Class 21 : Other galaxies • The distance to other galaxies • Cepheid variable stars. • Other methods. • The velocities of galaxies • Doppler shifts. • Hubble’s law. • The morphology of galaxies • Hubble Types.
I : Standard Candles • How to measure distances in astronomy: • Find an object whose luminosity you know. • Measure the energy (flux) that you observe. • Use the inverse square law to figure the distance to the object… • Such an object (i.e., one with a known luminosity) is called a “standard candle.”
II : Cepheid variables as distance indicators • Very important type of star for measuring distance. • Luminous variable stars. • “Breathe” in and out. • Periodicities range from 3-30 days. • Period and luminosity closely related! • Good standard candles. • Have to calibrate the luminosity relation of Cepheids using parallax to nearby Cepheids. From web site of Davison Soper (Univ. of Oregon)
Hubble found Cepheids in M31 • Edwin Hubble • Found a Cepheid in the Andromeda nebula (M31). • M31 is 750 kpc (about 2 million light years) away. • Firm evidence for the “island universes” hypothesis… M31 is a whole other galaxy. The Andromeda galaxy (M31)
Cepheid in the Virgo galaxy cluster with Hubble Space Telescope (16 Mpc away…)
Beyond the Cepheids… • Hubble could only find Cepheids in the closest few galaxies (1-2 Mpc). • Even Hubble Space Telescope cannot find Cepheids beyond the Virgo cluster (16 Mpc). • Beyond 1-2Mpc, Hubble used… • Brightest star method – identify the brightest “star” in the galaxy and assume that it is the same as the brightest star in nearby galaxies. BUT, brightest object may not be a star at all! • Overall galactic apparent brightness method – for distant galaxies, simply use overall brightness of galaxy to gauge distance. Very error prone!
III : The Doppler effect & the velocity of galaxies • Think about sound waves… • Let = frequency (number of waves passing certain fixed point in one second). • Let = wavelength (distance between two “crests” of the wave). • Let c = speed of the wave. c =
Suppose source is moving towards you with speed v… • Waves get squeezed in direction of motion (i.e., wavelength decreases). • So, frequency must go up. • Same thing happens for light! v
Technique for measuring a galaxy’s velocity: • Measure the spectrum of light from the galaxy. • Look for characteristic “fingerprint” in the spectrum whose wavelengths are known from lab experiments. • Measure actual wavelength and interpret shift as due to Doppler effect.
Blueshifts and redshifts • If galaxy is moving towards us, wavelengths are shortened spectrum blueshifted. • If galaxy is receding from us, wavelenths are lengthened spectrum redshifted. • Slipher measured velocities of nearby galaxies – by 1922, he found that 36 out of 41 were moving away from us! • [The first hint of Hubble’s remarkable result…]
IV: Hubble’s Law • Hubble measured distance and plotted it against velocity…
Everything’s rushing away! • Hubble found that all distant galaxies are rushing away from us! • Found that speed of recession is proportional to distance of galaxy (Hubble’s law): • H0 is called Hubble’s constant. • Modern measurements : H0 = 65 km/s/Mpc. • Can be used to determine distance to galaxies that are very far away… (just need spectrum.) v = H0d
V : Morphology of galaxies • Spiral galaxies… • Disk of younger stars (+ spiral structure). • Color of disk can be dominated by massive/hot stars…makes disk look blue! • Bulge of older stars. • All massive/hot stars have died. • Only cooler stars still live. • So bulge looks red.
M51 Credit : Tony and Daphne Hallas
Elliptical galaxies… • Featureless, often elongated ball of stars. • Very similar to bulge component of spirals.
Why is there a variety of morphologies? • Nature vs. nurture … • Are ellipticals & spirals “born” different? • Do spirals turn into ellipticals or vice versa? • Current thinking… • Galaxy collisions are basic driving force for galaxy structure…