Dark Matters

Star Words

The dark halo is the region surrounding the Milky Way and

Taking this approach, we find that the mass of the Milky Way within 15,000 parsecs of the Galactic center—that is, the radius of the visible galaxy (diameter ~30,000 parsecs)—is 2 x 1011 solar masses.

Star Words

We would expect the mass of the Galaxy to drop off precipitously when we run out of matter—at the visi ble outer edge of the Galaxy. But the puzzling fact is that more mass is contained beyond this boundary

The dark halo is the region surrounding the Milky Way and than within it!

other galaxies that contains dark matter. The shape of the dark halo can be probed by examining in detail the effects its mass has on the rotation of a galaxy. Dark matter is a catch-all

Within a radius of 40,000 parsecs from the Galactic center, the mass of the Milky Way is calculated to be about 6 x 1011 solar masses. This means that there is as much, if not more, of the Milky Way unseen as there is seen. Spiral galaxies like the Milky Way have mass-to-light ratios ranging up to 10, even in the visible part of the disk.

phrase used to describe an apparently abundant substance of unknown composition.

What is all this other mass?

Whatever its makeup, it apparently emits no radiation of any kind—no visible light, no X-rays, no gamma radiation. But it cannot hide completely. We see it simply because it has mass, and its mass is affecting the way in which the stars and gas of the Milky Way orbit.

Astronomers call the region containing this mass the dark halo. And the Milky Way is not alone in possessing such a region. Many, if not all, galaxies have the same signature in the rotation of their stars and gas. The dark halo presumably contains dark matter, a catch-all term that is used to describe a variety of candidate objects. The truth is, dark matter is a bit like Spam™.We're not sure what it is, but we know it's there because we can see its effects.

The nature of dark matter remains one of the greatest mysteries of science. Some astronomers have suggested that difficult-to-detect low-mass stars (brown dwarfs or faint red dwarfs) may be responsible for the mass in this region—although recent Hubble Space Telescope observations have suggested an insufficient quantity of such objects to account for so much mass. It has recently been established that neutrinos do have non-zero mass, so their presence might contribute. Yet others propose that dark matter consists of massive neutrinos hitherto unknown subatomic particles, which pervade the universe.

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