What are stars?

What is a star? How does a star form and die? Find out in our guide packed with facts about stars and answers to some of the most commonly-asked questions.

What are stars? Credit: Eric Lowenbach / Getty Images

A starry night sky is one of the greatest sights visible on planet Earth, but what exactly are those points of light twinkling at us from across the vast cosmos?

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Stars are glowing celestial bodies consisting of mostly hydrogen (90%) and helium (10%) that can be seen in the night sky because nuclear reactions at their core give off heat and light energy.

They are a fundamental component in the Universe in that they collectively form star clusters, galaxies and galaxy clusters.

Find out more about star names, star clusters to see with the naked eye, how to photograph stars and the science of stardust.

Composite image of the globular cluster NGC 3201, obtained with the WFI instrument on the ESO/MPG 2.2-m telescope at La Silla. Image Credit: ESO
Globular cluster NGC 3201. Globular clusters can contain up to millions of stars and are thought to be among the oldest objects in the Universe. Credit: ESO

Astronomers now also think that for every star we can see in the night sky, there is at least one planet in orbit around it (these are known as exoplanets).

That means that when you look up at a starry nightscape, you’re potentially looking at multiple stellar systems just like our own Solar System.

Within each stellar system, the star sits at the centre, providing heat and light that shapes and characterises the planets and other bodies in orbit around it, and may even be the basis for life on some of those worlds, like the Sun in our Solar System.

How do stars form?

Against the red glow of the Orion Molecular cloud is a dark curl of dust that forms the unmistakable shape of a horse’s head. The larger cloud is a giant stellar nursery, and it’s thought the Horsehead Nebula has enough mass to create 30 Sun-like stars. Credit: ESO
Against the red glow of the Orion Molecular cloud is a dark curl of dust that forms the unmistakable shape of a horse’s head. The larger cloud is a giant stellar nursery, and it’s thought the Horsehead Nebula has enough mass to create 30 Sun-like stars. Credit: ESO
ESO

A star is a glowing ball of very hot gas called plasma born in a huge cloud of hydrogen, known as a stellar nursery, where pockets of gas clump together. Over time, gravity draws in more gas until there are several trillion trillion tons gathered into a ball.

The gas is so tightly packed, it starts to fuse generating a huge amount of light and voila! You have a star.

Sometimes several stars form at the same time. Most commonly, these occur in pairs called binary stars or in trios called triplets, but as many as seven stars have been found together in systems such as AR Cassiopeiae. 

What are stars made of?

Stars consist mostly of hydrogen and helium, which provides the ‘fuel’ they need to produce energy in the form of heat and light, as a result of nuclear reactions occurring in their cores.

Over time, stars begin to use up their fuel and die, but there is so much fuel and matter within a star that it will typically take billions of years to die.

The dynamo effect of the Earth’s spinning molten core produces our planet’s magnetic field, which prevents the solar wind from stripping away our atmosphere. Credit: Naeblys / Getty Images
The dynamo effect of the Earth’s spinning molten core produces our planet’s magnetic field, which prevents the solar wind from stripping away our atmosphere. Credit: Naeblys / Getty Images

But before that happens, they continue to release heat and light energy that can be observed by astronomers on Earth, and also release bursts of charged particles out into space, known as a stellar wind.

In the case of our Sun, this is known as the solar wind, and it has a tremendous effect on the bodies of the Solar System, such as when it interacts with Earth’s magnetosphere to produce space weather like the colourful light displays known as the aurora.

Why do stars shine?

Field of stars in the constellation of Crux Very large Telescope, 6 July 2020. Credit: ESO
Field of stars in the constellation of Crux. Credit: ESO

Stars get their energy from fusion, where their gas atoms fuse together to form heavier elements.

The exact chain of reactions is complex, but the end result is that hydrogen atoms fuse together to form helium, which join together to form carbon, which then goes on to become oxygen and so on, up the periodic table until you reach iron.

This fusion process generates an intense amount of energy in the form of heat and light.

It’s also the outwards force of the energetic atoms – known as gas pressure ­– that pushes against gravity and stops the star from collapsing under its own weight.

This light passes through Earth’s atmosphere on its way to our eyes, and this is one reason why stars twinkle.

Why are stars different colours?

Both a red and blue star are clearly found in Orion. Credit: Mixetto/iStock/Getty Images
Both a red and blue star are clearly found in Orion. Credit: Mixetto/iStock/Getty Images

If you’ve looked at the stars for any length of time you’ve probably noticed they’re not all the same colour.

You can clearly see this in the constellation of Orion (above), where its left shoulder Betelguese is orangey-red, while its right foot Rigel is blue.

The colour of a star depends on how hot the star is. The hotter something burns, the bluer it will appear, which is why the relatively cool embers of a fire glow red, but the intense flame of a welding torch is blue. Things in between look white.

There are also binary pairs that look like green stars.

The temperature of a star depends on how much gas it gathered before igniting. The more massive a star is, the more intense the pressure at its core, the faster its fuel burns and the hotter it will glow.

The OBAFGKM classification system

Stellar classTemperatureColour
O>33,000KBlue
B10,000 – 30,000KBlue-white
A7,500 – 10,000 KWhite
F6,000 – 7,500 KYellow-white
G5,200 – 6,000 KYellow
K3,700 – 5,200 KOrange
M3,700 KRed

Astronomers group stars together by how hot they are. But rather than using their temperature in Kelvin (degrees Celsius above absolute zero), they’re usually referred to by the OBAFGKM classification system (seen above)

The unusual letter order arose when Harvard astronomers Willimina Fleming and Antonia Maury were classifying the light patterns of stars in photographs in the 1890s.

