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Milky Way Halo
There’s a lot more to the Milky Way than meets the eye.
Most of what we think of as the Milky Way galaxy is contained in a thin disk of stars and gas that’s about a hundred thousand light-years wide. But there’s a lot more material outside the disk than inside.
The region outside the disk is known as the halo. It forms a ball that completely surrounds the disk, and it’s several times wider than the disk.
There aren’t all that many individual stars in the halo. But the halo is home to the galaxy’s globular clusters — tightly packed balls of hundreds of thousands of stars. Most of the clusters lie outside the disk, but even those that are inside the disk are just passing through — they belong to the halo.
The stars in globular clusters are the oldest in the entire Milky Way; we’ll have more about that tomorrow.
A second component of the halo is hot gas. It produces an X-ray glow that was discovered a few years ago. The gas may equal the mass of all the stars in the galaxy — doubling the amount of the Milky Way’s “normal” matter.
Most of the stuff that makes up the Milky Way is dark matter, which is the halo’s main component. Dark matter produces no energy, but its gravity tugs at the visible matter around it. It may consist of subatomic particles that are still awaiting discovery.
The dark matter is several times more massive than all the galaxy’s stars and gas combined — so there’s a lot more to the Milky Way than meets the eye.