There are many different types of stars in the universe and they can be categorised according to their mass and temperature. They can also be classified by their spectra (the elements they absorb) and their brightness.
Our Milky Way galaxy contains an estimated 300 billion stars. Each one is brighter than our Sun, but much further away so they appear much smaller or not at all to the naked eye. Stars have been used for celestial navigations and religious practices for many thousands of years. Astronomers grouped stars into constellations to track their movements and the position of the Sun.
Stars form in huge clouds of gas and dust. Gravity causes the clouds to contract, drawing the gas closer and, as these materials accumulate in the centre, density rises and pressure increases. This then causes the matter to heat up and glow while the mass increases.
The temperature and pressure continues to grow until hydrogen can be fused. The heat generated by this nuclear fusion causes the gas to expand and, when hydrostatic equilibrium is reached, the star is born. Most stars form in groups called star clusters, but many are eventually ejected from these clusters.
The 7 Main Spectral Types of Stars
There are 7 main spectral types of stars — O (Blue), B (Blue), A (Blue), F (Blue/White), G (White/Yellow), K (Orange/Red) and M (Red). The stars are classified based on their temperature, with the hottest being O and the coolest being M. The temperature of each spectral class is then subdivided by the addition of a number, 0 stands for the hottest while 9 for the coolest.
This system is known as the Morgan-Keenan (MK) system and was introduced by William Wilson Morgan and Philip C Keenan in 1943.
The most common types of stars in our night sky, are dwarf stars. The most common type of dwarf star is a main-sequence red dwarf star. The Sun is a main-sequence, yell0w dwarf G-type star, but most of the stars in the Universe are much cooler and have a low mass. In fact, most of the main-sequence red dwarfs are too dim to be seen with the naked eye from Earth. With the naked eye, we can perceive around 2,000–2,500 stars.
A protostar is a collection of gas that has collapsed down from a giant molecular cloud. It is what exists before a star forms. This stage usually lasts around 100,000 years and, over time, gravity and pressure increases, forcing the protostar to collapse.
T Tauri Stars
A T Tauri star is the stage a star is at before it becomes a main sequence star. This occurs at the end of the protostar phase when the gravitational pressure holding the star together is the source of all its energy.
T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion. However, they do resemble main-sequence stars — they’re about the same temperature but brighter because they’re larger. The T Tauri stage will last about 100 million years.
Main Sequence Stars
Main Sequence stars are young stars that are powered by the fusion of hydrogen into helium in their cores. About 90% of the stars in the Universe are main-sequence stars, including our Sun. The main sequence stars typically range from between one-tenth to 200 times the Sun’s mass.
A star in the main sequence is in a state of hydrostatic equilibrium, which means gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, which allows the star to maintain a spherical shape.
Blue stars are typically hot, O-type stars that are commonly found in active star-forming regions, particularly areas such as the arms of spiral galaxies, where their light illuminates surrounding dust and gas clouds making these areas typically appear blue.
They are characterised by the strong Helium-II absorption lines in their spectra. They have weaker hydrogen and neutral helium lines in their spectra than B-type stars.
Their temperatures are around 30,000 K, with a luminosity around 100 to 1 million times that of the Sun. They usually have a mass around 2.5 to 90 times that of the sun and last about 40 million years.
Blue stars usually have relatively short lives that end in violent supernova events because they are so hot and massive. Examples of blue stars are Delta Circini and Theta1 Orionis C.
Yellow dwarfs are of the spectral type G and have a mass between 0.7 and 1 times the mass of the Sun. Around 10% of stars in the Milky Way are yellow dwarfs.
Yellow dwarfs have a surface temperature of about 6000°C and shine a bright yellow, almost white. Our Sun is a G-type star, but it is in fact white.
They have temperatures between 5,200 K to 7,500 K and luminosities around 0.6 to 5.0 times that of the Sun. They last about 4 to 17 billion years. G-type stars convert hydrogen into helium in their cores, and will evolve into red giants as their supply of hydrogen fuel is depleted.
