… 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?

Angular Size Linear size = how big something really is Meters, inches, light years, feet Angular size = how big something looks Degrees, arcminutes, arcseconds, milliarcseconds Circle = 360 degrees 1 degree = 60 arcmin 1 arcmin = 60 arcsec 1 arcsec = 1000 mas

Distance One proof of a heliocentric Universe is stellar parallax. Tycho Brahe saw no parallax. Copernicus: stars 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 This distance is 1 parsec (pc) 1 pc = 206,265 AU = 3.3 ly 1 lightyear = distance light travels in one year.

Concept Test If Star A has a parallax of 2 arcseconds, and Star B has a parallax of 0.25 arcseconds: Star A is closer to us than Star B. Both are farther from us than 1 pc. Star A is closer to us than Star B. Both are closer to us than 1 pc. Star A is closer to us than 1 pc. Star B is farther than 1 pc. Star B is closer to us than 1 pc. Star A is farther than 1 pc. Star B is closer to us than Star A. Both are farther away than 1 pc.

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

Terms Brightness = How intense is the light I see from where I am. Magnitude is numerical term for this. Luminosity = how much light is the thing really giving off.

Magnitude Scale The SMALLER the number the BRIGHTER the star! Every difference of 1 magnitude = 2.5x brighter or dimmer. Difference of 2 magnitudes = 2.5x2.5 = 6.3x brighter or dimmer

Magnitude vs. Brightness Mag. Difference Factors of 2.5 Brightness Diff. 1 2.51 = 2.5 2.5 2 2.52 = 2.5 X 2.5 6.3 3 2.53 = 2.5 X 2.5 X 2.5 16 4 2.54 = 2.5 X 2.5 X 2.5 X 2.5 40 5 2.55 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 100 6 2.56 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X 2.5 250

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 = brightness (magnitude) of a star as seen from Earth.  m Depends on star’s total energy radiated (Luminosity) and distance Absolute Magnitude = brightness (magnitude) of a star as seen from a distance of 10 pc.  M Only depends on a star’s luminosity

Concept Test Polaris has a an apparent magnitude of 2.5. It’s absolute magnitude is 3.1. Polaris is: Closer to us than 10pc. Farther from us than 10 pc. Exactly 10 pc away. Can’t know without the parallax angle. None of these is correct.

Concept Test Polaris has a parallax angle of 0.01 arcsconds. Polaris is therefore: Closer to us than 1pc. Farther from us than 1pc. Exactly 1pc away.

Concept Test Polaris has a parallax angle of 0.01 arcsconds. Polaris is therefore: 1pc. 10pc. 100pc. 1000pc. None of the above.

example Our Sun: Polaris: distance = 4.8 x 10-6 pc So: M = 4.8 Polaris: m = 2.5, distance = ~100 pc So: M = -3.1 Polaris is 1500 times more luminous than the Sun!

Concept Test Star Distance m M A 5 pc 1.0 B 10 pc 2.5 C 20 pc -1.0 The most likely absolute magnitudes (M) for each is: A = 2.5, B = -2.5, C = 2.5 A = 2.5, B = 2.5, C = -2.5 A = -2.5, B = 2.5, C = 2.5 A = 2.5, B = 2.5, C = 2.5 None of the above.

A Spectrum Emission lines Absorption lines Continuum A spectrum = the amount of light given off by an object at a range of wavelengths.

Three Reasons All objects do one or more: Reflect light because of color or smoothness (same as scatter) Emit light because of their temperature (thermal radiation) Emit or absorb light because of their composition (spectral lines) A person, house, or the Moon: reflects visible light, and because each is warm, emits infrared light.

Concept Question A red chair appear red because: it emits red light. it transmits red light. it absorbs red light. it reflects red light. all of the above D. it reflects red light.

Temperature and Light Warm objects emit light. Thermal radiation

Stellar Temperatures Hot How hot are stars? Stellar Spectra Hot Cool How hot are stars? Thermal radiation and temperature. Different stars have different colors, different stars are temperatures. Different temp, different trace compositions

Spectral Classification

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.

0.005 arcsec

Stellar Masses How massive are stars? Most stars have masses calculated this way. Find: 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.

Betelgeuse Angular size = 50 mas Parallax = 7.6 mas = 0.0076 arcsec Apparent mag = 0.6 Distance = 1/parallax = 132 pc True size = distance * angular size = 1400 Rsol Model solar system: 97 yards in radius. Absolute Mag = m – 5log(d/10pc) = -5 Our sun M ~5, Betelgeuse = 10,000x luminosity

Angular versus Linear Supergiants, Giants and Dwarfs

H-R Diagram Can order the stars by: Temperature (or spectral type) Luminosity (or absolute magnitude). And see where other qualities fall: Mass Radius

Luminosity Class I V III The roman numerals. Stars at same temp can have different luminosities. Corresponds to different classes: MS, giant, supergiant. I V III

Concept Test Which star is: Of Main Seq. Stars? Star Spec Type m M A F0 V 0.0 B G2 V 10.0 4.4 C K5 III -2.0 D F7 I -1.0 -5.0 E K3 V 5.0 6.5 Which star is: Hottest? Coolest? Faintest as seen from Earth? Most luminous? Of Main Seq. Stars? Most massive? Most like the Sun?

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