# Star systems like our Milky Way. Galaxies

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2 Galaxies

3 Star systems like our Milky Way Galaxies

4 Contain a few thousand to tens of billions of stars,as well as varying amounts of gas and dust Large variety of shapes and sizes

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12 Gas and Dust in Galaxies Spirals are rich in gas and dust Ellipticals are almost devoid of gas and dust Galaxies with disk and bulge, but no dust are termed S0

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16 Irregular Galaxies Often: result of galaxy collisions / mergers Often: very active star formation ( starburst galaxies ) Some: small ( dwarf galaxies ) satellites of larger galaxies ( e.g., Magellanic clouds )

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18 Measuring Distances: The Cepheid Method The more luminous a cepheid variable, the longer its pulsation period. Instability Strip Observing the period yields a measure of its luminosity and thus its distance!

19 Distance Measurements to Other Galaxies a) Cepheid method : using period luminosity relation for classical cepheids: measure cepheid s period find its luminosity compare to apparent magnitude find its distance b) Type Ia Supernovae ( collapse of an accreting white dwarf): type Ia aupernovae have well known standard luminosities compare to apparent magnitudes Find its distances Both are Standard-candle methods: Know absolute magnitude (luminosity) compare to apparent magnitude find distance

20 Distance Measurements to Other Galaxies (II): The Hubble Law Distant galaxies are moving away from our recession velocity, v r, proportional to their v r = H 0 d H 0 70 km/s/mpc is the r infer the distance E. Hubble (1913):

21 Milky way, with a distance d:

22 Hubble constant => Measure v through the Doppler effect

23 The Extragalactic Distance Scale Many galaxies are typically millions or billions of parsecs from our Galaxy. Typical distance units: Mpc = megaparsec = 1 million parsecs Gpc = gigaparsec = 1 billion parsecs Distances of Mpc or even Gpc the light we see has left the galaxy millions or billions of years ago!! Look-back times of millions or billions of years

24 Rotation Curves of Galaxies: Mass From blue / red shift of spectral lines across the galaxy Plot of rotational velocity vs. Infer rotational velocity distance from the center of the galaxy and use Kepler s law: Rotation curve -> Mass

25 Gravitational lensing may lead to mass NASA image

26 Dark Matter The only way dark matter can be detected is by mapping its gravitational affects. MACHOs ( brown dwarfs and/or small, dense chunks of heavy elements ) WIMPs ( exotic particles -not normal matter )

27 Supermassive Black Holes From the measurement of stellar velocities near the center of a galaxy: Infer mass in the very center central black holes! Several million, up to more than a billion solar masses! Supermassive black holes

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29 Dark Matter

30 Adding visible mass in stars, interstellar gas, dust, etc., we find that most of the mass is invisible! The nature of this dark matter is not understood at this time. Some ideas: brown dwarfs, small black holes, exotic elementary particles

31 Interacting Galaxies Particularly in rich clusters, galaxies can collide and interact Galaxy collisions can produce ring galaxies and tidal tails, often triggering active star formation: Starburst galaxies.

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33 Clusters of Galaxies Galaxies generally do not exist isolated, but form larger clusters of galaxies. Rich clusters: 1,000 or more galaxies, diameter of ~3 Mpc, condensed around a large, central galaxy. Poor clusters: Fewer than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center.

34 Our Galaxy Cluster: The Local Group Milky Way Andromeda galaxy Small Magellanic Cloud Large Magellanic Cloud

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37 Active Galaxies Galaxies with extremely violent energy release in their nuclei (plural of nucleus) Active Galactic Nuclei (= AGN) Up to many thousand times more luminous than the entire Milky Way; the energy is released within a region approximately the size of our solar system!

38 Seyfert Galaxies Unusual spiral galaxies: Very bright cores Emission line spectra. Variability: ~50% in a few months Most likely power source: Accretion onto a supermassive black hole (~10 7 to 10 8 M Sun )

39 Formation of Radio Jets Jets are powered by accretion of matter onto a supermassive black hole. Accretion disk Black hole Twisted magnetic fields help to confine the material in the jet and to produce synchrotron radiation.

40 Quasars Active nuclei in elliptical galaxies with even more powerful central sources than Seyfert galaxies Show strong variability over time scales of a few months Also show very strong, broad emission lines in their spectra

41 The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see the Milky Way as a faint band of light across the sky. From outside, our Milky Way might very much look like our cosmic neighbor, the Andromeda galaxy.

42 The Structure of the Milky Way 75,000 light years Sun Disk Nuclear bulge Halo Open clusters, O/B associations Globular clusters

43 The Mass of the Milky Way Total mass in the disk of the Milky Way: Approximately 200 billion sola masses Additional mass in an extended halo: Total: approximately 1 trillion sola masses Most of the mass is not emitting any radiation: Dark matter!

44 Stellar Populations Population I : Young stars: metal rich; located in spiral arms and disk Population II : Old stars: metal poor; located in the halo ( globular clusters) and nuclear bulge

45 Star Formation in Spiral Arms Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation. Spiral arms are stationary shock waves, initiating star formation.

46 Star Formation in Spiral Arms Spiral arms are basically stationary shock waves. Stars and gas clouds orbit around the Galactic center and cross spiral arms. Shocks initiate star formation. Star formation self-sustaining through O/B ionization fronts and supernova shock waves.

47 Radio View of the Galactic Center Many supernova remnants; shells and filaments Arc Sgr A Sgr A Sgr A*: The center of our galaxy The galactic center contains a supermassive black hole of approximately 3.7 million solar masses.

48 Measuring the Mass of the Black Hole in the Center of the Milky Way By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole can be determined to ~3.7 million solar masses.

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