On the cosmic scheme of things we are nothing but an insignificant bump in orbit around an ordinary star located in the suburbs of a typical galaxy, but how do we know this? And if we are so ordinary, what are the extremes like?
Temperature
A close approximation of a star's temperature can be found by just looking at
its color, but can you identify the color of the Sun? Most people would identify
the Sun as yellow, but it is in reality a white star. This misrepresentation most
likely comes from the Sun being colored yellow in popular media and entertainment.
The color of stars range from red/orange being the coolest, yellow in the middle,
and white/blue the hottest.
Looking at a star to determine temperature is not a very scientific way to go
about things, so scientists turn to something called a spectrum. A spectrum is
created when a star's light is passed through a prism and seperated into it's
different colors (Mother Nature calls this process a rainbow.). Every chemical
element present in a star produces a line in the spectrum and by studying these
lines scientists can determine a star's temperature and what the star is made of.
Once a star's temperature is precisly determined, it is given a letter identification
from one of the following : O B A F G K M with O being the hottest and M the
"coolest" (Our Sun is type G). Below is the list of letters corresponding to their
temperature range :
| Spectral Letter | Temperature (F) | Temperature (C) |
|---|---|---|
| O | more than 37,000 | more than 20,500 |
| B | 17,000 - 37,000 | 9,430 - 20,500 |
| A | 12,500 - 17,000 | 6,930 - 9,430 |
| F | 10,300 - 12,500 | 5,700 - 6,930 |
| G | 8,000 - 10,300 | 4,400 - 5,700 |
| K | 5,500 - 8,000 | 3,040 - 4,400 |
| M | less than 5,500 | less than 3,040 |
Our Sun is type G so it must be between 8,000 and 10,300 degrees F, which is correct. It is actually close to 10,000 which places it near the middle of the spectral classes. Now that we have proven the Sun is ordinary as far as temperature goes, what about its brightness?
Brightness
Anyone can tell that the star Sirius appears brighter than Rigel in Orion, but did you know that Rigel is thousands of times more luminous than Sirius? Everyone knows that stars are at different distances, therefore because Sirius is so much closer to us than Rigel, it appears brighter. Scientists needed a level playing field in order to compare the "absolute" brightness of many stars, so they decided to determine what a star's magnitude would be if it were placed 32.6 light-years (10 parsecs) away from Earth. After placing the stars at this distance we find a range from -8 to +15 magnitudes with Sirius being at +2, Rigel equal to -7 and the Sun at +5.
Once again we have shown the Sun to be near the middle of the chart. We have now discovered how to determine temperature and luminosity, but is there a way to combine the two? and are there any other variables among stars?
H-R Diagram
The Hertzsprung-Russell Diagram is named after the astronomers Einar Hertzsprung and Henry Russell, who in the 1910's began to see a relationship between a star's temperature and brightness. The H-R Diagram is simply a graph of stars with absolute magnitude plotted on the y-axis and temperature on the x-axis. After plotting thousands of stars on the H-R Diagram, scientists began to see four main groups of stars, which are : Dwarfs, Main Sequence, Giants, and Super-Giants.
Now that we understand how to classify stars, let us take a look at each group individually :
Main Sequence
As you can see, main sequence stars cover a large area of the H-R Diagram, from bright, hot stars to dim, cool ones. So what sets them apart? Equilibrium. These stars are almost identical to the state that they were born in. They neither expand nor contract great amounts, and will die fairly non-explosive deaths. Famous examples are our Sun, Sirius, Spica, and Vega.
Giants and Super-Giants
As the Sun, and others in the main sequence, grow older they begin to expand. While expanding, more light is given off and the Sun will climb the H-R diagram until it reaches giant status. (Don't worry, it will not be for at least five million years.) Stars that start off much more massive than the Sun will be able to expand that much more and reach super-giant status. Famous examples are Betelgeuse, Polaris, and Rigel.
Dwarfs
Dwarfs are stars at the end of their life-cycle. It is impossible to keep up
giant status for very long without running out of fuel. As a star begins to die,
the remaining stellar mass gets blown into space, called a nova (sometimes
violently, i.e. supernova). After the material has been ejected, all that remains is
a small, very-dense dwarf star. If the star started off heavy enough and the right
conditions existed, the star might become a black hole.
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