Overview of the Galaxy
Note: This page is a draft (October 2015), incomplete sections are flagged: [DRAFT].
This page outlines the features and structures of the Galaxy as they are conceived by modern astronomy: a single, gravitationally bound system of enormous mass, energy and dimensions. The presentation begins and concludes with the Milky Way, the Galaxy as it appears to the naked eye from our location near the Sun.
Many basic research papers are linked from Further Reading at the bottom of this page, with instructions on how to find papers of interest online.
From the Milky Way to the Galaxy
To the naked eye, our Galaxy appears as the Milky Way: an irregular, unevenly luminous band of dim light. Invisible from urban habitats and barely visible from many suburban locations, the Milky Way is actually bright enough, when located at the zenith of a dark sky site on a moonless night, to cast shadows on the ground. It will be useful to summarize briefly how our understanding has progressed from this naked eye view to the Galaxy model of modern astronomy.
The Naked Eye Era. The earliest and one of the finest naked eye descriptions of the Milky Way appears in Ptolemy's Almagest (c.150 CE). Ptolemy's intention was to carefully describe and measure the naked eye celestial sphere, which is the foundational task of every aspect of astronomy first, describe or measure as clearly as possible what appears in the sky, then use all available knowledge to interpret its physical nature and origin. His description begins this way:
"It is easily seen that the Milky Way is not simply a circle but a zone having quite the color of milk, whence its name; and that it is not regular and ordered, but different in width, color, density, and position; and that in one part it is double. These particulars we find in need of careful observation.
Now, the double part of this zone has one of its junctions near the Censer [Ara] and the other at the Bird [Cygnus]. The western zone nowhere touches the other, for it divides at the junction at the Censer and at the junction at the Bird, the eastern zone joining the other part of the Milky Way and making one zone which the great circle drawn in its middle would traverse. We shall first discuss this zone beginning with its southernmost parts.
These parts go through the Centaur's feet, and are thinner and fainter. The star in the bend of the right hind-foot is a little south of the northern line of the Milky Way; likewise the star in the left fore-knee and the star under the right hind-ankle. The star in the left hind-shank lies in the middle of the Milky Way, the star in the same ankle and the star in the right fore-ankle being very nearly 2° north of the southern arc. And the parts arout the hind feet are rather denser. ..." (Almagest, Book VIII, Chapter 2)
Ptolemy's analysis exemplifies the earliest human understanding of the Milky Way an irregular band of light of unknown nature that persisted up through the 17th century, when Galileo first resolved parts of the Milky Way into
through the late 18th century. In the last decades of the 19th century, before photographic astronomy made visual study of the Milky Way superfluous, the celestial artist Étienne Leopold Trouvelot expressed the esthetic power of this delicate and immense feature:
"During clear nights when the Moon is below the horizon, the starry vault is greatly adorned by an immense belt of soft white light, spanning the heavens from one point of the horizon to the opposite point, and girdling the celestial sphere in its delicate folds." (The Trouvelot Astronomical Drawing Manual, 1882)
Several "Milky Way skyscapes" depicting its naked eye appearance were made in the 19th century: Otto Boeddicker (observing in Ireland) and John Herschel (in South Africa) completed several fine examples. Trouvelot's portrait combines a Milky Way skyscape with the positions of bright stars and an ocean horizon (image, right), and is centered on the divided area from Cygnus to Sagittarius described by Ptolemy. Aside from their historical importance, these drawings are all touched by the visual beauty of the Milky Way, and underscore the importance of beauty in motivating the difficult work of observational astronomy.
Probably the last major Milky Way skyscape was the Lund panorama (image, below), a 2 meter long painting completed in 1955 and currently held at the Lund Observatory, Sweden. Unlike the many 19th century renderings based on naked eye observations, this painting relied on detailed isophote charts compiled from photometric surveys by Anton Pannekoek and colleagues in 1949.
The Lund panorama stands for the naked eye phenomenon that we must explain as a physical feature. The path to greater understanding progressed in three giant steps, relying first on the telescope, then on long exposure astrophotography, and finally on radio astronomy.
The Telescopic Era. In 1610 Galileo's Siderius Nuncius announced four dramatic telescopic discoveries: the cratered and mountainous surface of the Moon, the four revolving moons of Jupiter, the resolution of nebulae into clusters of stars, and the discovery of many new stars in the Milky Way too faint to be seen by the naked eye: "For the Milky Way is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, ... the multitude of small stars is truly unfathomable." This was the first indication that the universe was not identical to the apparent dome of the night sky but contained far more objects, and possibly extended to a greater distance, than the naked eye could see.