Examples of a yellow dwarf are, our own Sun, Alpha Centauri A and Tau Ceti.
Orange dwarfs are of the spectral type K. They emit markedly less UV radiation than G-type stars and they remain stable on the main sequence for up to about 30 billion years, as compared to about 10 billion years for the Sun. This makes them of particular interest in the search for extraterrestrial life.
Orange dwarfs are also very favourable for exoplanets that might reside in their habitable zone as they are about four times as common as G-type stars.
They have temperatures between 3,700 K to 5,200 K and luminosities around 0.08 to 0.6 times that of the Sun. They have a mass of between 0.45 to 0.8 times that of the Sun.
Examples of an orange dwarf are Alpha Centauri B and Epsilon Indi.
Red dwarf stars are the most common kind of stars in the universe. They have such a low mass that they’re much cooler than stars like our Sun, and therefore appear very faint.
Red stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. This means that some of these stars live to up to 10 trillion years.
Their temperatures are usually around 4,000 K, with luminosities around 0.0001 to 0.8 times that of the Sun. The smallest red dwarfs are 0.075 times the mass of the Sun, but they can have a mass of up to half of the Sun.
Examples of a red dwarf are Proxima Centauri and Trappist-1.
Types Of Stars: Giant and Supergiant Stars
Giants and supergiants form when a star runs out of hydrogen and begins burning helium. These are the largest stars in the universe.
As a star’s core collapses and gets hotter, the resulting heat subsequently causes the star’s outer layers to expand outwards.
Stars that are either low or medium in mass evolve into red giants, and stars with high-mass, around 10+ times bigger than the Sun, become red supergiants. A star can contract itself and become a blue supergiant during periods of slow fusion. The blue colour is usually present when temperatures are spread over a small surface area, making them hotter. Oscillations between red and blue can also occur.
Blue giants are very rare because they only develop from more massive and less common stars, and because they have short lives.
Stars with luminosity classifications of III and II (bright giant and giant) are referred to as blue giant stars. Their spectral types are O, B and A.
The term blue giant applies to a variety of stars in different phases of development. They are evolved stars that have moved from the main sequence but have little else in common. However, the true blue giants have temperatures above 10,000 K.
The temperature of a blue giant can vary all the way up to 33,000+ K, with luminosities around 1,000 times that of the Sun. They have a mass of between 2 to 150 times that of our sun and usually last between 10 to 100 million years.
Examples of a blue giant are Meissa and Iota Orionis.
Blue supergiants are also rare. They are scientifically known as OB supergiants, and generally have luminosity classifications of I, and spectral classifications of B9 or earlier. They are usually larger than the Sun, but smaller than red supergiant stars, with a mass of between 10 and 100 solar masses.
Blue supergiants have temperatures between 10,000 K to 50,000 K and luminosities around 10,000 to 1 million times that of the Sun.
They live very short lives, around 10 million years. Because of their mass, blue supergiants quickly burn their hydrogen supplies. Some stars evolve directly into Wolf-Rayet stars, jumping over the normal blue supergiant phase.
Examples of a blue supergiant are Rigel and Tau Canis Majoris.
Red giant stars are of the spectral types M and K and are much smaller than red supergiants and much less massive. When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together.
Therefore, a shell of hydrogen around the core ignites, continuing the life of the star but causes it to increase in size dramatically. This is what creates a red giant.
Red giants can be 100 times larger than the star was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions.
They normally have temperatures of around 3,300 to 5,300 K, and luminosities around 100 to 1,000 times that of the Sun. They also have a mass of between 0.3 to 10 of the Sun’s. Red giants live for around 0.1 to 2 billion years, before they run out of fuel completely and become a white dwarf. Examples of a red giant are Aldebaran and Arcturus.
