the surface of the Sun. Cooler stars glow red or orange, while hotter stars are white or blue.

Antares is one of many designations for the star. Because it’s the brightest star of Scorpius, it’s also known as Alpha Scorpii. And it’s also called the heart of the scorpion – Cor Scorpii.

Antares also has designations in many catalogs – lists of stars that have something in common. It’s in the bright-star catalog as HR 6134. It has a companion star, so it’s in the binary-star catalog. And it’s in several catalogs of objects that produce a lot of infrared light. In all, Antares has dozens of names and catalog numbers – an impressive list for an impressive star.

The gibbous Moon slips past Antares the next couple of mornings. The star will be to the left or lower left of the Moon at dawn tomorrow. And it will stand a little closer to the upper right of the Moon on Wednesday.

Tomorrow: more stars for the Pleiades.

Script by Damond Benningfield

Family members don’t always stay close together – they can be separated by thousands of miles. But one member of the Milky Way Galaxy’s family takes the separation to extremes. It’s 300,000 light-years from the center of the galaxy – one of the most distant residents of the Milky Way yet seen.

NGC 2419 is a globular cluster – a group of about a million stars. They form a dense ball a few hundred light-years across.

Any star near the middle of the cluster would have thousands of neighbors within a few light-years. Compare that to our own neighborhood – only three stars reside less than five light-years from the Sun.

NGC 2419 is one of the Milky Way’s oldest family members. The cluster was born more than 12 billion years ago – not long after the galaxy itself. All of its big, bright stars burned out long ago. So almost all of the remaining stars are much less massive than the Sun.

The cluster follows a highly stretched-out orbit around the center of the Milky Way. That’s led to suggestions that it was born elsewhere, then captured by the Milky Way. But there’s no confirmation of that idea. So NGC 2419 is still considered a far-away relative of the rest of the Milky Way.

The cluster is in the uber-faint constellation Lynx, which is in the east-northeast at nightfall. NGC 2419 is an easy target for just about any telescope.

Script by Damond Benningfield

Alpha Lyncis is only about a third of the age of the Sun. Yet the star has already zoomed through the prime phase of life. Now, it’s nearing the end of its life. And it’s letting us know about it – it’s the brightest star of the constellation Lynx.

That’s not necessarily saying much. Lynx is a large constellation, but it’s faint – only a few of its stars are bright enough to see from light-polluted cities or suburbs. In fact, the astronomer who created it, in the 1600s, called it “Lynx” because you needed the eyes of one to see it.

Alpha Lyncis is classified as a red giant. It’s about half-again the mass of the Sun. Heavier stars age more quickly. Such a star “burns” through the original hydrogen in its core in a hurry. As the core adjusts to the change, the star’s outer layers puff up. Today, Alpha Lyncis is more than 50 times the diameter of the Sun. As it got bigger, the star got cooler and redder – making it a red giant.

Puffing up also made the star hundreds of times brighter than the Sun. So Alpha Lyncis is visible – faintly – even though it’s a little more than 200 light-years away. That makes it one of the few stars in this faint constellation that you don’t need the eyes of a lynx to see.

Lynx is well up in the east-northeast at nightfall. It’s about half way between the Big Dipper and the twins of Gemini. But you need nice, dark skies to see much.

More about the constellation tomorrow.

Script by Damond Benningfield

The Moon snuggles close to the bright star Spica late tonight. They climb into good view by about midnight, and are high in the sky at dawn. At their closest, they’ll be separated by just a couple of degrees – about the width of your finger held at arm’s length.

That closeness is just an illusion – the Moon and the star are separated by a vast gulf.

The Moon is our closest neighbor. Tonight, it’s a little less than a quarter of a million miles away. At that distance, sunlight reflecting from the lunar surface takes about one and a third seconds to reach Earth. That means we see the Moon as it looked one and a third seconds earlier.

The Moon is moving farther from us – by an inch and a half per year. That’s a result of the tides. Earth and Moon exert a gravitational grip on each other. That slows Earth’s rotation, making the days a little longer. To balance the books, the Moon moves farther away.

Spica is more than nine billion times farther than the Moon. Its light takes about 250 years to reach Earth – the star is 250 light-years away. So as you look at Spica tonight, you’re actually seeing the bright star as it looked 250 years ago – about the time of the American Revolution.

And it’s moving away as well – by about 60 million miles per year. That’s a result of Spica’s motion around the center of the galaxy – an orbit that’s carrying the brightest star of Virgo into the distance.

Script by Damond Benningfield

The star cluster M79 is messy. It’s shedding some of its stars, creating a “tail.” Over the eons, in fact, the cluster might have lost most of the stars it was born with.

Messier 79 is a globular cluster – a ball-shaped family of about 150,000 stars. The cluster is more than 11 billion years old, so its stars are among the oldest in the entire Milky Way Galaxy.

There’s a trail of stars behind the cluster. The stars probably were stripped away by the gravity of the rest of the galaxy – especially its dense core.

Today, M79 is about 42,000 light-years from the Milky Way’s heart. But its orbit might bring it within just a few hundred light-years of the center. At that distance, the gravity of the galaxy’s core overpowers the gravity of the cluster. So stars in M79’s outskirts are pulled away. Eventually, they move away, and follow their own paths across the galaxy.

Some simulations have suggested that M79 has lost up to 85 percent of its original population of a million stars or so. And every future passage through the heart of the galaxy will pull away more stars – leaving only a glimmer of M79’s original glory.

M79 is in Lepus, the hare. The constellation is close to the lower right of bright Orion, in the southeast at nightfall. M79 is below the outline of the rabbit. You need binoculars to pick it out.

Script by Damond Benningfield