White Dwarf

• 25 Oct 2021

Why in News

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.

Key Points

• About White Dwarfs:
  
  • Formation:
    
    • 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.
  • Chandrasekhar Limit:
    
    • 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).
• Switch on and off Phenomena:
  
  • The white dwarf, which is discussed, is part of a binary system called TW Pictoris, where a star and a white dwarf orbit each other.
    
    • The two objects are so close to each other that the star transfers material to the white dwarf.
  • As this material approaches the white dwarf it forms an accretion disk or a disk of gas, plasma, and other particles around it.
  • As the accretion disk material slowly sinks closer towards the white dwarf it generally becomes brighter.
  • Also there are cases when the donor stars stop feeding the white dwarf disk. However, reasons for this are still not clear.
  • When this happens the disk is still bright as it “drains” material that was previously still there.
    
    • It then takes the disk about 1-2 months to drain most of the material.
  • However, TW Pictoris' drop in brightness in 30 mins was totally unexpected and it may be due to the process called magnetic gating.
    
    • Magnetic gating happens when the magnetic field is spinning so rapidly around the white Dwarf it creates a barrier disrupting the amount of matter the white dwarf can receive.
• Significance: This discovery will help understand the physics behind accretion – how black holes and neutron stars feed material from their nearby stars.

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.

Source: IE