INTRODUCTION
Stars are astronomial objects known to every one. These are the information carriers of the universe. They tells us about the evolution of the galaxy in which they formed. In addition stars are responsible of forming heavy elements such as carbon, nitrogen and oxygen during nuclear fusion. If we talk about the stars composition, mainly they composed of hydrogen and helium a fraction of heavier elements are also present.
Hydrogen cloud ( past of a star ):
Stars are born within the clouds of dust and scattered throughout most galaxies. These vast clouds can be hundreds of light years across and contain the raw material for thousands or even millions of times the mass of our Sun. These clouds are the birth place of stars.
Fig: Hydrogen Clouds (Credits)
The starting impulse come from
- A nearby supernova explosion
- Collision of hydrogen cloud with another cloud
- The pressure wave of galaxy's spiral arms passing through the region.
The turbulence give rise to knots in the centre of cloud. The gas and the dust begins to collapse due to massive gravitional attraction. As the cloud collapses, the material at the center begins to heat up called a protostar the mass of the knot is nearly the mass of the future star, it is this hot core at the heart of the collapsing cloud that will one day become a star. It is not necessary that all the dust would be comsumed in the formation of star it can be converted into planet, asteroid, planet or remain dust.
Three-dimensional computer models of star formation predict that the spinning clouds of collapsing gas and dust may break up into two or three blobs; this would explain why the majority the stars in the Milky Way are paired or in groups of multiple stars.
STAR ( Present ):
If a protostar has enough material, it can generate enough pressure and temperature at its core to begin deuterium fusion – a heavier isotope of hydrogen. This slows the collapse and prepares the star to enter the true main sequence phase. This is the stage that our own Sun is in, and begins when hydrogen fusion begins. The further collapse stops because the pressure due to nuclear fusion outward balance the gravitational force towards the centre.
Fig: A Star (Credits)
The question is, what is the life of this newly born star and the answer is it depends on its mass and how quickly it consume hydrogen. The life of a newly born star may vary from few million years to hundred of billion of years.
The next episode is how a star disintegrates. what happens next depends on the mass of the core. There are various possibilities but I will discuss only about black hole.
Black Hole (Future of a Star ):
Fig: Black Hole (Credits)
If the collapsed mass of the core is about three times of the solar mass, it collapses completely to form a black hole. it is infinitely dense object whose gravitational attraction is so strong that nothing which goes near to it can escape not even light.
Fig: Black Hole (Credits)
In the centre of a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate. We cannot observe the black hole directly they are detected indirectly by the considerable quantities of X-rays and Gamma-rays emitted by the matter dragged towards the black hole.
Reference:
https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve
https://www.universetoday.com/24190/how-does-a-star-form/