Astronomy 113 Dr. Joseph E. Pesce, Ph.D. Distances & the Milky Way 14-2 Historical Overview: the Curtis-Shapley Debate ³What is the size of our galaxy? ³What is the nature of spiral nebula? The Curtis View ³Our galaxy is small (10,000pc) ³Spiral nebula are not part of our galaxy 14-3 ³Occurred in 1920 in DC ³Did not resolve issues ³Both partly right and wrong 14-4 The Shapley View 14-5 Our Galaxy ³Our galaxy is large (100,000pc) ³Spiral nebula are gas clouds within our galaxy You are here ³Measuring the shape of our galaxy is hard: we re inside! 1
14-6 View From Earth 14-7 The Milky Way The Milky Way is a faint band of stars that circles the sky ³We ve known the Milky Way is composed of stars since Galileo (400 years ago) ³What does this tell us about the shape of the galaxy? You are here A Disk A big heap of stars! 14-8 ³Most stars are in a narrow band on the sky ³Most stars seem to be in a disk-shaped arrangement 14-9 Early Observations ³William Herschel, in Britain in the 1780s ³Assumed stars were of uniform brightness and distributed evenly ³Didn t know about interstellar extinction ³Counted stars in different directions to infer shape Herschel s Results You are still here A big heap of stars! ³We are at the center of the galaxy ³Spiral nebula are probably other galaxies 14-10 14-11 Interstellar Extinction ³Interstellar gas and dust absorb, redden, and polarize light that passes through it. ³Didn t know about extinction prior to about 1900 ³Affects all observations 2
What Herschel Saw 14-12 Limit of Herschel s observations You are here Globular Clusters 14-13 ³Shapley noticed that globular clusters are located in one area of the sky, near the center of the Milky Way ³Perhaps they are orbiting the galaxy ³How do you measure distances to these clusters? ³He was only observing a very small portion (5%) of the galaxy ³His view was distorted by interstellar extinction Cepheid Variable Stars 14-14 ³Very high luminosity stars (red giants) ³Found in globular clusters (and elsewhere) ³Are variable ³Period-luminosity relationship ³By measuring the period, get luminosity ³By knowing luminosity, get distance (How?) 14-15 Cepheid Period-Luminosity Relationship Distribution of Globular Clusters 14-16 Dust? 14-17 What about dust? You are here ³Most interstellar medium is in disk of galaxy ³Most globular clusters are outside of disk 3
3/27/18 14-18 Herschel & Shapley s Results Combined 14-19 What About Other Galaxies? ³Cepheids can be observed out to about 200 million ly Halo Bulge Disk Nucleus 14-20 14-21 Standard Candles ³Objects with known intrinsic luminosity ³Luminosity is the same wherever object is ³Luminosity is known fairly accurately ³Comparing absolute and apparent magnitudes gives distance ³Types The Current Picture ³ Our galaxy is 30 kpc in diameter ²It contains 100 billion stars ³There are other galaxies beyond ours ²100s of billions ³The universe is really big! ²Cepheids ²Supernovae (Type I) ²Red Giants ²Others 14-22 14-23 Our Galaxy Schematic Picture ³ Disk Halo ²Where all the blue stars and ISM are found Bulge Halo ³Bulge ²Spherical region near galaxy s center; old stars Disk ³Nucleus ²The central region of the galaxy ³Halo ²Spherical region surrounding disk and bulge; old stars Nucleus 4
The Disk 14-24 The Disk 14-25 ³Disk has lots of gas and dust the ISM ³Optical telescopes aren t very useful ³Infrared and radio observations Radio Observations 14-26 ³Atomic hydrogen (HI) emits a 21cm spectral line (in the radio part of the spectrum) ³Most of ISM is hydrogen ³Can thus map the distribution of hydrogen gas in the disk (velocity comes from doppler shift) 21cm Emission Line ³ Spin flip of electron orbiting proton: proton electron proton 14-27 Photon emitted electron High energy (from collision) Lower energy (Spontaneous de-excitation) 14-28 Spiral Arms Spiral Arms ³ISM in disk is distributed in spiral arms ³Most of the young stars in the galaxy are in the spiral arms ³The arms are sites of on-going star formation 5
