Galaxies

What this part of the map covers

A galaxy is a huge collection of gas, dust and billions of stars, with their planetary systems, all bound together by gravity.^{[1, 2]} Galaxies come in many shapes and sizes.^[2]

The main shapes

Scientists sometimes sort galaxies by their shape and physical features.^[3] The European Space Agency (ESA) names three main groups: elliptical, spiral and barred spiral.^[4]

• Spiral galaxies look like giant rotating pinwheels: a flat disc of stars with a central bulge and arms winding out from it.^{[3, 4]} The centre holds older, reddish stars, while the arms are full of young blue stars and places where new stars are forming.^{[4, 3]}
• Barred spirals are a subtype of spiral, and both the Milky Way and Andromeda belong to it.^{[3, 5]} Scientists think a bar may mean a galaxy has reached full maturity, but this is an interpretation, not a settled fact.^[3]
• Elliptical galaxies range from round to oval, usually contain little gas and dust, and show very little structure.^[3] Scientists think elliptical galaxies originate from collisions and mergers with spirals.^[3]
• Lenticular galaxies have a bulge and a disc like a spiral, but no arms.^[3] Their origin is debated: they may be old spirals whose arms have faded, or the product of merging spirals.^[3]
• Irregular galaxies have unusual shapes, such as rings or little groupings of stars.^[3]

Our home: the Milky Way

The Milky Way is a large barred spiral galaxy.^{[2, 5]} Its disc of stars is about 100,000 light-years across.^{[2, 9]} Estimates of how many stars it holds vary: some sources give about 100 billion, others a few hundred billion or roughly 400 billion.^{[2, 5, 9, 4]} ESA’s team describes it as roughly 13 billion years old.^[9]

The Sun is not near the centre.^[8] It lies roughly 25,000 to 26,000 light-years out, about halfway to the edge of the disc, on what is called the Orion Arm.^{[2, 5, 9, 8]} By one estimate, the Sun takes about 250 million years to circle the centre once.^[11]

At the very centre sits a supermassive black hole called Sagittarius A*, with a mass of about four million Suns; stars have been seen orbiting it as close as a few light-days away.^[5]

Neighbours and clusters

Gravity pulls galaxies together into groups and clusters.^[4] The Milky Way belongs to the Local Group: more than 30 galaxies spread across nearly 10 million light-years.^{[4, 12]}

Its most massive members are the Milky Way and the Andromeda Galaxy (M31), and Andromeda is the nearest large galaxy to ours.^{[15, 13, 12]} Sources put it roughly 2.3 to 2.5 million light-years away.^{[12, 13]} Andromeda is getting closer, and researchers predict it will collide with the Milky Way in roughly 4 to 5 billion years.^{[2, 1]} Some astronomers have speculated that the two could merge into one giant elliptical galaxy.^[12]

Active galaxies and quasars

About 10% of known galaxies are active: their centres look more than 100 times brighter than the combined light of all their stars.^[3] Astronomers think this extra energy comes from the region around a central supermassive black hole.^[3] Quasars are the brightest type of active galaxy.^[3]

The unseen part: dark matter

Astronomer Vera Rubin found that outer stars move about as fast as inner ones, which only makes sense if a great deal of unseen matter is present.^[10] Astronomers therefore infer that the Milky Way sits inside a larger halo of dark matter, detected only through its gravity.^{[5, 10]} In the standard picture it makes up about 85% of all the mass in the universe, but no one yet knows what it is.^[10]

How many galaxies?

Estimates range widely, and they cover only the observable universe, the part within our cosmic horizon.^[6] Earlier estimates were about 100 to 200 billion galaxies.^{[6, 14]} In a result released on 13 October 2016, a team led by Christopher Conselice of the University of Nottingham concluded that there are at least ten times as many.^{[6, 14]} That figure is inferred with mathematical models, not counted: the team estimated that about 90% of galaxies are too faint and distant for today’s telescopes.^{[6, 14]}

How we found other galaxies

The Persian astronomer al-Sufi gave the first known report of Andromeda in his Book of Fixed Stars, from the year 964.^[13] Harlow Shapley measured the distances of the globular clusters around our galaxy and found that the Sun lies far from its centre.^[8] In 1920 Shapley and Heber Curtis held a public debate over whether spiral “nebulae” were inside the Milky Way or beyond it.^[16]

Edwin Hubble settled it. In 1923 he picked out individual stars in the Andromeda “nebula”.^[17] One of them, V1, was a Cepheid variable, a pulsating star whose rhythm reveals its true brightness and so its distance.^{[16, 18]} By the end of 1924 his Cepheids gave a distance of 900,000 light-years, far beyond our own galaxy; later measurements placed Andromeda even farther away.^{[16, 12]} In 1929 he found that more distant galaxies are moving away faster, a pattern now called Hubble’s Law.^{[4, 17, 7]} This was evidence that the universe is expanding: the space between galaxies is stretching, which reddens their light.^{[7, 4]}

What telescopes see today

Webb’s infrared images of 19 face-on spiral galaxies show stars, gas and dust at small scales, and astronomers’ evidence indicates that galaxies grow from the inside out, with star formation starting in the core and spreading along the arms.^[20] Webb also watched Sagittarius A* for 48 hours spread across a year and saw five to six big flares a day.^[19] The researchers found no pattern, and the causes are not yet fully understood.^[19]

Common misconceptions

“Pictures of the whole Milky Way are photographs of it.” We live inside the galaxy, so no one can photograph its spiral shape from outside.^[8]

“When galaxies collide, their stars smash together.” Galaxies are so spread out that their planets and star systems often don’t come close to colliding.^[1]

“Galaxies are flying apart through space.” The universe appears to expand because the space between galaxies grows, not because the galaxies themselves are rushing through space.^[4]

Going further

See Stars for what galaxies are made of, Cosmology for the expanding universe, Solar System for our own neighbourhood, Gravity for the force that holds galaxies together and Space Telescopes for the instruments that study them.