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Announcements Reading for next class: Chapters 22.6, 23 Cosmos Assignment 4, Due Wednesday, April 21, Angel Quiz Monday, April 26 Quiz 3 & Review, chapters 16-23 Wednesday, April 28, Midterm 3: chapters 16-23 What are Galaxies?

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announcements
Announcements
  • Reading for next class: Chapters 22.6, 23
  • Cosmos Assignment 4,Due Wednesday, April 21, Angel Quiz
  • Monday, April 26Quiz 3 & Review, chapters 16-23
  • Wednesday, April 28,Midterm 3: chapters 16-23
what are galaxies
What are Galaxies?

Galaxies are vast collections of stars (~1011) and sometimes gas and dust as well

universe is expanding
Universe is Expanding
  • You and I are NOT expanding
  • The solar system is NOT expanding
  • The Milky Way Galaxy is NOT expanding
  • Our local group of Galaxies is NOT expanding
  • Nothing that is bound together by a force is expanding
  • SPACE between groups of galaxies IS expanding
hubble s law
Hubble’s Law

Velocity = Hubble’s Constant x Distance V = HDIf you are twice as far away,you are moving away twice as fast, so you started moving away at the same time!

How long ago was that?

age of the universe
Age of the Universe
  • V=HD
  • D = VT = V/H
  • T = D/V = 1/H Age if expansion not accelerated or decelerated
questions
Questions:

How did galaxies form?

Why are there different types of Galaxies?

galaxy formation
Galaxy Formation

Similar to star formation

  • H & He gas filled space almost uniformly
  • Where density slightly greater, gravity slightly greater
  • Matter falls into gravitational potential well, increases gravity
  • Matter pulled in by more gravity, density excess grows
  • Densest cores became 1st generation massive stars
galaxy formation models
Galaxy Formation Models
  • Assumptions:
  • Matter originally
  • filled all of space
  • almost uniformly
  • Gravity of denser
  • regions pulled in
  • surrounding
  • matter
clues to galaxy formation
Clues to Galaxy Formation

Halo stars are old, have randomly oriented orbits

Disk has young stars with orbits nearly in plane

  • Initially gravity pulled in matter from all directions. Stars formed during this stage have random orbits passing close to center
  • Later, rotation made any remaining gas flatten into disk. Stars forming after this have orbits in disk.
spiral or elliptical galaxy
Spiral or Elliptical Galaxy?

Possible explanations

  • Rate of star formation
  • Amount of Rotation
  • Collisions & mergers
density excess
Density Excess?

Higher density proto-galactic clouds form stars more rapidly,

use up all their gas before it can form a disk.

rotation
Rotation?

Larger rotation produces more disk-like distribution of matter.

collisions mergers
Collisions & Mergers

Question 1: If the Milky Way were the size of a grapefruit, where would the Andromeda galaxy (nearest comparable size galaxy) be?

  • About 1 cm away
  • About 1 m away
  • About 1 km away
  • About San Francisco
  • About the Moon
  • About the Sun
collisions mergers16
Collisions & Mergers

Question 1: If the Milky Way were the size of a grapefruit, where would the Andromeda galaxy (nearest comparable size galaxy) be?

  • About 1 cm away
  • About 1 m away (~3 m)
  • About 1 km away
  • About San Francisco
  • About the Moon
  • About the Sun
galaxies are close together
Galaxies are close together

Evidence of galaxy interactions via gravity

Burst of

star

formation

evidence of galaxy mergers
Evidence of Galaxy mergers

Super massive galaxies in the centers of clusters of galaxies

Merged galaxies

See also

Fig 21.11

elliptical galaxies are much more common in huge clusters of galaxies
Elliptical galaxies are much more common in huge clusters of galaxies

Denser cloud

More collisions

(hundreds to thousands of galaxies)

how do we know
How do we know?

When we look farther out in space we are looking farther back in time. See galaxies at different stages in their evolution

problem
Problem!
  • In an expanding universe, gravity takes longer to pull matter together.
  • Need stronger gravity
  • Need more mass
  • Don’t see any more mass
  • Postulate existence of DARK MATTER
other evidence for dark matter
Other evidence for Dark Matter
  • Rotation of galaxies
  • Motions in clusters of galaxies
  • Hot x-ray emitting gas in clusters of galaxies
  • Gravitational lensing
rotation curve
Rotation Curve

A plot of orbital velocity versus orbital radius

Solar system’s rotation curve declines because Sun has almost all the mass & gravity gets weaker with

Increasing distance

rotation curve27
Rotation Curve

of Milky Way stays flat with distance

Mass must be more spread out than in solar system

milky way
Milky Way

Significant mass exists outside radius of most stars!

mass of milky way
Mass of Milky Way

Mass within Sun’s orbit:

1.0 x 1011MSun

Total mass:

~1012MSun

-> Dark Matter

motions of galaxies in clusters
Motions of Galaxies in Clusters
  • Galaxies are moving too fast in clusters of galaxies to be held together by the gravity of the visible stellar material
  • Total Mass ~ 10x visible mass
hot gas in galaxy clusters
Hot Gas in Galaxy Clusters
  • Galaxy clusters are filled with hot gas that emits x-rays. Temperature ~ 100 million K
  • Pressure of hot gas must be balanced by gravity to hold it together in the cluster.
  • Total Mass ~ 10x visible mass
gravitational lensing
Gravitational Lensing
  • Mass produces gravity
  • Gravity bends light
  • Gravity can distort the image of an object behind the mass
what is dark matter
What is Dark Matter?
  • We DON’T KNOW!!!
  • We only know what it is not
    • It is not ordinary matter composed of protons, neutrons, electrons, etc.