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? Galaxies are vast collections of stars (~1011) and sometimes gas and dust as well
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 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 • V=HD • D = VT = V/H • T = D/V = 1/H Age if expansion not accelerated or decelerated
Questions: How did galaxies form? Why are there different types of Galaxies?
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 • Assumptions: • Matter originally • filled all of space • almost uniformly • Gravity of denser • regions pulled in • surrounding • matter
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? Possible explanations • Rate of star formation • Amount of Rotation • Collisions & mergers
Density Excess? Higher density proto-galactic clouds form stars more rapidly, use up all their gas before it can form a disk.
Rotation? Larger rotation produces more disk-like distribution of matter.
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 & 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 Evidence of galaxy interactions via gravity Burst of star formation
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 Denser cloud More collisions (hundreds to thousands of galaxies)
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! • 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 • Rotation of galaxies • Motions in clusters of galaxies • Hot x-ray emitting gas in clusters of galaxies • Gravitational lensing
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 Curve of Milky Way stays flat with distance Mass must be more spread out than in solar system
Milky Way Significant mass exists outside radius of most stars!
Mass of Milky Way Mass within Sun’s orbit: 1.0 x 1011MSun Total mass: ~1012MSun -> Dark Matter
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 • 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 • Mass produces gravity • Gravity bends light • Gravity can distort the image of an object behind the mass
Galaxy cluster acts as gravitational lens. Focuses image of galaxy behind it into blue arcs.
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.
Galaxies are arranged like soap bubbles with voids devoid of galaxies inside