In this article
Look carefully at the night sky and you will notice that not all stars look alike. Some are blue-white, some are yellow, and others lean towards orange or red. Those colours are not random. They tell us a great deal about a star’s temperature, size and age.
In this article we look at how astronomers sort stars into types, where our own Sun fits in, and how a star changes from birth to death.
Table of Contents
What is a star?
A star is a huge, extremely hot ball of plasma, made mostly of hydrogen and helium and held together by its own gravity. Deep in its core, nuclear fusion takes place: hydrogen nuclei combine to form helium, releasing enormous amounts of energy that we see as light and feel as heat.
The Sun is one such star. Its core sits at about 15 million °C, while the visible surface is about 5,500 °C. (Read more about the Sun in this article.)
How are stars classified?
Astronomers mainly look at two properties:
- Surface temperature – revealed by a star’s colour and by its spectrum.
- Luminosity – how much energy the star actually gives out each second, which also hints at its size.
Behind both lies the most important property of all: mass. A star’s mass decides how hot it gets, how long it lives and what it eventually becomes.
O, B, A, F, G, K, M – classes by colour
In the early 1900s, Annie Jump Cannon at Harvard College Observatory studied the spectra of thousands of stars. The spectral classification system she refined was essentially complete by 1912 and is still used worldwide. From hottest to coolest, the seven main classes are:
| Class | Surface temperature (approx.) | Colour | Examples |
|---|---|---|---|
| O | above 33,000 K | Blue | The companion star in Cygnus X-1 |
| B | 10,000 – 33,000 K | Blue-white | Rigel |
| A | 7,300 – 10,000 K | White | Sirius A, Vega |
| F | 6,000 – 7,300 K | Yellow-white | Procyon |
| G | 5,300 – 6,000 K | Yellow | The Sun, Alpha Centauri A |
| K | 3,900 – 5,300 K | Orange | Alpha Centauri B, Aldebaran |
| M | 2,300 – 3,900 K | Red | Proxima Centauri, Betelgeuse |
K here means kelvin; since 0 °C is about 273 K, the difference from Celsius is small at these temperatures. Generations of students have remembered the order with “Oh Be A Fine Girl/Guy, Kiss Me“.
Each class is split into subclasses from 0 to 9, and a Roman numeral is added to show the star’s size: V for a main-sequence star, III for a giant and I for a supergiant. That gives the Sun its full “address”: G2V.
Here is a surprise: about 76% of main-sequence stars are small red M-type stars, while the brilliant blue O-type stars make up only around 0.00003%.
The H-R diagram – a map of the stars
Working independently, Ejnar Hertzsprung of Denmark (1911) and Henry Norris Russell of the United States (1913) noticed that when stars are plotted by temperature and brightness, they do not scatter randomly – they fall into clear groups. This plot is now called the Hertzsprung–Russell (H-R) diagram.
- Main sequence – a long band running from the upper left to the lower right, where most stars sit.
- Giants – upper right: cool, but bright because they are large.
- Supergiants – across the very top: extremely luminous and rare.
- White dwarfs – lower left: hot, but faint because they are tiny.
A star’s position on the diagram tells astronomers which stage of life it has reached.
Main-sequence stars – the longest chapter
A star stays on the main sequence for as long as it fuses hydrogen in its core. This is the longest and most stable part of its life. Our Sun, about 4.6 billion years old, is roughly halfway through this stage.
Red dwarfs
Small, cool and dim, yet the most common stars of all. They use their fuel so slowly that they are expected to shine for trillions of years. Proxima Centauri, the nearest star to the Sun at 4.2 light-years, is a red dwarf – though too faint to see without a telescope.
Yellow dwarfs
G-type main-sequence stars like the Sun. Here “dwarf” does not mean tiny; it simply means the star is on the main sequence. A Sun-like star spends about 10 billion years in this phase.
Massive blue stars
O- and B-type main-sequence stars can be tens of times heavier than the Sun. They burn through their fuel so fast that they live only a few million years – the “live fast, die young” stars of the cosmos.
