1) Introduction to Galaxies

2) Types of Galaxies

3) The Milky Way4) Dark Matter

April 2, 2003

http://www.hep.fsu.edu/~tadams/courses/spr03/ast1002/Lecture040203.pdf

Galaxies

Messier Objects

1784 – Charles Messierhe was a comet hunter

Identified 103 objects in the sky which were not stars

these were fuzzy objectshe identified them so they would not be mistaken for comets

These were actually galaxies, globular clusters and such

far awayNow a very useful list of interesting objects for amateur astronomers to look at

M1 – Crab Nebular

Looking at Distant Objects

Objects look different depending on how they are viewedWe are unable to “walk around” an object which is millions of lightyears awaySo we have to try to interpret what we see

compare to objects oriented differently

Looking at the Dark Sky

A picture of a dark patch of the sky using the Hubble Space Telescope

Galaxies

Galaxies are large collections of starsmillions and billions of stars

The Milky Way is our own galaxyThere are hundreds of billions of galaxies in the UniverseMillions to hundreds of billions of stars in each galaxy

Types of GalaxiesThree main types of galaxies

based on shapeSpiral

rotating disk (with arms)

Ellipticaloval disk of starsmore chaotic motion

Irregularnone of the above

Active Galactic NucleiSome galaxies have supermassive black holes at the centerIf material is falling into the black hole, enormous amounts of energy are released

accretion diskCan shine with a luminosityof 1-100 million Suns!Quasars are a type of AGN

Galaxy Collisions

Occasionally galaxies collide

don’t actually slam into each other

Passage of one galaxy through/near another causes major “stirring”

due to gravityCauses new activity

star formation, AGNs,…

Think of walking around before a football game

Looking at the Milky Way

Viewed as a bright band of stars across the skyGalactic center appears in the southern part of the sky (from the northern hemisphereMuch of the Milky Way is blocked by dust

dark band through the middle of the Milky Way

But we can study it in longer wavelengthse.g. radio waves

Spiral Galaxies

Spiral arms area natural consequence of some rotationsIf gas/dust in a disk are rotating with the same speed, stuff further out will take longer to go around

it will lag behind

Pieces of a Spiral Galaxy

DiskArmsBulge

barred?black hole

HaloGlobular clusters

Globular Clusters

Groups of old stars in the galactic halomay have around 500,000 starsmay be around 15 lightyears across

Formed before the galaxyor at very early stages

Orbiting the galaxyNo new star formation going onUseful for studying the distribution of material in the galaxy

from gravity

Rotation Speeds of Galaxies

Rotation Speeds of GalaxiesFor spiral galaxies, stars farther out should be moving slowerBut we see them all moving with the same speed!

DarkDarkMatter!Matter!must have an explanation

What is Dark Matter?Dark matter is material we can’t seeWe have various evidence the Universe has lots of matter we can’t see

rotation speeds of galaxiesmovement of global clusters around galaxies

Two primary theoriesMACHOs – MAssive Compact Halo Objects

planets or brown dwarfs or low-mass black holesunlikely, people are looking, have seen a few, but not enough

WIMPs – Weakly Interacting Massive Particlesnew type of elementary particle

A Great Question

The nature of dark matter is one of the great questions of astronomyCould have very important implications on our understanding of the Universe

something is out there which we do not understandcould radically change how we think about many things

Pay attention to this issue in the coming decades…

Clusters of stars formed at the same time of the same materialsStudying them tells us about the life of stars

plot where stars fall on H-R diagram

Looking at many clusters tells us how stars leave main sequence

Studying Star Clusters

Summary

Galaxies are large collections of starsthree types/shapeshundreds of billions of galaxies

Spiral galaxies have structureMilky Way is a spiral galaxy

Globular clustersDark matter

from rotational velocities of stars in galaxies