The Spectral Sequence and Temperature

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  • 0:01 Classifying…
  • 0:37 How Star Spectra Form
  • 2:31 Spectral Classes and Sequence
  • 6:36 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will talk about spectral classes, spectral types, and the spectral sequence. We will break down each spectral class and point out some important characteristics of each.

Classifying Astronomical Information

Humans love to organize things into neat categories, like in a library, where we have history books in one section, thrillers in another, and drama in yet another aisle. Such organization or classification, in many different fields, allows us to compare and contrast categories or to remember information a lot better. Astronomers also classify stars based on their properties, with something known as the spectral sequence.

In this lesson, we will discuss the spectral sequence, along with taking a look at spectral classes and types.

How Star Spectra Form

Another lesson describes for you how star spectra are formed. An absorption line spectrum refers to dark spectral lines interspersed on a continuous spectrum. A continuous spectrum is like a rainbow.

The reason this type of spectrum is formed is because a star's lower atmosphere, where its gases are hot and dense, produce a continuous spectrum (a rainbow) when its light is passed through a prism. As this light floats towards the upper atmosphere of the star, where the gas is cool, specific wavelengths of light are absorbed by the atoms of these cooler and thinner layers of gas.

This means our light, one that leaves from the upper atmosphere and into space, is now devoid of certain wavelengths originally found on the continuous spectrum, leaving behind black empty lines.

Some star spectra have prominent hydrogen lines, pointing to hydrogen in its atmosphere. Other star spectra have prominent lines indicating heavier elements, like calcium or sodium. Further still, some other stellar spectra have broad dark lines, indicating that elements have combined into molecules.

Such spectral lines also depend on the temperatures involved, not just the elements in the atmosphere. This is because some lines will simply not show very well if it's not hot enough to excite atoms to energy levels that will produce spectral lines. The reverse is true, as if it's too hot, then an atom's electron (like that of hydrogen) may be torn away, disabling that atom's ability to produce an absorption line because these lines depend on an atom's electrons moving up and down certain energy levels.

Spectral Classes and Sequence

Since there are so many varieties of stellar spectra, astronomers have grouped similar spectra into spectral classes. A spectral class is the star's position in a temperature classification scheme based on the appearance of absorption lines in its spectrum. The arrangement of spectral classes based on temperature is known as the spectral sequence.

The spectral sequence, composed of the spectral classes, is arranged from hottest to coldest as follows:

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