Determining Distances in Space

There are several techniques for measuring distances in space: Geometric Methods For close stars Spectroscopic Measurements Close to medium distance stars (most of the stars in our galaxy) Use of “Standard Candles” Uses very bright objects in distant galaxies Using Redshifts For stars in distant galaxies

Geometric Methods Parallax: -the apparent difference in position of an object viewed along 2 different lines of sight

Parallax A triangle can be created, and if we know the length of the baseline, and angles x and y, we can calculate (using trigonometry) the distance to the object Notice that… The further away the object is, the closer the angles x and y are to each other The larger the baseline is, the further apart the angles will be Fortunately, we can use a very large baseline in astronomy: Earth’s orbit!

Since the objects in space are very far away, the angles x and y will be VERY close to each other In fact, the difference in angles is usually measure in units called arcseconds A degree is divided into 60 minutes, and a minute into 60 seconds, so an arcsecond is 1/3600th of a degree! …and no stars have differences even that large! This gives rise to a new unit: the parsec (pc) (short for “parallax arcsecond”) By definition, 1 pc = 3.08568025 × 1016 m or 3.26156 light years The first star to be measured this way was a star in the constellation of Cygnus. The difference in angles (x and y) was found to be 0.292 arcseconds. This gives a distance of 3.48 pc, or 11.36 light years

Since measuring such small differences in angles is very difficult, using this method on more distant stars is not possible. Nevertheless, it was a good starting point, that turns out to be invaluable in setting a standard for other measurement techniques.

Geometric Methods Angular Size If we know how large an object really is, and we know what angle it takes up in the sky, then we can calculate how far it must be. Pro: Can be used for even distant galaxies and quasars Con: Requires an assumption about the actual size of the object. (Sometimes this is not a problem, but other times it definitely can be!) This method was used determine the distance to the centre of our galaxy.