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Wienís law

Wienís law

Cooler objects produce radiation which peaks at longer wavelengths (redder colors), hotter objects produce radiation which peaks at shorter wavelengths (bluer colors).

Slide 13

A starís color depends on its surface temperature

A starís color depends on its surface temperature

Spectrum demonstration 6B40.55

Slide 14

Stars are assigned a `spectral typeí based on their spectra

Stars are assigned a `spectral typeí 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

Slide 15

Spectral type

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 Ö F9 where F1 is hotter than F3 . Sequence is O Ö O9, B0, B1, Ö, B9, A0, A1, Ö A9, F0, Ö

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

Slide 16

The spectrum of a star is most determined by

The spectrum of a star is most determined by

The temperature of the starís surface

The starís distance from Earth

The density of the starís core

The luminosity of the star

Slide 17

Classifying stars

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 Ė the Hertzsprung-Russell (HR) diagram

Slide 18

HR diagram

HR diagram

Slide 19

HR diagram

HR diagram

Originally, the HR diagram was made by plotting absolute magnitude versus spectral type

Now, itís better to think of the HR diagram in terms of physical quantities: luminosity and surface temperature

Slide 20

If we plot lots of stars on the HR diagram, they fall into groups

If we plot lots of stars on the HR diagram, they fall into groups

Slide 21

These groups indicate types of stars, or stages in the evolution of stars

These groups indicate types of stars, or stages in the evolution of stars

Slide 22

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