Life in the Galactic Suburbs (and what a pigeon can teach an astronomer)

“So can we see the earth from here?”

We're still too far away to see the earth, but we're heading toward that yellow star straight ahead. That's the sun.

"It doesn't look very special, just a normal looking star."

That's because it isn't really special, other than being home to us humans. It's just a nice, ordinary, stable star. Just the sort you’d want to live near. Not too big and not too small. Not a bad place to live around.

“Good neighbourhood?”

Not really, a typical quiet galactic suburb only thirty thousand light years from the centre of the galaxy.

“It's not as impressive as the star we came from.”

No, it's tiny compared to Deneb, but remember that big stars burn their nuclear fuel much faster. If the sun wasn't so average star we wouldn't be here talking about.

“How come?”

The sun's been burning for over four billion years and it took one and a half billion years for the earth to cool down and primitive life to start. Deneb will have blown up before life could possibly start. Even though the sun's not huge it's still brighter than most of the stars in this neighbourhood. Here is a map of this part of the galaxy from www.atlasoftheuniverse.com. We're just over 12 light years away now and this shows all the stars that are around the sun. If we humans ever get to travel into the galaxy these are the first stars they will reach.

“So which is the closest star?”

That little brown one over there, called Proxima Centauri. That's just over four light years from the sun but a rather dull little star. The one just near it called Alpha Centauri is almost a twin of the sun, it is exactly the same type of star.

“They still all just looks like dots of lights. When will the sun stop looking like star and start looking like the sun?"

Well, if you only had a pair of human eyes then you'd need to be within 5 billion miles.

"That's nothing. It’s...it’s...less than a thousandth of a light year."

Very good, just under 8 light hours. That's why from Earth all the other stars look like points of light, they are simply too far away. See those two stars up ahead.

“The two little dim ones, I see them.”

Those two stars are a binary system, so they are going around each other. A lot of stars form that way. That system is called Cygnus 61 as it was originally thought of as one star which is the 61st brightest star in our constellation Cygnus.

“How can they be in our constellation if we have been travelling through three thousand light years of space to get here?”

The constellations are not real groups of stars but just appear close when we look into space from earth. Because all stars look like dots of light, when we look at them we can't tell how far they really are away. So our constellation Cyngus is made up stars that are scatted all the way from here back to where we started.

“Is that the same for all constellations?”

Nearly all of them. One of the most recognisable constellations Orion, the hunter, is spread out over a huge distance too. This picture show what the constellation looks like from earth and how the stars are really spread out in space.

“If all the stars look the same no matter how far they are away, how can you measure their real distance?”

Scientists have a few tricks for that but one of the best is called parallax. Move your head from side to side. What happens?

“Not much.”

Don't things close up seem to move?

“Yes but that's because I'm moving my head.”

I know, but things close up seem to move more than things far away don't they?

“Yes, I suppose they do.”

Well the same thing can work for stars but rather than move your head you have to look from two different places millions of miles apart.

“Oh, that sound's very simple.”

Of course it is. You just have to wait 6 months for the earth to go half way around the sun and you will be looking from a position that over 160 million miles different. The most distant stars, like things on the horizon when you shake your head, will appear to stay still, but nearer stars will seem to move position over six months. From measuring that movement, which is called parallax, scientists can measure the distance of nearer stars very accurately. It's not just scientists that use parallax to judge distance, pigeons do too. That's one of the reasons they nod their heads.

“Wow, Albert. That's a bit random even for you. Explain that one to me.”

Well a pigeon has an eye on each side of its head which makes judging depth difficult. So by bobbing their heads as they walk they can judge how far things are away from them with parallax.

“I suppose I did ask.”

The Praying Mantis, a large insect does the same thing.....but perhaps we should come back to that some other time.

“I think so Albert.”