Stars, galaxies and the universe

Space distances are so large that astronomers use special units. A light-year is the distance light travels in a year, about 9.5 trillion kilometres. The nearest star beyond the sun, Proxima Centauri, is about 4.2 light-years away, so the light we see from it left more than four years ago.

Looking out into space therefore means looking back in time. Light from distant galaxies has travelled for billions of years, showing them as they were when the universe was young.

A star is a huge ball of hot gas, mostly hydrogen and helium, held together by gravity. In its core, pressure and temperature are high enough for nuclear fusion, which joins hydrogen nuclei into helium and releases the energy that makes stars shine.

A star's mass decides its life. Small red dwarfs burn slowly and can shine for trillions of years. Stars like the sun last about 10 billion years, then swell into red giants and shed their outer layers, leaving a dense white dwarf. The most massive stars live only a few million years and die in supernova explosions, leaving neutron stars or black holes.

Stars gather in galaxies. Our galaxy, the Milky Way, is a barred spiral about 100,000 light-years across containing somewhere between 100 and 400 billion stars. The sun lies in one of its spiral arms, roughly 26,000 light-years from the centre, where a supermassive black hole called Sagittarius A* sits.

Until the 1920s, many astronomers thought the Milky Way was the whole universe. In 1923 Edwin Hubble measured the distance to a variable star in the Andromeda nebula and showed it lay far outside our galaxy. Andromeda is a separate galaxy, and observations suggest the observable universe contains hundreds of billions to perhaps two trillion galaxies.

In 1929 Hubble, building on work by Vesto Slipher and others, showed that distant galaxies are moving away from us, and the farther away they are, the faster they recede. Light from them is stretched to longer, redder wavelengths, an effect called redshift.

This does not mean we are at the centre. Space itself is expanding, carrying galaxies apart, so observers in any galaxy would see the same pattern. Run the expansion backward, and everything converges into an extremely hot, dense state in the distant past.

• The expansion of the universe, measured through the redshift of galaxies.
• The cosmic microwave background, faint radiation filling all of space, discovered by Arno Penzias and Robert Wilson in 1965, left over from when the universe cooled enough for light to travel freely.
• The measured abundance of light elements such as hydrogen, helium and lithium, which matches predictions of nuclear reactions in the first minutes.
• Observations that distant, younger galaxies look different from nearby, older ones.
• Current measurements put the age of the universe at about 13.8 billion years.

Ordinary matter, everything made of atoms, appears to make up only about 5 percent of the universe's contents. Galaxies rotate too fast to be held together by their visible matter alone, pointing to invisible dark matter, which seems to make up about 27 percent. Nobody yet knows what it is.

In 1998 two teams discovered that the expansion of the universe is speeding up. The cause has been named dark energy, and it accounts for roughly 68 percent of the universe. Understanding dark matter and dark energy is among the biggest open problems in physics.