GALAXIES

                                                   GALAXIES

       A galaxy is a family of stars, gas, and dust held together by gravity. Much of a galaxy is empty space, with distances between each star that are hard to imagine.

Types of galaxies

       There are billions of Galaxies in the Universe. Some are very small with only a few million stars. While others could have as many as 400 billion stars, or even more. There are three kinds of Galaxies, Spiral, Elliptical, and Irregular. The only difference between the three is what shape they are our solar system is a tiny part of gigantic spiral galaxy, the Milky Way this is made up of billions of stars, which look as if they have been sprinkled thickly onto the night sky. Do you want to know more about types of galaxies and Milky Way?

Spiral Galaxies                                                            

         The most beautiful type of galaxies is Spiral Galaxies. Spiral galaxies consist of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. Their long twisting arms are areas where stars are being formed.

Where do the spirals come from?
Like ripples in a pond, the spiral arms seen in this kind of galaxy are circling waves.  These waves cause new stars to form. That’s right; they are like star farmers, planting star seeds where ever they go. 

 What causes the waves to glow?

Some of the new stars created in the wave are very large. Because of their size these large stars glow brighter than their smaller cousins, causing the nearby dust clouds to glow brightly. Thus any area near one of these waves glows like a fluorescent light. In other words you can't actually see the waves, the spirals that we see are the glowing clouds illuminated by large, hot stars. As the waves move on the clouds behind them dim down, no longer glowing until another wave passes through.

 Why doesn't the whole galaxy shine brightly?

The large bright stars created in the waves don't live very long. Their large size makes them burn all their fuel quickly. Usually they die before they ever leave the wave. Only the smaller stars which do not glow brightly survive to leave the waves they formed in.

  Elliptical Galaxies    

     The stars found in Elliptical Galaxies are often very old. This is because elliptical galaxies don't actively create new stars.  The only stars found within them were created a long time ago. Although they are usually smaller, this type of galaxy can be large. Most have only a few thousand stars, but some can have billions of stars. The stars in an elliptical galaxy are often very close together making the centre look like one giant star. If the Earth were inside an elliptical galaxy it would be bright both day and night    

  Irregular Galaxies                              

             Irregular Galaxies are simply all the galaxies which are not spiral or elliptical. They can look like anything and have many different characteristics. The distribution of stars is not in a definite pattern. Many irregular galaxies probably used to be spiral or elliptical until they had some kind of accident which changed them such as crashing with another galaxy. Many other irregular galaxies probably were never spiral or elliptical; they simply didn't evolve that way

       Milky Way

         The Milky Way is a large galaxy that is shaped like a disk. The Milky Way is a spiral galaxy. The Milky Way contains our sun and the solar system. The Milky Way got its name because of its appearance in the sky. At night, you can see a faint light band stretched across the sky. This band is where our solar system is. In the night sky, it has an unclear appearance because of the combined light of stars too far away to be seen individually by the naked eye, but the stars that lie close to the solar system can be individually in identified.

             The Milky Way is best seen on a clear, moonless summer night, when it appears lit up. It looks like an irregular band circling the sky from the north-eastern to the south-eastern horizon. This irregular shaped band extends through the constellations Perseus, Cassiopeia, and Cepheus. The Milky Way is a large spiral galaxy. It has several spiral arms wrapping around a central bulge. The central bulge is about 10,000 light-years thick. The stars in the central bulge are closer together than they are in the arms but in the arms, clouds of dust and gas are usually found. The Milky Way rotates around an axis which joins the galactic poles. The Milky Way rotates clockwise and all the spiral arms trail in the same direction.

       The stars in the centre of the Milky Way are closer together than the stars on the spiral arms. The stars on the spiral arms are farther apart from each other because there are clouds of dust and gas in between the stars. The Milky Way is surrounded by a large cloud of hydrogen gas and surrounding this is a spherical halo that holds many globular clusters. Astronomers have discovered that the Milky Way is surrounded by undetected matter that is larger than the halo.
             The Milky Way contains both blue stars and giant red stars. The central part of the Milky Way and the halo both contain giant red stars. By using tools like photoelectric cells, infrared filters, and radio telescopes, astronomers have found radiation. These observations also indicated compact objects near the galactic centre. In the galactic centre, there might be a black hole. On one of the arms, it contains the great nebula in Orion.