What Do Spectra Tell Us?

Most bright astronomical objects shine because they are hot. In such a case, the continuum they emit tells us what the temperature is. Here is a very rough guide.

We can learn a lot more from the spectral lines than from the continuum.

* The chemical composition of stars

The studies of the Solar spectrum (Joseph Fraunhofer is the most famous, and probably also the most important, early contributor to this field), however, revealed absorption lines (dark lines against the brighter continuum). The precise origin of these 'Fraunhofer lines' as we call them today remained in doubt for many years, until Gustav Kirchhoff, in 1859, announced that the same substance can either produce emission lines (when a hot gas is emitting its own light) or or absorption lines (when a light from a brighter, and usually hotter, source is shone through it). Now scientists had the means to determine the chemical composition of stars through spectroscopy!

One of the most dramatic triumph of astrophysical spectroscopy during the 19th century was the discovery of helium. An emission line at 587.6 nm was first observed in the Solar corona during the eclipse of 1868 August 18th, although the precise wavelength was difficult to establish at the time (due to the short observation using temporary set-ups of instruments transported to Asia). Two months later, Norman Lockyer used a cleaver technique and managed to observe the Solar prominence without waiting for an eclipse. He noted the precise wavelength (587.6 nm) of this line, and saw that no known terrestrial elements had a line at this wavelength. He concluded this must be a newly discovered element, and called it 'helium'. Helium was discovered on Earth eventually (1895) and showed the same 587.6 nm line. Today, we know that helium is the second most abundant element in the Universe.

We also know today that the most abundant element is hydrogen. However, this fact was not obvious at first. Many years of both observational and theoretical works culminated in 1925, when Cecilia Payne published her PhD thesis entitled 'Stellar Atmospheres' (Footnote: this was the first ever PhD awarded at Harvard; it was also praised as "undoubtedly the most brilliant PhD thesis ever written in astronomy" nearly 40 years later. She later turned to studies of variable stars, and coined the term 'cataclysmic variables'.) In this work, she utilized many excellent spectra taken by Harvard observers, measured the intensities of 134 different lines from 18 different elements. She applied the up-to-date theory of spectral line formation, and found that the chemical compositions of stars were probably all similar, the temperature being the important factor in creating their diverse appearances. She was then able to estimate the abundances of 17 of the elements relative to the 18th, silicon. Hydrogen appeared to be more than a million times more abundant the silicon, a conclusion so unexpected that it took many years to become widely accepted.

* The motion of stars and galaxies

If the spectrum of a star is red or blue shifted, then you can use that to infer their velocities along the line of sight. Such 'radial velocity' studies have had at least three important applications in astrophysics.

The first is the study of binary star systems. The component stars in a binary revolves around each other. You can measure the radial velocities for one cycle (or more!) of the binary, then you can relate that back to the gravitational pull using Newton's equations of motion (or their astrophysical applications, Kepler's laws). If you have additional information, such as from observations of eclipses (see Light Curve), then you can sometimes measure the masses of the stars accurately. Eclipsing binaries in which you can see the spectral lines of both stars have played a crucial role in establishing the masses and the radii of different types of stars.

The second is the study of the structure of our Galaxy. Stars in the Galaxy revolves around its center, just like planets revolve around the Sun. It's more complicated, because the gravity is due to all the stars in the Galaxy combined in this case (in the Solar system, the Sun is such a dominant source that you can ignore the pull of the planets --- more or less). So, radial velocity studies of stars (binary or single) have played a major role in establishing the shape of the Galaxy. It is still an active field today: for example, one of the evidence for dark matter comes from the study of the distribution of velocities at different distances from the center of the Galaxy. Another exciting development is the radial velocity studies of stars very near the Galactic center, which strongly suggest that our Galaxy contains a massive black hole.

The third is the expansion of the Universe. Edwin Hubble established that more distant galaxies tended to have more red-shifted spectra. Although not predicted even by Einstein, such an expanding universe is a natural solution for his general relativity theory. Today, for more distant galaxies, the redshift is used as primary indicator of their distances. The ratio of the recession velocity to the distance is called the Hubble constant, and the precise measurement of its value is one of the major goals of astrophysics today, using such tools as the Hubble Space Telescope.