… how I wonder what you are. Stars … how I wonder what you are.

Goals Stars are Suns. Are they: What categories can we place them in? Near? Far? Brighter? Dimmer? Hotter? Cooler? Heavier? Lighter? Larger? Smaller? What categories can we place them in?

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

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.

Distances Closest star: Proxima Centauri parallax = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears

Brightness How bright are they really? What is due to distance? What is due to luminosity? Luminosity: Total energy radiated every second.

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.

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.

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

example Our Sun: Polaris: 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!

Stellar Temperatures Hot How hot are stars? Cool Stellar Spectra 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.

Spectral Classifications

Orion Copyright – Tyler Nordgren

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” Where M is the Total Mass. Can calculate the mass of stars this way.

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.

Stellar Masses How massive are stars? 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.

Stellar Radii How big are stars? 50 mas 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.

Atmospheric Seeing

Stars are small Betelgeuse is the only star big enough to directly see its surface with a normal telescope.

Interferometry Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope. NPOI - optical VLA - radio

Optical Interferometry NPOI simulates a single optical telescope 65 meters in diameter. Resolve stars as small as 1.5 mas! PTI - infrared

Angular versus Linear Supergiants, Giants and Dwarfs

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

The Main Sequence “Stars are characterized by what holds them up.” 90% held up by heat of Hydrogen fusion? 4H  He + Energy

Main Sequence & Thermal Radiation The Main Sequence makes sense! Hotter stars are bluer – Wien’ Law Hotter stars are brighter – Stefan’s Law

Homework #6 For 10/9: Read B15.6, 16.1 – 16.2, Ty10 Do: Review Questions 1, 5, Problems 1, 2