What's the closest star [to Earth] other than the Sun, and how far away is it?

Question posed by Emi. And she wants it in perspective... I'll see what I can do!

The closest star to Earth is, of course, the Sun. I know that's not what Emi's asking, but for the 'put in perspective' bit, it helps to start out smaller, I think.

How far away is the Sun?

So, we, on Earth, orbit the Sun at an average distance of about 150 million kilometres. Even such small distances (in astronomical terms) are difficult to get some perspective on, so we'll try to get a handle on it one step at a time.

Try this for size: if you wanted to travel 150 million kilometres without leaving Earth you'd have to travel around the world almost four thousand times. That's a heck of a holiday.

Or what about this one: light reflected from the Moon takes a bit more than a second to reach your eye. Light from the Sun takes a bit longer- about eight minutes (never, ever look directly at the sun. Ever), so we could say that the Sun is eight 'light minutes' away from us.

Astronomers use units in terms of light (usually light years) to talk about the massive distances between stars because it helps to make the numbers smaller (because light is just so fast), and to put things in some kind of perspective, so I'll use the same idea to try to get perspective across in this post. If you're not sure what a light year actually is*, I suggest you read this post first to get a handle on it.

How far away is Pluto?

Proxima Centauri, taken by the Chandra X-Ray observatory

Pluto's not a planet any more, but it's the famous object that is furthest away from us yet still in our solar system**. It's about 40 AU away, which is nearly six billion kilometres***. That doesn't mean much in my mind, so let's think about it in terms of light again: it's about 5 and a half light hours away, which means that light from the Sun takes about 5.5 hours to get to Pluto.

What's the next nearest star, and how far away is it?

The nearest star system to our own is called Alpha Centauri and is actually a binary star system. This means that it is made up of two stars in orbit around each other. These are called, imaginatively, Alpha Centauri A and Alpha Centauri B. Which one's closest to us? Neither. There's a third, much smaller star in the system**** called Proxima Centauri.

Proxima is currently closer to us than either A or B and is therefore the closest star to us that isn't the Sun. How far away is it? Around 39,000,000,000,000 (thirty-nine trillion) kilometres. That's about 4.22 light years, so light from there takes something like four years, two and a half months to reach us.

That sounds like a long way, but you don't have to go much further in astronomy to realise that, in the grand scheme of things, it really is nothing at all. The image below is an artist's impression of the Milky Way (our home galaxy) viewed from above, with the position of our Sun marked on it. The Alpha Centauri system isn't marked on because, at this scale, it's pretty much in the same place.

[REPLACE IMAGE]

[UPDATE FOOTNOTES]

1. * It's not a year with 15% fewer calories.
2. ** Actually, that's probably the Voyager 1 probe, but I think Pluto's more famous.
3. *** That's actually the distance from the Sun, but it's a good average to use- Earth and Pluto are constantly moving so are different distances from each other at different times, and Earth is so close to the Sun in comparison it doesn't make a lot of difference for our purposes.
4. **** Actually, not all astronomers are agreed that Proxima is actually a part of the Alpha Centauri system- some believe that it's just passing through.