Astrophysics & Cosmology – Stellar Distances: Light Years, Parsecs and Astronomical Units.

Light Years: (ly)

A light year. Most people for some reason think a light year is a measure of speed or time, a light year is in fact a measure of distance. One light year is the distance which light can travel in one year, and being as light travels at 299,792,458 metres per second (m/s) and there are 31,557,600 seconds in a year; one light year is a very long way, around 9467280000000000 metres in fact. That is further than going to the sun and back over 650,000 times! The distance to our sun is 0.00000158 light years. Strangely enough this bring us on to Astronomical units:

Astronomical Units: (AU)

An Astronomical unit is a common unit of measure when looking at objects within our solar system and nearby stars. We derive the value of 1 AU (astronomical unit) from the distance between the earth and the sun. As the earth orbits a sun in a near circular (but technically parabolic) path, an average is taken for the distance, of which the exact value is always changing. It is agreed currently that one AU is 149,597,870,691 +/- 6 metres, which equates to 1.58e-6 ly.

We use the units of Au and the distance of it when calculating anual paralax and similar phenomena. Paralax is the percieved shift of position of objects in the night sky at different times of year (due to the observer moving with the earths orbit), using the angle which the object appears to have moved by and knowing the value of one astronomical unit we can work out how far away the object in the sky is.

Parsecs: (pc)

One parsec is another distance used in astronomy, but used to describe much more distant objects, and you will have heard this term used if you are a fan of any sci-fi or space films like star trek! One parsec is defined as the distance from an object to an observer with parallax of one arc-second. Now that may sound like a lot of nonsense to many people so allow me to explain:

As explained above the distance between the sun and earth on average is one AU, if you look at a star in the sky in June on earth, it will appear slightly off place to the left of its actual position in the sky, in January the earth is exactly the other side of its orbit, the observer is 2 AU away from were they observed the star from before and this time the star is again off its real position, by the same amount as in June, but this time it appears to the right of where it should be. this effect is called parallax.

The best way to see this is to hold your finger around a foot away from your eyes, then shut your left eye. Notice the position of your finger appears to have shifted slightly, then quickly switch eyes and shut your right eye, the finger will appear to have jumped slightly to the right. This shift is called Para-shift. Your two eyes are effectively the two positions of the observers and the finger is the star you are tring to see, since we see the sky in 2 dimensions, you see a flat image and you see your finger move in comparison to its surroundings, this is equally true with the stars.

By working out the distances between sun and earth and the positions of the stars we can work out the angle at which the star appears to have moved through, and using this we use the equation:

sin(A) = 1 AU / d

where A is the angle, d is the distance to the observed object and 1 AU, represents the distance between the earth and the sun. This diagram may help.

So knowing this, what is a parsec? When the angle of parallax is one arc second (1/360th of a degree) one parsec is the distance from the centre of the observers orbit (the sun) and the object being watched. One parsec is equal to roughly 3.26 light years or 30.857e15 metres.

Using these distances we can record and compare different distances of stars and planets in our and other galaxies, taking us one step closer to mapping out the seemingly infinte expananse of other worlds out there.