Types of Galaxies, and the One That Is Coming for Us

For most of history, the Milky Way was assumed to be the whole universe. Faint elongated smudges catalogued as "spiral nebulae" were taken to be gas clouds within it.

The question was settled in 1924, when Edwin Hubble identified Cepheid variable stars in the Andromeda "nebula". Cepheids pulse with a period that depends on their intrinsic brightness — a relationship discovered by Henrietta Swan Leavitt — so measuring the period gives the true luminosity, and comparing that with observed brightness gives the distance. Andromeda came out far beyond any plausible boundary of the Milky Way.

It was not a nebula. It was another galaxy, and the universe abruptly became much larger than anyone had assumed.

The Hubble sequence

Hubble sorted galaxies by appearance into a scheme still in use, usually drawn as a tuning fork.

Elliptical (E0–E7)

Smooth, featureless ellipsoids ranging from nearly spherical (E0) to markedly flattened (E7). Little gas or dust, minimal ongoing star formation, and a population dominated by old red stars.

Ellipticals include both the largest galaxies known — supergiant ellipticals at the centres of galaxy clusters, containing trillions of stars — and vast numbers of dwarf ellipticals. Stellar orbits within them are randomly oriented rather than organised into a disk, giving them a three-dimensional swarm structure rather than a rotating plane.

Spiral (Sa–Sc) and barred spiral (SBa–SBc)

A flattened rotating disk with spiral arms, a central bulge, and a surrounding halo of old stars and globular clusters. Gas-rich and actively forming stars, with the arms marking regions where star formation is concentrated.

The subclass letter describes how tightly the arms wind and how large the bulge is, from Sa (tight arms, large bulge) to Sc (loose arms, small bulge). Barred spirals have a straight bar of stars crossing the centre, with the arms trailing from its ends.

The Milky Way is a barred spiral, classified around SBbc. Establishing this took a long time for the obvious reason — we are inside it, viewing the disk edge-on through obscuring dust.

A point often misunderstood: spiral arms are not solid structures. They are density waves — regions of compression moving through the disk, like a traffic jam that persists while individual cars enter and leave it. Stars and gas pass through the arms rather than being permanently attached to them. Compression triggers star formation as gas is squeezed, and the bright, short-lived massive stars that result die before leaving the arm, which is why the arms look bright and blue while the disk between them does not.

Lenticular (S0)

Sitting at the junction of the fork: a disk and central bulge like a spiral, but with no spiral arms and little gas. These appear to be spirals that have exhausted or lost their gas, possibly through interaction with a cluster environment.

Irregular (Irr)

No organised structure. Often gas-rich and actively star-forming. Frequently the result of gravitational disruption by a larger neighbour — the Large and Small Magellanic Clouds, satellites of the Milky Way, are the nearest examples.

Where Hubble was wrong

Hubble read his sequence as an evolutionary path, with galaxies beginning as ellipticals and developing into spirals. He called ellipticals "early type" and spirals "late type", and that terminology survives despite being backwards.

The modern picture largely inverts it. Galaxies build up through mergers, and major mergers of spirals tend to produce ellipticals. The collision scrambles the ordered rotation of both disks into randomised orbits, and it triggers an intense burst of star formation that consumes the available gas, leaving a red, gas-poor, structureless remnant.

So the sequence is useful as a description and misleading as a history. This is worth knowing because "early-type" and "late-type" appear constantly in the literature and mean nearly the opposite of what they sound like.

The collision that is already scheduled

Almost every galaxy is receding from us, redshifted by cosmic expansion. Andromeda is blueshifted. It is approaching at roughly 110 kilometres per second, because within the Local Group, local gravity comfortably overwhelms cosmic expansion.

Current estimates put the first close encounter around 4 to 4.5 billion years from now. The two galaxies will then interact over a further few billion years, pulling out long tidal tails of stars and gas, before merging into a single large galaxy — most likely an elliptical, which some astronomers have already nicknamed "Milkomeda".

Recent analyses using Gaia and Hubble data on Andromeda's sideways motion have introduced real uncertainty about whether the first pass is a direct hit or a glancing encounter, and some studies now put the probability of a merger within the next ten billion years at closer to even odds than the near-certainty once assumed. The measurement is difficult and the conclusion is not yet stable.

Why almost nothing will collide

The word "collision" gives the wrong impression. Galaxies are overwhelmingly empty.

In our region of the Milky Way, the average separation between stars is around 5 light-years, while a star is on the order of a million kilometres across. The ratio is such that scaling a star to a marble would place the next marble roughly a thousand kilometres away.

When two galaxies pass through each other, the probability of any two stars physically colliding is essentially zero. The stars simply pass by, their orbits gravitationally rearranged.

What does collide is the gas. Interstellar gas clouds are vast and diffuse, they cannot pass through one another, and they shock and compress — triggering an enormous burst of star formation. Merging galaxies frequently light up as starburst systems for this reason.

What the sky would look like

From a hypothetical Earth, Andromeda would grow from its present faint smudge into a structure spanning a large fraction of the sky, gradually distorting over hundreds of millions of years as the two disks tear at each other.

But the Sun will be roughly 4.5 billion years older by then and approaching the end of its main-sequence life. Its gradual brightening will have rendered Earth uninhabitable long before the merger — likely within one to two billion years, well ahead of the collision.

The solar system itself would very probably survive intact, though it might be flung into a wildly different orbit, possibly into the outer halo of the merged galaxy or ejected entirely.

Finding Andromeda tonight

At 2.5 million light-years, the Andromeda Galaxy is the most distant object visible to the unaided eye.

The light reaching you left before the genus Homo existed on Earth. It is worth going to find it for that reason alone.

Under a genuinely dark sky it appears as a faint elongated smudge — do not expect the photographs, which are long exposures. Binoculars show it clearly and are arguably the best instrument for it, since it spans several times the width of the full Moon and a telescope's narrow field shows only the bright core.

Look in autumn, from the northern hemisphere or the northern tropics. Find the Great Square of Pegasus, follow the chain of stars leading northeast from its corner, and Andromeda sits a short distance off that chain. A star chart or a phone app makes the first attempt considerably easier.