Stars come in a wide range of sizes, colors and temperatures. There are 7 main types of stars that astronomers categorize based on their temperature and color. Below we will explore the 7 main star types from hottest to coolest.
O Type Stars
O type stars are the hottest, brightest and rarest stars in the universe. They have temperatures of over 30,000 Kelvin. At these extreme temperatures, O type stars emit large amounts of ultraviolet radiation. This high energy radiation ionizes the gas around O type stars, causing it to glow brightly.
O stars tend to have blue-white or blue color. Examples of O type stars include Zeta Puppis, Naos and HD 93129A. O stars don’t live very long. Their lifespans are only a few million years because they consume their fuel so quickly.
B Type Stars
B type stars are the second hottest type of star, after O stars. They have temperatures between 10,000 Kelvin and 30,000 Kelvin. This high temperature gives B stars a blue-white hue.
Like O stars, B stars are very luminous because of their high temperatures. But they are less massive and have longer lifespans than O type stars. Examples of B type stars include Regulus and Rigel.
A Type Stars
A type stars have temperatures between 7,500 Kelvin and 10,000 Kelvin. This makes them white or blue-white in appearance. The sun is a G type star with a temperature of around 5,800 Kelvin. So A stars are hotter than our sun.
A type stars are among the more massive main sequence stars. They tend to be rapid rotators and can have large starspots on their surfaces. Sirius and Vega are two examples of A type stars that can be seen without a telescope.
F Type Stars
F type stars have temperatures between 6,000 Kelvin and 7,500 Kelvin. This gives them a white color. F type stars emit a good portion of their radiation in the ultraviolet. But they emit significantly less UV than hotter O and B type stars.
Examples of F type stars include Procyon and Canopus. At an estimated temperature of 6,750 Kelvin, Canopus is one of the hottest F type stars. F stars are larger and more luminous than our G type sun. But they aren’t as massive as A type stars.
G Type Stars
Our sun is a G type star with a surface temperature of around 5,800 Kelvin. This yellow-white color makes G stars among the coolest of the main sequence stars. Besides the sun, other examples include Alpha Centauri A, Tau Ceti, and 51 Pegasi.
G stars make up about 7% of the main sequence stars in our galaxy. They are smaller and less luminous than larger, hotter star types. But they last much longer, burning steadily for 10 billion years or more. This provides the stable conditions needed for life to evolve on planets orbiting G type stars.
K Type Stars
K type stars have temperatures between 3,700 Kelvin and 5,200 Kelvin. This makes them orange stars, distinctly cooler than our yellow sun. Their orange color is caused by increased absorption of violet and blue light. As a result, K stars emit more red and infrared light from their cooler atmospheres.
Examples of K type stars are Epsilon Indi, 61 Cygni, and Alpha Centauri B. With a temperature around 5,300 K, Alpha Centauri B is among the hottest of the K type stars. Many K stars are smaller stars that are stable and long-lived.
M Type Stars
M type stars are the coolest and most common main sequence stars. They have temperatures below 3,700 Kelvin, giving them a red color. The coolest M stars have temperatures that can approach 2,000 Kelvin. At these cool temperatures, M stars emit very little visible light. Instead, most of their radiation is concentrated in the infrared.
Some examples of M type stars include Antares, Betelgeuse, and Barnard’s Star. Smaller M type stars are known as red dwarfs. Red dwarfs make up about 75% of the stars in our Milky Way galaxy. Though cool and dim, red dwarfs have very long lifespans and may harbor many billions of habitable planets.
The Different Star Types by Temperature
This table summarizes the 7 main star types ordered from the hottest to the coolest:
| Star Type | Temperature (Kelvin) | Color |
|---|---|---|
| O | Over 30,000 | Blue-white |
| B | 10,000 to 30,000 | Blue-white |
| A | 7,500 to 10,000 | White or blue-white |
| F | 6,000 to 7,500 | White |
| G | 5,000 to 6,000 | Yellow-white |
| K | 3,700 to 5,200 | Orange |
| M | Less than 3,700 | Red |
As shown in the table, O and B type stars are hot, blue-white stars while K and M type stars are cool, red stars. In between are the white A and F stars, yellow-white G stars, and orange K stars. A star’s temperature determines its color and brightness. This, along with its mass, also determines how long each star will live.
