Galaxies stars come in large groups (20-200 billion stars) called Galaxies >2 trillion observable galaxies. Come in Shapes and Sizes depending on how they were formed Elliptical (football shape) Spirals (frisbee shape) Irregulars (ill-defined shape) PHYS 162 1
Hubble Ultra Deep Space View PHYS 162 2
Partial Galaxy Map Sloan Digital Sky Survey. redshift axis is distance out to 7 billion light years PHYS 162 3
Galaxy Types Elliptical ^ Spiral < Irregular v PHYS 162 4
Galaxy Classification (don t need to know) PHYS 162 5
Elliptical Galaxies usually old Type II stars (little heavy elements) limited new star formation range in size from 10 5 to 10 13 times the mass of the Sun PHYS 162 6
Spiral Galaxies both young Type I (with heavy elements) and old Type II stars (little heavy elements) active new star formation range in size from 10 9 to 10 12 times the mass of the Sun most common type of galaxy Andromeda M31 visible by eye 2.2 MLY away PHYS 162 7
Irregular Galaxies mostly young Type I (with heavy elements) active new star formation range in size from 10 8 to 10 10 times the mass of the Sun mostly near larger galaxies whose gravity impacts shape Large and Small Magellanic Clouds visible by eye 170,000 LY away (angular size of LMC ~ 44 times larger than Moon) PHYS 162 8
Local Cluster = Local Group spiral: MW, Andromeda(M31), M33 elliptical: NGC205, M32, NGC185, NGC147 irregular: LMC, SMC, NGC 6822, IC 1613 Andromeda mving towards Milky Way collide in 4 billion years PHYS 162 9
Measuring Distances Directly using parallax (triangulation) - good within 1000 LY or so spectroscopic parallax - compare spectrum of distant star to similar close star. Large uncertainty. Variable Stars especially Cepheids give small error during certain stages of star s evolution it is nonstable. Luminosity varies with time PHYS 162 10
Variable Stars Brightness of some stars change with time. Period absolute luminosity Observe up to 60MLY. Parallax of close by calibrates. GAIA telescope PHYS 162 11
Variable Star Galactic Distances Cepheid Variables: common (Polaris is one). Bright, 100,000 L(Sun). First discovered in 1784. 1908: Henrietta Leavitt studies thousands in Large Magellanic Cloud - was a computer working at Harvard; usually females who studied photographic plates in US. Similar to Hidden Figures story where Katherine Johnson and other women (black and white) did computations. Type Ia supernovas (white dwarf collapse): Rare but very bright. 10 billion times L(Sun). Observe up to 8 billion LY Digital cameras making finding variable stars easier high priority PHYS 162 12
Hubble Law measure velocities of galaxies - almost all are moving away - larger velocity if further away observe V = H x D Hubble Law V = recessional velocity D = distance H = Hubble constant PHYS 162 13
Edwin Hubble 1889-1953 grew up in Wheaton, IL, college at U. of Chicago used Mt. Wilson Observatory in Pasadena, CA 1922-23. Observed Cepheid variable stars in spiral nebula far away and so galaxies 1927-29 combined his distance measurements with redshift measurements Slipher, Humason measurement of the Hubble Law (though not the first and wrong value by factor of 7) PHYS 162 14
Hubble Law plotted 46 galaxies in 1929, some indication of straight line relationship in distance vs recessional velocity plane. Later data at greater distances confirmed this (all being done by hand ) PHYS 162 15
Hubble Law measuring recessional velocity vs distance and understanding it is now one of the highest priorities in astrophysics mostly use Supernovas for most distant objects Can now use v=hd to determine the distance to a galaxy if you measure its recessional velocity PHYS 162 16
Meaning of Hubble Law distant objects are moving away from us Universe is expanding was predicted by Einstein s general theory of relativity (~1915) but Einstein thought it was a mistake in the theory prior to results from Hubble and others observations in 1926-1935 Measurement of Hubble constant gives age of Universe - about 13-14 billion years - depends on how Hubble constant is changing with time PHYS 162 17
Hubble Law more complete history We tend to give credit to Hubble for the first observation that the Universe is expanding. However Hubble wasn t the first to show the linear relationship between velocity and distance (the Hubble Law) and did not connect this to an expanding Universe George Lemaitre (Belgian priest) did this in 1926 (using same data as Hubble in 1929) plus he used Alexander Friedmann s (Russia s) 1922 derivation of the Hubble Law from Einstein s general theory of relativity PHYS 162 18
Rubber Band or Balloon or Bread Model mark points on rubber band or balloon or use raisin bread stretch outward at uniform rate velocity away from any point is larger if other point is further away 5th grad math gives Hubble Law PHYS 162 19
Hubble Law Age of Universe Hubble Law v = Hd Work backwards to estimate when time began assume constant velocity (not true) time = distance/velocity = d/v but v = Hd time = d/hd = 1/H so 1/H is about the age of the Universe Time = 0 Velocity v Time = now Distance = d PHYS 162 20
