An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)

A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae^[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named for the spiral structures that extend from the center into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,^[2] extending from the central bulge, at the ends of which the spiral arms begin. Our own Milky Way has recently (in the 1990s) been confirmed to be a barred spiral, although the bar itself is difficult to observe from our position within the Galactic disk.^[3] The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the Galactic center.^[4]

Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe.^[5] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.^[6]

Structure[link]

Barred spiral galaxy UGC 12158.

Spiral galaxies consist of four distinct components:

• A flat, rotating disc of (mostly newly created) stars and interstellar matter
• A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
• A near-spherical halo of stars, including many in globular clusters
• A supermassive black hole at the very center of the central bulge

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

Spiral arms[link]

NGC 1300 in infrared light.

Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.

Galactic bulge[link]

A bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a supermassive black hole at their centers. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. There is a tight correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge, the M-sigma relation.

Galactic spheroid[link]

Spiral galaxy NGC 1345

The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic center), or in a spheroidal galactic bulge around the galactic core.

However, some stars inhabit a spheroidal halo or galactic spheroid, a type of galactic halo. The orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion.

Origin of the spiral structure[link]

Spiral galaxy NGC 6384 taken by Hubble Space Telescope.

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

Since the 1960s, there have been two leading hypotheses or models for the spiral structures of galaxies:

• Star formation caused by density waves in the galactic disk of the galaxy.
• The SSPSF model – star formation caused by shock waves in the interstellar medium.

These different hypotheses do not have to be mutually exclusive, as they may explain different types of spiral arms.

Density wave model[link]

Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.

Explanation of spiral galaxy arms.

This idea was developed into density wave theory by C. C. Lin and Frank Shu in 1964.^[7] They suggested that the spiral arms were manifestations of spiral density waves, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars.

Historical theory of Lin and Shu[link]

The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves, they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.^{[citation needed]}

Star formation caused by density waves[link]

The following hypotheses exist for star formation caused by density waves:

• As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.

• As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.

• As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.

The bright galaxy NGC 3810 demonstrates classical spiral structure in this very detailed image from Hubble. Credit: ESA/Hubble and NASA.

More young stars in spiral arms[link]

The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the darker background stellar distribution behind the waves, hence making the waves visible.

While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.

Alignment of spin axis with cosmic voids[link]

Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids.^[8] That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.^[9]

Spiral arms seen as transient structures[link]

In April 2011 a presentation to the Royal Astronomical Society's April 2011 National Astronomy Meeting in Llandudno, Wales by postgraduate student Robert Grand, suggested that spiral arms do not rotate rigidly at a constant angular velocity about the galactic centre. This contradicts a 50 year old theory called Spiral Density Wave Theory (SDWT), which states that the spiral arm pattern we see is actually a wave pattern, that rotates independently of star and inter-stellar matter that follow the standard rotation curve of the galaxy. Stars that move faster than the arms can overtake them and move through them. Consequently, stars outside this radius move slower than the arm and fall behind. Rather than a long-lived rigidly rotating wave pattern found in SDWT, Grand's simulations suggested that the arms rotate with a pattern speed that decreases with radius, and that they are transient features, with some arms breaking up and new ones being formed over periods of 80 to 100 million years.^[10] Instead of stars rotating independently of the arm pattern, they co-rotate at every radius, as the arm pattern speed traces very well the rotation curve of matter. The destruction of the arms is due to a declining pattern speed, which means that the arms begin to wind up and so break to avoid the well-known Wind-Up problem. This pattern of arm formation and destruction has not been observed in real galaxies, mainly because this pattern would take tens of millions of years to observe from start to finish. As observers, we can only observe what is a relative "snapshot" of a galaxy's evolution, and since there are always new arms forming as older ones die, there is always a spiral pattern present. Therefore, a simple glance at a galaxy will not yield evidence either way. A real observational contribution will come from the Gaia satellite, due to be launched in the coming years.

