Spiral galaxy

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An example of a spiral galaxy, the Messier 77 (also known as NGC 1068) Messier 77 spiral galaxy by HST.jpg
An example of a spiral galaxy, the Messier 77 (also known as NGC 1068)

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae [1] and, as such, form part of the Hubble sequence. Most 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 often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

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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, 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. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe, 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 the galaxies in the visible universe (Hubble volume) have bars. [3]

The Milky 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 the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope. [5]

Together with irregular 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]

Structure

Tuning-fork-style diagram of the Hubble sequence HubbleTuningFork.jpg
Tuning-fork-style diagram of the Hubble sequence

Spiral galaxies may consist of several distinct components:

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

Spiral arms

Barred spiral galaxy UGC 12158 UGC 12158.jpg
Barred spiral galaxy UGC 12158

Spiral arms are regions of stars that extend from the center of barred and unbarred 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 many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.

Bulge

A bulge 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.

NGC 1300 in infrared light HAWK-I NGC 1300.jpg
NGC 1300 in infrared light

Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population 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 [8] 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. In our own galaxy, for instance, the object called Sagittarius 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 of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106.

Bar

Spiral galaxy NGC 2008 Open Arms - potw2006a.jpg
Spiral galaxy NGC 2008

Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies. [9] [10] 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 [11] [12] which typically have a maximum visibility at half the length of the in-plane bar.

Spheroid

19 face-on spiral galaxies from the James 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. PHANGS image mosaic.jpg
19 face-on spiral galaxies from the James 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.

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 exhibit 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 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 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 dwarfs 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, these stars often display unusually high proper motion.

Oldest spiral galaxies

The oldest spiral galaxy on file is BX442. 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 companion dwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years. [14] [15]

A1689B11 is an extremely old spiral galaxy located in the Abell 1689 galaxy cluster in the Virgo constellation. [16] A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang. [17] [18]

BRI 1335-0417 is the most distant known spiral galaxy, as of 2021. The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth. [19]

In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating. [20] [21]

Origin of the spiral structure

Spiral galaxy NGC 6384 taken by Hubble Space Telescope NGC 6384 HST.jpg
Spiral galaxy NGC 6384 taken by Hubble Space Telescope
The spiral galaxy NGC 1084, home of five supernovae A spiral home to exploding stars.jpg
The spiral galaxy NGC 1084, home of five supernovae

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. 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 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:

These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.

Density wave model

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. [23]

Historical theory of Lin and Shu

Exaggerated diagram illustrating Lin and Shu's explanation of spiral arms in terms of slightly elliptical orbits Spiral galaxy arms diagram.svg
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 by C. C. Lin and Frank Shu in 1964, [24] 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. [25]

Star formation caused by density waves

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.

More young stars in spiral arms

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, [26] their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent. [23]

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. [23] 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. [25]

Gravitationally aligned orbits

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. [27]

Distribution of stars in spirals

The similar distribution of stars in spirals Similar stellar distribution in spiral galaxies.jpeg
The similar distribution of stars in spirals

The stars in spirals are distributed in thin disks radial with intensity profiles such that [28] [29] [30]

with being the disk scale-length; is the central value; it is useful to define: as the size of the stellar disk, whose luminosity is

.

The spiral galaxies light profiles, in terms of the coordinate , do not depend on galaxy luminosity.

Spiral nebula

Spiral galaxy LEDA 2046648, about one billion light-years away A Spiral Amongst Thousands (potm2301a).jpeg
Spiral galaxy LEDA 2046648, about one billion light-years away
Drawing of the Whirlpool Galaxy by Rosse in 1845 M51Sketch.jpg
Drawing of the Whirlpool Galaxy by Rosse in 1845

Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae, due to Lord 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 in Messier 99 and Messier 33 respectively. In 1850 he made the first drawing of Andromeda Galaxy's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula. [31]

The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mount Wilson Observatory. Beginning in 1923, Edwin Hubble [32] [33] observed Cepheid variables in several spiral nebulae, including the so-called "Andromeda Nebula", proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use.

Milky Way

Milky Way Galaxy's spiral arms and barred core - based on WISE data PIA19341-MilkyWayGalaxy-SpiralArmsData-WISE-20150603.jpg
Milky Way Galaxy's spiral arms and barred core – based on WISE data

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 1960s. [34] [35] Their suspicions were confirmed by Spitzer Space Telescope observations in 2005, [36] which showed that the Milky Way's central bar is larger than what was previously suspected.

Famous examples

See also

Classification

Other

Related Research Articles

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Globular cluster</span> Spherical collection of stars

A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".

<span class="mw-page-title-main">Galactic astronomy</span> Study of the Milky Way galaxy and its contents

Galactic astronomy is the study of the Milky Way galaxy and all its contents. This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies.

