Sections
Galaxies are large, relatively dense, gravitationally-bound systems of stars and interstellar medium (ISM) (i.e., dust and gas).
But they are a fairly diverse lot and exact boundaries of the term galaxy are perhaps??? set by tradition and convenience rather than any absolute physical criteria: no single one sentence definition seems to adequately delimit galaxies from other gravitationally-bound systems of stars.
Perhaps the best definition is just to specify the four main types of galaxies: each type itself being reasonably well specified.
The four main types are (unbarred) spirals (having spiral arms), barred spirals (having a bar structure in the center and spiral arms), ellipticals (which are just swarm of stars with relatively random orbital orientations), and irregulars (which, oddly enough, are rather irregular in structure).
Galaxies range in luminous mass from about 10**5 M_Sun dwarf ellipticals to about 10**13 M_sun giant ellipticals (FK-582). Not counting some unknown number of dwarf ellipticals, ellipticals make up about 20 % of all galaxies in the modern (i.e., nearby) universe (FK-582).
Spirals (barred and unbarred) range in luminous mass from about 10**9 to 4*10**11 M_Sun and make up about 77 % of all galaxies in the modern (i.e., nearby) universe (FK-582).
Irregulars range in luminous mass from about 10**8 to 3*10**10 M_Sun and make up about 3 % of all galaxies in the modern (i.e., nearby) universe (FK-582).
We will consider how galaxies are spread through space (i.e., the large-scale structure) in IAWL Lecture 28: Galaxies. Here one can say that they seem be distributed on cell walls surrounding voids.
In images, yours truly always thinks galaxies are hanging space like a mobile over a baby's crib.
Intergalactic nearest neighbor distances vary widely, a typical nearest neighbor intergalactic distance is of order a Megaparsec.
HCG 87: Hickson Compact Group of Galaxies.
The sources with points and many of the faint sources are foreground stars in the Milky Way.
The points are an artifact of the optical imaging system.
Credit: NASA/HST.
There is a big distinction between thinking about stars and thinking about galaxies.
Stars are pinpricks compared to the distances between nearest neighbors, unless they are in a close binary or other multiple-star system.
Galaxies are typically not pinpricks compared to the distances between nearest neighbors.
Since galaxies range in size scale in luminous matter from about 1 kpc to 250 kps (FK-582), the ratio of size scale to spacing is typically of order 1/1000 to 1/1.
Galaxies are NOT not like stars where the ratio of size scale to spacing (except in multiple star systems) is typically of order 1/10**7 to within factor of 10**3 or so.
But we have trouble seeing the structure of the Milky Way: we are EMBEDDED in the galactic disk and that is laced with obscuring interstellar dust.
It's a classic ``can't see the forest for the trees'' situation.
Before about 1750, no one had thought of a universe of galaxies (No-405) and before 1923--1924, when solid evidence appeared for it (No-509--510), relatively few believed in it.
We now give a brief introduction to the Milky Way, our galaxy.
Galaxy is derived from the the Greek name for the Milky Way, galaxias (No-401): gala means milk in Greek (Ba-496).
In Latin, the name is Via Lactea (HI-405): Milky Way is a straight translation of this.
After or maybe sometime before???? other galaxies were proven to exist, the term galaxy was made generic for star systems like the Milky Way.
The great circles of the Milky Way, celestial equator, and ecliptic do NOT intersect at two points, alas.
The Milky Way passes through the constellation Cassiopeia in the northern sky (which can easily be recognized: it's the big W), passes over the north-east shoulder of Orion (where one finds Betelgeuse), and passes through the SUMMER TRIANGLE formed by the bright stars VEGA, ALTAIR, and DENEB (FK-S-8, S-13). Cassiopeia is circumpolar at mid-northern latitudes, and so can be found pretty much at any time of the year. Orion can be easily located in the winter. The SUMMER TRIANGLE can, of course, be found in the summer.
The northern constellations: a mid-winter night-time
view judging from the position of old man Orion.
The Milky Way is not displayed, but it passes through Cassiopeia and over the north-east shoulder of Orion (where one finds Betelgeuse).
Credit: Mount Wilson Observatory StarMap program by Bob Donahue. StarMap is fortran program, but it's been broke since 2000jan03. Download site: Univ. of Tennessee, Knoxville Astro course; more precisely here.
