Stars form in regions of Molecular Clouds when some event cause it to began to collapse down under gravity.Which is cause by shockwave of a nearby supernova or from when galaxies collide

As the stellar material pulls tighter and tighter together, it heats up pushing against further gravitational collapse. At this point, the object is known as a Protostars.

The Protostars phase of stellar evolution lasts about 100,000 years

Surrounding the Protostars is a circumstellar disk of additional material. Some of this continues to spiral inward,layering additional mass onto the star.The rest will remain in place and eventuallyform a planetary system.

After a star has formed, it creates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors.All main-sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward gravitational pressure from the overlying layers.

The coolest of the common star types, red stars are classified as M-type.They have very cool surface temperatures below 3,500 K, allowing more complex molecules to form. Among the brightest red stars in the sky are Betelgeuse (M2),Antares (M1), Gacrux (M4) and Mirach (M0).The Sun's nearest neighbor in space, Proxima Centauri, is also a red star, classified as M5.

K-type stars are occasionally referred to as Arcturian Stars, after the brightest of their number. Their surface temperatures are between 3,500 K and 5,000 K, low enough for simple molecules to form. K-type stars are orange in colour, and among the brightest in the sky are Arcturus (K2), Aldebaran (K5),Pollux (K0) and Atria (K2).

The cooler a star, the more complex its chemistry tends to be. G-type stars, with temperatures ranging between 5,000 K and 6,000 K, have spectra that betray the existence of 'metals' (in this context, 'metal' refers to any element heavier than helium). Examples of yellow G-type stars are Alpha Centauri (G2),Capella (G5), Kraz (G5) and Mufrid (G0).The Earth's Sun is a G2 star, and also belongs to this type.

F-type stars lie between the A-type white stars and G-type 'true' yellow stars, and have a distinctly yellowish light. Their surfaces have a temperature between 6,000 K and 7,500 K.Sometimes called Calcium Stars, examples of this type include Canopus (F0), Procyon (F5), Algenib in Perseus (F5) and Wezen (F8).

A-type stars are those whose surface temperatures lie in the approximate range 7,500 K to 11,000 K.They are white in color, and some of the brightest and most famous stars in the sky belong to this classification, including Sirius (A0), Vega (A0), Altair(A7) and Deneb (A2).

The B type is the first of the really populous classes. Stars of this type are blue in colour and burn hotly, with surface temperatures lying between 11,000 K and 25,000 K. Prominent examples of blue B-type stars are Rigel (B8), Achernar (B3), Agena (B1) and Spica (also B1).

O-type stars are also relatively uncommon, but far more numerous than those of type W. These are bright blue stars which also have very high surface temperatures, in the range 25,000 K to 50,000 K. Examples are Alnitak (O9.5), Naos (O5), Hatysa(O9) and Heka (O8).

Planetary Nebula

At the end of the star's life, during the red giant phase, the outer layers of the star are expelled via pulsations and strong stellar winds. Without these opaque layers, the hot, luminous core emits ultraviolet radiation that ionizes the ejected outer layers of the star. This energized shell radiates as a planetary nebula.

White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become supernovae—over 97% of the stars in our galaxy. After the hydrogen–fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf.

A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool down.

Neutron stars/PulsarsNeutron stars are composed of degenerate neutron matter with a density about that of atomic nuclei, 10∼ 17 kg. m-3. A thimble-full of this material has a mass of almost 109 tonnes. They range in mass from a lower value equal to the Chandrasekhar Limit of 1.4 solar masses up to about 3 solar masses. This upper limit is not well-defined and may be up to 5 solar masses in some models. A neutron star is typically about 10 km across. We thus have a very exotic object with twice the mass of the Sun packed into a sphere the size of a small city! Due to the conservation of angular momentum, a neutron star spins at a high rate. Whereas a star such as the Sun rotates on its axis roughly once a month, a neutron star can rotate dozens of hundreds of times a second. This is analogous to an ice skater spinning faster as they draw their arms in close to their body.

The high rotational speed means that the surface of neutron stars are travelling at relativistic speeds.

Black holes are even more exotic objects than neutron stars. With all the mass concentrated at a point they have extremely high gravitational fields. They are referred to as black because not even light can escape from them once it has crossed a region known as the event horizon. At the event horizon, the escape velocity equals the speed of light, c. Black holes are therefore hard to observe because they do not emit light at any waveband. Rather than look for a black hole itself, astronomers infer their presence due to their effect on surrounding matter.