What is a galaxy?
The constellation of Andromeda rises 3n the autumn sky right after the great square of Pegasus, It is a slender V of stars and, about half way along the palest side, you can see a faint blur which could be either a dim star or a faraway cloud of haze Actually it is a galaxy of about 100 thousand million brilliant stars, You are seeing this heavenly splendor across half a million light years of space, a distance which could be written in miles as ten, followed by 18 zeros
Two other galaxies are barely visible from south of the equator They arc the Megallanio Clouds, smaller than the great galaxy in Andromeda and much closer to us Still nearer to home is the great Galaxy we see when we look up at the hazy Milky Way which loops over our skies, Galileo trained his little telescope on the Milky Way and was the first to discover that its pale blur comes from billions of faraway stars like our sun
Our sun and Solar System are but grains of duet in the great Galaxy which is our home in the heavens Other galaxies of assorted shapes and sizes are scattered through the vast reaches of space as far as the ages of our giant telescopes can probe The endless universe is populated with countless numbers of these glorious star systems Telescope pictures can reveal the individual stars in many of them, though these details cannot be seen by the naked eye
A picture of the great galaxy in Andromeda looks somewhat like a giant daisy, seen slightly from one side The center is round and brilliant with closely packed stars, The wide outer rim is flat and dimmer Actually, it is formed from two vast arms of stars spiraling out from the center From side to side, this galaxy measures 12,000 light years Each flat arm
Each flat arm is about 2,00 light years wide and the two spiraling arms
are separated by a space, about 10,000 light years wide and each of
those light years equals about six million, million miles
Because of its shape, the galaxy in Andromeda is called a spiral nebula Our own Galaxy, though slightly smaller, is of the same type When we look at the Milky Way, we are looking out across the surface of our vast cartwheel of stars, out across the center where the system is thicker and more densely packed with stars
Some distant galaxies are called barred nebulae Here the central nucleus is rod‑shaped, rather than lens‑shaped like our own In a barred nebula, two spiraling arms whirl off one from each end of the rod Other distant galaxies are irregular in shape and seem to be cloudy collections of stars arranged in no particular order
There is also a group of round and oval‑shaped galaxies Some are great balls of stars some slightly flattened and some very flat However, they do not have the curved arms of the spiral nebulae„ They have less duet and cloudy gases than the spiral nebulae and seem to be more like the central core of our Galaxy