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Spectra - What Do They 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.

Astronomical examples
600KInfrared Planets, warm dust
6,000KOptical The photosphere of Sun and other stars
60,000KUV The photosphere of very hot stars
600,000Ksoft X-rays The corona of the Sun
6,000,000KX-rays The coronae of active stars

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

The motion of stars and galaxies

In such an analysis of chemical abundances, the wavelength of each line is treated as fixed. However, this is not true when the star is moving towards us (the lines are observed at shorter wavelengths, or 'blueshifted, than those measured in the laboratory) or moving away from us (observed at longer wavelengths, or 'redshifted'). This is the phenomenon of 'Doppler shift'.

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.

The chemical composition of stars

During the first half of the 19th century, scientists such as John Herschel, Fox Talbot, and Willam Swan studied the spectra of different chemical elements in flames. Gradually, the idea that each element produces a set of characteristic emission lines has become 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 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!

ASCA Spectrum of CasA
Image Credit: NASA

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.

What's So Special About X-ray and Gamma-ray Spectra?

What's so special about X-ray and gamma-ray spectroscopy?

One reason is that, sometimes, X-ray spectroscopy is simpler to interpret than optical spectroscopy. This is because, at X-ray temperatures, atoms are highly ionized (most of the electrons have been taken away from the atoms), leaving only a few electrons per nucleus. This makes theoretical calculations much easier! Thus it is, in principle, much easier to relate the strengths of X-ray lines to, for example, the abundances of various elements.

A more important reason is that there are many classes of astronomical objects that contain high temperature gases (at millions of degrees K). At these temperatures, more of their energies are radiated as X-rays (both continuum and lines) than at other wavelength ranges, so it makes sense to observe them in the X-rays. Such hot gases can be found, for example, in the corona of the Sun: the observation of the Solar corona is very important because Solar flares and other activities there can affect satellite communication links and the health of astronauts in orbit. Many of the elements that you and I are made of are produced by, or dispersed by, supernova explosions --- and supernova remnants are prominent in the X-ray sky because they are also at X-ray temperatures. Another example is the clusters of galaxies: studies show that the X-ray emitting (and otherwise hard to detect) gas may add up to for far more (in terms of their total mass) than the stars that we can see in the form of these galaxies.

Simulated X-ray spectrum
Simulated X-ray spectrum of what could be observed from radio galaxy Hydra A by the future Constellation-X mission. Credit: NASA
Lines in the optical to X-ray range are produced by atomic (and molecular) processes. In the gamma-rays, though, the lines are produced by different processes. Gamma-ray lines may be produced by nuclear processes --- each species of atomic nuclei has its own own set of characteristic lines, just like atoms do. Nuclear reactions (fission or fusion) leave the nuclei in an unstable state, which then emit gamma-ray lines. In normal stars, all the nuclear fusion takes place at their cores, so the Gamma-rays cannot escape unhindered. However, supernova explosions leave some of these unstable nuclei in the open, and gamma-ray lines from supernova remnants have been detected.

In some X-ray binaries and active galactic nuclei, positrons (anti-particle of the more familiar electron) are produced. A positron and an electron can annihilate each other, creating two gamma-ray photons of 511 keV each.

The technology to observe these gamma-ray lines is not as well developed as at other wavelengths. Still, there have been many important discoveries in the last decade or so, using the Compton Gamma Ray Observatory, among others. The technology is developing rapidly, and we hope to learn a lot about the most violent processes in the Universe using these gamma-ray line in the near future.

Info Click here for information on the Doppler shift and how is affects a galaxy's spectrum.
Quiz Click here for a quiz on what spectra can tell us!
Return Click here to return to observing the Spectrum of M31 to determine its velocity.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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