L27. Galaxies Beyond the Milky Way

Unit 6 · Galaxies and the Universe

R-report

L27. Galaxies Beyond the Milky Way

A century ago, most astronomers believed the Milky Way was the entire universe. Then one measurement showed that a faint smudge in Andromeda was not a cloud inside our galaxy — it was another galaxy, unimaginably far away.

The universe got much bigger in 1924

For a long time, astronomers argued about faint spiral-shaped smudges called nebulae. Were they gas clouds within the Milky Way, or separate galaxies far outside it? Edwin Hubble settled it. Using the largest telescope of the era, he identified a particular type of pulsating star in the Andromeda nebula. These stars pulse at a rate directly linked to their true brightness, so comparing true brightness with apparent brightness gives a distance. The answer was far beyond any plausible edge of the Milky Way. Andromeda was a separate galaxy, and the known universe expanded overnight from one galaxy to countless galaxies. Current estimates put the number of galaxies in the observable universe at roughly two trillion. Our galaxy went from being everything to being one ordinary member of an enormous population — a demotion that has happened repeatedly in astronomy, from Earth to the Sun to the Milky Way.

Shapes and what they mean

Galaxies fall into a few broad types, and the shape carries information about the galaxy's history. - Spiral galaxies: a flat rotating disc with arms, plenty of gas and dust, and active star formation. Young blue stars concentrate in the arms. - Barred spirals: spirals with a straight bar through the centre. The Milky Way is one, and so are most spirals. - Elliptical galaxies: smooth, rounded collections of mostly old stars with little gas left, so few new stars form. These include the largest galaxies known. - Irregular galaxies: no clear structure, often small, and frequently distorted by the gravity of a larger neighbour. Galaxies also change. Collisions and mergers are common, and when two spirals merge the result is often an elliptical. That is thought to be how many large ellipticals formed. Galaxy collisions rarely involve stars hitting each other, because the distances between stars are so vast. It is the gas clouds that collide, triggering bursts of star formation.

Galaxies cluster together

Galaxies are not scattered evenly. Gravity gathers them into groups and clusters, and those into larger structures still. The Milky Way belongs to the Local Group, about 80 galaxies spanning roughly 10 million light-years. It is dominated by two large spirals — the Milky Way and Andromeda — plus the smaller Triangulum galaxy and many dwarfs. Andromeda is about 2.5 million light-years away and, unusually, is moving toward us. In roughly 4 billion years the two galaxies will merge. Individual stars will almost certainly not collide; the two discs will be pulled apart and reassembled into a single larger galaxy. On larger scales, groups gather into clusters of thousands of galaxies, and clusters string together into filaments separated by enormous voids. Maps of this cosmic web show the largest structures known. All of this is held together by gravity, and once again the amount of mass required is far greater than the visible matter can supply — the same dark matter problem, at a much larger scale.

Putting the investigation together

Galaxies are vast gravity-bound collections of stars, gas, dust, and dark matter, and there are roughly two trillion of them in the observable universe. They come in spiral, barred spiral, elliptical, and irregular forms, and they cluster into groups, clusters, and a cosmic web of filaments.

The 1924 result is worth pausing on. A single careful distance measurement expanded the known universe by a staggering factor and reduced our galaxy to one ordinary example among many. Astronomy has repeatedly moved humanity away from the centre of things, and each move came from measurement rather than argument.