February 7, 2006 Astronomy Chapter 16: Analyzing Starlight

February 7, 2006 Astronomy

February 7, 2006 Astronomy Starlight stars: bright lights in the sky like our sun? different? in what ways? why? Nearest: Proxima Centauri 4.2 LY 100,000 years for fastest spacecraft red dwarf, 7% of Sun's diameter 1/18,000 as bright as Sun nearest star of a triple system observable from southern hemisphere Alpha Centauri 4.3 LY 40% brighter than Sun (G2 V spectral class) yellow orange (main sequence) magnitude -0.3 apparent, +4.4 absolute

February 7, 2006 Astronomy All Stars are Different colors: blue-white to red brightness: bright to very faint Orion: Constellation with many different star types Betelgeuse: orange-red supergiant Rigel: blue-white supergiant

February 7, 2006 Astronomy Betelgeuse A red supergiant star

February 7, 2006 Astronomy Key Properties of Stars Sun's key properties mass = 333,400 x Earth mass surface temperature: color yellow  5860 K composition: spectrum  mostly hydrogen size = 110 x Earth diameter luminosity = 3.8 x watts (spectral type G2V) Magnitude – 26.7 apparent, +4.8 absolute Deduce: core “burns” hydrogen, converting it to helium by thermonuclear fusion. Stars: how do we infer mass, temperature, chemical composition, size from observations?

February 7, 2006 Astronomy Key Properties of Stars Stars: how do we infer mass, temperature, chemical composition, size from observations? once we know distance, we can say a lot Chapter 18: how we measure distance assume for now we know the distance Distance Units: 1 AU handy unit for distances in Solar system light year: distance light travels in one year, 9.46 x m New unit: parsec (pc) New unit: parsec (pc) 1 pc = 3.26 LY is roughly the average distance between stars 1 kiloparsec = kpc = 1,000 parsecs is roughly the size of galaxy natural unit for measuring distances – see Ch. 18

February 7, 2006 Astronomy Star Brightness Star brightness specified with the magnitude system. Devised by the Greek astronomer Hipparchus around 150 B.C.E. brightest stars into the first magnitude class, next brightest stars into second magnitude class, and so on, until he had all of the visible stars grouped into six magnitude classes. dimmest stars were of sixth magnitude. brighter objects have smaller magnitudes than fainter objects! magnitude system was based on how bright a star appeared to the unaided eye.

February 7, 2006 Astronomy Extra magnitude? Some objects go beyond Hipparchus' original bounds of magnitude 1 to 6. Very bright objects can have magnitudes of 0 or even negative numbers. Very faint objects have magnitudes greater than +6. Remember: brighter objects have smaller magnitudes than fainter objects!

February 7, 2006 Astronomy Apparent Magnitude Apparent brightness of a star observed from the Earth is called the apparent magnitude. The apparent magnitude is a measure of the star's flux received by us. Examples of apparent magnitudes:  Sun = -26.7,  Moon = -12.6,  Venus = -4.4,  Sirius = -1.4,  Vega = 0.00,  faintest naked eye star = +6.5,  brightest quasar = +12.8,  faintest object = +27 to +28.

February 7, 2006 Astronomy Absolute Magnitude Measure of star luminosity. Luminosity is the total amount of energy radiated by the star every second If you measure a star's apparent magnitude and know its absolute magnitude, you can find the star's distance If you measure a star's apparent magnitude and know its absolute magnitude, you can find the star's distance If you know a star's apparent magnitude and distance, you can find the star's luminosity A quantity that depends on the star itself, not on how far away it is Provides information about the structure of the star – this is the real luminosity More important quantity than the apparent brightness need the distance to determine the absolute magnitude

February 7, 2006 Astronomy Star Luminosity vs Temperature stars are luminous because they are hot they are large or both! Luminosity of an object = the amount of energy every square meter produces multiplied by its surface area. Luminosity =  x T 4, Luminosity of a star increases very quickly with even slight increases in the temperature.

February 7, 2006 Astronomy Star Luminosity vs Size Luminosity ~ surface area. 1,000 watt bulb has same luminosity as a row of ten 100 watt bulbs Luminosity of a bigger star larger than a smaller star at the same temperature. From the apparent brightness, temperature, and distance of a star, one can determine its size.

February 7, 2006 Astronomy Absolute vs Apparent Star brighter if closer brightness fades with distance inverse square law if stars were all the same brightness than apparent luminosity would measure distance

February 7, 2006 Astronomy Famous stars Most famous apparently bright stars are also intrinsically bright (luminous). Can be seen from great distances away. Most nearby stars are intrinsically faint. Not necessarily representative of all stars… Magnitudes and Distances of some stars (from the precise measurements of the Hipparcos mission) Star Apparent Magnitude Distance (pc) Absolute Magnitude Luminosity (relative to Sun) Sun × Sirius Arcturus Vega Spica Barnard's Star /2310 Proxima Centauri /17700

February 7, 2006 Astronomy Color and Temperature Stars are dense hot balls of gas Their spectrum is close to that of a perfect thermal radiator Which produces a smooth continuous spectrum So called blackbody spectrum. Color of stars depends on their temperature: hotter stars are bluer cooler stars are redder. cooler stars are redder.

