Star Properties

Apparent Magnitude System of Hipparchus Group of brightest stars 1m Stars about ½ as bright as 1m 2m Stars about ½ as bright as 2m 3m • Naked Eye Limit 6m

Apparent Magnitude 19th century photographers learn how eye responds to light (Pogson) Doubling the brightness is not perceived as a doubling by the eye Eye response is logarithmic Ratio of 100 in brightness corresponds to a Difference of five magnitudes Dm of 5  100X in light Dm of 1  2.512X in light

Some Apparent Magnitudes Sun -26.8 Full Moon -12.6 Venus at brightest -4.4 Sirius -1.5 Naked Eye Limit 6.0 Faintest Objects +30.0 Hubble

Learning the Brightness Is a star bright... Because it really is a bright star? Because it is close to the Earth? Stellar brightness depends on Luminosity Distance

Measuring Distance Stellar Parallax June Sun January

Stellar Parallax June January Sun Parallax 1 AU

Measuring Parallax 1 arcsec 1 AU 1 parsec

Stellar Parallax When p is measured in arcsec and d is measured in parsecs One parsec: 206,265 AU 3.26 light years

Stellar Parallax Nearest star to Sun (largest parallax) a Cen p = 0.7 arcsec Limit of accurate parallax  200 pcs (angles of 0.005 arcsec) Hipparcos satellite (120,000 stars measured to 0.001 arcsec)

Absolute Magnitude The magnitude a star would have at 10 parsecs from the Sun. The apparent (m) and absolute (M) magnitudes of a star at 10 pcs are the same. M, m, and d are related. Knowing two allows you to compute the third.

Putting the Pieces into Place Ejnar Hertsprung 1911 Henry Norris Russell 1913

Luminosity Classes I Supergiants II Bright Giants III Giants IV Subgiants V Dwarfs

Luminosity Class implies Size Consider the Sun and Capella The Sun G2V M=5 Capella G2III M=0

Luminosity Class implies Size Equal sized pieces of each star are equally bright Capella is 100X brighter (5 magnitudes) Capella must have 100X as much area Surface area  radius2 Capella must be 10X larger than Sun.

Luminosity Class in the Spectrum Supergiant A3 Giant A3 Dwarf

Sun G2V Vega A1V Betelgeuse M1I

Which of these stars is hottest? Sun G2V Vega A1V Betelgeuse M1I Can’t compare

Which of these stars is brightest? Sun G2V Vega A1V Betelgeuse M1I Can’t compare

Which of these stars is smallest? Sun G2V Vega A1V Betelgeuse M1I Can’t compare

Which of these stars is most distant? Sun G2V Vega A1V Betelgeuse M1I Can’t compare

Spectroscopic Parallax Observe the spectrum and apparent magnitude of a star Classify the spectrum Main Sequence Plot it on the H-R Diagram Read off the M From m and M compute distance

Color Index 12000 K B V * * 7000 K * *

Color Index Star Temperature mB mV . 1 12000 K 2.0 2.4 Color Index = mB - mV = B-V 1 B-V = 2.0 - 2.4 = -0.4 2 B-V = 3.0 - 3.1 = -0.1

Spectroscopic Parallax Can now get distances to any object whose spectrum can be measured. Limit  5000 pcs

Study Tools Review 1 Review 2

The Advantage of Color Index Measures temperature just like Spectral Type Much easier to obtain requires two measurements of brightness spectral type requires getting the spectrum

Color-Magnitude Diagrams Standard H-R Diagram Color-Magnitude Diagram M Spectral Type mV B-V

Color-Magnitude Diagrams Useful for star clusters Can substitute mV for MV since you know all the stars are the same distance away. Star Clusters Open (galactic) Globular

Structure of the Milky Way

Open Clusters Irregular shape Few tens to few hundred stars In the plane of the galaxy Young stars

Open clusters M37 M16 M45

Color-Magnitude Diagram M45

Globular Clusters Spherical in shape Hundreds of thousands of stars Halo distribution about galactic nucleus Old stars

Globular Clusters SFA Observatory M5 M3

Color-Magnitude Diagram M3