Outline - April 6, 2010

• Hubble’s Tuning Fork Diagram (pg. 639)

• Galaxy groups and galaxy clusters (pgs. 639-640)

• Measuring distances to galaxies (pgs. 640-644)

• Hubble’s Law, the Cosmological Principle, and the cosmological redshift (pgs. 644-653)

Different Galaxy Morphologies

Hubble’s Classification of Galaxy Morphologies(note: Hubble didn’t know about S0/Lenticular galaxies)

Ellipticals:

En where n = 10(1-b/a)

Spirals:

S (normal) or SB (barred),

a (big bulge, tightly wound spiral arms)

b (medium bulge, medium wrapping of spiral arms)

c (tiny bulge and loosely wrapped spiral arms)

Is this an evolutionary sequence?(Hubble thought so)

Galaxies are Gregarious

• Collection of a few dozen galaxies

• Usually dominated by one or two large spiral galaxies, remainder are “dwarf” galaxies (mostly irregular and elliptical)

• Good example is our own “Local Group” of galaxies

• Galaxies are all orbiting about a common center

• Collection of typically 100’s but up to 1000’s of galaxies

• Largest elliptical galaxies are only found at the centers of clusters

• Ellipticals make up about 1/2 of the large galaxies in the centers of clusters

• S0/lenticular galaxies live in clusters

• Galaxies are all orbiting about a common center

Galaxy Groups Galaxy Clusters

The “Local Group” of Galaxies

Large Magellanic Cloud (LMC)

Small Magellanic Cloud (LMC)

Andromeda / M31

About 30 galaxies in total, dominated by two large spiral galaxies (Milky Way and M31); remaining galaxies are “dwarf” galaxies. Local Group is about 2.5x106 ly = 2.5 Mly in diameter. Milky Way and M31 are approaching each other at a relative speed of about 300 km/s.

Galaxy Groups(note that most of the big galaxies are spirals)

Galaxy Cluster

Small portion of the Virgo Cluster, the closest large galaxy cluster (contains about 1,500 galaxies, distance about 60 Mly)

M87 - One of the Largest Galaxies in the Universe

M87 is a giant elliptical galaxy that lies at the heart of the Virgo Cluster. It’s mass is about 1013 Msun, which is the same as the total mass of a typical GROUP of galaxies. At the heart of M87 is the largest black hole to be discovered so far (MBH = 3x109 Msun). M87 and galaxies like it probably got so large by “cannibalizing” other galaxies in the cluster.

Galaxy Cluster

A portion of the Coma cluster of galaxies, which contains 1000’s of galaxies. Most of the galaxies are ellipticals.

Distances to Galaxies, I

Why would you want to know the distances to galaxies?

1. Galaxy Luminosities

2. Galaxy Evolution (more distant = farther back in time)

3. Expansion of the universe (Hubble’s Law)

4. Structure of the universe

Distances to Galaxies, II

“Parallax” doesn’t work for measuring distances to galaxies:

p = 1 / d

Galaxies are so far away (d is so large) that p is not measurable!

Instead, astronomers resort to a suite of “Standard Candles” to measure distances to galaxies.

Standard Candles

A standard candle is an object for which we are (reasonably) sure we know the luminosity (L)

The brightness (b) of a standard candle is measured at the telescope

The distance (d) to the standard candle (and, therefore, the galaxy that it lives in) is derived using the formula

b = L / (4 d2)

Cepheid Variable StarsLu

min

osity

Stars with masses between about 5 Msun and 20 Msun that are in late middle-age. They are physically pulsating, which causes a periodic change in their luminosity.

The period-luminosity relationship has been recently calibrated using Cepheid variable stars in our own Galaxy with measured parallaxes. (Book is a bit out of date.)

Spiral Galaxy NGC4603

Cepheid Variable Stars in NGC4603

Procedure:

1. Identify Cepheid Variable stars by their light curve and measure their period

2. Measure average brightness at the telescope

3. Use the Period-Luminosity graph to determine the luminosity

4. Use b = L / (4 d2) to derive

the distance

This works for all types of galaxies (elliptical, spiral, irregular), and can be used to measure distances to about 60 Mly using Hubble Space Telescope.

