1. Galaxies

2. The Hubble Tuning-Fork Diagram This is the traditional scheme for classifying galaxies:

3. Elliptical Galaxies Elliptical galaxies have smooth, round distributions of stars. They resemble globular clusters, except they are much larger. They contain very little gas and dust, no star formation, and only older stars. Ellipticals are abbreviated as “E” galaxies.

4. Spiral Galaxies A spiral galaxy has a round distribution of stars at the center and a flat disk of stars. The stars in the disk are concentrated in spiral arms. The disks of spirals contain lots of gas and dust. As a result, new stars continue to be born in spirals, and these galaxies have both old and young stars. Spirals are abbreviated as “S” galaxies.

5. Barred Spirals Some spirals contain a straight elongation of stars at their centers, which is called a bar. Barred spirals are abbreviated as “SB”.

6. Sa and SBa Spiral galaxies are divided into 3 categories based on the relative sizes of the central bugles and the disks. In Sa and SBa galaxies, the bulge is very large, almost as large as the disk in some cases.

7. Sb and SBb In Sb and SBb galaxies, the bulge has a moderate size; not very large, and not very small.

8. Sc and SBc In Sc and SBc galaxies, the central bulge is very small compared to the disk.

9. Irregular Galaxies Irregular galaxies are do not have an ordered structure, and instead have random, messy shapes. They have lots of dust, gas, and young stars.

13. Active Galaxies The light from most galaxies is just the sum of light from all of the stars within it, so like starlight, a galaxy’s light is brightest at visible wavelengths and fainter at shorter and longer wavelengths.

14. Active Galaxies The light from most galaxies is just the sum of light from all of the stars within it, so like starlight, a galaxy’s light is brightest at visible wavelengths and fainter at shorter and longer wavelengths.

15. Active Galaxies But a small fraction of galaxies are different; they are much brighter and produce more long- and short- wavelength emission. They are called active galaxies.

16. Quasars: the first active galaxies In 1962, a radio survey of the entire sky revealed a few peculiar “stars” that were very bright at radio wavelengths (normal stars do not produce much radio emission). They were named quasi-stellar radio sources (quasars, or QSOs):

17. Quasars: the first active galaxies In 1962, a radio survey of the entire sky revealed a few peculiar “stars” that were very bright at radio wavelengths (normal stars do not produce much radio emission). They were named quasi-stellar radio sources (quasars, or QSOs): Instead of this… …Quasars look like single points of light

18. Quasars: Fast, distant, and very bright Quasars have enormous redshifts, indicating that they are moving away from us at more than 90% of the speed of light. Stars in the Milky Way cannot move that fast. The only way to achieve such a high speed is if they are incredibly far away. They are therefore incredibly bright – as bright as 1000 supernovae, and much brighter than galaxies like the Milky Way.

19. Size of a Quasar’s Energy Source Since many quasars vary in brightness we have a crude way to estimate their size. Imagine that there is some mechanism near the center of the quasar that controls the object’s brightness. It says “get bright”. That command goes forth no faster than the speed of light. Within a few months, the object gets bright. Since no signal can go faster than the speed of light, the object must be no bigger than a few light-months across.

20. A Quasar’s Energy Source black hole What can outshine ~1000 supernovae for millions of years, and be just slightly larger than our Solar System? Only the accretion disk around a “supermassive” black hole (>1,000,000 M  ).

21. Feeding the Monster If a star comes too close, the tidal forces of the black hole will rip it apart. The remains of the star spiral into the black hole via an accretion disk.

22. The Light from Quasars Because the spiraling matter is moving very fast, there is a lot of friction, producing a lot of light. This is why quasars are very bright. Closer to the event horizon, the gas moves faster, producing more friction, higher temperatures and light at shorter wavelengths. As a result, these accretion disks contain matter across a large range of temperatures, which is why quasars are bright at all wavelengths.

23. Instead of this… …Quasars look like single points of light Seeing the Galaxies that contain Quasars

24. The light from the accretion disk around a supermassive black hole is very bright and the glare makes it hard to see the surrounding galaxies. This is why they often appear as points of light. But sharp images with Hubble have been able to detect the faint light from the galaxies that contain these supermassive black holes.

25. Making a supermassive black hole Small black holes produced by supernovae sink to the centers of galaxies to merge into a single, more massive black hole.

26. Making a supermassive black hole Large galaxies form by the merger of smaller galaxies. When this happens, their supermassive black holes also merge together. This X-ray image from the Chandra Observatory shows two black holes orbiting each other in the center of a galaxy. They will probably merge together in about 400 million years.

27. Making a Quasar In a galaxy that has just formed by the merger of smaller ones, the stars have all kinds of orbits, some of which come too close to the supermassive black hole. As the black hole consumes these stars, an accretion disk forms around the black hole. Quasars are the accreting supermassive black holes made billions of years ago, which is why they have very large distances from us.

28. Normal Galaxy: no stars left to eat After a while, the black hole consumes all of the stars within its reach, and only the stars in safe orbits remain. Without matter to accrete, we don’t see anything from the black hole, and it is dormant. This is a normal galaxy (like ours).

29. The Milky Way’s Sleeping Monster The Milky Way is a normal galaxy that contains a non-accreting supermassive black hole at its center (4,000,000 M  ). None of the stars in the images below are close enough to the black hole to be ripped apart, so an accretion disk is not present.

31. Active Galaxies: fresh food If normal galaxies interact with each other, the orbits of the stars are disrupted, providing fresh victims for the black hole. This is an active galaxy.

32. How to Make an Active Galaxy

39. When Galaxies Collide It is not uncommon for galaxies to gravitationally interact with each other, and even collide.

40. When Galaxies Collide When galaxies interact or collide, their supermassive black holes can become “active”. But what happens to rest of each galaxy?  The stars don’t collide; they’re much too far apart compared to their sizes.  Galaxies can be tidally distorted, or even torn apart.  The galaxy types (elliptical or spiral) can change.  The gas clouds within each galaxy can collide. The increased density of gas can cause lots of star formation.  Because of the large number of newborn massive stars, there are lots of supernova explosions in the galaxy.

41. Two Large Galaxies make an Elliptical Galaxy

42. “Young” Ellipticals A few ellipticals even show traces of past interactions

43. “Young” Ellipticals A few ellipticals even show traces of past interactions

44. Explaining the Properties of Ellipticals When two large galaxies merge, the collision of clouds of gas and dust in these galaxies triggers the formation of a huge number of stars, including a lot of massive stars. When these massive stars die, they produce a lot of supernova explosions, which eject most of the dust and gas from the galaxy. As a result, no new stars can be made, and when we see ellipticals today, only old stars are present.

45. Mergers of Smaller Galaxies Probably Make Spirals past present

46. The Milky Way and a Dwarf Galaxy  A small dwarf galaxy

47. The Milky Way and a Dwarf Galaxy

51. past present

52. Seeing some of the first galaxies By obtaining images that can see very faint objects, we can look for galaxies that are very far away. Because it takes light a long time to reach us, we are seeing these galaxies as they were in the distant past. The deepest images show galaxy fragments (irregular galaxies) smaller than the normal galaxies near us today.