Gravitationally aligned orbits[link]

Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs), that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained.^[11]

Spiral nebula[link]

"Spiral nebula" is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objects like the Whirlpool Galaxy were just one more form of nebula that were within our own Milky Way galaxy. The idea that they might instead be other galaxies, independent of the Milky Way, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mt. Wilson Observatory. In 1926, Edwin Hubble^[12] observed Cepheid variables in several spiral nebulae, including the Andromeda Galaxy, proving that they are, in fact, entire galaxies outside our own. The term "spiral nebula" has since fallen into disuse.

Milky Way[link]

The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s.^[13] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005^[14] which showed the galaxy's central bar to be larger than previously suspected.

Famous examples[link]

See also[link]

Components[link]

Classification[link]

Other[link]

References[link]

External links[link]

Wikimedia Commons has media related to: Spiral galaxies

• Giudice, G.F.; Mollerach, S.; Roulet, E. (1994). "Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?". Physical Review D 50 (4): 2406–2413. arXiv:astro-ph/9312047. Bibcode 1994PhRvD..50.2406G. DOI:10.1103/PhysRevD.50.2406. 
• Stephens, Tim (March 6, 2007). "AEGIS survey reveals new principle governing galaxy formation and evolution". UC Santa Cruz. http://www.ucsc.edu/news_events/press_releases/text.asp?pid=1080. Retrieved 2006-05-24. 
• Spiral Galaxies @ SEDS Messier pages
• SpiralZoom.com, an educational website about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.
• Spiral Structure explained
• GLIMPSE: the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire

Robert Smithson
Spiral Jetty from atop Rozel Point, in mid-April 2005. It was created in 1970 and still exists although it has often been submerged by the fluctuating lake level. It consists of some 6500 tons of basalt, earth and salt.
Born	(1938-01-02)January 2, 1938 Passaic, New Jersey
Died	July 20, 1973(1973-07-20) (aged 35) Texas
Nationality	American
Training	Art Students League of New York
Movement	Land art
Works	Spiral Jetty, 1970
Patrons	Virginia Dwan
Influenced by	Frederick Law Olmstead

Robert Smithson (January 2, 1938–July 20, 1973) was an American artist famous for his land art.

Background and education[link]

This section requires expansion.

Smithson was born in Passaic, New Jersey and studied painting and drawing in New York City at the Art Students League of New York.

Career[link]

Early work[link]

His early exhibited artworks were collage works influenced by "homoerotic drawings and clippings from beefcake magazines",^[1] science fiction, and early Pop Art. He primarily identified himself as a painter during this time, but after a three year rest from the art world, Smithson emerged in 1964 as a proponent of the emerging minimalist movement. His new work abandoned the preoccupation with the body that had been common in his earlier work. Instead he began to use glass sheet and neon lighting tubes to explore visual refraction and mirroring, in particular the sculpture Enantiomorphic Chambers. Crystalline structures and the concept of entropy became of particular interest to him, and informed a number of sculptures completed during this period, including Alogon 2. Smithson became affiliated with artists who were identified with the minimalist or Primary Structures movement, such as Nancy Holt (whom he married), Robert Morris and Sol LeWitt. As a writer, Smithson was interested in applying mathematical impersonality to art that he outlined in essays and reviews for Arts Magazine and Artforum and for a period was better known as a critic than as an artist. Some of Smithson's later writings recovered 18th- and 19th-century conceptions of landscape architecture which influenced the pivotal earthwork explorations which characterized his later work. He eventually joined the Dwan Gallery, whose owner Virginia Dwan was an enthusiastic supporter of his work.

Mature work[link]

In 1967 Smithson began exploring industrial areas around New Jersey and was fascinated by the sight of dump trucks excavating tons of earth and rock that he described in an essay as the equivalents of the monuments of antiquity. This resulted in the series of 'non-sites' in which earth and rocks collected from a specific area are installed in the gallery as sculptures, often combined with mirrors or glass. In September 1968, Smithson published the essay "A Sedimentation of the Mind: Earth Projects" in Artforum that promoted the work of the first wave of land art artists, and in 1969 he began producing land art pieces to further explore concepts gained from his readings of William S. Burroughs, J.G. Ballard, and George Kubler.

