GALAXIES
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- GALAXIES ARE GIANT GROUPS OF MILLIONS OF TRILLIONS OF STARS.
- THERE MAY BE 20 TRILLION GALAXIES IN THE UNIVERSE.
- OUR LOCAL GALAXY IS THE MILKY WAY.
- ONLY THREE GALAXIES ARE VISIBLE TO US FROM THE NAKED EYE-THE ANDROMEDA GALAXY, THE LARGE AND SMALL MAGELLANIC CLOUDS.
- GALAXIES ARE OF THREE TYPES-ELLIPTICAL, SPIRAL AND IRREGULAR.
- A SPIRAL GALAXY RESEMBLES A PINWHEEL, WITH SPIRAL ARMS COILING OUT FROM A CENTRAL BULGE.
- ELLIPTICAL GALAXIES RANGE IN SHAPE FROM ALMOST PERFECT SPHERES TO FLATTENED GLOBES. THE LIGHT FROM AN ELLIPTICAL GALAXY IS BRIGHTEST IN THE CENTER AND GRADUALLY BECOMES FAINTER TOWARD ITS OUTER REGIONS. AS FAR AS ASTRONOMERS CAN DETERMINE, ELLIPTICAL GALAXIES ROTATE MUCH MORE SLOWLY THAN SPIRAL GALAXIES OR NOT AT ALL. THE STARS WITHIN THEM APPEAR TO MOVE IN RANDOM ORBITS. ELLIPTICAL GALAXIES HAVE MUCH LESS DUST AND GAS THAN SPIRAL GALAXIES HAVE, AND FEW NEW STARS APPEAR TO BE FORMING IN THEM.
- GALAXIES OF A THIRD KIND, IRREGULAR GALAXIES, LACK A SIMPLE SHAPE. SOME CONSIST MOSTLY OF BLUE STARS AND PUFFY CLOUDS OF GAS, BUT LITTLE DUST. THE MAGELLANIC CLOUDS ARE IRREGULAR GALAXIES OF THIS TYPE. OTHERS ARE MADE UP MOSTLY OF BRIGHT YOUNG STARS ALONG WITH GAS AND DUST.
- GALAXIES ARE DISTRIBUTED UNEVENLY IN SPACE. SOME HAVE NO CLOSE NEIGHBOR. OTHERS OCCUR IN PAIRS, WITH EACH ORBITING THE OTHER. BUT MOST OF THEM ARE FOUND IN GROUPS CALLED CLUSTERS. A CLUSTER MAY CONTAIN FROM A FEW DOZEN TO SEVERAL THOUSAND GALAXIES. IT MAY HAVE A DIAMETER AS LARGE AS 10 MILLION LIGHT-YEARS.
- CLUSTERS OF GALAXIES, IN TURN, ARE GROUPED IN LARGER STRUCTURES CALLED SUPERCLUSTERS. ON EVEN LARGER SCALES, GALAXIES ARE ARRANGED IN HUGE NETWORKS.
- NEW STARS ARE CONSTANTLY FORMING OUT OF GAS AND DUST IN SPIRAL GALAXIES. SMALLER GROUPS OF STARS CALLED GLOBULAR CLUSTERS OFTEN SURROUND SPIRAL GALAXIES. A TYPICAL GLOBULAR CLUSTER HAS ABOUT 1 MILLION STARS.
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