Spiral Galaxies
A spiral galaxy is a flat, disc-shaped galaxy with long curving arms wrapping around a bright central bulge. Spiral galaxies are the most spectacular looking type of galaxy, and they are also where most active star formation happens. Around two thirds of all the bigger galaxies in the universe are spirals, including our own Milky Way and our closest neighbour, the Andromeda Galaxy.
- ShapeFlat disc + bulge + armsWith or without a central bar
- % of galaxiesapprox. 66%The most common type of big galaxy
- Typical size20,000 to 200,000 light yearsAcross
- Number of arms2 to 6Most have 2 main arms
- Our galaxyMilky WayA barred spiral with 4 main arms
- Most famous exampleThe Whirlpool (M51)23 million light years away, in Canes Venatici
The parts of a spiral galaxy
A typical spiral galaxy has three main parts.
- A bright bulge in the centre, full of older yellow and red stars, and almost always with a supermassive black hole at its core.
- A flat disc of stars, gas and dust spinning around the bulge. The disc is where the famous spiral arms live and where new stars are being born.
- A faint halo of much older stars and dark matter that surrounds the whole galaxy in a spherical cloud.
Most spirals also have a straight bar of stars across the centre of the bulge, with the spiral arms wrapping out from the ends of the bar. These are called barred spiral galaxies. Spirals without a bar are called ordinary or unbarred. Our Milky Way is a barred spiral.
How do spiral arms form?
For many years astronomers were puzzled by spiral arms. If the arms were really made of fixed groups of stars, they should wind up tighter and tighter as the galaxy rotated, then eventually disappear. But spiral galaxies keep their arms for billions of years.
The current best explanation is the density wave theory. The arms are not solid structures of stars; they are travelling waves of high density moving through the disc. Stars and gas slow down slightly as they pass through an arm, like cars slowing down through a traffic jam, before continuing on the other side. The wave itself looks stable and keeps its spiral pattern for a very long time, even though the actual stars passing through it are always changing.
Why most stars in the universe are born in spiral arms
The squeezing inside a spiral arm is what triggers most star formation. As gas slows down in the high-density region, it gets compressed enough for gravity to take over, and pockets of gas collapse into bright new stellar nurseries. That is why spiral arms are dotted with brilliant young blue stars and pink nebulae like the ones in the famous Pillars of Creation image. The bulge and the outer halo of a spiral are full of older red stars, but the arms are always young and bright.
Famous spiral galaxies
- The Milky Way: our own home, a barred spiral about 100,000 light years across with 4 main arms.
- Andromeda (M31): our nearest big neighbour, 2.5 million light years away. Slightly bigger than the Milky Way.
- The Whirlpool Galaxy (M51): a stunning face-on spiral 23 million light years away. Currently devouring a smaller companion galaxy that is pulling its outer spiral arm out of shape.
- The Pinwheel (M101): a giant face-on spiral 21 million light years away, with arms full of star-forming regions.
- NGC 1300: one of the best-known examples of a barred spiral, 70 million light years away in the constellation Eridanus.
How fast does a spiral galaxy spin?
Spiral galaxies are not solid like a CD or a Frisbee; different parts of them rotate at different speeds. The Sun, sitting in the disc of the Milky Way about 27,000 light years from the centre, is moving around the galactic centre at approximately 220 km/s. Even at that pace, one full orbit around the galaxy takes the Sun about 225 million years. Since the Sun was born, it has made only around 20 trips around the galaxy.
Surprisingly, stars on the outer edge of a spiral galaxy do not slow down nearly as much as they should. This is one of the main pieces of evidence for dark matter: an invisible halo of extra mass surrounds the galaxy and keeps the outer stars whizzing along faster than visible matter alone could explain.
Deeper dive: dark matter and spiral galaxy rotation
One of the deepest mysteries in modern astronomy was uncovered in the 1970s by the American astronomer Vera Rubin. Rubin carefully measured how fast stars rotate around the centre of nearby spiral galaxies, including Andromeda. She expected to see the same pattern as in our solar system: things further from the centre orbit slower, because the gravity that holds them in falls off with distance.
What Rubin actually found was completely different. The outermost stars in a spiral galaxy are travelling almost as fast as the ones near the middle. The rotation curve goes flat instead of dropping off. The only way this can work, given the laws of gravity, is if there is a huge amount of extra mass spread out in a giant invisible halo around the galaxy. That extra mass is what astronomers call dark matter.
Many decades of follow-up work in dozens of galaxies, plus studies of how galaxy clusters bend light and how the cosmic microwave background looks, have all backed up Rubin's discovery. Dark matter appears to make up roughly 27% of the entire universe, compared with just 5% for the ordinary matter we can see. Scientists still do not know what dark matter is made of, but spiral galaxies are one of the main reasons we are sure it exists.
For our own home spiral, see the Milky Way. For our nearest big spiral, see the Andromeda Galaxy. For the smooth alternative, see elliptical galaxies.