The Milky Way is quite visible in dark skys, but even in the city under very clear conditions its faint luminous band can sometimes be seen---maybe with a bit of imagination.
The center of the Milky Way is in Sagittarius which is low in the southern sky in summer (Shu-257; HI-405; ???).
A cartoon of the
Milky Way
(FK-556,566;
CK-379;
Shu-257;
Cox-570).
He, however, apparently thought of the stars pasted on a sphere of the stars: a real celestial sphere (Fu-???).
Galileo (1564--1642) and probably other early telescopic observers discovered that much of the Milky Way is indeed resolvable into stars (No-401).
Interest in the overall structure of the Milky Way was slow in developing it seems until the mid 18th century.
The obscure surveyor, antiquarian, amateur theologian, and architect Thomas Wright of Durham (1711-1786) from 1742, the not-so-obscure philosopher Immanuel Kant (1724--1804) shortly thereafter and following Wright's lead, and the mathematician Johan Heinrich Lambert (1728--1777) by his own account in 1749 all speculated on structure of the Milky Way (No-404--407).
Wright even proposed that the Milky Way was supported against gravitational collapse by orbital motion about the center (the ``divine center'') of the Milky Way (No-405).
This is essentially true: rotational kinetic energy holds up the Milky Way from collapse to its center. But this idea was not completely accepted until the 1920s???.
The greatest observational astronomer of the 18th and early 19th centuries William Herschel (1738--1822) attempted to map the Milky Way using star counts (star gauges he called them) and statistics (No-407--408).
He could NOT measures distances directly or in absolute units.
William Herschel (1738--1822)
HERSCHEL was the greatest observational astronomer of the 18th and early 19th centuries and one of the great visionaries of astronomy. He was born Hanover, Germany, but in 1757? migrated permanently to England which was then under a Hanoverian dynasty. His initial profession was a musician, but he gradually developed into a full-time astronomer and was given a pension by George III to support his research (No-398).
His sister CAROLINE HERSCHEL (1750--1848) assisted him and became a significant observer on her own (St. Andrews Mathematics Archive). She was also given a royal pension (No-399).
One of HERSCHEL greatest endeavors was a program to map the universe (No-399).
It was during the course of this program that he discovered Uranus on 1781mar13 using 6.2 inch reflector (No-399).
Uranus, blue and extremely bland, but not as Herschel saw it. Credit NASA.
HERSCHEL was also the maker of his own telescopes. The most useful of these was his 0.48 m (18.7 inch) reflector erected at Slough, England near Windsor Castle (No-400).
Herschel's 0.48 m reflector at Slough, England near Windsor Castle.
Credit: 18th century artists; modern credit: ????; download site Wolfgang Steinicke's List of NGC/IC observers.
Herschel's project to map the Milky Way was a brilliant pioneering effort, but doomed to failure in its main aim given the limited astronomical techniques and astrophysical knowledge of his time.
Observations and statistical methods continued to develop through the 19th and early 20th centuries---or, as in the old movies, one could say time passed.
In 1901, Jacobus Cornelius Kapteyn (1851--1922) relying on statistical methods proposed a model of the Milky Way that put the Sun, more or less, in the center of disk-shaped structure of stars that was 10 kpc in diameter and about 2 kpc thick at its thick point. (No-453,490).
By 1922, Kapteyn had revised his model and gave it dimensions of 16 kpc by 3 kpc with the Sun about 0.65 kpc from the center in the central plane of the disk (No-491).
Kapteyn and other early modelers were fooled by their ignorance of extinction caused by interstellar dust which was only conclusively demonstrated exist in the 1930s (No-491).
Interstellar dust limits the distance we can see in the disk of the Milky Way to less than 3 kpc in most directions (FK-563).
Harlow Shapley (1885--1972) took a different approach to determining the structure of the Milky Way.
He measured the distances to globular clusters in the Galactic halo using Cepheids.
Because their luminosities can be calibrated, they are be used as distance indicators using the inverse-square law for light.
L
F= ____________ gives r = sqrt[L/(4*pi*F)] .
4*pi*r**2
We assume, you can neglect
interstellar dust.]