February 7, 2006 Astronomy Color and Temperature One can observe the stars through different filters to get an approximate temperature. Filter allows only a narrow range of wavelengths (colors) through. By sampling the star's spectrum at two different wavelength ranges (“bands”), one can determine if the spectrum is that a hot, warm, cool, or cold star. Hot stars have surface temperatures around 60,000 K while cold stars have surface temperatures around 3,000 K.

February 7, 2006 Astronomy Star’s Color  Temperature

February 7, 2006 Astronomy B-V Color Index Measure of the temperature based on apparent color. Based on two different filters. A blue (B) filter that only lets a narrow range of colors or wavelengths through centered on the blue colors. A “visible”' (V) filter that only lets the wavelengths close to the green- yellow band through. A hot star has a B-V color index close to 0 or negative, while a cool star has a B-V color index close to 2.0. Other stars are somewhere in between. Defined as the difference in magnitude between the B and V bands.

February 7, 2006 Astronomy Spectra of Stars primary reason stellar spectra look different is stars have different temperatures hydrogen most abundant element – most stars show hydrogen absorption lines hottest may not so hot that hydrogen is completely ionized coolest stars hydrogen atoms are all in lowest state no hydrogen transitions seen

February 7, 2006 Astronomy Spectral Classes O B A F G K M

February 7, 2006 Astronomy Spectral Class Characteristics OIonized Helium and metals; weak Hydrogen BNeutral Helium, ionized metals, stronger Hydrogen A Balmer Hydrogen lines dominant, singly-ionized metals FHydrogen weaker, neutral and singly-ionized metals G Singly-ionized Calcium most prominent, Hydrogen weaker, neutral metals KNeutral metals, molecular lines begin to appear M Titanium Oxide molecular lines dominant, neutral metals

February 7, 2006 Astronomy Doppler Effect Case (a) Object (source) moving towards observer A at velocity “v” Observer “A” sees compressed wave, I.e. shorter wavelength, higher frequency. Observer “B” see stretched wave, I.e. longer wavelength, lower frequency. Case (b) Stationary source Observer “A” and “B” see same wavelength. Source Observer A Observer B v (a) Source Observer A Observer B (b)

February 7, 2006 Astronomy Doppler Effect with Stars Motion of the light source (star) causes the spectral lines to shift positions. An object's motion causes a wavelength shift  = new - rest Depends on speed and direction of moving object. Shift given by:  = rest × V radial / c, c is the speed of light, rest is the wavelength measured if object is at rest. rest is the wavelength measured if object is at rest. V radial is object speed along the line of sight.

February 7, 2006 Astronomy Red and Blue Shift If object is moving toward you, the waves are compressed, So their wavelength is shorter. Lines are shifted to shorter (bluer) wavelengths. This is called a blueshift. If the object is moving away from you, the waves are stretched out, So their wavelength is longer. The lines are shifted to longer (redder) wavelengths. This is called a redshift.

February 7, 2006 Astronomy Spectral Shifts Doppler effect doesn’t affect overall color of an object unless it is moving at a significant fraction of the speed of light (VERY fast!). For an object moving toward us, the red colors will be shifted to the orange and the near- infrared will be shifted to the red, etc. All of the colors shift. The overall color of the object depends on the combined intensities of all of the wavelengths (colors).

February 7, 2006 Astronomy Spectral Shifts Sun spectrum at 3 speeds (0, 0.01c, 0.1c). Hydrogen-alpha line (at 656.3nm) is shown. Objects in our galaxy move at speeds much less than 0.01c. Doppler-shifted continuous spectrum for the Sun moving at 0.01c almost indistinguishable from the Sun at rest.

February 7, 2006 Astronomy Spectral Shifts (cont’d) Doppler shift of spectral lines measurable even for slow speed. Astronomers can detect spectral line doppler shifts for speeds as small as /sec or lower (less than  c). Astronomers can detect spectral line doppler shifts for speeds as small as 1 km/sec or lower (less than  c).

February 7, 2006 Astronomy Proper Motion Doppler effect provides speed along the line of sight. Most stars move at an angle to our line of sight. We measure this by watching stars move over time.

February 7, 2006 Astronomy Doppler Shift

February 7, 2006 Astronomy Summary A wealth of information is contained in the spectra of stars. Astronomers can learn about: luminosity surface temperature composition radial motion from the doppler shift rotation from broadening of spectral lines