Limitations of Standard Candles
Several types of challenges exist for any class of standard candle, limiting the ability of astronomers to measure accurate distances.
Calibration
The principal one is calibration, determining exactly what the absolute magnitude of the candle is. It is essential to be able to find enough members of the class with distances obtained from another Standard Candle method, so that their luminosity can be calibrated accurately.
Recognition
Another problem lies in recognising members of the class, and not mistakenly using the standard candle calibration for an object which does not belong to the class. At extreme distances, which is where one most wishes to use a distance indicator, this recognition problem can be quite serious.
Availability
Different stellar clusters or galaxies do not generally have all types of stars in them, so standard candle techniques cannot always be used.
Without alternative
In some cases the use of a particular class of objects is known to be of limited accuracy, but when no other class is available, the class is useful simply because there is no alternative.
These issues cause major limitations and uncertainties for measuring distance in the Universe and are subject to continuing research and discussion.
| ``It is much easier to make measurements than to know exactly what you are measuring.'' J. W. N. Sullivan |