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# Standard candle

A standard candle is an astronomical object that has a known luminosity. They are 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

$5 \log \frac{D}{kpc} = m -M +5,$

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 distances, 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 fo 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.

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