Hubble Law Measure H PHYS 162 21
Hubble Law Measure Age of Universe 1/H gives approximate age of Universe need to convert 71 km/sec/mpc to inverse years (DON T NEED TO KNOW. On exercise) 1Mpc 3.3MLY 1LY 1 H 1 H 300,000km/ sec 1year Mpc 71km/ sec 14billion years 3.3 10 6 5 3 10 km/ sec 71km/ sec year PHYS 162 22
Expanding Universe Hubble constant H changes with time Gravity can slow the expansion rate (by pulling inward ) expand forever Open Universe stop expanding and then contracts Closed Universe How rapidly H changes depends on the amount of matter - we seem to be very close to the Critical Density which separates an Open from a Closed Universe (hint of underlying physics?) recent (1998)(perplexing) data indicated Universe is accelerating! Like anti-gravity! More on this later. PHYS 162 23
Expanding Universe Hubble constant H changes with time expand forever Open Universe stop expanding and then contracts Closed Universe Expansion increases what seems to be observed PHYS 162 24
Expanding Universe most distant supernovas are dimmer then expected (2011 Nobel Prize) shows rate of expansion of Universe is increasing dark energy used to explain though no one quite knows what this is PHYS 162 25
Galaxy Clusters and Superclusters galaxies come in clusters of 10-1000 galaxies: gravitationally bound, impacts formation clusters usually part of superclusters which reflect distribution of matter in early Universe Formax Cluster 60 MLY from us PHYS 162 26
Galaxy Superclusters Measure motion gives masses of galaxies PHYS 162 27
Milky Way Galaxy Milky Way spiral galaxy - flattened disk 150,000 LY in diameter with about 200 billion stars we sit in a gas/dust arm - active star formation - absorbs visible light study using IR/radio/gamma or by looking at other galaxies PHYS 162 28
Milky Way Structure Nucleus Disk Halo PHYS 162 29
Galaxy Nucleus Nucleus - high density of stars - little gas or dust; limited new stars very active star formation in past - many black holes - super massive BH at center >1,000,000 Mass(Sun) PHYS 162 30
Galactic Center measure motion over time get mass in center (like Kepler) PHYS 162 31
Milky Way Galaxy - Disk and Halo Stars In Disk - lots of bright, young stars - lots of gas and dust - stars abundant in heavy elements (Type I) earlier generations formed heavy elements active star formation in Halo - mostly old red stars - no gas/dust - no heavy elements (just H and He) (Type II) no current star formation PHYS 162 32
Globular Clusters Halo Stars M10: 85 LY across, 16,000 LY from Earth PHYS 162 33
Size and Shape of Galaxy measure distances to globular clusters outside plane of galaxy not obscured by gas and dust (done early in 20th century) tell where center of galaxy is shape of early Galaxy Center of Galaxy sun Visible stars Glob cluster PHYS 162 34
Radio Maps of Galaxy use Doppler shift to get velocity of different regions, identify different arms PHYS 162 35
21 cm line in H (like MRI) Doppler Shifts As radio penetrates through gas/dust can use to map out Milky Way PHYS 162 36
Measure mass by: motion within a galaxy Masses of Galaxies motion of different galaxies about each other gravitational lensing Gives - most mass isn t in stars and normal matter (gas, dust, neutron stars, black holes) DARK MATTER PHYS 162 37
Mass of Galaxy PHYS 162 38
Differential Rotation of Galaxy PHYS 162 39
Mass of Milky Way Sun is 30,000 light years from center (2,000,000,000 AU); period of 200,000,000 years same as for planets: Dist 3 / Period 2 = M (inside) giving 200 billion mass(sun) inside the Sun s radius repeat for 150,000 LY >1000 billion Mass(Sun) for Galaxy mass inside that distance PHYS 162 40
Differential Rotation of Galaxy PHYS 162 41
Mass of Galaxies mass doesn t match observed amount of matter unseen dark matter unknown composition extends beyond visible part of Milky Way observed in other Galaxies (1959) PHYS 162 42
Other Dark Matter Observations look at velocities of individual galaxies in a cluster about each other missing mass first observed by Fritz Zwicky in the 1930s (Caltech; he also introduced name supernova ) look at gravitation lensing by a nearby galaxy of a more distant galaxy (many including NIU students Donna Kubik and Matt Weisner. see their theses at www.physics.niu.edu/physics/academic/grad/theses1.shtml) PHYS 162 43
Galaxy Clusters and Superclusters galaxies come in clusters of 10-1000 galaxies: gravitationally bound, use to measure mass, impacts formation reflects distribution of matter in early Universe Formax Cluster 60 MLY from us PHYS 162 44
Gravitational Lensing by Galaxies PHYS 162 45
Donna Kubik Thesis NIU students work with Fermilab astrophysicists. Use Sloan Digital Sky Survey data to find Einstein ring candidates. Then use better telescope to improve image. Size of ring tells mass in closer galaxy amount of dark matter PHYS 162 46
Composition of the Universe 95% not understood Graphics courtesy: NASA PHYS 162 47
Extra Slides PHYS 162 48
Hubble Law value vs time Hubble made a few mistakes (confused variable star types ) and so had distance wrong PHYS 162 49