16 July 2009

Pinwheels, lenses and eggs: a gallery of galactic shapes

The universe’s galaxies are the nurseries of stars, and in one known case life, and come in an impressive range of shapes and sizes. But only recently has it become possible to simulate how that diversity came about.

Explore the images below to discover the different kinds of galaxy in the universe, and read our feature to learn how astronomers are starting to build their own virtual galaxies using complex simulations.

M104, the Sombrero galaxy. Spiral; type Sa


M31, Andromeda. Spiral; type Sb

Andromeda is the nearest large galaxy to the Milky Way, and the two are the biggest members of our "Local Group" of nearby galaxies. It is classified as an "Sb" galaxy because its arms are not wound too tightly around it.

A trillion stars light up the galaxy, which lies about 2.5 million light years away. In recent years, astronomers have found tentative evidence for the first exoplanet ever found beyond the Milky Way inside Andromeda. (Image: Rob Gendler)

M51, the Whirlpool galaxy. Spiral; type Sc

M51 is classified as an Sc galaxy because it has a relatively small central bulge and loose, well-defined arms. Clusters of young stars dot the arms, which swirl around a central core of older stars.

This galaxy is also interacting with a satellite galaxy, seen here at the end of one of its spiral arms. Eventually, the two will merge. Both galaxies lie about 35 million light years away.

(Image: NASA/ESA/S Beckwith/Hubble Heritage Team/STScI/AURA)


NGC 6050 and IC 1179. Interacting spiral galaxies

Two spiral galaxies, NGC 6050 and IC 1179, are colliding inside the Hercules galaxy cluster some 450 million light years away. The cluster is part of the "Great Wall" of galaxy clusters and superclusters, the largest known structure in the universe. (Image: NASA/ESA/Hubble Heritage Team/STScI/AURA/Hubble Collaboration/K. Noll)

M64. Spiral, post merger

M64 is known as the "Black Eye" galaxy because of the dark band of dust that absorbs light from its bright nucleus. While it looks like an ordinary spiral galaxy, observations in the 1990s showed that gas in its outer regions is rotating in the opposite direction to gas and stars close to its centre. That suggests M64 ate a small neighbour more than a billion years ago. The gas in M64's outer regions may have come from that small galaxy. (Image: NASA/Hubble Heritage Team/AURA/STScI)

NGC 5866. Lenticular galaxy; type S0

Lenticular galaxies boast a disc, like spiral galaxies, but they also share properties with elliptical galaxies, since they are usually low in dust and gas and don't tend to have obvious spiral arms. They are named for their lens-like shapes when seen along their edges.

Theorists are divided over how lenticular galaxies lost their gas in the first place – some think they used it up to form stars, while others believe it was stripped away as the galaxies passed through hot gas in galaxy clusters.

Some lenticular galaxies have retained a little gas, though. The gas in NGC 5866 is thought to be coupled to a thin lane of dust, seen as a long, dark bar. In 2006, Hubble spotted wisps of dust emanating from both sides of this dust lane. The wisps may have been created by young massive stars, which stir up the gas and dust around them. The observation suggests that the galaxy continues to form some stars, although not enough to form spiral arms. (Image: NASA/ESA/Hubble Heritage Team/STScI/AURA)

NGC 2787. Barred lenticular galaxy; type SB0

Some lenticular galaxies sport bars through their centres, like some spiral galaxies. Astronomers are studying the centres of these galaxies to find clues about how the galaxies formed, and whether galaxy collisions and colossal black holes were involved in the process. NGC 2787, which harbours a subtle bar at its centre, lies about 25 million light years away. (Image: HST/STScI/AURA)

M87. Elliptical galaxy; type E0

Unlike spiral galaxies, which have flat discs and tend to be relatively rich in interstellar gas and dust, elliptical galaxies are blob-shaped and usually have little gas and dust. They are classified with a number that describes their appearance – E0 galaxies are nearly perfectly circular, while E7 galaxies are quite stretched out.

Some are very small, such as the dwarf galaxies that orbit the Milky Way, while others, like M87, are gigantic. M87 lies at the heart of the Virgo cluster of galaxies about 60 million light years away and is spewing out a jet of matter (not visible in this image) from around the supermassive black hole at the galaxy's heart. (Image: Canada-France-Hawaii Telescope/J.-C. Cuillandre/Coelum)

NGC 1316. Elliptical, post merger

This unusual jumble of stars, gas and dust is thought to result from galaxies that have collided with each other in the past few billion years. After such collisions, many star clusters are destroyed in the dense galactic centre. The dark clumps and lanes of dust in this galaxy suggest that at least one of the devoured galaxies was a spiral galaxy. NGC 1316 lies about 75 million light years away. (Image: P. Goudfrooij/STScI/Hubble Heritage Team/AURA/ESA/NASA)

NGC 1427A. Irregular galaxy

Most galaxies are either spirals or ellipticals, but about 3 per cent are very different. These "irregular galaxies" share few common properties, but many get their unusual shapes from gravitational interactions with other galaxies.

NGC 1427A (large bluish smudge at centre) shows how gravity can deform a galaxy's shape. The arrow-shaped galaxy is being pulled towards the centre of a cluster of galaxies known as Fornax (upwards in this image) at 600 kilometres per second. Its movement through the cluster's gas compresses its own gas clouds, triggering star birth. But it will not survive the passage through the cluster – within a billion years, its shape will be completely scrambled, and its stars and gas will be subsumed into the gas inside the Fornax cluster. (Image: NASA/ESA/Hubble Heritage Team/STScI/AURA)