Presentation on theme: "9 Stars … how I wonder what you are.. 9 Goals Stars are Suns. Are they: –Near? Far? –Brighter? Dimmer? –Hotter? Cooler? –Heavier? Lighter? –Larger? Smaller?"— Presentation transcript:
9 Stars … how I wonder what you are.
9 Goals Stars are Suns. Are they: –Near? Far? –Brighter? Dimmer? –Hotter? Cooler? –Heavier? Lighter? –Larger? Smaller? What categories can we place them in?
9 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
9 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
9 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.
9 Concept Test If Star A has a parallax of 2 arcseconds, and Star B has a parallax of 0.25 arcseconds: a.Star A is closer to us than Star B. Both are farther from us than 1 pc. b.Star A is closer to us than Star B. Both are closer to us than 1 pc. c.Star A is closer to us than 1 pc. Star B is farther than 1 pc. d.Star B is closer to us than 1 pc. Star A is farther than 1 pc. e.Star B is closer to us than Star A. Both are farther away than 1 pc.
9 Distances Closest star: Proxima Centauri parallax = 0.76 arcsec Distance = 1.3 pc or 4.3 lightyears
9 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.
9 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
9 Magnitude vs. Brightness Mag. Difference Factors of 2.5Brightness Diff = = 2.5 X = 2.5 X 2.5 X = 2.5 X 2.5 X 2.5 X = 2.5 X 2.5 X 2.5 X 2.5 X = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X
9 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.
9 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
9 Concept Test Polaris has a an apparent magnitude of 2.5. It’s absolute magnitude is 3.1. Polaris is: a.Closer to us than 10pc. b.Farther from us than 10 pc. c.Exactly 10 pc away. d.Can’t know without the parallax angle. e.None of these is correct.
9 Concept Test Polaris has a parallax angle of 0.01 arcsconds. Polaris is therefore: a.Closer to us than 1pc. b.Farther from us than 1pc. c.Exactly 1pc away.
9 Concept Test Polaris has a parallax angle of 0.01 arcsconds. Polaris is therefore: a.1pc. b.10pc. c.100pc. d.1000pc. e.None of the above.
9 example Our Sun: –m = -26.8, –distance = 4.8 x 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!
9 Concept Test The most likely absolute magnitudes (M) for each is: a.A = 2.5, B = -2.5, C = 2.5 b.A = 2.5, B = 2.5, C = -2.5 c.A = -2.5, B = 2.5, C = 2.5 d.A = 2.5, B = 2.5, C = 2.5 e.None of the above. StarDistancemM A5 pc1.0 B10 pc2.5 C20 pc
9 A Spectrum A spectrum = the amount of light given off by an object at a range of wavelengths. Emission lines Absorption lines Continuum
9 Three Reasons All objects do one or more: 1.Reflect light because of color or smoothness (same as scatter) 2.Emit light because of their temperature (thermal radiation) 3.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.
9 Concept Question A red chair appear red because: a.it emits red light. b.it transmits red light. c.it absorbs red light. d.it reflects red light. e.all of the above
9 Temperature and Light Warm objects emit light. –Thermal radiation
9 Stellar Temperatures How hot are stars? Thermal radiation and temperature. Different stars have different colors, different stars are temperatures. Different temp, different trace compositions Stellar Spectra Hot Cool
9 Spectral Classification
9 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.
9 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.
9 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.
9 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
9 Betelgeuse Angular size = 50 mas Parallax = 7.6 mas = arcsec Apparent mag = 0.6 Distance = 1/parallax = 132 pc True size = distance * angular size = 1400 R sol –Model solar system: 97 yards in radius. Absolute Mag = m – 5log(d/10pc) = -5 –Our sun M ~5, Betelgeuse = 10,000x luminosity
9 Angular versus Linear Supergiants, Giants and Dwarfs
9 H-R Diagram Can order the stars by: –Temperature (or spectral type) –Luminosity (or absolute magnitude). And see where other qualities fall: –Mass –Radius
9 Luminosity Class The roman numerals. Stars at same temp can have different luminosities. Corresponds to different classes: MS, giant, supergiant. V III I
9 Concept Test Which star is: 1.Hottest? 2.Coolest? 3.Faintest as seen from Earth? 4.Most luminous? Of Main Seq. Stars? 1.Most massive? 2.Most like the Sun? StarSpec Type mM AF0 V0.0 BG2 V CK5 III DF7 I-5.0 EK3 V5.06.5
9 The Main Sequence Stars characterized by what holds them up. 90% held up by heat of Hydrogen fusion? 4H He + Energy