StarDate

Two Greek goddesses huddle close together in the sky this evening: Selene and Aphrodite – the Moon and Venus. They’re in good view well before night falls, shining through the twilight.

Aphrodite was the goddess of love and beauty. She was identified with the brilliant morning and evening star. We know them today by her Roman name, Venus. Selene was one of several Moon goddesses, but the only one considered to be the human form of the Moon itself.

Selene’s name has been applied to a lot more than just the Moon. Among other things, it’s the name of a mineral – selenite – and an element – selenium.

Selenite is a crystal form of the mineral gypsum. It’s translucent, so it was sometimes used as window panes. There are several possible explanations for the name. One says it might have been bestowed because the light that passed through the windows looked milky white – like the light of a bright Moon.

Selenite doesn’t contain any selenium, though. The only thing the mineral and the element have in common is the name. Selenium was discovered in the early 1800s. It’s rare – usually found mixed in with the ores of various metals. Its appearance and properties were similar to those of another recently discovered element – “tellurium” – a word that means “Earth.” So it made sense to name the new one for our satellite world – the Moon.

Tomorrow: a different seasonal dividing line.

Script by Damond Benningfield

All of the Sun’s major planets are a bit flattened – they’re bigger through the equator than through the poles. For Mercury and Venus, the difference is only a few miles. For Earth, it’s about 20 miles. But for Saturn, it’s more than seven thousand miles. Such a big difference is obvious even to the eye alone: The giant planet looks like a squashed beachball.

The planets are flattened because they’re spinning. The rotation pushes material outward. Solid bodies like Earth are stiff, and they don’t rotate all that fast, so they’re not deformed by much. But Saturn and the other giant worlds don’t have solid surfaces. That makes it easier to push stuff away from the center as the planet spins.

Saturn is flatter than the other giants. That’s mainly because it’s less dense than the others. That means its surface gravity isn’t as strong, so it doesn’t grip material at the surface as tightly.

And Saturn spins in a hurry – roughly once every 10 and a half hours, although the exact timing is still uncertain. That means the planet’s equator rotates at more than 21,000 miles per hour – 20 times faster than Earth’s equator. That slings material outward with great force – making Saturn the most “smushed” planet in the solar system.

Look for Saturn near the crescent Moon early this evening. The planet looks like a bright star above the Moon as twilight fades. They set a couple of hours after sunset.

Script by Damond Benningfield

The constellation Auriga is high in the eastern sky as night falls. It’s usually drawn as a pentagon. The brilliant star Capella is at the top left corner, with fainter Elnath – a star that’s shared with Taurus – at the lower right.

A trail of three prominent star clusters winds through the pentagon – Messier 36, 37, and 38. M36 and 38 are inside the pentagon. M37 is just below it. All three are easy targets for binoculars.

M37 is about 4500 light-years away. And it’s the largest of the trio of clusters. It spans a few dozen light-years. Astronomers have cataloged more than 500 member stars, but the total population could be much higher. The cluster includes more than 50 dead stars known as white dwarfs. The details of such stars help reveal the cluster’s age – roughly 400 million to 550 million years.

And that’s a long time for a star cluster to hang together. M37 has orbited the center of the Milky Way Galaxy at least twice. As it orbits, the gravitational fingers of other stars and gas clouds pull away some of the stars on the fringes of the cluster.

So far, Messier 37 appears to be maintaining a firm grip on most of its stars. But that won’t last forever. More stars will be pulled away on each trip through the galaxy – whittling down this tight family of stars.

Script by Damond Benningfield

For decades, the DSS-28 radio antenna in the California desert listened to the faint whisper of spacecraft exploring the solar system. It’s still listening for artificial radio signals today – the faint whisper of other civilizations. It’s controlled by students around the country and around the world.

DSS-28 is a radio dish more than a hundred feet in diameter. It was built for the Army, then transferred to NASA. It was decommissioned in the 1980s. But 40 years ago, a kindergarten teacher in California found a new use for it – as an educational tool for teachers and students.

