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=^= 0.3.2 - Star Classification

Created by Captain Lirha Saalm on 14 Sep 2012 @ 6:17pm

Star Classification List

The Hertzsprung-Russell Star Classification Chart.

Introduction to Stars

Stars are classified according to a spectral scale which measures the temperature of a star's protosphere. There are four main categories into which they are grouped, including main sequence stars, giant stars, supergiant stars, and white dwarfs. Other variations include brown dwarfs and neutron stars. Most stars found in the galaxy are main sequence stars which use a nuclear reaction to burn hydrogen and convert it into helium to create massive amounts of energy. Most of these stars spend almost their entire lifespan as a main sequence star, but turn into giants or supergiants when they exhaust their supply of hydrogen and then expand. Low-mass sequence stars typically turn into giants, and high-mass sequence stars turn into supergiants. After a giant expands to critical mass and sheds its outer layer, it implodes and leave remnants which become white dwarfs. In rare cases, a high-mass star will implode and create a black hole.

For each spectral class, there are ten subcategories, (0-9) with 0 being the hottest and 9 being the coolest. In order to classify a star, the spectral class must first be identified, followed by an exact assessment of its surface temperature. For example, an A0 is a Spectral Class A star that is burning extremely hot for its class, around 8,500 to 8,700 K. Stars are then sub-categorized by luminosity. Luminosity subclasses include: 1a (luminous supergiants), 1b (less luminous supergiants), II (luminous giants), III (normal giants), IV (subgiants), and V (main sequence and dwarf stars). Using the previous example, an A0V designation now tells us that the star is main sequence Spectral Class A star that is burning extremely hot for its class.

Spectral Class O

Class O stars are the largest and hottest in the galaxy, ranging anywhere between 50 and 60 times the size of Earth's sun with 100,000 times the luminosity. They also happen to be the rarest type in abundance, only accounting for roughly 0.00001% of all known stars. They are composed primarily of ionized atoms and helium, and can reach extreme temperatures anywhere between 30,000 and 40,000 K. Their intense surface temperatures give these celestial bodies a deep blue color. Due to the immense size and energy put out, they burn for only a (relatively) short period of time, only about 10 million years.

Spectral Class B

Class B stars are much smaller than Class Os, yet are still at least ten times the size of Earth's sun. They have a surface temperature of around 20,000 K which gives them a blue color and gives off a luminosity of about 1,000 Sols. Their composition is mostly helium but also has a light concentration of hydrogen as well. This allows them to live longer than Class Os, typically about 100 million years. Like their larger cousin, they are rare to find and only account for 0.1% of all known stars.

Spectral Class A

Burning bright white, Class A stars are smaller than Class O and B stars, however are still twice the size of Earth's sun and have 20 times the luminosity. Their surface temperature is usually near 8,500 K, and they are composed of strong concentrations of hydrogen and ionized metals. Their smaller size and composition allows them to live close to one billion years, and they are one of the most optimal star types when searching for habitable life in their solar systems. Though still rare, they account for 0.7% of all known stars.

Spectral Class F

Class F stars are in the middle of the spectral chart yet still only compose about 2% of all stars in the galaxy. Smaller than Class A stars, they are only slightly bigger than Earth's sun, about 1.5 times the size. Their surface temperatures are anywhere between 6,000 and 7,000 K which yields a luminosity of four Sols and gives they a white-yellow color. They are composed of a variety of elements including hydrogen, ionized metals, iron and even calcium. Their average life span is three billion years, and these stars are known to play host to diverse planetary bodies.

Spectral Class G

Class G stars are yellow in color and are the same type as Earth's sun, Sol. A good balance between size and longevity, these stars can live up to 10 billion years with sustained surface temperatures of 5,000 to 6,000 K. They are composed of hydrogen, ionized calcium, and ionized and neutral metals. Their mass and radius are within +/- 0.1 Sols. They make up 3.5% of all known stars, and can be excellent breeding grounds for life-sustaining solar systems.

