- How big is a galaxy?
- The Milky Way's disc is about 100,000 light years across and holds a few hundred billion stars. Dwarf galaxies are a thousand times smaller; the largest ellipticals are ten times wider and hold a trillion.
- Where is the Sun in ours?
- About 26,000 light years from the centre, in a minor spiral arm, roughly halfway out. One trip around takes something like 230 million years, so the Sun has made about twenty of them since the Earth formed.
- Are they all moving apart?
- Every distant galaxy is receding, and the further away it is the faster it goes, because the space between galaxies is expanding. Inside a gravitationally bound group it is not: Andromeda is coming towards us at about 110 km/s.
- What is a quasar?
- A galaxy whose central black hole is feeding hard enough that the infalling matter outshines every star in the galaxy put together. They were common in the early universe and are rare now, which is one of the reasons the supermassive holes are thought to have grown early.
- How far away is the furthest one seen?
- Light from the most distant galaxies confirmed so far left them when the universe was a few hundred million years old, around 13.5 billion years ago. Seeing further means seeing earlier, which is why a large telescope is also a way of looking back in time.
- How many galaxies are there?
- Hundreds of billions within reach of current telescopes, and the true figure depends on how faint a dwarf still counts. Estimates in the trillions come from extrapolating to galaxies too faint to have been seen, which makes them a model result rather than a count. Either way the number is beyond intuition, and every one of them holds billions of stars.
- Will the Milky Way and Andromeda collide?
- Probably, though it is no longer the certainty it was long presented as. Andromeda is approaching at about 110 km/s, but whether the encounter a few billion years from now is a hit or a miss depends on its sideways motion, and the latest analysis — folding in the pull of the other Local Group galaxies and the measurement errors — finds roughly even odds of a merger within ten billion years. If they do merge, the two settle into a single elliptical over a couple of billion years. Almost no stars would hit each other — the gaps between them are enormous — but both discs would be destroyed as structures.
- Can I see another galaxy without a telescope?
- Yes. The Andromeda Galaxy is visible from a dark site as a faint elongated smudge, and at 2.5 million light-years it is the most distant thing most people ever see unaided. The Magellanic Clouds are obvious from the southern hemisphere. Everything else needs optics.
- Why are some galaxy distances far less certain than others?
- Because a redshift can be measured or fitted. A spectroscopic redshift reads the lines of the spectrum where they actually are and is good to a few per cent. A photometric redshift is inferred from the galaxy’s colours through a handful of filters matched against templates, and carries a standard deviation of about a fifth of itself. A wide error bar beside a distance usually means a fitted redshift rather than a more distant galaxy.
- Is the Milky Way a typical galaxy?
- It is an ordinary large spiral — unremarkable in type, but larger than most. The great majority of galaxies are dwarfs far smaller than ours, so a random galaxy is much more likely to be a faint dwarf than anything resembling the Milky Way.