Lesson - Photometry

Astronomers use the light they see from the sky to get information about the stars we look at. One of the ways we do this is called photometry. Basically, what we do is collect light at different wavelengths, and compare how much blue light we get to how much red light we get. This is then compared to a model of the light that would come from a hot object, and the temperature of the object can be determined. If we know the distance to the object, we can determine the size, and if we know the size of the object, we can determine the distance.

Astronomers find out how much light they get at certain wavelengths by filtering out the light at other wavelengths. These filters are called band pass filters, or sometimes just bands or filters for short.

Some typical bands that get used in astronomy are given in the following table. The band name and the primary (or peak) wavelength of the band are given. Each band does not just contain the peak wavelength, but also allows light from a range of wavelengths around the peak wavelength.

The Blackbody Radiation Data Fitting Tool lets you enter the magnitude of stars in the UBVRIJHK bands. The magnitudes are converted to fluxes, and plotted as crosshairs. Also plotted is the expected flux of a pure blackbody. You can change the temperature, distance to, and size of the blackbody source. A common measurement in astronomy is the difference between the magnitudes in the B and V bands. B-V for the blackbody is calculated for you, and displayed.

Data on many astronomical objects can be found at the Simbad Astronomical Database.

1. What are B and V for the star Betelgeuse in the constellation Orion?
2. What are B and V for the star Rigel in the constellation Orion?
3. What are the temperatures of Betelgeuse and Orion?

If we know the distance to a star, we can use photometry to tell use the size of the star. For nearby stars, we can measure the parallax motion of the star. The distance to the star in parsecs is equal to 1 over the parallax in arcseconds. Stars which are very far away don't appear to have any parallax at all, so this only works for nearby stars. This technique of finding the distance to a star is called stellar parallax.

5. Get the parallax for Betelgeuse and Orion from the SIMBAD database. How far away is each star?
6. What is the size of Betelgeuse? Of Rigel?