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38-664: Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae 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

76-422: 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,

114-586: 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-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies. Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies,

152-483: A three-dimensional version of Hubble's tuning fork, with stage (spiralness) on the x -axis, family (barredness) on the y -axis, and variety (ringedness) on the z -axis. De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme. Values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies (ellipticals and lenticulars) and positive numbers to late types (spirals and irregulars). Thus, as

190-423: A density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms. The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964, 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

228-422: A more elaborate classification system for spiral galaxies, based on three morphological characteristics: The different elements of the classification scheme are combined — in the order in which they are listed — to give the complete classification of a galaxy. For example, a weakly barred spiral galaxy with loosely wound arms and a ring is denoted SAB(r)c. Visually, the de Vaucouleurs system can be represented as

266-723: A rough rule, lower values of T correspond to a larger fraction of the stellar mass contained in a spheroid/bulge relative to the disk. The approximate mapping between the spheroid-to-total stellar mass ratio (M B /M T ) and the Hubble stage is M B /M T =(10−T) /256 based on local galaxies. Elliptical galaxies are divided into three 'stages': compact ellipticals (cE), normal ellipticals (E) and late types (E ). Lenticulars are similarly subdivided into early (S ), intermediate (S ) and late (S ) types. Irregular galaxies can be of type magellanic irregulars ( T = 10) or 'compact' ( T = 11). The use of numerical stages allows for more quantitative studies of galaxy morphology. The Yerkes scheme

304-475: Is RA 12 51 26.282 , Dec 27° 07′ 42.01″. This position is in Coma Berenices , near the bright star Arcturus ; likewise, the south galactic pole lies in the constellation Sculptor . The zero of longitude of galactic coordinates was also defined in 1959 to be at position angle 123° from the north celestial pole . Thus the zero longitude point on the galactic equator

342-635: Is an extremely old spiral galaxy located in the Abell 1689 galaxy cluster in the Virgo constellation. A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang. 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. The pioneer of studies of

380-432: 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. The following hypotheses exist for star formation caused by density waves: Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than

418-406: Is often represented in the form of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the two parallel prongs of the fork on the right. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”. To this day, the Hubble sequence

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456-467: Is the plane on which the majority of a disk-shaped galaxy 's mass lies. The directions perpendicular to the galactic plane point to the galactic poles . In actual usage, the terms galactic plane and galactic poles usually refer specifically to the plane and poles of the Milky Way , in which Planet Earth is located. Some galaxies are irregular and do not have any well-defined disk. Even in

494-524: Is the central value; it is useful to define: R o p t = 3.2 h {\displaystyle R_{opt}=3.2h} as the size of the stellar disk, whose luminosity is L t o t = 2 π I 0 h 2 {\displaystyle L_{tot}=2\pi I_{0}h^{2}} . The spiral galaxies light profiles, in terms of the coordinate R / h {\displaystyle R/h} , do not depend on galaxy luminosity. Before it

532-472: Is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy . Nonetheless, 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. The de Vaucouleurs system for classifying galaxies is a widely used extension to

570-557: Is the oldest and most distant known spiral galaxy, as of 2024.The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth. The oldest grand design 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. A1689B11

608-618: The Hubble sequence , first described by Gérard de Vaucouleurs in 1959. De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies —based on the tightness of the spiral arms and the presence or absence of a bar—did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses are important structural components of spiral galaxies. The de Vaucouleurs system retains Hubble's basic division of galaxies into ellipticals , lenticulars , spirals and irregulars . To complement Hubble's scheme, de Vaucouleurs introduced

646-555: 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

684-570: 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 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

722-430: 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. 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

760-739: The Milky Way's central bar is larger than what was previously suspected. Galaxy morphological classification The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubble's scheme divides galaxies into three broad classes based on their visual appearance (originally on photographic plates ): These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies , which have no obvious regular structure (either disk-like or ellipsoidal). The Hubble sequence

798-482: The case of a barred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar . In 1959, the IAU defined the position of the Milky Way's north galactic pole as exactly RA = 12 49 , Dec = 27° 24′ in the then-used B1950 epoch ; in the currently-used J2000 epoch, after precession is taken into account, its position

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836-470: 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, 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

874-470: The different components varies from galaxy to galaxy. 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

912-584: The first drawing of Andromeda Galaxy 's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula. 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 observed Cepheid variables in several spiral nebulae, including

950-531: 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 . BRI 1335-0417

988-404: 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

1026-411: 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

1064-430: 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. The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc. The most convincing evidence for

1102-480: The presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures which typically have a maximum visibility at half the length of the in-plane bar. 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

1140-409: The rest of the galaxy. As massive stars evolve far more quickly, their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent. 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. As stars move through an arm,

1178-462: 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

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1216-414: 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. 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. Their suspicions were confirmed by Spitzer Space Telescope observations in 2005, which showed that

1254-567: 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. 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

1292-422: 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. 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

1330-536: The stars forming a bar in the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope . Together with irregular galaxies , spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters. Spiral galaxies may consist of several distinct components: The relative importance, in terms of mass, brightness and size, of

1368-418: 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. The stars in spirals are distributed in thin disks radial with intensity profiles such that with h {\displaystyle h} being the disk scale-length; I 0 {\displaystyle I_{0}}

1406-565: Was created by American astronomer William Wilson Morgan . Together with Philip Keenan , Morgan also developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy; the shape, real and apparent; and the degree of the central concentration to classify galaxies. Thus, for example, the Andromeda Galaxy is classified as kS5. Galactic plane The galactic plane

1444-572: 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

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