- Edwin Hubble developed a classification system for galaxies that included spiral, elliptical, and irregular galaxies. Spiral galaxies have spiral arms and central bulges, elliptical galaxies have spherical or egg-shaped structures, and irregular galaxies have no defined shape. - Galaxies exist in groups and clusters held together by gravity, with galaxy clusters containing dozens to thousands of galaxies. Superclusters are clusters of galaxy clusters and are the largest known structures in the universe. - The Milky Way is part of the Local Group, which also includes the Andromeda Galaxy and other dwarf galaxies. The Magellanic Clouds are two small, nearby galaxies that orbit the Milky Way.

1. Astronomy Topic 06 : Galaxies (part 2) Stephen’s Quintet

2. Galaxy Classification Edwin Hubble (1926) Three Classes Spiral Barred-spiral Regular spiral Elliptical Irregular Hubble’s famous “tuning fork”

3. Hubble Tuning Fork http://www.astro.psu.edu/users/niel/astro1/slideshows/class21/003-hubblefork3.gif

4. Elliptical Galaxies (E) Spheres of hundreds of billions of stars. Largest known galaxies E, are egg shaped E0, nearly perfect spheres, to flattest, E7 No gas and dust The oldest galaxies, population II stars

5. M87-Elliptical Small dots represent M87’s globular clusters Some large ellipticals emit jets of matter, perhaps from massive black holes at their centers

6. M87’s Jet Kitt Peak image Hubble

7. M87-CFH Telescope M87’s jet is visible at the 1:00 position

8. Spiral Galaxies Normal Spiral (S), and Barred Spiral (SB) Flat, pancake shaped with central bulge and spiral arms Young, large amounts of gas and dust Population I and II stars

9. Normal Spiral Galaxies (S) Normal spiral galaxies, S Sa, Sb, and Sc, how tightly wound the spiral arms appear If no apparent spiral arms, SO

10. M51-Whirlpool Galaxy (Spiral) http://antwrp.gsfc.nasa.gov/apod/image/0207/m51_hallas_big.jpg

11. Barred-Spiral Galaxies (SB) Designated SB Spiral arms unwind from bar-shaped mid section NGC 1365

12. M109 http://www.astronomysight.com/as/images/pics/

13. Irregular Galaxies Irregular galaxies, Irr No regular, geometric shape Contain gas and dust Ex: Small and Large Magellanic Clouds

14. Irregular NGC1427A, an example of an irregular galaxy This photo was taken by the Hubble Space Telescope, and it reveals many other galaxies in the background Note how the HST can resolve this galaxy’s stars

15. Galaxy Comparisons http://www.glyphweb.com/esky/_images/diagrams/galaxies.gif Elliptical galaxies, such as M87, are the largest known galaxies

16. Galaxy Orientation Spiral galaxies are flat disks Appearance from earth depends upon how they are tilted compared to our line-of-sight “ Edge-on”, “face-on”, or in between NGC 891 (above) is an “edge-on” spiral; M74 (below) is a “face-on” spiral

17. M77 & NGC 2055 http://antwrp.gsfc.nasa.gov/apod/ap061215.html NGC 2055 is an edge-on spiral, and M77 is a face-on spiral Both galaxies are located in Cetus

18. Galaxy Summary Table Values Spiral Elliptical Irregular Mass (Milky Way = 1) 0.005 to 2.0 0.000001 to 50.0 0.0005 to 0.15 Diameter (Milky Way = 1) 0.2 to 1.5 0.01 to 5.0 0.05 to 0.25 Luminosity (Milky Way = 1) 0.005 to 10.0 0.00005 to 5.0 0.00005 to 0.1 Star Population Old and Young Old Old and Young Interstellar Matter (gas and dust) Moderate Little Plentiful

19. Groups of Galaxies Galaxy clusters, contain dozens to thousands of galaxies held together as a unit by gravity A supercluster is a cluster of galaxy clusters, the largest known structures in the universe

20. Hercules Galaxy Cluster http://home.earthlink.net/~stanleymm/images/NGC6050sm.jpg

21. Local Group Our galaxy cluster The Local Group contains about 30 galaxies Spirals Milky Way, Andromeda Galaxy, and M33. Others are small elliptical and irregular (dwarf galaxies) Local Group is part of the Virgo Cluster of galaxies, part of the Local Supercluster

22. Local Group http://www.public.asu.edu/~rjansen/localgroup/NVWS99B_03.gif Some Members : Milky Way Large Magellanic Cloud, Small Magellanic Cloud NGC 6822 Andromeda Galaxy (M31) Triangulum Galaxy (M33)

23. Magellanic Clouds Magellanic Clouds are two small, nearby galaxies Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) Appear as hazy spots in the night sky, visible only from the southern hemisphere Clouds are gravitationally bound satellites of the Milky Way They lie at distances of 169,000 LY and 210,000 LY A supernova in the LMC was discovered in 1987, the first bright nearby supernova in 300 years (since Kepler and Tycho)

24. Magellanic Clouds LMC, red glows are nebulae in the LMC Clouds from Lake Titicaca, Bolivia http://www.ctio.noao.edu/~mcels/gallery/lmc.1.jpg

25. Magellanic Clouds & Milky Way http://www.atnf.csiro.au/news/press/images/magellanic_pics/MSsim_labels.jpg

26. Milky Way & Clouds in infrared from 2MASS http://astsun.astro.virginia.edu/~mfs4n/sgr/2mass_allskyatlas.jpg http://astsun.astro.virginia.edu/~mfs4n/sgr/sgr.flyaround.mpg

27. Andromeda Galaxy Our nearest large neighbor Similar in size and shape to Milky Way Andromeda Galaxy is visible to the naked eye as a glowing spot in the constellation of Andromeda 2.3 million light years distant (wave!)

28. Andromeda Galaxy (M31, M32, M110)

29. Andromeda Constellation Locate M31 and M33

30. Triangulum Galaxy (M33)

31. M33 The pink glow of emission nebulas can be seen in this photo

32. Virgo Virgo Supercluster About 200 galaxies are visible in small telescopes Galaxies range from Ursa Major through Coma Berenices to Virgo Sombrero Galaxy

33. Center of Virgo Cluster Virgo Cluster near M84, M86 M86

34. Sombrero Galaxy (M104) from HST

35. Sheets and Voids Superclusters are located in thin sheets of galaxies Sheets border empty voids Voids resemble gigantic bubbles The universe resembles a sponge

36. Large-Scale Structure of the Universe http://www.astro.princeton.edu/~cen/PROJECTS/p1/DARKallz0.jpeg Sheets & Voids: http://heasarc.nasa.gov/docs/cosmic/sheets_voids.html

37. Active Galaxies (AGN) An active galaxy emits large amounts of energy from its nucleus AGNs exhibit jets of hydrogen racing outward along an axis Massive black holes? Mysterious quasars might represent extremely active galaxies

38. Galactic Cannibalism Galaxies can merge together to form larger galaxies Elliptical galaxies may be the result of multiple galaxy collisions NGC 2207 (left) and IC 2163 (HST)

39. Quasars Resemble faint stars, are strong radio sources Acronym for Quasi-Stellar Radio Source Small objects, yet emit energy egual to thousands of galaxies combined Most distant and powerful objects known Might represent early stage of galaxy growth Seyfert galaxies are active galaxies with nuclei that resemble quasars

40. 3C 273-Quasar HST image of 3C 273 showing disk details (quasar blocked) 3C 273 Starfield in Virgo 3C 273 is one of the first identified quasars, it resembles an 11 th magnitude star in Virgo and is visible in small amateur telescopes