In our Universe Many ISM are present where many dust particles, Hydrogen, He and ions are present .
We divide the ISM into 4 categories
1. H1 Region: It has Hydrogen.
2. H11 region: It has mostly ionized hydrogen.
3. Intercloud medium: It has ionized and neutral Hydrogen gas.
4. Molecular cloud: matter exits in molecular form in ISM.
Stellar Nebula: It is where occurring of formation of star happens. This is birth place of star. And after birth of stars, next step is to form Protostar. Molecular cloud collects in the Protostar because gravity works on it. When gravity starts working, the formation of Core starts. When gravity starts working continuously, nuclear fusion reaction starts in the core of the stars. This reaction is called Nuclear fusion or thermonuclear reaction and this process is called Nuclear Synthesis.
Fig: Stellar Nebula
Fig: Main sequence star
In Nuclear Synthesis, there are 4 type of reaction.
1. Hydrogen burning: (P-P chain and CNO cycle).
2. Helium burning: (3 alpha reaction)
3. Burning of Carbon and heavier nuclei
4. S- and –R Process.
Then transform into Main sequence star or we can say that these stars comes into the main sequences stars. But when formation of protostar is going on then due to gravity molecular cloud comes to closer to each other. But due to internal pressure clouds wants to expand. When internal pressure is more as compare to the gravity which acts on this Protostar then clouds burst. If gravitational collapse occurs then clouds form Protostar but for collapsing clouds need a minimum mass which is called Jeans Mass.
When stars leave the main sequence they transform into a giant star. Size of the this giant star is more and temperature is less and seems red in color and we call it Red Giant Star.
If after leaving the main sequence, if mass of the star >10 times solar mass then its transform into a Supergiant star.
After leaving the main sequence of mass of star:
Approx. equal to solar mass: then its core start the helium burning reaction temperature is increase and blast(explosion). Then core looks like a planet and we call it a Planetary nebula.
After that planetary nebula get more contracted and inside the core degenerate gas has formed. According to Pauli's exclusion principle they are getting more contrast and after a long time these stars has more contrasted and these stars are called white dwarf star.
The maximum mass of white dwarf star is approximately 1.4 times of the solar mass and this limit is called Chandrasekhar limit and these stars do not have any energy that’s why we called it Dead star.
Is > approx. 5 times of solar mass: these star comes into a horizontal line in H.R. diagram. And reaction start in the core of these stars.
Fig: HR Diagram
Is >10 times of solar mass: The helium reactions starts gradually. When helium in the core is exhausted the core contracts once again and helium reactions are now ignited in a thin shell around the core. The temperature of the contracting core rises and becomes high enough for carbon-carbon reactions to occur. The cycle of core contraction and burning of the next heavy element continues till the core consists of iron. No more fusion reactions are possible now or in this core , reaction going on. Yet when iron not form after that fusion reaction so, S-and R- process star process starts. Large no. of neutron are released. Then Bi-209 are formed. And finally explosion in core envelop (at that point where heat reached. due to this at that place reaction starts.) this explosion is called Supernova.
If you want to know more about supernovas, click here.
And those element released from this explosion are used as to give birth to 2nd generation star. And remaining core, if initial mass of star is maximum 12 times then mass of core becomes 2-3 time of solar mass and this convert into Neutron star.
If initial mass of star is closed to 15 times of solar mass then core mass is around 3 times of solar mass then it transform into a Black hole.
You can know more about blackholes. Click Here and here.