Much later, toward the end of the 18th century, William Herschel manufactured telescopes of sufficient aperture and optical quality to compile 698 "star gauges" or methodical counts of the number of stars within a standard telescopic field of view in different parts of the sky. Assuming that stars were approximately equally bright and evenly distributed in space, so that a greater density implied a longer view through the cloud, Herschel was able to deduce the Sun's position near the center of an extended mass of stars (On the Construction of the Heavens, 1785). He summarized his findings in a diagram (above left) representing the Galaxy in a cross section extending from the divided band in Cygnus on the left to Puppis on the right; the Sun is located slightly to the right of center. This is the first empirical conception of the Galaxy: "a very extensive, branching, compound Congeries of many millions of stars."
Herschel estimated his telescopic "sounding line" could measure out to "not less than 497 times the distance of Sirius from the sun" (1300 parsecs in modern units), and because he found no stars beyond roughly 100 times the distance to Sirius in the direction of the galactic poles, he concluded that empty space lay beyond and therefore we inhabited a "detached nebula" similar to the nebulae widely separated in space that Herschel was cataloguing at the time.
This star count approach was adopted and greatly refined by Jacobus Kapteyn (1922). Using photographic star counts and kinematic analysis based on measures of parallax and proper motion in 206 "Selected Areas" of the sky, Kapteyn placed the Sun about 650 parsecs from the center of a disk shaped conglomeration of 47 billion stars having an estimated radius of 8500 parsecs and a center of rotation in Cygnus.
In contrast to the Herschel/Kapteyn approach, which assumed an approximately uniform distribution of stars, others attempted to explain the visible lack of uniformity in the Milky Way the clumpy variations in brightness, clearly outlined dark areas, and streams or bands of faint stars, as memorably described by John Herschel (1847): "The Milky Way is like sand, not strewed evenly as with a sieve, but as if flung down by handfuls (and both hands at once), leaving dark intervals, and all consisting of stars ... down to nebulosity in a most astonishing manner."
The Photographic Era. The advances in long exposure astrophotography due to the work of Andrew Ainslie Common (1883), Isaac Roberts (1899), James Keeler (1908) and E.E. Barnard (1913) produced views of nebulae, the Milky Way and other galaxies in a level of detail and consistency of magnitude estimation not possible with the naked eye at any practicable aperture.
Richard Proctor (1869) suggested that the Milky Way was composed of "a distinct ring of matter out younder in space, [which] is of nearly circular section throughout its length." But the Dutch astronomer Cornelis Easton (1900) argued that this annular hypothesis was incompatible with available evidence. Adapting a photograph by Roberts of the spiral M 74 galaxy, he proposed that the Galaxy was a spiral nebula and the annular appearance was actually composed of spiral arms. He associated an inner "ring" with "'the belt of bright stars' of John Herschel and Gould" (the Gould belt), which was inclined 20° to an outer ring of distant stars that comprised what we could see of the farther Galaxy, whose center of rotation he located in the Cygnus star cloud between β and γ Cygni at some distance from the Sun (diagram, right).
Confusion about the actual dimensions and nature of the Galaxy appear as late as the "Great Debate" between Harlow Shapley and Herber Curtis in April, 1920. This event was held at a time when information about many aspects of the Milky Way (globular clusters, H II regions, stellar magnitudes and proper motions) was rapidly increasing. For example, Shapley (1918) had recently used the photographically measured period/luminosity relation of RR Lyrae variable stars found in globular clusters to estimate the distance of these clusters; their three dimensional distribution suggested the center of the Galaxy was in the direction of the constellation Scorpius at a distance of between 13,000 to 20,000 parsecs.
In the debate, divergent conclusions were drawn as to the validity or interpretation of the limited information then known about the Galaxy. Shapley argued that the Sun was eccentrically placed in an enormous (~90,000 parsec diameter) "island universe", surrounded by nearby spiral objects that were most likely to be much smaller gaseous nebulae; he suggested other galaxies similar to ours might exist but were too far away to detect. Curtis replied that the Sun was centrally placed in a small (less than ~9,000 parsec diameter) galaxy surrounded by other similar galaxies at distances of 3 million parsecs or more; he suggested these distant nebulae were only visible near the poles of the Milky Way due to obscuring matter within the disk. Remarkably, the two scientists were both right and wrong in about equal measure.
