Spiral galaxy

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.

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 their barless cousins 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]

Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the 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]

M101 hires STScI-PRC2006-10a
An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)

Structure

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

HAWK-I NGC 1300
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 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.

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[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. 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.

Bar

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

NGC 1345 HST
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 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.

A mess of stars
NGC 428 is a barred spiral galaxy, located approximately 48 million light-years away from Earth in the constellation of Cetus.[13]

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 galaxy

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 think 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]

Origin of the spiral structure

NGC 6384 HST
Spiral galaxy NGC 6384 taken by Hubble Space Telescope.
A spiral home to exploding stars
The spiral galaxy NGC 1084, home of five supernovae.[16]

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:

  • star formation caused by density waves in the galactic disk of the galaxy.
  • 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.

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.

Historical theory of Lin and Shu

Spiral galaxy arms diagram
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,[17] 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.[18]

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.
NGC 3810 (captured by the Hubble Space Telescope)
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

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.

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.[19]

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[20] [21] [22]

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 nebula" was a term used to describe galaxies with a visible spiral structure, such as the Whirlpool Galaxy, before it was understood that these objects existed outside our Milky Way galaxy. 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 Mt. Wilson Observatory. Beginning in 1923, Edwin Hubble[23][24] 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 into disuse.

Milky Way

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.[25][26] Their suspicions were confirmed by Spitzer Space Telescope observations in 2005,[27] which showed that the Milky Way's central bar is larger than was previously suspected.

PIA19341-MilkyWayGalaxy-SpiralArmsData-WISE-20150603
Milky Way Galaxy Spiral Arms - based on WISE data.

Famous examples

See also

Classification
Other

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. Archived from the original on 2012-09-29.(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 |archive-url= requires |archive-date= (help).
  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.
  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. ^ "A mess of stars". Retrieved 11 August 2015.
  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. ^ "A spiral home to exploding stars". ESA / Hubble. Retrieved 2 April 2014.
  17. ^ 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.
  18. ^ 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.
  19. ^ 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.
  20. ^ F. Shirley Patterson (1940), The Luminosity Gradient of Messier 33
  21. ^ Gerard de Vaucouleurs (1957), Studies of the Magellanic Clouds. III. Surface brightness, colors and integrated magnitudes of the Clouds.
  22. ^ Freeman, K. C. (1970). "On the Disks of Spiral and so Galaxies". Astrophysical Journal. 160: 811. Bibcode:1970ApJ...160..811F. doi:10.1086/150474.
  23. ^ "NASA - Hubble Views the Star That Changed the Universe".
  24. ^ 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.
  25. ^ Gerard de Vaucouleurs (1964), Interpretation of velocity distribution of the inner regions of the Galaxy
  26. ^ 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.
  27. ^ McKee, Maggie (August 16, 2005). "Bar at Milky Way's heart revealed". New Scientist. Retrieved 17 June 2009.

External links

Barred spiral galaxy

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in between one third and two thirds of all spiral galaxies. Bars 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 our own Solar System is located, is classified as a barred spiral galaxy.Edwin Hubble classified spiral galaxies of this type as "SB" (spiral, barred) in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy. A new type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Clouds, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy.

Dwarf spiral galaxy

A dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having low luminosities, small diameters (less than 5 kpc), low surface brightnesses, and low hydrogen masses. The galaxies may be considered a subclass of low-surface-brightness galaxies.

Dwarf spiral galaxies, particularly the dwarf counterparts of Sa-Sc type spiral galaxies, are quite rare. In contrast, dwarf elliptical galaxies, dwarf irregular galaxies, and the dwarf versions of Magellanic type galaxies (which may be considered transitory between spiral and irregular in terms of morphology) are very common.It is suggested that dwarf spiral galaxies can transform into dwarf elliptical galaxies, especially in dense cluster environments.

Eyes Galaxies

The Eyes Galaxies (NGC 4435-NGC 4438, also known as Arp 120) are a pair of galaxies about 52 million light-years away in the constellation Virgo. The pair are members of the string of galaxies known as Markarian's Chain.

Flocculent spiral galaxy

A flocculent spiral galaxy is a type of spiral galaxy. Unlike the well-defined spiral architecture of a grand design spiral galaxy, flocculent (meaning "fluffy") galaxies are patchy, with discontinuous spiral arms. Self-propagating star formation is the apparent explanation for the structure of flocculent spirals. Approximately 30% of spirals are flocculent, 10% are grand design, and the rest are referred to as "multi-armed". The multiple-arm type is sometimes grouped into the flocculent category.The prototypical flocculent spiral is NGC 2841.

Grand design spiral galaxy

A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy's radius. As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, including M81, M51, M74, M100, and M101.

Intermediate spiral galaxy

An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification system devised by Gerard de Vaucouleurs. Subtypes are labeled as SAB0, SABa, SABb, or SABc, following a sequence analogous to the Hubble sequence for barred and unbarred spirals. The subtype (0, a, b, or c) is based on the relative prominence of the central bulge and how tightly wound the spiral arms are.

List of NGC objects (1–1000)

This is a list of NGC objects 1–1000 from the New General Catalogue (NGC). The astronomical catalogue is composed mainly of star clusters, nebulae, and galaxies. Other objects in the catalogue can be found in the other subpages of the list of NGC objects.

The constellation information in these tables is from The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, which was accessed using the VizieR Service. Galaxy morphological types and objects that are members of the Small Magellanic Cloud are identified using the NASA/IPAC Extragalactic Database. The other data of these tables are from the SIMBAD Astronomical Database unless otherwise stated.

Magellanic spiral

Magellanic spiral galaxies are (usually) dwarf galaxies which are classified as the type Sm (and SAm, SBm, SABm). They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy. They can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies.

