Astronomy 113. Dr. Joseph E. Pesce, Ph.D. Dr. Joseph E. Pesce, Ph.D.

Transcription

1 Astronomy 113 Dr. Joseph E. Pesce, Ph.D.

2 The Nature of Stars

3 8-2 Parallax For nearby stars - measure distances with parallax July 1 AU d p A A A January ³ d = 1/p (arcsec) [pc] ³ 1pc when p=1arcsec; 1pc=206,265AU=3 x km ³ P = VERY small!! (<<1 ) ³ Using satellites, can measure start to d~1,000pc away

4 8-3 Brightness Apparent Magnitude (log scale - the eye s response) (naked-eye limit) Bright Faint 1 magnitude = 2.5 times real brightness (so 1 to 6 is 100 times difference: = 100) Magnitudes can be negative too. So, Sun = -25 Moon = -12 Sirius = 0 Faintest objects = +30 (with HST, for example)

5 8-4 Magnitudes ³ Apparent magnitude (m) is not real brightness ³ Apparent brightness decreases inversely with square of distance - the inverse square law : Double distance apparent brightness decreases (1/2) 2 =1/4 Triple distance (1/3) 2 =1/9 Need to be able to compare real brightness, so correct for distance Absolute magnitude (M) = apparent magnitude an object would have if it were at 10 pc Sun = +4.8 (range for stars: -10 to +17) Measure m and d, then: M = m - 5log(d/10)

6 8-5 Inverse-square Law

7 8-6 Luminosity ³ Absolute magnitude is related to Luminosity (the physical brightness of an object) L =sat 4 = s 4p r 2 T 4 (r = Radius; s is a constant) ³ Solar luminosity = 3.9 x Watts ³ Call this 1 L Stars with: M = -10 have 10 6 L M = +17 have 10-5 L

8 8-7 Stellar Temperatures ³ Remember Wien s law? (l max = 1/T; color related to surface tempature) Use a photometer to measure light intensity and filters to measure intensity at different bands : Intensity U B V Wavelength U (UV), B (blue), V (visual) That is, 3 apparent magnitudes which tell us where most energy in spectrum is (B-V = color index; if small, object is blue, if large, it is red) From blackbody curves, get T vs. B-V, and thus a surface temperature At different temperatures, different elements produce different emission lines - can measure temperature this way too

9 8-8 Spectral Types ³ A star s surface temperature determined from color index or spectral line strengths ³ In the 1920s, Cecilia Payne classified stars based on spectral features visible (and ordered them by surface temperature) ³ Spectral types: Hottest T~35,000 K O B A F G K M HeII (singly ionized) SiIV (triply ionized) Sun = G2 Coolest T~3,000 K Molecules (TiO) Further subdivided: B0 - B9 Hottest Coolest

10 8-9 Spectral Types

11 Types of Stars

12 9-2 The Hertzsprung-Russell Diagram Hot 40,000K Cool 2,500K Supergiants Bright Giants Luminosity Absolute Magnitude Sun Main Sequence +10 Faint O0 B0 A0 F0 G0 K0 M0 Surface Temperature White Dwarfs

13 9-3 The Hertzsprung-Russell Diagram ³ Pattern when color-index is plotted against absolute magnitude ² Also called Color-Magnitude diagram Surface Temperature and Absolute Magnitude are Related! ³ Main Sequence: 90% of stars in Solar neighborhood are on Main Sequence (called dwarf stars) ² M-type stars most common ² O-type stars rarest ² Most M.S. stars are like the Sun

14 9-4 The Hertzsprung-Russell Diagram

15 9-5 Giants ³ From Stefan-Boltzmann Law: E = T 4 ³ Cool objects radiate less E than hot (per unit surface area) ³ So, for Giants to be so bright, they must be huge! T ~ 3,000-6,000K R ~ x Solar Radius Red Example: Arcturus

16 9-6 Supergiants ³Even bigger and brighter than Giants ³Example: Betelgeuse ³1% of all stars in Solar neighborhood 9% of stars in Solar neighborhood are white dwarfs - more later

17 9-7 Binary Stars ³ Two stars gravitationally bound ³ Orbital motion of binaries shifts spectral lines (Doppler Shift) Approaching Radial Velocity Curve Radial Velocity 0 Time Receding

18 9-8 Eclipsing Binaries ³ We see stars along their orbital plane ³ Causes effects in light curve: Intensity Time to cross disk Time ³ Total eclipses allow us to measure radii of stars

19 9-9 Stellar Masses ³ How do you measure mass? ³ Newton s adaption of Kepler s Law: M 1 + M 2 = a 3 / p 2 (both measured in binaries) Mass-Luminosity Relationship: On the Main Sequence, the more massive a star, the more luminous L/L Mass 0.1

20 9-10 Contact Binaries ³ Roche Lobe - Sphere of gravitational influence Detached Binaries Semi-detached Binaries Contact Binaries

21 Stellar Motion ³ Proper Motion: True motion on the plane of the sky ³ Radial Motion: 3-D motion along the line-of-sight

22 Thank You!