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Temperature and Color, Classifying Stars. Spectrum of lightHow the eye sees colorTemperature and color/spectrumColors/spectra of starsClassifying starsReading: sections: 16.5-16.6, 6.2. Electromagnetic spectrum. The
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1. Announcements Observing on the roof of Van Allen Hall has started and will run Tuesday to Thursday from 9-11 pm this week.
The first hour exam will be on Friday, September 17.
3. Electromagnetic spectrum
4. How your eye sees light and color
5. Rods and cones on the retina sense light
6. Rods and cones Cones are color sensors
There are cones for red, green, and blue
The color ones perceives depends on the firing rates of the red vs. green vs. blue cones
Cones need relatively bright light to work
Rods give finer, more detailed vision
Rods can work with less light
At night, color vision is less effective because only the rods function
7. Sensitivity of cones
8. A star will produce light overlapping the response of all three cones. The color of the star depends on how strong its spectrum is in the ranges covered by the different cones.
9. A star will produce light overlapping the response of all three cones. The color of the star depends on how strong its spectrum is in the ranges covered by the different cones.
10. A star will produce light overlapping the response of all three cones. The color of the star depends on how strong its spectrum is in the ranges covered by the different cones.
11. What can we learn from a stars color? The color indicates the temperature of the surface of the star.
The same is true for the filament in a light bulb or any other hot object. In general, we call radiation from a hot body `black body radiation (do demonstration 6B40.10).
12. Wiens law Cooler objects produce radiation which peaks at longer wavelengths (redder colors), hotter objects produce radiation which peaks at shorter wavelengths (bluer colors).
13. A stars color depends on its surface temperature
14. Stars are assigned a `spectral type based on their spectra The spectral classification essentially sorts stars according to their surface temperature
The spectral classification also uses spectral lines, which will discuss on Friday
15. Spectral type Sequence is: O B A F G K M
O type is hottest (~25,000K), M type is coolest (~2500K)
Star Colors: O blue to M red
Sequence subdivided by attaching one numerical digit, for example: F0, F1, F2, F3 F9 where F1 is hotter than F3 . Sequence is O O9, B0, B1, , B9, A0, A1, A9, F0,
Useful mnemonics to remember OBAFGKM:
Our Best Astronomers Feel Good Knowing More
Oh Boy, An F Grade Kills Me
(Traditional) Oh, Be a Fine Girl (or Guy), Kiss Me
16. The spectrum of a star is most determined by The temperature of the stars surface
The stars distance from Earth
The density of the stars core
The luminosity of the star
17. Classifying stars We now have two properties of stars that we can measure:
Luminosity
Color/surface temperature
Using these two characteristics has proved extraordinarily effective in understanding the properties of stars the Hertzsprung-Russell (HR) diagram
18. HR diagram
19. HR diagram Originally, the HR diagram was made by plotting absolute magnitude versus spectral type
Now, its better to think of the HR diagram in terms of physical quantities: luminosity and surface temperature
20. If we plot lots of stars on the HR diagram, they fall into groups
21. These groups indicate types of stars, or stages in the evolution of stars
22. Luminosity classes Class Ia,b : Supergiant
Class II: Bright giant
Class III: Giant
Class IV: Sub-giant
Class V: Dwarf
The Sun is a G2 V star
