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Star Properties. Apparent Magnitude System of Hipparchus Group of brightest stars1 m Stars about ½ as bright as 1 m 2 m Stars about ½ as bright as 2 m.

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Presentation on theme: "Star Properties. Apparent Magnitude System of Hipparchus Group of brightest stars1 m Stars about ½ as bright as 1 m 2 m Stars about ½ as bright as 2 m."— Presentation transcript:

1 Star Properties

2 Apparent Magnitude System of Hipparchus Group of brightest stars1 m Stars about ½ as bright as 1 m 2 m Stars about ½ as bright as 2 m 3 m Naked Eye Limit6 m

3 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  m of 5  100X in light  m of 1  2.512X in light

4 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

5 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

6 Measuring Distance Stellar Parallax June January Sun

7 Stellar Parallax June January Sun 1 AU Parallax

8 Measuring Parallax 1 AU 1 parsec 1 arcsec

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

10 Stellar Parallax Nearest star to Sun (largest parallax)  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)

11 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.

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

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14 Luminosity Classes ISupergiants IIBright Giants IIIGiants IVSubgiants VDwarfs

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

17 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  radius 2 Capella must be 10X larger than Sun.

18 Luminosity Class in the Spectrum A3 Supergiant A3 Giant A3 Dwarf

19 SunG2V VegaA1V BetelgeuseM1I

20 Which of these stars is hottest? 1.SunG2V 2.Vega A1V 3.Betelgeuse M1I 4.Can’t compare

21 Which of these stars is brightest? 1.SunG2V 2.Vega A1V 3.Betelgeuse M1I 4.Can’t compare

22 Which of these stars is smallest? 1.SunG2V 2.Vega A1V 3.Betelgeuse M1I 4.Can’t compare

23 Which of these stars is most distant? 1.SunG2V 2.Vega A1V 3.Betelgeuse M1I 4.Can’t compare

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

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

26 Color Index Star Temperature m B m V. 1 12000 K 2.0 2.4 2 7000 K 3.0 3.1 Color Index = m B - m V = B-V 1 B-V = 2.0 - 2.4 = -0.4 2 B-V = 3.0 - 3.1 = -0.1

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

28 Study Tools Review 1 Review 2

29 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

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

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

32 Structure of the Milky Way

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

34 Open clusters M16 M45 M37

35 Color-Magnitude Diagram M45

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

37 Globular Clusters M5M3 SFA Observatory

38 Color-Magnitude Diagram M3


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