Stars and Galaxies
Stars:
All stars come from nebulas. Nebulas are vast clouds of gas and dust in which new stars form. Some nebulae make their own light from stars forming inside them, and some nebulae reflect light from nearby stars. Some nebulae block out light of stars behind them. The remains of exploded stars also form nebulas.
Stars create energy by nuclear fusion. This is where two or more atoms combine to become a larger atom. The most common of all nuclear fusions is where two hydrogen atoms combine to make a single helium atom. The star�s gravity equals the energy of the explosions inside the star, so the star stays intact. A prominence is when too much energy is released and a part of the star is blown out.
Scientists classify stars by their color. They give each color a letter. The biggest star is an O star. It�s blue, reaches 35,000 degrees Celsius, and its main component is helium. Next is a B star, which is bluish white, can get up to 21,000 degrees Celsius, and its main component is helium. Next is an A star, which is white, can get up to 21,000 degrees Celsius, and its main component is calcium. Our sun is a G star. It�s yellow, can get up to 6,000 degrees Celsius, and its main components are metals. Smaller than out sun, a K star is orange, can get up to 4,700 degrees Celsius, and its main components are hydrocarbons. The smallest star is an M star. It�s red, can get up to 3,700 degrees Celsius, and its components have a complex mix.
Scientists believe that the larger the star, the smaller the remains of the star after death. Scientists are convinced that stars smaller than our sun go on forever. Scientists conclude that stars the same size as our sun end in a white dwarf. After millions of billions of years, the star uses up its materials. When this happens, the nuclear fusions are overwhelmed by gravity. With this new gravitational force, new reactions are started. The new energy is stronger than the gravity, so the star begins to expand. This force of the new energy expands the star so far that the gravitational forces aren�t strong enough to hold the star together. Scientists believe that the outer parts of the star then fly off into space and all that�s left is the small, inner core. Scientists believe that stars bigger than the sun have very dramatic endings. They believe that eventually the cores of these stars completely collapse, and the temperature goes to 35 billion degrees Celsius. The star then explodes and makes a supernova. Some supernovas are so bright they are visible from Earth in the daytime.
The core of a star is where nuclear fusions occur. All of the sun�s light that reaches Earth comes from the photosphere. Sun spots are areas of cooler gasses surround by hotter gasses in the photosphere.
Galaxies:
A galaxy is an enormous collection of billions of stars, gas, and dust, held together by gravity. One galaxy can have hundreds of billions of stars, and can be up to 200,000 light years across. Spiral galaxies have a bulge at the center, and a flattened disk containing spiral arms. These galaxies rotate somewhat like a Frisbee, or a hurricane. Spiral galaxies are classified according to the size of the bulge, and the tightness of the arms. The spiral arms that wrap around the bulge contain mostly round, blue stars, yellow stars like our sun, and lots of gas and dust. Older red stars are usually in the bulge. Elliptical galaxies don�t have a disk or arms, and don�t rotate. They are characterized by a smooth, ball shape appearance. Elliptical galaxies contain older stars, and posses little gas or dust. The stars move within the galaxy like swarms of bees. Irregular galaxies tend to be smaller galaxies with no definite shape, and tend to have very hot, newer stars mixed in with lots of gas and dust.