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:1. Find an object whose luminosity you know.2. Measure the energy (flux) that you observe.3. 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.”

Type Ia supernova in NGC4526 (HST pic)

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 ofDavison Soper(Univ. of Oregon)

Edwin Hubble

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)

M100

Cepheid in the Virgo galaxy cluster with Hubble Space Telescope (16 Mpc away…)

The Virgo cluster

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.

Example of a real galaxy spectrum…

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…

Latest data (from SNe Type Ia)

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.

Credit : Tony and Daphne HallasM51

M81

NGC891

NGC4622

NGC4414

Elliptical galaxies… Featureless, often elongated ball of stars. Very similar to bulge component of spirals.

M87

M87

… finally, irregular galaxies

LMC

The Hubble sequence…

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…