Tully-Fisher Relationship for Spiral Galaxies(whole galaxy is the “standard candle”)

The disks of all spiral galaxies rotate, and the rotation speed correlates with the luminosity of the galaxy.

The faster the disk rotates, the larger is the galaxy’s luminosity.

Measure rotation speed of the galaxy, use the graph to determine the luminosity of the galaxy.

Measure the brightness of the galaxy and again use b = L / (4 d2) to get the distance

Note: the Tully-Fisher relationship was calibrated using nearby spiral galaxies with Cepheid variable stars!

Works only for spiral galaxies, and only for distances less than about 100 Mly.

White Dwarf Supernovae(“Type Ia” Supernovae)

Supernovae are extremely bright, and they can be seen in very distant galaxies

White dwarf supernovae make good standard candles because the white dwarfs all have about the same masses (1.4 Msun) so they all have about the same luminosities

Has be used to measure (!!), distances to galaxies as far as 1 billion light years but it’s a bit of a potshot…

Note: exploding massive stars (“Type-II” supernovae) make rotten standard candles.

Distances to Galaxies

Why would you want to know the distances to galaxies?

1. Galaxy Luminosities (see 04/01 notes)

2. Expansion of the Universe (Hubble’s Law)

3. Galaxy Evolution (next time)

4. Structure of the universe

Hubble’s LawWhat can galaxies tell us about the universe?

1912: Vesto Slipher discovered that the spectra of galaxies were redshifted (galaxies appeared to be moving away from us).

Distances to galaxies were unknown at the time.

“Redshifted” Galaxy Spectrum

“Spectrum” = plot of intensity of light as a function of wavelength (or frequency) of the light

“Red” shift = shift to longer wavelengths than if the object were at rest with respect to you

Object at rest

Object “receding”

Galaxy Redshifts

The redshift (z) of a galaxy is:

For z < 0.2, the recessional speed of the galaxy is: v = cz

For z > 0.2, need to use Special Relativistic Doppler Shift formula to compute v

Note: largest redshifts measured (so far) give z = 7, which does not mean v = 7c (!!)

How to Get a Law Named After You(discover something really fundamental)

Edwin Hubble

Hubble’s Law: the farther a galaxy is from us, the faster it is receding from us

Hubble’s Law is proof that the universe as a whole is expanding (getting larger)!

In the 1920’s Edwin Hubble measured the distances to many galaxies (using Cepheid

variable stars) and found that the recessional speeds of the galaxies

increased with distance!

Hubble’s Law

H0 is called “Hubble’s Constant”. Usually astronomers measure H0

in units of km/s/Mpc, but the book uses km/s/Mly. The value of H0 is about 71 km/s/Mpc or 22 km/s/Mly.

Raisin Bread Analogy to the Expanding Universe

http://spiff.rit.edu/classes/phys301/lectures/lambda/RaisinBread.swf

The program plots the distance of each of the raisins, as viewed from one particular raisin, and the rate at which the distances to the raisins is increasing (i.e., the velocity)

What’s getting larger?

• Distances between objects that are not held together by gravity or chemical bonds

• Primarily, it is distances between clusters and groups of galaxies

• Clusters and groups of galaxies are held together by gravity (note: some galaxies in our Local Group are blueshifted!)

• Galaxies and stars are held together by gravity

• People, chairs, etc. held together by chemical bonds

Hubble’s Law Does Not Say We are at the “Center of the Universe”

All observers on all galaxies will observe Hubble’s Law (that’s what the previous animation demonstrates). There is nothing special about our vantage point in space (i.e., the Milky Way)

Not only does Hubble’s law not say that we are at the center of the universe, in modern cosmology there can be no center to the

universe at all.

Cosmology = the study of the origin, evolution, and structure of the entire universe

The Cosmological Principle

On a large enough scale, the universe is

both isotropic and homogeneous

ISOTROPY: There is no preferred direction in space. (All directions are alike.)

HOMOGENEITY: One randomly-chosen large volume of the universe will have the same physical properties (and identical physical laws) as another randomly-chosen large volume of the universe. (All places are alike.)

2-Dimensional Examples of Isotropy and Homogeneity

• Surface of plain white“cue” ball used for playing pool (billiards)

• Infinite forest of identical treesWarning: at some level all analogies fail…

Isotropic Forest

Anisotropic Forest (trails = “preferred direction”)