At best guess, there are over 200 billion trillion stars in the universe and somewhere around 200 billion in our galaxy alone. These stars come in all sorts of sizes and colors, and we perceive each one uniquely from here on Earth. Read on to discover what the different types of stars are and the particular qualities of each.
How Are Stars Formed?
In the seemingly infinite universe, gas and dust particles find each other and form clouds. As sections of this cloud reach a critical density, they collapse into dense cores. These cores, also known as protostars, continue to heat and shrink. Once a protostar reaches a core temperature of 10,000,000 Kelvin (K), nuclear fusion begins, and a star is officially born.
Not All Clouds Become Stars
As interstellar clouds collapse, they don’t always meet the criteria to successfully form stars.
If a cloud is too small in size, it will contract and heat up but will never reach a temperature to begin nuclear fission. While the surface of the forming protostar shows some heat, it will never become a star. Such objects are called brown dwarfs.
Brown dwarfs never reach the size of an average star and are very dim. They are incredibly rare in our universe and only make up about 1% of all star-like objects.
If a condensing cloud is too big, nuclear reactions occur so quickly that these forming stars explode from the inside out. This can lead to the complete destruction of a protostar, or a star can save itself by blowing off only its outermost layers.
Classifying Stars
The most common way we classify stars is through the Morgan–Keenan (MK) system, which groups these celestial objects based on color and temperature. This system uses letters to designate stars in decreasing temperature.
| Spectral Type | Color | Surface Temperature (in K) |
|---|---|---|
O |
Blue |
28,000 – 50,000 |
B |
Light Blue |
10,000 – 28,000 |
A |
White |
7,500 – 10,000 |
F |
Whitish-yellow |
6,000 – 7,500 |
G |
Yellow |
5,000 – 6,000 |
K |
Orange |
3,500 – 5,000 |
M |
Red |
2,500 – 3,500 |
Each star’s spectral type is supplemented by a number that depicts its absolute brightness. The brightest stars have the number 0, whereas the dimmest is number 9.
Types of Stars
Astronomers place the stars in our universe into one of four categories: main sequence stars, giants, supergiants, and dead stars. Let’s take a look at each of these.
Main Sequence Stars
Main sequence stars represent protostars that successfully begin nuclear fusion in their core. These stars make up the vast majority of objects in the night sky, encompassing roughly 90% of all the stars in the universe.
Main sequence stars sit in perfect hydrostatic equilibrium between the force of gravity pulling inward and the forces created from nuclear fusion pushing back outwards. As such, they remain a particular size and a spherical shape.
These stars are young, full of life, and can range anywhere from just 0.08 times the mass of our Sun all the way to a size that’s 100 times bigger. Main sequence stars can be one of four colors.
Blue Stars
Just like a scorching flame here on Earth emits a blue light, blue stars burn hotter than any other color star in the universe. These stars fall into the O and B spectral types and tend to run about 30,000 K.
Because they burn so hot, blue stars are some of the most luminous stars out there. This intensity also causes them to burn out much faster than other stars and only typically last about 40 million years. These stars owe their heat to their mass, which can range anywhere from double that of our Sun to 90 times its mass.
Spica in the constellation Virgo is one of the few blue stars in our galaxy.
White Stars
White stars, type A, are quite a bit cooler and less dense than blue stars. That being said, they are still very bright and quite detectible in the night sky. Sirius A, the brightest star in the night sky, is a white star.
Yellow Dwarfs
We are most familiar with yellow dwarfs, since our planet revolves around one. These stars can either fall into type F or G on the Morgan-Keenan system and share a similar size and mass to that of our Sun.
Even though quite a bit smaller than blue stars, yellow dwarfs are still brighter than most other stars in the universe. They burn at temperatures between 5,000 and 7,000 Kelvin and are expected to last between 10 and 15 billion years.
Orange Dwarfs
Orange dwarfs are lighter than yellow dwarf stars, having masses between one-half and three-quarters than that of our Sun. They are also considerably cooler, with surface temperatures ranging between 3,500 and 5,000 Kelvin.
These stars represent spectral type K and, thanks to cooler temperatures, may survive as long as 30 billion years. Alpha Centauri B is one of the few orange dwarfs that we can see from Earth.
Red Dwarfs
Red dwarfs are by far the most numerous in our universe, making up nearly 75% of all existing stars. They are no more than half the size of our Sun, and scientists predict they can live for trillions of years due to their low temperatures.
