 |
| Suresh in front of black board |
Evolution of stars, cold stars (white dwarf Neutron stars) and Black holes:
Life cycle of a star:
a) A star is formed when. A large amount of interstellar gas, mostly and He, starts to collapse on itself, due to the gravitational attraction between the gas atoms or molecules.
b) As the gas contracts it heats up due to atomic collisions.
c) As the gas continues to contract, the collision rate increases to such an extent, that the gas becomes very hot and the gas atoms are stripped off their electrons, and the matter is in a completely ionized state, containing bare nuclei and electrons. Such a state of matter is called plasma state.
d) Under the conditions specified above, the bare nuclei have enough energy to fuse with each other (Nuclear fusion). Hydrogen nuclei fuse in such a manner to form Helium with a release of a large amount of energy in the form of radiation.
e) The radiation emitted in this process is mostly emitted in the form of visible light, UV light, IR light etc., from its outer surface. This radiation is what causes the star to shine, which makes them visible (Ex.: Sun and other visible stars)
f) The star at this stage is halted from gravitational collapse (contraction) since the gravitational attraction of matter in it, towards the centre of the star is balanced by the outward radiation pressure. A star will remain stable like this for millions of years, until it runs out of nuclear fuel such as and He.
g) The more massive a star is, faster will be the rate at which it will use its fuel because greater energy is required to balance the greater gravitational attraction, owing to greater mass i.e. massive stars burn out quickly.
h) When the nuclear fuel is over, i.e., when the star cools off, the radiation pressure is not sufficient to halt the gravitational collapse. The star then begins to shrink with tremendous increase in density. The star eventually settles into a white dwarf, Neutron star or a Black hole depending upon its initial mass.
Formation of White dwarfs:
a) For a star to become a white dwarf, initial mass must be less than ten solar masses. (M < 10 where is the mass of the Sun)
b) As the star collapses after cooling off, the radiation emitted due to fusion of remaining nuclei at the outer edge of the core, causes the lighter outer mantle of the star to expand to several times its original diameter. Such an expanding star is called a “Red Gaint”.
c) After several millions of years, the fuel is exhausted and the material from the outer mantle of the Red giant is blown off and the remaining core left over, which is very dim is called white dwarf.
d) A white dwarf is barely visible and has a mass of less than 1.4 .
e) Chandrashekar limit: The maximum mass that a white dwarf can have is called Chandrashekar limit, which is 1.4 . A white dwarf cannot have a mass greater than 1.4 .
f) The volume of a white dwarf is about times the volume of the original star.
g) In the white dwarf stage further gravitational collapse is halted due to the balance between repulsion of electrons and gravitational attraction. The repulsion between electrons is caused degeneracy pressure.
h) The degeneracy pressure is because all the lower available quantum energy states, is filled up by electrons. The pauli exclusion principle prevents further filling up of these energy states. This causes the remaining electrons to fill up higher energy levels causing the required effect. In the white dwarf there is complete break down of atomic structures.
i) Matter in a white dwarf has a very high density. A white dwarf having the mass of the Sun has approximately the size of the earth.
j) If the mass is greater than 1.4 , the degeneracy pressure between electrons will not be able to halt further gravitational collapse. The star then collapses into a neutron star or a black hole.
Formation of Neutron stars:
a) For a star to become a neutron star, its initial mass must be greater than ten solar masses. (M > 10 )
b) As a star wit initial mass > 10 cools off, the large mass of the star causes it to contract abruptly, and when out of fuel it explodes violently. The explosion flings most of the star matter into space and is called a supernova. A supernova explosion is very bright and outshines the entire light from the galaxy.
c) The mass of the matter left behind is greater than 1.4 .
d) If the mass of the left over matter is between 1.4 and 3 , Neutron stars evolve.
e) If the mass of left over matter is greater than 3 , it becomes a black hole.
f) When the mass of the left over matter lies in the range 1.4 and 3 , the repulsion between electrons will not be sufficient to halt gravitational collapse. Under such conditions, the protons and electrons present in the star combine to form neutrons. After the formation of neutrons, the outward degeneracy pressure between neutrons prevents further gravitational collapse, and the matter left over is the Neutron star.
g) Neutron star has a density much larger than white dwarf and has a radius of about 20 kms.
h) Neutron stars are also called pulsars, because they emit regular pulses of radio waves.
i) Neutron stars are not visible.
Formation of Black Holes:
a) Black holes are objects in space, whose gravitational field is so strong that even light cannot escape from it. We cannot see black holes because light emitted by them, would not reach us, however its gravitational effect, will be felt by other objects.
b) The possible existence of a black hole was first pointed out by John Michell (1783).
c) The phrase black hole was first coined by John Wheeler.
d) Using Einsteins General theory of relativity, Karl Schwarzchild determined the radius of an object below which the escape velocity would be greater than speed of light. This radius is called Schwarzchild radius.
Schwarzchild radius is R =2GM/C2 [2 as power]
M = Mass of object, G = universal gravitational constant and c is the velocity of light.
e) Any object would be a black hole if and only if all of its mass is inside a sphere with a radius equal to Schwarzchild radius. At the Schwarzchild radius, escape velocity is equal to the speed of light. This boundary is called the event horizon.
f) Any event that occurs, within the event horizon cannot be observed from outside.
g) Black holes have infinitely large density. The matter present inside is called singularity.
h) Any object, even light, present within the even horizon will be sucked into the black hole.
i) Stars turn into black holes, when the mass of the remaining matter after a supernova explosion is greater than 3 . The initial mass being greater than 10 .
j) When the mass of the remaining star is greater than 3 even the degeneracy pressure between neutrons cannot prevent, the gravitational collapse and Black holes are formed.
k) Evidences for existence of Black Holes:
i. Cygnus XI is a binary star, which contains a visible star moving around an unseen companion and which emits X rays. It is believed that the unseen companion is a black hole, which sucks off matter from the visible star. As the matter moves towards the Black hole it gets very hot (about 100 billion degrees) and emits X-rays.
ii. At almost all the galactic centers, huge amounts of radiation jets are observed perpendicular to the plane of the galaxy. This radiation is emitted by the matter falling into massive black holes present at the galactic center.
iii. Quasars (Quasi Stellar Radio Sources): Are very distant objects which emit powerful radio waves. A Quasar is an entire galaxy undergoing a gravitational collapse due to the presence of a super massive black hole at the galactic centre.
 |
| Stars collapse in black hole |
