-
- The more astronomers learn about the
universe, the less exalted our place in it seems.
-
- Ever since Nicolaus Copernicus in the
1540s demonstrated that the Earth revolves around the Sun, and so is not
the center of the universe, the prominence of our blue-green planet has
faded quickly.
-
- In studying the stars, astronomers came
to realize that the sun is a garden-variety specimen -- not especially
large or small -- and that our solar system resides in a galactic backwater
out toward the end of one of the spiral arms of the Milky Way.
-
- In the 1920s, astronomers discovered
that the fuzzy smudges they spotted beyond the stars were not clouds of
gas, but separate galaxies, each containing billions of stars. Still, their
initial measurements suggested that among galaxies, the Milky Way was unusually
large: perhaps the biggest, at least in its class of spiral galaxies.
-
- Now astronomers from Great Britain, using
the Hubble Space Telescope to measure distances to other galaxies, have
burst that bubble too.
-
- Calculating the diameters of galaxies
based on new, more accurate measurements of their distance from Earth,
astronomers Simon Goodwin, John Gribbin and Martin Hendry declared that
the Milky Way is not especially big. In fact, the average size for spiral
galaxies now appears to be slightly bigger than the diameter of the Milky
Way.
-
- As astronomer Jay Gallagher of the University
of Wisconsin-Madison put it: "It looks like we're kind of a Chevy
of a galaxy."
-
- The measurements were made possible by
Hubble's ability to resolve more clearly than ever before individual stars
called Cepheid variables in distant galaxies. Cepheids are special kinds
of stars that vary in brightness in regular pulses that astronomers have
come to use as astronomical measuring rods.
-
- Mostly, they have been used to measure
distances within the Milky Way. In recent years, astronomers have spotted
Cepheid variables in other galaxies, but the measurements derived from
them have been relatively crude because of the limited resolving power
of Earth-based telescopes.
-
- "It's as if you were sitting on
the moon looking at the Earth and you detected a glow along Lake Michigan,
but you couldn't detect whether it was Milwaukee, Racine or Chicago,"
said Gallagher.
-
- The same applies to Cepheids in distant
galaxies. "With Hubble," Gallagher said, "you can separate
them. You can pick out the individual Cepheid stars. That's important in
judging distance."
-
- The British astronomers selected 17 spiral
galaxies where astronomers working on Hubble had identified Cepheids and
measured their diameters. They found two that were at least twice as big
as the Milky Way and others that were smaller.
-
- The findings, they wrote, support the
notion that "there is nothing special about where or when we live
or observe from. We seem to live on an ordinary planet, orbiting an ordinary
star, and it is natural to infer that the Solar System resides in an ordinary
galaxy."
-
- Gallagher said that assessment may be
a little generous.
-
- Residing as we do near the outer edge
of a middling spiral galaxy, we are probably a little bit further out of
the action than most. Closer to average, he said, likely would be a star
in the midst of a massive elliptical galaxy 100 times bigger than the Milky
Way.
-
- It may seem a little deflating to have
our place in the universe reduced to sub-par. For astronomers, though,
it's actually a good sign. It means that our astronomical neighborhood
is fairly typical, which means that what we learn here probably applies
to most of the rest of the universe.
|
