- A Brief Introduction to Galaxies
Since there are lectures to come on the Milky Way (IAWL Lecture 27: The Milky Way) and galaxies (IAWL Lecture 28: Galaxies), we just give a brief introduction to galaxies here.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).
A cartoon of galaxy types.
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.
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We can put some numbers on these comparisons.
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.
- A Brief Introduction to Milky Way
The Milky Way as an unaided-eye object has always been with us: known since prehistory.-
[The
Milky Way is
also known as THE Galaxy with a capital G.
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).
- A History of the Discovery of the Milky Way to Shapley
The idea that the Milky Way was mass of stars unresolved to the unaided eye goes back to Democritus of Abdera (circa 460--360 BCE) (No-401).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???.
-
[It seems people until circa 1920 accepted that the
Milky Way
was essentially held static by some means not known in
terrestrial physics.]
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.
-
Cepheids
are very luminous stars with calibratable luminosities.
Because their luminosities can be calibrated, they are be used as distance indicators using the inverse-square law for light.
-
[If you know luminosity and can measure flux, the distance
follows from the
inverse-square law:
L F= ____________ gives r = sqrt[L/(4*pi*F)] . 4*pi*r**2We assume, you can neglect interstellar dust.] - A Brief Introduction to Milky Way