Cepheids have an interesting discovery history and properties, but they are NOT discussed in further detail in the Introductory Astronomy Web Lectures because we can't do everything.
We will not go into Shapley's method in detail, but we can mention a couple of points.
Interstellar dust is much less significant away from the disk of the Milky Way.
By 1916, Shapley had estimated that the diameter of the system of globular clusters of the disk of Milky Way was of order 100 kpc (which is about 3 times too large) and that the Sun was far from the Galactic center.
Although Shapley's determinations had their own errors, he was on the more correct path than Kapteyn and other early modelers.
This was NOT clear to everyone circa 1920 when we leave off this thread to pick up another one.
The tenable alternate hypothesis was that space was infinite emptiness beyond the boundary of the Milky Way.
But if there are other galaxies what are they observationally?
Well from Greco-Roman Antiquity there had always been a few cloudy stars: nebulae which is just Latin for clouds.
Ptolemy (circa 100--170 CE) listed 6 or 7 cloudy stars in his 2nd century CE catalog of 1022 stars in 1048 constellations. (No-113,402).
Ptolemy with an armillary sphere.
Ptolemy surprisely missed one of the most obvious nebulae: the Andromeda nebula.
The Andromeda nebula was first recorded by Al-Sufi (903--986) in his Book of the Fixed Stars (No-188,402), but Tycho Brahe (1544--1601) missed it his star catalog (No-299,308,402). Simon Mayr observed it telescopically in 1612 and put it permanently on the sky map??? (No-402).
After the invention of the telescope in 1608 and its utilization in astronomy, more nebulae were gradually discovered, but even well into the 18th century they still numbered only in the tens.
He compiled a what we call the Messier Catalog of nebulae.
The final published version contained 101 objects (No-403--404), but several others were added in his notes, and so today there are conventionally 110 Messier objects: M1, M2, M3, ... , M110.
M2, NGC7089 is a Galactic
globular cluster in
Aquarius.
This image was made using the Kitt Peak National Observatory 0.9-meter telescope.
M2 is the 2nd object in the Messier Catalog.
It is about 15 kpc away and is located almost directly below the south Galactic pole in the Galactic halo.
M2 is one of the richer denser globular clusters with about 10**5 star and a diameter of about 50 pc.
It has an overall visual magnitude of 6.3 which makes it just barely unobservable with the unaided eye. But it is a good object for small telescopes and binoculars.
Like all Galactic globular clusters M2 is very old. Current calculations put the ages of Galactic globular clusters at about 12.5 Gyr (FK-638).
This age is a lower bound on the age of the observable universe which in the concordance model is 13.7+/-0.2 Gyr (FK-653).
Credit: Doug Williams, REU Program/NOAO/AURA/NSF.
There are, of course, far more nebulae than in the Messier Catalog, but the Messier objects are just about all the largest and brightest ones as seen from Earth, and so are especially important objects for both amateur and professional astronomers.
Comet Ikeya_Seki in 1966.
Credit: Roger Lynds/NOAO/AURA/NSF.
Faint comets and nebulae look much alike---they both look like fuzzy little clouds. comets.
Messier aim in making his catalog was to be able avoid mistaking nebulae for comets.
It seems the fainter you go, the more nebulae you find: this is still true today, of course: you keep finding less intrinsically luminous or farther ones.
Herschel himself verified that some nebulae were resolvable into stars and that others clearly seemed to be gas surrounding a single star.
The former turned out to be Galactic star clusters and the latter planetary nebulae??? (No-407).
I believe Herschel also believed that there could be other galaxies (No-407): he was familiar Lambert's ideas (No-407).
In the 19th century, William Parsons, 3rd Earl of Rosse (1800-1867), a wealthy Irish landowner was a believer in other galaxies and was also a pioneer in large reflector telescope construction (No-435--438).
In 1845 February, his Lordship completed the construction at his Irish estate of the LEVIATHAN of Parsontown: a reflector of diameter 1.83-meter (72.05 in or 6 feet) made of speculum with a mass nearly 4 metric tons. It took 5 attempts in 5 years to construct the mirror.
Reflectors could be made larger than refractors (lens telescopes), but were disfavored for various reasons by most astronomers up to about 1900.
Early reflectors were all made of speculum.