<span class="mw-page-title-main">Hubble sequence</span> Galaxy morphological classification scheme advocated by Edwin Hubble

The Hubble sequence is a morphological classification scheme for galaxies published by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tuning fork. It was invented by John Henry Reynolds and Sir James Jeans.

<span class="mw-page-title-main">Andromeda Galaxy</span> Barred spiral galaxy in the Local Group

The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a D25 isophotal diameter of about 46.56 kiloparsecs (152,000 light-years) and is approximately 765 kpc (2.5 million light-years) from Earth. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

<span class="mw-page-title-main">Lenticular galaxy</span> Class of galaxy between an elliptical galaxy and a spiral galaxy

A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.

<span class="mw-page-title-main">Galactic bulge</span> Tightly packed group of stars within a larger formation

In astronomy, a galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

<span class="mw-page-title-main">Ring galaxy</span> Galaxy with an annular appearance

A ring galaxy is a galaxy with a circle-like appearance. Hoag's Object, discovered by Arthur Hoag in 1950, is an example of a ring galaxy. The ring contains many massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars. However, the gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy. Other astronomers think that rings are formed around some galaxies when external accretion takes place. Star formation would then take place in the accreted material because of the shocks and compressions of the accreted material.

A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo:

The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the galactic halo of the Milky Way. A similar body of very hot and tenuous gas in the halo of any spiral galaxy may also be described by these terms.

<span class="mw-page-title-main">Barred spiral galaxy</span> Spiral galaxy with a central bar-shaped structure composed of stars

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where the Solar System is located, is classified as a barred spiral galaxy.

The Canis Major Overdensity or Canis Major Dwarf Galaxy is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.

<span class="mw-page-title-main">Dark matter halo</span> Theoretical cosmological structure

In modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).

<span class="mw-page-title-main">Milky Way</span> Galaxy containing the Solar System

The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος, meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

<span class="mw-page-title-main">Satellite galaxy</span> Galaxy that orbits a larger galaxy due to gravitational attraction

A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.

<span class="mw-page-title-main">Interacting galaxy</span> Galaxies with interacting gravitational fields

Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, which may lead to a galaxy merger.

<span class="mw-page-title-main">Galaxy merger</span> Merger whereby at least two galaxies collide

Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.

<span class="mw-page-title-main">Thin disk</span> Structural component of galaxies

The thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre density, of such matter. That of the Milky Way is thought to have a scale height of around 300–400 parsecs (980–1,300 ly) in the vertical axis perpendicular to the disk, and a scale length of around 2.5–4.5 kiloparsecs (8.2–14.7 kly) in the horizontal axis, in the direction of the radius. For comparison, the Sun is 8 kiloparsecs (26 kly) out from the center. The thin disk contributes about 85% of the stars in the Galactic plane and 95% of the total disk stars. It can be set apart from the thick disk of a galaxy since the latter is composed of older population stars created at an earlier stage of the galaxy formation and thus has fewer heavy elements. Stars in the thin disk, on the other hand, are created as a result of gas accretion at the later stages of a galaxy formation and are on average more metal-rich.

The following outline is provided as an overview of and topical guide to galaxies:

