D.2a Stellar spectra (application)

THE IB SAYS

Understandings: Stellar spectra

Applications and skills: Explaining how the chemical composition of a star may be determined from the star’s spectrum

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Metallicity

Analysis of spectral lines in stars will reveal the presence of hydrogen (by far the most common element), helium (the next most common) and other elements. Astronomers call all elements heavier than helium metals. The proportion of "metals" is a star is known as its metallicity. Most stars have a metallicity of around 0.02 (or 2%).

Determining the metallicity of a star allows us to assign it to one of three populations - the relative metallicity of a star is an indication of it's age. The more metal poor a star is, the older it probably is.

Red shift / blue shift

Another thing we can do with the spectral lines of a star is compare them to where they are found in a lab. We sometimes observe that all of the lines are shifted towards the blue end of the spectrum, or towards the red end of the spectrum. This is an application of the Doppler Effect and generally indicates the the object is moving towards us (blue shift) or away from us (red shift). We will look more at this effect and its implications in our study of cosmology.

Spectroscopic binaries

Sometimes when observing the spectrum of a star over time, you may see the spectral lines of the star split and then come back together again. This can be caused by large magnetic fields, but is more commonly the result of two or more stars orbiting their common centre of mass.

Stellar classification

By combining tour analysis of the surface temperature of stars with an analysis of the relative strength of hydrogen lines, we can assign strs to one of several class - each with a different temperature and colour.

Questions answered:

How do we use the doppler shift in astronomy?

Resources from textbooks

Oxford Physics: pp 650 - 651, including a worked example on page 652

Hamper HL (2014): p 542

Relevant questions from Dot Point Physics (E. Astrophysics)

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