Speculum is an alloy of copper and tin with a dash of arsenic invented for telescopic mirrors by Newton. It is brittle, tarnishes easily, and reflects only about 16 % of incident light. Newton also constructed the first working reflector telescope in about 1668 based on earlier ideas by Scottish mathematician and astronomer James Gregory published in 1663 (No-347).
From the later 19th century, reflector mirrors were made of rigid glass with a thin reflective coating (No-438).]
With the LEVIATHAN his Lordship made his greatest discovery: that some nebulae had spiral structure: they are, of course, the spiral galaxies.
M51 is actually two galaxies: NGC 5194, a large Sc spiral and a smaller companion NGC 5195 which a sort of barred galaxy.
M51 is about 8.5 Mpc away and 20 kpc across.
The spiral nature of some galaxies (historically nebulae) was first discovered from M51 by the 3rd Earl of Rosse (1800--1867; AKA William Parsons) in 1845apr at Birr Castle, Parsontown, Ireland using the Leviathan of Parsontown (1.83 m diameter telescope) (CK-366; No-435--437).
His Lordship circulated the sketch at the 1845jun meeting of the British Association for the Advancement of Science.
His Lordship did, in fact, believe that the spiral nebulae were other galaxies??? (No-437).
Credit: DJ Jeffery, WU 2005.
The sketch was made from the image of the original at the Sandburg Center for Sky Awareness where more of His Lordship's images can be viewed. It seems Cambridge University Press claims copyright on the images. Since they are just images of out-of-copyright original documents, this seems disputable to me. But I can't be bothered with others claims to owning copyright on historical images: I just make my own ``historical'' images as needed.
Now for a modern image of M51.
M51, NGC 5194/5195 (Sc/SB?): The Whirlpool Galaxy in Canes Venatici.
M51 is actually two galaxies: NGC 5194, a large Sc spiral and a smaller companion NGC 5195 which a sort of barred galaxy.
M51 is about 8.5 Mpc away and 20 kpc across.
The spiral nature of some galaxies (historically nebulae) was first discovered from M51 by the 3rd Earl of Rosse (1800--1867; AKA William Parsons) in 1845apr at Birr Castle, Parsontown, Ireland using the Leviathan of Parsontown (1.83 m diameter telescope) (CK-366; No-435--437).
Credit: Todd Boroson/NOAO/AURA/NSF. For more information see the SEDS M51 page.
The LEVIATHAN continued in operation until 1878, but its initial discovery of the spiral nebulae were its greatest achievement.
Smaller telescopes did not have the:
The closest spiral galaxies are quite big objects on the sky. They don't need much magnification or resolution.
For example, Andromeda galaxy (M31) has a disk angular diameter of about 3 degrees (Cox-578).
But it and the other spiral galaxies are rather faint and look like clouds in small telescopes used for visual astronomy.
This is why small-telescope viewing of galaxies is often a little disappointing. What you see doesn't look like the pictures which result mainly from long-exposure photography.
You need a lot of light-gathering to see the structure of spiral galaxies visually.
In a battle of technologies, LEVIATHAN'S size beat out astrophotography.
Photography developed in a series of steps starting from about 1816 (No-442) and was still only being applied to bright astronomical objects in the 1840s: i.e., Sun and Moon (No-443).
In the time from Lord Rosse to the 1920s considerably more data and theory were developed about the nature of nebulae. This included data from astrophotography and spectroscopy.
We will not here rehearse this the argument about the nature of nebulae.
By the 1920s, however, the argument was coming to a head.
In fact, on 1920 April 26, there was a formal debate between Shapley of Mount Wilson Observatory and Heber D. Curtis (1872--1942) of Lick Observatory entitled The Scale of the Universe which turned essentially on whether the some nebulae were other galaxies (Hoskin, M. A. 1976, Journal for the History of Astronomy, ``The `Great Debate': What Really Happened,'' vii, 169 [p.174]).
This debate has gone down in history as the Shapley-Curtis debate or THE GREAT DEBATE. Curtis was PRO on the subject of some nebulae being other galaxies; Shapley was CON.
On the night of, Curtis was probably the winner in a formal debate sense.
However, in subsequent papers the two ``GLADIATORS'' came off with more equal honors in a fairly sophisticated review of the evidence (Shu-286--291).
