1. Discovering galaxies Bursting through the limits of space and time
Hubble Science Briefing
Discovering galaxies Bursting through the limits of space and time
Title:  Discovering Galaxies: Bursting the Limits of Space and Time
Abstract: With today's marvelously detailed images and knowledge that
there are perhaps a hundred billion throughout the universe,
it is easy to forget that there was a time, only a century
ago, when we knew of only one galaxy. The key findings in
the history of the discovery of galaxies make a complex
story. It is, for example, easier to discuss how things were
found than who was the exact discoverer. Finding and
understanding galaxies has been fundamental in establishing
properties of our universe, such as its origin, age, size
and future. Bio:
Jean-René Roy is currently Senior Scientist with the Space Telescope Science Institute and the Gemini Observatory. He received his Ph. D. in astronomy from the University of Western Ontario in 1973.Following work at several research institutes and observatories, he held a staff position at the Department of Physics of Laval University in Quebec City from 1977 to He was a member of the directorate of the Gemini Observatory in Hawaii and in Chile from 2000 to Recently, he worked with the Large Facilities Office of the National Science Foundation in Arlington, VA from 2009 to His research areas include the role of bars in the evolution of spiral galaxies, the formation of massive stars and the dispersal of elements in the interstellar medium. More recently, he has explored the issues and challenges of governance and management of large research infrastructures and facilities.
Jean-René Roy
Space Telescope Science Institute
5 July, 2012

2. The Hickson compact group of galaxies H87 at a distance of about 400 million light-years imaged with the Gemini North Telescope of the
Gemini Observatory.
Galaxies are giant systems of stars/planets, gas and dust (+ cosmic rays and magnetic fields) & dark matter held together by the force of gravity. Dark matter is the major constituent of most galaxies, but we do not know yet the nature of DM. 2

3. The key ideas Many people “discovered” galaxies
Many people found the distances to galaxies
A few people found that the universe was expanding and accelerating
Establishing priority is a complex exercise
The discovery of the “sidereal universe” is a fascinating story of humankind curiosity, astuteness and ingenuity
The 5 points I wish to share with you.
The Stiegler Law of eponomy: “No scientific discovery is named after its original discoverer”.
To demonstrate his point, Stiegler wrote that the Stiegler Law had been discovered by the science historian Robert K. Merton. Stiegler was a Professor of Statistics at the University of Chicago. 3

4. This is a dramatic picture of the whole sky projected in galactic coordinates.
For a few centuries, nebulae were known to exist within the galactic plane (gaseous, with some with stars), and most others lying away from the plane. Finding and understanding they they belong to two fundamentally distinct categories (by their location, content and size) took almost 10 centuries. 4

5. This is a dramatic picture of the whole sky projected in galactic coordinates.
Nebulae were known to exist within the galactic plane (gaseous, with some with stars), and most others lying away from the plane.
In this image, one can see the Andromeda nebula, left of center south of the plane, and the Small and Large Magellanic Clouds at the right, also south of the plane. 5

6. How were galaxies discovered?
This is a long and complex story starting in the 10th century. 6

7. Pre-history
Abd al-Rahman Al Sufi ( ) was one of several famous muslim astronomers. He worked at Isfahan, Persia (today Iran). He was to first to refer directly to the Andromeda galaxy as a “small cloud”.
Andromeda is a princess from Greek mythology, chained to a rock as a sacrifice to a sea monster. She was saved by Perseus, her future husband. This is the theme of many spectacular renaissance paintings, e.g. Vassari.
The Andromeda galaxy is at a distance of 2.6 million light-years, referred to as the “Great Andromeda Nebula” in older texts. It is the largest galaxy in the Local Group. Spitzer Space Telescope observations in the infrared indicate that it contains one trillion stars; this is at least three times as many stars as the Milky Way has.
Abd al-Rahman ibn Umar al-Sufi ( ) refers to a “small cloud” in Andromeda
Book of the constellations of the fixed stars in year 964
Al-Sufi ( ) 7

8. The Herschel family and the 40 ft telescope: it used an alt-az mounting and metal mirror with polished surface. William and Caroline did most of the observing work with the smaller 20 ft telescope. The larger instrument proved awkward to operate. The Herschels used their large telescopes to systematically survey the sky, first to find binary stars, then to map and catalog “nebulae”.
William : (aged 83)
Caroline : (aged 97) - struck by typhus at age of 10, never grew to be > 4’ 3”
John: ( ) born from William and Mary Baldwin
The Herschels - William ( ), Caroline ( ) & John ( ) 8

