Stars form in huge clouds of gas and dust called nebulae. The gravity of the gas and dust in the clouds pulls everything inwards. The clouds slowly collapse onto a number of points (or cores). Deep in the centre of these cores, there is lots of dense material squashed together, and it is very hot. Eventually it is hot enough for nuclear fusion to start. Nucelar fusion is the process that powers a star.

Image: Hubble’s sharpest view of the Orion Nebula | Credit: ESA/Hubble

Stars are not true stars until they can fuse hydrogen into helium. This point is called stellar ignition because it is when a star starts to shine. Before that point, they are called protostars. The protostellar phase is the earliest one in the process of stellar evolution. The sudden burst of light made by a new star blows away much of the nearby gas cloud. However, it can leave enough material behind to form a number of planets.

Image: Artist’s impression of a baby star  | Credit: ESO/L. Calçada

After stellar ignition, the star becomes relatively stable. Stars shine for many millions of years but do not last for ever. A star like the Sun will shine for around 10 billion years. Stars in the main-sequence stage give out energy as light and heat. This energy is released by nuclear fusion reactions deep in their cores. Stars contain a lot of mass which has a lot of gravity. The gravity pulls inwards, which could cause a star to collapse in on itself. In a main sequence star, this inward pull is balanced by the outward gas pressure due to the nuclear fusion reactions deep in the star’s core. This balance is called hydrostatic equilibrium and stops main sequence stars from collapsing.

Image: The Sun in high resolution
Credit: ESA & NASA/Solar Orbiter/EUI team; Data processing: E. Kraaikamp (ROB)

Once a star has exhausted its supply of hydrogen in its core, leaving nothing but helium, the outward force created by fusion starts to decrease and the star can no longer maintain equilibrium. The force of gravity becomes greater than the force from internal pressure and the star begins to collapse. Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star. While the core collapses, the outer layers of material in the star to expand outward. The star expands to larger than it has ever been. At this point the star is called a red giant.

The results of this collapse depend on the mass of the star.

Image: VLTI reconstructed view of the surface of Antares | Credit: ESO/K. Ohnaka
Background image: Artist’s impression of the red supergiant star Antares
Credit: ESO/M. Kornmesser

Low-mass stars turn into planetary nebulae towards the end of their red giant phase. At that point the star becomes highly unstable and starts to pulsate. This produces strong stellar winds which throw off the outer layers of the star. The outer layers drift away from the star leaving a small hot, bright core behind, called a white dwarf. The white dwarf gives of ultraviolet radiation which lights up the layers of gas around the star. White dwarfs are made of carbon and oxygen. This was created by the star by nuclear fusion during its main sequence and red giant phases. This material is compacted into a relatively small space, which makes white dwarfs very dense. White dwarfs do not release energy through nuclear fusion reactions. Over many thousands of millions of years, they will stop glowing completely and become cold black dwarfs.

Image:  Citizen Scientist Finds Ancient White Dwarf Star Encircled by Puzzling Rings
Credit: GSFC – Photographer: NASA’s Goddard Space Flight Center/Scott Wiessinger
Image background: NASA’s Hubble Space Telescope Finds Dead Stars
Credit: NASA Goddard

When a massive star reaches the final stages of its evolution, its core is made mostly of iron. The star cannot fuse elements heavier than iron. This means fusion stops. At this point, there is no outward pressure to balance the inward pull of its gravity. Gravity pulls all the material in the star towards its middle. This starts a sudden, rapid collapse of the star. The outer layers of the collapse inwards until they reach the core. They bounce of the surface of the dense iron core at around 30,000 kilometres per second. This sends shock waves through the star. The shock waves cause the star to explode as a supernova. Huge amounts of energy are created during the collapse and new elements form in the process. The star brightens quickly, then gradually fades away leaving only core. During the explosion the core collapses down to create either a neutron star or a black hole

Image: Supernova 1994D in Galaxy NGC 4526 | Credit: NASA & ESA
Background image: Progenitor star to a type Ic supernova | Credit: NASA, ESA, and J. Olmsted (STScI)

A neutron star is the collapsed core of a massive star. It is what is left of the star, after a supernova explosion. When a high-mass star comes to the end of its lifetime, its outer layers collapse onto the core. This squashes the star’s core to the point where the atoms are smashed apart, leaving only neutrons. Neutrons are sub-atomic particles with no electric charge. The outer layers are then thrown out into space by a shock wave. This leaves a rapidly spinning neutron star behind. Some neutron stars have been found to rotate at several hundred times a second. A neutron star can have the same mass as 1 or 2 Suns. However, it will only be about 20 km across. The only object denser than a neutron star is a black hole.

Image:  The hibernating stellar magnet | Credit: ESO/L.Calçada.
Image background: Artist’s impression of merging neutron stars | Credit: ESO/L. Calçada/M. Kornmesser

After the supernova, anything left of the star is squashed and compacted into an incredibly small, dense object. This is the black hole. Once a black hole has formed, it grows by pulling in gas, dust, stars, and even other black holes around it. Close to a black hole, its gravity is so strong that nothing can get away, not even light. This is why we cannot see into a black hole – because they do not reflect or emit light. The distance at which light cannot escape from a black hole is known as its event horizon.

Image: First image of our black hole | Credit: EHT Collaboration
Background image: Black Holes: Monsters in Space Artist Concept
Credit: JPL | Secondary Creator Credit: NASA/JPL-Caltech