Look at the night sky and most stars appear to be little more than points of white light. Look more carefully, however, and differences begin to emerge. Some stars have a noticeably warm orange or reddish tint, while others appear blue-white. Some are relatively modest stars like the Sun, while others are giants hundreds of times wider. And some of the most common stars in the Galaxy are so faint that we cannot see a single one with the naked eye.
Learning the major types of stars gives these differences meaning. Betelgeuse and Rigel in Orion, orange Arcturus, brilliant Sirius and even our own Sun are not simply different-looking stars. They represent different temperatures, masses, sizes and stages of stellar life.
There are several ways of classifying stars, which can make the subject seem more complicated than it needs to be. For now, it helps to think of stars as belonging to several broad populations: main-sequence stars, giants, supergiants and stellar remnants such as white dwarfs and neutron stars. Brown dwarfs sit just outside the family because they are not technically stars.
We will meet those groups first through real examples in the night sky. Later, we’ll see how astronomers use spectral classes such as O, B, A, F, G, K and M to describe stars more precisely.
The Main Types of Stars at a Glance
Most stars belong to the main sequence, the long period during which they generate energy by converting hydrogen into helium in their cores. Main-sequence stars span an enormous range, from hot, massive blue stars through stars like the Sun to small, cool red dwarfs. Around 90% of stars are in this stage.
When stars leave the main sequence, some expand enormously and become giants or, in the case of massive stars, supergiants. Between the principal groups are intermediate classes such as subgiants and bright giants, so the boundaries are not as abrupt as a simple list might suggest.
Eventually, stars can leave behind compact stellar remnants. These include white dwarfs and neutron stars, while sufficiently massive stars can ultimately produce stellar-mass black holes. These are different possible endpoints rather than successive stages, with the outcome depending largely on the star’s mass and how it evolves. A black hole itself is no longer a star.
Brown dwarfs belong outside this sequence. They are substellar objects that did not acquire enough mass to sustain the ordinary hydrogen fusion that powers main-sequence stars.
So how many types of stars are there? There isn’t one single number, because stars can be grouped in different ways. The often-quoted “seven types” are the seven traditional spectral classes O, B, A, F, G, K and M. Stars can also be grouped by their physical and evolutionary state, including main-sequence stars, giants and supergiants.
Main-Sequence Stars
The Sun is a main-sequence star, and this is where stars spend most of their active lives. What makes a star “main sequence” isn’t its color or size, but what is happening inside it: hydrogen is being fused into helium in its core.
That simple definition encompasses a remarkable variety of stars. At one end are very massive, extremely hot stars that appear blue or blue-white. Moving along the sequence, stars become progressively cooler and generally less massive and less luminous, passing through white and yellow stars such as the Sun, then orange stars, and finally the small, faint red dwarfs.
Astronomers describe this temperature sequence using the spectral classes O, B, A, F, G, K and M, but there’s no need to memorize those letters yet. The important point is that the main sequence isn’t one type of star in the everyday sense. It is an enormous family ranging from some of the brightest stars in the Galaxy to objects far too faint to see without a telescope.
Hot Blue and White Main-Sequence Stars
At the hot end of the main sequence are O-, B- and A-type stars. The most massive members are intensely luminous and burn through their available hydrogen much faster than smaller stars do.
Their color provides a clue to their temperature. Hotter stellar surfaces appear bluer, while cooler ones appear progressively whiter, yellowish, orange and red. That means some of the differences between stars can actually be seen rather than merely measured.
Sirius A, the brightest star in the night sky, provides an accessible example. It is an A-type main-sequence star and appears distinctly white to blue-white. Hotter B-type main-sequence stars can be found among the bright stars of Orion.
Sun-Like and Orange Main-Sequence Stars
Our Sun lies much closer to the middle of the main sequence. It is a G-type main-sequence star, neither among the hottest and most massive stars nor among the coolest and smallest.
That makes the Sun a useful ruler throughout this article. Instead of trying to visualize an unfamiliar radius or luminosity, we can ask whether another star is hotter or cooler than the Sun, larger or smaller, more or less massive, and brighter or fainter.
Cooler still are K-type main-sequence stars, often informally called orange dwarfs. Alpha Centauri B is a familiar nearby example. These stars are generally smaller, cooler and less luminous than the Sun.
The progression from hot blue stars through Sun-like stars to orange and red stars isn’t a collection of unrelated categories. It is part of the continuous main sequence.
Red Dwarfs
At the cool, faint end of the main sequence we find one of the most important stellar populations of all: red dwarfs.
Red dwarfs are small, low-mass stars, mostly belonging to spectral class M, with some warmer examples in class K. Despite the name, they would generally appear more orange than deep red to human eyes. They produce so little visible light that not one red dwarf is bright enough to be seen with the naked eye from Earth.
Yet red dwarfs make up around 75% of the stars in the Milky Way. Our nearest stellar neighbor, Proxima Centauri, just over four light-years away, is one of them.
That reveals something important about the night sky. The stars we see aren’t a representative sample of the Galaxy. Bright and intrinsically luminous stars are much easier to notice across large distances, while the enormous population of faint red dwarfs remains largely invisible to us.
