StarDate

A galaxy merger can be messy. Stars in the two galaxies are stirred up like chocolate chips mixed into a batch of cookie dough. Some stars are tossed out of the galaxies, while gas clouds slam together to give birth to new stars. And the central black holes of the two galaxies spiral together, setting off their own fireworks when they merge.

Such a merger appears to be underway in a system in Ursa Major, the great bear, which is high in the north on May evenings. The black holes are a few thousand light-years apart, so any merger is still millions of years away. But they reveal something about the individual galaxies in the merger.

The system is known as J1126+2944, for its coordinates in the sky. One of its black holes appears to be hundreds of times more massive than the other. That suggests that this isn’t a merger of equals. Instead, one galaxy may be big and heavy, like our own galaxy, the Milky Way. The other may be a dwarf galaxy, only a fraction the size of its partner.

If so, then its center may contain an intermediate-mass black hole — one that’s perhaps tens of thousands of times the mass of the Sun. So far, there’s evidence of only a handful of these black holes. It’s possible that they’re the “seeds” from which supermassive black holes grow — the monsters that inhabit the hearts of big galaxies. In fact, its merger with the other black hole will make that black hole even bigger — adding to the heft of a monster black hole.

Script by Damond Benningfield

A dead star in a distant cluster is facing a fate worse than death: complete annihilation. That’s because the star may orbit just 600,000 miles from a black hole — closer than any other black-hole companion yet seen.

This odd system is known as 47 Tucanae X9. It’s in a globular star cluster in the southern constellation Tucana, the toucan. The cluster is about 15,000 light-years from Earth.

Astronomers have watched X9 for years because it’s one of the brightest sources of X-rays in the cluster. X-rays are an indication that something powerful is going on.

Recent observations with the space-based Chandra X-Ray Observatory suggest that the system consists of a white dwarf and a black hole. A white dwarf is the dead core of a once-normal star. It’s about as big as Earth, but almost as heavy as the Sun.

The recent X-ray observations show that the likely white dwarf orbits the black hole about twice an hour, which tells astronomers that the two are quite close.

They’re so close, in fact, that the black hole appears to be stealing gas from the surface of the white dwarf. This material forms a wide, thin disk around the black hole. As material in the disk spirals toward the black hole, it gets extremely hot, so it emits X-rays.

Over time, the black hole could consume most of the white dwarf, leaving behind an even denser knot of material. On the other hand, it might eat the whole thing — cannibalizing its unlucky companion.

Script by Damond Benningfield

Different stars face different fates. The smallest will shine feebly for trillions of years — many times longer than the current age of the universe — then simply fade away. The heaviest, on the other hand, will blast themselves to bits, briefly shining brighter than billions of normal stars.

A bright star that faces such a violent end is in good view tonight. Spica stands close to the right of the Moon as night falls, with the brilliant planet Jupiter above them.

Spica actually consists of two stars.

The heavier star, Spica A, is about 10 times the mass of the Sun. Such stars burn through their nuclear fuel in a hurry. That makes them extremely bright. But it also means they won’t live long. Spica A, for example, will live a “normal” lifetime of less than 30 million years, compared to about 10 billion years for the Sun. When it can no longer produce nuclear reactions in its core, the core will collapse. The star’s outer layers will fall inward, then rebound violently, blasting the star apart as a supernova.

Its companion, Spica B, is about six times the Sun’s mass. Assuming it’s not destroyed by the nearby supernova, it’ll live more than a hundred million years. At the end of its lifetime, it’ll swell up to many times its current size, just as Spica A will. But it’s not massive enough to explode. Instead, it’ll lose its outer layers in a less-violent process. That’ll leave only its dead core, shining feebly through the long cosmic night.

Script by Damond Benningfield

Most of the solar system’s planets are surrounded by radiation belts — zones where charged particles are trapped by the planet’s magnetic field. Earth, for example, is encircled by the Van Allen belts. Spacecraft that pass through these belts need special shielding to protect their electronics.

But the most powerful belts encircle Jupiter, the giant of the solar system. Its magnetic field is far stronger than Earth’s, so it traps more charged particles from the Sun. Other particles come from its volcanic moon Io. These sources create radiation belts that could kill an unprotected person in hours.

