----- The following copyright 1991 by Dirk Terrell
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 Lecture #6  "Making Sense of All Those Stars"

   I have always been fascinated by the stars. When I was very young I would 
sit for hours on end staring at all those stars and wondering just what they 
were. I can still remember the moment that I learned that the stars were 
suns that were just very far away as I breathlessly turned through an 
astronomy book I found in the school library. As an undergraduate I had to 
work a full time job to pay for tuition and support myself. At times I would 
get very tired and depressed, wondering if it was really worth it. All I had 
to do was walk outside and look up to see all those stars twinkling away, 
and I knew that I had to press on. My momentary depression and exhaustion 
evaporated as my curiosity flooded me with the desire to know what those 
stars were.

   The theory of stellar evolution must certainly rank as one of the 
greatest achievements in astrophysics in this century. Its success had come 
from the work of theorists and observers from a variety of disciplines. In 
this lecture I want to talk about the observational end of stellar 
astrophysics. 

   If you have reasonably dark skies and you aren't color blind, you have 
probably noticed that stars have different colors. The winter sky has some 
fine examples of this phenomenon. Examine the stars Betelgeuse and Rigel in 
the connstellation Orion. You will see that Betelgeuse has a deep red color 
while Rigel is bluish-white. What is the significance of stellar colors? It 
turns out that the same physics that describes your electric stove eye also 
describes the situation at the surface of a star. WHen you turn the burner 
on high, it begins to glow as it heats up. The first color you notice is a 
deep red, then orange, and then a yellowish-orange as it reaches its highest 
temperature. This explains the colors of stars. The colors of stars indicate 
their surface temperatures. Red stars are cool (around 4500 C), yellow stars 
like the sun are hotter (around 6000 C), blue stars like Sirius are even 
hotter (10,000 C). The hottest stars have temperatures in excess of 40,000 
C! Now we can measure the temperature of a star by measuring its color.

   Stars emit all colors of the rainbow, but their color to the eye results 
from the fact that they emit more of some colors than others. Hot stars 
appear blue because they emit more blue light than yellow, orange, or red 
light. Cool stars emit more red light than blue light, so they look red to 
our eyes (and instruments). If you split the light of a star up into a 
spectrum with a prism or diffraction grating, you can see that all the 
colors are there. Well, almost all of them. Some colors (wavelengths) are 
conspicuously absent. These absences of colors show up as dark lines in the 
spectrum of the star, and we call them the spectral lines. These lines arise 
because elements in the atmosphere of the star absorb certain colors of 
light. The nice thing about it is that a particular element always absorbs 
the same colors (i.e. always produces the same spectral lines). Hydrogen 
always absorbs a particular set of red, green, and purple wavelengths. If 
you look at a star and see those colors missing, then you know that the 
star's atmosphere has hydrogen in it. Helium absorbs a different set of 
colors. The spectral lines of an element act as its 'fingerprint'. By 
looking at the spectral lines in a star, we can tell what it's made of. 
Later on I will amaze you with what other information we can get out of a 
star's spectrum!

   As is often the case with scientific studies of new phenomena, there was 
a lot of data and very little theory. By 1890, Harvard astronomers had taken 
spectra of thousands of stars and placed them into categories labeled A 
through Q based on the strength of the hydrogen lines(i.e. how dark and fat 
the lines were). The A stars had very strong hydrogen lines and subsequent 
classes had weaker hydrogen lines. About 1896, Annie Jump Cannon at Harvard 
arranged the spectral sequence so that it reflected a progression in the 
temperature of the stars rather than the strength of the hydrogen lines. She 
established the spectral sequence that we use today: O B A F G K M from 
hottest to coolest. These spectral classes are further divided into 10 parts 
to represent a finer classifciation scheme- a B2 star is hotter than a B5 
star for instance. Our sun is a G2 star. Betelgeuse is an M2 star. Rigel is 
a B8, Sirius A1. Ms Cannon had a remarkable ability to look at the spectrum 
of a star and classify it. I have done some spectral classification myself 
and I can assure you it is not easy to do. It took me about a week to 
classify 100 spectra. Ms. Cannon averaged 5000 per month between 1911 and 
1915! I have great respect for her work!

   Early attempts to explain stellar evolution proposed that stars started 
off being very hot (O stars) and cooled off as they aged to become M stars. 
O stars were therefore referred to as 'early' and M stars as 'late', 
something that is still done today. For instance we refer to a B8 star as a 
'late B' and a B0 is an 'early B'. Astronomers are very slow to give up old 
habits!