Galaxy file

The galaxy file

Sourced facts about galaxies and the universe, topic by topic. Every fact links to its source, checked 31 Oct 2026.

Galaxies

Galaxies come in a variety of shapes, mostly spirals and ellipticals, plus less orderly ones usually called irregular. [source]
Galaxies are still sorted using the scheme proposed by the American astronomer Edwin Hubble, with three general classes: ellipticals, spirals and irregulars. [source]
Most galaxies are between 10 billion and 13.6 billion years old. [source]
Most large galaxies have a supermassive black hole at the centre, some with billions of times the Sun’s mass. [source]
The Milky Way sits in a neighbourhood of over 50 other galaxies called the Local Group. [source]

The Milky Way is a large spiral of several hundred billion stars, one of which is the Sun. [source]
Its disc of stars spans more than 100,000 light-years. [source]
Earth sits along one of the galaxy’s spiral arms, about halfway from the centre. [source]
The Sun lies near a small, partial arm called the Orion Arm, or Orion Spur. [source]
Our solar system takes about 240 million years to go round the Milky Way just once. [source]
From Earth it looks like a faint, milky band of light arching across the sky, which is how it got its name. [source]

Closer to home, planets.im has sourced facts about the planets of our solar system.

A black hole is so dense that, just beneath its surface (the event horizon), gravity is strong enough that nothing, not even light, can escape. [source]
The Milky Way’s central black hole is called Sagittarius A* (said “A-star”). It has 4 million times the Sun’s mass. [source]
Astronomers proved it is there by tracking stars orbiting the centre of the Milky Way, a discovery that won the 2020 Nobel Prize. [source]

The universe formed around 13.8 billion years ago. [source]
One second after the big bang, the universe was an extremely hot soup of light and particles, at 10 billion degrees Celsius. [source]
After five minutes, most of today’s helium had formed. [source]
Around 380,000 years after the big bang, the universe had cooled enough for atomic nuclei to capture electrons. [source]

The Andromeda galaxy is our nearest major galactic neighbour, 2.5 million light-years away. [source]
The Local Group ranges from dwarf galaxies with up to a few billion stars to Andromeda, our nearest large galactic neighbour. [source]
It can be seen with the naked eye, even in places with moderate light pollution. [source]
Only in the 1920s did Edwin Hubble show for certain that Andromeda is a separate galaxy beyond the Milky Way. [source]
In January 2025 Hubble released a panorama of Andromeda made from 7,398 exposures, its largest mosaic so far. [source]

Planning a night of stargazing? moon.planets.im explains the phases of the Moon.

Dark matter is called “dark” because it doesn’t absorb, reflect or give out any light, so we can’t see it. [source]
In 1933 the Swiss-born astronomer Fritz Zwicky, studying the Coma Cluster of galaxies, suggested that invisible matter might be holding the galaxies together. [source]
In the 1970s the American astronomer Vera Rubin studied stars on the outer edges of spiral galaxies, and her work gave strong evidence for dark matter. [source]

The Hubble Space Telescope was launched on 24 April 1990. [source]
The James Webb Space Telescope was launched on 25 December 2021. It is the largest, most powerful telescope ever launched into space. [source]
Webb doesn’t orbit the Earth like Hubble. It orbits the Sun 1.5 million kilometres from Earth, at a point called L2. Hubble orbits 560 kilometres above the Earth. [source]
Webb has a five-layer sunshield against the heat of the Sun, Earth and Moon. [source]

Telescopes and spacecraft explore our own solar system too: see the missions to Jupiter on jupiter.planets.im.

A quasar is an extremely bright object at the centre of some galaxies, powered by gas spiralling at high speed into an enormous black hole. [source]
The brightest quasars outshine all the stars in their own galaxies, so they can be seen from billions of light-years away. [source]
3C 273 is the brightest known quasar. [source]

A supernova is the explosion of a star, and the biggest explosion humans have ever seen. [source]
Astronomers think about two or three supernovae happen each century in galaxies like the Milky Way. [source]
The Crab Nebula is what is left of a massive star in our Milky Way that died 6,500 light-years away. [source]

A nebula is a cloud of gas and dust in interstellar space. [source]
Nebula is Latin for “mist” or “cloud”. [source]
Stars form in large clouds of gas and dust. Clouds full of young star clusters are called stellar nurseries. [source]
The Helix Nebula, a typical planetary nebula, lies 650 light-years away in the constellation Aquarius. [source]

The Magellanic Clouds are two irregular galaxies that are satellites of the Milky Way. [source]
They are named after the Portuguese navigator Ferdinand Magellan, whose crew discovered them on the first voyage around the world (1519–22). [source]
They lie about 22° apart in the sky, near the south celestial pole. [source]

Distances, ages and sizes are the sources’ own estimates, and they can change as astronomers learn more.

Ad · affiliate link

For a dark night in the garden, the classroom or the bedroom shelf: Astronomy books · Planispheres · Stargazing binoculars · Beginner telescopes · Red-light torches · Galaxy posters. These are Amazon UK search links, not endorsements of particular products. As an Amazon Associate I earn from qualifying purchases.