So one of the most common kinds of star in the universe is almost entirely absent from the naked-eye sky.
Evolved Stars
Stars don’t remain on the main sequence forever. As they exhaust the hydrogen in their cores, their structure begins to change. Depending largely on their mass, they can evolve into very different kinds of stars, including giants and supergiants.
Giant Stars
Stars such as the Sun can expand enormously after leaving the main sequence, producing giant stars. A giant isn’t defined simply by being red. Giants exist at different temperatures and spectral types. Astronomers classify them primarily by their luminosity and spectral characteristics, with ordinary giants assigned luminosity class III. Between main-sequence stars and giants lies an intermediate subgiant class.
The red giants and orange giants are nevertheless the easiest examples to understand because several are prominent naked-eye stars.
Arcturus, the brightest star in Boötes, is an excellent example. Its warm orange color is noticeable to the naked eye, and it can be found by following the curve of the Big Dipper’s handle—the familiar observing instruction to “arc to Arcturus.”
Arcturus is cooler at its surface than the Sun yet far more luminous. There’s no contradiction. A giant has an enormously larger surface area from which to radiate energy. This relationship between temperature, size and luminosity is one of the important things the H-R diagram will help us visualize later.
Aldebaran in Taurus is another familiar orange giant. Once you know what these stars are, their warm colors become more than an attractive feature: you’re looking at stars fundamentally different from the Sun.
Giants and supergiants together account for fewer than about 1% of stars in representative stellar populations, yet their high luminosities make them disproportionately conspicuous in the night sky.
Supergiant Stars
Supergiants take stellar size and luminosity to much greater extremes. They are evolved massive stars occupying the upper regions of the H-R diagram and include stars at very different surface temperatures.
This is one stellar difference you can experience particularly well simply by finding Orion.
Look toward Betelgeuse, marking one of Orion’s shoulders, and then toward Rigel, at one of his feet. Betelgeuse has an obvious orange-red tint, whereas Rigel appears much more blue-white.
Both are supergiants, but they occupy very different parts of the temperature range.
Blue Supergiants
Rigel is a hot blue-white supergiant. Stars such as this are massive and extraordinarily luminous, which is why they can remain conspicuous despite lying hundreds of light-years away.
Its blue-white appearance tells us something immediately useful: its surface is much hotter than that of the Sun—and much hotter than Betelgeuse.
Red Supergiants
Betelgeuse provides the opposite extreme. It is a red supergiant with a substantially cooler surface than Rigel, yet it is enormously luminous because its outer layers have expanded to a tremendous size.
This is why color, brightness and size shouldn’t be treated as interchangeable properties. A reddish star may have a cooler surface than a blue star while still pouring out vastly more total energy than the Sun because it is so enormous.
There are supergiants at intermediate temperatures too, and astronomers also recognize bright giants between ordinary giants and supergiants. The important point isn’t to memorize every intermediate class. It is to recognize that giant and supergiant describe a star’s luminosity/physical state rather than simply its color.
Stellar Remnants
When a star reaches the end of its active life, what remains depends largely on its mass and how it has evolved. The possible outcomes include white dwarfs and neutron stars, while sufficiently massive stars can form stellar-mass black holes. These are alternative endpoints, not successive stages.
White Dwarfs
Not every small star is a red dwarf. White dwarfs are completely different objects.
A white dwarf is a stellar remnant—the compact core left behind after a lower-mass star has passed through its giant stages and expelled its outer layers. It no longer generates energy through ordinary stellar fusion. Instead, it begins extremely hot and gradually cools over immense periods of time.
White dwarfs are extraordinary because they combine stellar-scale mass with roughly Earth-sized dimensions. Their small surface area also explains an initially puzzling feature of the H-R diagram: a white dwarf can be extremely hot while remaining relatively faint.
And we can connect this exotic object with a star we’ve already met.
The brilliant main-sequence star Sirius A has a white-dwarf companion, Sirius B. You cannot see Sirius B with the naked eye—the glare of Sirius A overwhelms it—but the system contains two objects representing radically different stellar states.
Earlier Sirius helped us understand the main sequence. Its companion now lets us glimpse what can remain after a star’s active life has ended.
What Is a Black Dwarf?
Given enough time, a white dwarf should continue cooling until it eventually becomes a cold, dark black dwarf.
But none should exist yet. White dwarfs cool so slowly that the universe hasn’t existed long enough for one to reach that state.
So unlike the other objects in this article, black dwarfs are theoretical inhabitants of the distant future, not stars astronomers can currently observe.
Neutron Stars
More massive stars can leave behind something considerably more extreme: a neutron star.
A neutron star packs more mass than the Sun into an object roughly the size of a city. Matter is compressed to densities so extraordinary that familiar comparisons with ordinary materials quickly become inadequate.
We don’t generally experience neutron stars as points of light in the way we do Sirius or Arcturus. Instead, astronomers often detect them through radio waves and X-rays.
Some rapidly rotating neutron stars produce beams of radiation that sweep through space. If one of those beams repeatedly crosses Earth, we detect regular pulses, which is why these objects are called pulsars.
