StarDate

Galaxies are like businesses: They grow bigger through mergers, with the big guys gobbling up the smaller ones.

Our home galaxy, the Milky Way, is a prime example. It’s an amalgam of many galaxies — perhaps hundreds of them. These mergers have pumped up the Milky Way to gigantic proportions. It spans a hundred thousand light-years or more, and consists of hundreds of billions of stars.

Astronomers have detected remnants of many of the smaller galaxies the Milky Way has gobbled up. They form streamers of stars that can stretch across thousands of light-years. The stars in a streamer all follow the same path, and they have a similar composition. That suggests that the stars were born elsewhere, then incorporated into the Milky Way.

One of the biggest mergers happened when the galaxy was just a few billion years old. Astronomers found its remains in observations by Gaia, a European space telescope. The craft found tens of thousands of stars that move in the opposite direction from most of the Milky Way’s stars. The stars also have slightly different make-up. Even though they’re found throughout the entire galaxy, the stars probably were born elsewhere, then consumed by the Milky Way

The original host galaxy of these stars probably was about a quarter the mass of the Milky Way when they merged — about 10 billion years ago. Today, it still hasn’t been fully absorbed by the larger galaxy, so it preserves hints of the Milky Way’s hectic past.

Script by Damond Benningfield

Every star sits at the middle of its own “bubble” — a region of space dominated by the star’s magnetic field and its “wind” of hot gas. For the Sun, the bubble spans billions of miles — at least 150 times the distance between the Sun and Earth. For other stars, though, it can be much larger.

An example is the bubble surrounding Aldebaran, the bright star that represents the “eye” of Taurus, the bull. The star lurks quite close to the Moon tonight, so it’s hard to miss.

Aldebaran itself is a giant. It’s at the end of its life, so it’s puffed up like a hot balloon. As it gets puffier, more hot gas escapes from its surface and races out into space. Right now, in fact, Aldebaran is losing enough material to make a planet as massive as Earth in about a quarter of a million years.

As the gas expands, it gets cooler — cool enough for atoms to begin linking together to make molecules. Astronomers have detected molecules of water, carbon monoxide, and other compounds around the star.

Even as the dust forms, though, the wind keeps on blowing. It doesn’t stop until the gas and dust get thin and slow enough that they’re blocked by the material in interstellar space. For Aldebaran, that happens at about a thousand times the distance from Earth to the Sun.

At that boundary, the wind piles up, so it gets hotter and denser. That outlines the edge of Aldebaran’s bubble — and the end of the giant star’s realm.

Script by Damond Benningfield

A wave of starbirth has rolled across a massive cloud of gas and dust in Monoceros, the unicorn. It started a few million years ago, and has given rise to more than a thousand stars. And the process is continuing even now.

The complex is NGC 2264. It consists of two main components. One is a cluster of bright young stars known as the Christmas Tree. And the top of the tree intersects a pillar of dark dust known as the Cone Nebula.

A recent study of the region found that its first stars probably formed about five million years ago, in the northern part of the Christmas Tree. They’re clustered around a brilliant star that has blown away much of the remaining gas and dust. Without that supply of raw materials, fewer stars are being born in that area.

Instead, star formation has moved southward, to the peak of the Cone Nebula. Most of the stars in that area are only about a million years old. They’re still embedded in dense clouds of gas and dust. Many of them are still pulling in some of that material, so they’re still getting bigger. They’re surrounded by wide, flat disks of gas and dust that spiral toward the stars.

These dense “cores” of star formation are surrounded by a “halo” of slightly older stars, which are beginning to move away from their birthplace.

NGC 2264 is in the southeast at nightfall. It’s between the bright stars Procyon and Betelgeuse. But you need a telescope to spot this busy stellar nursery.

Script by Damond Benningfield

A unicorn follows Orion across the sky: Monoceros, which is to the lower left of Orion at nightfall. It’s not much to look at, though — its brightest stars are so faint that you need pretty dark skies to pick them out.