Lifespans of the Different Star Types
In general, the more massive and hotter stars burn through their fuel faster and have shorter lifespans than smaller, cooler stars:
– O and B type stars live for just a few million years.
– A stars may live for up to a billion years.
– F stars live for 3 to 10 billion years.
– Our sun, a G star, will live for about 10 billion years total.
– Smaller K stars may live for 15 to 30 billion years.
– Cool M dwarf stars have such long lifespans they have not existed long enough in the universe for astronomers to measure!
So while large O type stars shine brightly, they do so for only a brief cosmic moment. But tiny red M dwarf stars may shine steadily for trillions of years, long after stars like our sun have died.
Star Formation & Evolution
The life cycle of stars depends on their initial mass during formation. When giant molecular clouds of gas collapse, gravity causes them to fragment into smaller pockets that form protostars. More massive protostars have stronger gravity and pull in more gas.
These more massive protostars evolve into larger, hotter main sequence stars like B and O type stars. While low mass protostars become cooler, smaller main sequence stars like red dwarfs.
Over time, stars fuse lighter elements into heavier ones in their cores. When a star has consumed its core hydrogen fuel, fusion continues in shell around the core. For low mass stars like red dwarfs, this fusion continues for trillions of years.
But in more massive stars over 8 times the sun’s mass, fusion occurs quickly until iron builds up in the core. Fusion of iron absorbs energy, causing the star to collapse until a supernova explosion occurs. This leaves behind a dense neutron star or black hole remnant.
So a star’s initial mass determines its lifespan, temperature and ultimate fate. Lower mass stars burn longer and cooler, while high mass stars have shorter, hotter lives.
Locations of Different Star Types
The different types of stars also follow distinct patterns of distribution in galaxies:
– O and B type stars reside almost exclusively in the spiral arms of spiral galaxies like our Milky Way. Here star formation occurs more actively in the presence of dust and gas clouds.
– A stars exist more broadly throughout the spiral disk. While F, G, and K stars make up the central bulge of spiral and elliptical galaxies.
– M dwarf stars dominate the central halo region surrounding the core. Several dwarf stars also orbit the outskirts of galaxies as satellite galaxies.
So the hottest, most massive O stars are concentrated in star forming regions of galaxy disks. While cooler, lower mass M dwarf stars occupy the central and outermost regions of galaxies. This distribution results from galactic evolution over billions of years.
Notable Stars of Each Type
Here are some standout examples of stars representing each spectral type:
– O Stars – Zeta Puppis, HD 93129A, Naos
– B Stars – Rigel, Regulus, Spica
– A Stars – Sirius, Vega, Altair
– F Stars – Canopus, Polaris, Procyon
– G Stars – The Sun, Alpha Centauri A, Tau Ceti
– K Stars – Alpha Centauri B, Epsilon Indi, 61 Cygni
– M Stars – Proxima Centauri, Barnard’s Star, Trappist-1
Many of the brightest stars visible in the night sky are massive, hot O and B type stars. Sirius, the brightest star, is an A type star. Our sun is the most famous G type main sequence star. While tiny cool red dwarfs like Proxima Centauri and Trappist-1 have captured attention recently for their potentially habitable planets.
Classifying Distant Stars
Astronomers classify most stars based on their spectra. Stellar spectra act like fingerprints that allow astronomers to determine a star’s spectral type, temperature, composition, and velocity.
Comparing absorption lines in stellar spectra reveals the types and amounts of elements present. And combining spectroscopy with the Doppler shift of lines enables measuring a star’s radial motion.
So even for stars trillions of miles away, detailed analysis of their light spectra enables classification of their properties and types. This allows building a broader understanding of stellar populations and evolution throughout our galaxy and the universe.
Conclusion
While stars may appear as pinpoints of light in the sky, they actually range widely in their intrinsic properties and characteristics. The 7 main types of stars – O, B, A, F, G, K and M – encompass this diversity. Their classification depends fundamentally on temperature, which ranges from over 30,000 K for the hottest O stars down to under 3,700 K for the coolest red M dwarfs.
Temperature regulates a star’s color, luminosity, size, lifespan and ultimate fate. Massive blue-white O and B stars shine brightly and fuse fuel quickly, while lower mass red dwarfs smolder coolly for trillions of years. Spectroscopy allows astronomers to classify even distant stars by their spectra. Stellar classification provides insight into the origins, physics, evolution and distributions of different star populations in our galaxy and throughout the cosmos.