To learn about why and how stars are classified, read our “Stellar classification” article here. This article aims to explain all the classifications mentioned in the “Stellar Classification” in brief.
Summary of the Types of Stars
Stars are classified under the modern stellar classification system named the Morgan-Keenan system. This classification includes the letters O, B, A, F, G, K, or M. Stars classified as ‘O-type’ are the hottest whereas stars that are classified ‘M-type’ are the coldest. Each letter is sub-divided into 10 categories of numbers, from 0 to 9 where 0 is the hottest and 9 is the coldest. Few classification examples- K8, O0 etc. Some other classifications include D, L, T and Y for Red and Brown Dwarf Stars etc.
Understanding each type
O-type stars are the rarest of all the types. These stars emit very strong Ultraviolet radiation. They have a luminosity of 10,000 to 1,000,000 suns (the most luminous of any star type) and have temperatures of 30,000 to 50,000 Kelvin (the hottest of any star type). They ted to be some of the most massive stars ever observed. Their mass on the lower side is around 16 Solar masses. Their mass on the higher side is still unclear. Some examples of O-type stars include 9-Sagittarii, Iota Orionis, etc.
B-type stars have a mass range of 2-16 solar masses and a temperature range of 10,000 to 30,000 Kelvin. They are very luminous and are blue in color. Due to there high energy output, they (O and B stars) tend to live very short lives. These kinds of stars are also known to have planets around them. Some examples of these type of stars are Regulus, Argol A, etc.
A-Type stars are white or blue white stars that mostly emit infrared radiation and are most commonly visible to the naked eye. These stars are 5 to 25 times more luminous than our sun and usually have temperatures ranging from 7,500 K to 10,000 K . In the latter stages of their life, these stars turn into red giant stars. A-Type stars are also known to host planets. Some famous examples of A-type stars are Sirius and Vega (shown in image on the right).
F-type stars are hydrogen-fusing stars that are white or yellowish white in color. They are not much bigger than our sun- 1.15 to 1.4 times the radii of our sun and 1.04 to 1.4 times the mass of our sun. These stars have a temperature of around 6,000 K to 7,500 K. Astronomers say that planets around F-type stars can be habitable for life. But, these stars emit relatively higher UV radiation which can damage basic DNA molecules over time unless the concerned planet has sufficient atmospheric shielding.
G-Type stars are commonly known as yellow-dwarf stars. Our Sun falls into the category of G-Type stars. These stars have a mass between 0.8 and 1.04 solar masses and a radius between 0.96 to 1.15 solar radii. These type of stars also have a very long lifespan of about 10 billion years. After that time period, they expand multiple times their size and become a Red Giant star. Some well known examples of these stars other than the Sun include Alpha Centauri A, Tau Ceti etc. G-types also have planets around them that may be habitable for life.
K-type stars are slightly cooler than the sun, temperatures ranging from 3700 K to 5200 K. These stars are orangish in color. K-types are favorites in the search for life. Their small size of 0.7 to 0.96 solar radii and mass of 0.45 to 0.8 solar masses mean that they have a very long lifespan- 18 to 34 billion years. They also emit less UV radiation, therefore, they are less harmful to DNA. Some well known K-type stars are Alpha Centauri B and Epsilon Indi.
M-type stars commonly known as red dwarf stars are the most common type of stars. They are very dim and small, making them invisible to the naked eye. They too have planets around them that may be habitable for life. But, due to the small size of M-type stars- <0.7 solar radii, 0.08-0.45 solar mass- the planets around these stars will be tidally locked. This means that planets orbiting these stars will have one face that will always face towards the star and the other face will always face away from the star. In addition, planets in orbit around these stars are very prone to Solar flares. These factors make it harder for life to exist. A well known example of M-Type star is Proxima Centauri.
White dwarf Stars– D.
Cool stars like Red and Brown Dwarfs– L, T and Y
Carbon stars- C- Carbon stars are old red giant stars with a great abundance of carbon in atmosphere
Planetary Nebulae– P