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Galaxies

A galaxy is an enormous system of stars, gas, dust and dark matter held together by gravity. Our Sun is one of about 100 billion stars in the Milky Way, a barred spiral galaxy, and beyond it lie countless other galaxies — a fact only proved in the 1920s. How fast galaxies spin reveals that most of their mass is invisible dark matter.

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Rotation Curves and Galaxy Mass

Treating a star as on a circular orbit, its speed obeys v² = GM(r)/r. If all the mass were in the luminous disk and bulge, v would fall roughly as 1/√r beyond them. Rubin and Ford found Andromeda's rotation curve stays roughly flat instead, so M(r) ∝ r out to large radii; they estimated about ten times as much invisible mass as visible.

M(r)=v2rGM(r) = \dfrac{v^2 r}{G}

Enclosed mass implied by a measured orbital speed v at radius r.

Independent methods agree. Zwicky argued in 1933 that galaxies in the Coma Cluster move too fast for their visible mass. Gravitational lensing maps a cluster's total mass from the distortion of background galaxies; in the colliding Bullet Cluster, the lensing mass sits in a different place from the hot X-ray gas that holds most of the ordinary matter.

Info: Two-thirds of spiral galaxies, including the Milky Way and Andromeda, are barred spirals, with a ribbon of stars, gas and dust cutting across the centre.
Full explanation — the complete reference version every reading depth is based on

What it is

A galaxy is a vast collection of stars, planets, gas and dust bound together by gravity. The largest contain trillions of stars and stretch more than a million light-years across; the smallest hold only a few thousand stars. Most large galaxies have a supermassive black hole at their centre.

  • Spiral galaxies, like the Milky Way and Andromeda, have a central bulge and a flat disk with spiral arms; two-thirds of spirals, including both of these, have a bar of stars across the centre.
  • Elliptical galaxies range from round to oval and have no spiral arms.
  • Irregular galaxies have no orderly shape.

Our galaxy, the Milky Way

The Milky Way is a spiral galaxy of about 100 billion stars whose disk is more than 100,000 light-years across. Earth lies along one of the spiral arms about halfway from the centre, and the Solar System takes about 240 million years to go round once. Seen from inside the disk, the Milky Way appears as a faint band of light across the night sky. It belongs to the Local Group, a neighbourhood of more than 50 other galaxies whose largest member is the Andromeda galaxy, about 2.5 million light-years away.

Why it matters: the dark matter clue

Measuring how fast stars orbit at different distances from a galaxy's centre gives a rotation curve. If visible matter were all there is, outer stars should move more slowly. In the 1970s Vera Rubin found that the outer stars of spiral galaxies move so fast that a large amount of unseen matter must hold them in place; for Andromeda, she and Kent Ford estimated about ten times as much invisible mass as visible. Gravitational lensing of galaxy clusters points the same way.

v=G M(r)rv = \sqrt{\dfrac{G\,M(r)}{r}}

Orbital speed at radius r depends on the mass M(r) enclosed within that radius; speeds that stay high far out mean M(r) keeps growing.

Worked example

The Sun formed about 4.6 billion years ago and takes about 240 million years to orbit the Milky Way. Dividing 4,600 million by 240 million gives about 19, so the Sun has travelled round the galaxy roughly 19 times (our own calculation from the two sourced figures, assuming the orbital period has stayed about the same).

Common misconception: A common mix-up is between a galaxy and a solar system. The Solar System is one star and the bodies orbiting it; a galaxy contains billions of stars, many with their own planets. The Milky Way alone holds about 100 billion stars.

Where it connects

Galaxies are made of stars (Stars) whose life cycles enrich them with new elements (Stellar evolution). Their centres host supermassive black holes (Black holes), Cepheids in galaxies were discovered with telescopes (Telescopes), and the way galaxies move apart is the evidence for an expanding universe (Cosmic expansion).

