1. Stars By: Sana Gill

2. Characteristics Stars are spheres of plasma kept together by gravity. An example of a star is our sun. Are sun is one of at least 100 billion and up to 400 billion stars estimated to be in our galaxy. For the most part the bigger a star is, the shorter its lifespan. At the center of this Hubble Telescope image is a red supergiant star. The star is around 20,000 light years away from Earth, and is near the outer edge of our Milky Way galaxy.

3. Star Formation Stars are formed in regions called molecular clouds. Molecular clouds are groups of interstellar gas and dust left over from when the galaxy formed. The Orion Nebula is an example of a nebula near the sun where massive stars are forming. The formation of a star begins with gravitational instability in a molecular cloud. The gas and dust collapse making the center of the collapsing molecular cloud hot. This hot core of the cloud is known as a protostar, and will eventually become a star. The hot core gathers dust and gas. The left over materials may become parts of planets, asteroids, or comets. Main Sequence stars like our sun take 50 million years to reach adulthood. This picture was taken by the Hubble telescope, and is known as Pillars of Creation. Stars are forming in the Pillars of Creation, which is in the Eagle Nebula.

4. Classification and Types of Stars Stars are classified by size and color. Dwarf star, Giant star, and Supergiant star are the names used to classify stars in order from smallest to largest. The color of a star shows how hot it is. Red stars are the coldest and white stars are the hottest. Yellow stars are after red stars in heat, and then come blue stars. Hypergiants are the largest stars, and may be 100 or more times as massive as the sun. Hypergiants are bright, emitting hundreds of thousands of times more energy than the Sun, but have lifetimes lasting only a few million years. Hypergiants are very rare, and the Milky Way galaxy contains few hypergiants. Red dwarfs are the smallest, but are the most plentiful stars in our universe. They can be as little as 10% of the size of the sun, and emit 0.01% as much energy. Main Sequence stars account for 90% of all stars. Red dwarfs glow weakly, but can last for tens of billions of years. This is a Hubble Space Telescope image of Eta Carinae, a supergiant.

5. Life Cycle of Stars The mass of a star determines its future transformation. The greater a stars mass the shorter its life cycle. In the center of stars like the sun hydrogen is converted to helium. At some point the hydrogen necessary to balance gravity runs out, and the center of the star contracts. The star expands and as it cools it glows red. At this stage it is called a red giant. For stars with a low mass, when the helium in the core fuses into carbon the core collapses. The core collapsing causes the outer layers of the star to be expelled. The core is left as a white dwarf and later becomes a black dwarf. A planetary nebula is formed from the outer layers that were expelled. Massive stars will have a supernova explosion. Depending on the size of the remaining matter it will either turn into a neutron star or a black hole.

6. Constellations Constellations are groups of stars that form an imaginary figure. Today there are 88 official constellations. People started creating constellations in order to know what season it was because in some places it was difficult to distinguish between the seasons. Using the fact that different constellations are visible at different times of the year, you can use them to know what month it is. Some people suspect that constellations were invented to help farmers know when to plant and harvest crops. Majority of the constellations have Latin originating names. An example is Scorpius, which got its name from the Latin word for scorpion.