The absorption lines seen in spectra of stars provide information about their su

Question

The absorption lines seen in spectra of stars provide information about their surface temperatures and luminosities. The graph below shows the relative strengths of absorption lines due to different atoms and molecules as a function of spectral type. Consider 4 stars. (a) The spectrum of Star 1 has strong absorption lines due to ionized helium. Star 2's spectrum shows strong lines of neutral helium. Which star is hotter? (b) Star 3 has a surface temperature of 1000 K, while Star 4 has a surface temperature of 5000 K. In which star would you expect to see strong absorption lines due to molecules like titanium oxide (TiO)? (c) Order Stars 1-4 in terms of increasing surface temperature. (d) Why would one not see strong lines due to molecules or ionized helium in an F star? In other words, why is the surface temperature of an F star not ideal for the presence of ionized helium or molecules?

Explanation / Answer

a) Star 2 is hotter as can be seen from the graph. He spectra peaks at around 10000 K while He I spectra peaks around 25000 K.

b) Star 3 would show absorption line due to TiO, as clearly visible in the graph.

c) 3< 4 <1 < 2

d) F stars how characteristic absorption lines due to neutral metals. They have temperatures around 6000-7000 K, hence they can't show ionized helium presence, due to low temperature.

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