To make an H-R diagram, you must look at many stars. But for each star, you need to measure only two quantities: luminosity and temperature. You can’t directly measure either quantity. But astronomers have developed several clever ways to find stellar luminosities and temperatures from quantities that we can directly observe.
Luminosity
The most difficult part of making an H-R diagram is finding the luminosities of stars. The animation to the right lets you control the luminosity of the sample star; drag the slider to the right to make the star more luminous.
Since we can’t go to stars, all we can know about them is what we can see from Earth. But from Earth, we don’t know how bright a star really is; we only know how bright it looks to us.
Two stars with the same luminosity at different distances will look different; a nearby star will look brighter than an identical star farther away. Astronomers measure the apparent brightnesses of stars with a number called apparent magnitude. In the magnitude scale, a lower number means a brighter object (see the Color project for more information). If two stars have the same luminosity, the star that is closer to us will appear brighter and have a lower apparent magnitude. A more distant star will appear dimmer and will have a higher apparent magnitude. Later in this project, you will learn how to calculate the distances to stars, and how to use these distances to find the stars’ luminosities.
Temperature
Astronomers have several ways to find the temperatures of stars, but the simplest way is to look at the stars’ colors. In astronomy, a star’s color is defined as the difference between its magnitudes as seen through two different filters – telescope attachments that block out all light except light with a specific wavelength. It doesn’t matter which two filters you use – you should calculate the same temperature (to learn how to calculate temperature from color, see the Color project). Traditionally, astronomers have taken images through blue, yellow, and red filters denoted by the letters b, v, and r. The animation at the right shows what a heating and cooling pan might look like through b and v filters.
If you subtract a star’s v magnitude from its b magnitude, you get a color called b-v. Stars with lower b-v colors have higher temperatures, so you can use b-v color to make an H-R diagram.
The SDSS does not use the traditional b, v, and r filters; instead, it uses five filters that see ultraviolet, green, red, and two wavelengths of infrared light. These five filters are denoted u, g, r, i, and z. To make your H-R diagram, use the green and red filters, which both fall in the visible part of the spectrum. From the magnitudes of stars in these filters, you can calculate the color g-r.