Star Brightness and the H-R Diagram

1. Star Brightnessand the H-R Diagram

2. A Star’s perceived brightness (how we see it) depends on three things: • 1) Temperature of the Star • 2) Distance from the Earth • 3) Star Magnitude A Star’s Absolute Magnitude is the star’s magnitude if it were to be found at a distance of 32.6 Light Years from our Sun. A Star’s Apparent Magnitude is the brightness of a star as it is observed by the naked eye or through a basic telescope.

3. When comparing two stars at the same distance from Earth, the hotter star will appear brighter. 6000˚C Appears Brighter 10 000 ˚C

4. If comparing two stars at different distances from Earth but of the same size and temperature, then the star that is closest to the Earth will appear brighter. 6000˚C Closer Star Appears Brighter! 6000˚C

5. H-R Diagram • This is a diagram that plots the temperature of stars against absolute magnitude • The main sequence of an H-R Diagram shows that most stars are brighter as they are hotter in temperature. Exceptions to this rule are Red Giants and Supergiants which appear very bright because of their size, even though they may be cooler in temperature compared to smaller, very hot, “blue” stars.

6. The H-R Diagram http://aspire.cosmic-ray.org/labs/star_life/hr_interactive.html

7. The Colour Spectrum • Although stars appear to be one colour (yellow, red, blue, etc.), they radiate all the colours of the colour spectrum (from ultra-violet to red). • They appear on colour because they tend to radiate one of the colours of the spectrum more than the rest.