Star Classification: A Look at the Different Types of Stars

The stars, at its initial process of formation are termed as protostars. The contraction phenomenon occurs inside the protostar for around 10 to 15 million years and when the process of contraction ends in the protostar, it transforms into a pre-main sequence star. Depending on the mass of the star, it is classified as T Tauri star or Herbig Ae/Be star.

The lifetime of a main sequence star, mainly depends upon its mass and chemical compositions. Some of the main sequence stars are the Sun, Sirius, Alpha Centauri, etc. All the main sequence stars are in hydrostatic equilibrium condition, which means that the gravity and pressure gradient force inside the star is balanced and the star is stable.

The main sequence stars with extremely high temperatures are also called dwarfs. It should be clear that, all main sequence stars are not dwarfs, example, white dwarfs. The initial stage of a main sequence star is called brown dwarf.

The main sequence star, after converting most of the hydrogen present in it to helium, becomes a giant star, with increased radius and luminosity as compared to the main sequence star. In a giant star, the helium in its core, is fused into carbon and oxygen and this process is known as 'triple-alpha process'. There are two types of giant stars.

Blue Giant Star: These are the biggest and the hottest stars in the whole universe. Rigel is one fine example of a blue giant star which belongs to Orion constellation. The mass and luminosity of Rigel is 17 times and 40,000 times, respectively, more than that of the Sun.

Red Giant Star: Red giant stars are comparatively cooler than blue giant stars and their radius is 10 to 100 times more than the Sun. If a red giant star does not have sufficient temperature to fuse the carbon and oxygen elements present in its core, then it becomes a white dwarf. Some of the red giant stars are Aldebaran, Alpha Orionis, Antares, etc.

Stars with an initial mass, which is less than four times the mass of sun (yellow dwarf ), are termed as 'white dwarfs'. White dwarfs are highly dense objects in the universe and their density is figured out to be 1 million times more than that of the Sun. The pressure exerted by the electrons in the white dwarfs are independent of the velocity.

You may think that it is contradictory to the law of quantum mechanics (no two identical fermion particles with half-integer spin may occupy the same quantum state simultaneously), but subatomic particles don't always obey the laws of quantum mechanics.

Because of intense gravitational pull inside the star, it is small in size, but burns inside with very high temperature. It radiates heat, thereby becoming cooler and fainter, but still stable. White dwarfs are sorted out as dA, dB, dO, dAO and dAB, according to the primary constituent present in it.

In these classifications,

• 'd' means degenerative
• 'A' means hydrogen
• 'B' means neutral helium
• 'O' means ionized helium

Eventually, when the temperature of the white dwarf decreases to a certain level, it turns out to be a neutron star or black hole. In the in-between stage, it is called 'black dwarf'. IK Pegasi B is a white dwarf star.

Gravitational collapse of neutron stars, with heavy mass due to high density, results in a violent explosion called 'supernova'. The neutron star rotates very fast and its radius is eventually reduced due to conservation of angular momentum. The core of the neutron star is filled with subatomic particles like protons and electrons.

The magnetic flux density of a neutron star is of the order of 10¹² Gauss , which is approximately 10¹³ times of the magnetic field of our earth. For a massive star to become a neutron star, it should have a mass, which is 8 times that of the solar mass.

But for the same star to transform into a black hole, it must have a mass, which is 30 times more than our Sun's mass. Some examples of neutron stars are PSR J0108-1431, LGM-1, PSR B1257+12, etc.