These basic uncertainties were resolved by Edwin Hubble (1929), who obtained a series of photographs of the Andromeda and Triangulum galaxies (M31 and M33) that were sufficiently detailed to identify individual Cepheid variables within them, then used the Cepheid period/luminosity relationship to deduce distances to these galaxies of about 275,000 and 263,000 parsecs, respectively (about one third of current estimates). The same year, using spectrophotography to measure the radial velocity of many nearby galaxies, Hubble published his first demonstration of a "velocity-distance relation among extra-galactic nebulae" the red shift that has since been used to estimate cosmological distances exceeding several billion parsecs.
In the same decade, large scale proper motion and radial velocity surveys of individual stars within the Galaxy provided data that allowed Bertil Lindblad (1927) to outline the spiral kinematics of the Galaxy and the formation of spiral arms, and Jan Oort (1927) to estimate the rotational velocity of the Sun and to locate the center of the Galaxy in the direction of the constellation Sagittarius at a distance of approximately 6300 parsecs (which he later revised to ~9000 parsecs, only 8% greater than the current value). Thus, by 1930 the "disk" structure, rotational speed, center of rotation, and dimensions of the Galaxy, and the relative size and distances of the galaxies around it, had been established.
The Wide Spectrum Era. Astrophotography extended the reach of observation to galaxies distant from our own, but it could not penetrate far within our own Galaxy due to the obscuring clouds of gas and dust hypothesized by Curtis. The presence of this obscuring matter was verified by Robert Trumpler (1930), who showed that the diameter of open star clusters diminished with distance less rapidly than the brightness of the stars inside them.
This interstellar medium of gas and dust, confined to within a few hundred parsecs of the galactic plane, obscures (reduces) stellar brightnesses by about 0.7 magnitudes per kiloparsec of distance from the Sun, mostly in "blue" wavelengths, which produces a corresponding reddening in the photographic color of stars. In the extreme, this obscuration becomes an interstellar extinction that blocks visual examination of the total extent of the Galaxy. Later, Walter Baade (1951) demonstrated that dark clouds and star forming regions in the Andromeda galaxy were confined to the visible spiral arms.
Following that lead, the spiral structure of the Galaxy was first detected by Morgan et al. (1953) who applied the method of spectroscopic parallax to determine the distance of 27 OB associations, H II regions and K giant stars. These appeared to identify segments of three spiral arms closest to the Sun, although the method could not trace the spiral arms to any great distance. Becker & Fenkart (1970) and others augmented Morgan's method with larger and more accurate samples of young open star clusters and H II regions.
A more powerful method followed the conjecture by Hendrick van den Hulst (1949) that the 21 cm emission line of atomic hydrogen (H I) could be used to make observations in radio wavelengths that would easily pass through obscuring gas and dust. During the 1950's, Oort, Kerr & Westerhout (1958), along with many collaborators worldwide, used radio telescopes in Europe and Australia to trace the position and radial velocity of hydrogen clouds within the Milky Way, then applied Oort's kinematic analysis to plot the velocity data as a spiral structure the first reasonably accurate image of the structure of most of the Galaxy (diagram, left).
During the 1960's these 21 cm line radio surveys were extended to greater coverage and resolution (cf. Westerhout & Wendlandt, 1982), as reported in the 1970 IAU symposium The Spiral Structure of Our Galaxy (cf. Bok, 1970). These early studies tend to identify four spiral arms: a "very conspicuous" Perseus arm, a "quite poorly defined" Orion arm (the Local Arm), a "well defined" Sagittarius arm, and a Norma arm [now called the Scutum-Crux arm] beyond and clearly separated from the Sagittarius arm (Courtès & alia, 1969). Later surveys used carbon monoxide (CO) emissions as a radio marker for otherwise invisible molecular hydrogen (H II; Dame & alia, 1987).
Subsequent studies improved on these results by combining spectroscopic parallax with radio astronomy to improve the estimated distances to the star forming regions and young star clusters that were presumed to be tracers of spiral arm structure. Typical of work at this time is the detailed map of the local region of the Galaxy and a large arc of the Sagittarius-Carina spiral arm by Roberta Humphreys (1976). She combined data on supergiant stars, OB associations, galactic star clusters, H II regions and stellar proper motions, and concluded that our Galaxy had two prominent spiral arms and, in overall structure, most resembled an Sc or "M 101 type" spiral galaxy such as NGC 1232 (also cited as an exemplar by Becker & Fenkart.)
