A beginner’s guide to stars

Stars. They fill the night sky with their twinkling, have inspired countless tales and helped people navigate since the dawn of time. But what actually are they?

The Pleiades, as seen through a telescope. This cluster is known as 'The Seven Sistsers' in Greek and also Aboriginal mythology. NASA, ESA, AURA/Caltech, Palomar Observatory The science team consists of: D. Soderblom and E. Nelan (STScI), F. Benedict and B. Arthur (U. Texas), and B. Jones (Lick Obs.)

What is a star?

A star is a glowing ball of plasma (very hot gas). They’re born in huge clouds of hydrogen, known as stellar nurseries, when pockets of gas begin to clump together. Over time, gravity draws in more gas until there are several trillion trillion tons gathered into a ball.

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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 binaries or in trios called triplets, but as many as seven stars have been found together in systems such as AR Cassiopeiae. 

What are stars’ different colours?

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, where its left shoulder (Betelguese) is orangey-red, while its right foot (Rigel) is blue.

A sense of wonder: from a dark-sky site a wealth of deep-sky objects can be seen even with the naked eye, including the Milky Way, Orion Nebula and star clusters such as the Pleiades and Hyades. Credit: Mixetto/iStock/Getty Images
Both a red and blue star are clearly found in Orion. Credit: Mixetto/iStock/Getty Images

These colours depend 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.

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.

Stellar classes

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 O-B-A-F-G-K-M classification system.

O

>33,000K

blue

B

10,000 – 30,000K

blue white

A

7,500 – 10,000 K

white

F

6,000 – 7,500 K

yellow white

G

5,200 – 6,000 K

yellow

K

3,700 – 5,200 K

orange

M

<3,700 K

red

 

 

 

 

 

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.

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 unusual order.

Why do stars shine?

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.

How do stars age?

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.

A blue giant star
Blue giants live fast and die young. Credit: NASA’s Goddard Space Flight Center/S. Wiessinger
Credit: NASA's Goddard Space Flight Center/S. Wiessinger

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

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 will happen to our own Sun in five 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, 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).

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

Planetary nebula NGC 2022. Credit: ESA/Hubble & NASA, R. Wade
Planetary nebula NGC 2022. Credit: ESA/Hubble & NASA, R. Wade

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.

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 infalling gas bounces off, creating a massive explosion known as a supernova.

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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.

Type IA

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 gauge how far away objects are in the Galaxy, though recent observations are beginning to throw doubt as to how ‘standard’ they are. 

A white dwarf syphons gas off a red giant.
NASA/Swift/ Aurore Simonnet, Sonoma State Univ.