How we know other galaxies exist

In the early 1900s many astronomers thought the Milky Way was the whole universe, and that fuzzy 'spiral nebulae' were clouds inside it. In 1912 Henrietta Swan Leavitt had shown that a Cepheid variable star's rhythm of brightening and dimming reveals its true brightness. In 1923 Edwin Hubble found a Cepheid, V1, in the Andromeda nebula. Comparing how bright it should be with how bright it looks gave a distance far beyond the edge of the Milky Way: Andromeda had to be a separate galaxy.

Assumptions and limits

  • Star counts for the Milky Way are estimates: it is very hard to count stars from our position inside the disk.
  • Galaxy masses from rotation assume stars move on near-circular orbits under ordinary gravity.
  • Rotation curves reveal how much unseen mass there is, not what it is made of; that remains unknown.

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Sources and methodology

  • A galaxy is a system of stars, planets, gas and dust bound together by gravity; the largest contain trillions of stars and can be more than a million light-years across, while the smallest hold a few thousand stars across a few hundred light-years. (awaiting scientific review)
    • Galaxies — Government or standards body
  • Galaxies come mainly in spiral and elliptical shapes, with less orderly galaxies usually called irregular. (awaiting scientific review)
    • Galaxies — Government or standards body
  • The Milky Way is a spiral galaxy whose disk of stars spans more than 100,000 light-years; Earth lies along one of its spiral arms about halfway from the centre, and the Solar System takes about 240 million years to orbit the galaxy once. (awaiting scientific review)
    • Galaxies — Government or standards body
  • Best estimates put the number of stars in the Milky Way at about 100 billion. (awaiting scientific review)
  • The Sun formed about 4.6 billion years ago. (awaiting scientific review)
  • Most large galaxies have a supermassive black hole at the centre, some with billions of times the Sun's mass. (awaiting scientific review)
    • Galaxies — Government or standards body
  • In 1923 Edwin Hubble found a Cepheid variable star, V1, in the Andromeda 'nebula' using the 100-inch Hooker Telescope at Mount Wilson, and showed it lay far beyond anything in the Milky Way — proof that the universe extends beyond our galaxy. (awaiting scientific review)
  • In 1912 Henrietta Swan Leavitt showed that the period over which a Cepheid variable star brightens and dims is related to its luminosity, which lets astronomers use Cepheids to measure distances. (awaiting scientific review)
  • For Cepheid variables, the brighter the star, the slower its brightness changes, so comparing a Cepheid's apparent brightness with the brightness predicted from its changes gives its distance. (awaiting scientific review)
  • The Andromeda galaxy, a larger galaxy than the Milky Way, is about 2.5 million light-years away and can be seen without a telescope from the northern hemisphere under a very dark sky. (awaiting scientific review)
  • Galaxies gather into groups of about 100 or fewer members held together by their mutual gravity, and into larger clusters of up to thousands; the Milky Way sits in the Local Group with more than 50 other galaxies, including Andromeda. (awaiting scientific review)
    • Galaxies — Government or standards body
  • Most galaxies are between 10 billion and 13.6 billion years old. (awaiting scientific review)
    • Galaxies — Government or standards body
  • The Milky Way and the Andromeda galaxy are both barred spirals, the subtype that makes up two-thirds of spiral galaxies. (awaiting scientific review)
  • In the 1970s Vera Rubin found that stars on the outer edges of spiral galaxies move so fast that a large amount of unseen matter must be holding them in place, which convinced the scientific community that dark matter exists. (awaiting scientific review)
  • Rubin and Kent Ford's measurements of Andromeda's rotation suggested that, besides its visible matter, the galaxy contains about ten times as much invisible mass. (awaiting scientific review)
  • In 2006 observations of the Bullet Cluster, two colliding galaxy clusters, used gravitational lensing to show that most of their mass lies in a different place from their hot X-ray-emitting gas — some of the best direct evidence for dark matter. (awaiting scientific review)
  • Dark-matter candidates such as WIMPs and axions are being searched for, but no evidence has yet confirmed any of them. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.