9. While William Herschel is
surveying the sky for binary stars, his sister searches for new comets.
With her comet finder,
Caroline discovers several new “nebulae”
Drawings of nebulae by William Herschel, from Philosophical Transactions of the Royal Society of London, 101, , Plate IV
After his search of binary stars (chance alignments that would reveal parallaxes), Herschel switched to his all sky survey. Following Gingerich (in The Astronomy Revolution, CRC Press, 2012, p. 279), Herschel made a comet finder for Caroline. She found 9 comets, but began finding new nebulae. Herschel turned his sweeps of the sky into finding new nebulae and clusters. He published various catalogs in 1786, 1789 and 1802.
Color impression. R. Hawkins, N. 53 Old Comton St., Soho. From the Hale Scrapbook, p.16. This is a representation of Caroline Herschel, sister of astronomer William Herschel of Bath who discovered the planet Uranus in 1781.
William switches his program
to the “study of the sidereal Universe” 9

10. Construction of the Heavens, 1785
.... the naked eye, which, as we have before estimated, can only see the stars of the seventh magnitude so as to distinguish them; but it is nevertheless very evident that the united lustre of millions of stars, such as I suppose the nebula in Andromeda to be, will reach our sight in the shape of a very small, faint nebulosity; since the nebula which I speak may easily be seen in a fine evening.
William Herschel,
Construction of the Heavens, 1785
After initially supporting the view of nebulae being “other” Milky Ways, William Herschel turned around in his opinion to support the “local” hypothesis. He was greatly influenced by his discovery of the association of several nebulae with stars in them, and promoted the idea that nebulae were condensing into planetary systems. 10

11. “Island Universes”
Immanuel Kant ( ) from Königsberg, now known as Kaliningrad. Kant is roughly contemporary with William Herschel.
On the “nebulous stars” mentioned by Pierre-Louis de Maupertuis, Kant wrote that “I easily persuaded myself that these stars can be nothing else than a mass of many fixed stars... On account of their feeble light, they are removed to an inconceivable distance from us.” (1755)
Immanuel Kant
“I easily persuaded myself that these stars can be nothing else than a mass of many fixed stars...
On account of their feeble light, they are removed to an inconceivable distance from us.” (1755) 11

12. William Parsons, 3rd Earl of Rosse
( )
William Parsons, 3rd Earl of Rosse
72-inch Leviathan
William Parsons ( ), also known as the 3rd Earl of Rosse, made major accomplishments in pushing forward the technique of building large reflectors and conducting observations with them. We see his 30-inch and 72-inch (the “Leviathan”) telescopes, used for the observation of nebulae and comets. The mirrors were of polished brass.
Birr Castle, Ireland 12

13. Drawings of “nebulae” from visual observations with the six-foot and three-foot reflectors. Modern CCD pictures of two of objects are also shown for:
GC 2301 = NGC 3521 SABbc at 35 million light-years
GC 1861 = NGC 2903 SB(s)d at 30 million light-years 13

14. Whirlpool Galaxy (M51 or NGC 5194/95)
Leviathan
HST
M51 SA(s)bc pec at 23 million light-years. M 51 was observed several times at Birr Castle, and there are multiple descriptions of it. This is the drawing (one of many) that is reproduced from the Scientific Transactions of the Dublin Royal Society (1978) - copy from the Johns Hopkins University Peabody Library, Baltimore MD. To the right is a mosaic picture from the Hubble Space Telescope
Whirlpool Galaxy (M51 or NGC 5194/95) 14

15. How far away? How we found out
How far away? was also a key element for the answer to the question: What are they? This was the subject of an accelerated debate that took place in the early part of the 20th century. Key to the developments were new technologies introduced at the end of the 19th century, e.g. the photographic plate, successful silver coating on glass mirrors, and new sturdy and high precision mounting for telescopes - equatorial mount type. 15

16. Extragalactic Renaissance
Opinions change dramatically between 1900 and 1920
Opposing views for “in” (Shapley/van Maanen) and for “out” (Curtis, Lundmark, Öpik, Hubble)
Strong observational evidence builds up quickly for most nebulae to be “extragalactic”
Several new technologies provide the key tools 16

17. Isaac Roberts introduces astrophotography for nebular work
Roberts observatory, Crowborough - Sussex
The British engineer and amateur astronomer Isaac Roberts ( ) introduces astrophotography. This technique would revolutionize astronomy in the 20th century.
Roberts took this first photograph of the Andromeda galaxy
on December 29, 1888 17

