Stars are giant spinning balls of hot gases. They produce vast amounts of energy, including heat and light, which they radiate across space as they shine. They vary enormously in size (Betelgeuse in the constellation of Orion is 800 times the size of the Sun), and by the amount of light they give off. Stars come in different colours according to how hot they are. Blue stars are hot; red stars are cooler, although still thousands of degrees Celsius—much hotter than an oven. Orange and yellow stars (including the Sun) are moderately hot.

How stars form

A star is born when a part of cloud of dust and gas in space, known as a giant molecular cloud (a kind of nebula, 1), compresses under the force of gravity to become a dense blob, called a protostar (2). The shock waves from an exploding star at the end of its life in the same neighbourhood are often responsible for triggering this process. The protostar becomes so hot that it starts to produce its own energy in its core. A spinning disc of gas and dust surrounds the newly forming star, which may eventually become the birthplace of new planets.

Main sequence stars

If the star has a mass around that of the Sun, it now spends the next few billion years of its life as a main sequence star (3). The star produces massive amounts of energy as hydrogen atoms fuse together to form helium at extremely hot temperatures in its core. When the hydrogen fuel starts to run out, the star swells into a red giant (4). Arcturus, in the constellation of Boötes, is an example of a red giant. It is 25 times the size of the Sun and the fourth brightest star in the night sky.

​​​​​​​A red giant does not explode. Instead, its outer layers expand to form a planetary nebula (5) with a white dwarf (6) at its core. This, in turn, fades and cools to become a black dwarf (7).

Supergiants and supernovae

Almost all stars are born in giant molecular clouds (1) as protostars (2). Massive stars (3) later become supergiants (4) All stars are born in nebulae (1) and become protostars (2). A very few massive stars (3) become supergiants (4). Betelgeuse is an example of a red supergiant, Rigel (also in the constellation of Orion) a blue supergiant. Eventually, a supergiant blasts apart in a mighty explosion called a supernova (5). It ends its days as a black hole (6), a region of space so dense that nothing, not even light, can escape the pull of its gravity, or a neutron star (7).

Betelgeuse

The red supergiant Betelgeuse (often pronounced “beetlejuice”) is one of the stars of the constellation of Orion in Greek astronomy. Its name comes from the Arabic yad al-jawzāʾ (“the giant’s hand”).

​​​​​​​One of the largest stars visible to the naked eye, it is reckoned to be about 800 times the size of the Sun, although as a pulsating variable, its diameter changes with time. If it were at the centre of our Solar System, its surface would lie within the Asteroid Belt and it would engulf the orbits of Mercury, Venus, Earth and Mars.

In May 2023, astronomers reported a strange event. Known to become brighter and dimmer in 400-day cycles (the variability—though not the period—was discovered by Aboriginal Australians long before western astronomers), from late 2019 to early 2020 Betelgeuse underwent what is now known as the Great Dimming. Scientists explained that the star had "sneezed" a huge mass of gas from its surface far out into space, where it cooled and became a dust cloud. This blocked much of the light of the star for that period.

Scientists expect Betelgeuse to explode in a supernova some time in the next 100,000 years. It is just possible—but unlikely—its dramatic death could be witnessed in our lifetimes. Since light from Betelgeuse takes more than 500 years to reach us, it could have already exploded and we do not yet know whether this has happened (although the chance of this is less than 1 in 100).
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In 2026, it was reported that Betelguese had a companion star, orbiting within its outer atmosphere. Named Siwarha, this star is about 1.5 times the mass of the Sun (but hundreds of times smaller than Betelguese itself). It was found to be responsible for the 2100-day cycle over which the supergiant regularly dims and brightens. By analysing the spectra of light emitted by elements in Betelguese's atmospheric gases, astronomers found that Siwarha was stirring up these gases, creating a dense trail of material behind it as it orbited the main star. Siwarha's "wake" was causing Betelgeuse's brightness to dim as it crossed our line of sight of it from Earth.

Horsehead Nebula

A nebula is a vast cloud of gas and sometimes dust—the remains of dying stars. Seen through a powerful telescope, some of the Galaxy’s nebulae take on spectacular shapes, like, for example, the Horsehead Nebula in Orion (part of a much larger giant molecular cloud), which is made up of a dense region of dust. Some nebulae provide the “raw material” from which new stars begin to form. 

White dwarf

Stars like the Sun are so small when compared with red giants and supergiants that astronomers call them dwarfs. A spoonful of the Sun, depending on where it was taken from, would, on Earth, weigh about 2 kilos (5 pounds). After the Sun swells to become a red giant—in about five billion years—it will lose its outer layers. Just the small, very hot, dead core will remain. Called a white dwarf, it may measure about 10,000 kilometres (6000 miles) across—perhaps even smaller than the Earth—and be extremely dense. A spoonful of white-dwarf material would weigh 5 tonnes on Earth.

Neutron stars
​​​​​​​and pulsars

The core that remains after a supernova explosion is a tiny star no more than 25 kilometres (15 miles) across, known as a neutron star. Surpassed only by black holes, neutron stars are the second smallest and densest known type of objects. A spoonful of material from a neutron star would weigh an incredible one billion tonnes on Earth. Although they no longer generate heat and become cooler over time, neutron stars have powerful magnetic and gravitational fields.
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Newly formed neutron stars rotate at up to several hundred times per second. As they spin, they send out large amounts of energy in the form of beams of electromagnetic radiation. This radiation can be detected from Earth only when the beam is pointing towards us. So it appears as a series of pulses as the star spins on its axis—in the same way that the beam from a lighthouse can only be seen when it is pointed towards us as the light spins round. For this reason, rapidly spinning neutron stars are called pulsars.
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In 2025, astronomers detected an unusual star in deep space named ASKAP J1832. To find it, they combined data from NASA’s Chandra X-ray Observatory space telescope and the Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia. The object emits pulses of radio waves and X-rays every 44 minutes. It is known as a "long period radio transient", an object that emits a regular pulse of radiation at intervals thousands of times longer than those seen in pulsars. Some scientists think the object could be a neutron star, though one with an extremely strong magnetic field, a type called a magnetar. Others say it could be a white dwarf (see above) with a companion star.
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Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram shows the relationship between luminosity (the amount of light and other radiation given off) and temperature for many types of stars. Stars of greater luminosity are placed towards the top of the diagram, while stars with higher temperatures are plotted towards the left side of the diagram. Astronomers use the Sun as a kind of measuring rod for luminosity: it is given a luminosity of 1, and sits halfway up on the vertical scale.
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Most distant star

A star named Earendel (officially called WHL0137-LS), in the constellation of Cetus, is the most distant known star. The light detected from Earendel was emitted "only" 900 million years after the Big Bang and reached Earth 12.9 billion years later. Due to the expansion of the Universe, however, the star now lies 28 billion light-years away. Its discovery, made by astronomers using the Hubble Space Telescope, was reported on 30th March 2022. Earendel is more than twice as hot as our Sun and about a million times more luminous.
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