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Chapter 17: Problem 22

Some giant and supergiant stars are of the same spectral type (G2) as the Sun. What aspects of the spectrum of a G2 star would you concentrate on to determine the star's luminosity class? Explain what you would look for.

Short Answer

Expert verified
To determine the luminosity class of a G2 star, one should examine the widths of the hydrogen H and K lines, and the metal lines in its spectrum. Broad lines indicate higher luminosity classes (such as giants or supergiants), while narrower lines suggest the star could be a dwarf like our Sun.

Step by step solution

01

Explaining the Stellar Classification

Stellar classification is a classification scheme for stars based on their spectral characteristics. The spectral sequence OBAFGKM is a classification in the visible spectrum. 'G' type in this sequence means the star has a surface temperature similar to the Sun and the spectral lines indicate the presence of certain elements. The Sun is a G2 star. The second element of the classification '2' tells that the star is towards the hotter part of the 'G' classification. Therefore, a star being G2 does not provide any direct information about its luminosity class.
02

Understanding Luminosity Classes

Luminosity classes are categories that stars can be divided into according to their luminosity. There are seven luminosity classes from I (supergiant stars) to V (dwarf stars). Luminosity of a star could be identified through observing and interpreting the features of its spectral lines that reflect its size and temperature gradients. The classification affects the pressure in the star’s atmosphere and thus affects the broadness of the absorption lines in the star's spectrum - in a broader line, the luminosity class is higher.
03

Analyzing Spectrum to Determine Luminosity Class

To determine the luminosity class of a G2 star, the width of the spectral lines should be studied. The width can be related to the pressure, which in turn allows us to determine the relative size and thus the luminosity of the star. In a spectrum of a G2 star, the focus must be on the widths of both the hydrogen H and K lines and the metal lines. If these lines are very broad, then the star could potentially be a giant or supergiant (luminosity classes I, II or III). If the lines are rather narrow, the star could potentially be a dwarf star (luminosity class V), like our Sun.

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