A spacecraft that’s currently orbiting Jupiter has special protection against the radiation. Juno’s computer and much of its other electronics are housed inside a titanium vault. The vault is about as big as the trunk of an SUV, and its walls are almost half an inch thick. And even with that extra level of protection, Juno’s orbit is carefully controlled to keep the craft out of the most dangerous radiation zones.

Juno is measuring Jupiter’s magnetic and gravitational fields. Its readings should help scientists determine how Jupiter is put together. And that will help them learn more about how it generates its magnetic field — creating “dead zones” around the giant planet.

Jupiter is in good view tonight. It stands quite close to the Moon, and looks like a brilliant star. The true star Spica stands below the pair. More about Spica tomorrow.

Script by Damond Benningfield

The Moon swings past a couple of bright companions the next few nights — the planet Jupiter and the star Spica. Jupiter is the solar system’s largest planet, and looks like a brilliant star to the lower left of the Moon as darkness falls tonight. Fainter Spica, the leading light of Virgo, stands below Jupiter.

The gap between Jupiter and Spica will stay about the same for another couple of months. As we get deep into summer, though, Jupiter will start moving toward Spica. And by the start of autumn, it’ll have passed the bright star, and will be leaving it behind at a pretty good clip.

That changing gap is the result of Jupiter’s orbit around the Sun, and our changing viewing angle on the planet.

It takes Jupiter about 12 years to complete one full turn against the background of stars, so it moves slowly across the sky. That means it can linger near a particular star for a good while.

But Jupiter’s position as seen from Earth also depends on Earth’s orbit. We follow a smaller, faster path around the Sun, so we pass by Jupiter once every 13 months or so. In fact, we just passed it last month.

As we pass Jupiter, it appears to stop its normal eastward motion across the sky, and actually move backwards for a while. When Earth gets far enough ahead, though, Jupiter once again resumes its eastward crawl. So the changing gap between Jupiter and Spica is the result of the orbital motions of two planets.

More about Jupiter and the Moon tomorrow.

Script by Damond Benningfield

Spring is already halfway through here in the northern hemisphere. But the season is just beginning in the northern hemisphere of Mars. Today is the vernal equinox on the Red Planet. The Sun crosses the planet’s equator from south to north, ushering in spring.

Mars has seasons for the same reason that Earth does: the planet is tilted on its axis. In fact, Mars is tilted at almost the same angle as Earth. So as the planet orbits the Sun, the northern and southern hemispheres take turns receiving more sunlight.

But the Martian seasons are a bit more complicated than those on Earth. That’s because Mars’s orbit is much more lopsided than Earth’s. The planet’s distance from the Sun varies by about 25 million miles, compared to only about three million miles for Earth. So when Mars is closest to the Sun, it receives a lot more energy than when it’s farthest from the Sun.

Right now, Mars is near the middle of that range in distance. As the northern spring progresses, though, Mars will move farther from the Sun. It’ll then start moving closer, and get closest to the Sun near the end of autumn. As a result, northern winters and summers are much more temperate than the southern seasons.

As the distance to the Sun changes, so does Mars’s orbital speed. That means there’s a good difference in the length of the seasons. Northern spring is the longest season. It lasts more than six months, so it’ll continue until the middle of fall here on Earth.

Script by Damond Benningfield

Most of the matter that passes too close to a black hole will get a one-way trip to oblivion: It’ll vanish into the black hole. But some of that matter may instead get a quick trip back out into space. That’s because many black holes produce jets of charged particles. The biggest of them can span thousands of light-years, and can squirt away at close to the speed of light.

In fact, similar jets are found in other astronomical objects — from infant stars to dead stars.

Scientists still aren’t certain just how these jets form. But the process involves disks of gas and dust and strong magnetic fields. The material in a disk spirals toward the central object — a young star, for example, or a dead star such as a white dwarf, a neutron star, or a black hole.

Friction in the disk heats the material enough to rip atoms apart, creating streamers of charged particles. Magnetic fields then grab some of these particles and shoot them into space from the poles of the central object as narrow high-speed jets.

