17 (2) Flashcards
Measuring colors is only one way of analyzing starlight. Another way is to use a ……………. to spread out the light into a spectrum
spectrograph
A spectrograph is an instrument that separates incoming light by its wavelength or frequency and records the resulting spectrum in some kind of multichannel detector, like a photographic plate. Many astronomical observations use telescopes as, essentially, spectrographs.
In 1814, the German physicist ……….. ………… observed that the spectrum of the Sun shows dark lines crossing a continuous band of colors.
Joseph Fraunhofer
In the 1860s, English astronomers Sir ……….. ……….. and Lady ………… ………… succeeded in identifying some of the lines in stellar spectra as those of known elements on Earth, showing that the same chemical elements found in the Sun and planets exist in the stars.
William Huggins / Margaret Huggins
The hypothesis that the spectra differ from one start to another because stars are not all made of the same chemical elements turned out to be wrong. What is the real reason?
The primary reason that stellar spectra look different is because the stars have different _temperatures_. Most stars have nearly the same composition as the Sun, with only a few exceptions.
In the atmospheres of the hottest stars, hydrogen atoms are completely ……….
Because the electron and the proton are separated, …………. hydrogen cannot produce absorption lines.
ionized
The hydrogen lines in the visible part of the spectrum (called ………… ……….) are strongest in stars with intermediate temperatures—not too hot and not too cold.
Calculations show that the optimum temperature for producing visible hydrogen lines is about …………. K.
Balmer lines / 10,000
Astronomers use the patterns of lines observed in stellar spectra to sort stars into a ………… ……….
spectral class
Because a star’s …………. determines which absorption lines are present in its spectrum, these spectral classes are a measure of its surface ………………..
temperature
There are seven standard spectral classes. From hottest to coldest, these seven spectral classes are designated ………………………., ………….. Recently, astronomers have added three additional classes for even cooler objects—…………,………, and …………..
O, B, A, F, G, K, and M
L, T, and Y.
What is a mnemonic the can be used to remember the Classification of Stellar Spectra’s 10 letters?
Oh Be A Fine Girl, Kiss Me Like That, Yo!
Each of these spectral classes, except possibly for the Y class which is still being defined, is further subdivided into …….. subclasses designated by the numbers ……. through ………..
10 / 0 9
A B0 star is the hottest type of B star; a B9 star is the coolest type of B star and is only slightly hotter than an A0 star.
And just one more item of vocabulary: for historical reasons, astronomers call all the elements heavier than helium ………….., even though most of them do not show metallic properties.
- metals*
- (If you are getting annoyed at the peculiar jargon that astronomers use, just bear in mind that every field of human activity tends to develop its own specialized vocabulary. Just try reading a credit card or social media agreement form these days without training in law!)*
To see how spectral classification works, let’s use Figure 17.5. Suppose you have a spectrum in which the hydrogen lines are about half as strong as those seen in an A star. Looking at the lines in our figure, you see that the star could be either a B star or a G star. But if the spectrum also contains helium lines, then it is a B star, whereas if it contains lines of ionized iron and other metals, it must be a G star.
