Stars are divided into spectral classes, which in turn help to identify their color, size, and luminosity. The seven main types of stars are assigned one of the letters O, B, A, F, G, K, and M, remembered by the classic mnemonic “Oh Be A Fine Girl (Guy), Kiss Me,” with their individual star colors, effective temperatures, size and masses compared to the Sun (sol) as follows:
Harvard Spectral Classification
- O: Blue, 28,000-50,000K, radius 20, mass 40,
- B: Blue-white, 10,000-28,000K, radius 5, mass 0.1,
- A: White, 7,500-10,000K, radius 2, mass 10,
- F: White-yellow, 6,000-7,500K, radius 1.2, mass 1.5,
- G: Yellow, 4,900-6,000K, radius 1, mass 1,
- K: Orange, 3,500-4,900K, radius 0.3, mass 0.5,
- M: Red, 2,000-3,500K, radius 0.1, mass 0.1,
The stellar classification sequence has since been extended to include the spectral types L, T, and Y, with L-type stars ranging in temperature from 1,300K to 2,000K, and usually red-brown in color; T-type stars between 700K and 1,300K and of a purplish-red hue; and Y type stars showing temperatures of less than 600K.
Life Cycle of Stars
Nebulae are large expanses of interstellar gas which mostly contain vast amounts of hydrogen and helium, and when a dense region of a nebula starts to gravitational collapse and heat up, stars begin to form. This may in turn result in the birth of just a few dozen stars to many thousands. To put the process into perspective, our Sun, which has a diameter of 864,938 miles (1.392 million km), would require an amount of gas hundreds of times the size of our solar system to form.
Protostars/T Tauri phase or Brown Dwarfs
The process of star formation begins with hot clumps of molecules forming inside a gas cloud to create a protostar, with the object remaining in this contraction stage as long as material continues to fall inward. For our Sun, this protostar phase would have lasted around 100,000 years, after which it would have entered the T Tauri phase for 100 million years, in which it shines using only energy produced by its ongoing gravitational collapse. Eventually, it would have acquired enough size and mass, as well as temperatures and pressures at its core to sustain nuclear reactions (hydrogen fused into helium), after which the outward force of its emitted radiation is balanced by its own inward gravity resulting in a hydrostatic equilibrium state referred to as the main sequence.
Those balls of gas whose mass is less than 8% that of the Sun, however, are unable to ignite nuclear fusion, and end up as Brown Dwarfs, or a failed star. These dim and cool objects fall into the M, L and T spectral class, and have between 13 and 90 times the mass of Jupiter. They also emit so little light and energy that they are difficult to detect.
The main sequence is where a star will spend 90% of its life fusing hydrogen into helium in its core. These type of stars account for around 90% of all stars in the universe, and range in mass from 1/10th to 200 times that of the Sun, with their life spans mostly depending upon their mass and chemical compositions; the least massive stars last for tens of billions of years, while for the heaviest stars their estimated lifetimes may only be a few million years.
Leaving the Main Sequence
Mass also determines how a star leaves the main sequence phase of its life, and what type of star it then becomes.
1) Those stars with solar masses less than 0.5 do not have enough size or pressure in their core to fuse helium, and so collapse directly into a ‘dead’ star known as a White Dwarf. These type of stars can be a million times denser than than the Sun, but have only 1% the Sun’s diameter and luminosity. Over several billion years, the leftover heat it still emits will subsequently radiate away to leave a Black Dwarf, which is a hypothetical stellar remnant that has no heat or light.
2) Those stars with solar masses between a 0.5 and 8 continue to fuse hydrogen into helium in their core until the hydrogen available runs out and hydrogen fusion takes place in a shell surrounding the core, which then expands to the star’s outer layers, resulting in it growing in size and luminosity to form a Subgiant, and then a Red Giant. In the meantime, the star’s helium-rich core starts to fuse helium into carbon and oxygen, and after its helium supply is exhausted the star’s outer layers will be ejected to form a planetary nebula, while its core becomes a white dwarf.
3) More massive stars will either evolve into Red Giants, or even Red Supergiants as they fuse heavier and heavier elements in their cores. Over time, they may oscillate between existence as a red and Blue Supergiant before being unable to fuse the iron which has formed in its core, leading to it becoming unstable and collapsing. A massive explosion then causes the star to go supernova, in the process creating many elements heavier than iron, such as uranium and plutonium, with those stars with 8 or more solar masses leaving behind a Neutron Star, and those with 30 or more Sun’s masses transforming into a Black Hole.