Ask anyone to draw a galaxy and they will draw a spiral: a bright centre with curved arms winding out. It is one of the best-known shapes in science, and one of the most beautiful. It is also the shape of our own home. This post explains what a spiral galaxy is, and why artists return to it.
What a galaxy is
A galaxy is a great gathering of stars, gas and dust, held together by its own gravity. A large one holds hundreds of billions of stars. There are thought to be hundreds of billions of galaxies, perhaps far more, in the part of the universe we can observe.
Until about a hundred years ago, nobody knew that. The fuzzy spirals seen through telescopes were called spiral nebulae, and most astronomers thought they were clouds inside our own system of stars. In 1923 and 1924, Edwin Hubble measured the distance to the one in Andromeda and found that it lay far outside the Milky Way. It was a separate system of stars, as large as our own. The universe became very much bigger overnight.
The kinds of galaxy
Hubble sorted galaxies by shape, and his scheme is still used.
- Spiral: a flat, rotating disc with arms.
- Barred spiral: a spiral with a straight bar of stars across the centre. The arms start from the ends of the bar. The Milky Way is one of these.
- Elliptical: a smooth ball or oval of mostly old stars, with little gas and no arms.
- Lenticular: a disc without clear arms, between the two.
- Irregular: no regular shape.
Most of the large, bright galaxies near us are spirals.
The parts of a spiral galaxy
- The bulge. A dense, rounded mass of older stars at the centre. It looks yellowish.
- The central black hole. Nearly every large galaxy has a supermassive black hole at its heart. Ours is about four million times the mass of the Sun. See black holes and wormholes.
- The disc. A thin, flat plate of stars, gas and dust, turning round the centre. It is very wide and very thin, in roughly the proportions of two fried eggs back to back.
- The arms. Bright lanes that wind out through the disc.
- Dust lanes. Dark streaks along the arms, where dust blocks the light of the stars behind.
- The halo. A large, faint sphere round the whole galaxy, with scattered old stars and ball-shaped clusters of stars. Most of a galaxy's mass is here, in the form of dark matter, which can't be seen and is known only by its gravity.
Why there are arms
You might suppose the arms are fixed streams of stars, like the arms of a pinwheel. They can't be. The inner parts of the disc go round faster than the outer parts. Fixed arms would wind themselves up tight in a few turns and vanish.
The accepted explanation is that the arms are waves. They are regions where stars and gas are crowded a little closer together, and the pattern moves round the disc at its own speed. Stars pass into an arm, slow down, and pass out again. The usual comparison is a traffic jam on a motorway. The jam stays in one place, or creeps along, while individual cars drive into it and out of it.
When gas enters an arm, it is squeezed. That sets off the birth of new stars. The largest and hottest of those are blue, very bright and short-lived. They burn out before they have travelled far from the arm. So the arms are marked out in blue light, with pink patches of glowing hydrogen where stars are still forming. See what a nebula actually is.
Our own spiral
The Milky Way is a barred spiral about 100,000 light years across. The Sun lies about 26,000 light years from the centre, in a minor arm, and takes roughly 230 million years to go round once.
We live inside the disc, so we see it edge on, from within. That is the pale band across the night sky: the combined light of billions of stars in the disc, with dark rifts of dust. Nobody has ever seen the Milky Way from outside. Every picture of it as a spiral is a reconstruction, built from measurements.
From most towns and cities, the band can no longer be seen, because of artificial light. Our piece Space Cityscape on the Shore imagines it back above a city.
Andromeda
The nearest large spiral is the Andromeda galaxy, about two and a half million light years away. It is the most distant thing that can be seen without a telescope, as a faint oval smudge on a dark autumn night in the northern hemisphere. It is tilted towards us, so we see it as a long oval, with small companion galaxies beside it.
Andromeda and the Milky Way are moving towards each other. In about four billion years they are expected to meet and, over a long time, merge. Stars are so far apart that almost none will collide.
What colour is a galaxy?
Through a telescope, by eye, a galaxy is a soft grey-white glow. The eye can't see colour in light that faint. Long-exposure photographs show the true colours: a yellow-white core, blue arms, pink clouds, brown dust.
Many published images go further and use false colour. Telescopes record kinds of light we can't see, such as infrared, ultraviolet and X-rays, and astronomers assign each a visible colour. An image may be green, orange and purple, and be a perfectly accurate map of real data.
Why the spiral works in a picture
- It pulls the eye in. A spiral leads to its centre. You can't look at one without following it.
- It suggests motion in a still image. The curve reads as turning.
- It is familiar. The same shape appears in shells, storms, whirlpools, ferns and the seeds of a sunflower. It appears in the oldest carvings known. People respond to it.
- It is simple. One shape, a bright centre, a dark ground. It reads from across a room.
- It can be shown from any angle: face on as a pinwheel, tilted as an oval, edge on as a line.
Vincent van Gogh's The Starry Night, of 1889, has a great swirl at its centre. Some historians think it may owe something to a famous drawing of a spiral galaxy made through a giant telescope in Ireland in 1845, which had been widely published.
Spirals at Kiyo Arts
- Vaporwave Green and Blue Spiral Galaxy. A tilted spiral in neon green and electric blue, with a small companion beside it. The angle is that of the classic view of Andromeda. The colour is false colour, in the manner of a scientific image.
- Purple Pink Wormhole. A spiral seen face on, in purple and pink, turning into a dark centre.
- Black Hole. The centre of the story, in silver and white.
- Light Nebula. The kind of glowing cloud found along a spiral arm.
Hanging a spiral
- Give it space. A spiral draws attention to its centre. It works best alone, or as the middle of a group.
- A dark wall lets the dark background of the picture disappear. See wall art for dark walls.
- A tilted spiral has a direction. Hang it so that the sweep leads into the room.
- Light it. Space pictures are dark and need a lamp.
For more on space in art, see cosmic and celestial canvas art, the moon in art and northern lights art. Browse the canvas art collection.