As well as works of art, Smithson produced a good deal of theoretical and critical writing, including the 2D paper work A Heap of Language, which sought to show how writing might become an artwork. In his essay "Incidents of Mirror-Travel in the Yucatan" ^[2] Smithson documents a series of temporary sculptures made with mirrors at particular locations around the Yucatan peninsula. Part travelogue, part critical rumination, the article highlights Smithson's concern with the temporal as a cornerstone of his work.

Smithson's interest in the temporal is explored in his writings in part through the recovery of the ideas of the picturesque. His essay "Frederick Law Olmsted and the Dialectical Landscape" was written in 1973 after Smithson had seen an exhibition curated by Elizabeth Barlow Rogers at the Whitney Museum entitled “Frederick Law Olmsted’s New York” as the cultural and temporal context for the creation of his late-19th-century design for Central Park. In examining the photographs of the land set aside to become Central Park, Smithson saw the barren landscape that had been degraded by humans before Olmsted constructed the complex ‘naturalistic’ landscape that was viscerally apparent to New Yorkers in the 1970s. Smithson was interested in challenging the prevalent conception of Central Park as an outdated 19th-century Picturesque aesthetic in landscape architecture that had a static relationship within the continuously evolving urban fabric of New York City. In studying the writings of 18th- and 19th-century Picturesque treatise writers Gilpin, Price, Knight and Whately, Smithson recovers issues of site specificity and human intervention as dialectic landscape layers, experiential multiplicity, and the value of deformations manifest in the Picturesque landscape.

Smithson further implies in this essay that what distinguishes the Picturesque is that it is based on real land ^[3] For Smithson, a park exists as “a process of ongoing relationships existing in a physical region” ^[4] Smithson was interested in Central Park as a landscape which by the 1970s had weathered and grown as Olmsted’s creation, but was layered with new evidence of human intervention.

Now the Ramble has grown up into an urban jungle, and lurking in its thickets are “hoods, hobos, hustlers, and homosexuals,” and other estranged creatures of the city…. Walking east, I passed graffiti on boulders… On the base of the Obelisk along with the hieroglyphs there are also graffiti. …In the spillway that pours out of the Wollman Memorial Ice Rink, I noticed a metal grocery cart and a trash basket half-submerged in the water. Further down, the spillway becomes a brook choked with mud and tin cans. The mud then spews under the Gapstow Bridge to become a muddy slough that inundates a good part of The Pond, leaving the rest of The Pond aswirl with oil slicks, sludge, and Dixie cups”^[5]

While Smithson did not find “beauty” in the evidence of abuse and neglect, he did see the state of things as demonstrative of the continually transforming relationships between man and landscape. In his proposal to make process art out of the dredging of The Pond, Smithson sought to insert himself into the dynamic evolution of the park ^[6]

Smithson became particularly interested in the notion of deformities within the spectrum of anti-aesthetic dynamic relationships which he saw present in the Picturesque landscape. He claimed, “the best sites for ‘earth art’ are sites that have been disrupted by industry, reckless urbanization, or nature’s own devastation.” ^[7] While in earlier 18th-century formal characterizations of the pastoral and the sublime, something like a “gash in the ground” if encountered by a “leveling improver”, as described by Price, would have been smoothed over and the whole composition returned to a more aesthetically pleasing contour ^[8] For Smithson, however, it was not necessary that the deformation become a visual aspect of a landscape; by his anti-formalist logic, more important was the temporal scar worked over by natural or human intervention. He saw parallels to Olmsted's Central Park as a “sylvan” green overlay on the depleted landscape that preceded his Central Park ^[9] Defending himself against allegations that he and other earth artists “cut and gouge the land like Army engineers”, Smithson, in his own essay, charges that one of such opinions “failed to recognize the possibility of a direct organic manipulation of the land..” and would “turn his back on the contradictions that inhabit our landscapes” ^[10]

In revisiting the 18th- and early 19th-century treatises of the Picturesque, which Olmsted interpreted in his practice, Smithson exposes threads of an anti-aesthetic anti-formalist logic and a theoretical framework of the Picturesque that addressed the dialectic between the physical landscape and its temporal context. By re-interpreting and re-valuing these treatises, Smithson was able to broaden the temporal and intellectual context for his own work, and to offer renewed meaning for Central Park as an important work of modern art and landscape architecture.

