Lecture #21: Plan. Normal Galaxies. Classification Properties Distances

Transcription

1 Lecture #21: Plan Normal Galaxies Classification Properties Distances

2 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

3 Late 20 th Century: A Sky Full of Galaxies! Zone of avoidance

4 The Zone of Avoidance Dust and the center of our own Milky Way galaxy merely blocks our view there is no zone of avoidance!

5 Early 21 st Century: Wide variety of types!

6 Spiral Galaxies (fast rotator) Rotating disk & spiral arms Gas and dust Young stars in the disk

7 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!

8 Elliptical Galaxies (slow rotator) No disk, no spiral arms Ellipsoidal shape Smooth, featureless appearance Almost no gas and dust Old stars

9 Irregular Galaxies (irregular motion) No particular shape Often rich in gas and dust (as much as ~50% in mass) Young stars

10 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

11 The Galaxy Zoo Project Citizen science: You get to classify galaxies! If you re intrigued by this idea, check it out:

12 Distances to Galaxies Measure apparent brightness of standard candles with known luminosity within these galaxies B ~ L / d 2

13 Cepheid Variables Cepheid Variables (Henrietta Leavitt, 1912) Time-averaged luminosities of Cepheid variables are directly related to their pulsation periods L P

14 Light Curves: Brightness vs time

15 Cepheid and RR Lyrae Variables Period-Luminosity Relation

16 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 )

17 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

18 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 )

19 New (>1990) Method: Supernovae Use supernovae as standard candles to get an independent measurement of the distance (more on this later )

20 Distances to other galaxies Results: distances to galaxies are huge! d ~ s x 10 6 pc ~ s Mpc! The universe is mostly empty!

21 Distances to other galaxies Nearest companions: Large & Small Magellanic Clouds (~50 60 kpc < 0.1 Mpc) Andromeda Galaxy (Messier 31; ~0.7 Mpc) ~10o

22 Distances to other galaxies Nearest companions: Large & Small Magellanic Clouds (d ~ kpc < 0.1 Mpc) Andromeda Galaxy (Messier 31; d ~ 0.7 Mpc)

23 Distances to other galaxies Most distant resolved galaxy: SPT0615-JD (HST image published in 2018) 13.3 billion light-years away

24 Distances to other galaxies Most distant galaxy: GN-z11 (discovered in 2016 with HST) 13.4 billion light-years away

25 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,