Astronomy 113 Dr. Joseph E. Pesce, Ph.D.
Distances & the Milky Way
Historical Overview: the Curtis-Shapley Debate ³What is the size of our galaxy? ³What is the nature of spiral nebula? 14-2 ³Occurred in 1920 in DC ³Did not resolve issues ³Both partly right and wrong
14-3 The Curtis View ³Our galaxy is small (10,000pc) ³Spiral nebula are not part of our galaxy
14-4 The Shapley View ³Our galaxy is large (100,000pc) ³Spiral nebula are gas clouds within our galaxy
14-5 Our Galaxy You are here ³Measuring the shape of our galaxy is hard: we re inside!
14-6 View From Earth The Milky Way is a faint band of stars that circles the sky
14-7 The Milky Way 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?
14-8 A Disk A big heap of stars! You are here 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
14-10 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
14-11 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
14-12 What Herschel Saw Limit of Herschel s observations You are here ³He was only observing a very small portion (5%) of the galaxy ³His view was distorted by interstellar extinction
14-13 Globular Clusters 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?
14-14 Cepheid Variable Stars 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
14-16 Distribution of Globular Clusters You are here
14-17 Dust? What about dust? ³Most interstellar medium is in disk of galaxy ³Most globular clusters are outside of disk
14-18 Herschel & Shapley s Results Combined Halo Disk Bulge Nucleus
14-19 What About Other Galaxies? ³Cepheids can be observed out to about 200 million ly
14-20 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 ²Cepheids ²Supernovae (Type I) ²Red Giants ²Others
14-21 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!
14-22 Our Galaxy ³ Disk ²Where all the blue stars and ISM are found ³Bulge ²Spherical region near galaxy s center; old stars ³Nucleus ²The central region of the galaxy ³Halo ²Spherical region surrounding disk and bulge; old stars
14-23 Schematic Picture Halo Bulge Halo Disk Nucleus
14-24 The Disk ³Disk has lots of gas and dust the ISM ³Optical telescopes aren t very useful ³Infrared and radio observations
14-25 The Disk
14-26 Radio Observations 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)
14-27 21cm Emission Line ³ Spin flip of electron orbiting proton: proton proton electron Photon emitted electron High energy (from collision) Lower energy (Spontaneous de-excitation)
14-28 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
Spiral Arms
Spiral Arms
14-31 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
14-32 Red Stars 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
14-33 Population I and II Stars ³Population I Like the sun ²A younger generation of stars found in the disk/spiral arms ³Population II Older stars ²An older generation of stars, found in the halo an bulge
14-34 Orbits of Stars ³Stars in the disk have circular orbits ³Stars in the bulge have random, radial orbits
14-35 Nucleus of galaxy Disk Orbits Orbits of stars and gas Disk of galaxy Side View Stars and gas are rotating in the same direction
14-36 Nucleus of galaxy Top View
14-37 Where do the Arms come ³Winding? from? *** don t see this, so arms aren t from orbiting stars What causes them?
14-38 Density Waves ³Ripples waves travel through galaxy s dust/gas: Spiral Density Waves ²Like traffic jams ³Galaxy rotates causing spiral structure
14-39 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
14-40 Spiral Density Waves
14-41 Halo and Bulge Orbits ³Radial orbits through nucleus ³At any given time, stars are in various positions, spherically around nucleus
14-42 Disk and Bulge
14-43 Disk and Bulge
14-44 Our Galaxy Halo (old stars) Globular Clusters (mostly old) Bulge (old stars) Young stars in spiral arms Nucleus Disk (old stars throughout) Side View
14-45 Weighing the Galaxy ³Use orbits of stars to measure mass of Galaxy ³(Use Newton s laws of gravity) ³Measure orbital velocity ³Measure distance from nucleus
14-46 Rotation Curves ³Plots of orbital velocity versus distance from nucleus ³Gives measure of mass WITHIN each orbit
14-47 Results ³The mass of the Galaxy is about 10 11 times the mass of the sun (100 billion) 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
14-48 Dark Matter ³Matter detected through its gravity 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
14-49 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
14-50 Dark Matter Candidates ³Black holes? (Probably not) ²Primordial ones expected but they have evaporated ³Brown dwarfs? (Probably not) ³Neutrinos? (Lots and lots of them, but not massive enough) ³Subatomic particles ²WIMPs ²Higgs particles ²Etc ³Detection may come soon.
14-51 Nucleus of the Galaxy What s in the center of the Galaxy?
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 10 6 solar masses within a space 1AU ³A Black hole!
Thank You!