I. Stars have color, brightness, mass, temperature and size. II. Distances to stars are measured using stellar parallax a. The further away, the less offset b. Parallax angles are extremely small c. Measured using photographs six months apart d. Distances reported in light years--light travels 9.5 trillion km/year III. Binary star pairs a. Mutually orbit one another around a center of mass b. At least half of stars are binary c. Speed of orbits can tell us mass of each star in system IV. Stellar brightness function of distance, temperature and size a. Color of stars tells us their temperature i. Blue stars are hotter ii. Red stars are cooler b. Most stars have a specific ratio of absolute brightness to color i. Shows mass of star 1. larger are hotter, bluer, brighter 2. smaller are cooler, redder, dimmer ii. Hertzsprung-Russell diagram iii. Main sequence stars 1. blue, bright, large, hot 2. yellow, medium 3. small, dim, red, cool 4. Sun a. in middle of main sequence between ends b. More stars have been observed smaller, cooler than larger and hotter iv. exceptions include red giants and white dwarfs 1. color vs brightness a. color shows temperature b. brightness shows size 2. red giants a. main sequence stars larger brighter are bluer b. these are brighter, but not bluer, must be cool 3. white dwarfs a. hotter by color b. smaller by faintness 4. there is a progression through star types with age c. some stars have variable brightness i. pulsating of cepheid variables 1. Polaris has four day cycle light varies about 10% 2. absolute brightness related to period of brightness: longer period are brighter stars ii. eruptive variables perhaps stellar pair swapping H 2 irregular period of nova stars: Nova Herculus, Nova Persei Beyond the Solar System 2006 Oct 17 Page 1 of 5
V. Interstellar matter dust and gases of Nebulae large and massive a. emission (bright) nebula absorbs ultraviolet, emits visible light fluorescence: Orion Nebula b. reflection nebula composed of interstellar dust reflects nearby star light: Pleiades star cluster c. dark nebula has no nearby star light to reflect appear opaque VI. Stellar evolution a. Knowledge of age of stars i. Know distance to stars by parallax calculations ii. Know speed of light 1. calculate how long it takes to get light here 2. what we see happened a long time ago when star was young iii. Distant stars are different uniformly than closer ones!! iv. Implication that older stars are different than young ones b. Stars exist due to gravity i. Gravity compresses H 2 into He nuclear fusion releases energy ii. Gases of stars held out of complete collapse by its gravity by thermonuclear energy released by the fusion of H 2 into He c. Stages i. Star birth from contracting cloud of 92% H 2, 7% He ii. Protostar gravity heats mass to 10 million K, begins fusion iii. Main sequence stars stable because gravity balanced with outward pressure of release of thermonuclear energy of fusion 1. massive ones fuse H 2 ( burn fuel) more quickly than smaller stars 2. depletion of fuel after ~90% of lifespan a. upon depletion of H 2 rapid evolution b. dies shortly unless: 3. may extend lifespan by becoming red giant iv. Red Giant Stage 1. hydrogen becomes depleted in core, burning continues in outward migrating area a. no fusion in core has no support b. gravity begins to contract He core 2. core collapses, a. gravitational contraction increases temperature i. heat invigorates hydrogen fusion in outer star shell ii. expands outer envelope of star to giant iii. outer surface is cooler, because of large size compared to total energy iv. can becomes oscillating in size from porpoise of expansion and contraction b. increased gravitational contraction in core raises temp. to point where He fuses to C c. can fuse up to element 26: Fe, in hotter stars Beyond the Solar System 2006 Oct 17 Page 2 of 5
