A very important task in modern astronomy is the measurement of distances to things that are very far away. We have seen earlier some methods for measuring distances to relatively nearby objects.
At those distances, astronomers turn to a series of methods that use standard candles: objects whose absolute magnitude is thought to be very well known. Then, by comparing the relative intensity of light observed from the object with that expected based on its assumed absolute magnitude, the inverse square law for light intensity can be used to infer the distance.
Because type Ia supernovae are so bright, it is possible to see them at very large distances. Cepheid variables, which are supergiant stars, can be seen at distances out to about 10-20 Mpc; supernovae are about 14 magnitudes brighter than Cepheid variables, which means that they can be seen about 500 times further away. Thus, type 1a supernovae can measure distances out to around 1000 Mpc, which is a significant fraction of the radius of the known Universe.
|Source: An Introduction to Modern Astrophysics, B. W. Carrol and D. A. Ostlie (Addison-Wesley, 1996)|
We shall not discuss the details of how all these methods work, but we note that there is reasonably good agreement on the distance to the Virgo Cluster (the average among the different techniques is approximately 15 Mpc). This gives us some confidence that these methods can be used to measure large distances. The last column (Range) gives the largest distance at which these methods can be used. We see that distances in excess of 1000 Mpc may be measured.