The Universe within 500 million light years

This entire page and website are excerpted from "You and the Universe," a personalized fine art book on astronomy, mythology and astrology through which is woven each recipient's complete astrological reading.

Galaxies, like stars, also form groups. The Milky Way and the Andromeda Galaxy belong to such a group of 20 to 30 galaxies called the Local Group. And finally, clusters of galaxies group in clusters of clusters, called superclusters. Our Local Group is about 50 million light years from a supercluster of about 2000 galaxies called the Virgo Supercluster.

Each dot on the map of the universe above represents an entire galaxy of hundreds of billions of stars, and we, the Milky Way, are at the center. Superclusters are not isolated in space, but together with many other smaller concentrations of galaxies form parts of extensive "walls" of galaxies that it turn surround large voids. On this scale our own Milky Way galaxy is very insignificant, since there are several hundred thousand large galaxies within 500 million light years of us. And there are estimated to be hundreds of billions of galaxies in the universe.

Although it’s too early in our observational experience to give a percentage of the stars that have planets, that condition seems to be the rule rather than the exception. Most stars are probably also encircled by billons of small bodies like our asteroid and Kuiper belts that never coalesced into planets as the system evolved. And further out still is each star’s own Oort cloud, a sphere of trillions of frozen objects left over from that system’s primordial past. By this point we’ve come a few light years from the star and have reached the edge of it’s gravitational and magnetic influence.

The distance light travels in a year - a "light year," about six trillion miles - is the yardstick astronomers use for measuring large distances. Because other stars and galaxies are so far away, it takes time - and lots of it - for their "pictures" to get here. So as we look out into the universe, we are, by necessity, looking back in time. If, for example, the Sun exploded, we wouldn’t know it for 8.3 minutes, because that’s how long it takes light to get to us from the Sun. Pluto is five light hours away, and the next nearest visible star, Alpha Centauri, is 4.4 light years distant.

Everything else we see in the night sky that’s not in our solar system is older still. The nearest galaxy to us, the Andromeda galaxy, is just under three million light years away. That’s how old its picture is when we see a photograph of it. In fact, astronomers are now looking out so far into the universe (billions of light years) that they hope to see the universe begin, about 13.7 billion light years away and just that many years ago!

In order to see as far out in space (and therefore as far back in time) as possible, astronomers had to find an area of the sky with no bright stars in it that would otherwise overexpose a very long time exposure. Having found such a small area near the big dipper, they took a long exposure of it, magnified that, and then chose a tiny area in that photograph with the least number of bright stars. Then they put the Hubble telescope on it for ten days straight.

The result was the Hubble Deep Field (HDF) shown on page 116 of "You and the Universe."  Taken in 1995, the HDF image holds over 1500 galaxies in an area of the sky that can be covered by a grain of sand held at arm’s length. The bright, white object with diffraction spikes at the bottom just to the left of center may be a 20th magnitude star in our own galaxy, millions of times closer than the menagerie of far more distant background galaxies.