StarDate

That’s the moan of a supermassive black hole. The black hole inhabits the heart of NGC 1275, a giant galaxy at the heart of one of the largest galaxy clusters in the universe. The interplay of the material around the black hole creates sound waves that ripple through giant clouds of gas. Scientists pitched the sound waves into the range of human hearing.

NGC 1275 belongs to the Perseus Cluster — a collection of thousands of galaxies. They’re bound to each other by their mutual gravitational pull. And they’re embedded in a vast cloud of gas that’s heated to millions of degrees.

Some of that gas is falling into NGC 1275. It feeds the black hole, which is about 800 million times the mass of the Sun. Some of the gas forms a swirling disk around the black hole. The disk shines brilliantly, especially in X-rays. That energy pushes some of the gas outward. The gas ripples through the surrounding clouds, producing sound waves.

The waves are far, far below human hearing. So astronomers pitched them up by 57 or 58 octaves — allowing us to “hear” the moaning of a supermassive black hole.

NGC 1275 is in Perseus. The constellation is low in the northeast around midnight, and high in the east at dawn. But you need a telescope to see this busy galaxy.
 

Script by Damond Benningfield

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Astronomers usually look for things to get bright. But this month, they’re looking for a black hole in a distant galaxy to get dark — the result of a break in the black hole’s dinner.

The black hole is about 50 million times the mass of the Sun. It’s getting a little heavier by eating a nearby star. And it appears to refill its plate every 3.3 years.

That situation is rare. Astronomers do see black holes ripping stars apart. But in almost every case, it’s a one-time feast — the black hole completely destroys the star.

But in the case of AT 2018ftk, astronomers have seen a couple of outbursts. A recent study says the star survived the first pass, and is making repeated encounters, losing some of its gas each time.

The star probably had a close companion. They were ripped away from each other by the black hole’s gravity. The companion star was kicked away fast enough to escape the galaxy.

The other star swung into a lopsided orbit around the black hole. Each pass pulls away more of the star’s gas, which spirals into the black hole. The gas gets extremely hot, so it emits X-rays. It stays bright for about half of the star’s orbit, then goes dark, which should happen this month — a disruption in the black hole’s dinner.

AT 2018ftk is in Grus, the crane. For skywatchers in the southern half of the U.S., the constellation stands quite low in the south in the wee hours of the morning.
 

Script by Damond Benningfield

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A recent discovery by Hubble Space Telescope sounds like a cosmic version of “Beauty and the Beast.” A beastly black hole appears to be leaving a trail of beauty in its wake: a ribbon of bright stars.

Astronomers discovered the system as a small streak of light on a Hubble image. Follow-up observations revealed that it’s more than seven billion light-years away and 200,000 light-years long. There appears to be a hot spot at one end of the streak, and a small galaxy at the other. A trail of young stars connects them.

The team that discovered the system says the hot spot probably contains a black hole up to 20 million times the mass of the Sun. It’s plowing through clouds of gas, heating them and making them shine brightly. The gas flows around the black hole. It cools and condenses, forming new stars.

The black hole might have started its jaunt almost 40 million years ago. Three galaxies merged and their black holes fell toward each other. A complex dance sent one of the black holes careening off into space.

A few months after the initial announcement, though, other astronomers suggested that the system is a lot less exciting. It’s simply a thin galaxy seen edge-on, so it looks like a bright streak of light — beauty without the beast.

Whatever its true nature, the system is in the head of Cetus, the whale or sea monster, which is in the southeast at dawn, close to the brilliant planet Jupiter.
 

Script by Damond Benningfield

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According to an old saying, “from tiny acorns do mighty oak trees grow.” And mighty black holes must do the same thing. The problem is, astronomers aren’t sure just what form the black hole “acorns” take.

Supermassive black holes inhabit the hearts of most galaxies — and perhaps all of them. They can grow to billions of times the mass of the Sun. And some of them did so in the blink of a cosmic eye — in the first few hundred million years after the Big Bang.

Despite their reputation, though, black holes can’t gobble up enough stars and gas to grow that big that fast. So they must have grown from some sort of “seed” — something that was fairly heavy to begin with.

One possibility is the merger of a bunch of smaller black holes. The combined black hole would be massive enough to pull in enough material to grow to supermassive proportions.

Another possibility is giant clouds of dark matter — material that we can’t see, but that pulls on the stars and galaxies we can see. And yet another is mid-sized black holes that formed when compact but heavy clouds of gas collapsed.

It’s also possible that the first supermassive black holes formed from collapsing gas clouds directly. The clouds were so tightly packed that they couldn’t resist gravity’s powerful grip — making them “acorns” from which mighty black holes grew.

We’ll talk about a possible trail of stars behind a black hole tomorrow.
 

Script by Damond Benningfield

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The giant planet Saturn keeps company with the Moon tonight. They climb into good view by 10:30 or 11. Saturn looks like a bright star close to the left of the Moon. It’ll stand even closer to the Moon at dawn tomorrow.

Saturn is best known for its beautiful rings. They probably represent the shattered remains of one or more small moons — countless bits of ice from the size of dust grains to boulders.

The major rings span more than two-thirds the distance from Earth to the Moon. But they’re amazingly thin — no more than a few hundred feet thick. And across most of their great span, they’re only about 30 or 40 feet thick.

To put that into perspective, consider this: If you reduced the rings to the length of a sheet of printer paper, they’d be far thinner than the paper. They got so thin as bits of material rammed into each other, grinding them down to smaller and smaller pieces. Each piece settled into its own orbit around Saturn’s equator.