It’s the centerpiece of the Goldstone Apple Valley Radio Telescope project. Teachers train to use the telescope, and pass that knowledge on to their students. The students then work with scientists to observe stars, planets, and other objects.

Current projects include studies of the radio waves from Jupiter; the busy regions around supermassive black holes in other galaxies; and the connection between sunspots and the Sun’s hot outer atmosphere.

Another project is SETI – the search for extraterrestrial intelligence. Students have been using the telescope to scan the galactic plane – the region that outlines the disk of the Milky Way Galaxy. The first phase found no signals from other civilizations. But the telescope will scan more slices of the galaxy in the coming years – still listening for radio signals from beyond Earth.

Script by Damond Benningfield

The Big Dipper is low in the northeast at nightfall, ready to climb high across the north during the night. The prominent constellations Gemini and Orion are far to its upper right. But the space between them looks dark and empty. There’s just not much to see – especially from light-polluted cities. In fact, the main feature in the gap is named for its faintness. The constellation Lynx was given its name because you need the sharp eyes of a lynx to see any of its stars.

A telescope reveals some impressive sights, though. That includes the galaxy NGC 2273 – a galaxy within a galaxy. It’s about as big and as massive as our own home galaxy, the Milky Way. It has a small, tight spiral in its middle, with a wider spiral around it.

A supermassive black hole inhabits the galaxy’s heart. The black hole produces enormous amounts of radio waves. That tells us the black hole is feeding voraciously. As gas and dust spiral toward the black hole they form a wide, hot disk around it. Some of the material in the disk is funneled into “jets” that shoot into space at almost the speed of light. The jets produce radio waves.

The jets and other evidence show that the black hole is about seven and a half million times as massive as the Sun. That’s about twice as heavy as the black hole at the center of the Milky Way – an impressive zone of darkness in a dark region of the night sky.

Script by Damond Benningfield

Astronomers will keep a close eye on a distant galaxy later this year. They’ll be looking to see if a giant black hole is once again feasting on a star.

The system is known as AT2018fyk. It was discovered in 2018, when a galaxy 860 million light-years away flared brightly. The flare-up happened when a star flew too close to a black hole about 50 million times the mass of the Sun. The black hole appeared to rip the star to bits and gobble up the debris.

A couple of years later, though, the system flared again. That means the star survived the encounter – or at least some of it did. When the star once again came close, the black hole took another bite.

Last year, researchers concluded that the star originally was a member of a binary – two stars locked in orbit around each other. But the black hole split the binary apart. One of the stars was fired away like a cosmic bullet. The other entered a lopsided orbit around the black hole. It orbits once every three and a half years.

When the star gets close to the black hole, it loses some of its gas. The gas spreads out along the star’s orbit. Every year and a half, the gas begins to pour into the black hole. It heats up and glows brightly.

If that model is right, the system should brighten again this summer. So astronomers will be watching – to see if a black hole takes another bite of a star.

We’ll talk about another black hole tomorrow.

Script by Damond Benningfield

For the casual skywatcher, Camelopardalis, the giraffe, is a dud. Only a few of its stars are visible to the unaided eye, and you need dark skies to see even those.

With binoculars, though, you can pick out some nice sights within the giraffe’s borders. One example is Kemble’s Cascade – a string of about 20 stars along the giraffe’s neck. The stars aren’t related – they just happen to line up in the same direction.

Near one end of the cascade, though, is a group of several dozen stars that are related: the star cluster NGC 1502. It’s high above Polaris, the North Star, in mid evening.

The cluster is a few thousand light-years away, but the exact distance is uncertain. The cluster’s age is uncertain, too, although it’s probably only a few million years.

One clue to its age is the types of stars it contains. Many of the stars of NGC 1502 are much bigger, brighter, and hotter than the Sun. Such stars live no more than a few tens of millions of years, compared to billions of years for stars like the Sun. Since quite a few of these stars are still around, the cluster can’t be very old.

