ASTR 1120 REVIEW General Astronomy: Stars & Galaxies � NNOUNCEMENTS • Midterm #3 this Thursday 11/17; covering lect. 15-21 (and corresponding reading in Ch. 19-22) • last part of class today will be review • HW #8 due today, 5pm • HW #9 due Tue, 12/01, by 5pm ALTERNATE FINAL on Monday, Dec.7th, 5:30pm- 7:00pm, in Muenzinger E131 Numerical simulations of structure formation Lessons from Imaginary Universes • Cold (Slow) dark matter works better than hot (fast) dark matter • Neutrinos are too fast– structure would be smeared out • What is slow and dark enough? We don’t know yet! – Particle experiments under way…..
Very important diagram Dark Matter and the Fate of the Universe “Average distance SIZE between • Expansion begins with the Big Bang (we’ll talk about galaxies” this after the break) measure of “expansion factor of Universe” • At that point, everything in the universe is flung apart at outrageous speeds! NOW Hubble constant NOW TIME • Several different models for Past and Future sets how fast depending upon the amount of dark matter universe is expanding NOW Big Bang = when distance zero The expansion rate of the universe is Since gravity is what pulls everything not necessarily constant for all time back in, there must be a magic number • GRAVITY should SLOW • Just the right amount of mass (in our current expansion rate � deceleration universe) to slow down expansion but not enough to cause recollapse • Different models for different amounts of dark • We call this exact amount of matter, the matter CRITICAL DENSITY – Let’s ignore accelerating for now ~10 -29 grams/cm 3 = a few atoms in a closet
Critical Universe Recollapsing Universe Flat Universe Closed Universe • Dark matter density is • Density of greater than matter = “critical density” “critical density” • Expansion will stop in the • Will expand future, will forever, but collapse back more and in more slowly – Big Crunch with time – Oscillations? Coasting Universe Three models for fates of universe Open Universe • The universe has always expanded at the same rate (no deceleration due to gravity!) • The age of the Universe = 1/H o CLOSED FLAT OPEN
Clicker Question Which model predicts the youngest • A. Recollapsing age for the universe today? • Age of universe is A. Recollapsing how far to left curves hit (closed) horizontal axis B. Critical (distance between (flat) galaxies = 0) C. Coasting (open) Clicker Question Clicker Question What determines the future average What determines the future average distance between galaxies? distance between galaxies? A. The rate of expansion, with a slower expansion A. The rate of expansion, with a slower expansion rate meaning a greater average distance rate meaning a greater average distance B . The rate of expansion, with a faster expansion B . The rate of expansion, with a faster expansion rate meaning a greater average distance rate meaning a greater average distance C. Only on the density of matter in the universe. C. Only on the density of matter in the universe.
Clicker Question Clicker Question Which model has the slowest future rate of Which model has the slowest future rate of expansion? expansion? A. Recollapsing A. Recollapsing B . Critical B . Critical C. Coasting C. Coasting Which is it? What is the fate of the Universe? Is there enough dark matter • Recollapse to big crunch: to recollapse the universe? – Crushing heat Baryonic matter: only a few % of critical density – Destruction of all matter – Rebirth? Dark matter: only about 25% of what is needed • Eternal expansion: • Universe should be in between the “coasting” and “critical” models – Cold, galaxies dimming – Star formation slowing The Universe will expand forever – Everything winds up as a brown dwarf, black dwarf, neutron star or black hole … and with an unexpected twist…
A New Twist for the 21 st Using supernovae to determine Century the fate of the Universe • Redshifts of the supernovae gives • Scientists using white their vertical position dwarf supernovae to – Space has measure distances stretched since discovered something they gave off their light quite strange • Apparent brightness gives their horizontal position – Dimmer=more distant = high lookback Supernovae are not consistent even with How can the universe be the expectations of a coasting universe!!! accelerating??????? A force that counteracts gravity? “ Dark energy ”– outweighs every other form of mass/energy! Truly an unknown force in all of physics The Cosmological Constant actually exists! Universe is accelerating?!?! (“Einstein’s Greatest Blunder” )
Clicker Question Four models for fates of universe What is meant by “dark energy”? A. The energy associated with dark matter through E=mc 2 B. An unknown form of energy that counteracts gravity and causes the expansion of the universe to accelerate. C. Any unknown force that acts like gravity D. Highly energetic particles that are believed to CLOSED constitute dark matter FLAT E. The total energy in the universe after the Big OPEN ACCELERATING Bang but before the first stars Clicker Question What is meant by “dark energy”? REVIEW FOR MIDTERM III A. The energy associated with dark matter through E=mc 2 to follow….. B. An unknown form of energy that counteracts gravity and causes the expansion of the universe to accelerate. C. Any unknown force that acts like gravity D. Highly energetic particles that are believed to constitute dark matter E. The total energy in the universe after the Big Bang but before the first stars
Disk, Bulge & Halo Disk is very thin! • Disk: includes spiral arms -- young, new star formation • Bulge & Halo: older stars, globular clusters Artist’s sketch Galaxies: Ultimate Recyling Plants
We observe star-gas-star cycle operating in Milky Way’s Summary of Galactic Recycling disk using many different wavelengths of light From HOT to COLD • Stars make new elements by fusion • Dying stars expel gas and new elements, producing Gas Cools hot bubbles (~10 6 K) • Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K) • Further cooling permits molecules to form, making molecular clouds (~30 K) • Gravity forms new stars (and planets) in molecular clouds Dark matter halo for galaxies Stars appear to be • Dark matter extends beyond visible part of orbiting something the galaxy -- mass is massive but ~10x stars and gas! invisible … a black hole! • Probably not normal mass that we know of (protons, neutrons, electrons). Orbits of stars indicate a mass of • Most likely subatomic about 3-4 million particles, as yet M sun within 600 unidentified (weakly interacting massive R Schwarzchild particles – WIMPs?)
Summary “Hubble’s Law” v = H o � d “Distance Ladder” to measure universe Velocity of Hubble’s Distance Recession Constant (Doppler Shift) (km/sec) (km/sec/Mpc) (Mpc) velocity Best current values for expansion H o = 71 +/- 4 distance Different standard candles are useful km/s/Mpc for different distances Balloon analogy for Forming a disk expanding universe with spiral • Each dot on the • As more material balloon can be collapses, angular thought of as a momentum spins it galaxy. into a disk As the balloon • Stars now formed expands, galaxies in dense spiral move farther away arms – disk stars from each other are younger! Angular momentum of protogalactic cloud important in spiral galaxy formation
Or perhaps a different scenario…. Making ellipticals 1. Higher density: much • Spiral galaxy collisions faster star formation destroy disks, leave uses up all the gas behind elliptical – Nothing left to make a disk • Burst of star formation or uses up all the gas 2. Lower spin – Gas used up before • Leftovers: train wreck angular momentum took over • Ellipticals more common • Now we see a sphere in dense galaxy clusters of old stars (centers of clusters contain central dominant galaxies) NGC 4038/39 Antennae “Central Engine” -- artist’s conception REVIEW AGNs Central Engines • Accretion disk around super- How do AGNs emit so massive black much light in so little hole space? • Disk itself may or • They are powered by may not be obscured accretion disks around JET by dust supermassive black holes • If bright nucleus is visible, looks like a • In some AGNs, huge DISK quasar, if not, then jets of material are shot its a radio galaxy out at the poles. These jets are strong radio sources.
Prototypical “radio galaxy” Giant elliptical galaxy NGC 5128 with dust lane (from spiral galaxy?) + Centaurus A radio source (color lobes)
Recommend
More recommend
