Why do stars have dark lines in their spectra?

Answer 1

Those are absorbtion lines meaning that star is comprised of atoms that absorb that particular frequency of light. Scientists can use that information to determine what a star is burning and ultimately how old the star is.

Answer 2

Dark Line Spectra

Answer 3

The chemical composition of stars
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. 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 this 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 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 triumphs 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 clever 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 that 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 than silicon, a conclusion so unexpected that it took many years to become widely accepted.

Answer 4

Hi. They don't. The dark lines are caused by light energy being absorbed by cooler gas between the star and you. Energy is absorbed a particular frequencies as electrons jump from one shell to another.

Answer 5

The above response was a blatant copy/paste from this page: http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectral_what.html


You might want to research Fraunhofer lines (also called absorption lines)

Put in its simplest terms, each atom has specific absorption lines - these help identify them, spectrally.

Obviously you are taking classes on this subject... haven't you been paying attention?

JT