The halo

The halo is perhaps the least understood part of the Galaxy, even though, at first glance, it appears to be mostly empty.

Hundreds of globular clusters float in the Milky Way's halo, from the galactic center out to the edge of the stellar halo. (See chapter 2, figure 2.6, for an illustration of a globular cluster.) These systems contain hundreds of thousands or millions of stars — as many as in a small galaxy — in a compact volume only a few hundred light-years across, which makes them very luminous. Their beauty derives from their spherical symmetry: they are as round as dandelion puffballs, often with bright centers and more diffuse edges. Their orbits around the galactic center cause many of them to plunge through the plane of the disk, then to swing slowly out into comparatively empty space. Isolated or ''field'' stars in the halo may have come from the gradual dispersal of some globular clusters, early in the formation of the Galaxy.

The stars that populate the halo, either as field stars or as members of globular clusters, are generally much older than the stars in the disk. While some disk stars may be 10 billion years old, those in the halo are at least 13 billion years old, and that difference is significant. The halo stars formed almost exclusively from the primordial elements of hydrogen and helium, present when the Galaxy itself was young. The disk stars that we currently see, on the other hand, incorporated material from earlier generations of stars as they formed. Stars burn hydrogen and helium in their nuclear furnaces, creating more complex elements, and spew that material into interstellar space when they die. In this way, the stars in the disk, having formed more recently than the halo stars and in a more densely populated environment, have had a chance to enrich their chemical composition.

The advanced age and primitive composition of halo stars is not the only interesting aspect of the halo. While a substantial fraction of the matter in the disk is luminous — i.e., in the form of stars—most of the halo matter is dark. The halo stars, it turns out, contribute a meager fraction of the halo mass.

We can infer the presence of dark matter in the Galaxy through its effect on luminous matter in the disk and on the stellar component of the halo. Dark matter, whatever it may be, shares one fundamental characteristic with ordinary matter in dust, gas and stars: it exerts a gravitational force. Furthermore, the distribution of dark matter in space—how concentrated it is around the center of the Galaxy—affects the speed of stars and gas clouds in the disk as they orbit the center of the Galaxy. Charts of disk-component speeds, called rotation curves, allow us to determine the amount of unseen matter as a function of distance from the center of the Galaxy. The Milky Way's rotation curve shows that at least 90% of the matter composing it is dark, and that most of the dark matter must be in an extended halo that reaches beyond even the limit of the globular clusters, perhaps as far as a million light-years. Figuring out what the dark or missing matter is, is one of the great challenges astronomers currently face.5

0 0

Post a comment