Measuring Properties of Stars Important Slides marked with a

Measurable Properties of Stars Position Apparent Brightness Color Distance –many methods Velocity –Doppler Shift, proper motion Luminosity –Distance + Apparent Brightness Temperature –Color Composition –Spectral Lines Size (radius) –Stephan-Boltzmann Law Mass –Binary star orbit Age

Measuring Distance Parallax Angular Diameter Method Standard Candle Method

Measuring Distance - Parallax Parallax is the change in position of a star, relative to other stars, based on change in the observer’s position. One parsec (pc) = distance of an object with a parallax of one arc-second = 3.26 ly

Measuring Distance - Parallax Only good out to ~300 ly Or d (pc)=1/p(arcsec)

Measuring Distance - Angular Diameter Method

Angular diameter is easy to measure D (distance) = L (real size) x A (angular diameter, in radians) If you somehow know real size, you can calculate distance Does not work for stars (too small) Humans judge long distances with this method

Measuring Distance - Standard Candle Method Apparent Brightness –How bright something looks from the Earth –Easily measured –Measured as Apparent Magnitude Luminosity –How bright something really is –Units of power (energy/sec emitted, or watts) –Also called Absolute Magnitude (App Mag at 10pc)

Measuring Distance - Standard Candle Method Distance relates Apparent Brightness and Luminosity Inverse-square Law shows this “Apparent Brightness-Luminosity- Distance” –Any two yield the third

You should review pages “Explorations, Third Edition by Arny – Class Text” “Apparent Magnitude” was defined by Hipparachus in 150 BC. He devised a magnitude scale based on: Magnitude ConstellationStar 1 (Orion) Betelgeuse 2 Big Dipper various 6 stars just barely seen However, he underestimated the magnitudes. Therefore, many very bright stars today have negative magnitudes. Magnitude Difference is based on the idea that the difference between the magnitude of a first magnitude star to a 6 th magnitude star is a factor of 100. Thus a 1 st mag star is 100 times brighter than a 6 th mag star. This represents a range of 5 so that = the fifth root of 100. Thus the table hierarchy is the following. Magnitude Difference of 1 is 2.512:1, 2 is :1 or 6.31:1, 3 is = 15.85:1 etc. (2) Astronomy Magazine Sept issue defines the faintest naked eye star at 6.5 apparent magnitude. Absolute Magnitude is defined as how bright a star would appear if it were of certain apparent magnitude but only 10 parsecs distance.

Images courtesy of Nick Strobel's Astronomy Notes. Go to his site at for the updated and corrected version.Nick Strobel's Astronomy Noteswww.astronomynotes.com

Images courtesy of Nick Strobel's Astronomy Notes. Go to his site at for the updated and corrected version.Nick Strobel's Astronomy Noteswww.astronomynotes.com

Measuring Distance - Standard Candle Method Standard Candle –An object with a luminosity that can be found without knowing its distance We will cover many types over the semester Examples: 75W light bulb, second brightest galaxy in a cluster, main sequence star spectral type, Cepheid Variables

If luminosity and temperature are known, we can find radius F =  T 4

Spectral Types Around 1901 Annie Jump Cannon finished Henry Draper’s work categorizing stars by the strength of their Hydrogen lines Categories: A,B,C,D,…,M,N,O,P A-> strong lines, P-> weak/no lines No physical basis for these categories (“stamp collecting”)

Spectral Types Around 1920 M. Saha realized temperature determines strength of lines –All stars made of Hydrogen –Spectral Types -> temperature Cecilia Payne made the modern system –Spectral Sequence OBAFGKM –Hottest to coolest –Letters still represent strength of H-lines

Other Properties Composition –Spectral lines Velocity –Doppler-shifts and proper motion Mass –Weigh the star with a form of Kepler’s 3 rd Law

Summary of Spectral Lines

Putting it Together - The Herztsprung-Russell Diagram Measure surface temperature from color or spectral-type Measure distance from parallax (or other method) Calculate luminosity from apparent brightness and distance Plot luminosity vs. surface temperature

Radius From Stephan-Boltzmann Law Shows three groups of stars.

Luminosity=Mass 3 This was empirical and is now supported by computer models. Masses come from binary stars

Herztsprung-Russell Diagram Plot Luminosity vs. Temperature Showed three classes of stars –Main Sequence, Red Giants, White Dwarfs For MS stars Temperature -> Luminosity –MS stars are ‘standard candles’ Binaries show variations between Main Sequence stars are due to mass

Binary Systems At least 70% of all stars are binaries or multiple-star systems Optical Doubles: Not actual binaries Visual binaries: Two stars seen as binaries Spectroscopic binaries –Two sets of spectral lines seen, or –One set of Doppler-shifting lines Eclipsing binaries: Stars eclipse each other

Algol - An Eclipsing Binary This blue ‘star’ turns red every ~3 days Arabic: Algol –‘Demon’s Head’ Chinese: Tseih-She –‘Piled-up corpses’ Hebrew: Rosh he Satan –“Satan’s Head” Hebrew: Lillith –Adam’s first, demonic wife