Everything about Extragalactic Distance Scale totally explained
The Extragalactic Distance Scale is a series of techniques used today by astronomers to determine the distance of cosmological bodies (beyond our own galaxy) not easily obtained with traditional methods. Some procedures utilize properties of these objects, such as
stars,
globular clusters,
nebulae, and galaxies as a whole. Other methods are based more on the statistics and probabilities of things such as entire
galaxy clusters. It is important to note that the following methods are for, as the name implies, extragalactic objects, so the more traditional and well known method of trigonometric parallax won't be covered as it's only accurate to around a kiloparsec or so.
Wilson-Bappu Effect
Discovered in 1956 by
Olin Wilson and M.K.
Vainu Bappu,
The Wilson-Bappu Effect utilizes the use of
spectroscopic parallax. Certain stars have features in their emission/absorption spectra which makes it relatively easy to calculate their absolute magnitudes. Certain spectral lines are directly related to an objects magnitude, such as the K absorption line of calcium. From there one can use the
distance modulus
»
Where C is a constant which depends on the distance to the galaxy clusters.
This method has the possibility of become one of the strongest methods of galactic distance calculators, perhaps exceeding the range of even the Tully-Fischer method. As of today, however, elliptical galaxies aren’t bright enough to provide a calibration for this method through the use techniques such as Cepheids. So instead calibration is done using more crude methods.
Summary
All the methods mentioned above are used today by astronomers for measuring objects beyond our own galaxy. Like all methods, different techniques and calibrations are used by other astronomers. The following table shows at-a-glance information for most of the methods mentioned above. It lists each error (in magnitudes), distance to the Virgo Cluster as calculated by each technique, and overall range of how far out each method can be used effectively.
Further Information
Get more info on 'Extragalactic Distance Scale'.
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