18. Herschel and Parsons telescopes had metal mirrors (polished) and were mounted on primitive alt-azimuth mountings; the observers used the naked eye for observations and made drawings.
New telescopes had glass mirrors (coated with reflecting silver) and new sturdy metal mounts, with precision equatorial and motor driven mechanisms. The photographic plates became of common use. Telescopes became housed in enclosures, often with the typical dome shape. This ensured their protection against bad weather and deterioration.
New technologies: sturdy metal mount, precision motor drive, glass mirror 18

19. Harvard College Observatory astronomer and director Harlow Shapley ( ) held first the view that the Milky Way was a very large system. He had measured its size, based on the geometrical distribution of the globular cluster system, to be 300,000 light-years in diameter. In his opinion, everything was part of the Milky Way. Although some nebulae might be located at the edge, they were part of the great Milky Way system. Shapley later accepted the extragalactic hypothesis, and he became an active researcher on galaxies and their distribution.
Harlow Shapley ( )
Shapley: The Milky Way is big (300,000 l-y) and is everything = the universe, but ... 19

20. Novae! ~24 novae/yr are observed every year in the Andromeda galaxy
Novae are a spectacular type of variable stars. Novae result from nuclear detonations from material being dumped by an evolving companion star onto its white dwarf. The brightness of the star surges by several orders of magnitude. Novae have been well known in our Milky Way and often become visible to the naked eye. Their light curve betrays them even at large distances such as that of the Andromeda galaxy.
~24 novae/yr are observed every year
in the Andromeda galaxy 20

21. In 1917, novae in the Andromeda “nebula”:
Heber Curtis, Knut Lundmark and George W. Ritchey discovered “novae” in “nebulae” - this was a major observational evidence for them to be extragalactic. (However some that were thought to be novae, e.g. in NGC 6946 by Ritchey, were clearly supernovae, a more more energetic and luminous stellar explosion).
Heber Curtis ( )
Knut Lundmark ( )
In 1917, novae in the Andromeda “nebula”:
they are located at 650,000 light-years =
well outside the Milky Way 21

22. Vesto Slipher ( ) of Lowell Observatory, Flagstaff/Arizona was the first astronomer to measure the Doppler shift in the spectra of galaxies, making him the discoverer of galaxy rotation and of galaxy redshifts. He hired Clyde Tombaugh, who discovered Pluto in 1930.
Vesto Slipher ( ) 22

23. Ernst Opik (1893-1985) was an Estonian astronomer
Ernst Opik ( ) was an Estonian astronomer. He spent the second half of his career at Armagh Observatory in Northern Ireland. He had powerful insights in several areas of knowledge, the source of energy in stars and the ancient climate of Earth.
In 1922, Ernst Öpik uses stellar orbital velocity and starlight density of Andromeda:
Distance = 1,440,000 l-y
Mass = 4.5 billions “suns”
Ernst Öpik ( ) 23

24. “Cracking the nut” of galaxies24

25. variable stars: Cepheids
Henrietta Leavitt
and a new class of
variable stars: Cepheids
Henrietta Swan Leavitt ( ) worked at Harvard College Observatory as a “computer” at $10.50/week and was assigned to count images on photographic plates. She found and formulated the period-luminosity relationship for Cepheids (a type of variable stars), that provided the foundation for a paradigm shift in modern astronomy. She received almost no recognition during her lifetime. She had been nominated for the Nobel Prize in 1924 ! .... paperwork stopped when it was learned she had died 3 years earlier. She had provided the key to open the lock of stellar distances. Light curves shown are from Cepheids in M31, the Andromeda galaxy.
Text 25

26. Andromeda Galaxy, M31 by Edwin Hubble
N not a nova
it is a
variable
Cepheid
Edwin Hubble ( )
In 1924 (1925), Edwin Hubble ( ) - then 36 years old - showed that the variable stars observed in M31, the Andromeda galaxy, were not members of the Milky Way. They were Cepheid variables located in M31. His results settled (almost) the “Island Universe” debate. In addition to M31, he observed M33, IC 1613 and NGC 6822.
The “great debate” - on the scale of the universe - held on 26 April 1920 between Curtis and Shapley had never really taken place. Both presented their view on the size of the Milky Way and on the nature of the nebula. The “debate” took place in the Baird auditorium of the Smithsonian Museum of Natural History on the Mall, DC.
Andromeda Galaxy, M31 by Edwin Hubble
“The spiral is 7.25 times as far away as the Cloud, or about 680,000 l-y.” 26

