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

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?

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?

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

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 q = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears

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

The Solar Neighborhood

Star light, star bright In lecture 5 we talked about stellar magnitudes. Sirius is magnitude –1.5 Polaris is magnitude 2.5 While Sirius is brighter than Polaris, Sirius is also a lot closer to us.

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 = 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!

Stellar Temperatures Hot How hot are stars? Cool Stellar Spectra How hot are stars? In Lecture 3 we learned about blackbody spectra 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 The Sun is in the minority by being single.

Spectroscopic Binaries Sometimes you can’t see both stars. Too close Too far Too faint But if a star is orbiting something it must be moving.

0.01 arcsec

Stellar Masses How massive are stars? Where M is the Total Mass of the binary. 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.

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 the property of temperature and luminosity (or absolute magnitude). Brightest Stars 1000 pc Stars Nearby Stars Prominent stars