Spectral Class K

Smaller and cooler than the Earth's sun, these stars have lower surface temperatures usually between 4,000 and 5,000 K. They produce an orange color which can vary depending on their composition, but their luminosity is only 0.2 Sols and they are roughly only 0.6 to 0.7 Sols in radius and mass. Composed of less hydrogen and more metals, they are able to survive for much longer than most stars, up to 50 billion years. They are somewhat common in the galaxy and account for 8% of all stars. Unfortunately, due to their low heat/energy output, they rarely have many habitable planets in their solar systems, if any.

Spectral Class M

Class M stars are the most populous in the galaxy and make up almost 80% of all stars. This is because they also happen to live the longest, sometimes in excess of 200 billion years. Unfortunately, these stars are cold and small, with surface temperatures only around 3,000 to 3,500 K. They are less than half the size of Earth's sun, and produce only 0.01 Sols of luminosity. Habitable planets are almost never found in these systems, however their enormous life span allows them to accumulate many celestial bodies which fall into orbit over time. Class Ms are red in color and composed of very little hydrogen, and mostly ionized atoms and helium.

Giants (Red)

Giant stars are low-mass stars which are at the end of their lifespan. They are created when a main sequence star runs out of fuel and the surface begins to expand. Compared to supergiants, giants are small, and are mainly composed of Spectral Class G, K, or M stars, with surface temperatures ranging anywhere between 3,000 and 10,000 K. Typically they are between one and five times the mass of Earth's sun, but have a much larger radius of 10 to 50 Sols due to their expanded surface. Despite their lower surface temperatures, their luminosity is high, between the ranges of 50 to 1,000 Sols, and they are very bright. Giant stars compose about 0.4% of all known stars, and have a lifespan of about one billion years.


Supergiant stars are similar to giant stars, however are the result of high-mass stars which are nearing the end of their lives. They are created through the same process as giants and require a main sequence star to run out of fuel prior to expanding. Due to their huge size, supergiants are made of Spectral Class O, B, A, or F stars. Their surface temperatures are very hot and can range anywhere between 4,000 and 40,000 K. Compared to giants, supergiants are huge, and possess a mass between 10 and 70 Sols, with a radius of 30 to 500 Sols. Their luminosity is immense, from 30,000 to 1 million Sols, yet they do not live very long before imploding and typically last 10 million years at best. They are very rare and only make up 0.0001% of all stars in the galaxy.

White Dwarf

After a giant star becomes unstable and loses mass, it blows its surface and outer layers off creating a planetary nebula, yet the core still remains. The star now becomes a white dwarf. They are not very large and typically have a mass of only 0.1 to 1.4 Sols, and a tiny radius under 0.01 Sols. Luminosity is also very low but surface temperatures can still reach anywhere up to 10,000 K. It takes billions of years for a white dwarf to cool, but it is theorized that once this occurs, white dwarfs will turn into black dwarfs, a non-radiating ball of glass. No black dwarfs have been found to exist, therefore it is assumed that the universe is simply not old enough for this process to have taken place. For the time being, a white dwarf represents the final stage in a star's life.

Brown Dwarf

Brown dwarfs are commonly referred to as failed stars. This is usually because, for some reason or another, they lack the required mass to initiate the hydrogen-nuclear reaction necessary to turn into full stars. They still put out substantial heat, but nothing above 2,500 K and are more akin to a hot planet than a star. They are typically no bigger than Earth's sun, and have extremely low luminosity, 0.004 Sols at best.

Neutron Star

A neutron star is the remains of a supernova. They are created when the core of a massive star collapses on itself during the explosion, and as their name suggests, they are composed mostly of neutrons. Tiny in size and radius, they have incredible mass and can generate enormous gravity wells despite their deceptively small appearance. Neutron stars are no larger than a small city, yet have the mass of half a million Earths. They are one of the densest objects known to exist.

  • Pulsars: Pulsars are neutron stars which emit visible beams of electromagnetic (EM) radiation due to their highly-magnetized nature. Similar to how a lighthouse functions, pulsars emit beams of light which can only be seen as the star rotates through certain angles.

(Created by Lirha Saalm, USS Galileo)

Categories: Science Database