SciBar at The Vat & Fiddle: Julain Onions on Galaxies - One Gigayear at a Time

Words: Gav Squires
Wednesday 06 June 2018
reading time: min, words

Julian Onions is back at SciBar to tell us all about galaxies, one gigayear at a time.


Galaxies are big - each one contains 100 billion stars in average and they operate on timescales that are almost unimaginable to the human mind. Rather than being measured in years, centuries or even millennia, the progress of galaxies is measured in gigayears, with each gigayear being equivalent to a billion years.

For a long time we didn’t know if there was anything outside of our galaxy. Since it is difficult to measure distances in astrology, we didn’t know if the Milky Way was the all that there was. We could see distant “fuzzy clusters” but couldn’t tell if they were inside the Milky Way or outside it. Then Edwin Hubble managed to calculate the brightness of one of the stars in these fuzzy clusters and he could show that they were too far away to be inside our galaxy. Even though later calculations showed him to be off by a factor of three, these things were still further away than the edge of the Milky Way. Since then, we’ve examined a good chunk of the local space and we’ve discovered that Earth is in a backwater cul-de-sac on the edge of a galactic arm where it takes us 250 million years to orbit the centre of our galaxy.

Scientists have categorised galaxies into four types - spiral (like our own Milky Way), elliptical, lenticular and “other” for those that don’t fit into the other three. Our nearest neighbouring galaxy is Andromeda, which is 2 million light years away but can be seen from Earth with the naked eye. Actually, it’s the furthest thing that you can see with the naked eye. We are also moving towards Andromeda at the speed of 150 kilometres per second and the two galaxies will actually collide in 4 billion years (or just 4 gigayears!) This is also roughly the same time that our sun will burn itself out. When they do come together, it’s expected that there will only be a couple of actual star collisions - the distances between individual stars in galaxies is just so vast.

Spiral galaxies have a bulge in the middle and very thin edges, like two fried eggs back to back. There is also a lot of blue in the spiral arms - this means that there are a lot of blue giant stars. The colour of stars shows how hot they are so our Sun is a yellow star and is around 6000oC while a red dwarf, as the name implies, is red. They are half the size of the sun and burn at 3500oC. The blue giants can be 30 times the size of our star and can be as hot as 30000oC. However, blue giants do not last very long, only around 10-15 million years while a red dwarf can go on for 150 billion years. The presence of a lot of blue giants in the spiral arms means that they must be forming all of the time and the Milky Way, for example, forms a new star every year, on average. Lenticular galaxies are basically spiral galaxies but without the arms. They are also very red, which implies that there isn’t much star formation happening. Elliptical galaxies have their stars moving around in a haphazard manner, the opposite of the well-ordered orbits in a spiral galaxy. There is also a much newer classification - active galaxies. These have an incredibly bright centre, far brighter than it should be.

We now know a lot about other galaxies - there are between 2 and 4 billion galaxies in the universe and the biggest one is the excitingly named IC1101. The furthest one away is 13.4 billion light years away (the edge of the universe is 13.8 billion years away) But there is always more to learn - the Gaia satellite has been up for four years, mapping the Milky Way. The James Webb space telescope has a mirror that measures 8 metres across (compared with just 2 metres for the Hubble space telescope) It has had its launch date put back to 2020 but if that massive mirror doesn’t unfold correctly, that’s $8billion down the drain. In South Africa and Australia there is the Square Kilometre Array being contracted as well as the Extremely Large Telescope in Chile. Who knows what the next wonders they will reveal…

SciBar returns to The Vat & Fiddle on the 27th of June at 7:30 where David Quain will talk about beer quality in his talk Few Things Are Finer Than A Pint Of Draught Beer

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