Giants, supergiants and hypergiants
Once the hydrogen in its core runs out, a star leaves the main sequence. The core contracts while the outer layers swell enormously, and as the surface cools the star turns redder.
- Red giants – a stage Sun-like stars pass through. NASA says that in about 5 billion years the Sun will become a red giant large enough to engulf Mercury and Venus, and possibly Earth. Aldebaran in Taurus is a good example of a swollen giant star.
- Supergiants – the late stage of massive stars. In Orion, red Betelgeuse (a red supergiant) and blue-white Rigel (a blue supergiant) are classic examples. (Is Betelgeuse about to explode? See this article.)
- Hypergiants – the most luminous and among the largest stars known. They are unstable and shed huge amounts of material in powerful winds. VY Canis Majoris is one of the largest known stars, and Eta Carinae is another famous example.
The life cycle of a star
Birth: the protostar
Stars are born inside cold clouds of gas and dust (nebulae). When a dense clump collapses under gravity it heats up and becomes a protostar, still wrapped in a cocoon of dust and not yet hot enough for fusion. Once hydrogen fusion ignites in its core, it becomes a true star.
Death: three paths, decided by mass
- Sun-like stars → white dwarf – after the red-giant stage, the outer layers drift away as a planetary nebula, leaving the core behind as a white dwarf. No more fusion happens; it simply cools over billions of years. Sirius B, the companion of Sirius, packs roughly the Sun’s mass into a body about the size of Earth.
- Massive stars → neutron star – stars more than roughly eight times the Sun’s mass end in a supernova. The leftover core can become a neutron star only about 20 km across; a sugar-cube-sized piece of it would weigh around a billion tonnes on Earth. Rapidly spinning neutron stars that sweep beams of radiation across space are called pulsars. (More: neutron stars and pulsars and supernovae seen with the naked eye.)
- The heaviest stars → black hole – if the core is heavy enough, nothing can stop its collapse, and a black hole forms. Strictly speaking it is not a star but a star’s remains. Cygnus X-1, about 7,000 light-years away, holds a black hole of around 21 solar masses.
Special kinds of “stars”
- Brown dwarfs – objects of roughly 13 to 80 Jupiter masses, too light to sustain hydrogen fusion, which is why they are nicknamed “failed stars”. The first one confirmed was Teide 1 in the Pleiades, in 1995.
- Variable stars – stars whose brightness changes over time. Cepheids are especially important: in 1912 Henrietta Leavitt found that the period of a Cepheid’s pulsation is linked to its true brightness, turning these stars into cosmic measuring tapes.
- Magnetars – neutron stars with magnetic fields about 1,000 times stronger than those of ordinary neutron stars. They occasionally release bursts of X-rays and gamma rays.
Frequently asked questions
What decides a star’s type?
Mainly its mass. Heavy stars are hot, bright and short-lived; light stars are cool, dim and extremely long-lived.
Can we see star colours with the naked eye?
Yes, for bright stars. From Sri Lanka, Orion is well placed in the evening sky in December and January – compare orange-red Betelgeuse with blue-white Rigel. For tips on naked-eye stargazing, see this guide.
Do stars collide?
In our part of the galaxy stars are so far apart that collisions are extremely rare. In crowded places such as globular clusters, close encounters and mergers can happen.
Next time you look up at a starry sky, remember: every one of those points of light is a distant sun with its own story and its own life cycle.
Sources
How much do you remember?
1. According to the article, what mainly decides a star's type?
The article says the most important property is mass, and that a star's mass decides how hot it gets, how long it lives and what it becomes.
2. What does the article say about the Sun's current stage of life?
The article states that the Sun, about 4.6 billion years old, is roughly halfway through the main-sequence stage.
3. Which star is given as an example of a red dwarf in the article?
The article says Proxima Centauri, the nearest star to the Sun at 4.2 light-years, is a red dwarf.
