Stellar parallax:The parsec
We have seen that the distances of the stars are so great that any measured parallax is less than 1 second of arc. There is, therefore, a need to introduce a unit of length for use in describing such distances that will lead to convenient numerical values.
The parsec is the more usual unit used by astronomers: it is the distance of a celestial body whose parallax is 1 second of arc. Now 1 second of arc is approximately 1/206 265 of a radian; therefore, a distance of 206 265 astronomical units will be the distance at which one astronomical unit subtends 1 second of arc. Hence, we may write
1 parsec = 206 265 AU
or, using the accepted value of the AU, namely 149·6 × 10⁶ km,
1 parsec = 30·86 × 10¹² km approximately.
A larger unit, the kiloparsec (= 10³ parsec) is often used in expressing the distances of stars or the size of galaxies. In popular books on astronomy, the light-year is often employed as a unit of length. As its name implies, it is the distance travelled by light in 1 year. The velocity of light is 299 792 km s⁻¹ and there are 31·56 × 10⁶ s in 1 year. Hence,
1 light-year = 9·46 × 10¹² km, approximately
so that
1 parsec is equal to about 3·26 light-years.
The use of the light-year as the unit of length does emphasize one important aspect of astronomical observations, namely that distant objects are seen not as they are now but as they were at a time when the light entering the observer’s telescope left them. In the case of galaxies, this ‘timescope’ property of a large telescope is particularly important, enabling information to be obtained about the Universe in its remote past.