The Types of Stars in Astronomy

Mankind has been looking up into the heavens since the dawn of time, until recent times, we knew very little about the world around us and even less about the lights in the night’s sky.

Many civilizations have cataloged the lights they observed at night, some even understanding the differences between stars and planets. Mapping stellar locations, movements and even describing their levels of brightness was essential for the evolution of understanding the types of stars in astronomy.

Before we dive into the discoveries and classifications of the different types of stars in astronomy, let’s take a look under the hood and see what makes a star, a star.

How Do the Stars in Astronomy Work?

Without going into a long and boring graduate level physics lecture, the simple explanation for how stars work is through the process of nuclear fusion.

This is probably better known by Albert Einstein’s famous equation of E=mc². E represents Energy, M stands for Mass and C is the letter used to mathematically express the constant for the speed of light (186,282 miles per second), the super-scripted number 2 represents the mathematical notation for the power of 2. Said as, energy is equal to the mass of an object multiplied by the speed of light (squared).

Nuclear fusion takes place at an atomic level, meaning super, super small particles. Nuclear fusion is the process of combining many nuclei (the center of atoms) into one nucleus. When this occurs, the original many nuclei are one element and after the process takes place, the final single nucleus takes the form as a different element.

Stars fuse together through the process of nuclear fusion; 2 hydrogen nuclei create 1 helium nucleus. After the process takes place, the overall mass is less than it was as two hydrogen nuclei. The additional mass gets expelled in the form of energy. This energy can be observed as the light emitted from a star or even the heat felt by a star, if close enough, obviously.

Our Sun fuses together roughly 620 million metric tons of hydrogen each second and is now roughly 60% helium.

Who Discovered the Different Types of Stars in Astronomy?

In the early 1900’s, a Danish Astronomer Ejnar Hertzsprung and an American Astronomer Henry Norris Russell began plotting the various points on a diagram where a star’s brightness (luminosity in solar units) on the vertical axis intersected with its temperature (Kelvin, at the surface level) on the horizontal axis.

What they found was that there was a very strong correlation between brightness and temperature. So much so that this diagram helped them determine that there are 3 very different types of stars in astronomy. The Hertzsprung–Russell diagram became the first Astronomical classification of stars using their brightness and temperature on a diagram.



“HRDiagram” by Richard Powell – The Hertzsprung Russell Diagram. Licensed under CC BY-SA 2.5 via Wikimedia Commons –


A Solar Luminosity value of 1 is equal to that of the star of our Solar System; the Sun. Increasing in scale +1, means the star is brighter than our Sun. Values that are -1 represent stars that are dimmer than our Sun.

The horizontal axis measures temperature in units of Kelvin. When looking at a star’s light through a prism, you’re able to split out the different colors of a rainbow and observe what are called absorption lines. These absorption lines or often called spectral lines, they help identify what ions are present and which chemical element is abundant. With this information, you’re able to infer the temperature of the types of stars in astronomy.

What are the Different Types of Stars in Astronomy?

After Hertzsprung and Russell plotted tens of thousands of different stars, they identified a pattern and realized there are 3 very different types of stars based on where they’re located on the HR Diagram.

The three different types of stars in astronomy are radically different from one another and vary in temperature, brightness, size and even quantity; main sequence stars, supergiants and white dwarfs.

Main Sequence Stars – Types of Stars in Astronomy

Main sequence stars are the most common stars in our galaxy and even in the Universe. Our Solar System’s star (the Sun) is a main sequence star, even the closest stars to Earth are main sequence stars; Alpha Centauri A and Sirius.

Main sequence stars convert hydrogen into helium, changing these elements in the core of a star releases a massive amount of energy; observed in the form of light and if close enough, felt by their heat.



“Sun920607” by NASA – Licensed under Public Domain via Wikimedia Commons –


Main sequence stars have a push/pull type of lifestyle. Their gravity pulls their matter inwards and the pressure from the fusion of the elements in their core pushes matter outwards. This push/pull relationship creates an astronomical balance or equilibrium and is responsible for the formation of a star being round or spherical in shape.

Supergiants Stars – Types of Stars in Astronomy

Supergiant stars are old and nearing the end of their life. This type of star is the largest known type of star, some can be as large as our entire Solar System.

Supergiants must have a minimum mass of ten times as much as our Sun. Some Supergiant stars can even be as big as 1,700+ times the size of our Sun’s solar radius.

Because of how big these stars are, they go through their “fuel” much faster than other stars. The conversion of hydrogen into helium (nuclear fusion) can take as little as just a few million years for Supergiants.



“Rigel, Rutherfurd Observatory, 09 September 2014” by Haktarfone at English Wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons –


Supergiants don’t live very long and will eventually explode! Depending on their mass, they can explode into planetary nebula’s or go supernova. At the end of their lives, supergiants end up becoming white dwarfs, neutron stars, or even in some cases – black holes!

White Dwarfs Stars – Types of Stars in Astronomy

White dwarf stars are similar to that of Supergiants, only in that they are very close to the end of their lives. At this point in a white dwarf’s life cycle, they will have nearly converted all of its hydrogen into helium and almost all of its helium into carbon and oxygen, the remaining elements in nuclear fusion.

