Accretion Disks

Introduction figure

Note → This chapter explains the disks of matter, which surround the two binary stars. This summary continues the excretion disk, accretion disk, and science within them in more detail. Therefore, I request the respected readers to peruse the previous chapters before proceeding here(4^th chapter). Links are provided at the end of the story.

  1. Matter from the mass ejecting star, when is not accepted by the companion star, forms a ring around the both in common, which is known as an Accretion disk. The accretion disk is also called a flat star.
  2. The first-ever accretion disk is discovered around a binary of white dwarfs.
  3. Accretion disks are the primary sources of radiation (mostly- X-rays) from the binary systems.
  4. One star provides matter and another provides Gravity to the disc. Being hot, the disk generates heat and light on its own.
  5. This disk forms through a simple process → the mass gaining star captures the mass/matter and creates the stream of particles. This stream, due to high gravity, creates a ring around the binary and finally a disc.
  6. Here, the velocity of the disc is inversely proportional to angular momentum. Also, the velocity of the disc is inversely proportional to the size of the orbit of the disk.
  7. The disk leaves its own life as a flat star by driving the energy from the matter of the companion star (Fig 1).
  8. The situation in Fig 1 applies to every type of binary. Here, the matter, in the end, collides to the surface. But, in case of black holes, the matter continues to move and radiate, thus jets of radiation seem to radiate from the black hole.
  9. The heat- due to friction- is released from the disk as radiation.
  10. The generation of heat depends on the temperature of the disc. The potential decrease in temperature makes the matter transparent, thus low heat and friction appear, but more radiation. A hotter disk makes the matter opaque, thus less radiation is released. (Fig 2)
  11. The cooling of the disk after the heating stage leads to a sudden loss of density, due to which the matter rarely interacts and the disc at the central part swells and gets fatter. Here, the fatness stays uniform.
  12. At this stage, the matter moves in so rapidly that it Falls into a black hole increasing the energy and decreasing the heat and temperature as well. The process of energy generation in this process is called advection-dominated accretion flow(ADAF).
  13. This heat is carried in the black hole and radiates in the form of X-rays and gamma rays. However, the central structure continues to be flat and is called Convection-Dominator accretion flow(CDAF).
  14. The outer portion of the disc releases the optical radiation, while the inner portion is known for its ultraviolet and X-rays (Fig 3). However, the gamma rays are also emitted.
  15. Later, in case of the white dwarf, the matter collides into the surface and it’s the end. But, the case is different for neutron stars or black holes.
  16. For the neutron star and black holes, the velocity of spiraling can even get near to the speed of light.
  17. The magnetic field in the disc becomes naturally stretched, thus amplifies and stretches the matter and carries it inwards (Fig 4). This magnetic field from the center can drive the energy into the matter and fasten the spinning. This process is called ‘Magnetic-rotational instability’ or ‘Magnetorotational instability.
  18. Due to the large X-ray radiation at the center of neutron stars and black holes, there occur ‘X-ray transients’.
  19. The black holes are defined as ‘Magnetically-dominated accretion flow(MDAF).
Figure- 1
Figure- 2
Figure- 3
Figure- 4







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Rohan Gharate

Rohan Gharate


I am a high school student and an excellent astrophysics enthusiast