Standard candle
A standard candle is an astronomical object that has a known luminosity.
This is an extremely important method of deriving distances of objects in extragalactic astronomy and cosmology.
Comparing this known luminosity (or its derived logarithmic quantity, the absolute magnitude) and its observed brightness (apparent magnitude) the distance to the object can be calculated as
where D is the distance, kpc is kiloparsec (103 parsec),
m the apparent magnitude and M the absolute magnitude (both in the same band at rest).
At relatively close stellar distances, main sequence stars are the preferred choice. For somewhat more distant stars and nearly galaxies, cepheid variables are the
preferred choice.
For larger distances many objects have been used, some better that others.
Currently, for long distances the best available standard candles are Supernovae Ia, that have a very well-determined maximum absolute magnitude as a function of the shape of their light curve.
In galactic astronomy, X-ray bursts (thermonuclear flashes on the surface of a neutron star) are used as standard candles.
Observations of X-ray burst sometimes show X-ray spectra indicating radius expansion.
Therefore, the X-ray flux at the peak of the burst should correspond to Eddington luminosity, which can be calculated once the mass of the neutron star is known (1.5 solar masses is a commonly used assumption).
This method allows distance determination of some low-mass X-ray binaries.
Low-mass X-ray binaries are very faint in the optical, making measuring their distances extremely difficult.
One issue with standard candles is the question of how standard they are. For example, all observations seem to indicate that type Ia supernovae that are of known distance have the same brightness. However, it is not known why they should be the same brightness, and the possibility that the distant type Ia supernovae have different properties than nearby type Ia supernovae exists.
That this is not merely a philosophical issue can be seen from the history of distance measurements using cepheid variables. In the 1950's, Baade discovered that the nearby cepheid variables used to calibrate the standard candle were of a different type than the ones used to measure distances to nearby galaxies. The nearby cepheid variables were population II stars which much higher metal content than the distant population I stars. As a result, the population I stars were actually much brighter than believed, and this had the effect of doubling the distances to the globular clusters, the nearby galaxies, and the diameter of the Milky Way.
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