<P> Other individual objects can have fundamental distance estimates made for them under special circumstances . If the expansion of a gas cloud, like a supernova remnant or planetary nebula, can be observed over time, then an expansion parallax distance to that cloud can be estimated . Those measurements however suffer from uncertainties in the deviation of the object from sphericity . Binary stars which are both visual and spectroscopic binaries also can have their distance estimated by similar means, and don't suffer from the above geometric uncertainty . The common characteristic to these methods is that a measurement of angular motion is combined with a measurement of the absolute velocity (usually obtained via the Doppler effect). The distance estimate comes from computing how far the object must be to make its observed absolute velocity appear with the observed angular motion . </P> <P> Expansion parallaxes in particular can give fundamental distance estimates for objects that are very far, because supernova ejecta have large expansion velocities and large sizes (compared to stars). Further, they can be observed with radio interferometers which can measure very small angular motions . These combine to provide fundamental distance estimates to supernovae in other galaxies . Though valuable, such cases are quite rare, so they serve as important consistency checks on the distance ladder rather than workhorse steps by themselves . </P> <P> Almost all astronomical objects used as physical distance indicators belong to a class that has a known brightness . By comparing this known luminosity to an object's observed brightness, the distance to the object can be computed using the inverse - square law . These objects of known brightness are termed standard candles . </P> <P> The brightness of an object can be expressed in terms of its absolute magnitude . This quantity is derived from the logarithm of its luminosity as seen from a distance of 10 parsecs . The apparent magnitude, the magnitude as seen by the observer (an instrument called a bolometer is used), can be measured and used with the absolute magnitude to calculate the distance D to the object in kiloparsecs (where 1 kpc equals 1000 parsecs) as follows: </P>

Which two things are needed to determine a standard candle distance from earth