Perhaps it’s fitting, then, that one of its most interesting denizens is a black hole — the closest one to Earth.

A0620-00 is more than 3,000 light-years away. The black hole produces no energy. But it has a companion — a “normal” star about half as massive as the Sun. And it’s the interactions between these two objects that reveals the black hole’s presence.

For one thing, over the last century or so, A0620 has flared up twice. The second of those events was observed by an X-ray telescope in space. It found that the system’s X-ray brightness jumped to a hundred thousand times its normal level.

The outbursts may have been produced by gas that the black hole “stole” from its companion. The black hole takes gas from the star all the time, forming a disk around the black hole. As the gas piles up, though, it may become unstable. Lots of gas may plunge toward the black hole in a rush, heating to millions of degrees and shining brightly just before it vanishes.

Measuring the orbit of the visible star and its dark companion suggests that the black hole is about six times the mass of the Sun — a small black hole that’s a fairly close neighbor.

More about Monoceros tomorrow.

Script by Damond Benningfield

A giant gash slices across the surface of Mars. It’s a network of canyons and valleys that’s among the most impressive features in the solar system.

Valles Marineris consists of about a dozen major canyons. Together, they span about 2500 miles — equal to the width of the United States. Individual canyons are up to about 125 miles wide. Several of them merge in the middle of the system, where they form a basin that’s 375 miles wide and almost six miles deep.

Valles Marineris has a complicated history. The first canyons probably formed billions of years ago, when the planet was quite young. A nearby network of massive volcanoes got so heavy that it created cracks in the crust. The cracks then grew wider, creating long, deep canyons.

Over the eons, though, other processes have helped carve the canyons. That includes erosion by wind and water. In fact, water may have partially filled some of the canyons, creating lakes that deposited layers of sediments. Some of the water may have burst out of the eastern end of Marineris in massive floods, carving deep channels.

And today, landslides along the canyon walls help make the channels wider — making the Grand Canyon of Mars even more spectacular.

Look for Mars to the upper right of the crescent Moon this evening. The little planet looks like a bright orange star. And as a bonus, the planet Uranus is nearby, too, although you need binoculars to see it.

Script by Damond Benningfield

A study in contrasts is on display in the western sky the next few evenings as two planets slide past each other. Although they’re siblings, everything about them is different — their size, composition, and even their color.

Tonight, Mars and Uranus are separated by about twice the width of your finger held at arm’s length. Over the next few nights, Mars will move past Uranus, so they’ll be even closer.

Mars is the second-smallest planet in the solar system. It’s a bit more than half the diameter of Earth. It’s made of rock, and it has a series of layers like those that make up our own planet. A craft that landed on Mars last year is probing the details of those layers.

Mars is topped by dark rock and rusty orange dirt. They give the planet an overall orange color.

Uranus, by contrast, looks blue-green. Methane in its upper atmosphere absorbs red light, so only bluer wavelengths shine through.

Uranus is the third-largest planet in the solar system — about four times the diameter of Earth. It probably has a dense, rocky core, surrounded by layers of ice and gas — a major contrast with its sibling planet.

Mars is well up in the southwest at nightfall, and looks like an orange star. Tonight, it’s directly above the Moon. Uranus is a little to the upper left of Mars. Mars will move closer to Uranus tomorrow, stand almost side by side with it on Monday, then begin pulling away on Tuesday.

More tomorrow.

Script by Damond Benningfield

Some of the oldest rocks on Earth aren’t from Earth at all. Instead, they’re pieces of an asteroid. They formed millions of years before Earth itself — at the dawn of the solar system.

The rocks fell to Earth 50 years ago today. They landed near Pueblito de Allende in Mexico, so they’re known as the Allende meteorites. And they’re some of the most thoroughly studied rocks in history.

The meteorites were part of a larger rock that slammed into Earth’s atmosphere at high speed. It formed a brilliant streak of light that was visible across thousands of square miles of Mexico and the United States. The stress of its flight through the atmosphere caused the rock to explode. Fragments rained across more than a hundred square miles.