Finally, Georgelin & Georgelin (1976) combined a meticulously measured and validated sample of more distant H II regions with radio observations of spiral arm velocity tangents and compact radio sources to create a widely cited and remarkably robust model of the Galaxy with four prominent spiral arms (diagram, right). This model placed the Sun (S) 10 kiloparsecs from the galactic center (GC), located it between the Sagittarius (no. 1) and Perseus (no. 2') spiral arms, and demoted the Orion "arm" to the status of a spur or branch between them; renamed no. 2 the Scutum-Crux arm, and identified a new arm (no. 1'), named the Norma arm, closest to the galactic center. The two pairs of arms 1/1' and 2/2' are symmetrically opposite each other and curve outward in identical spiral slopes. Four decades later, only the solar radius is known to be significantly in error. In all other respects, our current consensus model of the Galaxy is nearly identical to this late 20th century "draft" of the Galaxy structure.
Four Forms of Measurement. The fundamental attributes in astronomy include position, time, distance, dimension, mass, energy, temperature, spectral radiance profile, magnetic flux, chemical composition, structure, motion and age. These are related to each other in ways that often allow one attribute to be inferred from the others. Remarkably, these variables are studied using four principal methods: celestial position, physical appearance, spectral analysis, and computational prediction or simulation.
Celestial Position. The earliest steps in astronomy, developed in Middle Eastern civilizations, attempted to the determine the positions of the Sun, planets and stars on the celestial sphere and predict lunar and solar eclipses. Positional measurement allowed the basic distinction between the "fixed" stars and the rapidly moving bodies of the Solar system. Ptolemy's Almagest epitomizes this tradition by recording the positions (to within 10 arcminutes) of approximately 1000 naked eye stars and calculating basic orbital parameters for the known planets.
Lunar and planetary orbits were measured with increasing precision up through the work of Tycho Brahe in the 16th century, and Brahe's measurements of the orbit of Mars allowed Johannes Kepler to infer his three "laws" of orbital dynamics, which Isaac Newton generalized to elliptical orbits of any eccentricity in his Principia mathematica.
Further advances required several innovations: optical instruments to measure stellar altitude and azimuth at the arcsecond level, accurate clocks to measure the time of transit to within a second, a fixed celestial coordinate scheme, and the tools of spherical geometry and trigonometry necessary to determine relative distances and planetary orbits on the celestial sphere. Eighteenth century visual measurements of absolute celestial position were made using a transit telescope and an accurate sidereal clock, while measures of the relative position of binary stars were made in the 19th century with an eyepiece filar micrometeter.
Astrometry the measurement of celestial position at a specific time laid the foundation for celestial catalogs, star atlases, calculation of gravitational dynamics, and the refined forecasting of planetary positions, lunar and solar eclipses and lunar occultations. Astrometric surveys were extended to the southern hemisphere in the 19th century, and star catalogs incorporated fainter stars by means of larger transit instruments. By measuring at opposite times of the year the relative position between nearby and distant stars that were visually close together, astronomers could calculate spatial distance using geometric parallax, identify proper motion as changes in photographic positions, and calculate the orbital parameters of a small number of binary stars. These binary measurements gave the first indications of interstellar distances, galactic motion, and stellar dynamics ultimately the key to the calculation of relative stellar masses.
Positional data and changes in position over time are so critical in astronomy that they are now collected by astrometric satellites and ground based telescopes that provide an unparalleled precision and reliability of measurement. By the year 2020 the Gaia satellite will extend the horizon of positional and dynamic astrometry almost to the galactic center.
Coordinate Systems. Coordinate systems provide a way to systematically record celestial position and to compare positional measures of motion across time.
The earliest system of mesurating the sky was based on the twelve constellations of the zodiac, comprising the celestial paths of the Sun, Moon and six naked eye planets. A complete solar cycle around the Zodiac determined the time interval of the year; the twelve "houses" (constellations) of the Zodiac divided the year into 12 months.
The path of the Sun across the sky is the ecliptic, which defines a great circle on the celestial sphere. A great circle is defined by a plane that passes through the center of a sphere, so the ecliptic is defined on the celestial sphere by a plane that passes through the center of the Earth and contains the Sun's apparent orbit around the Earth. It is also, within a few degrees inclination, the average path in the sky traced by the Moon and planets (excepting Mercury).
The ecliptic is chiefly used as an astronomical coordinate system for calculating the position of solar system bodies such as comets and asteroids. For objects outside the solar system, the equatorial (celestial) coordinate system takes the Earth's equator as the fundamental plane and the projection of the Earth's axis of rotation as the celestial north and south poles. Location around the celestial equator is measured as right ascension (RA or α) in units of time (hours, minutes and seconds; one hour of RA is equal to 15°), starting at the vernal equinox and increasing from west to east. The distance from the celestial equator to a celestial pole is measured in degrees of declination (Dec or δ), with a minus sign indicating measurement toward the celestial south pole.
