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Stars … how I wonder what you are. 7 Goals • Stars are Suns. • Are they: – – – – – Near? Far? Brighter? Dimmer? Hotter? Cooler? Heavier? Lighter? Larger? Smaller? • What categories can we place them in? 7 Distance • One proof of a heliocentric Universe is stellar parallax. – Tycho Brahe saw no parallax of stars. – Copernicus thought stars must be too far away. • Nearest star: Proxima Centauri Parallax angle = 0.76 arcsec – Tycho’s precision = 1 arcmin 7 The Parsec • What is the distance of an object with a parallax angle of 1 arcsec? Distance = 206,265 AU • Call this distance 1 parsec (pc) 1 pc = 206,265 AU =3.3 lightyears • 1 lightyear = distance light travels in one year. 7 Distances 1 Distance (in parsecs) parallax (in arcsec) • Closest star: Proxima Centauri parallax = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears 7 Brightness • How bright are they really? • What is due to distance? • What is due to luminosity? • Luminosity: – Total energy radiated every second. 7 Magnitude Scale • The SMALLER the number the BRIGHTER the star! – Every difference of 1 magnitude = 2.5x brightness. – Every difference of 5 magnitudes is a 100x difference in brightness. 7 Star light, star bright • Sirius is magnitude –1.5 Polaris is magnitude 2.5 • Is Sirius really more luminous than Polaris? • No, Sirius is just closer. 7 Apparent and Absolute • Apparent Magnitude = the brightness (magnitude) of a star as seen from the Earth. m – Depends on star’s total energy radiated (Luminosity) and its distance • Absolute Magnitude = the brightness (magnitude) of a star at a distance of 10 pc. M – Only depends on a star’s luminosity distance m M 5log 10 10pc 7 example distance m M 5log 10 10pc • Our Sun: – m = -26.8, – distance = 4.8 x 10-6 pc So: M = 4.8 • Polaris: – m = 2.5, – distance = 132 pc So: M = -3.1 • Polaris is 1500 times more luminous than the Sun! 7 Stellar Temperatures How hot are stars? • In Lecture 3 we learned about thermal radiation and temperature. • Since different stars have different colors, different stars must be different temperatures. Stellar Spectra Hot Cool 7 Spectral Classifications 7 Orion Copyright – Tyler Nordgren 7 Stellar Masses How massive are stars? • Kepler’s Laws – devised for the planets. • Apply to any object that orbits another object. • Kepler’s Third Law relates: – Period: “how long it takes to orbit something” – Semimajor axis: “how far you are away from that something” – Mass: “how much gravity is pulling you around in orbit” 3 a P M 2 • Where M is the Total Mass. • Can calculate the mass of stars this way. 7 Binary Stars • Most stars in the sky are in multiple systems. • Binaries, triplets, quadruplets, etc…. – Sirius – Alcor and Mizar – Tatooine • The Sun is in the minority by being single. 7 Stellar Masses How massive are stars? 3 a P M 2 • Most stars have masses calculated this way. • Result: – The more massive the star, the more luminous it is. – The more massive the star, the hotter it is. 7 Stellar Radii How big are stars? • We see stars have different luminosities and different temperatures. • Stars have different sizes. • If you know: 50 mas – Distance – Angular size • Learn real size. 7 Atmospheric Seeing 7 Stars are small • Betelgeuse is the only star big enough to directly see its surface with a normal telescope. 7 Interferometry • Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope. NPOI - optical VLA - radio 7 Optical Interferometry • NPOI simulates a single optical telescope 65 meters in diameter. • Resolve stars as small as 1.5 mas! PTI - infrared 7 Angular versus Linear Supergiants, Giants and Dwarfs 7 H-R Diagram • Can order the stars we see by: – Temperature (or spectral type) – Luminosity (or absolute magnitude). • And see where other qualities fall: – Mass – Radius 7 7 The Main Sequence • “Stars are characterized by what holds them up.” • 90% held up by heat of Hydrogen fusion? 4H He + Energy 7 Main Sequence & Thermal Radiation • The Main Sequence makes sense! • Hotter stars are bluer – Wien’ Law • Hotter stars are brighter – Stefan’s Law 7 Homework #6 • For 10/9: • Read B15.6, 16.1 – 16.2, Ty10 • Do: Review Questions 1, 5, Problems 1, 2 7