• Recently, an international team saw a white dwarf losing its brightness in 30 minutes, which usually takes a period of several days to months.
  • This peculiarity in brightness of white dwarfs can be referred to as switch on and off phenomena.
  • Using the Hubble Space telescope and Transiting Exoplanet Survey Satellite (TESS)astronomers have identified several white dwarfs over the years.

Important points:

  • White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel.
  • Such stars have very high density.
  • A typical white dwarf is half the size of our Sun and has a surface gravity 1,00,000 times that of Earth.
  • Stars like our sun fuse hydrogen in their cores into helium through nuclear fusion reactions.
  • Fusion in a star’s core produces heat and outward pressure (they bloat up as enormous red giants), but this pressure is kept in balance by the inward push of gravity generated by a star’s mass.
  • When the hydrogen, used as fuel, vanishes and fusion slows, gravity causes the star to collapse in on itself into white dwarfs.



Black Dwarfs:

  • Eventually – over tens or even hundreds of billions of years – a white dwarf cools until it becomes a black dwarf, which emits no energy. Because the universe’s oldest stars are only 10 billion to 20 billion years old there are no known black dwarfs.
  • It must be noted that not all white dwarfs cool and transform into black dwarfs.
  • Those white dwarfs which have enough mass reach a level called the Chandrasekhar Limit.
  • At this point the pressure at its center becomes so great that the star will detonate in a thermonuclear supernova (explosion).

Chandrasekhar Limit

  • Chandrasekhar Limit is the maximum mass theoretically possible for a stable white dwarf star.
  • A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 times the mass of the Sun.
  • Any degenerate object more massive must inevitably collapse into a neutron star or black hole.
  • The limit is named after the Nobel laureate Subrahmanyan Chandrasekhar, who first proposed the idea in 1931.
  • He was awarded the Nobel Prize in Physics in 1983 for his work on the physical processes involved in the structure and evolution of stars.


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