Universe Sandbox: Stars Guide 2022

You want to make a star system, or perhaps you just want fun with stars, but… why do different stars have different colors? What are the different kinds of stars? No problem, today I’ll show you about stars and the flavors they come in.



Astronomers classify stars on a spectrum, and this classification is referred to as the star’s spectra. You may or may not have heard of the OBAFGKM. These seven letters are used to classify stars based on their temperature, listed from hottest to coolest. They can be remembered by the phrase “Oh Be A Fine Guy/Girl Kiss Me.”

Anyways, let’s take a look at these stars, shall we?

The Main-Sequence

Introduction to the Main-Sequence

Main-Sequence stars are in equilibrium. They are happily burning hydrogen as fuel in nuclear fusion and converting it into helium. These are the most common stars, and are represented in their classification by the Roman numeral V. The Sun is a main-sequence star.

O-type stars

O-type main-sequence stars are the most massive stars (aside from Wolf-Rayet stars), and are blue. Their temperature is at least 30,000 K, and generally have a mass of 16 to 100 times that of the Sun (I can’t seem to have a randomly generated star be that massive in-game). They are short-lived and are extremely rare, comprising of about 0.00003% of all main-sequence stars!

O-type stars will lose much of their mass as they age (not represented in-game), and they will spend only a few million years in the main-sequence, as they burn through their fuel quickly. They probably won’t have planets, since they will blow their planetary disk material away from them.

B-type stars

B-type main-sequence stars are the more common blue stars. Their coloration is similar to that of the O-type, but with a slight whitish tint. Their temperature ranges from 10,000 to 30,000 K, and have masses generally ranging from 2.1-16 times that of the Sun.

B-type stars have short lifespans, but still longer than the O-type. Expect them to leave the main-sequence at some time between 10 to 250 million years after they are born. They comprise about 0.13% of all main-sequence stars.

A-type stars

A-type main-sequence stars are white stars with a bluish tint. Their temperature ranges from 7,500 to 10,000 K with masses ranging from 1.4 to 2.1 times the mass of the Sun. They comprise about 0.6% of all main-sequence stars, and are short lived.

F-type stars

F-type stars are white main-sequence stars. They have a white color with temperatures ranging from 6,000 to 7,500 K. Their mass ranges from 1.04 to 1.4 times the mass of the Sun.

White main-sequence stars remain in the main-sequence for at least a billion years, and they make up about 3% of main-sequence stars. If your system has a habitable, life-bearing planet with a higher mass star, aim for the lower side of the mass range, as this will allow adequate time for complex lifeforms to develop.

G-type stars

G-type stars are called “yellow dwarves”, though they actually exhibit a white color with a slight yellowish tint. Their temperatures range from 5,200 to 6,000 K, and their mass generally ranges from 0.8 to 1.4 times the mass of the Sun.

These stars make up about 7.6% of main-sequence stars, and will last for about 10 billion years. The Sun is a G-type star.

K-type stars

K-type stars, also called “orange dwarves”, have a yellow-orange color, and temperatures range from 3,700 to 5,200 K. Their mass generally ranges from 0.45 to 0.8 times that of the Sun, and represent about 12.1% of all main-sequence stars.

These stars will last for a while, at least 14 billion years.

M-type stars

M-type stars, also known as “Red dwarves”, are the least massive stars, ranging from 0.08 to 0.45 times that of the Sun, and their temperatures range anywhere from 2,400 K to 3,700 K. These are the most common main-sequence stars, as they make up 76% of them. Despite this, they are so faint that not one is visible to the unaided eye on Earth.

Planets orbiting these stars will likely be tidally-locked, meaning one side will be faced with never-ending daylight, and the other, in never-ending darkness.

Also, many red dwarves are UV Ceti or “flare stars”, which means that they often erupt solar flares from their surface. This will prove challenging for any lifeforms on nearby planets.

Post-main-sequence Stars


Post-main-sequence stars have left the main-sequence and are running out of hydrogen in their cores. They will swell to many times their original size, and their brightness will increase greatly. However, their temperature will decrease. They are still classified using the OBAFGKM system, but are represented instead by the Roman numerals IV (sub-giants), III (giants), II (bright giants), Ib (dim supergiants), Iab (moderate-luminosity supergiants) Ia (bright supergiants), or 0 (hypergiants, also represented with Ia+).


Sub-giants are just beginning to end the main-sequence stage of their life-cycle, and their temperature varies from star to star, depending on their mass and age. They will be larger and brighter for main-sequence stars of the same temperature.


Giants are stars that have progressed from the sub-giant stage. They will be much larger than the main-sequence stars, and brighter than the sub-giants.

If they have a mass similar to that of the Sun, they will eventually return to the sub-giant phase with a reduced mass before returning to the giant phase again in the asymptomatic giant branch (AGB). They will then pulsate, losing mass each time, until all that is left is the core, which from that point onward will be called a white-dwarf (more on that later).

If, however, they are more massive than the Sun, they will progress further…

Bright giants

These are similar to giants, but are brighter and larger.


Supergiants have progressed from massive giant stars, and these will be huge. These stars will end their lives in a Type-II Supernova, but the most massive of all stars can progress even further…


Hypergiants are HUGE. Their radius can be up to 6 AU! They are also VERY rare, and aare about to go Supernova like the Supergiants, after which they will either become a neutron star or a black hole.

Dead Stars


After the post-main-sequence phase, stars will usually leave behind a stellar remnant, all of which are small in size, but have a high mass for said size.

White dwarves

White dwarves are the result of a Sun-like star after its death. They will have lost a significant amount of mass, and will gradually cool as they age, though this will take quite a while. Eventually (and hypothetically) they will cool to absolute zero (0 K or -273 Celsius), at which point they will be called a “black dwarf”.

White Dwarves have a radius similar to that of the Earth’s radius, but they will have masses more comparable to that of the Sun. Even stranger, their radius will decrease when the mass is increased.

If white dwarves exceed around 1.4 times the mass of the Sun, they will explode in a type-Ia supernova.

Neutron Stars

Neutron stars are the result of type-II supernovae, which will occur when the mass of the giant star exceeds 8 times that of the Sun. These are even smaller than white dwarves, around several kilometers, but their mass will be much more comparable to that of the Sun!

If a neutron star has a magnetic field and spins rapidly, it will be a pulsar.

Black Holes

Black holes are classified based on size, but we will focus on the stellar-mass black holes. They form when a star whose mass is at least 15 times that of the Sun dies (though in-game this seems to be lower). Black holes aren’t measured by radius, but rather by the schwartzchild radius. This represents the “black” portion of the black hole. That is, any light crossing this will not be able to escape.

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