The Earth's axis of rotation is not fixed in relation to the stars but appears to move in a precession circle caused by the long period (~26,000 year) wobble in the Earth's axis of rotation. This wobble performs a circle around the pole of the ecliptic and the radius of the precession circle is about 23.5°. This causes the location of celestial north to change and the vernal (March) and autumnal (September) equinoxes to progress through all the constellations of the Zodiac. Due to precession, the fixed celestial coordinate system periodically must be shifted so that the coordinate celestial poles are consistent with the dynamic poles. For most astronomical atlases and references this epoch is revised every twenty-five or fifty years (the current epoch is J2000), but in astrometry the epoch is often recorded in fractions of a day.
William Herschel (1785) first recognized the importance of a galactic coordinate system, but developing the system was thwarted by a lack of clarity about the Galaxy itself. In 1958 the International Astronomical Union (IAU) established the system currently in use (Blaauw et al., 1960). Galactic coordinates are stated in degrees of latitude (b) and longitude (l), with longitude increasing from west to east. The galactic fundamental plane was defined by radio telescope measurements of the remarkably flat (~50 parsec thick) disk of galactic neutral hydrogen within 60° on either side of the galactic center, which placed the north galactic pole (NGP) in the constellation Coma Berenices. The origin of galactic longitude (0°), in earlier times placed in Cygnus or Aquila, was centered on the complex radio source Sagittarius A* (spoken as Sagittarius A star), which was later identified as a super massive black hole at the barycenter of the Galaxy and the best physical marker of the galactic barycenter. (The SMBH itself is located about 4 arcminutes from the radio marker used in the 1958 system.)
The figure (below) shows the locations of the ecliptic north pole (NEP) in the constellation Draco (located almost exactly at the Cat's Eye Nebula, NGC 6543), the celestial north pole (NCP) in the constellation Ursa Minor, ecliptic and celestial equator in relation to the galactic north pole (NGP) and cardinal points of galactic longitude.
Only galactic coordinates are used on this page to locate objects. Note that the precession of the celestial pole will appear to move counterclockwise around the NEP from the vantage of "observer". Note also that the distance of the Sun above the galactic fundamental plane (Z⊙) is about 23 parsecs (the average of 22 separate estimates). This causes objects relatively near the Sun that actually lie in the fundamental plane to appear below the galactic equator.
A galactic rectilinear system of three Cartesian coordinates, denoted x, y and z, can be constructed to represent the galactic coordinate system axes as linear distances. The x dimension extends from the Sun through the galactic barycenter (galactic coordinates l = 0°, b = 0°); z is a dimension from the Sun through the galactic north pole (b = 90°), and y is the dimension perpendicular to both x and z in the direction of the galactic rotation (l = 90°, b = 0°). All dimensions are usually measured in parsecs. The xy plane conveniently divides the galactic disk into four quadrants, labeled in counterclockwise order (see figure).
The UVW system, fundamental for the study of Galaxy kinematics, is a vector coordinate system of stellar velocities relative to the local standard of rest (LSR), an imaginary point in a circular orbit in the Galaxy fundamental plane and comoving with the average rotational speed of stars at the same distance as the Sun from the galactic center; UVW values are normally expressed in kilometers per second. Only the positive directions of the UVW dimensions are shown in the figure.
Note that neither the xyz nor UVW systems are projected onto the celestial sphere. The xyz system is a true spatial frame of reference: a Euclidean distance calculated from xyz values is the real distance from the Sun. As a vector system, the UVW coordinates have no center: an object with zero values of U, V and W is moving in exactly the same direction at the same speed as the LSR, but can be located at any distance in any direction from the Sun.
Physical Appearance. Before the turn of the 20th century, astronomical research methods were exclusively visual: humans looked into the sky with various instruments and documented what they saw. Even after astrophotography became the principal research tool for astrometry and deep space astronomy, visual study remained the primary tool in lunar and planetary astronomy up to the satellite era and the probes launched to the Moon from 1964 (Ranger 7) and to the planets from 1969 (Mariner 6 & 7): but a principal measurement delivered by these probes was video imagery.
The invention of the telescope revealed that: the Sun and planets were solid bodies rotating in fixed periods; they differed in dimension and surface appearance; they had satellites; the Sun had changing spots and Saturn had enduring rings. Deep space objects were cataloged for the first time, nebulae distinguished in size and form, and the structure of the Milky Way probed with "star gauges" or star counts. Visual cartography focused on Mars and the Moon. In the second half of the 19th century, astrophotography produced more accurate positional information on stars of fainter magnitudes and allowed for the first time the definitive categorization of visual "faint fuzzies" as planetary nebulae or emission nebulae and globular clusters from very distant spiral and elliptical galaxies, and provided large scale surveys of the Milky Way, including the first clear portraits of galactic interstellar medium.
