Assessing the absolute magnitude and colour of a star

By using increasingly powerful telescopes, fitted with various instruments to analyse the quality of the light collected from objects such as stars and galaxies, and by making calculations based on their observations, astronomers down the centuries have been able to gather a considerable amount of information about a large number of stars and other objects.

One item of information that can be assessed is the “absolute magnitude” of a star. It is not surprising that stars that are relative close to Earth will appear brighter than those that are further away. However, it is only possible to compare the actual visual luminosity of stars if it is possible to calculate their distances and make the necessary adjustments. When this is done, astronomers can set a standard and declare the absolute, as opposed to the apparent, magnitude of every object in the night sky. The standard that is set is the magnitude that a star would have if it were at a distance of 10 parsecs from us (a parsec is equivalent to 3.26 light years).

Absolute magnitude is expressed as a factor of its apparent magnitude, either plus or minus. Thus the brightest objects, in absolute terms, record a figure of minus 10 and the dimmest are at plus 15. The Sun’s absolute magnitude is plus 4.8.

Star colour

Astronomers are also interested in how hot stars are, and this is directly related to their colour. Thus a relatively cool star such as Betelgeuse produces light at long wavelengths, which make the star look red, and a hotter star, such as Rigel, has its intensity curve skewed towards short wavelengths, so it appears blue. By using a light-sensitive device such as a photomultiplier, and a standard set of colour filters, it is possible to arrive at an accurate assessment of a star’s colour.

By using a spectroscope it is possible to obtain a full spectrum of the light coming from a star, and the nature of the spectrum is also directly related to the surface temperature of a star. Astronomers use a spectral scale of star types which are given the letters O, B, A, F, G, K and M, with O stars being the hottest (surface temperatures above 35,000 degrees K) and M stars the coolest (at around 3,000 degrees K).

© John Welford