Our Solar System’s star, the Sun, will eventually turn into a white dwarf star. At this point white dwarfs are usually around the size of the Earth, but are nearly unimaginably heavy. Because of the nuclear fusion process, the elements at the white dwarf’s core make it extremely dense.



Photo by NASA HubbleSite


Imagine if you scooped up dirt from your backyard and put it in a small camping matchbox or something about the size of a pack of gum. It would weigh nearly nothing, you probably wouldn’t even notice it in your pocket.

Now imagine doing that on a white dwarf star, scooping up enough of its matter to fit inside that same small sized matchbox or pack of gum. Well, it’d be amazing if you could even do that, because that matter would weigh over half a million pounds (250 Metric Tons or 551,156 Pounds). You’d probably notice that!



The Morgan–Keenan Spectral Classification System:

In more recent years, additional classifications for the types of stars in astronomy have been added to the traditional Hertzsprung-Russell model. These new additions build off of the 3 main types of stars that were originally identified in astronomy.

Please welcome; Hypergiants, Brightgiants, Giants, Subgiants, Sub-dwarfs, Red-dwarfs and brown-dwarfs.



“HR-diag-w-text” by User:Rursus – Self made diagram. Licensed under CC BY-SA 3.0 via Wikimedia Commons –


This new system categorizes all the different types of stars into a spectral class. This system relies on a star’s effective temperature, measured in Kelvin, to determine which class it should belong to.

This is a wonderful and easy to use tool to convert temperatures in Kelvin to Fahrenheit.

Class O – Morgan–Keenan Classification:

Class O stars are the big dogs and have temperatures equal to or in excess of 30,000 Kelvin, 53,540+ degrees Fahrenheit.

They’re referred to as blue colored and have solar masses and solar luminosity equal to or bigger than 16 and 30,000, respectively.

Basically 16+ times larger mass and 30,000+ times brighter than our own Sun.

Class B – Morgan–Keenan Classification:

Class B stars have temperatures between 10,000 – 30,000 Kelvin, which is 17,540 to 53,540+ degrees Fahrenheit.

They have a blueish-white color and have solar masses between 2.1 and 16 times that of our Sun. Their solar luminosity is between 25 and 30,000 times brighter than our Sun.

Class A – Morgan–Keenan Classification:

Class A stars have temperatures of 7,500 to 10,000 Kelvin, 13,040 to 17,540 degrees Fahrenheit.

They have a whitish color and a solar mass of 1.4 to 2.1 times that of our Sun. Their solar luminosity is 5 to 25 times brighter than our Sun.

Class F – Morgan–Keenan Classification:

Class F stars have temperatures of 6,000 to 7,500 Kelvin, 10,340 to 13,040 degrees Fahrenheit.

They have a yellowish-white color and a solar mass of 1.04 to 1.4 times that of our Sun. Their solar luminosity is 1.5 to 5 times brighter than our Sun.

Class G – Morgan–Keenan Classification:

Class G stars have temperatures of 5,200 to 6,000 Kelvin, 8,900 to 10,340 degrees Fahrenheit.

They have a yellowish color and a solar mass of 0.8 to 1.04 times that of our Sun. Their solar luminosity is 0.6 to 1.5 times brighter than our Sun.

Class K – Morgan–Keenan Classification:

Class K stars have temperatures of 3,700 to 5,200 Kelvin, 6,200 to 8,900 degrees Fahrenheit.

They have an orange color and a solar mass of 0.45 to 0.8 times that of our Sun. Their solar luminosity is 0.08 to 0.6 times as bright as our Sun.

Class M – Morgan–Keenan Classification:

Class M stars have temperatures of 2,400 to 3,700 Kelvin, 3,860 to 6,200 degrees Fahrenheit.

They have a reddish color and a solar mass of 0.08 to 0.45 times that of our Sun. Their solar luminosity is less than 0.08 times as bright as our Sun.



“Morgan-Keenan spectral classification”. Licensed under CC BY-SA 3.0 via Wikimedia Commons –


For more information on Stellar Classifications, please check out this article on Wikipedia.


Star Class

Temperature (Kelvin)


Solar Masses

Solar Luminosity

Class O
≥ 30,000 K
≥ 16 M☉
≥ 30,000 L☉
Class B
10,000–30,000 K
Bluish White
2.1–16 M☉
25–30,000 L☉
Class A
7,500–10,000 K
1.4–2.1 M☉
5–25 L☉
Class F
6,000–7,500 K
Yellowish White
1.04–1.4 M☉
1.5–5 L☉
Class G
5,200–6,000 K
0.8–1.04 M☉
0.6–1.5 L☉
Class K
3,700–5,200 K
0.45–0.8 M☉
0.08–0.6 L☉
Class M
2,400–3,700 K
0.08–0.45 M☉
≤ 0.08 L☉




Featured image “Hertzsprung-Russel StarData” by ESO – Licensed under CC BY 4.0 via Wikimedia Commons –


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