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Wien’s law

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

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A star’s color depends on its surface temperature

Spectrum demonstration 6B40.55

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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

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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

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The spectrum of a star is most determined by

The temperature of the star’s surface

The star’s distance from Earth

The density of the star’s core

The luminosity of the star

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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

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HR diagram

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HR diagram

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

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

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

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

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