27. American Astronomical Society 1st Jan 1925
Hubble did not present his results in person. This is surprising. Some explain his absence by his reluctance to do the long cross-continent train trip, others say that it can be explained by Hubble still being troubled by the contradictory measurement of proper motions claimed by Adrian van Maanen to be associated with the rotation to the galaxies, which would imply velocities of more than 100,000 km/sec.
GWU Corcoran Hall (725 21st St., DC) was dedicated in Oct and was first GWU building on its new Foggy Bottom campus. From 1934 to 1956, George Gamow taught and did research there. Its basement was the birthplace of the “bazooka”.
Corcoran Hall, George Washington University, DC
American Astronomical Society 1st Jan 1925
“Cepheids in Spiral Nebulae”, Edwin Hubble 27

28. ... there is the shadow of Adriaan van Maanen
Hubble wrote in “Contributions from the Mount Wilson Observatory”, No. 514, p : “The outstanding discrepancy in the current conception of nebulae as extra-galactic systems lies in the large angular rotations announced more than a decade ago by Dr. van Maanen. The extraordinary significance of the phenomena, if they are real, has lead the writer to remeasure four of the principal nebulae, M81, M51, M33 and M A careful study of the longest interval with the blink comparator gives no indication of systematic displacements.... The results establish the existence of some systematic errors in the rotation...”
M 33 28

29. Edwin Hubble
Combining several proposed schemes of galaxy classification, Hubble proposed in 1936 his own, the so-called “Hubble Sequence”. The scheme was later expanded by Gérard de Vaucouleurs and Allan Sandage, refined by Ron Buta et al. in the 1990s.
Hubble in 1936, in The Realm of the Nebulae: “... nebulae are closely related members of a single family. They are constructed on a fundamental pattern which varies systematically through a limited range. The nebulae fall naturally into an ordered sequence of structural forms and are readily reduced to a standard position in the sequence.” 29

30. How was the expansion of the universe found?
As we came to understand the nature of galaxies and their distances, we could establish their distribution in space. Galaxies served as probes in the newly discovered space-time. We found that they were flying apart like drifting icebergs on the ocean. The expansion of the universe was discovered almost simultaneously with establishing the extragalactic nature of most “nebulae” in the sky. 30

31. The expanding universe
The theory of general relativity by Albert Einstein ( ) described how the universe with matter behaves. Einstein believed a spherical static universe was philosophically preferred; he wanted the universe to be eternal and unchanging. To his astonishment, this was not consistent with general relativity which has solutions of dynamic universes: it either expands or contracts depending on the amount of mass in it. To fix this he introduced a new notion, the cosmological constant.
Text
Albert Einstein ( )
General Relativity: A theory of space-time and gravitation 31

32. Slipher Humason Hubble
Alexander Friedman ( ) discovered the dynamic solution in He did not connect his finding with astronomical observations.
Georges Lemaître ( ) also found the expanding-contracting universe solution and published it in He derived an approximation of the “Hubble Law”.
Lemaître and Einstein met at the Fifth Solvay Congress in Brussels in October Einstein said to the young Lemaitre (33 yr old) “Your calculations are correct, but your physical insight is abominable”. Lemaître was also shunned by de Sitter who had “no time for an unassuming theorist without the proper international credentials.”
Slipher had measured galaxy redshifts, observing from the Flagstaff Observatory, since 1912.
Lundmark did draw the first “distance-velocity diagram” of spirals in 1924 and Eddington had been very supportive of Lemaître. Eddington was responsible for having Lemaître’s 1927 paper translated and published in MNRAS (Monthly Notices of the Royal Astronomical Society) in 1931.
Georges Lemaître ( )
Slipher Humason Hubble
Alexander Friedman ( ) 32

33. 1924: First “distance-velocity diagram”
by Knut Lundmark
Several researchers tried to find if, as predicted by some interpretations of General Relativity like that of de Sitter, space was expanding. In 1924, Carl Wirtz found a relationship between the apparent photographic diameters and the radial velocities for spirals. Also in 1924, Knut Lundmark published in MNRAS the first distance-velocity diagram expressed in units of distance to Andromeda. His article “The Determination of the Curvature of Space-Time in de Sitter’s World” was published in MNRAS, vol. 84, p. 747, It has 8 citations ! But there was a lot of confusion regarding the distances of “nebulae” and of globular clusters. No one was yet able to find a trend.
Lundmark
( ) 33