Over the decades, scientists and collectors have recovered more than two tons of the meteorites. Most of the pieces are no bigger than a sugar cube. And scientists have studied them in labs across the world.

They found that the Allende meteorites are carbonaceous chondrites. That means they’re dark, and they contain a fair amount of iron. But they also contain veins of white minerals that are rich in calcium, aluminum, and titanium. These minerals were among the first to form in the solar system — 4 billion, 567 million years ago. And they’ve changed very little since their creation. So scientists are learning about the birth of the solar system from some of its earliest residents.

Script by Damond Benningfield

Lasers are good pointers. Teachers use them in the classroom, pet owners use them to rev up their cats, and other civilizations may use them to draw attention to themselves. And someday, it might be possible for us to do the same thing.

A student-led research team at the University of California-Santa Barbara has been looking for laser signals from M31, the Andromeda Galaxy. It’s high in the west at nightfall right now. Under dark skies, it’s visible as a small, hazy patch of light.

The California students have been using a network of telescopes to photograph M31 at different times. It’s a small enough target that they can take in the whole thing — hundreds of billions of stars and perhaps a trillion planets.

They’re looking for changes — for a bright light that flickers on and off. Such a light could be a signal from an advanced civilization — a beacon designed to announce its presence to the universe. Since M31 is two and a half million light-years away, though, that civilization could have vanished long before a signal could reach Earth.

Another study, by a student at MIT, found that we could send our own signals — just not to another galaxy. But using high-power lasers beamed through giant telescopes, we could let nearby star systems know that we’re here. And with the right equipment, we could reach out as far as 20,000 light-years or more — through a good bit of our own galaxy.

Script by Damond Benningfield

Astronomers rarely pay much attention to objects that are no more than half a mile long and that are out of sight within months of their discovery. When such an object comes from outside the solar system, though, they’re all in.

That was the case with ‘Oumuamua, which was discovered in late 2017, shortly after it passed closest to the Sun. Calculations of its path showed that it came from another star system, and that it would leave our own system in a hurry.

So astronomers tracked the object with telescopes on the ground and in space. They found that it probably was shaped like a cigar, that it was tumbling, and that it was brighter than the comets and asteroids in the solar system. And they found that it sped up after it passed the Sun — something that comets and asteroids don’t do.

Yet most studies have concluded that it probably was a comet or asteroid. As it got close to the Sun, some of its frozen gases vaporized. They rushed into space as “jets,” causing the object to tumble — and perhaps causing its acceleration. Some of the gas may have fallen back on to 'Oumuamua, coating it with fresh, bright ice.

One study, though, said the object could have an artificial origin. Its odd acceleration and other factors suggest that it was acting like a solar sail, which is pushed forward by sunlight. So the researchers said that perhaps ‘Oumuamua was a probe dispatched to study other star systems — including our own.

Script by Damond Benningfield

Most of the planets discovered in other star systems aren’t considered good homes for life. They’re too hot, too cold, or too “gassy” to support life as we know it. But a few planets have better chances. They’re rocky worlds, like Earth. And most important, they’re in the habitable zone. That’s the distance from the parent star where temperatures are just right for liquid water — a key ingredient for life like that on Earth.

Defining the habitable zone is a bit tough. It depends on the size and temperature of the star, for example. A big, hot star has a wide zone, although it’s a long way from the star. A small, cool star, on the other hand, has a narrow zone that’s quite close to the star.

And conditions that might be habitable for one planet might not be for another. Rocky worlds that are more massive than Earth, for example, have stronger surface gravity. That would allow them to hold on to a thicker atmosphere, which could hold on to more heat — extending the size of its star’s habitable zone.

Uncertainties in these numbers mean there’s some wiggle room in the number of habitable-zone planets. But most lists include several dozen planets — up to perhaps 60 or more. And the list is likely to just keep growing, as planet hunters find more and more worlds that could be just right for life — planets that live in the habitable zone.

We’ll have more about life in the universe tomorrow.

Script by Damond Benningfield