The fact was that more evidence was needed to decide the issue.
That evidence would be provided by Hubble.
He was also an athlete and is the only known astronomer to have boxed a world heavyweight champion: Georges Carpentier: a non-title bout one assumes (No-508).
After serving in the infantry in WWI he joined the staff of Mount Wilson Observatory in southern California in 1919 (No-509).
Hubble, remarkably for an astronomer, became a well known person in Hollywood during its golden age and he possibly turns up in small parts in Hollywood novels under different names: e.g., in James Hilton's Morning Journey.
The great discoveries Hubble was to make at Mount Wilson were predicated on the facts that Mount Wilson in those days (the 1920s) was one of the best observing sites in the world---this was before the smog and the light pollution of Los Angeles mostly ruined things---and on having the largest telescope to date at his disposal: the 100-INCH TELESCOPE: i.e., 2.54-meter Mount Wilson reflector, formally called the Hooker Telescope.
Hubble from his Ph.D. student days had been interested those nebulae that we now classify as galaxies (No-508).
He had developed what we now call the Hubble types by 1923 which can be illustrated in a Hubble tuning fork diagram.
Hubble tuning fork diagram
(CK-394;
FK-585;
No-508--509).
This classification scheme will be discussed in detail in IAWL Lecture 28: Galaxies.
Using the 100-INCH TELESCOPE, Hubble was able to resolve stars in the outer regions of the spiral nebulae M31 (the Andromeda galaxy) and M33 by 1923 (No-510).
M31: The Andromeda Galaxy.
M31 is a large spiral galaxy in the Local Group: it is the nearest large galaxy to the Milky Way and possibly a similar to the Milky Way.
M31 is visible to the unaided eye as a cloudy star. Remember, most of the stars seen in this image are foreground stars in the Milky Way.
M31 is about 725 kpc away (Cox-578) The disk diameter is about 20 kpc.
It is approaching us at 266 km/sec.
Credit: T.A.Rector and B.A.Wolpa /NOAO/AURA/NSF.
Hubble found that 34 stars were Cepheids by their periodic brightness variation (No-510).
Using the known luminosity of Cepheids and the inverse-square law, he was able to put the Andromeda nebula well beyond the confines of the Milky Way as established by Shapley.
Alien Hubble discovers a galaxy.
By the 1924, Hubble had established the distance to the Andromeda galaxy to be 285 kpc (No-510).
This is NOT a very accurate result. Hubble had various errors in his measurements and calibrations that are entirely understandable given his time.
The modern distance to the Andromeda galaxy is about 725 kpc (Cox-578): this about 2.5 times Hubble's value.
But even if Hubble's contemporaries suspected large errors---and they may have---they did concede??? that Andromeda galaxy had to be a remote large system of stars comparable to Milky Way.
Now if Andromeda galaxy is another galaxy like the Milky Way, then their was little reason to doubt that all the spiral nebulae were spiral galaxies too.
Thus suddenly their were thousands of spiral galaxies extending out as far as the 100-INCH TELESCOPE could see.
There were also other nebulae with star-like spectra (which spiral nebulae have too) that were NOT obviously gaseous nebulae in the Milky Way.
These turned out to elliptical galaxies: yours truly does not know how quickly this became this was recognized, but it was probably pretty soon after 1924????.
M87 (NGC 4486) in Virgo.
M87 is the giant elliptical galaxy at the center of the Virgo cluster, the nearest large cluster of galaxies at about 15 Mpc away (FK-593,615).
M87 is about 100 kpc in diameter and since it is rather spherical it has much more luminous mass than the Milky Way. It probably grew so large by galactic cannibalism.
M87 is surrounded by a rich system of globular clusters which are fairly clear in the image. (FK-615).
Credit: NOAO/AURA/NSF.
Hubble along with collaborators continued to work on extragalactic distance measurements for the rest of his life.
By 1929 he had distances to 18 galaxies including 4 in the Virgo cluster of galaxies (No-510).
How could it be anything else.
Hubble remained a bit old-fashioned in that he continued to refer to galaxies as nebulae which we now no longer do, except when speaking historically.
He titled his famous book The Realm of the Nebulae (1936) (No-509).
That vast realm which we inhabit---and though some suspected as much, we never knew until 1923.