The brightest member of the cluster is called SZ Cam. It’s a family of six stars, split into three tight pairs. Two of the pairs huddle close together, while the third is much farther out. SZ Cam helps make the cluster worth looking for, in a constellation that’s a little short of skywatching wonders.

Script by Damond Benningfield

There’s a double burst of color in the sky right now. The planet Mars and the star Pollux stand side by side. They’re in the east at nightfall and soar high across the sky later on. Mars is by far the brighter of the two, with Pollux close to its left.

Mars and Pollux both shine bright orange. But they achieve that color in different ways.

Mars is a planet – a chunk of rock and metal about half the size of Earth. Much of its surface is coated with dust. It’s made mostly of iron oxide – it’s basically rust. And if you have any rusty iron pipes, you know it looks orange.

That dust could be a problem for future Mars explorers. It’s so fine that it could get into machines and electronics. That could jam any moving parts, and short out the electronics. So mission planners will have to find ways to prevent astronauts from tracking the dust into their habitats.

Pollux, on the other hand, is a star. And it’s an impressive one: It’s about twice as massive as the Sun, almost 10 times wider, and more than 30 times brighter.

Pollux is so large because it’s burned through the original hydrogen fuel in its core. That’s made the core smaller and hotter. Radiation from the hotter core pushes on the surrounding layers of gas, making them puff up. As the gas expanded it got cooler. So the surface of Pollux is a couple of thousand degrees cooler than the Sun. At that temperature, the gas glows orange – a bit of color in the night sky.

Script by Damond Benningfield

Two of the eight planets of the solar system highlight the western evening sky. Venus is the brilliant “evening star.” Saturn is below it. It looks like a fairly bright star, although not nearly as bright as Venus.

A bunch more planets lurk in the background. They’re in the star system Trappist-1. It’s in the constellation Aquarius, right next to Saturn’s current location.

The star is a red dwarf. It’s only a fraction the size and mass of the Sun. In fact, it’s just about as puny as it’s possible for a true star to be.

Trappist-1 has seven known planets. Only our own solar system is known to have more. All seven planets are extremely close to the star – no more than a few million miles away. But several of them are in the star’s habitable zone – the region where conditions are most comfortable for life. And the system is about three billion years older than our own, so there’s been plenty of time for life to evolve there.

In fact, a project that’s looking for extraterrestrial civilizations took a look at the system. Scientists wondered whether more than one planet might be inhabited, and whether they might talk to each other via radio. We see the system edge-on, so any transmissions between planets might leak out in our direction. So the project tried to “eavesdrop” on the conversations.

The search came up empty. So if the planets of Trappist-1 are inhabited, the folks there don’t seem to be talking to each other.

Script by Damond Benningfield

In Greek mythology, Orion and Scorpius were mortal enemies. So the gods placed them on opposite sides of the sky. When one is rising, the other is dropping from view.

The constellations have a lot in common. And so do their best-known stars – Betelgeuse in Orion, and Antares in Scorpius. Both stars are red supergiants. Both are many times more massive than the Sun, and a hundred thousand or more times brighter. And each is destined to explode as a supernova – probably within the next million years.

Betelgeuse and Antares also are hundreds of times wider than the Sun. So even though they’re hundreds of light-years away, they’re big enough for astronomers to see them as disks instead of just pinpoints of light. In fact, they’re among the first stars to have their size measured directly.

Antares was measured 100 years ago. Astronomers used a special device attached to the 100-inch telescope at Mount Wilson in California – the largest telescope in the world at the time. It gave them a diameter of more than 400 times the size of the Sun. As technology has improved, astronomers have found that Antares is probably about 50 percent larger than that. So if Antares took the Sun’s place, it would engulf the four innermost planets – including Earth.

Betelgeuse is in the east at nightfall. Antares is on the other side of the sky – low in the south-southeast at dawn. Tomorrow, it’s close to the lower left of the Moon.

Script by Damond Benningfield