Williamina Fleming. Public Domain

As they developed a better understanding of what these patterns meant, colleague Annie Jump Cannon reorganised the two astronomer’s classifications to from the O-B-A-F-G-K-M system.

People later started using the mnemonic ‘Oh Be A Fine Girl/Guy Kiss Me’ to remember the order.

The life of a star

A blue giant star. Credit: Credit: NASA's Goddard Space Flight Center/S. Wiessinger
A blue giant star. Credit: Credit: NASA’s Goddard Space Flight Center/S. Wiessinger
Credit: NASA's Goddard Space Flight Center/S. Wiessinger

How long stars live depends on how much fuel they have. It seems counter intuitive, but the more fuel there is, the shorter the lifespan of the star.

Giant stars with many times the mass of our Sun burn through their gas much faster. The largest only last a few million years. Meanwhile stars like our Sun last for around 10 billion years.

In both cases, the stars eventually begin to run out of hydrogen in their cores.

A stream of charged particles emanating from red giant star R Aquilae. Credit: ALMA (ESO/NAOJ/NRAO), Decin et al.
A stream of charged particles emanating from red giant star R Aquilae. Credit: ALMA (ESO/NAOJ/NRAO), Decin et al.

Though hydrogen continues to burn in the outer layers, and heavier elements still fuse in the core, the change upsets the delicate balance of gravity and gas pressure.

The outer layers balloon outward, increasing the star’s size hundreds of times over.

These fluffed up layers cool off, resulting in a huge, red-hued star called a red giant (or a red supergiant if the start star was particularly big). This is predicted to be how our Sun will die in 5 billion years or so.

Meanwhile, stars with less than half the mass of the Sun – red dwarfs – burn their gas so slowly they last for trillions of years, longer than the Universe has existed.

How do stars die?

Once again, how a star meets its end depends on its size. Because red dwarfs live so long no human has seen one die, but astronomers have witnessed the death of many large stars.

Things begin to go downhill for most stars once they reach the red giant phase, as the gas near the outer layers begins to get blown away.

If the star was originally around the mass of the Sun (or indeed, the two Sun-like stars in the Zeta Reticuli system), this creates a cloud of gas called a planetary nebula (named so because it resembles a planet through a telescope, not because it has anything to do with forming planets).

A bubble of ultra-hot gas in the middle of planetary nebula IC 4593 CHANDRA X-RAY OBSERVATORY/HUBBLE SPACE TELESCOPE, 12 NOVEMBER 2020. Credit: X-ray: NASA/CXC/UNAM/J. Toalá et al.; Optical: NASA/STScI
A bubble of ultra-hot gas in the middle of planetary nebula IC 4593 CHANDRA X-RAY OBSERVATORY/HUBBLE SPACE TELESCOPE, 12 NOVEMBER 2020. Credit: X-ray: NASA/CXC/UNAM/J. Toalá et al.; Optical: NASA/STScI

At the nebula’s centre, the core of the original star continues to burn as a small white dwarf.

Astronomers think these will eventually cool to a black dwarf, but this takes tens of billions of years so hasn’t had time to occur anywhere in the Universe yet.

For larger stars, however, things are more dramatic. The cores of red supergiants burn heavy elements at a prodigious rate, but these reactions don’t produce as much energy as fusing hydrogen, so the gas pressure isn’t as high.

A multi-wavelength view of Supernova 1987A, as seen by the Hubble Space Telescope, the Chandra X-ray Observatory and the Atacama Large Millimeter/submillimeter Array. Credit: NASA, ESA, and A. Angelich (NRAO/AUI/NSF); Hubble image: NASA, ESA, and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) Chandra image: NASA/CXC/Penn State/K. Frank et al. ALMA image: ALMA (ESO/NAOJ/NRAO) and R. Indebetouw (NRAO/AUI/NSF).
A multi-wavelength view of Supernova 1987A, as seen by the Hubble Space Telescope, the Chandra X-ray Observatory and the Atacama Large Millimeter/submillimeter Array.

Eventually, the star burns itself out to the point where the gas pressure loses the fight against gravity and the star collapses.

The core folds up to form a dense blob, which the in-falling gas bounces off, creating a massive explosion known as a supernova.

After the gas has dissipated, the core remains behind as a super dense neutron star or, if the star was big enough, a black hole.

If you’re really interested in cosmic death, read our interview with US astrophysicist Katie Mack.

Type 1a supernovae

Type 1a supernova. Credit: ESA/ATG medialab
Artist’s impression of a Type 1a supernova. Credit: ESA/ATG medialab

There is a variety of supernova, called a Type Ia, that work a little differently. They happen in binary star systems with a white dwarf and a red giant close to each other.

The white dwarf syphons off gas from the red giant, slowly building up mass. When the white dwarf reaches a critical mass (around 1.5 times that of the Sun), the star collapses and goes supernova.

Because these stars are always the same size when they explode, Type Ia supernova always have the same brightness.

As such, they’re used as ‘standard candles’, where astronomers use their brightness to measure distance in space, though recent observations are beginning to throw doubt as to how ‘standard’ they are.

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Ezzy Pearson is BBC Sky at Night Magazine’s News Editor. Iain Todd is BBC Sky at Night Magazine’s Staff Writer.