Scientists rewrite guide to Milky Way galaxy

By Ker Than

updated 1:16 p.m. ET Feb. 27, 2006

The Irish novelist George Moore once wrote, “A man travels the world in search of what he needs and returns home to find it.”

Astronomers sometimes follow a similar philosophy when they want to learn about our galactic home, the Milky Way.

Because Earth is located on the same plane as the Milky Way’s disk, astronomers can’t look down upon our galaxy to study it the way they can for others, like Andromeda. So for a long time, even basic things about the Milky Way, such as its shape and size, were difficult to determine.

Astronomers came up with a variety of ways to solve this problem. They invented tools that see in ways human eyes can’t, devised clever measuring techniques, and, as Moore suggested, they “travel.”

With penetrating telescopes, astronomers roam the entire universe, exploring billions of galaxies in their virtual spaceships. They take the lessons, some of them learned billions of light-years away and billions of years back in time, and apply them closer to home.

As a result, our picture of the Milky Way is constantly changing as technology improves and astronomers learn more about distant galaxies. The current picture is richer than even just a few years ago as astronomers have filled in knowledge gaps and added new details.

They’ve recently learned, for example, that the mysterious dark matter saturating our galaxy is actually “warm,” and they verified by various indirect means the existence of a supermassive black hole at its center. Studies have also shown that the Milky Way is more massive, more crowded and its stars more lonely than previously thought.

If our virtual travelers could fly home from distant galaxies, approaching the Milky Way from afar and then soaring to its center, here is what they would find.

First stop: The halo
The Milky Way is a member of a collection of more than 50 galaxies called the Local Group. In terms of space occupied, Andromeda, or M31, is the biggest galaxy in this posse, but the Milky Way is the most massive.

Were an intergalactic traveler to approach the Milky Way edge-on, the first thing she would notice is a luminous halo made up of gas and stars enveloping the galaxy. The halo is about 100,000 light-years in diameter and 1,000 light-years thick.

A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion km).

This halo contains 170 orbiting star clusters and about a dozen small galaxies. The gravitational tug of the Milky Way is so great that it can sometimes tear these passing satellites apart, stripping them of gas and even stars. One star cluster, Messier 12, is thought to have been robbed of as many as a million stars in this way.

Orphan stars stripped from their parent galaxies and clusters form streamerlike “tidal tails,” or else they linger in the galactic halo, where they intermingle with other lone stars. These other stars are mostly ancient, around 12 billion years old and older, and they don’t rotate around the galactic center in any organized way.

Orbiting satellites can also affect the shape of the Milky Way. According to one hypothesis, the strange warp in the Milky Way’s hydrogen disk is caused by the movement of two dwarf galaxies—the Large and Small Magellanic Clouds — and their interactions with dark matter as they orbit our galaxy.

Dark matter is an unknown sort of material that has never been seen. Astronomers know it permeates our galaxy and others because the collections of stars could not hold together without some other, invisible source of gravity.

Next stop: The spiral disk
Astronomers estimate that the Milky Way contains about 100 billion stars. Recently, however, this number was upped by about a billion after the discovery that very old, nearly invisible stars had escaped earlier detections.

Most of the Milky Way’s stars are concentrated in a main disk, which lately has been described as a series of disks, none of which are entirely distinct, but instead overlap one another. The largest is known as the thick disk; this disk is fairly flat and spirals like a slow-spinning hurricane because of our galaxy’s rotation.

Nestled within the thick disk is an even flatter disk of stars, known as the thin disk. The stars in this thin disk rotate even faster around the galactic center than those in the thick disk.

Further in is yet another disk, known as the extreme disk, where stars and clouds of gas are moving fastest of all.

NASA Scientists believe our sun lies in a relatively short spur from one of the four major arms known to exist in the Milky Way. The four arms are called Perseus, Sagittarius, Norma and Scutum-Crux. Our region is called the Orion Arm or the Local Spur Arm.

Our sun, which is 4.6 billion years old, is located 26,000 light-years away from the galactic center on one of the spiral arms. It is a location considered more suitable than others for harboring life, in part because the central region is too chaotic, and in part because the concentration of metals there is too heavy, and it’s too light in the galaxy’s outer fringes.

The sun makes one complete orbit around the galaxy about once every 225 million years. In contrast, stars near the galactic center complete a lap in a few million years or less. These stars as a group tend to be younger than the galactic average, most ranging in age from 1 billion to 10 billion years old.

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