Welcome to the summary series of the book- “Cosmic catastrophe”. Make sure that you have read the previous chapters too. This is the 5th chapter. White dwarfs are the most common corpuses in the Galaxy.
- The white dwarfs are older than a Galaxy, but they die earlier if they are in the Binary System.
- These are supported by Quantum pressure with a mass of approximately 0.6 solar mass.
- To get the clues to the evolution of white dwarf it needs to probe inside it. This is done by the technique that uses the Seismology of the white dwarfs to reveal their internal structures.
- The oscillation of white dwarf causes small variations in the light output to stop and is never constant.
- By using seismology astronomers discovered the inner structure of the white dwarf forms mostly carbon and oxygen, while the outer surface of hydrogen(Fig 2).
- The age of the by god is inversely proportional to its temperature.
- The huge number of white dwarfs in the Galaxy are binaries. There are cases of binary → first, both are white dwarf, second, one is a white dwarf and other is the main sequence star.
- The binary system, in which the mass flows from one star to the acceleration disk then to white dwarf is called the Cataclysmic Variable.
- Most light in such a binary system is radiated from the region called Hotspot (Fig 1).
- There are several types of cataclysmic variables but all fall under the bracket of Novae.
- Dwarf Novae, however, flare up irregularly but are 10 times brighter than the star.
- The idea of flare occurrence → a mass-losing star- gradually- throws extra mass towards the outer surface. The matter gets piled up in the outer region. The mass, then, moves towards the white dwarf and builds friction; then heat; and eventually, light. This dazzling light is nothing but the flare.
- The reason for the white dwarf Novae is the disk heating instability.
- Recurrent Novae used to be 10³ times brighter than their original brilliance before the outburst. It occurs every 10- 100 years.
- Classical Novae used to be 1000³ times dazzling (Fig 2). It occurs every 10000 years. This is a thermonuclear explosion, however, only the outer layer is excluded.
- Initially, the stars in the binary stay apart, then come closer due to the formation of the common envelope that is already discussed in the second chapter.
- The rapid mass-loss makes it difficult for the white dwarf to accept it again, thus the matter revolves around the star. However, this is not an excretion disk but a red-giant sphere along with the star.
- Revolving in this way results in the stars come much closer that they touch each other and nuclear explosion blows the common envelope. while some matter remains around. Now, it has become a completely formed cataclysmic variable(Fig 3). However, now, the mass-gaining star, again, loses mass to the white dwarf.
- There are two possible fates of cataclysmic variables. First, the system could fizzle out forming a new star and stopping the mass transfer; second, could result in cataclysmic implosion or explosion.
- Now, death depends on mass with respect to the Chandrasekhar limit.
- The star with 10% of the Chandrasekhar limit increases its mass steadily, but slowly. In case of a star with the mass- closer to Chandrasekhar limit can trigger the nuclear reaction and explodes due to the high Quantum pressure.
- The star with the mass that equals Chandrashekhar limit thus implodes and the energy releases in the form of neutrinos. Thus, it results in the neutron star around the main sequence star. Also, this implosion releases nearly no ‘optical’ radiation.
- If the white dwarf possibly is made of iron, magnesium etc. it approaches the Chandrasekhar limit and carries implosion. But, here, a white dwarf is formed as a companion. As a result, mass transfer ends and, finally, they crackle. For a normal star, mass is directly proportional to the size, but for the White dwarf, which is supported by the quantum pressure, mass is inversely proportional to the size. Therefore, when the white dwarf loses its mass its size increases and ultimately it consumes its companion star.
CURRENTLY, YOU ARE ON CHAPTER-5