SOME UNEVEN TOPICS OF PHYSICS

            SOME UNEVEN TOPICS OF PHYSICS

                                                    1.CHANDRASEKHAR LIMIT

The core of a star that has exhausted its nuclear fuel can end up in different ways depending on its mass. In the case of a star like our sun as a result of gravitational compression, the core is left with protons, with electrons moving around like the molecules of a gas. This electron gas is able to withstand the gravitational force. As a result, a stable equilibrium can be achieved. Such a star is called a white dwarf. White dwarfs are quite different from all the usual stars. The particles of classical gases obey classical mechanics. The particles of quantum gases obey quantum mechanics. Quantum gases help us to understand the behaviour of white dwarfs. Fowler regarded the core of a white dwarf as a Fermi gas. The pressure in such a gas is of quantum mechanical origin. It is called degeneracy pressure and has its origins in the Pauli Principle. According to Fowler, it was degeneracy pressure that arrested the total collapse of a white dwarf. Fowler applied quantum mechanical ideas to stars. When the white dwarf gradually cools, at some stage it ceases to be self-luminous and ends up as a black dwarf. S. Chandrasekhar discovered theoretically in the 1930s that such a stable condition can occur only for cores which have masses up to 1.4M where M is the mass of the sun. This is known as the Chandrasekhar limit. If the mass of the core exceeds the Chandrasekhar limit, the gravitational compression becomes so large that the electrons are forced into the nuclei. This process is known as inverse beta decay. The star is now composed only of neutrons. These neutrons can withstand the gravitational compression.

When a massive star starts rapidly collapsing, the interior gets heavily compressed and therefore also very hot. The core becomes converted into neutrons. If the compression is sudden and the heat generated is very high, then a violent explosion called supernova explosion takes place. It tears off the outer layers of the star and hurls them into space. What is left after a supernova explosion may be a remnant whose mass is greater than 1.4M, where M is the mass of the sun. The Crab Nebula is the nearest supernova.

                                                              3. BIOASTRONOMY

A new discipline of science was given birth to in 1982 at the 18th General Assembly of the International Astronomical Union (IAU) held in Patras, Greece. The new discipline of science has been named BIOASTRONOMY. The search for Extraterrestrial Intelligence (SETI) and a new commission, Commission 51, was created by the IAU for study, research and development of this discipline. The creation of this commission was a major triumph for bioastronomers. The name Bioastronomy is a discovery of Michael Papagiannis of Boston University who was the first President of the IAU Commission 51 (1982-85). Bioastronomy is emerging as a multidisciplinary science where planetary science, planetary system science, the origin of life studies and the search for extraterrestrial intelligence- all converge. Since according to several estimates up to 0.5% of all stars could have a planet similar to our earth, it is a natural question 'Are there intelligent beings elsewhere in our Galaxy?'. There are several controversial aspects to consider, to find a satisfactory answer. The most significant one is N, the number of technological civilizations, if any, in an average spiral like Milky Way. N has been debated at various meetings and extreme values between 10^10 and 10^-24 have been suggested.

                                                        4. DARKE EQUATION

In 1960, Franke Drake formulated his equation to calculate the number of technological civilizations(N) in Green Bank meeting held at Green Bank, West Virginia that established SETI as a scientific discipline.