1. Galaxies

2. Edwin Hubble (1889-1953) Discovered that the universe goes beyond the Milky Way He was the first person to establish the distances to other galaxies. Calculated distance to Andromeda galaxy. 2.2 million light years not in the Milky Way Discovered that the velocity of distant galaxies increases with distance Developed a classification scheme for galaxies.

4. 4 General Types of Galaxies Spiral – like a propeller (Sa, Sb, Sc) Spiral – like a propeller (Sa, Sb, Sc) Barred Spiral – similar to a spiral, but with a straight bar through the nucleus Barred Spiral – similar to a spiral, but with a straight bar through the nucleus Elliptical – spherical to football- shaped Elliptical – spherical to football- shaped Irregular – no well-defined shape Irregular – no well-defined shape

5. Spirals have a disk component, central bulge & halo disk contains an interstellar medium (ISM) of gas & dust relative sizes of bulge/disk & amount of gas/dust vary appear white because they contain both blue & red stars The spiral arms contain only slightly more stars than the regions between the arms, even though active star formation happens there

6. Spirals How do the tightness of the arms and the size of the central bulge differ?

7. Spirals Variations? Sa: Tightest arms and largest central bulge Sc: Loosest arms and smallest central bulge.

8. M104 – the Sombrero Galaxy, an Sa type, large nucleus & very tight spiral arms.

9. The Andromeda Galaxy, M81, is a type Sb. It has a medium nucleus & less tight arms.

10. This type Sc has a small nucleus and very open arms.

11. Spirals “Edge-on”

12. Arms in spiral galaxies can vary in definition. Flocculent spiral galaxies do not have well- defined spiral arms (fluffy) Grand design spiral galaxies have highly defined spiral arms.

13. The usual circular ripples expanding from the place where a rock was thrown into the water. Ripples in rotating water creating spiral arms, as do ripples in the gas and dust of a disk galaxy. Formation of Spiral Arms

14. Spiral Arms as Density “Traffic Jams” Compression Wave in Traffic Flow: When normal traffic flow is slowed down, cars bunch together. In a grand design galaxy, a density wave moves through stars and gas. The wave is a region of slightly denser matter which creates more gravitational force. This force compresses the gas and enhances star formation, which makes the spiral density wave appear brighter.

15. Barred Spirals also classified according to the tightness of their spiral arms and sizes of the central bulges. SBa galaxies have the most tightly wound spirals and largest central bulges SBc galaxies have the least tightly wound spirals and the smallest central bulges. The Milky Way is probably an SBb type galaxy, similar to Andromeda, but with a central bar.

16. The bar is very noticeable in this SBc type.

17. In a barred spiral, the arms don’t “grow” directly from the nucleus. This is NGC 1365.

18. Types of Galaxies The Hubble classification system groups galaxies into four major types: spiral, barred spiral, elliptical, and irregular. The Hubble classification system groups galaxies into four major types: spiral, barred spiral, elliptical, and irregular. The arms of spiral and barred spiral galaxies are sites of active star formation. The arms of spiral and barred spiral galaxies are sites of active star formation. Arms of flocculent galaxies are caused by the births and deaths of stars over large regions. Arms of flocculent galaxies are caused by the births and deaths of stars over large regions. Differential rotation: The stars at the center of a spiral galaxy revolve faster than the outer stars Differential rotation: The stars at the center of a spiral galaxy revolve faster than the outer stars

19. Elliptical Galaxies only have a spheroidal component; no disk very little gas/dust, no active star formation appear red because they contain mostly red stars

20. Elliptical Galaxy Variations Classified according to the degree of flatness or elongation. An E0 galaxy is spherical An E7 galaxy is very elongated

21. Irregular Galaxy Variations neither spiral nor elliptical appear white & dusty: lots of gas and dust (ISM) have more in common with disk component of spirals distant galaxies more likely to be irregular more common when the Universe was young

22. Irregular Galaxies Several famous galaxies are irregular and have no definite shape. Several famous galaxies are irregular and have no definite shape. The Large & Small Magellanic Clouds, and the Cartwheel Galaxy are irregular. The Large & Small Magellanic Clouds, and the Cartwheel Galaxy are irregular. These galaxies are full of regions of new star formation! These galaxies are full of regions of new star formation!

23. Irregular Galaxies Irregular galaxies are often the result of collisions between galaxies. Irregular galaxies are often the result of collisions between galaxies. Our own Milky Way cut a single smaller galaxy like a buzz saw blade. Our own Milky Way cut a single smaller galaxy like a buzz saw blade. The Large & Small Magellanic Clouds are the “chips” left over from that round of cannibalism. The Large & Small Magellanic Clouds are the “chips” left over from that round of cannibalism.

24. The Large Magellanic Cloud.

25. The Cartwheel Galaxy.

26. A Composite Image of the Cartwheel Galaxy: The ring-shape probably formed when the smaller galaxy passed through the middle of the larger one. A circular density wave developed that stimulated a burst of star formation, creating many bright blue and white stars.

27. Hubble’s Classification System

28. Final Cosmic Distance Ladder!

29. The light from supernovas observed in remote galaxies help astronomers to determine the distances to these faraway objects.

30. Hubble’s Law: the velocity of distant galaxies increases with distance Closer Galaxies move away from us slower Farther Galaxies move away from us faster

31. Expanding Universe Analogy

32. Clusters and Superclusters Galaxies group into clusters rather than being randomly scattered through the universe. Galaxies group into clusters rather than being randomly scattered through the universe. Rich clusters: Rich clusters: At least 1000 galaxies At least 1000 galaxies Contains mostly elliptical and lenticular galaxies Contains mostly elliptical and lenticular galaxies Giant elliptical galaxies often found towards the center. Giant elliptical galaxies often found towards the center. Poor clusters: Poor clusters: Less than 1000 galaxies Less than 1000 galaxies Contains mainly spiral and irregular galaxies Contains mainly spiral and irregular galaxies Regular clusters: Spherical shape with most galaxies in the center. Regular clusters: Spherical shape with most galaxies in the center. Irregular Cluster: Irregular shape with galaxies spread throughout. Irregular Cluster: Irregular shape with galaxies spread throughout. Our Galaxy is a member of a poor, irregular cluster, called the Local Group. Our Galaxy is a member of a poor, irregular cluster, called the Local Group.

33. Clusters and Superclusters No cluster of galaxies has an observable mass large enough to explain the motions of its galaxies; a large amount of dark matter must be present between the galaxies. When two galaxies collide, their stars initially pass each other, but their interstellar matter (ISM) collide violently. This can either strip the ISM from the galaxies or trigger star formation. Gravity can cast stars out of a galaxy. Galaxies can combine; a large galaxy in a rich cluster may grow through galactic cannibalism, sometimes producing a giant elliptical galaxy.

34. Galactic “Cannibalism” - examples

35. Superclusters in Motion Hubble’s law states that closer galaxies move away from us slower and farther galaxies move away from us faster. Hubble’s law states that closer galaxies move away from us slower and farther galaxies move away from us faster. Astronomers use objects of known brightness (called standard candles) to calculate distances. Some of these objects include Cepheid variables, the brightest supergiants, globular clusters, and supernovas. Astronomers use objects of known brightness (called standard candles) to calculate distances. Some of these objects include Cepheid variables, the brightest supergiants, globular clusters, and supernovas.

36. This cluster of galaxies, called the Fornax cluster, is about 60 million light-years from Earth. Both elliptical and spiral galaxies are easily identified.

37. Superclusters in Our Neighborhood

38. The Hercules Cluster

39. A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation. Sometimes form when galaxies collide Starburst Galaxies