Red supergiant stars are stars that have exhausted their supply of hydrogen at their cores, and therefore their outer layers expand hugely as they evolve off the main sequence. They are of the spectral types K and M and are among the biggest stars in the universe, though they are not among the most massive or luminous.
They have temperatures of between 3,500 to 4,500 K, and luminosities between 1,000 to 800,000 times that of the Sun. Red Supergiants have a mass of between 10 to 40 times that of the Sun and live for around 3 to 100 million years.
Some red supergiants which still can create heavy elements eventually explode as type-II supernovas. Examples of a red supergiant are Antares and Betelgeuse.
Types Of Stars: Dead Stars
Dead stars no longer have fusion processes taking place in their cores.
A white dwarf star is formed when a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity.
The reason a white dwarf shines is because it was a hot star once, but there’s no fusion reactions happening anymore. They normally have temperatures of between 8,000 to 40,000 K, and luminosities around 0.0001 to 100 times that of the Sun. They have a mass of about 0.1 to 1.4 times that of our Sun.
A white dwarf can live for between 100,000 to 10 billion years. Examples of a white dwarf are Sirius B and Procyon B.
Neutron stars are the collapsed cores of massive stars that were compressed beyond the white dwarf stage during a supernova explosion.
A neutron star is an unusual type of star that is composed entirely of neutrons — particles that are marginally more massive than protons, but carry no electrical charge.
Neutron stars can collapse into black holes if they have more than 3 solar masses. They usually have temperatures of around 600,000 K and very low luminosities. They have a mass of about 1.4 to 3.2 times that of our Sun and live for between 100,000 to 10 billion years.
Examples of a neutron star are PSR J0108-1431 and PSR B1509-58.
Black dwarfs are theorised to be white dwarfs that have radiated away all their leftover heat and light. However, because white dwarfs have relatively high life spans, no black dwarfs have had enough time to form yet. This means black dwarfs are somewhat hypothetical. If a black dwarf were to form, it will be after our Sun has died.
While small stars eventually become white dwarfs or neutron stars, stars with a high mass become black holes after a supernova explosion.
Because the remnant has no outward pressure to oppose the force of gravity, it will continue to collapse into a gravitational singularity and eventually become a black hole. A black hole is so strong that not even light can escape it.
Examples of a black hole are Cygnus X-1 and Sagittarius A.
Types Of Stars: Failed Stars
Failed stars are stars that do not have sufficient mass to ignite and fuse hydrogen gas, which means they do not shine. It is common for brown stars to be known as failed stars.
Brown dwarfs range between spectral types M, L, T and Y and don’t emit visible light. A Brown dwarf usually fills the gap between the most massive gas planets and the least massive stars.
They normally have a temperatures of between 300 K to 2,800 K and a mass of about 0.01 to 0.08 times that of our Sun. They can live for possibly trillions of years.
Examples of brown dwarf are Gliese 229 B and Luhman 16.
Types Of Stars: Binary Stars
A double star is two stars that appear close to each other in the night sky. Some are binaries, which means the revolve around one another, while others just appear together because they are in the same line of sight.
A binary star is a system of two stars that rotate around a common centre of mass. About half of stars are in a binary star system.
An eclipsing binary is two close stars that appear to be a single star varying in brightness. The variation in brightness is due to the stars periodically eclipsing and obscuring one another.
X-Ray Binary Star
An X-ray binary star is a special type of binary star in which one of the stars is a collapsed object such as a white dwarf, neutron star or black hole. As matter is stripped from the remaining star, it falls into the collapsed star, which produces X-rays.
Types Of Stars: Variable Stars
Variable stars are stars that vary in luminosity.
Cepheid Variable Star
A Cepheid variable star is a star that regularly pulsates in size and changes in brightness. As the star increases in size, its brightness decreases, and then vice versa happens.
Stars are not always permanently variable and it can just be a phase that they are going through. An example includes the Whirlpool Galaxy.