Properties of Stars

Unit 5: Stars

How do we Classify Stars?

1. Size

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• Temperature and Color

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• Luminosity

Stars 101

Size

• Giant Stars: old, large stars – red giants, blue giants, and super giants
• Main Sequence Stars: young stars that make up 90% of all stars – the sun is a main sequence star – dwarfs, yellow dwarfs, and red dwarfs
• Faint, Virtually Dead Stars: small stars near the end of their lives – white dwarfs, brown dwarfs, neutrons, and pulsars

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Size

Temperature and Color

• The color of the star is dependent on the temperature of the star
• Blue – hottest stars
• Red – coolest stars
• Order from hottest to coolest
• Blue, white, yellow, red-orange, red

35,000K 6,000K 3,500K

The letter is the spectral class. We’ll get back to it later – along with more spectroscopy!

Stellar Luminosity

Brightness = Magnitude = Luminosity

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but the stars don’t shine, they burn…

Stellar Luminosity

• Apparent Magnitude – a star’s brightness as it appears to Earth
• Three factors control the apparent brightness of a star as seen from Earth: how big it is, how hot it is, and how far away it is
• Higher magnitude = dimmer star

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Stellar Luminosity

• Absolute Magnitude – how bright a star actually is
• To determine absolute brightness, astronomers measure how large the star is, what temperature it is, and what its apparent brightness would be at 32.5 light-years

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Absolute Magnitude ≠ Apparent Magnitude

Absolute vs Apparent Magnitude