The Great Debate

! What was being debated at the Great Debate? Why is this a difficult problem? How was the debate resolved?

! What are the basic properties (mass, shape, color, luminosity) of elliptical, spiral and irregular galaxies?

! Why are spiral galaxies (their arms) blue? For the same reason, it’s true that irregular galaxies are blue (young) and ellipticals galaxies are red (old)

! How do rotation curves tell us that most of the mass in galaxies (and so in the Universe) is made of dark matter?

! How do galaxies form? What is one observational clue to why galaxies are so different? What is a giant elliptical galaxy? What is a rich cluster? A poor cluster (or group?)

Learning Objectives

Rosse’s M51 sketch

Modern M51 picture

“Leviathan” Lord Rosse’s 1.8 m reflecting telescope

“Spiral Nebulae”! Dim, diffuse “nebulae”

with spiral patterns! Spiral structures noted

in the mid-1800s by Lord Rosse (Ireland)

! Much, much disagreement:! Kant: Our Galaxy is a spiral

“island universe” and the other spiral nebulae are the same asour Galaxy and far away

! Herschel and others: Our Milky Way Galaxy is the whole Universe, and the other spiral nebulae are nearby

Sun*Galaxy

*Sun wrongly in center; this is before Shapley’s globular cluster studies

The Great Debate - How Far Away Are They?

The Great Debate! Think about how difficult a

problem was being discussed at the Great Debate

! We see these weird things in the sky…but we see lots of weird things...star clusters, novae, giant molecular clouds!Why should these nebulae

be special?! How might we prove these

things are either quite close to us (within our Galaxy), or really far away from us (outside of our Galaxy)?

The Andromeda Galaxy: The largest “spiral nebula”

Edwin Hubble!1923: Hubble resolved M31, the

Andromeda “nebula”, into stars!After resolving stars, Hubble went

looking for Cepheid Variables (stars that have a pulsation period-absolute magnitude relation)

!Found M31 was ~300,000pc away (modern estimate is ~770,000pc)

!Andromeda is an “island universe” like our own Galaxy

The Andromeda “Nebula”

Hubble at Mt. Wilson Observatory

The three classes of galaxies

Irregular Galaxies Spiral Galaxies

Elliptical Galaxies

Classes of Galaxies!Spirals (S)

!Structure: disk, halo, bulge!Medium to large galaxies!The disk has the young

blue stars, while the bulge and halo have older red stars

!Ellipticals (E)!Pure bulge, no disk component!Large range in sizes!All older red/yellow stars

! Irregulars (Ir)!Well, erm, irregular!Smaller galaxies!Mostly young blue stars

Galaxy Types: Overview

Spirals Ellipticals Irregulars

Mass (MSun) 109 – 1012 105 – 1013 108 – 1010

Luminosity (LSun) 108 – 1010 105 – 1011 107 – 109

Diameter (kpc) 5 - 200 1 - 200 1 - 10

Color

Disk: Blue-to- white

Halo/Bulge: Red-to-yellow

Reddish-yellow Bluish-white

Why do we see Spiral Arms?! They are easily

seen as the arms contain numerous bright, blue O and B stars!These stars

also illuminate dust in the arms

! However, stars in general are evenly distributed throughout the disk

Face-on

Edge-on

Effect of Viewing Angle

Easier to seethe spiral arms

Easier to see the dust in the disk

Rotation Curves‣ A rotation curve is a plot of the orbital

speed of stars vs. the distance of the stars from the center of their host galaxy

‣ Example: rotation curve of the planets

Masses of Galaxies! As with the Milky

Way, we measure the speed that stars orbit in a galaxy’s disk!We use (Newton’s

version of) Kepler’s Laws to calculate the enclosed mass

! Like the Milky Way, all other galaxies have flat rotation curves

! The mass is mostly in the outer parts of galaxies, it is not where the stars exist

! This indicates a halo of dark matter

Dark Matter! Normal matter makes up less than

5% of our Universe! Dark matter is spherically

distributed all around galaxies in huge halos

! What is this stuff that emits no light!Neutrinos? Black holes? Brown or

red dwarfs? Cooled-off white dwarfs?! MACHOs: Massive Compact Halo Objects

!Something exotic – particles as yet unknown?! WIMPs: Weakly Interacting Massive Particles

! We don’t know exactly what dark matter isbut we categorically, absolutely know it’s there

How did our galaxy form?(a) Proto-galactic clouds are

drawn together by gravity; halo stars begin to form

(b) Merged cloud collapses(c) Gas & dust settle into a

spinning disk; halo star formation stops. As of now there are only old and long-lived objects in the halo (red dwarfs, white dwarfs, black holes etc.)

(d) Stars continue to form in the disk. As of now there are still young stars in the disk This model is, of course, a

tad simplified for ASTR-1050!

M87NGC 4414

But why do some galaxies end up looking so different?

Observational Clue: Galaxy Clusters!Galaxies occur in a wide

range of sizes of clusters! “Rich” clusters have many

galaxies, mostly elliptical!At the center of each rich

cluster there is usually a giant elliptical galaxy

! “Poor” clusters contain fewer than ~100 galaxies!Our Galaxy is in a “poor”

group of ~35 galaxies!For some reason “ending up close” to other

galaxies makes galaxies tend to be elliptical

Elliptical galaxies dominate rich clusters of galaxies

Next Time

Objects in the Rear-view Mirror May Appear Closer Than They Are