1AST 103Final Exam ReviewMay 6 2008, 9-10 pmVanVleck B130Notes about the final exam:• Saturday May 17th, 7:45 AM-9:45 AM• Chamberlain 2103• If you have a CONFLICT email me or Ella before theend of this week. No excuses accepted after exam.• Comprehensive, covering all material in the course.• Focus (at least half) on new material since last exam.• 50 questions,multiplechoice•Somefractionwillbequestionsyouhaveseeninquizzesorclass.•25%ofyourgrade2• 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 the center 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.Two 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.3The 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 IstarsThe 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 Galaxy4Emission 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!Reflection Nebulae• When the cloud of gasand dust is simplyilluminated by nearbystars, the light reflects,creating a reflectionnebula• Typically glows blue5Dark 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.6Our Galactic Neighborhood• The smallest organization ofgalaxies are called galaxy groups• Our local group is called the LocalGroup• The Local Group contains 40 knownmembers, including the AndromedaGalaxy and the Large and SmallMagellanic clouds, dwarf satellitegalaxies of the Milky Waywww.astronomy.com/asy/objects/images/7Spiral Galaxies• Spiral arms and a central bulge• Type SElliptical Galaxies• No spiral arms• Ellipsoidal shape• Smooth, featureless appearance• Type E8Irregular Galaxies• Stars and gas cloudsscattered in random patches• No particular shape• Type IrrDifferences in Star andGas Content• Ellipticals:– Low in gas and dust, so containsmostly older Pop II stars– Contain very high temperature, verylow density clouds of gas that cannotcondense into stars.• Spirals:– Lots of gas and dust, so have activeregions of star formation– Have both Pop II and younger Pop Istars• Irregulars:– Many hot, young stars– Large amounts of interstellar matter– Might be young galaxies9Barred Spirals• Related to regular spiral galaxies,barred spirals are noted for theirlarge bar of stars across the centralbulge• Type SB• The arms of the spiral begin at theend of the bars• Recently discovered that the MilkyWay is a barred spiral!S-zero galaxies• S0 galaxies are in theshape of a disk, buthave no spiral arms• It is likely that the gasand dust have beenblown out of suchgalaxies• The lack of gas anddust means that no newstars can form, so thereare no spiral arms10The Tuning Fork• Edwin Hubble organized thesedifferent galaxy types into a tuningfork shaped diagram• Ellipticals are labeled E0-E7– E0 is almost perfectly spherical, E7is quite flattened• Spirals are labeled Sa – Sd– Sa galaxies have tightly woundarms and a large central bulge– Sd galaxies are loosely woundand have a small central bulge• Barred Spirals are labeled SBa– SBd– Same flow as the SpiralsA look back in time• The Hubble Space Telescopewas pointed at a part of the skythat looked empty, taking a 100-hour exposure• Very distant galaxies weredetected, some closer than others• This technique allows us to seegalaxies at various stages offormation• These early galaxies tend to besmaller than the Milky Way, andto not fall into Hubble’sclassification scheme11Distances to other galaxies• We can use Cepheid variablestars to measure the distance toother galaxies• A Cepheid’s luminosity isproportional to its period, so ifwe know how rapidly it brightensand dims, we know much energyit emits• If we see a Cepheid in anothergalaxy, we measure its period,determine its luminosity, andcalculate its distance!• Distance between galaxies ishuge!– M31 is 2 million Lightyears away– M100 is 55 million Light yearsaway.Tully- Fisher Relation12Calculating the Mass of the Galaxy• The rotational velocity of the Sunaround the center of the galaxy canbe used to estimate the galaxy’s mass• The combined gravitational effect ofall mass within the Sun’s orbit isequivalent to one large lump of massat the center of the galaxy• Newton’s Law of Gravitation showsthat the mass of all matter within theSun’s orbit is 9×1010 solar masses!• We can estimate the mass of theentire galaxy be measuring theorbital velocity of small satellitegalaxies in