What's a parsec?

18 June 2019

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Question

Big things are measured in parsecs, what is a parsec?

Answer

Carolin: Well yes that's infamously wrong because the parsec is a measure of length and there Han Solo was using it as a measure of time. So that was a big faux pas but it's a measure of length, it's three point two six light years or about 31 trillion kilometres and it comes from the way we measure distances to stars. So it's using the method triangulation or parallax, where you look at the way a relatively nearby star is displaced relative to the background when you observe from one side of Earth's orbit and then you know six months and 300 million kilometres later the other side of Earth's orbit. And if that shift against the more distant stars is one arc second an anglicised one arc second then that star is one parsec away. And just to say how small one arc second is, it’s a measure of angular size, that is the size that a pound coin would subtend at a distance of four kilometres. So it's a really tiny amount. So parsecs measure distances to stars but they start to get unwieldy before too long. If you want to think about distances in the galaxy we talk about kilo-parsecs, so thousands of parsecs. So like the center of the Milky Way is eight kilo-parsecs away from us, Andromeda galaxy, our nearest large galaxy, is 800 kilo-parsecs away, but after that, actually we use mega-parsecs, so millions of parsecs. So m87, which was observed by the event horizon telescope, the image of the event horizon around it, that is sixteen mega-parsecs away. So it's not a particularly useful unit of distance given that we have to go into mega and kilo quite soon, but it's more as, I say, more historical because of that original way of measuring distances to nearby stars.

Chris: If all of the bodies in the universe are in motion because our star is moving around the center of our galaxy, our galaxy is moving around as part of a big cluster of other galaxies, how do we kind of keep track of where everything is? Because presumably it's all changing over time and it's all changing all the time?

Carolin: Well now if you think about modern satellites like the gaia satellite who do all this meticulous measurement and motions they actually use a reference frame which is way external to our galaxy. They'll use that the grid of very distant quasars which are so far away we're not seeing their motion relative to us so that is a fixed grid that is external to our galaxy.

Chris: By referencing that,  even though things locally might move around a bit, we've got those fixed points to rely on?

Carolin: That's right. That's the nearest we can get to fix points. And you know now we can measure things to within about 10 micro-arc seconds or that's about the equivalent of your pound coin, not a distance of four kilometres, but on the surface of the moon. So gaia satellite is refining all these distances to fantastic accuracy.

Chris: I think Gerry Gilmore said to me it's got a giga-pixel camera on it or more isn’t it. I mean that's some camera; it's eye watering detail that they're getting isn't it?

Carolin: And the amount of data that they get is... just handling that is a huge challenge.

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