Despite the enormous advances in instrumentation and the extension of imagery to wavelengths and flux levels invisible to the human eye, the importance of "appearance astronomy" is evident in the many features first identified or primarily studied through their visual structure or visual evidence of change: binary stars, variable stars, planetary nebulae, open star clusters, Bok gobules, Herbig-Haro jets, novae, pulsars, quasars, star forming regions, galaxy Hubble types, galaxy clusters, the earliest galaxies, the cosmic microwave background. The achievements of the Hubble and other space telescopes have been principally in high quality imagery of nearly every type of astronomical object; the value of high quality imagery motivates the optimal siting of new terrestrial observatories, the use of adaptive optics and speckle interferometry, computer image processing, and the imagery from planetary survey vehicles in the Voyager, Mariner, Cassini and Mars Explorer programs. To a remarkable degree, astronomy remains a visual (optical) science.
Spectral Analysis. The previous two methods, celestial position and physical appearance, developed using the eye and photographic emulsions as the two principal detectors. These sum within the image all the visual radiation (roughly 800 to 400 nanometers) from the celestial object. This method is epitomized as the brightness or magnitude of celestial objects, originally based on the visual comparison of stars but standardized as various forms of photometry.
At first, the advent of photography allowed magnitude to be more accurately measured as the size (density) of the photographic star image, which extended catalogs of stellar position and magnitude far below the naked eye limit. But around the turn of the 20th century it became possible to decompose the visual radiation into its constituent spectrum, and photography was used to record the spectral profile of stars as the location and intensity of specific spectral absorption or emission lines imaged through a prism or reflected from a diffraction grating. These early spectrum images laid the foundation for the large scale spectral classification of stars, published as the Henry Draper Catalog (1890). The original (and still most accurate) method relies on the detailed visual inspection of a diagnostic part of the spectrum; later the spectral type was inferred from the color index or relative magnitude (intensity of radiation) in two photographic plates imaged in short versus long wavelengths by means of two different colored filters.
In addition to the classification of stellar types, spectral decomposition allowed two other important insights. First, spectral lines are produced by light emitted from specific atomic elements, and these lines (as identified in terrestrial laboratories) were used to determine the chemical composition of the Sun, planets, stars and nebulae. In stars, chemistry is summarized as metallicity or the log ratio between the spectral presence of hydrogen or helium and some heavier element such as iron; metallicity tracks the sequence of star formation in cosmological time and can tag "star streams" in the Galaxy of common origin. The spectral analysis of emission nebulae and galaxies verified that these were composed primarily of hydrogen and helium and allowed more precise estimation of their temperatures; it also indicated the stellar composition of other galaxies.
Second, spectral lines are shifted in position by motion toward or away from Earth, and these Doppler shifts allowed measurement of radial motion which, in combination with proper motion, allowed a complete kinematic analysis of galactic structure and rotation. The variation of Doppler shifts and light curves in eclipsing binary stars allowed the association of spectral and luminosity types with characteristic stellar radii and provided the critical missing piece that, combined with astrometric measurements of proper motion or change in celestial position across time, allowed the three dimensional spatial analysis necessary to fully specify the dynamics of binary star orbits and the galactic orbits of individual stars. Radial velocity by itself revealed the recession of galaxies (the cosmological red shift) and the large scale expansion of the universe.
Combined with parallax or geometric distance estimates, the absolute magnitude (intrinsic brightness) of different spectral types could be estimated; combined with binary star dynamics the relative masses of spectral types could be derived. Around 1900 the link between a stars spectral profile and the blackbody temperature of its photosphere allowed the temperature of stars to be inferred from the spectral "color". This unlocked the relationships between mass, luminosity, effective temperature and spectral type at the heart of the Hertzsprung-Russell mass-luminosity diagram, and the eventual development of theories of nuclear fusion, nucleosynthesis, stellar evolution and star formation. Accurate time series measures of magnitude allowed for a precise analysis and classification of variable stars, including the RR Lyrae and Delta Cephei variables whose temporal periods are closely linked to their intrinsic brightness. These relationships, discovered in early 20th century, allowed the brightness distance relationship to be extended from stars whose distances could be measured by parallax to distances estimated by the apparent brightness of these "standard candle" variable stars. These permitted the first distance estimates to globular clusters and nearby galaxies, establishing the scale of the solar radius and intergalactic space.
