Galaxy

Full definition

If the Sun were a grain of sand, the Milky Way would be roughly the size of Europe. A galaxy is that kind of structure: a gravitational archipelago gathering hundreds of millions to several trillion stars, gas, dust and a massive halo of dark matter. All of this orbits a common barycenter, most often hosting a supermassive black hole at the center.

Galaxies are not isolated. They live in groups, clusters and superclusters, linked by cosmic filaments — the cosmic web mapped out by large surveys like SDSS. The Milky Way belongs to the Local Group (about fifty galaxies including giant Andromeda M31), itself on the edge of the Virgo Cluster, part of the Laniakea Supercluster.

The ingredient holding it all together is invisible: dark matter accounts for about 85 % of the mass of a typical galaxy. It explains why outer stars orbit as fast as inner ones — the rotation-curve anomaly first systematically documented by Vera Rubin in the 1970s.

The conceptual lineage is recent. Kant spoke of 'island universes' as early as 1755, but proof only arrived in 1925, when Edwin Hubble identified Cepheids in what was then called the 'Andromeda Nebula'. Henrietta Leavitt's period-luminosity relation (1912) allowed him to compute its distance: far beyond our own galaxy. M31 was another universe. Galaxies became an object of study in their own right, and astronomy changed scale.

Morphology and the Hubble classification

Edwin Hubble proposed in 1926 the morphological classification that bears his name, often drawn as a 'tuning fork'.

Elliptical galaxies (E0 to E7). Spherical to elongated shape, little gas and dust, old red stellar population. Little ongoing star formation. Masses range from dwarf ellipticals (10⁷ M☉) to cD giants like M87 (10¹³ M☉).

Spiral galaxies (Sa, Sb, Sc). Flat disk with spiral arms, central bulge, spherical halo. Plenty of gas, dust and active star formation. The Milky Way is a barred spiral of type SBbc.

Barred spiral galaxies (SBa, SBb, SBc). Variant with a bar of stars crossing the central bulge. About two thirds of spirals have one.

Lenticular galaxies (S0). Hybrids between ellipticals and spirals: visible disk but no well-drawn arms, little gas.

Irregular galaxies (Irr). No clear structure, often young, rich in gas and star formation. The two Magellanic Clouds are irregulars.

Typical masses span 10⁷ M☉ (dwarfs) to over 10¹³ M☉ (cD giants at the hearts of clusters), diameters from 1 kpc to over 100 kpc. The Milky Way: ~10¹² M☉, ~30 kpc across.

Famous examples

The Milky Way. Our host galaxy. Barred spiral, ~200 to 400 billion stars, disk ~30 kpc across, with Sagittarius A* (4.3 × 10⁶ M☉) at its center. The Sun orbits 8.2 kpc from the center every ~230 million years.

Andromeda (M31). Our giant spiral neighbor, 780 kpc away (2.5 million ly). Visible to the naked eye from a dark site, magnitude 3.4. More massive than the Milky Way (~1.5 × 10¹² M☉). Scheduled to merge with the Milky Way in ~4.5 billion years.

Triangulum (M33). Third large spiral of the Local Group, 860 kpc away. Smaller, more open.

Magellanic Clouds. Two dwarf satellite galaxies of the Milky Way, visible to the naked eye from the Southern Hemisphere. The Large Cloud hosts the Tarantula Nebula and hosted supernova SN 1987A.

M87 (NGC 4486). Giant elliptical in the Virgo Cluster, ~16.4 Mpc. Its central black hole (M87*, 6.5 × 10⁹ M☉) was the first black-hole shadow ever photographed directly, on April 10, 2019 by the Event Horizon Telescope.

M104 (Sombrero), the Antennae (NGC 4038/4039 in ongoing collision), the Cartwheel: all iconic targets seen by Hubble and then JWST.

How do we observe them?

With the naked eye, only three galaxies are normally visible from Earth: M31 (Andromeda) and the two Magellanic Clouds. Everything else requires at least binoculars.

Amateur telescope. A 150 mm under a dark sky already reveals dozens of bright galaxies: M51 (the Whirlpool, in Canes Venatici), M81 and M82 (Ursa Major), M87, M104 (the Sombrero), the Leo Triplet (M65/M66/NGC 3628). Beyond 200 mm, you enter the NGC/IC catalogue world and tens of thousands of galaxies accessible in long-exposure imaging.

Professional telescopes. Hubble (since 1990) revolutionized the study of distant galaxies: Hubble Deep Field (1995), Ultra Deep Field (2004), eXtreme Deep Field (2012). JWST (since 2022) pushes further in the infrared and has already detected candidate galaxies at z > 13, less than 350 million years after the Big Bang.

Large surveys. SDSS mapped several million galaxies; DESI is producing tens of millions of spectra; Euclid (ESA, 2023-) is mapping 1.5 billion galaxies to constrain dark matter and dark energy.

To spot galaxies visible from your location and plan a session, use our sky map tool.

Not to be confused with

Star cluster. A globular cluster (like M13 in Hercules) contains a few hundred thousand stars, but it belongs to a galaxy — usually ours. An open cluster (Pleiades, Hyades) contains a few hundred. Star clusters have no supermassive black hole and no significant dark matter halo, and are a thousand times smaller in diameter than a galaxy.

Cluster of galaxies. A higher-order hierarchical structure: the cluster contains tens to thousands of galaxies bound by gravity. The Virgo Cluster (~1,300 galaxies) is our nearest neighbor, Coma the most studied.

Nebula. Historically, galaxies were called 'spiral nebulae'. That confusion was resolved in 1925 by Hubble. Today, 'nebula' specifically refers to a cloud of gas and dust INSIDE a galaxy (Orion, Eagle, Cat's Eye…), whereas a galaxy is the entire structure.

Quasar. A quasar is an extremely luminous active galactic nucleus, usually seen at great distance. It is therefore A PHENOMENON occurring at the core of certain galaxies, not a parallel category. Many quasars are in fact spiral or elliptical galaxies whose ultrabright core is all we can see.