New work finds that the Milky Way stretches
nearly 2 million light-years across, more than 15 times wider than its luminous
spiral disk. The number could lead to a better estimate of how massive the
galaxy is and how many other galaxies orbit it.
Background
Astronomers have long known that the
brightest part of the Milky Way is some 120,000 light-years across.
Beyond this stellar disk is a disk of gas. A vast halo of dark matter,
presumably full of invisible particles. But because the dark halo emits no
light, its diameter is hard to measure.
Details
Ø Scientists have used nearby galaxies to
locate the Milky Way’s edge. The precise diameter is 1.9 million
light-years, give or take 0.4 million light-years.
Ø To find the Milky Way’s edge, team conducted
computer simulations of how giant galaxies like the Milky Way form. In
particular, the scientists sought cases where two giant galaxies arose side by
side, like the Milky Way and Andromeda, our nearest giant neighbor,
because each galaxy’s gravity tugs on the other.
Ø The simulations showed that just beyond the
edge of a giant galaxy’s dark halo, the velocities of small nearby
galaxies drop sharply.
Ø Using existing telescope observations, they
found a similar plunge in the speeds of small galaxies near the Milky Way. This
occurred at a distance of about 950,000 light-years from the Milky Way’s
center, marking the galaxy’s edge. The edge is 35 times farther from the
galactic center than the sun is.
Ø In the future, astronomers can refine the
location of the Milky Way’s edge by discovering additional small galaxies
nearby. Astronomers could also search for individual stars out at the boundary.
Ø The measurement should also help astronomers
tease out other galactic properties. For instance, the larger the Milky Way,
the more massive it is and the more galaxies there should be revolving around
it.
Ø So far, there are about 60 known Milky Way
satellites, but astronomers suspect that many more await discovery.
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