34. Milton Humason Edwin Hubble
Working together with the large reflectors of Mount Wilson Observatory, Milton Humason and Edwin Hubble conducted a systematic program of imaging and spectroscopy of several tens, then hundreds, of galaxies. The imaging provided the information on the distance of the galaxies and the spectroscopy of their apparent relative velocities. These observations confirmed that there existed a strong trend of “a wavelength increasing with distance”. However, Hubble himself refrained from interpreting this redshift as due necessarily to a velocity recession.
Milton Humason
Edwin Hubble 34

35. The “Hubble law” of 1929 v = H0 d H0 = 72 km/sec per megaparsec
From “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae”. This diagram (from The Realm of the Nebulae) was first published by Hubble in the Proceedings of the National Academy of Sciences of the United States, vol 15, no. 3, pp , 1929
v = recessional velocity
H0 = Hubble constant
d = distance
H0 = 72 km/sec per megaparsec
or 20 km/sec per million light-years 35

36. Hubble, like Einstein, had doubts about an expanding universe, strongly preferring (believing) that the universe was static. Sandage said that Hubble never believed in the expansion of the Universe. For a more detailed discussion, see Nussbaumer & Bieri (2011) see arXiv: v2 [physics.hist-ph] 13 Jul 2011 and their book “Discovering the Expanding Universe”, Cambridge University Press, 2009.
H. Nussbaumer and L. Bieri (2011) write that Hubble was very dubious that the recessional velocities represented the expansion of the universe: “Actually Hubble never believed in such a thing.” 36

37. The evolving universe
As we go back into the early history of the universe, we see a different universe
Stars are forming at a much higher rate
This rate is changing rapidly over the first Gyrs (1 Gyr = 109 years)
Galaxies are merging at a higher rate
Galaxy morphology is different 37

38. The expansion of the universe as observed today and other properties of our universe indicate that all space-time-matter had a beginning about 13.7 billion years ago. Expansion and a finite lifetime of the universe since its beginning implies that as we look back in time (or deeper in space) we will see changes. Objects like galaxies and clusters of galaxies should appear younger, closer together (more interaction) and less evolved, e.g. having a much higher gas/star ratio than galaxies of the local universe.
The universe was condensed in a “fireball” of space-time 13.7 billion years ago 38

39. Georges Lemaître 40 rue de Namur Louvain 39
Georges Lemaître (in Nature May 9, 1931) vol. 127, p “The Beginning of the World from the Point of View of Quantum Theory”. This led to the Big Bang theory and its multiple variations that help us to understand the universe as seen at all wavelengths. In a short article, not even filling half a page of the May 1931 issue of Nature, Georges Lemaître proposed his idea of the beginning of the universe using the framework of quantum mechanics. This short article was the proposed “Big Bang” cosmological model. Note the article on the left discussion the insect remains in the gut of a cobra.
Georges Lemaître
40 rue de Namur
Louvain 39

40. Galaxy cluster Abell 2744 ~3.5 Giga-ly
Hubble Deep Fields
If the universe has a finite age (e.g billions years), one should see galaxies differently as one looks further away or a longer time in the past. One should come to a point where galaxies will look different because they are younger, when more of their content was gaseous than stellar. We should observe a change in the star formation rate as a function of time. The powerful imaging capabilities of the Hubble Space Telescope and of other space- and ground-based telescopes have demonstrated that galaxy numbers and properties indeed change as we look further in distance or in the past.
More distant field
Galaxy cluster Abell 2744 ~3.5 Giga-ly 40

41. Examples of galaxy shapes and colors from Hubble Space Telescope images. Galaxies in the current/local universe appear more “stellar” and more settled in their shape and morphology. Galaxies seen further in the past show a greater ratio of gas, stronger star forming activity, and their morphology appears disturbed. 41

42. Halton Arp and peculiar galaxies.
The world of galaxies is much more varied and strange than inferred by Hubble’s elegant scheme. 42

43. A Computer Simulation of Galaxies Colliding
“Cosmic Collisions”
A Computer Simulation of Galaxies Colliding
Download movie here: 43
Movie Credits: Frank Summers (STScI), Chris Mihos (Case Western Reserve Univ.) & Lars Hernquist (Harvard)