Because the rings are so thin, when we see them edge-on, they all but vanish. That happens every 14 or 15 years, around Saturn’s equinoxes — the start of spring or fall. Northern fall will arrive in May of 2025, so that’s when the rings will next appear edge-on. For now, their angle is pretty shallow, so the view through a telescope isn’t great. Even so, it’s still one of the most amazing sights in the solar system — the incredible rings of Saturn.
 

Script by Damond Benningfield

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The modern calendar owes its existence to the most famous of all Roman emperors, 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 revisions, we still use that calendar today.

One of those revisions is the names of the months of July and August. Under the original Roman calendar, the year began with March. The two hot summer months were called Quintilis and Sextilis — names that mean the fifth month and sixth month. 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., 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 imperial summer calendar.

Script by Damond Benningfield

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Real estate markets have been pretty volatile in recent years. And that’s true even beyond Earth. Astronomers have been trying to figure out whether it’s possible for anything to live on planets orbiting red-dwarf stars. That’s important to know because red dwarfs are by far the most common stars in the galaxy. So if such star systems can support life, they could increase the Milky Way’s habitable real estate by a factor of 10.

Red dwarfs are the smallest, coolest, and faintest of all stars. But they’re also the longest-lived — the smallest could shine for trillions of years. And they account for three-quarters or more of all the stars in the galaxy.

Small, rocky planets similar to Earth should be common around such stars. And a large fraction of the planets could be in the best location — inside the habitable zone — the region where temperatures are just right for liquid water. That could mean tens of billions of potentially habitable worlds.

But there are lots of caveats. Because a red dwarf is so feeble, its habitable zone is narrow and close to the star. But red dwarfs produce powerful outbursts of X-rays, so any close-in planets would be zapped by radiation. And close planets would be locked so that the same side always faces the star, creating big differences in the daytime and nighttime temperatures.

So astronomers are still trying to figure out the real estate market around red-dwarf stars.
 

Script by Damond Benningfield

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Some well-known binary star systems are in good view as night falls. Antares, the heart of the scorpion, is due south. Blue-white Spica is at about the same height in the southwest. And Polaris, the pole star, stands due north, just as it does every night of the year.

Many of the other stars visible to the naked eye are binaries as well — two stars that orbit each other, bound by their mutual gravitational pull.

In fact, binaries are quite common. But astronomers aren’t sure just how common. One study put the fraction of stars that are in binaries at about half. But others have come up with higher numbers.

Binaries probably are born in the same way as single stars — from the collapse of giant clouds of gas and dust. As the cloud gets especially tight, though, it splits apart. That creates two newborn stars — and sometimes three, four, or even more.

Most of the biggest and heaviest stars have binary companions. Binaries are less common among stars that are roughly the mass of the Sun. And they’re least common among the smallest and faintest of stars, known as red dwarfs. Because they’re so puny, they’re hard to study, so we’re not sure just how likely they are to be members of binaries. But red dwarfs account for most of the stars in the Milky Way Galaxy. So zeroing in on that likelihood will give us a better idea of just how many stars have companions — and how many travel through the galaxy alone.
 

Script by Damond Benningfield

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To modern eyes, the brightest stars of the constellation Sagittarius form the outline of a teapot, with the handle to the left and the spout to the right. To ancient eyes, though, it was the archer — a centaur holding a bow and arrow. It’s tough to pick out the centaur, but the bow is easy. It’s traced by an arc of three stars in the teapot, including the brightest star in the whole constellation.

The stars are Kaus Borealis, Kaus Media, and Kaus Australis — names that mean the northern, middle, and southern points of the bow. Kaus Borealis forms the top of the teapot. Kaus Australis is at the teapot’s bottom right, and is the brightest member of the constellation. And Kaus Media bows outward between them.

All three stars are giants — they’re much bigger and brighter than the Sun. And even though they’re much younger than the Sun, they’re much later in life. That’s because they’re all more massive than the Sun. Heavy stars consume their nuclear fuel quickly, so they burn out in a hurry. That process causes their outer layers to puff up, which makes them much brighter.

Kaus Media is the farthest member of the bow, which means that it’s actually the most impressive of the three stars — it’s more than a thousand times brighter than the Sun.

And if there’s a bow, there must be an arrow as well. It forms the outer point of the spout of the teapot — one more giant star that’s near the end of its life.
 

Script by Damond Benningfield

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Pull out your binoculars and make sure you have a clear western horizon after sunset the next few nights. You’ll need them to see an amazing close encounter between the planet Mercury and the star Regulus, the heart of the lion. This evening, they’ll pass just one-tenth of a degree from each other — less than the width of a pencil held at arm’s length.

Regulus has frequent encounters with solar system bodies because it’s almost on top of the ecliptic, which is the Sun’s path across the sky.

The Moon and planets stay close to the ecliptic as well, so they periodically pass close to Regulus. Just this month, the list of close visitors has included Mars, Venus, and the Moon.

Mercury is the least frequent caller. It’s the closest planet to the Sun, so it never appears far from the Sun in our sky. That means Mercury has to wait for Regulus to come to it, shortly before sunrise or shortly after sunset.

Mercury has been creeping up on Regulus for a few nights. And it will remain close for a few more nights as it pulls away.

Look for Mercury and Regulus beginning about a half-hour after sunset, almost due west. They’re so low in the sky that any trees or buildings along your horizon will hide them from view. Mercury is the brighter of the two targets. The view is better from the southern United States because the ecliptic stands at a slightly more favorable angle — a better view of a beautiful encounter.
 

Script by Damond Benningfield

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