When we look up at the night sky, we notice that there are many stars
in our sky. Stars must like to live together in star cities -
galaxies. Our city of stars is called the Milky Way, and it is home to
100 billion stars, including the Sun. The stars stay together because
of the force of gravity. There are as many other galaxies in our
universe as there are stars in our Galaxy!
Galaxies come in different shapes and sizes. They can be circular or
shaped like an egg (elliptical), or appear as
pretty pinwheels (spirals).
Some galaxies don't seem to fit into these groups, so they are called
irregulars.
During the early 1900's, which is not very long ago, astronomers were
unaware that there were other galaxies outside our own Milky Way Galaxy. When they
saw a small fuzzy patch in the sky through their telescopes, they
called it a nebula. When
examined closely, some of the nebulae had a spiral shape. So
astronomers at first called these "spiral nebulae". These nebulae
were all believed to be part of our Galaxy, our community of stars.
Edwin Hubble studied the "spiral nebulae" and found that they were composed of stars. These nebulae were not nebulae at all, but other communities of billions of stars held together by gravity - galaxies! Suddenly, our universe was much bigger. We realized that our Galaxy was just one of many billions of galaxies in the universe.
Hubble studied galaxies for a very long time, and after seeing many,
many galaxies, he realized that he could put them into groups based on
their shape: spirals, ellipticals, or irregulars. His work helped
us to understand that the appearance of galaxies depends on our point
of view, and on what's happening in the galaxies.
The introduction of telescopes to the study of astronomy opened up the
universe, but it took some time for astronomers to realize how vast
the universe could be. Telescopes revealed that our night sky was not
only populated with stars, but with other, more nebulous objects.
Some of these objects were nebulae within our Galaxy, the Milky Way. As telescopes
became more powerful, it was possible to see that some of the nebulae
had a spiral-like structure. These were also believed to be part of
our Galaxy and thus relatively nearby.
In 1920, two important astronomers, Harlow Shapley and Heber D. Curtis, held a great debate about the nature of these "spiral nebulae". Were they objects within the Milky Way, or were they communities of stars distinct from our Galaxy? Edwin Hubble studied these "spiral nebulae" and found that they were composed of stars, and thus resolved the debate. These nebulae were not nebulae at all, but galaxies! Suddenly, our universe was much bigger. We realized that our Galaxy was just one of many billions of galaxies in the universe.
Hubble continued to study galaxies his entire career, and we owe much
of our understanding of galaxies to him. His observations led to the
current classification of galaxies as spirals, ellipticals, or irregulars, and to our
knowledge that the appearance of these galaxies depends both on our
perspective, and on the forces which form and power galaxies.
The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-01 University Corporation for Atmospheric Research. All Rights Reserved. Site policies and disclaimer