Those from young stars can stretch across billions of miles. But those from supermassive black holes at the hearts of galaxies can span thousands of light-years. And the black hole’s powerful gravity can accelerate them to much higher speeds — in some cases, just a bit below lightspeed.

As these jets ram into material in the galaxy around the black hole, they produce shockwaves and other dramatic effects — the fate of matter escaping from a black hole.

Script by Damond Benningfield

Regulus is one of the brightest lights in the night sky. But just how bright depends on how you look at it.

Seen with the eye alone, the heart of the lion ranks as the 21st brightest star in the night. On the astronomical scale, it has a visual magnitude of about 1.4. In this scale, the brighter an object, the lower the number. The brightest star in the night sky, Sirius, has a magnitude of minus 1.4. And the faintest stars visible under a dark sky have a magnitude of about six, which is only about a thousandth as bright as Sirius.

Since the stars are all at different distances from Earth, though, that number doesn’t tell you a star’s true brightness. For that, astronomers calculate its absolute magnitude. That’s how bright the star would look at a distance of 10 parsecs — about 32 light-years.

In this scale, Regulus has a magnitude of point five. So if you lined up Regulus and the Sun at that distance, Regulus would be more than a hundred times brighter.

Even that doesn’t give you the star’s total brightness, because it counts only a star’s visible light. But stars emit many other forms of energy as well. Because Regulus is quite hot, for example, it produces a lot of ultraviolet energy. When you add that to the visible light, the star is about 350 times brighter than the Sun. So Regulus is a brilliant beacon any way you look at it.

And you can get a good look at brilliant Regulus tonight, standing just a whisker away from the Moon.

Script by Damond Benningfield

At first glance, the largest resident of the asteroid belt looks kind of dull. Its brownish surface is pockmarked by thousands of impact craters, much like the Moon and many other bodies — a world of little interest.

But a more detailed look reveals some remarkable features. There’s a mountain made of ice, for example, along with deposits of salts that may have bubbled up from an ocean below the surface.

Ceres is about 600 miles across — a quarter the diameter of the Moon. And it contains more than a quarter of all the material in the asteroid belt — a ring of debris between the orbits of Mars and Jupiter.

The Dawn spacecraft has been orbiting Ceres for more than two years. Its observations have supported the idea that there’s a lot of water in Ceres. Much of it is mixed with a layer of rock below the crust. But some could form a subsurface ocean.

Some of that water could have bubbled to the surface just four million years ago, inside a wide crater. The water quickly evaporated. But it left behind mineral deposits that are almost pure white — the brightest features on the entire world.

Water also may have formed the largest mountain on Ceres. It’s two-and-a-half miles tall, and its slopes are bright and smooth. An impact by an asteroid may have cracked the crust halfway around Ceres, allowing a mixture of water, ice, and minerals to push up from below on the opposite hemisphere. That built a mountain of ice on this intriguing little world.

Script by Damond Benningfield

The realm of the planets is neatly divided into two regions. There’s the region close to the Sun, which contains the small, rocky planets: Mercury, Venus, Earth, and Mars. And there’s the region far from the Sun, with the giant planets Jupiter, Saturn, Uranus, and Neptune.

The two regions are separated by a “dotted line”: the countless bits of rubble that make up the asteroid belt.

When the first asteroids were discovered, more than two centuries ago, many astronomers thought they were the remains of an exploded planet.

It turns out, though, that nearby Jupiter prevented the asteroids from clumping together to make a planet. Its powerful gravity stirred things up so much that when asteroids hit each other, instead of sticking together, they blasted each other apart. Much of the debris from these collisions escaped the asteroid belt. That depleted the supply of planet-making materials. Instead of a planet, the leftovers formed a wide, thin ring around the Sun.

Hundreds of thousands of asteroids have been found so far, with tens of millions more likely awaiting discovery. They don’t add up to much, though — only a few percent of the mass of the Moon.

And despite what you see in the movies, the asteroid belt is mostly empty space. The average distance between asteroids is a million miles or more. In fact, many spacecraft have flown through the asteroid belt with no trouble at all — unmolested by the debris from a planet that never was.

Script by Damond Benningfield