Presentation on theme: "Agenda Review magnitude scale and HR diagrams"— Presentation transcript:
1 Agenda Review magnitude scale and HR diagrams Another way to measure distance: Spectroscopic ParallaxPlease pick up essays after classStars 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.
2 Apparent Magnitude scale SunNakedeyePolarisFaintest objectWith binocularsVenusSiriusFull moon4 m telescope
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 CandlesThe Sun has an apparent magnitude of –26. What might its absolute magnitude be?–26–234.85–33Cannot determine
5 Recall from last time… We know temperature O B A F G K MWe can determine luminosity (if we know distance)Temperature and luminosity are related!main sequence15510-10-5
6 The HR Diagram!! Bright Dim Main Sequence Stars Red Giant Stars 15510-10-5Spectral ClassHOTCOOLBrightDimSunMain Sequence StarsWhiteDwarf StarsRedGiant StarsAbsolute magnitudeO B A F G K M
7 Given two stars, which one looks brighter in the night sky? Star AStar BLook the sameCannotconclude-10-5BAbsolute magnitude510ALet them talk to their neighbor --- convince your neighbor and try again!15O B A F G K MSpectral Class
8 Given two stars, which is larger (more surface area)? Star AStar BLook the sameCannotconclude-10-5BAbsolute magnitude510A15O B A F G K MSpectral Class
9 Spectroscopic “parallax” Another way to measure distanceIt’s not parallax!How does it work?Measure star’s spectral typeGet luminosity (absolute magnitude) from spectral type (how?)*Use the “standard candle” method to get distance.
10 *Temperature => Luminosity 15510-10-5Spectral ClassHOTCOOLBrightDimSunMain Sequence StarsWhiteDwarf StarsRedGiant StarsAbsolute magnitudeLuminosityO B A F G K MSpectral typeWorks 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 laboratoryCan only observe what happens to be availableCan’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 Star Clusters — and how they hint at Stellar Life Cycles Next timeStar Clusters — and how they hint at Stellar Life Cycles