January 9 – Far, Far Away

Today’s Factismal: Alpha Centauri is 25,800,000,000,000 miles away.

Have you ever wondered how we know how far away the stars are? You aren’t alone. Indeed, until 1839, everyone more or less assumed that the answer was “really far”. The reason for that is that stars don’t display parallax and don’t occult; that is, the nearer stars don’t seem to move relative to those farther away and the stars don’t move in front of each other as the Earth moves in its orbit.

Perhaps the best example of parallax is the view outside of a car window. As the car moves down the street, the things nearer to you appear to zip right past while the things that are more distant change position only very slowly. That happens because the things that are close to you change the angle between your position and theirs very quickly and those that are far away change the angle only very slowly. And, as you ride in that car, you also see occultation happen. The nearby tree blocks your view of the far away lake for a moment, and then the tree has moved on and you can once more see the lake.

As you move, the closer things zoom by faster. That's parallax in action!

As you move, the closer things zoom by faster. That’s parallax in action!

Since ancient times, these effects have been seen in the Solar System. We can see the Moon move in front of just about everything else in the sky and we can see the angle change every month; from this, we know that the Moon is the closest body to the Earth. We see the same effect for the other planets and even for the Sun. But not for the stars. They never seem to occult each other, and never appear to change their parallax.

From the Earth's orbit, the stars move only a very, very tiny amount.

From the Earth’s orbit, the stars move only a very, very tiny amount.

There are two possible reasons for that. Either the stars are all at the same distance from us, embedded in a crystalline sphere, or the stars are so very, very far from us that it takes a really good instrument and a lot of patience to measure the parallax. And in 1839, Thomas Henderson showed that it was the latter case. Using a year’s worth of observations made with one of the most powerful telescopes available, he was able to show that Alpha Centauri had a parallax of just 747 milliarcseconds; that’s a change in angle of less than 1/2500th the width of the Moon. And Alpha Centauri is the nearest bright object in the sky (it has a dimmer companion known as Proxima Centauri that wasn’t seen until 1915 that is about 1,208,425,490,000 mi or 4% closer).

Other stars are even farther away. 61 Cygnus, the first star to have its parallax published (Henderson had sat on his results for several years because he didn’t think they were good enough to publish) is nearly three times farther away and has a parallax 1/3 that of Alpha Centauri. Procyon, the bright star in Sirius is about as far away as 61 Cygnus. And the nearest star like ours other than ours is Tau Ceti, with a parallax of just 274 milliarcseconds and a distance of 69,877,727,300,000 miles.

If you’d like to play a bit with parallax yourself (and see some amazing things while you are at it), why not join Galaxy Zoo? They need ordinary folks like us to sort  pictures of galaxies so that we can get a better understanding of how this whole shebang happened in the first place! To take part, fly on over to:

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