7B 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  = opposite/adjacent For small angles: tan  =   = (1 AU)/Distance Distance = (1 AU)/  What is the distance of an object with  = 1 arcsec? Distance = 206,265 AU Call this distance 1 parsec (pc) 1 pc = 206,265 AU = 3.3 lightyears

7B Distances 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  = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears

7B How far is this? The Sun New York Alpha Centauri Hawaii

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

7B example Our Sun: –m = -26.8, –distance = 1/206,265 = 4.8 x 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 Spectra Stellar Temperatures 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.

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. 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: –Distance –Angular size Learn real size. 50 mas

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. VLA - radio NPOI - optical

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 Deneb is A2Ia star –A2  Blue star –Ia  Supergiant –m = 1.25 –M = -8.8 So: distance = 1000 pc