(Courtesy to Microsoft Encarta for this photograph.)

There are many "star cities" out in the depths of space. These are called galaxies, and galaxies can be millions of light years in diameter. The galaxy that we live in is called the Milky Way, and all of the stars that we can see without the use of a telescope is in the Milky Way. Our Sun and all of the planets of the Solar System is in the Milky Way galaxy. Like the Milky Way, all other galaxies have stars, planets, nebulae, different celestial bodies (e.g. asteroids and meteors), and also different kinds of radiation (e.g. gamma rays and x-rays).

Have you ever seen different species of galaxies up in space? Our Milky Way is a form of galaxy called a spiral galaxy. Other kinds of galaxies are a barred spiral, irregular, and elliptical. Each of these kinds have their own unique areas. Elliptical galaxies have many old stars, very few new stars, and little amounts of other gas and dust. Spiral and barred spiral galaxies all have many middle age and newborn stars. They also have some older stars in their systems too. Pockets of gas and dust are to be found abundantly in spiral galaxies. Spiral galaxies are fairly flat and have a nucleus that bulges above and below the normal plane of stars in the arms of the galaxy. Irregular galaxies usually have plentiful gas and dust with many new stars. The irregular shapes of the galaxies are probably caused by other galaxies' gravitational pull.

At the beginning of the Universe, many galaxies were formed. The birth of all galaxies was when clumps of gas and dust came together and formed many stars. After when the formation of the galaxy, there was probably a quasar at the middle of the galaxy. A quasar is the very bright core of a newborn galaxy. At the center of the quasar is a huge black hole. These kinds of black hole probably lies dormant in many of the galaxies in the Universe today. The quasar then sent out huge jets of gas and dust at almost the speed of light. After a few million years, the quasar stage wears off and the galaxy evolves into a radio galaxy. The radio galaxy also spews out gas jets, but they are more like clouds now. These clouds have powerful radio waves, and the clouds can be millions of light years long. After this stage, the radio galaxy becomes what it is today, a normal spiral, irregular, or elliptical galaxy. At the center of our Milky Way, scientists think that there is a black hole that was the center of our galaxy when it was still in its quasar stage. This black hole weighs at an estimated 3 million star masses.

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