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Ch.11: Galaxies

Next: Ch. 12: Cosmology

Chapter 11: Galaxies

Spiral Arms! Spiral arms contain gas and denser concentrations of stars ! So most star formation takes place in spiral arms

! Stars pass through the arms as they rotate around the center of the galaxy

! The spiral pattern does not rotate rigidly.

Typical Spiral Galaxy

! Spiral arms are like traffic jams where cars pile up. ! Cars move slowly while in the jam, but speed up outside it ! Similarly, stars slow down while in a spiral arm

Elliptical Galaxies

… are round, not flat like spirals

They also contain billions of stars.

But they have less gas and dust than spirals.

M32Ellipticals have very little new star formation.

Most large galaxies (Spirals & Ellipticals) show signs of a black hole in their center. …

The black hole in M87 produces a huge jet of matter

….and one BH has now been imaged!

An elliptical with two spirals nearby

Some galaxies have characteristics of both ellipticals and spirals…

The Sombrero Galaxy

Irregular Galaxies •Lack any distinct shape•Are generally smaller than spirals and ellipticals• The Large & Small Magellanic Clouds are Irregulars

•LMC

•SMC

Galaxy Classes

Spiral Galaxies (S): Classified according to spiral arms (a,b,c) and presence of a bar (“B”)

Elliptical Galaxies (E): Classified according to shape (E0-E9)

Edwin Hubble devised a system of classifying galaxies…

You can classify galaxies yourself at:galaxyzoo.org

Clusters of Galaxies

! Many galaxies are found in Clusters with other galaxies

! The Milky Way is in a small cluster of galaxies called “The Local Group”

! The Local Group includes a few large spiral galaxies (like Andromeda) and several small irregulars

! Other clusters of galaxies contain many more members

Milky Way

Large Magellanic Cloud

Small Magellanic Cloud

Andromeda Galaxy

NGC 6822

M33

The “Local Group”, a small cluster of galaxies

The “Local Group”

Abell S740, a Galaxy Cluster containing about 40 galaxies

The Virgo Cluster is about 54 million light years away.

The Virgo Cluster is about 54 million light years away.

(Ellipticals and Spirals) The Coma Cluster contains over 1000 galaxies!

Superclusters of Galaxies! Stars group together into galaxies.! And galaxies group together to form clusters. ! AND, clusters of galaxies group together to form superclusters. ! The Local Group, (containing the Milky Way), and the Virgo Cluster

are in the Local Supercluster.

A Map of the Large Scale Structure of the Universe

Superclusters are found in “walls” that surround huge empty voids

Each dot is a galaxy

The large-scale structure of the universe looks like a sponge!

Known Superclusters

The Really Big Picture How Many Galaxies are There?

! To estimate the total number of galaxies in the universe, the Hubble Space Telescope took a picture of an “empty” region of space.

! The exposure time was 11 days!

! This is a small portion of the sky, just the size of a pencil lead held at arm’s length!

! But it is representative of the rest of space.

! The Hubble Ultra Deep Field (HUDF) contains over 10,000 galaxies!

! Light from some of these galaxies has taken almost 13 billion years to reach us.

! If this small portion of the sky is like the rest of space, then there must be over 200 billion galaxies in the Universe!

Cosmic Measurements

! The number of galaxies is huge! ! Can we ever figure out what’s going on?

! Yes! We can measure several properties of galaxies:

! Distance ! Motion ! Mass

! And use these to learn what the universe is doing

Extragalactic Distances

Other galaxies are millions of light years (or parsecs) from our Galaxy.

Typical distance units:

Kpc = kiloparsec = 1,000 parsecs

Mpc = megaparsec = 1 million parsecs

1 parsec = 3.3 light years

1 Mpc = 3.3 MILLION light years

Measuring Enormous Distances! A “standard candle” is any phenomenon whose absolute magnitude and

thus distance can be measured. ! Example: Cepheid Variable Stars. ! Another standard candle is a: Type Ia supernovae ! These happen in a binary star system when mass falls from one star onto

the white dwarf. ! The white dwarf’s mass goes over 1.4 Msun and it explodes!

Since exactly 1.4 Msun explodes every time, the same amount of energy is always released.

Which makes these supernovae good standard candles.

Supernova in galaxy NGC4526 (HST Image)

Type Ia Supernovae can be seen at very large distances.

Interacting Galaxies

Occasionally Galaxies CollideGravity distorts their shapes, sometimes producing an irregular galaxy.Gas clouds in each galaxy collide, triggering star formationHowever the stars pass right by each other!