44. We have found deeper forces at work on the largest scale.
By carefully monitoring the movements of objects on the galactic and intergalactic scale, we found evidence first for the expansion of space-time, then for dark matter. By mapping the footprint of the fossil radiation left by the big bang --going backward into the history of the universe -- we discovered that the expansion is accelerating, evidence for dark energy. 44

45. Signatures of dark energy and of dark matter
2011 Physics Nobel
Adam Riess
We discovered that the expansion is accelerating, evidence for dark energy. The sum of these two forms -- dark matter and dark energy -- of mass-energy comprises 96% of the content of the universe and we do not what they are, creating the greatest mystery of modern age astrophysics. Working in the footsteps of the theory of General Relativity of Albert Einstein, our colleagues of the 2011 Nobel Prize for Physics, Riess, Schmidt, Perlmutter (right) and their collaborators discovered that the expansion of the universe was accelerating.
Brian Schmidt
Albert Einstein
Saul Perlmutter
Signatures of dark energy and of dark matter 45

46. In the last 100 years, the volume of the observed universe has been multiplied by times, or “one million billion times !” 46

47. We found in the 18th and 19th centuries that all the stars and many nebulae were part of one large stellar system:
the Milky Way. 47

48. In the 20th century, we established the detailed geometry, properties and content of our stellar system, the Milky Way. We also established that the Milky Way was one of several billion similar large stellar systems in the universe. 48

49. From deep imaging done with several space- and ground-based telescopes, we estimate that there are about 200 billion galaxies in the observable universe.
Whether they fascinate or frighten, astronomically large numbers hide a fundamental characteristic of nature: diversity and complexity. They allow a cosmic landscape of incredible richness. Weird things like planets and moons can happen ! 49

50. From deep imaging done with several space- and ground-based telescopes, we estimate that there are about 200 billion galaxies in the observable universe.
Whether they fascinate or frighten, astronomically large numbers hide a fundamental characteristic of nature: diversity and complexity. They allow a cosmic landscape of incredible richness. Weird things like planets and moons can happen ! 50

51. 200 billion galaxies in the observable universe, each with
about 100 billion stars like the sun
20,000,000,000,000,000,000,000 stars
From deep imaging done with several space- and ground-based telescopes, we estimate that there are about 200 billion galaxies in the observable universe.
Whether they fascinate or frighten, astronomically large numbers hide a fundamental characteristic of nature: diversity and complexity. They allow a cosmic landscape of incredible richness. Weird things like planets and moons can happen ! 51

52. Saturn, occulting the Sun, viewed by the spacecraft Cassini
Saturn, occulting the Sun, viewed by the spacecraft Cassini. Dramatic view of the rings but also of Earth located about 1.5 billion km away. Cassini, orbiting around Saturn, drifted in the giant planet’s shadow for about 12 hours in 2006 and looked back toward the eclipsed Sun.
This shows the rings in a spectacular way but also of Earth as a tiny luminous dot see through one of the faint outer rings (next slide). 52

53. Inset shows a zoom-in view on the little dot: this is the Earth with a faint extension at 10 o’clock that is the Moon. 53

54. The Suomi NPP Blue Marble, Credit:NASA/NOAA
Image of Earth recently released by NASA. This montage (and that of the opposite hemisphere) where created from photographs taken by the Visible/Infrared Imager Radiometer Suite instrument on board of the new Suomi NPP satellite, named after Verner Suomi, commonly deemed the father of satellite meteorology. 54
The Suomi NPP Blue Marble, Credit:NASA/NOAA

55. Discovering galaxies: some references
The Realm of the Nebulae, Edwin Hubble, Yale University Press, 1936
Man Discovers the Galaxies, R. Berendzen, Richard Hart & Daniel Seeley, Science History Publications, 1976
The Day We Found the Universe, Marcia Bartusiak, Pantheon Books, 2009
Discovering the Expanding Universe, H. Nussbaumer & L. Bieri, Cambridge University Press, 2009 55

56. Images from NASA and ESA Space Telescope Science Institute
Gemini Observatory
Canada France Hawaii Telescope
Peabody Library, Johns Hopkins University
Wikipedia Commons 56

57. Acknowledgments Zoltan Levay, STScI Carolyn Slivinski, STScI
Frank Summers, STScI
Paul Espinoza, JHU Peabody Library 57

58. Rings and arcs of starlight surrounding a galaxy, remnants and vestiges of recent interactions and mergers between galaxies.
Thank you 58