Finally, advances in aperture, instrumentation and detectors expanded spectral decomposition to include the entirely new bandwidth sections of the electromagnetic spectrum. Xray, infrared and microwave telescopes in particular capture radiation that is very little obscured by interstellar dust and gas, resulting in a much expanded view in space and a more penetrating view into densely obscured areas of star formation. Microwave and infrared surveys, including long baseline arrays, have established the kinematics of large star forming regions several kiloparsecs away, which have been used to trace the spiral arms of the Galaxy.
Computational Astronomy. During the latter half of the 20th century the computer became an essential astronomical research tool. Computers made it possible for the first time to assemble, compile and verify the enormous quantities of data from astrometric satellites, to exploit the potential of active and adaptive optics to overcome the deterioration of astronomical imaging due to atmospheric turbulence, and to develop numerical simulations of long term cosmological and galactic processes.
Computers have allowed the aggregation of astronomical catalogs at a scale well beyond anything possible by human hands in the previous era. The Tycho catalog lists the positions of roughly 2.5 million stars, and the forthcoming Gaia catalog will extend measurement to as many as 1 billion objects. Large baseline array interferometry is only possible through the digital interconnections among widely separated instruments, and computers are used to stack, process, filter and sharpen astronomical imagery, control and communicate with deep space probes, and reliably classify the spectra of hundreds of thousands of stars and galaxies.
This last resource computer simulation has provided some of the most basic insights into nucleosynthesis, stellar nuclear dynamics, star formation, gravitational dynamics, stellar collapse and supernova events, star cluster evolution, galactic dynamics and cosmology. Much of what we believe we know about astrophysics is the product of known physical laws applied by calculation or simulation to the known mass, temperature, density, dimension and dynamics of astronomical objects. The fundamental power of the method is that it transcends our extraordinarily limited sample of astronomical time and allows study of processes that may span millions or even billions of years.
It is hardly an exaggeration to say that high speed computers and computationally efficient simulation software represent the largest aperture astronomical instrument in use today. Computers can pierce the veil of time, which is far more opaque than the cloaks of distance and magnitude or the landslide mass of astronomical data.
Measurement Units. [DRAFT] For reference, this section defines the principal units of measurement used by professional astronomers; some of these are not among the International System of Units.
Distance. The METER (m) or KILOMETER (km) of 1000 meters is the standard unit of small dimensions (up to the diameter of planets or stars), equal to the distance in a vacuum that a photon travels in 3.336·109 seconds.
The ASTRONOMICAL UNIT (AU) is a relative unit of astronomical distance, equal to the average radius of the Earth's orbit around the Sun or 149,597,871 (1.496·108) kilometers; it is usually applied to dimensions on the scale of planetary or binary star orbits.
The PARSEC (pc) or KILOPARSEC (kpc) of 1000 parsecs is the standard astronomical unit of large dimensions, defined as the distance at which a spatial interval of 1 AU will subtend an angular interval of one arcsecond (") that is, a width of 1 AU viewed from a distance of 30,856,804,799,936 (3.086·1013) kilometers. By definition, one parsec is equal to 206,265 AUs, the number of arcseconds in a radian.
The parsec unifies astronomical distances across many scales. Thus, 0.1 parsec is roughly the maximum orbital radius between gravitationally bound stars, 1 parsec is the average distance between single stars in the Galaxy, 10 parsecs is the benchmark distance for the calculation of absolute magnitude, 100 parsecs is the diameter of a large globular cluster, 1000 parsecs (1 kiloparsec) is the width of a galaxy spiral arm, 10,000 parsecs (10 kiloparsecs) is the radius of an average spiral Galaxy, 100,000 parsecs is the outer orbital limit between parent and satellite galaxies, and 1,000,000 parsecs (1 megaparsec) is the average distance between galaxies in the Universe.
The antiquated light year, used primarily during the late 19th to mid 20th centuries, is now encountered in the lay literature of amateur astronomy, science fiction and science journalism. As simple conversions: 1 parsec = 3.26 light years, and 100 light years = 31 parsecs.
Mass. The GRAM (g) or KILOGRAM (kg) of 1000 grams is the standard unit of mass, roughly 2.2 US pounds; the Solar mass (M⊙) is the unit of relative mass, equivalent to 1.989 x1030 kilograms. The mass range of stars that fuse hydrogen or heavier elements is roughly 1.392·1029 kg (0.08 M⊙, the hydrogen burning limit) to about 2.983·1032 kg (150 M⊙).