Other theoretical writings explore the relationship of a piece of art to its environment, from which he developed his concept of sites and non-sites. A site was a work located in a specific outdoor location, while a non-site was a work which could be displayed in any suitable space, such as an art gallery. Spiral Jetty is an example of a sited work, while Smithson's non-site pieces frequently consist of photographs of a particular location, often exhibited alongside some material (such as stones or soil) removed from that location.

The journeys he undertook were central to his practice as an artist, and his non-site sculptures often included maps and aerial photos of a particular location, as well as the geological artifacts displaced from those sites. In 1970 at Kent State University, Smithson created Partially Buried Woodshed (1970) ^[11] to illustrate geological time consuming human history. His most famous work is Spiral Jetty (1970), a 1,500-foot (460 m) long spiral-shaped jetty extending into the Great Salt Lake in Utah constructed from rocks, earth, and salt. It was entirely submerged by rising lake waters for several years, but has since re-emerged. The lake waters may be pinkish due to high concentrations of β-carotene in the halophyte green alga Dunaliella salina.

In 1971 he created Broken circle/Spiral Hill for the exhibition for the Sonsbeek'71 art festival at Emmen, the Netherlands. The subject of the 1971 Sonsbeek exhibition was Beyond Lawn and Order (Dutch: Buiten de perken).

On July 20, 1973, Smithson died in a plane crash, while surveying sites for his work Amarillo Ramp in Texas.^[12] Despite his early death, and relatively few surviving major works, Smithson has a following amongst many contemporary artists. In recent years, Tacita Dean, Sam Durant, Lee Ranaldo, Vik Muniz, Mike Nelson, and the Bruce High Quality Foundation have all made homages to Smithson's works.

The Estate of Robert Smithson is managed by James Cohan Gallery.

• 

  Broken Circle, Emmen, The Netherlands

• 

  Broken Circle detail of materials, Emmen, The Netherlands

• 

  Spiral Hill, Emmen, The Netherlands

• 

  Spiral Jetty, Utah, USA

Notes[link]

References[link]

• Busch, Julia M. (1974), A Decade of Sculpture: the New Media in the 1960s, Philadelphia: The Art Alliance Press, ISBN 0-87982-007-1, OCLC 804815, OCLC 804815 .
• Kimmelman, Michael (2005-06-24), "Sculpture From the Earth, But Never Limited by It", The New York Times, http://www.nytimes.com/2005/06/24/arts/design/24kimm.html?ex=1180929600&en=97cea2a283bb6762&ei=5070 . Retrieved 2 June 2007.
• Smithson, Robert (1969), "Incidents of mirror-travel in the Yucatan", Artforum VIII (1), ISSN 0004-3532, OCLC 1514329, ISSN 0004-3532 OCLC 1514329 .
• Smithson, Robert (1996), Flam, Jack D., ed., Robert Smithson: The Collected Writings, Berkeley: University of California Press, OCLC 32853450, OCLC 32853450 .
• Roberts, Jennifer L. (Sept. 2000), "Landscapes of Indifference: Robert Smithson and John Lloyd Stephens in Yucatan", The Art Bulletin .

External links[link]

• www.robertsmithson.com
• Broken Circle/Spiral Hill (1971-2011) an historic Dutch program (video, exhibition and publication).
• Plans to Mix Oil Drilling and Art Clash in Utah, article by Kirk Johnson in the New York Times, 27 March 2008.
• 'An interminable avalanche of categories': conceptual issues in the work of Robert Smithson (or, once more, against 'sculpture') Lecture by Peter Osborne given at the Witte de With Center for Contemporary Art, Rotterdam, in 2008, which is available to download in audio form [2]
• Smithson Sightings Short essay on Smithson by Timothy Don of 3 Quarks Daily.
• Pictures of Robert Smithson's Spiral Jetty.
• 'Extra Terrestrial' - a 1993 monograph from frieze
• Robert Smithson exhibition at The Renaissance Society, 1976.
• Robert Smithson and Floating Island, by John Haber
• Nonsite from Smithson to New Media, by John Haber