Spectra

What Do Spectra Tell Us?

Most bright astronomical objects shine because they are hot. In these cases, the continuum emission tells us the temperature of the object. The following table shows a rough guide for the relationship between the temperature of an object and what part of the electromagnetic spectrum where we see it shine.

However, we can learn a lot more from the spectral lines than from the continuum. Two very important things we can learn from spectral lines is the chemical composition of objects in space and their motions.

Chemical composition

During the first half of the 19th century, scientists such as John Herschel, Fox Talbot, and William Swan studied the spectra of different chemical elements in flames. Since then, the idea that each element produces a set of characteristic emission lines has become well-established. Each element has several prominent, and many lesser, emission lines in a characteristic pattern.

Sodium, for example, has two prominent yellow lines (the so-called D lines) at 589.0 and 589.6 nm – any sample that contains sodium (such as table salt) can be easily recognized using these pair of lines.

The studies of the Sun's spectrum revealed absorption lines, rather than emission 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 absorption lines (when a light from a brighter, and usually hotter, source is shone through it). With that discovery, scientists had the means to determine the chemical composition of stars through spectroscopy.

Stars aren't the only objects for which we can identify chemical elements. Any spectrum from any object allows us to look for the signatures of elements. This includes nebula, supernova remnants and galaxies.

X-ray spectrum of supernova remnant Cas A from ASCA data. (Credit: Holt et al., PASJ 1994)

Motions of stars and galaxies

Once we have identified specific elements in a spectrum, we can also look to see if the emission lines from those elements has been shifted from where we might expect to find them. While we usually talk about emission spectra as though the wavelengths of the lines are fixed, that is only true when the source emitting the lines and the detector "seeing" the lines are not moving relative to one another. When they are moving relative to each other, the lines will appear shifted. For example, if a star is moving toward us, its lines will be observed at shorter wavelengths, which is called "blueshifted". If the star is moving away from us, the lines will appear at longer wavelengths, which is called "redshifted". This is called "Doppler shift."

Simplified star spectrum showing how it would appear if the star was at rest with respect to us (top), moving toward us (middle; "blueshifted"), and moving away from us (bottom; "redshifted").

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

1.      One application is in the study of binary star systems. For stars in some binary systems we can measure the radial velocities for one orbit (or more). Once we've done that, we can relate that back to the gravitational pull using Newton's equations of motion (or their astrophysical applications, Kepler's laws). If we have additional information, such as from observations of eclipses, then we can sometimes measure the masses of the stars accurately. Eclipsing binaries in which we 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 red giant star Mira A (right) and its companion, a close binary pair. (Credit: M. Karovska/Harvard-Smithsonian Center for Astrophysics and NASA)

2.      Another application is the study of the structure of our galaxy. Stars in the Galaxy revolve 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 that lie inside the stars' orbit combined; whereas in the Solar system, the Sun has so much more mass than the planets combined, we can ignore the pull of the planets, more or less. 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 form of the evidence for dark matter comes from the study of the distribution of velocities at different distances from the center of the Galaxy (and for other galaxies). Another exciting development is from the radial velocity studies of stars very near the Galactic center, which strongly suggest that our Galaxy contains a massive black hole.

An artist's conception of the Milky Way galaxy. (Credit: NASA/JPL)

3.      A third application 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 theory of relativity. 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.