Our GalaxyGuiding QuestionsHenrietta Leavitt & the Cepheid P-L Relationship Nature of the Spiral Nebulae and the Great Debate1923 - Hubble Measures Distance to M 31 using Cepheid VariablesStellar Photometric DistancesThis dilemma was resolved by observing parts of the Galaxy outside the diskDetermining the distance and direction of the globular clusters gave us the Sun’s locationObservations at nonvisible wavelengths reveal the shape of the GalaxyThere are about 400 billion (4 × 1011) stars in the Galaxy The spin-flip transition in hydrogen emits 21-cm radio wavesThese emissions easily penetrate the intervening interstellar dustSpiral arms can be traced from the positions of clouds of atomic hydrogenOB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral armsThe rotation of our Galaxy reveals the presence of dark matterOnly about 10% of this mass is in the form of visible stars, gas, and dust Spiral arms are caused by density waves that sweep around the GalaxyA strong radio source called Sagittarius A* is located at the galactic centerKey WordsOur GalaxyChapter Twenty-FiveGuiding Questions1. What is our Galaxy? How do astronomers know where we are located within it?2. What is the shape and size of our Galaxy?3. How do we know that our Galaxy has spiral arms?4. What is most of the Galaxy made of? Is it stars, gas, dust, or something else?5. What is the nature of the spiral arms?6. What lies at the very center of our Galaxy?Interstellar dust obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic diskWilliam Herschel maps out the distribution of stars and gets:The sunThe “universe”of HerschelWilliam Parsons, 3rd Earl of Rosse, builds the “Leviathan of Parsonstown” - draws “spiral nebulae”What were they?Stars & planetary systems forming in our own “universe”?Separate “island universes”?Henrietta Leavitt & the Cepheid P-L RelationshipLight curve of a Cepheid variableLarge & Small Magellaic Clouds Period versus magnitude of Cepheids in SMCH. Shapley maps distribution of Globular Star Clusters using “Cepheids”(“where’s the mass centered?”)We are NOT at the center.What happened?Globular cluster with variable stars“Dust Happened”At visible wavelengths, the center of our galaxy suffers ~ 30 mag of extinction by dust!! Even with big modern telescopes, we cannot see very far in the plane of our galaxy at visible wavelengthsThe Shapley-Curtis Debate (1920)MWGMWGShapleyCurtisThe debate solved nothing!Questions in science are not resolved by debates, but by observations & experimentsNature of the Spiral Nebulae and the Great DebateShapley Novae brightnessesincompatible with M31 being as big as MWG Rotation of M101Curtis Novae indicate a smaller MWG than Shapley’s Galaxy proper motions undetected Zones of avoidance in other systems1923 - Hubble Measures Distance to M 31 using Cepheid Variables100-inch Hooker Telescope, Mt. WilsonEdwin HubbleDebate OVER!Discovery of Cepheids in M 31Star cluster is really HEREBut the extinction makes it fainter, so we would incorrectly think that it is HERE based on brightness measurementsRegion with dust absorption: A magsIgnoring the extinction due to dust will result in deriving a photometric distance that is too large by a factor of 10A/5!Trumpler - 1929Shapley’s MWG was too big for a couple of reasons:Using RR Lyrae stars & Type II Cepheids, thinking they are Type I, will make the distances appear largerFor a given apparent brightness, a higher L star must be more distant. Observed Type II’s, but used L’s of Type I’s.Other problems:S Andromedae, a “nova” in the Andromeda Galaxy, was actually a supernova -with much higher L and hence distanceProper motions in galaxies “measured” would require speeds greater than light if they were distant - these measurements turned out to be wrong!Summary: Shapley’s MWG was too big, and his distances to the spiral nebulae too smallStellar Photometric DistancesFor an apparent (observed) magnitude m, absolute magnitude M, and distance d in parsecs:Without dust: m = M+5logd-5 and so d = 10(m-M+5)/5 pc(reminder: m = Md=10pc)With dust: m = M+5logd-5+A and d = 10(m-M-A+5)/5 pcwhere A is the extinction by dust in magnitudes(Note: sometimes astronomers use the “distance modulus”m-M = 5logd-5 to express the distance to some objects)This dilemma was resolved by observing parts of the Galaxy outside the diskDetermining the distance and direction of the globular clusters gave us the Sun’s locationOur Sun lies within the galactic disk, some 8000 pc (26,000 ly) from the center of the GalaxyObservations at nonvisible wavelengths revealthe shape of the GalaxyThere are about 400 billion (4 × 1011) stars in the Galaxy• Our Galaxy has a disk about 50 kpc (160,000 ly) in diameter and about 600 pc (2000 ly) thick, with a high concentration of interstellar dust and gas in the disk• The Sun orbits around the center of the Galaxy at a speed of about 790,000 km/h• It takes about 220 million years to complete one orbit• The galactic center is surrounded by a large distribution of stars called the central bulge• This bulge is not perfectly symmetrical, but may have a bar• The disk of the Galaxy is surrounded by a spherical distribution of globular clusters and old stars, called the galactic haloThe spin-flip transition in hydrogen emits 21-cm radio waves• This is the same physical principle behind magnetic resonance imaging (MRI),an important diagnostic tool of modern medicineThese emissions easily penetrate the intervening interstellar dustSpiral arms can be traced from the positions of clouds of atomic hydrogenOB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral armsThe rotation of our Galaxy reveals the presenceof dark matterFrom studies of the rotation of the Galaxy, astronomers estimate that the total mass of the Galaxy is about 1012MOnly about 10% of this mass is in the form of visible stars, gas, and dust• The remaining 90% is in some nonvisible form, called dark matter, that extends beyond the edge of the luminous material in the Galaxy• Our Galaxy’s dark matter may be a combination of MACHOs (dim, star-sized objects), massive neutrinos, and WIMPs (relatively massive subatomic particles)Spiral arms are caused by density waves that sweep around the Galaxy• There are two leading theories of spiral structure in