A star is formed by a Nebula, clouds of dust and gas fall and hit each other under gravitational forces which form prostars. The young stars, when collapsed  form main sequence stars. Stars expand as they start to grow old. When the core runs out of hydrogen and helium, then the outer layers of the core start to expand. It becomes a red giant or a super red giant. It will collapse then it may become a black dwarf, neutron star, or black hole. Most massive stars have the shortest lives. Stars die so quickly because they burn massive amounts of nuclear fuel. Stars like our sun live for about 10 billion years. Stars with less mass then the sun have even longer life spans. An evolved star is an old star that is near the end of its existence.

DWARF STARS are stars that are big. They are up to 20x larger then the sun and are up to 20,000x brighter. Our sun is also a dwarf star. YELLOW DWARFS are small mostly main sequence stars. The sun is also a yellow dwarf star. RED DWARF is a cool but very small star. It's also very faint and a main sequence star. SUPER GIANT, its diameter is about 100x bigger then it was originally supposed to be. It's about 20x as massive as the sun.

Stars twinkle because of their viewing through the thick layers of turbulent air in the earth's atmosphere. Stars appear so tiny in the sky as light travels through many layers of the earth's atmosphere. The light of the stars is bent many times in random directions. Stars closer to the horizon appear to twinkle more then stars that are overhead, this is because the light of the stars have to travel through more air than the light of stars overhead, because they are so close to us that they appear big enough that the twinkling is not noticeable.( Also planets do not twinkle because they are so close to us. )

A NEBULA is a huge cloud that is made of gas and dust in intergalactic space. Most of the gas in a nebula is hydrogen gas. A stellar nursery is a nebula in which a star formation is starting to occur. One example of a stellar nursery is the Eagle Nebula. The Ring Nebula is a nebula shaped like a ring, it is 2,000 light-years away from earth, it was discovered by Ch. Messier. Planetary Nebula  is a nebula that's formed by a shell of gas which was ejected from a certain  kind of extremely hot star which could be a Red Giant or Super giant. As the star explodes the core of the star starts to be exposed. ( Planetary Nebula has nothing to do with planets.)
 

MAJOR STARS IN CONSTELLATIONS   Aldebran is the brightest star in the constellation called Taurus (Taurus  is a bull). It is also the 13th brightest star in the sky. Aldebran is one of the bull's eyes. It is a giant, old, and orange star that is cooler than the sun. Aldebran is about 40x as big as the sun. Alnitak is a star in the constellation Orion. It is found at the eastern end of Orion's  belt. Alnitak is also known as Alnitah.  Bootes is a red giant located in the Northern  Hemisphere. It is the 4th brightest star in the sky. (Bootes the Herdsman.) There are other major stars like Sirus which is found in Canis or Auriga which is found in Capella.
 
 

Credits: Pictures and Information

http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/starbirth.shtml

http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml

http://www.enchantedlearning.com/subjects/astronomy/stars/nebulae.shtml

http://www.enchantedlearning.com/subjects/astronomy/stars/majorstars.shtml