Q & A – Island Universes

Mini-Science logo At the conclusion of each Mini-Science lecture, audience members submit their questions to the evening’s presenter, who answers as many as possible on the spot. Three of the unanswered questions are sent to the presenter for posting here. In addition, a quiz is held each week based on material from the lecture. Here are questions and the quiz from Prof. Tracy Webb’s lecture ‘Island Universes: The Nature and Origin of Galaxies’ (April 15, 2009).

Q: Are all elliptical and spiral galaxies disk-like? And if so, why?

A: One of the differences between elliptical galaxies and spirals is that spirals are disk-like while ellipticals are in general roundish. We think that these different shapes indicate different formation or evolutionary processes. Disks are very common in the universe and result from the collapse of a spinning object. That is, if a large cloud of gas begins to collapse under its own gravity, if it has even the smallest amount of initial spin it will collapse into a disk because of conservation of angular momentum (the same physics that makes a figure skater speed up during a spin when she pulls in her arms). Solar systems also form in disks for example. It may be that all galaxies start out as disks, but galaxy-galaxy mergers destroy the disks and result in elliptical galaxies.

Q: If black holes are at the centre of galaxies why does the centre appear bright when looking at pictures [of galaxies]?

A: Black holes are by definition dark, because their gravitational fields are so strong that not even light can escape. When we look at the centers of galaxies we don’t see black holes, but the stuff which surrounds them, and this is what is so bright. Moreover, the size of the black hole is quite small (not its mass but its radius) so it would be very small on images of galaxies and completely dwarfed by the material around it. An active black hole which is accreting (pulling in) material will be surrounded by very hot matter which is spiralling in toward the black hole in a disk (see previous question). This hot material is very bright and this is what we see. Even galaxies with dormant black holes are bright in the center because the number of stars in the center regions of a galaxy is very high.

Q: What is it that makes a black hole dormant or active? Can dormant ones ‘wake up’?

A: A black hole is active if there is a lot of matter around it that it can pull in. This accretion of matter is an extremely energetic processes and emits a lot of energy in the form of light which can escape the gravitational pull of the black hole, even though the matter cannot (closer in to the black hole neither matter nor light can escape). We think that in the early universe there was a lot more gas around which had not yet been made into stars or pulled into a black hole, so there was a lot of raw material for this process. As time went on black holes cleared out the areas around them and ran out of fuel (the remaining gas was far enough away from the black hole that it was safe) and thus became dormant. Dormant black holes can wake up if matter is somehow transported to them again and this has been observed to happen during galaxy-galaxy mergers – the violence of the collision causes matter to fall into the centers of the merging galaxies and onto their black holes.


Q: Which galaxy is our own Milky Way on a collision course with, and when will they collide?

A: The Andromeda galaxy (2.5 million light years away) – in approximately 3-4 billion years.

Q: In the early universe (approximately 2 billion years after the Big Bang) we see some key differences in galaxies compared to today which we think are important for understanding how they form. What are these processes?

A: An increase in

  1. the average amount of stars being formed in galaxies;
  2. the number of galaxies with active black holes;
  3. the number of galaxies undergoing galaxy-galaxy mergers.

Q: What are 2 key differences between spiral and elliptical galaxies?


  1. Shape: spirals have disks and spiral arms, while ellipticals are round and football shaped.
  2. Composition: spirals contain lots of gas and dust are still making new stars from this material while ellipticals are old and ‘dead’ with lots of old stars but no material from which to form new stars.
  3. Mass: ellipticals tend to be more massive than spirals.
  4. Location: ellipticals are often found in galaxy clusters while spirals are often found in isolation or smaller groups.

If you have any related comments or questions, please feel free to post them. We cannot promise a reply to every question but will answer what we can.

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