The Hertzsprung-Russell diagram (known in what follows as the HR diagram) takes its name from the Danish astronomer E. Hertzsprung, who established it in 1911, and from the American astronomer H.N. Russell, who independently rediscovered it in 1913. This diagram shows the absolute luminosity of a star (that is, independent of its distance from Earth) as a function of the effective temperature, or of any other quantity linked to it (such as, for example, the difference in luminosity of the object measured through two coloured filters of different colours and therefore of different passbands). This diagram is shown in Fig. A.2. The specific feature of this diagram is that it is simultaneously a tool for visualizing both the morphological diversity of stars, and equally their evolution over the course of time. In fact, stars occupy a position on the HR diagram that evolves from their birth to their death.
Of the stars that we see, 90 per cent are at a stage of their lives that we may term 'adult' and occupy a portion of the HR diagram that is known as the 'Main Sequence' (the portion encircled in Fig. A.2). This is the case with our Sun, which was born about 5000 million years ago, and will die in about 5000 million years time. The special feature of stars on the Main Sequence is that there is an unequivocal relationship between their effective temperature, their absolute luminosity, their mass, and their lifetime. As effective temperature and colour are directly linked by Stefan's law, we can see that the HR diagram is therefore an extremely powerful tool, because, from the colour of an object, it allows us to deduce the other characteristics. For example, the average lifetime (ilife) of a star depends on its luminosity or its mass via the relationship:
where M0, M*, LQ and L* are the masses and absolute luminosities of the Sun and the star, respectively.
The HR diagram may be used to illustrate the concept of spectral classification, which consists of distinguishing stars from spectroscopic criteria. The best-known and most-used spectral classification is the 'Harvard' one, which dates from 1872.
Bright Spectral type
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