Lecture #21: Plan Normal Galaxies Classification Properties Distances
Messier 31 = M31 Early 20 th Century The Great Debate (4/26/1920): Harlow Shapley (Mt Wilson) vs Heber Curtis (Lick Observatory) Smithsonian s National Museum of Natural History Three questions:! Nature of spiral nebulae?! Size of our MW Galaxy?! Is the Sun at the center of MW? Shapley: MW is the entire universe, so spiral nebulae are in the MW Curtis: spiral nebulae are MW-like and lie outside of the MW Shapley based part of his thinking on v ~ c rotation measurements of M31! Curtis didn t believe them and was proven to be correct in the end
Late 20 th Century: A Sky Full of Galaxies! Zone of avoidance
The Zone of Avoidance Dust and the center of our own Milky Way galaxy merely blocks our view there is no zone of avoidance!
Early 21 st Century: Wide variety of types!
Spiral Galaxies (fast rotator) Rotating disk & spiral arms Gas and dust Young stars in the disk
Spiral Galaxies: Barred (fast rotator) Same as spirals But have a large bar of stars across the central bulge Arms of the spiral begin at the end of the bar Example: Milky Way!
Elliptical Galaxies (slow rotator) No disk, no spiral arms Ellipsoidal shape Smooth, featureless appearance Almost no gas and dust Old stars
Irregular Galaxies (irregular motion) No particular shape Often rich in gas and dust (as much as ~50% in mass) Young stars
The Hubble Tuning-Fork Diagram (1920s) Ellipticals: E0 E7 E0 is almost perfectly spherical, E7 is quite flattened Spirals and barred spirals: Sa Sd and SBa SBd, respectively Sa and SBa galaxies have tightly wound arms and a large bulge Sd or SBd galaxies are loosely wound and have a small bulge
The Galaxy Zoo Project Citizen science: You get to classify galaxies! If you re intrigued by this idea, check it out: http://zoo1.galaxyzoo.org/
Distances to Galaxies Measure apparent brightness of standard candles with known luminosity within these galaxies B ~ L / d 2
Cepheid Variables Cepheid Variables (Henrietta Leavitt, 1912) Time-averaged luminosities of Cepheid variables are directly related to their pulsation periods L P
Light Curves: Brightness vs time
Cepheid and RR Lyrae Variables Period-Luminosity Relation
Distances to other galaxies Cepheid of period P in our MW Galaxy: B * ~ L / d * 2 Cepheid of same period P in other galaxy: B g ~ L / d g 2 B * /B g = d g 2 / d * 2 d g = sqrt (d * 2 B * / B g ) d g = d * sqrt (B * / B g )
Distances to more distant galaxies Cepheids are too faint to use for these galaxies Early 20 th century astronomers noted: Spectra from most galaxies are shifted towards red wavelengths Shift is largest for dimmer, more distant galaxies
The Hubble(-Lemaître) Law (1929) V = H d V: recession velocity d: distance to galaxy H: Hubble Constant. (~70 km/sec per Mpc) Can be used to determine the distances to galaxies without using the Cepheids! (more on this later )
New (>1990) Method: Supernovae Use supernovae as standard candles to get an independent measurement of the distance (more on this later )
Distances to other galaxies Results: distances to galaxies are huge! d ~ 1 1000s x 10 6 pc ~ 1 1000s Mpc! The universe is mostly empty!
Distances to other galaxies Nearest companions: Large & Small Magellanic Clouds (~50 60 kpc < 0.1 Mpc) Andromeda Galaxy (Messier 31; ~0.7 Mpc) ~10o
Distances to other galaxies Nearest companions: Large & Small Magellanic Clouds (d ~ 50 60 kpc < 0.1 Mpc) Andromeda Galaxy (Messier 31; d ~ 0.7 Mpc)
Distances to other galaxies Most distant resolved galaxy: SPT0615-JD (HST image published in 2018) 13.3 billion light-years away
Distances to other galaxies Most distant galaxy: GN-z11 (discovered in 2016 with HST) 13.4 billion light-years away
Midterm #2 Tuesday 9:30 10:45 am (PHYS 1412 here!) Be there early (~9:15 am) if you can Material: everything not covered by midterm #1, starting with Terrestrial Planets up to Neutron Stars and Black Holes, inclusively. 20 multiple choice questions + 4 problems No need to remember any formula Bring your student ID card Write on the scan sheet: your section #, student ID # Use #2 pencil only No books, calculators, phones, computers, hats,