14-31 Spiral Arms Young Blue Stars ³Stars form in gas clouds ³These clouds are found in the spiral arms ³Therefore young stars are found in spiral arms ³Massive, young stars are bright and die rapidly ³These bright, blue stars (and associated starforming regions) highlight the arms and make them easier to see ³Most stars are low mass, long-lived stars between the arms Red Stars 14-32 ³Most stars in the galaxy are low mass, longlived stars (red stars) ³They are distributed throughout the disk and are between the arms too ³Red stars are predominantly in the bulge and halo too Population I and II Stars ³Population I Like the sun ²A younger generation of stars found in the disk/spiral arms 14-33 ³Population II Older stars ²An older generation of stars, found in the halo an bulge 14-34 14-35 Orbits of Stars ³Stars in the disk have circular orbits ³Stars in the bulge have random, radial orbits Nucleus of galaxy Disk of galaxy Disk Orbits Orbits of stars and gas Side View Stars and gas are rotating in the same direction 6
Nucleus of galaxy Top View 14-36 14-37 Where do the Arms come from? ³Winding? *** don t see this, so arms aren t from orbiting stars What causes them? 14-38 14-39 Density Waves ³Ripples waves travel through galaxy s dust/gas: Spiral Density Waves ²Like traffic jams ³Galaxy rotates causing spiral structure Density Waves ³ Waves travel slower than gas/stars gas piles up ³ Compression causes star formation ³ See arms easier because lots of hot, O & B stars ² But most stars (faint M dwarfs) are between arms Spiral Density Waves 14-40 Halo and Bulge Orbits 14-41 ³Radial orbits through nucleus ³At any given time, stars are in various positions, spherically around nucleus 7
Disk and Bulge 14-42 Disk and Bulge 14-43 Halo (old stars) Globular Clusters (mostly old) Our Galaxy Bulge (old stars) Young stars in spiral arms 14-44 Weighing the Galaxy 14-45 ³Use orbits of stars to measure mass of Galaxy ³(Use Newton s laws of gravity) ³Measure orbital velocity ³Measure distance from nucleus Nucleus Disk (old stars throughout) Side View Rotation Curves 14-46 Results 14-47 ³The mass of the Galaxy is about 10 11 times the mass of the sun (100 billion) ³Plots of orbital velocity versus distance from nucleus ³Gives measure of mass WITHIN each orbit ³There is much more mass than all the stars, gas, & dust, that we don t observe directly ²Some mass is NOT detected through electromagnetic means 8
3/27/18 14-48 14-49 Dark Matter ³Matter detected through its gravity What is Dark Matter? ³We don t know ³It can t be anything too normal or we would detect it ³Good candidates, but no observations yet ²Has no detectable electromagnetic emission ³Over 90% of the mass of the Galaxy is composed of this dark matter ³Found everywhere we can measure it s gravitational influence. Universe is full of it 14-50 Dark Matter Candidates ³Black holes? (Probably not) ²Primordial ones expected but they have evaporated 14-51 Nucleus of the Galaxy What s in the center of the Galaxy? ³Brown dwarfs? (Probably not) ³Neutrinos? (Lots and lots of them, but not massive enough) ³Subatomic particles ²WIMPs ²Higgs particles ²Etc ³Detection may come soon. 14-52 Nucleus of the Galaxy SGr A* - The Dynamical Center 14-53 The Monster in the Middle ³Only radio, IR, and X-ray observations possible (due to large amounts of dust) ³Stars very close to dynamical center (Sgr A*) ³From long-term observations and doppler shifts of these stars => a large central mass ³3 x 106 solar masses within a space 1AU ³A Black hole! 9
Thank You! 10