These M class stars are very dim in the sky and can be difficult to see if far away from Earth. Perhaps the most notable red dwarf, Proxima Centauri, happens to be the closest neighbor to our Solar System.
Giants/Supergiants
Eventually, main sequence stars run out of hydrogen to fuse into helium. Without the energy from nuclear fusion to keep gravity at bay, these stars begin to collapse. This collapse causes the star’s core to superheat, pushing its surface layers outward. These now giant stars can reach sizes 100 to 1000 times larger than they previously were.
This change also cools down the star, with F, G, K, and M spectral types all becoming red in color. Blue stars retain much of their temperature and keep the color blue. To survive, giant stars begin burning off and fusing the helium they’ve created over the last millions or billions of years.
Blue Giants
Blue giants are even rarer than the blue stars they are created from. Although they cool off some, they continue to burn hot enough to keep their bluish hue. Stars from spectral types O, B, and A can become blue giants.
These stars are very bright in the night sky thanks to their temperature and also their size. Blue giants grow to be up to ten times the size of our Sun and 150 times more massive. The star Meissa in Orion’s head is a blue giant.
Blue Supergiants
The most massive blue stars can become blue supergiants when they run out of hydrogen in their cores. Blue supergiants can range anywhere from 10,000 to 50,000 Kelvin and are so large that they can shine with the brightness of one million Suns!
They live a very short time, roughly 10 million years, but they command a presence. These stars are often over 20 times the size and 1000 times the mass of our own Sun. Such super rare stars come from spectral types O and B. There are two in the constellation Orion; Rigel and Alnitak.
Red Giants
Red giants are exceedingly rare in our galaxy, although type F, G, K, and M stars can all become them. This is simply because it takes billions (or trillions) of years for these classes of stars to fuse away all their hydrogen.
The increase in size from a red giant’s expansion results in a size 20 to 100 times that of our Sun, boosting a brightness of up to 1000 times what it once was. These stars are some of the easiest to pick out in the night sky, such as Aldebaran in Taurus’s eye and Arcturus in Bootes.
Red Supergiants
Red supergiants are the biggest stars in the known universe, as the expansion from a superheated core pushes them out to up to 1500 times the radius of the Sun. These stars are not hot, averaging 4,000 Kelvin, but can be hundreds of thousands of times brighter than what they were before.
A slow burn of helium within a red supergiant’s core means they can survive upwards of 100 million years. Antares in Scorpius and Betelgeuse in Orion glow with a beautiful, luminous red in our night sky.
Dead Stars
Once a star has nothing left to give in the core, it once again collapses in on itself. This time around, a star effectively dies and ends up in one of a few different states.
White Dwarfs
Stars of around three solar masses or less crunch down into white dwarf stars once nuclear fusion has stopped taking place. Similar to when these stars became giants, they heat up during the collapsing process to anywhere from 8,000 to 40,000 K. At this point, white dwarfs will slowly cool off over a few billion years until they have no more heat or light to give.
White dwarfs are at most one-fifth the size of the Sun and typically not nearly as bright. However, larger stars before their collapse can end up more massive than the Sun and up to 100 times more luminous. Sirius B is a white dwarf.
Black Dwarfs
Only theoretical in nature, once a white dwarf cools completely, it is believed that these stars will become black dwarfs. Without a source of heat or light, these stars would be nearly impossible to detect out in space. With the universe estimated to be 13 billion years old, no star has lived long enough to become a black dwarf.
Neutron Stars
Massive stars typically go out with a bang in the form of a supernova. The left-behind core can collapse in on itself, crushing protons and electrons together to form neutrons. This newly formed object is aptly called a neutron star. As long as it maintains 1 to 3 solar masses, it will resist complete disintegration.
All that remains of a once-massive star is a super-condensed ball no greater than 13 miles (20 kilometers) across. With a mass similar to the Sun, neutron stars are the densest object known to man. A half-inch (one centimeter) cube of neutron star would weigh more than one billion U.S. tons (900 billion kilograms).
Black Holes
After a supernova, stars larger than 3 solar masses will continue to collapse inward into a singularity. This singularity becomes a black hole, with gravity so strong that not even light can escape.
Conclusion
In this article, we’ve taken a look at the different types of stars in our universe and the type of lives they can expect to have. While every star starts off the same way, each star has unique characteristics that sets it apart from all others. Keep this in mind when you’re taking photos of your favorite constellations on a clear, beautiful night!