If you find yourself away from city light pollution and under a clear haze-free sky this week, look almost straight overhead around 10 o'clock.* Your eye may catch upon a faint glow, like a patch of thin cirrus, nestled between the stars of Andromeda and Cassiopeia. It will probably be something you see out of the corner of your eye, and you may wonder if you're seeing anything at all.
Indeed you are. What you're looking at is the Andromeda Galaxy, a system of several hundred billion stars that lies across a gulf of interga-lactic space, 2/4 million light-years away. It is the nearest major galaxy to our Milky Way and ranks as one of the most distant objects visible to the unaided eye. The light arriving tonight from the Andromeda Galaxy was emitted over 2 million years ago, when early hominids like Australopithecus were fashioning rudimentary stone tools and establishing the first social order.
If you have binoculars handy (at least 7 X 50s) be sure to take a look at this object. Suddenly, what was a vague glow becomes a distinct oval patch of light with a bright center. If you could see the full extent of this galaxy in binoculars - as revealed by long exposure photographs - it would cover an area of sky more than twice that of the full Moon. Its most compelling features are its size, symmetry, and the dramatic way it stands out so starkly against the surrounding blackness.
Today, astronomers estimate there are billions of galaxies in the universe, each containing hundreds of billions of stars. During the early part of this century, however, such objects were thought to be gaseous disks residing in our galaxy. A great many exhibited striking whorl-like structures and were thus referred to as 'spiral nebulae.' Then in 1925, the American astronomer Edwin Hubble determined the distance to the spiral nebula in Andromeda by comparing the brightnesses of a class of nearby known stars called Cepheids to those in the Andromeda nebula.
Cepheids, named for the archetypal star of this class, Delta (8) Cephei, fluctuate regularly in brightness. Like lighthouse beacons, they
* If you live in the Southern Hemisphere (Sydney, Australia, Cape Town, South Africa, about latitude 33°S) you must look about 15° above your north horizon at this time; at lesser southern latitudes (Lima, Peru, latitude 12°S), look anywhere between 30° and 40° above the north horizon.
A stepping stone to the universe, overhead, 10 p.m., November 5.
can be used as 'standard candles' for estimating distance. Hubble discovered these same types of stars in the Andromeda nebula, as well as another spiral-type object, M33. By comparing the brightness of these stars to the ones in our galaxy, he showed that both the Andromeda nebula and M33 lay far beyond the boundaries of our galaxy.*
This epochal discovery forced astronomers to realize that the universe was vaster than had been imagined. For beyond even the nearest galaxies, themselves inconceivably remote, lay wave upon wave of galaxies stretching out to distances where, as Hubble himself once
* Many observers claim to be able to see M33 with the unaided eye. Located 14° southeast of M31 in the constellation Triangulum, this galaxy, known as the Pinwheel, is a bit larger on the sky than the apparent size of the full Moon, but its light is also spread out over this area, making it difficult to detect in binoculars, even in dark skies. Still, if you can see M33 with the naked eye, that would make this object the most distant one you can see without the aid of a telescope. Its distance is 2.5 million light-years.
remarked, 'we measure shadows and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.'
Galaxies are the largest, coherent building blocks of the universe. Together they make up clusters and superclusters of galaxies, some of which stretch for hundreds of millions of light-years in length.
Closer to home, however, the nearest group of galaxies comprise a humble little cluster astronomers call the Local Group. In addition to M31 and M33, the Local Group includes the Large and Small Magellanic Clouds (February 8 - 14), and over two dozen other elliptical, irregular, and spiral galaxies, including our own Milky Way. Galaxies may appear isolated in the universe, but in fact they, like stars, are more gregarious than they seem.
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