Spiral galaxy
Spiral galaxies form a
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
Spiral galaxies may consist of several distinct components:
- A flat, rotating disc of stars and interstellar matterof which spiral arms are prominent components
- A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
- A bar-shaped distribution of stars
- A near-spherical halo of stars, including many in globular clusters
- A supermassive black hole at the very middle of the central bulge
- A near-spherical dark matter halo
The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.
Spiral arms
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.
Using the Hubble classification, the bulge of Sa galaxies is usually composed of
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
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
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
Unlike the galactic disc, the halo seems to be free of
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]
Related
In June 2019,
Origin of the spiral structure
The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was
Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:
- star formation caused by galactic diskof 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
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
The first acceptable theory for the spiral structure was devised by
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 wavesthrough 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
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
Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae, due to
The question of whether such objects were separate galaxies independent of the Milky Way, or a type of
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.[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
- Andromeda Galaxy – Barred spiral galaxy in the Local Group
- Milky Way – Galaxy containing the Solar System
- Pinwheel Galaxy – Galaxy in the constellation Ursa Major
- Sunflower Galaxy– Spiral galaxy in the constellation Canes Venatici
- Triangulum Galaxy – Spiral galaxy in the constellation Triangulum
- Whirlpool Galaxy – Galaxy in the constellation Canes Venatici
- Black Eye Galaxy – Spiral galaxy in the constellation Coma Berenices
- Malin 1 – Spiral galaxy in the constellation Coma Berenices
See also
Classification
- Disc galaxy – Type of galactic form
- Dwarf elliptical galaxy – Elliptical galaxy smaller than normal ones
- Dwarf spheroidal galaxy – Small, low-luminosity galaxy with an old stellar population and little dust
- Flocculent spiral galaxy – Patchy galaxy with discontinuous spiral arms
- Galaxy color–magnitude diagram – Chart depicting the relationship between brightness and mass of large star systems
- Grand design spiral galaxy – Type of spiral galaxy with prominent and well-defined spiral arms
- Intermediate spiral galaxy – Galaxy type intermediate between a spiral galaxy and barred spiral galaxy
- Lenticular galaxy – Class of galaxy between an elliptical galaxy and a spiral galaxy
- Ring galaxy – Galaxy with an annular appearance
- Starburst galaxy – Galaxy undergoing an exceptionally high rate of star formation
- Seyfert galaxy – Class of active galaxies with very bright nuclei
Other
- Galactic coordinate system – Celestial coordinate system in spherical coordinates, with the Sun as its center
- Galactic corona – Hot, ionised, gaseous component in the Galactic halo
- Galaxy formation and evolution – From a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time
- Galaxy rotation curve – Observed discrepancy in galactic angular momenta
- Groups and clusters of galaxies – All of space observable from the Earth at the present
- List of galaxies
- List of nearest galaxies
- List of spiral galaxies
- Stellar halo
- Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure – Timeline of human understanding of large-scale structures in space
- Tully–Fisher relation – Trend in astronomy
References
- (pp. 124–151)
- ISBN 978-0-7167-0326-6.
- Science Daily. 16 January 2014.
- ^ "Ripples in a Galactic Pond". Scientific American. October 2005. Archived from the original on 6 September 2013.
- S2CID 14782284.
- .
- doi:10.1086/157753.
- ^ Alister W. Graham and C. Clare Worley (2008), Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations
- ^ de Vaucouleurs, G.; de Vaucouleurs, A.; Corwin, H. G., Jr.; Buta, R. J.; Paturel, G.; Fouqué, P. (2016), Third Reference Catalogue of Bright Galaxies
- ^ B.D. Simmons et al. (2014), Galaxy Zoo: CANDELS barred discs and bar fractions
- ^ Astronomy Now (8 May 2016), Astronomers detect double ‘peanut shell’ galaxies
- ^ Bogdan C. Ciambur and Alister W. Graham (2016), Quantifying the (X/peanut)-shaped structure in edge-on disc galaxies: length, strength, and nested peanuts
- ^ "Webb reveals structure in 19 spiral galaxies". www.esa.int. Retrieved 30 January 2024.
- ^ Oldest spiral galaxy is a freak of cosmos http://www.zmescience.com/space/oldest-spiral-galaxy-31321/
- ^ Gonzalez, Robert T. (19 July 2012). "Hubble Has Spotted an Ancient Galaxy That Shouldn't Exist". io9. Retrieved 10 September 2012.
- ^ "[BBC2005] Source 11 -- Galaxy". 24 June 2018.
- ^ "The most ancient spiral galaxy confirmed". PhysOrg. 3 November 2017.
- S2CID 119267114.
- ^ "ALMA Spots Candidate for Most Distant Known Spiral Galaxy". www.sci-news.com. 20 May 2021. Retrieved 20 May 2021.
- EurekAlert!. Retrieved 11 June 2019.
- .
- ^ "A spiral home to exploding stars". ESA / Hubble. Retrieved 2 April 2014.
- ^ ISBN 978-0-7503-0730-7.
- doi:10.1086/147955.
- ^ 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
. - ^ "Main Sequence Lifetime". Swinburne Astronomy Online. Swinburne University of Technology. Retrieved 8 June 2019.
- S2CID 12461073.
- ^ F. Shirley Patterson (1940), The Luminosity Gradient of Messier 33
- ^ Gerard de Vaucouleurs (1957), Studies of the Magellanic Clouds. III. Surface brightness, colors and integrated magnitudes of the Clouds.
- doi:10.1086/150474.
- ^ 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)
- ^ "NASA - Hubble Views the Star That Changed the Universe".
- doi:10.1086/142976.
- ^ Gerard de Vaucouleurs (1964), Interpretation of velocity distribution of the inner regions of the Galaxy
- Bibcode:1996A&AS..120C.315C.
- ^ McKee, Maggie (16 August 2005). "Bar at Milky Way's heart revealed". New Scientist. Retrieved 17 June 2009.
External links
- Giudice, G.F.; Mollerach, S.; Roulet, E. (1994). "Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?". Physical Review D. 50 (4): 2406–2413. S2CID 14500715.
- Stephens, Tim (6 March 2007). "AEGIS survey reveals new principle governing galaxy formation and evolution". UC Santa Cruz. Archived from the original on 11 March 2007. Retrieved 24 May 2006.
- Spiral Galaxies @ SEDS Messier pages
- SpiralZoom.com, an educational website about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.
- Spiral Structure explained
- GLIMPSE: the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire
- Merrifield, M. R. "Spiral Galaxies and Pattern Speed". Sixty Symbols. Brady Haran for the University of Nottingham.