Class of galaxy that has spiral structures extending from their cores
This article is about the class of galaxy. For the type of logic puzzle, seeSpiral Galaxies (puzzle).
An example of a spiral galaxy,Messier 77 (also known as NGC 1068)
Spiral galaxies form aclass of galaxy originally described byEdwin Hubble in his 1936 workThe Realm of the Nebulae[1] and, as such, form part of theHubble sequence. Most spiral galaxies consist of a flat, rotatingdisk containingstars,gas and dust, and a central concentration of stars known as thebulge. These are often surrounded by a much fainterhalo of stars, many of which reside inglobular clusters.
Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hotOB 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. The proportion ofbarred spirals relative tobarless spirals has likely changed over the history of theuniverse, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of thegalaxies in the visible universe (Hubble volume) have bars.[3]
TheMilky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.[4] The most convincing evidence for the stars forming a bar in theGalactic Center comes from several recent surveys, including theSpitzer Space Telescope.[5]
Together withirregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe.[6] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[7]
Spiral arms are regions of stars that extend from the center of barred andunbarred spiralgalaxies. These long, thin regions resemble aspiral and thus give spiral galaxies their name. Naturally, differentclassifications 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 many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.
Abulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light.
Using the Hubble classification, the bulge of Sa galaxies is usually composed ofPopulation II stars, which 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[9] and are composed of young, bluePopulation 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 asupermassive black hole at their centers. In our own galaxy, for instance, the object calledSagittarius A* is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence ofactive nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such asMessier 106.
Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.[10][11] Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures[12][13] which typically have a maximum visibility at half the length of the in-plane bar.
19 face-on spiral galaxies from theJames Webb Space Telescope in near- and mid-infrared light. Older stars appear blue here, and are clustered at the galaxies’ cores. Glowing dust, showing where it exists around and between stars – appearing in shades of red and orange. Stars that have not yet fully formed and are encased in gas and dust appear bright red.[14]
The bulk of the stars in a spiral galaxy are located either close to a single plane (thegalactic plane) in more or less conventional circularorbits around the center of the galaxy (theGalactic Center), or in aspheroidal galactic bulge around the galactic core.
However, some stars inhabit aspheroidal halo orgalactic spheroid, a type ofgalactic halo. The orbital behaviour of these stars is disputed, but they may exhibitretrograde and/or highlyinclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into andmerge with the spiral galaxy—for example, theSagittarius Dwarf Spheroidal 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 ofdust, and in further contrast, stars in the galactic halo are ofPopulation II, much older and with much lowermetallicity than theirPopulation 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 smallred dwarfs close to theSun are thought to belong to the galactic halo, for exampleKapteyn's Star andGroombridge 1830. Due to their irregular movement around the center of the galaxy, these stars often display unusually highproper motion.
BRI 1335-0417 is the oldest and most distant known spiral galaxy, as of 2024.[dubious –discuss] The galaxy has aredshift of 4.4, meaning its light took 12.4 billion years to reach Earth.[15][16]
The oldest grand design spiral galaxy on file isBX442. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companiondwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.[17][18]
The oldest multi-arm spiral galaxy, as of 2022, isA2744-DSG-z3. Its redshift is z=3.059, which corresponds to 11.5 billion light years to Earth.[19][20]
A1689B11 is an extremely old spiral galaxy located in theAbell 1689 galaxy cluster in the Virgo constellation.[21] A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang.[22][23]
The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms wasBertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. 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 thewinding problem. Measurements in the late 1960s showed that theorbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected fromNewtonian dynamics but still cannot explain the stability of the spiral structure.
Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:
the stochastic self-propagating star formation model (SSPSF model) – star formation caused by shock waves in the interstellar medium. The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self-propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy's disk.
These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.
Animation of orbits as predicted by the density wave theory, which explains the existence of stable spiral arms. Stars move in and out of the spiral arms as they orbit the galaxy.
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.[27]
Exaggerated diagram illustrating Lin and Shu's explanation of spiral arms in terms of slightly elliptical orbits
The first acceptable theory for the spiral structure was devised byC. C. Lin andFrank Shu in 1964,[28] 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. 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 to the right. 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.[29]
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 (theJeans 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 driveshock waves through the gas, which in turn causes the gas to collapse and form stars.
Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly,[30] their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent.[27]
Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms.[27] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm.[29]
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 themolecular clouds in whichnew stars form, and evolution towards grand-design bisymmetric spirals is explained.[31]
Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to asspiral nebulae, due toLord Rosse, whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the "Whirlpool Galaxy", and his drawings of it closely resemble modern photographs. In 1846 and in 1849 Lord Rosse identified similar pattern inMessier 99 andMessier 33 respectively. In 1850 he made the first drawing ofAndromeda Galaxy's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula.[35]
Milky Way Galaxy's spiral arms and barred core – based onWISE data
TheMilky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s.[38][39] Their suspicions were confirmed bySpitzer Space Telescope observations in 2005,[40] which showed that the Milky Way's central bar is larger than what was previously suspected.
^R. A. Benjamin; E. Churchwell; B. L. Babler; R. Indebetouw; M. R. Meade; B. A. Whitney; C. Watson; M. G. Wolfire; M. J. Wolff; R. Ignace; T. M. Bania; S. Bracker; D. P. Clemens; L. Chomiuk; M. Cohen; J. M. Dickey; J. M. Jackson; H. A. Kobulnicky; E. P. Mercer; J. S. Mathis; S. R. Stolovy; B. Uzpen (September 2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy".The Astrophysical Journal Letters.630 (2):L149 –L152.arXiv:astro-ph/0508325.Bibcode:2005ApJ...630L.149B.doi:10.1086/491785.S2CID14782284.
^de Vaucouleurs, G.; de Vaucouleurs, A.; Corwin, H. G., Jr.; Buta, R. J.; Paturel, G.; Fouqué, P. (2016).Third Reference Catalogue of Bright Galaxies. 9. New York: Springer-Verlag. Retrieved8 March 2025.{{cite book}}: CS1 maint: multiple names: authors list (link)
^abHenbest, Nigel (1994),The Guide to the Galaxy,Cambridge University Press, p. 74,ISBN9780521458825,Lin and Shu showed that this spiral pattern would persist more or less for ever, even though individual stars and gas clouds are always drifting into the arms and out again.
^Alexander, S. On the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulae. Astronomical Journal, vol. 2, iss. 37, p. 97-103 (1852)
^Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL26Al map features".Space Science Reviews.120:315–316.Bibcode:1996A&AS..120C.315C.