v. Burnout and death exhaustion of nuclear fuel 1. low mass stars collapse to white dwarf, a. never hot enough for helium fusion to carbon do not become red giants b. emission of heat energy of gravitational collapse to 25,000 K c. no nuclear fusion glow because of heat d. eventual cooling goes to black dwarf e. extremely dense spoonful=several tons 2. medium stars like Sun a. may go through giant phase before becoming white dwarf i. giant phase accelerates burning of H 2 ii. white dwarf is as dense of matter as physics will allow b. cast off their outer atmosphere into a nebula during red giant phase c. Helix Nebula in Aquarius nearest to us i. 450 light years away ii. ring shape is the greater material we see through in sides of sphere d. eventual collapse of core into white dwarf 3. large stars explode in a supernova a. consumes fuel rapidly b. collapses in without fusion pressure to hold it out c. implosion shock wave causes explosion as a supernova, 1 million times brighter than before i. blows outer shell into space ii. interior collapses into dense material: denser than matter iii. becomes neutron star or black hole iv. Crab Nebula 1. from supernova event observed in 1054 AD 2. 6300 light years away means? v. Veil Nebula in Cygnus 1. supernova 5000 to 10,000 years ago 2. 1400 l.y. away vi. Eagle Nebula in Serpens 1. example of supernova 2. creating stars in present nebula 3. stars have heavier elements from supernova event than original generation of star formation Beyond the Solar System 2006 Oct 17 Page 3 of 5
d. neutron star i. electrons collapsed to combine with protons neutron ii. pea-size=100 million tons iii. strong magnetic field, rotation, radio emissions=pulsar iv. Crab nebula supernova observed in 1054 AD has a pulsar at center e. black hole i. so much gravity that photons cannot escape ii. matter being engulfed emit x-rays Gravity of an object depends upon its mass. When stars collapse, those more massive have more gravity, to pull its material a greater amount inward. The more massive the star, upon its collapse, the smaller the resulting body becomes, with greater gravity than collapsed stars with lower-mass. VII. Stellar remnants a. White dwarfs extremely small with high density i. Degenerate matter with electrons displaced inward toward nucleus of atom ii. Very hot 25,000 K, 1. without energy source 2. remnant heat from gravitational collapse iii. will cool over time to become black dwarf none yet in universe, due to length of time and cooling time b. neutron stars smaller in radius, more massive than white dwarfs i. electrons combine with protons to become neutrons ii. Earth-sized body collapse to density of neutron star size of a football field iii. Supernova event collapses star to neutron body that 1. has a very strong magnetic field, 2. has high rotation rate, 3. generates radio waves from magnetic poles iv. result is pulsar 1. there is a pulsar in Crab Nebula 2. remnant of star that exploded in supernova c. black holes i. extreme collapse into ultra-dense material ii. gravity of extremely massive object so great that even light does not have velocity to escape its grasp iii. extremely hot iv. generate x-ray frequency of electromagnetic radiation when matter is engulfed v. can find them by looking at binary star systems 1. x-ray source in Cygnus found by satellites 2. orbiting supergiant star around x-ray source Beyond the Solar System 2006 Oct 17 Page 4 of 5
VIII. IX. Galaxies a. Milky Way Galaxy i. 100 billion stars, interstellar matter, ii. spiral arm structure 1. extremely flat 2. bulge in center iii. 100,000 light years across b. Types of galaxies i. Spiral like the Milky Way and Andromeda 1. most are large 2. mixed ages of stars a. older in central part b. youngest in arms ii. Elliptical galaxies often much smaller 1. most galaxies are of this type (60%) 2. stars in these are old stars iii. Irregular galaxies like the Magellanic Clouds stars are forming in the Magellanic Clouds mostly young stars in these c. Galaxies occur in clusters, and clusters occur in superclusters Red Shifts a. Most galaxies have red shift of spectrum b. Dimmest ones have greatest red shift c. Implications i. they are going away from us, and ii. farther ones going away faster d. The raisin bread dough analogy i. Every raisin is moving away from every other raisin ii. Those further apart are moving away faster than those close together X. Big Bang a. 14 billion years ago there was a cataclysmic explosion b. All matter and space were created at this moment c. Cooling and condensation created stars d. Will there be an end to the expansion?? i. Either we will keep moving away from other galaxies or ii. At some ultimate distance, the galaxies will begin to be drawn in by gravity to the Big Crunch e. Absence of evidence is not evidence of absence Beyond the Solar System 2006 Oct 17 Page 5 of 5