Galaxy
Galaxy
» A large group of stars, dust and light gases, bound together by their own gravity, is called a galaxy.
» There are 1011 galaxies in the universe.
» We live on the outer edge of a spiral type of galaxy called the Milky Way, which is about 100,000 light years in diameter and is rotating slowly.
Earth's Galaxy: The Milky Way
» The Milky Way is a large spiral-shaped galaxy.
» It spans about 1,00,000 light-years across and is about 10,000 light-years thick at the centre.
» It is called the Milky Way because it appears as a soft glowing light of billions of stars. These stars are so far that they can be seen only in constellation, not separately.
» Galileo discovered that this band of light was produced by countless individual stars which a naked eye can not see.
» It takes about 250 million years to complete one revolution.
Andromeda : Earth's closest Galactic neighbor
» Andromeda is a spiral galaxy and also our closest neighbour.
» It appears as a fuzzy patch of light and contains millions of stars.
» It is the farthest object that can be seen with the naked eye.
» Along with the Milky Way, it belongs to a group of galaxies known as the Local Group, which in turn is a part of Virgo Cluster of groups.
» Like stars, galaxies are grouped into clusters. Some clusters contain thousands of galaxies.
» About 30 galaxies, along with the Milky Way and the Andromeda are grouped together in one cluster called the Local Group.
» Clusters may group together into upper clusters.
» Super clusters are also spread randomly throughout the universe.
Nebulae
» Nebulae are huge interstellar clouds of gas and dust that appear as faint, misty patches of light scattered all over the sky.
» They appear either as bright luminous clouds or as dark patches against a brighter background.
» A nebula depends for its luminosity upon the presence of stars that have either arisen from it or are contained in it.
» If the stars are extremely hot, the hydrogen in the nebula is ionized and emits a certain amount of light of its own.
» If a star is less hot, the nebula shines only by reflection.
» If there are no suitable stars, the nebula does not shine and remains dark and can be detected only because it blots out the light of the stars beyond.