Most of what we know about the nature of stars comes from examining their spectra.
So let's talk a little bit about how we can classify stars according to their
spectra. When we look at the spectra of our Sun for example, there are noticeable
absences of light or colors of light within the Sun's spectra. And this of
course is caused by the presence of atoms subtracting light from the star as
it passes through the star's atmosphere. So let's break that down a little bit
and just kind of talk about the different components or the anatomy of a
star's spectra. First of all, we have the star itself which is shining very much
like a blackbody. The star is hot enough and dense enough to produce what would
otherwise be a fairly smooth Planck curve, or blackbody curve. But the star has an
atmosphere, so it's the presence of atoms in the atmosphere that intercept, or
rather, the electrons in those atoms are intercepting light at specific
wavelengths as they emerge from the surface of the star. So this results in
the absorption of those colors, or those wavelengths, from the resulting spectra
giving us the dips that you see here. Now, in some rare circumstances there may be
some portions of the star or maybe there's a shell around the star that is
superheated and gives off emission as a result. However this is very rare in
stars and most stars' spectra look like this, the absorption spectra that you see
here. As a matter of fact, this is not just an arbitrary spectra, this is the
actual spectra of the star Vega. So we can see the tell-tale hydrogen alpha,
beta, gamma, and delta lines. There's also some extra lines introduced by the
Earth's atmosphere since this was made by a telescope on the ground. And there's
a couple of other lines present due to the presence of other elements in Vega's
atmosphere. So how do we classify stars according to their spectra? Well, the
story begins with Edward Pickering at Harvard College at the time
and he developed a method by which he could take the spectra of multiple stars
simultaneously. These spectrum were taken on photographic plates, these glass
plates that we learned about earlier, and he asked his graduate students--who by
the way were all men at the time--to come up with a classification system based
upon their spectra well the fellows didn't get it done and
Pickering in frustration finally said 'I bet my secretary could do a better job'
so he asked Annie Jump Cannon, who eventually came up with a way of classifying stars
according to their spectra but identifying that the pattern or the
presence of certain spectral lines were the result of the star's temperature. So
these stars were arranged in the following alphabet soup that you see
here - O,B,A,F,G,K,M - and Annie Cannon didn't do the job all by herself, she had so
much data to work with that she enlisted the help of other secretaries so this
was "Pickering's harem", as it was known in the day. So Canon's initial classification
of stars by their spectra was due to temperature from hottest to coolest
however there's really a range of temperatures even in between the
individual spectral classifications so what we have instead is not O and B
stars, we have O0 through O9, B0 through B9, A0 through A9, and so forth.
These are just some of those spectral types shown here and as we've already
seen a more modern way is to characterize their spectra as a function
of their intensity. You can still see the presence of - in this case - the hydrogen
lines - the Balmer series of hydrogen - there's also some helium as well as some
sodium and even titanium oxide. So you notice that the M stars are cool enough
that even molecules can form, producing in this case, titanium oxide.
So when we think about stars arranged by their color indices, we know that the
lower the value the hotter the surface temperature, and since the spectral type
is also associated with the surface temperature, we can represent them like
you see here. So where does our Sun fit into all this?
Well, it turns out that it's a G2 spectral type star, it has a color index
of +0.67, and a surface temperature of about 5800 Kelvin. So it's a little bit
cooler than a G0 star but certainly not nearly so cool as a K0 star. So if you're
having difficulty remembering the order of the spectral types according to their
temperatures, just remember this mnemonic "Oh Be A Fine Girl (or guy) Kiss Me". Now, if
you're one of my students I certainly hope this does not apply but here's an
alternative mnemonic, "Oh Boy An F Grade Kills Me". Whichever way you want to think
about it, this is a convenient way of remembering the spectral types from
hottest to coolest.