The Milky Way: A Tourist's Guide

What a difference a couple of years make. Buzz Lightyear has given up on the whole "infinity and beyond" thing, instead focusing on his own backyard. (Actually, the front yard of his owner, Andy, and the toys that get stolen therefrom.)

In the same time frame, astronomers have undergone a similar transition of thinking.

Once looking far off into the deep black in search of answers to black holes, the big thinkers of space have returned, if not to Earth, at least to the cosmic backyard, solidifying the theory that there's a big ol' gravity monster just 26,000 light-years away, in the very center of our home galaxy. (One light-year is equal to 5.88 trillion miles, or 9.46 trillion kilometers.)

The revelation has researchers scrambling to study what is by far the closest black hole around. It also raises a simple question: What do we really know about our own galaxy?

In seven separate articles in the January 7 issue of the journal Science, all of this new thinking is discussed.

Were an intergalactic traveler to approach the Milky Way edge-on, she would first encounter a vast but sparsely populated galactic halo, with a few lone stars and about 170 star clusters, plus various clouds of gas, all of which does not rotate in sync with the rest of the galaxy.

A less obvious aspect to this halo would be an incredible amount of dark matter, stuff that can't be seen but is thought to comprise the bulk of the Milky Way's mass.

Our dauntless explorer would find that the stars in the halo are mostly ancient -- 12 billion years old and older. They would not appear to rotate around the galactic center in any organized manner, though gravitationally bound -- as such, the stars in the halo are moving every which way.

These old stars and clusters, and the streaming gas clouds, tell many stories. For one, they may represent the age of the galaxy itself.

They are also evidence of galaxy gobbling. Researchers are now convinced that the Milky Way has built itself up over the eons, in part, by swallowing smaller galaxies. Streams of stars and clouds of gas represent the remnants of this galactic feasting.

Recent evidence also shows that dwarf galaxies are still being pulled in. The spherical galactic halo, constituting the outermost reaches of the galaxy, is the Ellis Island for all these incoming galaxies.

"Direct and indirect evidence now available of such events leaves no doubt that the accretion of satellites and fragments into the galaxy's halo has been a continuing process since early in its formation," writes Roland Buser in one of the Science articles.

The feast continues. Streams of gas extending in front of and behind two nearby galaxies, the Large and Small Magellanic clouds, show that the galaxies are being distorted as they pass through our galaxy.

The two galaxies are expected to spiral into the Milky Way for the next 10 billion years.

Another nearby galaxy, the Sagittarius dwarf, is also orbiting our own, stretched into a long, odd shape by the Milky Way's gravity. The tiny Sagittarius, one-thousandth the size of our galaxy, has only about 750 million years to go before it is swallowed whole.

And at a meeting of the American Astronomical Society next week, scientists will present further evidence that miniature galaxies, which may be leftovers from the earliest days of the universe, are raining into the Milky Way continuously, providing fuel for new star formation.

Next, our traveler would encounter a more heavily populated, fairly flat disk of gas and stars known as the thick disk. Like the outer bands of a hurricane, this pancake of stuff would be seen spiraling slowly around the center of the galaxy.

Further in, the visitor would find a flatter disk of stars, like a thin pancake nestled inside the thicker one. The stars in this thin disk rotate even more rapidly around the galactic center, hence the more compact nature of this inner pancake. If our traveler knows her celestial coordinates, she might run across our own sun soon after entering this thin disk.

Further in is yet another disk, known as the "extreme disk." Stars and clouds of gas are moving even more rapidly. The stars here are younger, ranging in age from 1 billion to 10 billion years old.

Researchers suspect the galaxy formed out of a simple thin disk, building itself up via mergers with smaller galaxies and gradually developing the more complex spiral that is nestled inside the present halo. None of these regions is entirely distinct, but instead they all overlap.

Approaching the middle now, feeling a greater pull of gravity, the explorer would find a "central bulge" of stars, all obscuring the unexplained center of our galaxy, to which the far more numerous stars are bound ever more strongly.

At the center of our Milky Way is, almost certainly, a gigantic black hole from which our traveler, however intrepid, would want to stay clear. Researchers have suspected the matter-gobbling monster, called Sagittarius A*, for 25 years, but only recently have observations confirmed its existence -- though still not by any direct methods.

"In the last two years, scientists have firmed up evidence for this super-sized black hole, which has swallowed enough gas and dust to equal the mass of more than two million suns," writes Erik Stokstad in another Science article.

Stokstad notes that while it is huge, our black hole is small compared to others, which can be billions of times as massive as the sun.

The evidence for the black hole involves measurements of stars that orbit at incredible speeds -- 50 times faster than Earth orbits the sun -- in a tightly packed area of the central bulge. The gravity required to keep these stars in such a fast, tight orbit is calculable, and the tiny area into which it must fit indicates that it has to be a black hole, experts say.

But researchers have yet to figure out how the black hole works and where all the matter and energy goes once it is sucked inward. Seeing things in the center of our galaxy is difficult -- it's all obscured from our view by the central bulge -- and little evidence exists showing the outflow of high-energy radiation that has been seen coming from other black holes.

Researchers suspect the Milky Way's black hole formed early on, when a large cloud of gas collapsed on itself, Stokstad writes. It probably started out small, growing over billions of years while gradually feeding on inflowing clouds of gas.

Periodically, stars would be sucked in and swallowed in dramatic fashion. A star would be ripped apart and a flash of radiation would briefly outshine all the rest of the stars in the Milky Way combined.

Are we safe from this monster? Quite, Stokstad writes. While tremendous, the gravity of a black hole is not strong enough to pull in stars from the far reaches of its galaxy.

If it seems as though scientists are painting a pretty complete picture of the galaxy, consider this: The movements of stars tell researchers that they have yet to find about 95 percent of the mass in the Milky Way.

Much of the dark matter, as researchers call it, is thought to exist in a vast sphere that surrounds the visible portions of the galaxy. It could be composed of ancient dwarf stars that can't be seen or, as some suggest, it might exist in the form of small, elementary particles that aren't yet understood.

By figuring out the composition and role of dark matter, and understanding how the central black hole works, researchers will begin to put some of the final pieces of our galactic puzzle together.

The Chandra X-ray Observatory, launched last year, is one of three new telescopes that will probe the Milky Way for clues to all this.

While our home galaxy has become what it is by picking off smaller galaxies, it appears we are due for a dose of our own medicine.

Right now, the Andromeda Galaxy and the Milky Way are rushing toward each other at more than 310,000 miles (500,000 kilometers) per hour, explains Robert Irion in another Science article.

The pace will pick up as the 2.5 million-light-year gap closes.

"The momentum of the galaxies will carry them past each other initially, but the gravitational attraction of their dark matter halos will doom them to coalesce," Irion writes. The result won't be pretty. Andromeda is twice as big as the Milky Way and it will "distort our galaxy beyond recognition."

Irion says we've got between 1 billion and 3 billion years before our beloved spiral becomes a wrecked ellipse.

Meanwhile, colliding gas clouds will form new suns. And our own sun, along with the nine planets, might be cast off into virtual nothingness, so far away from the activity that the night sky on Earth would no longer be full of stars.

Worse, we could be flung inward, where the night would be bright as day from all the exploding stars, and where the intense radiation might render life on our formerly hospitable world impossible.

For either scenario to have any effect on Earth, our planet would have to still be around when the galactic merger takes place. And that's not certain.

Many experts think our own sun may have already exploded by then, turning Earth and any remaining Earthlings back into dust anyway. Or, as other theories suggest, our sun might have 6 billion years left to it.