Messier 100

Messier 100 (also known as NGC 4321) is an example of a grand design intermediate spiral galaxy located within the southern part of constellation Coma Berenices. It is one of the brightest and largest galaxies in the Virgo Cluster, located approximately 55 million light-years distant from Earth and has a diameter of 167,000 light years and contains 1 trillion stars, roughly the size of the Milky Way. It was discovered by Pierre Méchain on March 15, 1781 and was subsequently entered in Messier's catalogue of nebulae and star clusters after Charles Messier made observations of his own on April 13, 1781. The galaxy was one of the first spiral galaxies to be discovered, and was listed as one of fourteen spiral nebulae by Lord William Parsons of Rosse in 1850. Two satellite galaxies named NGC 4323--connected with M100 by a bridge of luminous matter--and NGC 4328 surround M100.

Messier 109

Messier 109 (also known as NGC 3992) is a barred spiral galaxy exhibiting a weak inner ring structure around the central bar approximately 83.5 ± 24 million light-years away in the constellation Ursa Major. M109 can be seen southeast of the star Phecda (γ UMa).

Messier 61

Messier 61 (also known as M61 or NGC 4303) is an intermediate barred spiral galaxy in the Virgo Cluster of galaxies. It was discovered by Barnaba Oriani on May 5, 1779. This was six days before Charles Messier observed the same galaxy, but had mistaken it as a comet.

Messier 66

Messier 66 or M66, also known as NGC 3627, is an intermediate spiral galaxy in the equatorial constellation of Leo. It was discovered by French astronomer Charles Messier on March 1, 1780, who described it as "very long and very faint". This galaxy is a member of a small group of galaxies that includes M65 and NGC 3628, known as the Leo Triplet, or the M66 Group. M65 and M66 make a popular pair for observers, being separated by only 20′.M66 has a morphological classification of SABb, indicating a spiral shape with a weak bar feature and loosely wound arms. The isophotal axis ratio is 0.32, indicating that it is being viewed at an angle. M66 is receding from us with a heliocentric radial velocity of 696.3±12.7 km/s. It lies 31 million light-years away and is about 95 thousand light-years across with striking dust lanes and bright star clusters along sweeping spiral arms. As of 2018, five supernovae have been observed in M66: SN 2016cok, 2009hd, 1997bs, 1989B, and 1973R.Gravitational interaction from its past encounter with neighboring NGC 3628 has resulted in an extremely high central mass concentration; a high molecular to atomic mass ratio; and a resolved non-rotating clump of H I material apparently removed from one of the spiral arms. The latter feature shows up visually as an extremely prominent and unusual spiral arm and dust lane structures as originally noted in the Atlas of Peculiar Galaxies.

Messier 88

Messier 88 (also known as M88 or NGC 4501) is a spiral galaxy about 50 to 60 million light-years away in the constellation Coma Berenices. It was discovered by Charles Messier in 1781.

Messier 91

Messier 91 (also known as NGC 4548 or M91) is a barred spiral galaxy located in the Coma Berenices constellation and is part of the Virgo Cluster of galaxies. M91 is about 63 million light-years away from the earth. It was the last of a group of eight nebulae discovered by Charles Messier in 1781.

Originally M91 was a missing Messier object in the catalogue as the result of a bookkeeping mistake by Messier. It was not until 1969 that amateur astronomer William C. Williams realized that M91 was NGC 4548, which was documented by William Herschel in 1784 (according to other sources, however, the nearby spiral galaxy NGC 4571 was also considered as a candidate for Messier 91 by him.)

NGC 2403

NGC 2403 (also known as Caldwell 7) is an intermediate spiral galaxy in the constellation Camelopardalis. NGC 2403 is an outlying member of the M81 Group, and is approximately 8 million light-years distant. It bears a striking similarity to M33, being about 50,000 light years in diameter and containing numerous star-forming H II regions.

The northern spiral arm connects it to the star forming region NGC 2404. NGC 2403 can be observed using 10×50 binoculars.

NGC 4565

NGC 4565 (also known as the Needle Galaxy or Caldwell 38) is an edge-on spiral galaxy about 30 to 50 million light-years away in the constellation Coma Berenices. It lies close to the North Galactic Pole and has a visual magnitude of approximately 10. It is known as the Needle Galaxy for its narrow profile. First recorded in 1785 by William Herschel, it is a prominent example of an edge-on spiral galaxy.

NGC 4945

NGC 4945 is a barred spiral galaxy in the constellation Centaurus, visible near the star Xi Centauri. The galaxy was discovered by James Dunlop in 1826 and is thought to be similar to the Milky Way Galaxy, although X-ray observations show that NGC 4945 has an unusual energetic Seyfert 2 nucleus that might house a supermassive black hole. This object has an estimated mass of 1.4+1.4−0.7×106 M☉.

NGC 55

NGC 55, also occasionally referred to as The Whale Galaxy, is a Magellanic type barred spiral galaxy located about 7 million light-years away in the constellation Sculptor. Along with its neighbor NGC 300, it is one of the closest galaxies to the Local Group, probably lying between the Milky Way and the Sculptor Group.

Unbarred spiral galaxy

An unbarred spiral galaxy is a type of spiral galaxy without a central bar, or one that is not a barred spiral galaxy. It is designated with an SA in the galaxy morphological classification scheme.

The Sombrero Galaxy is an unbarred spiral galaxy.

Barless spiral galaxies are one of three general types of spiral galaxies under the de Vaucouleurs system classification system, the other two being intermediate spiral galaxy and barred spiral galaxy. Under the Hubble tuning fork, it is one of two general types of spiral galaxy, the other being barred spirals.

Morphology
Structure
Active nuclei
Energetic galaxies
Low activity
Interaction
Lists
See also
Location
Galactic core
Spiral arms
Satellite galaxies
Related

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