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Agenda

Agenda. Review magnitude scale and HR diagrams Another way to measure distance: Spectroscopic Parallax Please pick up essays after class. Faintest object 4 m telescope. Faintest object With binoculars. Sirius. Full moon. Venus. Naked eye. Sun.

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Agenda

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  1. Agenda • Review magnitude scale and HR diagrams • Another way to measure distance: Spectroscopic Parallax • Please pick up essays after class

  2. Faintest object 4 m telescope Faintest object With binoculars Sirius Full moon Venus Naked eye Sun -25 -20 -15 -10 -5 0 5 10 15 20 25 Polaris Apparent Magnitude scale

  3. Absolute magnitude • Luminosity = how much light a star is giving off • (like wattage for light bulbs) • Compared to the Sun (Lsun) • Absolute magnitude = how bright (what magnitude) a star would appear at 10 parsecs (32.6 light years)

  4. Standard Candles The Sun has an apparent magnitude of –26. What might its absolute magnitude be? • –26 • –23 • 4.85 • –33 • Cannot determine

  5. -10 -5 0 5 10 15 Recall from last time… • We know temperature • O B A F G K M • We can determine luminosity (if we know distance) • Temperature and luminosity are related! • main sequence

  6. -10 Bright Main Sequence Stars -5 Red Giant Stars 0 5 Sun White Dwarf Stars 10 Dim 15 Spectral Class HOT COOL The HR Diagram!! Absolute magnitude 0123456789 O B A F G K M

  7. Given two stars, which one looks brighter in the night sky? • Star A • Star B • Look the same • Cannot conclude -10 -5 0 B Absolute magnitude 5 10 A 15 O B A F G K M Spectral Class

  8. Given two stars, which is larger (more surface area)? Star A Star B Look the same Cannot conclude -10 -5 0 B Absolute magnitude 5 10 A 15 O B A F G K M Spectral Class

  9. Spectroscopic “parallax” • Another way to measure distance • It’s not parallax! • How does it work? • Measure star’s spectral type • Get luminosity (absolute magnitude) from spectral type (how?)* • Use the “standard candle” method to get distance.

  10. *Temperature => Luminosity -10 Bright Main Sequence Stars -5 Red Giant Stars 0 5 Sun White Dwarf Stars 10 Dim 15 Spectral Class HOT COOL Absolute magnitude Luminosity 0123456789 O B A F G K M Spectral type Works only for main sequence!

  11. Summary • We can measure apparent magnitude and spectral type. • The main sequence lets us infer absolute magnitude from spectral type. • Comparing apparent magnitude to absolute magnitude gives us distance.

  12. Something to ponder... Astronomy, as an observational science, has limitations: Can’t build stars in our laboratory Can only observe what happens to be available Can’t go back in time to watch stars form, or go forward in time to see them die. How, then, can we learn about the life cycles of stars? Sample stars that exist now in various stages of development.

  13. Next time Star Clusters — and how they hint at Stellar Life Cycles

  14. Activity #7 Spectroscopic Parallax

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