CLEA - Classification of Stellar Spectra
How do we measure the distance to stars that are too far away to use trigonometric parallax? Also, how do we classify stars according to their spectra?
Objective
 In this experiment, you will use CLEA software to identify the class of a star and measure the brightness of a star. You will then find the luminosity of the star (by using a H-R diagram or simply a table of luminosities for a given class of stars). And finally, knowing the brightness and luminosity, you will calculate the distance to the star.
Files
Background
For nearby stars, we use trigonometric parallax to determine their distance. This method is very precise for measuring stellar distances, but only works for nearby stars. In fact, this method only lets us measure distances to less than 1% of the stars in our own galaxy. As a result, we need another method to measure distances to stars.
By studying the 118,000 nearest stars and plotting luminosity vs. temperature (i.e. the H-R diagram), astronomers found a relationship between luminosity and temperature for main sequence stars.
Now, assuming that these stars are typical of all main sequence stars in the universe, then we can use the H-R diagram to get distance to a main sequence star. This method is called spectroscopic parallax.
Spectroscopic parallax is the method used to get distance to stars that are too far away to use trigonometric parallax.
To summarize, the method is as follows:
- measure the apparent brightness of a star
- collect the absorption spectrum of a star
- compare it to standard spectra to determine the class of the star
- knowing the class of the star gives you the temperature of the star
- use the H-R diagram to determine the luminosity of a main sequence star at the given temperature
- knowing apparent brightness and luminosity, calculate distance using the inverse square law
Procedure
We will use the CLEA program called Classification of Stellar Spectra. You should have already installed it.
1. Download two files: (1) the handout and (2) a spreadsheet for doing the calculations.
2. Watch the following instructional videos that will help you understand how to use the software.
 CLEA: Classification of Stellar Spectra -- Part 1. This part shows you how to determine the class of a star.
CLEA: Classification of Stellar Spectra -- Part 2. This part shows you how to determine the distance to a star.
3. Make the measurements described in the handout and enter your data in the spreadsheet. The green boxes are where you type your data and the purple box is a calculation.
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