# Agenda Review magnitude scale and HR diagrams

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Agenda Review magnitude scale and HR diagrams
Another way to measure distance: Spectroscopic Parallax Please pick up essays after class Stars are born from H gas (nebula) and spend 90% of their lives on the main sequence. After becoming a red giant, the fate of a star depends on its mass.

Apparent Magnitude scale
Sun Naked eye Polaris Faintest object With binoculars Venus Sirius Full moon 4 m telescope

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)

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

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 15 5 10 -10 -5

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

Given two stars, which one looks brighter in the night sky?
Star A Star B Look the same Cannot conclude -10 -5 B Absolute magnitude 5 10 A Let them talk to their neighbor --- convince your neighbor and try again! 15 O B A F G K M Spectral Class

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

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.

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

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.

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.

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

Spectroscopic Parallax
Activity #7 Spectroscopic Parallax