Question: What is the closest star to the sun. Some sources say Alpha Centauri and others say Proxima Centauri is the closest. Both are reliable is the closest star???

Answer: The name "Alpha Centauri" actually denotes a triple star system in the constellation Centaurus (it appears as a single star to the naked eye). Two of these stars, denoted Alpha Centauri A and B, are very similar to the Sun and are approximately 4.336 light-years away. They orbit each other (better said, they orbit their common center of mass) with a period of about 80 years. Then, orbiting this pair is a small, faint star only a tenth the size of the Sun. This star takes one-and-a-half million years to orbit the larger pair; at the moment it is closer to us than the other two, a mere 4.205 light-years away. Because it is the nearest star to the Sun, it is named Proxima (from the Latin word for "nearest") Centauri. This information came from "Astronomy: The Cosmic Journey", an astronomy text by William Hartmann. I did not know this stuff myself 'til now!

Ron Winther

Many nebulae form from the gravitational collapse of diffuse gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the centre and illuminate the surrounding gas allowing it to be observed. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula.

Some nebulae are formed as the result of supernova explosions. One of the best examples of this is the Crab Nebula, in Taurus. It is the result of a recorded supernova in the year 1054. At the center of the nebula is a neutron star, created during the explosion.

Other nebulae may form as planetary nebulae. Again these are created near the end of a star's life; when a star with a mass of under 1.4 solar masses becomes a red giant. An outer layer of light hydrogen gas is ejected from the star as the fusion process slows, and the star becomes unable to sustain its entire mass. A nebula is made of luminiscent gases, rock and ash.