Stars are composed of very hot plasma (a gas where the electrons and nuclei of atoms are largely separated) that is held together by its own gravity. The sustaining energy output of a star is generated by nuclear reactions taking place in its centre, which initially fuse hydrogen into helium, via the proton-proton chain (and for more massive stars via the carbon-nitrogen-oxygen CNO cycle) before moving onto fusing higher elements. Stars are stabilised by the pressure sustained via the energy set free during their central fusion processes, which counteracts the star’s urge to collapse under its own gravity. In this way, most stars of similar or less mass to our Sun remain stable for a few billions or even tens of billions of years.
The gravitational collapse of giant cold molecular clouds gives birth to stars. As the cloud collapses, it fragments into cores whose central regions get ever denser and hotter. Beyond critical values for temperature and pressure, nuclear fusion ignites, and a star is born. This young star is initially surrounded by a protoplanetary disk of dust and gas. Over the course of millions of years, this disk differentiates into planets and smaller bodies.
With an equatorial diameter of about 1.4 million kilometres, the Sun, the nearest star to Earth, is so big that we could fit roughly 1.3 million Earths inside. Even though our star is enormous compared to our planet, there are much bigger stars in the Universe. The supergiant VY Canis Majoris, with about 1400 times the diameter of the Sun, is the largest star known to date. If placed in the centre of the Solar System, the surface of VY Canis Majoris would extend beyond the orbit of Jupiter. There are also stars much smaller than the Sun. The closest star, Proxima Centauri, is a red dwarf with a diameter of about 200,000 kilometres, only 16 times the diameter of Earth.
Although it seems uniform in appearance, the surface of the Sun can be mottled with dark spots. These sunspots, or regions of strong magnetic field, appear dark because they are cooler than the surrounding material. Every 11 years, the Sun cycles between producing many spots and producing a few spots. Sometimes, the Sun's magnetic field gets twisted up, builds a lot of energy, and releases this energy in a burst of light and particles. These bursts are called flares or coronal mass ejections. But even when it is calm, the Sun constantly spews about 1.5 billion kilograms of hot, magnetized gas into space every second. This solar wind flows through the solar system and interacts with planets. Other stars also produce flares and winds.
Stars can have surface temperatures between a few thousand degrees Celsius and fifty thousand degrees Celsius. Hot stars radiate away most of their energy in the blue and ultraviolet region of the electromagnetic spectrum (at short wavelengths), and thus look blueish to our eyes. Cooler stars look reddish, since they radiate away most of their energy in the red and infrared regions of the electromagnetic spectrum (at long wavelengths).
The space between the stars contains minute traces of matter in the form of gas, dust and high energy particles (“cosmic rays”). This matter content is called the Interstellar Medium. It can be more or less dense in different parts of the galaxy. However, even the most dense regions of the Interstellar Medium are still one thousand times less dense than the best vacuum created in a laboratory.
Computer simulations reveal that the first stars had life spans of some millions of years. In contrast, the average life expectancy of a star similar to the Sun is about 10 billion years. Low-mass red dwarf stars can live for trillions of years. A star with a mass similar to that of our Sun will eventually evolve into a red giant star and later on eject most of its mass to space, leaving behind a compact white dwarf star surrounded by what is called a planetary nebula. A star with at least eight solar masses will evolve into a red supergiant before exploding in an event called supernova, leaving behind a neutron star or a stellar black hole.
A black hole is a region of space whose extreme gravitational field prevents anything, even light, from escaping once it has crossed the event horizon. The event horizon is a boundary surface surrounding a black hole, where the speed needed to escape its gravitational field is greater than the speed of light. Theoretical models predict that at the centre of a black hole is a singularity, where the density of matter and curvature of spacetime approaches infinity. Stellar mass black holes have masses in the order of a few tens of solar masses, in a region having a radius of some few kilometres to tens of kilometres (depending on the mass).
Apart from hydrogen, most of the helium and a small amount of lithium, all elements in the present Universe have been produced inside stars by nuclear fusion. Low-mass stars, like the Sun, produce elements up to oxygen, while massive stars can create elements heavier than oxygen and up to iron. Elements heavier than iron, like gold and uranium, are created during high-energy supernova explosions and neutron star collisions. As they die, stars release most of their mass into the Interstellar Medium. From this matter, new stars form, in the cosmic version of a recycling process.
Elements other than hydrogen and helium and a small amount of lithium, were mainly created in the interior of stars, and released into space in the last stages of these stars’ lives. This is the origin of most of the elements that make up our bodies, such as the calcium in our bones, the iron in our blood and the nitrogen in our DNA. In the same way, the elements making up other animals, plants, and indeed most of the things we see around us, were produced by stars billions of years ago.