For our purposes, that’s enough physics. The important idea is the scale of the transformation: a star that was once vastly larger than Earth can ultimately leave a remnant only tens of kilometers across.
Stellar Black Holes
Some sufficiently massive stars can end their lives by forming stellar-mass black holes.
A black hole is not a type of star. It belongs here because it can be the end result of stellar evolution.
Unlike a normal star, a black hole doesn’t have a glowing stellar surface for us to observe. Astronomers instead infer stellar black holes from their gravitational effects and, in some systems, from radiation produced by hot material interacting with them.
That gives us another useful observational lesson. Astronomy isn’t limited to looking directly at an object. Sometimes we learn that something invisible is there because of what it does to everything around it.
The details of how massive stars produce neutron stars and black holes belong to the life cycle of a star rather than this guide to star types.
Brown Dwarfs
At the opposite end of the story are objects that never became ordinary stars in the first place.
Brown dwarfs form a bridge between stars and giant planets. They are generally more massive than planets but lack enough mass to sustain the ordinary hydrogen fusion that defines a main-sequence star. NASA gives a commonly used approximate range of 13 to 80 Jupiter masses.
Brown dwarfs can form from collapsing clouds of gas and dust much as stars do, but they remain relatively cool and faint. They emit very little visible light and are generally observed at infrared wavelengths rather than as visible light, and they cannot be seen with the naked eye or ordinary backyard telescopes.
Calling them “failed stars” provides an intuitive picture, although it shouldn’t be taken too literally. They are a genuine class of substellar objects rather than stars that somehow went wrong.
And they provide a useful boundary to our tour: having enough mass for sustained hydrogen fusion is what separates an ordinary star from a brown dwarf.
How Astronomers Classify Stars: O, B, A, F, G, K and M
We’ve now encountered blue-white Rigel, the Sun, orange Arcturus and reddish Betelgeuse. Their different colors aren’t cosmetic. They contain information about the stars themselves.
Astronomers analyze starlight by spreading it into a spectrum. The resulting spectrum contains features that allow stars to be classified and reveals information about their surface temperatures. The traditional stellar sequence runs:
O → B → A → F → G → K → M
from the hottest to the coolest ordinary stellar spectral classes.
Each class is subdivided numerically from 0 to 9, with 0 representing the hottest stars within that class and 9 the coolest. This allows astronomers to describe temperature more precisely. Our Sun, for example, has spectral type G2.
But here’s the distinction that makes the whole subject much easier to understand:
Spectral class and stellar population are not the same thing.
OBAFGKM principally tells us about a star’s spectrum and temperature. Main sequence, giant and supergiant tell us something different about its luminosity/physical state.
That means knowing that a star is G2 isn’t necessarily enough to know what kind of star it is. A G-type star could be a main-sequence star, a giant or even a supergiant.
Astronomers therefore add a luminosity class, commonly represented by Roman numerals:
I — supergiants
II — bright giants
III — giants
IV — subgiants
V — main-sequence stars
The Sun’s fuller classification is therefore G2 V: G2 describes its spectral type, while V tells us that it is a main-sequence star.
You don’t need to memorize the system to enjoy astronomy. Its real value is that labels such as M-type red dwarf, K-type giant or B-type supergiant now tell you something meaningful rather than looking like arbitrary astronomical jargon.
How the Different Types of Stars Fit on the H-R Diagram
We can now bring everything together.
The Hertzsprung-Russell diagram, usually shortened to the H-R diagram, plots stars according to luminosity and surface temperature or spectral class. When astronomers began doing this, stars didn’t scatter randomly across the graph. They clustered into recognizable populations.
Most form a diagonal band called the main sequence, running from hot, luminous stars toward cooler, fainter ones.
Above the main sequence sit the giants and supergiants. They’re exceptionally luminous for their temperatures because they’re physically enormous.
Down toward the hot but faint part of the diagram sit the white dwarfs. Their surfaces can be extremely hot, but their tiny dimensions keep their total luminosity relatively low.
And now the diagram needn’t be an abstract scientific chart, because we’ve already met stars occupying these regions:
- Rigel — supergiant
- Betelgeuse — supergiant
- Arcturus — giant
- Sirius A — main sequence
- Sun — main sequence
- Proxima Centauri — red dwarf at the faint end of the main sequence
- Sirius B — white dwarf
That is perhaps the most useful way to think about star types. They aren’t simply a list of names to memorize. Temperature, color, luminosity, size and evolutionary state are related properties, and the H-R diagram gives us a map for seeing those relationships.
The next time you look at Orion, Sirius or Arcturus, you’re therefore seeing more than bright points marking out the constellations. Rigel and Betelgeuse reveal different extremes of massive stellar evolution; Sirius is a main-sequence star accompanied by a tiny white-dwarf remnant; Arcturus is an evolved giant; and beyond naked-eye visibility lies the enormous population of red dwarfs that actually dominates our Galaxy.
Understanding the different types of stars doesn’t make the night sky more complicated. It gives the stars you already see a story.