References

  1. ↑ Hubble, E.P. (1936). The realm of the nebulae. Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven: Yale University Press. ISBN   9780300025002. OCLC   611263346. Alt URL(pp. 124–151)
  2. ↑ D. Mihalas (1968). Galactic Astronomy. W. H. Freeman. ISBN   978-0-7167-0326-6.
  3. ↑ "Hubble and Galaxy Zoo Find Bars and Baby Galaxies Don't Mix". Science Daily . 16 January 2014.
  4. ↑ "Ripples in a Galactic Pond". Scientific American. October 2005. Archived from the original on 6 September 2013.
  5. ↑ 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. S2CID   14782284.
  6. ↑ Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue". Monthly Notices of the Royal Astronomical Society . 278 (4): 1025–1048. arXiv: astro-ph/9603040 . Bibcode:1996MNRAS.278.1025L. doi:10.1093/mnras/278.4.1025.
  7. ↑ Dressler, A. (March 1980). "Galaxy morphology in rich clusters — Implications for the formation and evolution of galaxies". The Astrophysical Journal. 236: 351–365. Bibcode:1980ApJ...236..351D. doi: 10.1086/157753 .
  8. ↑ Alister W. Graham and C. Clare Worley (2008), Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations
  9. ↑ de Vaucouleurs, G.; de Vaucouleurs, A.; Corwin, H. G., Jr.; Buta, R. J.; Paturel, G.; Fouqué, P. (2016), Third Reference Catalogue of Bright Galaxies
  10. ↑ B.D. Simmons et al. (2014), Galaxy Zoo: CANDELS barred discs and bar fractions
  11. ↑ Astronomy Now (8 May 2016), Astronomers detect double ‘peanut shell’ galaxies
  12. ↑ Bogdan C. Ciambur and Alister W. Graham (2016), Quantifying the (X/peanut)-shaped structure in edge-on disc galaxies: length, strength, and nested peanuts
  13. ↑ "Webb reveals structure in 19 spiral galaxies". www.esa.int. Retrieved 30 January 2024.
  14. ↑ Oldest spiral galaxy is a freak of cosmos http://www.zmescience.com/space/oldest-spiral-galaxy-31321/
  15. ↑ Gonzalez, Robert T. (19 July 2012). "Hubble Has Spotted an Ancient Galaxy That Shouldn't Exist". io9. Retrieved 10 September 2012.
  16. ↑ "[BBC2005] Source 11 -- Galaxy". 24 June 2018.
  17. ↑ "The most ancient spiral galaxy confirmed". PhysOrg. 3 November 2017.
  18. ↑ Tiantian Yuan; Johan Richard; Anshu Gupta; Christoph Federrath; Soniya Sharma; Brent A. Groves; Lisa J. Kewley; Renyue Cen; Yuval Birnboim; David B. Fisher (30 October 2017). "The most ancient spiral galaxy: a 2.6-Gyr-old disk with a tranquil velocity field". The Astrophysical Journal. 850: 61. arXiv: 1710.11130 . Bibcode:2017ApJ...850...61Y. doi: 10.3847/1538-4357/aa951d . S2CID   119267114.
  19. ↑ "ALMA Spots Candidate for Most Distant Known Spiral Galaxy". www.sci-news.com. 20 May 2021. Retrieved 20 May 2021.
  20. ↑ Royal Astronomical Society (11 June 2019). "Citizen scientists re-tune Hubble's galaxy classification". EurekAlert! . Retrieved 11 June 2019.
  21. ↑ Masters, Karen L.; et al. (30 April 2019). "Galaxy Zoo: unwinding the winding problem – observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding". Monthly Notices of the Royal Astronomical Society . 487 (2): 1808–1820. arXiv: 1904.11436 . Bibcode:2019MNRAS.487.1808M. doi:10.1093/mnras/stz1153.
  22. ↑ "A spiral home to exploding stars". ESA / Hubble. Retrieved 2 April 2014.
  23. 1 2 3 Belkora, L. (2003). Minding the Heavens: the Story of our Discovery of the Milky Way. CRC Press. p. 355. ISBN   978-0-7503-0730-7.
  24. ↑ Lin, C. C.; Shu, F. H. (August 1964). "On the spiral structure of disk galaxies". The Astrophysical Journal. 140: 646–655. Bibcode:1964ApJ...140..646L. doi:10.1086/147955.
  25. 1 2 Henbest, Nigel (1994), The Guide to the Galaxy, Cambridge University Press, p. 74, ISBN   9780521458825, 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.
  26. ↑ "Main Sequence Lifetime". Swinburne Astronomy Online. Swinburne University of Technology . Retrieved 8 June 2019.
  27. ↑ Francis, C.; Anderson, E. (2009). "Galactic spiral structure". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 465 (2111): 3425–3446. arXiv: 0901.3503 . Bibcode:2009RSPSA.465.3425F. doi:10.1098/rspa.2009.0036. S2CID   12461073.
  28. ↑ F. Shirley Patterson (1940), The Luminosity Gradient of Messier 33
  29. ↑ Gerard de Vaucouleurs (1957), Studies of the Magellanic Clouds. III. Surface brightness, colors and integrated magnitudes of the Clouds.
  30. ↑ Freeman, K. C. (1970). "On the Disks of Spiral and so Galaxies". Astrophysical Journal . 160: 811. Bibcode:1970ApJ...160..811F. doi: 10.1086/150474 .
  31. ↑ 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)
  32. ↑ "NASA - Hubble Views the Star That Changed the Universe".
  33. ↑ Hubble, E. P. (May 1926). "A spiral nebula as a stellar system: Messier 33". The Astrophysical Journal. 63: 236–274. Bibcode:1926ApJ....63..236H. doi:10.1086/142976.
  34. ↑ Gerard de Vaucouleurs (1964), Interpretation of velocity distribution of the inner regions of the Galaxy
  35. ↑ Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL 26Al map features". Space Science Reviews. 120: 315–316. Bibcode:1996A&AS..120C.315C.
  36. ↑ McKee, Maggie (16 August 2005). "Bar at Milky Way's heart revealed". New Scientist . Retrieved 17 June 2009.
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