StarDate

Something small, dark, and heavy lurks at the heart of the Milky Way Galaxy. Most astronomers say it’s a black hole. But a recent study says “not so fast”- it could be a clump of dark matter.

The central object is called Sagittarius A-star. It’s about 4.3 million times the mass of the Sun. A few years ago, astronomers took a picture of its “shadow” against a glowing background.

Sagittarius A-star is encircled by stars and dusty clumps in tight orbits. They’re accelerated to millions of miles per hour by the gravity of the central object.

The new study suggested that object could be a knot of dark matter particles known as fermions. Dark matter produces no detectable energy, but its gravity pulls on the visible matter around it. It appears to make up about 85 percent of all the matter in the universe, but its nature remains unknown. The clump could account for many of the observed qualities of the central dark object.

Other studies have suggested that dark matter could produce fountains of gamma rays that shoot from the galaxy’s core. Dark-matter particles could produce the gamma rays when they ram together and cancel each other out.

The dark-matter model doesn’t explain all of the evidence of a black hole. But future instruments should be able to discern between the two models – providing a definitive explanation for the darkness at the galaxy’s heart.

More about dark matter tomorrow.

Script by Damond Benningfield

Our solar system is passing through one of the spiral arms that makes the Milky Way Galaxy look like a pinwheel. But the key words there are “passing through.” The solar system moves through the galaxy a little faster than the spiral arms do. So over hundreds of millions of years, we cross all of the galaxy’s major arms. And those crossings could be dangerous.

A spiral arm is a region where a passing wave squeezes giant clouds of gas and dust, triggering the birth of new stars. Many of the stars are especially hot and bright, so they light up the spiral arms.

The star-forming clouds are dense and turbulent. And a recent study suggested that could be where the “danger” comes in.

Researchers looked at tiny crystals in Earth’s crust. They found that the composition of the crystals varied over periods of hundreds of millions of years.

During some of those periods, Earth’s crust appeared to be especially hot. And the timing of those periods may correspond to passages through the spiral arms.

As we move through the dense clouds in the arms, big balls of ice and rock far from the Sun could be nudged inward. Some of them could ram into Earth, creating conditions that could account for some of the crystals. So while the Milky Way’s spiral arms may be beautiful, they may also be deadly.

Look for the glowing band of the Milky Way curving across the east as night falls, and arching high overhead later on.

Script by Damond Benningfield

The Milky Way shines at its best on summer nights. Right now, it arcs across the east as the sky gets fully dark, and passes high overhead later on. Under dark skies, it looks like a hazy band of light.

That band outlines the disk of our home galaxy. So for the astronomers who study the Milky Way Galaxy, it’s the go-zone – there’s lots to look at. But for those who study other galaxies, it’s been the no-go zone. In fact, it’s called the Zone of Avoidance, because it’s hard to see anything through it.

The main problem is giant clouds of dust scattered throughout the galaxy. The dust absorbs visible light. Depending on which part of the disk you look through, in fact, the clouds can block more than 99 percent of the light from objects behind them.

The other problem is that the Milky Way is crowded – millions upon millions of stars everywhere you look. So when you look into the band of the Milky Way, it’s hard to know whether you’re seeing a star or gas cloud in the galaxy or something beyond it.

Fortunately, some wavelengths that are invisible to the human eye do get through: infrared light and radio waves. The infrared is best seen from space, but the radio can be turned in by giant antennas on the ground. Galaxies typically emit more of both of those forms of energy than individual stars do – important ways to avoid problems from the Zone of Avoidance.

More about the Milky Way tomorrow.

Script by Damond Benningfield

It’s hard to map a forest when you’re standing in the middle of it. You see the trees that are close by, but most of the forest is blocked out. Astronomers have faced the same challenge when trying to map the Milky Way Galaxy. We’re right in the middle of it, surrounded by bright stars and dark dust clouds. So we can’t get an overall picture of the whole thing.

But nature has provided a way to see the forest through the trees: galactic radio. Big clouds of hydrogen gas emit radio waves at a wavelength of 21 centimeters – eight and a quarter inches. The radio waves pass through the intervening material, giving us a good outline of the structure of the Milky Way.

That wavelength is produced when hydrogen atoms get “bumped up” to a higher energy level. When the atoms drop back to their base level, they emit radio waves. This process plays out most commonly in clouds where new stars are being born.

Mapping the clouds revealed that the Milky Way is a spiral galaxy – a beautiful cosmic pinwheel. And measuring the motions of the clouds reveals how that pinwheel spins. So a lot of what we know about the Milky Way has come to us through the broadcasts of “galactic radio.”

The Milky Way arcs across the east as night falls. You need dark skies to see it. The center of the galaxy is in Sagittarius, which is low in the southeast. It’s easy to pick out because its stars form the outline of a teapot.

Script by Damond Benningfield

The center of the Milky Way Galaxy is in good view as night falls this evening. And as the night ends, at dawn tomorrow, the galactic “anticenter” is in view – the point directly opposite the center.

The Milky Way is our home galaxy. It’s a disk about a hundred-thousand light-years wide. Earth is about half-way between the center of the disk and its rim. In the night sky, the disk forms the faint path called the Milky Way. But you need nice dark skies to see it.

The center is in the constellation Sagittarius. Its most prominent stars form the outline of a teapot. Puffs of “steam” appear to rise from the spout of the teapot. The center of the galaxy is immersed in the steam.

We can’t see the center because intervening clouds of dust absorb its light. But if we could see it, it would be impressive. Billions of stars are jammed together – far more tightly packed than in our region of the galaxy.

The anti-center is in Taurus, which is low in the east at dawn. That point is marked by the star Elnath. It’s the bull’s second-brightest star, at the tip of one of his horns. And it’s easy to spot tomorrow because it’s quite close to the crescent Moon.

Except for Elnath, there’s not much to see in that direction. We’re looking toward the galaxy’s thinly settled outer precincts, with intergalactic space beyond. Enjoy the panorama of the Milky Way – our galactic home – all night long.

Script by Damond Benningfield