Time. The SECOND (s) is the standard unit of time, formerly defined as a small interval of a standard day but now defined as the interval required for 9.19·109 hyperfine electron transitions in a cesium atom; the relative units of time are the DAY (d) equal to 84,600 seconds and the YEAR (yr) equal to 31,557,600 seconds (3.156·107 s). The megayear (Myr) is one million years or 3.156x1013 s. The age of the Earth is roughly 1.433·1017 s (4.54 billion years), and the age of the universe is roughly 4.354·1017 s (13.8 billion years).
Energy. The NEWTON (N) is the standard unit of gravitational force or acceleration, equal to the energy necessary to accelerate 1 kg of mass at the rate of 1 meter per second squared (notated 1 kg m/s2 or 1 kg m1 s2). The gravitational acceleration (G) acting on 1 kilogram at the Earth's surface is equal to 9.807 N; at the photosphere (surface) of the Sun G = ~274 N.
The JOULE (J) is the standard unit of kinetic or mechanical energy, equal to 1 newton; this is equivalent to the energy required to raise a mass of 100 grams a vertical distance of 1 meter at the Earth's surface.
The WATT (W) is the standard unit of power (energy per unit time) or of radiant flux (energy per unit time emitted into a unit area), equal to 1 joule per second (power) or 1 joule per second per square meter (flux). The Solar luminosity (L⊙) is the relative unit of radiant flux across the entire electromagnetic spectrum, equal to roughly 3.846·1026 W.
Brightness. MAGNITUDE (m) is the log ratio of brightness between the brightness of a celestial object and the brightness of an arbitrary standard object; for stellar magnitudes the arbitrary standard has been the star Vega with a magnitude of 0.03. ABSOLUTE MAGNITUDE (M) is the magnitude of an object at a standard 10 parsec distance; the absolute magnitude of Vega is +0.6. Magnitude is normally measured within a limited spectral range, which must be specified unless the visible spectrum is understood. Note that in astronomy luminosity usually refers to radiance across all wavelengths, not just visible wavelengths.
The metric for the brightness of extended objects, such as nebulae or galaxies, is total magnitude or integrated magnitude, the brightness emitted by the entire object equated with the magnitude of a single star. Because the luminance is diffused over the visual extent of the object, the object will appear much fainter than a star of the same magnitude. To avoid this problem, the surface brightness is calculated as the integrated magnitude divided by the surface area of the object, in square arcseconds.
Temperature. The KELVIN (K) is the standard unit of temperature; the relative unit of temperature is the DEGREE (Celsius), which is not used in astronomical contexts although its scale units (degrees Centigrade) are identical in magnitude to kelvins. The zero point of the kelvin scale is absolute zero, or the complete absence of all thermal motion: this is a theoretical definition, since (according to the Third Law of Thermodynamics) it is not possible to achieve in physical systems. The temperature of the triple point of water (0° C) is 273 K, the effective temperature of the Sun's photosphere (surface) is roughly 5800 K, and the temperature at the Sun's core is estimated to be roughly 1.57·107 K.
Anatomy of the Galaxy
Galaxy Structure & Dimensions. [DRAFT] The basic attribute of a galaxy is its galaxy type in the Hubble original or revised (de Vaucouleurs) systems. The Galaxy is currently believed to be a Hubble type Sb (Vaucouleurs type SB(rs)bc II) galaxy, in other words it has a pronounced spiral structure that includes a prominent central bar and a ring structure.
Any component of the galaxy has a characteristic thickness, the scale height, and radius measured from the Galaxy barycenter.
Since the bulk of Galactic mass is found in dark matter, this acts as a frictionless gravitational medium. The primary consequence is that the Galaxy is an encounterless medium with respect to stars, which are spaced about one parsec apart.
differential rotation, "liquid" disk, collisionless medium. The cross section of stellar dimensions to empty space is about 500 trillion to 1, a ratio that is over 3000 times greater than the number of stars in the Galaxy. A physical example can clarify this point. If we define a main sequence star to have a diameter of 1 millimeter (a grain of sand), then the nearest star to the Sun (at 4.37 light years distance) would be a second grain of sand almost 30 kilometers away. At the center of a globular cluster, the spacing would be one grain every cubic kilometer.
Recent parallax measurements of water or methane maser sources distant star forming regions with the Very Long Baseline Array (a group of large radio telescopes interconnected from Hawaii to the Virgin Islands with an angular resolution in the infrared of less than one millionth of an arcsecond) put the Sun at about 8300 parsecs from the Galactic barycenter. Using plate scales and best estimates of distance, we can project the radius of the Solar orbit onto other, familiar spiral galaxies (images, below). This illustrates the astonishing grandeur of both the Galaxy and the Andromeda system.
The table (below) summarizes the physical parameters of the distinct parts of the Galaxy.