Difference Between Galaxies and Milky Way
Galaxies are immense collections of stars, gas, and dust held together by gravity. They are present throughout the known universe. In fact, they are considered the building blocks of the universe. Individual galaxies occur in groups called galaxy clusters, which clump together to form superclusters. And superclusters link up with each other to form filaments and sheets. These filmy structures resemble cobwebs. The gaps in these cosmic cobwebs are called voids.
Galaxies are very far away, so it is hard to see them in detail. And they change very slowly, so it is hard to understand how they evolve. To overcome these difficulties, scientists use powerful telescopes and other instruments to detect the visible light and other kinds of radiation that are emitted by distant galaxies. Typically, these galaxies are so far away that this radiation takes a long time to reach the instruments. This is because light travels slowly relative to the distances that must be crossed. In fact, light takes so long to cross astronomical distances that looking deep into space is the same as looking into the distant past.
By studying the radiation emitted by galaxies from different time periods, scientists collect clues about the history of the universe. Scientists work with supercomputers to find out if these clues fit with their ideas about the origin, evolution, and fate of the universe.
Milky Way, the galaxy that includes the sun, the earth, and the rest of the solar system, along with billions of stars, gas and dust clouds, and other objects. The solar system is just one very tiny part of the Milky Way galaxy, which in turn is just one very tiny part of a universe containing billions of galaxies.
The nighttime summer sky shows the edge of the Milky Way galaxy as a band of faint light arching across the sky from horizon to horizon—the combined light of millions of stars. This band, called the Milky Way, is broadest and brightest in the southern constellation Sagittarius, and thinner and fainter in the northern constellation Cygnus. The Milky Way also can be seen in the winter sky, but it is neither as bright nor as broad as the summer Milky Way. The ancients saw the band of light as a river of milk or a road joining earth with the heavens.
All the stars and other objects that the naked eye sees spread over the entire sky are part of the immense Milky Way galaxy. The central nucleus of the galaxy, which is in the direction of Sagittarius, is about 10,000 light-years thick. (One light-year is about 6 trillion miles.) The disk flattens toward its edge. In the vicinity of the sun, the thickness of the disk is about 3,000 light-years. The total mass of the Milky Way galaxy is about 100 billion times the mass of the sun.