1• Quiz #4 today covers Lectures 9-16• Exam #2. Next Monday (April 7th), covers Lectures9-16 (everything about stars).• “Sample exam”/review sheet posted.• Review sessions (Me on tonight 7-8 pm: Ella onThursday 6-7pm pm)• Next assignment is posted (Lab #1 write-up, dueApril 14th (mon 1 week after exam).Black Holes2What would happen to the earth if thesun were instantly turned into a solar-mass black hole?• A) We would fall into the center of the BH.• B) We would instantly be turned into spaghetti.• C) We would immediately be vaporized.• D) Nothing, the Earth would continue to orbit the BH,(although it would be cold.)The Escape Velocity Limit• The velocity necessary toavoid being gravitationallydrawn back from an object (theescape velocity) is:• Note that as R decreases, theescape velocity increases• A white dwarf’s escapevelocity is around 6000 km/s,ten times faster than the Sun’s• (a white dwarf is around 100times smaller than the Sun)• Also recall that nothing can travelfaster than the speed of light, c, or 3×108 m/s• If a stellar core is compressed somuch that its radius is smaller than(the Schwarzschild radius) thennothing can escape from itsgravitational force, including light!• The core would become a black hole! Vesc=2GMR! RS=2 " G " Mc23Black HolesIf you fell into a black hole what would you observe asyou crossed the event horizon?a) Everything would go black?b) Everything would go white hot like enetring thesurface of the sun?c) Nothing would appear to be different?d) I would see the universe, stars and galaxies grow oldand die?4Viewing a black hole• You may be asking, “If light cannotescape a black hole, how can we seeone?”• If a black hole is in orbit around acompanion star, the black hole canpull material away from it.• This material forms anaccretion disk outside ofthe event horizon andheats to high temperatures• As the gas spirals into theblack hole, it emits X-rays, which we can detect!Blackhole spectrum5PART 3 GalaxiesOur galaxy, the “Milkyway”Our Galaxy, the Milky Way• A galaxy is a largecollection of billions ofstars• The galaxy in which theSun is located is calledthe Milky Way• From our vantage pointinside the galaxy, theMilky Way looks like aband of stars across thenight sky, with dark dustlanes obscuring thecenter of the band.67• The Milky Way is a spiral galaxy approximately 30 kps (100K Ly) across.• The Sun is located around 8 kpc from the center, in one of the spiral arms.• Most of the stars are concentrated in the galactic plane, or in the central bulge at thecenter of the galaxy• Inside the bulge is the nucleus of the galaxy• Surrounding the disk is a roughly spherical distribution of stars called the halo.• Globular clusters are distributed throughout this halo, surrounding the center of the galaxy.8Two Stellar Populations: Pop I and Pop II• Stars in neighboring galaxies are segregated– Younger, blue, “Population I” stars were found mostly in the disksand spiral arms. Typically less than a few billion years old; Followcircular orbits in the galactic plane– Older, red, “Population II” stars were found mostly in the halo andcentral bulge; More than 10 billion years old; Follow randomelliptical orbits around the galactic center – not in the plane.The primary reason that massive O-type stars are notfound in the galactic halo is because they are a) too massive to be kicked into the halo from the disk. b) so massive that they settle into the thinner disk. c) too short-lived to have persisted from haloformation until today. d) closer to us in the disk than in the extended halo.9The Formation of the Milky Way• Our galaxy likely began 13 billionyears ago as a huge cloud of purehydrogen and helium, slowly rotatingand collapsing• The first stars formed (Pop III?)within this cloud, burning outquickly and violently. This addedheavy elements to the cloud• Population II stars formed next,capturing some of the heavyelements and settling into ellipticalorbits around the center of the cloud• As the collapse continued, a diskformed, and Population I starsformed from the ashes of dying Pop IstarsCompared to stars like the Sun in the disk of the Milky Way, stars that populate theextended spheroidal halo of the galaxy were born a) earlier, so have had time to accumulate more heavy elements. b) later, so have used up their heavy elements. c) earlier, from more nearly primordial material, so have fewerheavy elements. d) later, so have accumulated more heavy elements from previousgenerations of stars.10Mapping the Milky Way’sspiral arms• Once this difference betweenPopulation I and II stars wasnoted, astronomers couldmap our galaxy’s arms• Population I stars are mostlybright, blue stars (hot O andB stars) found in the disk• By measuring the location ofO and B stars near the Sun,the first pictures of the MilkyWay’s spiral structure wereproduced.• Dust and gas obscure thelight from more distant stars,so the map is incomplete.The Interstellar Medium• Space is far from empty!– Clouds of cold gas– Clouds of dust• In a galaxy, gravity pulls the dustinto a disk along and within thegalactic plane• This dust can obscure visible lightfrom stars and appear to be vasttracts of empty space• Fortunately, it doesn’t hide allwavelengths of light!The Sombrero Galaxy11Emission Nebulae• We frequently seenebulae (clouds ofinterstellar gas and dust)glowing faintly with a redor pink color• Ultraviolet radiation fromnearby hot stars heats thenebula, causing it to emitphotons• This is an emissionnebula!12Reflection Nebulae• When the cloud of gasand dust is simplyilluminated by nearbystars, the light reflects,creating a reflectionnebula• Typically glows blue13Dark Nebulae• Nebulae that are notilluminated orheated by nearbystars are opaque –they block most ofthe visible lightpassing through it.• This is a dark nebulaInterstellar Reddening• As starlight passes through a dustcloud, the dust particles scatter bluephotons, allowing red photons to passthrough easily• The star appears red (reddening) – itlooks older and dimmer (extinction)than it really is.14If one region of the sky shows nearby stars but nodistant stars or galaxies, our view is probably blockedby a) nothing, but directed toward a particularly emptyregion of space. b) an emission nebula of ionized gas. c) an interstellar gas and dust cloud. d) a concentration of