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Stars … how I wonder what you are. 7B Goals Tie together some topics from earlier in the semester to learn about stars: • How do we know how far away stars are? • How do we know how bright they really are? • What are they like? – Temperature – Radius – Mass • What categories can we place them in? 7B Distances • • • • How do we perceive distances here on Earth? How do we know A is closer than B? Can we apply these to objects in space? Can we apply these to objects beyond the solar system? • How do we know how far away the stars are? 7B Stellar Parallax • Recall from Lecture 1B: • One proof of a heliocentric Universe is stellar parallax. • Copernicus thought stars must be too far away. • Nearest star: Proxima Centauri Parallax angle = 0.76 arcsec • Recall: Tycho’s precision = 1 arcmin 7B The Parsec • Triangles: tan P = opposite/adjacent • For small angles: tan P = P P = (1 AU)/Distance Distance = (1 AU)/P • What is the distance of an object with P = 1 arcsec? Distance = 206,265 AU • Call this distance 1 parsec (pc) • 1 pc = 206,265 AU = 3.3 lightyears 7B Distances 1 Distance (in parsecs) parallax (in arcsec) • • • • 1 parsec = distance with a parallax of 1 arcsecond. 1 lightyear = distance light travels in one year. 1 pc = 206,265 AU = 3.3 lightyears Closest star: Proxima Centauri P = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears 7B How far is this? The Sun Hawaii Alpha Centauri New York 7B The Solar Neighborhood 7B Star light, star bright • In Lab 1 we talked about stellar magnitudes. • Vega is magnitude 0 Polaris is magnitude 2.5 • While Vega is brighter than Polaris, Vega is also a lot closer to us. 7B 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 7B example distance m M 5log 10 10pc • Our Sun: – m = -26.8, – distance = 1/206,265 = 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! 7B Stellar Temperatures Stellar Spectra How hot are stars? • In Lecture 2A we learned about blackbody spectra and temperature. • Since different stars have different colors, different stars must be different temperatures. Hot Cool 7B Spectral Classifications 7B Temperature and Spectral Type 7B Binary Stars • Most stars in the sky are in multiple systems. • Binaries, triplets, quadruplets, etc…. – Alberio – Alcor and Mizar • The Sun is in the minority by being single. 7B Types of Binaries • Visual – You see both stars • Spectroscopic – You see one star, but you see the Doppler shift (lecture 2B) due to its orbital motion. – Double-line – see lines from both stars – Single-line – see only one set of lines • Eclipsing – One star passes directly across the other. 7B •These categories are purely observer dependent. NPOI Observations of Mizar A (1 Ursa Majoris) Orbital Phase: 000o Mizar, 88 light years distant, is the middle star in the handle of the Big Dipper. It was the first binary star system to be imaged with a telescope. Spectroscopic observations show periodic Doppler shifts in the spectra of Mizar A and B, indicating that they are each binary stars. But they were too close to be directly imaged - until 2 May 1996, when the NPOI produced the first image of Mizar A. That image was the highest angular resolution image ever made in optical astronomy. Since then, the NPOI has observed Mizar A in 23 different positions over half the binary orbit. These images have been combined here to make a movie of the orbit. As a reference point, one component has been fixed at the map center; in reality, the two stars are of comparable size and revolve about a common central position. 7B Stellar Masses How massive are stars? • In Lecture 1B we learned about Kepler’s Laws. • Kepler’s Third Law relates Period to Semimajor axis. But also Mass. P a 2 3 4 3 P a GM 2 2 • Where M is the Total Mass of the binary. • Most stars have masses calculated this way. 7B 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. 7B Stars are small • Betelgeuse is the only star big enough to directly see its surface with a normal telescope. 7B Interferometry • Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope. NPOI - optical VLA - radio 7B Optical Interferometry • NPOI simulates a single optical telescope 65 meters in diameter. • Resolve stars as small as 1.5 mas! PTI - infrared 7B Angular versus Linear Supergiants, Giants and Dwarfs 7B H-R Diagram • Can order the stars we see by the property of temperature and luminosity (or absolute magnitude). Prominent stars Nearby Stars Brightest Stars 1000 pc Stars 7B Spectroscopic Parallax • If you know how luminous a star REALLY is and how bright it looks from Earth, you can determine how far away it must be to look that faint. • For any star in the sky, we KNOW: – Apparent Magnitude – Spectral Type (O, B, A, F, G, K, M) – Luminosity Class (Main Sequence, Giant, etc…). These are denoted by a roman numeral (V, III, I,…). • Use H-R Diagram to figure out how luminous the star really is (M). With (m) one gets distance. • Works well out to 10,000 pc. 7B example distance m M 5log 10 10pc • Deneb is A2Ia star – A2 Blue star – Ia Supergiant – m = 1.25 – M = -8.8 So: distance = 1000 pc 7B