Stellar spectra

6.7 Stellar spectra

When discussing the Sun, the formation of the Fraunhofer absorption lines observed in its spectrum was described. It turns out that the absorption lines that are seen in the spectra of stars are very closely related to the temperature of the gas through which the light passes on its way from their visible surfaces. If the temperature of a gas was at absolute zero, all the electrons would be in the low- est possible energy levels – the ground states of the atoms. As the temperature rises, fi rst some electrons will rise into their fi rst excited state and then to higher excited states until, at suffi ciently high temperatures, electrons may escape from their atoms which are then said to be ionized . The remnant of an atom will then have less than its full quota (one for each proton) of electrons and is called an ion (often called a positive ion as, having lost electrons having negative charge, it will have a net positive charge). Roman numerals are used to defi ne the state of ionization of atoms. For example, H I represents a neutral hydrogen atom and

H II an ionized hydrogen atom that has lost its one electron. In the same way He

I is neutral helium, He II singly ionized helium and He III doubly ionized helium.

The Properties of Stars

6.7.1 The hydrogen spectrum

Neutral hydrogen produces a distinctive set of spectral lines, called the Balmer series that range in wavelength from 363.46 nm in the ultraviolet to 656.3 nm in the red. The most prominent is the 635.3 nm red line which is called the Hα line

and is seen in clouds of gas where the electrons have been lifted into excited states by incident ultraviolet radiation and then drop back down into lower energy states. To form the Balmer series of visible lines the electrons drop down to the fi rst excited state called level 2. The Hα line is caused by a transition from the sec-

ond to the fi rst excited state and thus from level 3 down to level 2. The green line, produced by the transition from level 4 down to level 2, is called the Hβ line whilst the Hγ line, in the mid blue, is produced by the transition from level 5 down to level 2 (Figure 6.4).

Our eyes are not sensitive to the red light of the Hα line and, sadly, we see very little colour in the universe with our eyes, but our eyes are far more sensitive in the green, and using a telescope of ∼16 in. or more aperture some objects, such as the Dumbell Planetary Nebula, appear a vivid green – the light from the Hβ line.

Other atoms will produce similar series of lines either in neutral or ionized form dependant on the temperature of the stellar atmosphere. The absorption spectra that we observe are thus a mix of all these lines and depend strongly on tem- perature. The hydrogen Balmer series, for example, appears strongest when the star’s atmosphere is ∼9000 K. At very high stellar temperatures, virtually all the hydrogen atoms are ionized so the Balmer lines are very weak. However, it takes far more energy to fully ionize helium so lines of both neutral (He I) and singly ionized (He II) helium are seen.

Introduction to Astronomy and Cosmology

6.7.2 Spectral types

In the latter part of the nineteenth century, the spectra of thousands of stars were photographed by astronomers at Harvard University and the spectra were used to classify the stars into what are called their spectral types. For example, Type A stars were those where the hydrogen Balmer lines were seen to be at their stron- gest. Stars where the hydrogen lines were weak but helium lines were seen were called Type O. In all, the stars were split into seven spectral types: O, B, A, F, G, K and M. Here they have been listed in decreasing order of temperature, O the hot- test and M the coolest. Each type is split into tenths, so the hottest stars within

a spectral type will be classifi ed as, say, G0 and the coolest within that type G9. Our Sun is classifi ed as a G2 star and is thus towards the hotter end of the G type stars.

O type stars range from ∼60 000 down to 30 000 K. As we will see in Chapter 7, such stars have a very short lifetime so are relatively rare. They are indicated by lines of ionized helium (He II) in their spectra – only possible at very high tem- peratures.

B type stars are cooler, ranging from 30 000 down to 10 000 K. The hydrogen Balmer lines are stronger, and lines of neutral helium (He I) are seen rather than ionized helium.

A type stars range in temperature from 10 000 down to 7500 K. As previously mentioned, the hydrogen Balmer lines are strongest here and lines of singly ion- ized elements such as magnesium and calcium appear.

F type stars cover the range from 7500 down to 6000 K. The Balmer lines of hydrogen are weaker whilst those from singly ionized calcium (Ca II) are becom- ing prominent.

G type stars, the type which includes our Sun, cover the temperature range from 6000 down to 5000 K. (Remember our Sun is a G2 type star with a surface temperature that we calculated above to be 5800 K.) The H and K lines of singly ionized calcium are at their strongest.

K type stars range from 5000 down to 3500 K and the many spectral lines come largely from neutral metals such as iron and sodium. M type stars are the coolest with surface temperatures less than 3500 K. At such temperatures molecules can exist in the stellar atmosphere and so their spec- tra show many molecular lines.

Analysis of a star’s absorption spectrum is thus an excellent method of determining its temperature to add to those previously discussed (Figure 6.5). From the surveys that currently exist, the percentages of stars in the differing spectral classes are given in Table 6.1. From Table 6.1 it can be seen that the great majority of stars are cool M type stars and there are a very small percentage of O and B type stars.

The Properties of Stars

Figure 6.5 Typical stellar spectra.

Table 6.1 The percentages of stars in the diff ering spectral classes. Type

Colour

Proportion (%)

O Blue

B Blue-white

A White

F White-yellow

G Yellow

K Orange

M Red