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How do you find the distance to a star using the method of spectroscopic
> parallax?
Spectroscopic parallax is a term that I found confusing, although the idea
is simple. My confusion arose from the use of the word
"parallax," although parallax (the movement of a star in the sky due to
Earth's orbit around the sun) is only distantly related to this topic. It
would be better called "spectroscopic distance measurements."
The concept is this. Stars can be classified by means of a "spectral
type," such as "08V," "G2V," "M3III," etc. The letter and number
indicate the "color" of the star (really they indicate its surface
temperature, which in turn determines the color), and the Roman numeral
tells us how big the star is (V indicates smaller stars, I indicates
supergiants). [This is an over-simplification, but close enough unless
you are an astronomer.]
Stars of the same spectral type are thought to all have the same
luminosity, or energy output. For instance, the sun is a G2V star. Every
other G2V star in the galaxy should then have the same luminosity as the
sun. This is a lot like lightbulbs. Every bulb marked "60W" has the same
luminosity, and a bulb marked "120W" will have a different luminosity than
the 60 Watt bulbs.
How bright a star appears in our sky (its apparent magnitude) depends on
two things: the star's luminosity (how bright it is) and its distance from
us. You probably know that a 60 Watt light bulb as viewed from 10 miles
away is a lot fainter than a 60 Watt bulb at the end of your nose (unless
your nose is 10 miles long, of course!).
Spectroscopic parallax is simply looking at how bright a star appears to
be and, by knowing its luminosity, determining its distance. So let's
suppose that you look in the sky and see three A5IV stars. One has an
apparent magnitude of 4, one has an apparent magnitude of 6, and the other
has an apparent magnitude of 8. We know that each of these stars is
putting out the same amount of energy because they all have the same
spectral type. So the faintest star (m=8) must be the most distant, and
the m=4 star must be the closest.
If you know a star's absolute magnitude, you can calculate the distance
exactly. Hopefully your teacher or your text can explain this to you --
it requires a math equation, and email is horrible for explaining math.
Thanks for writing, and I hope this clears things up for you.
Sincerely,
Kurtis Williams
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