Galaxy morphological classification

Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage.

HubbleTuningFork
Tuning-fork-style diagram of the Hubble sequence

Hubble sequence

A remarkable galactic hybrid
Spiral galaxy UGC 12591 is classified as an S0/Sa galaxy.[1]

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.[2][3] 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)[4]:

  • Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter "E", followed by an integer n representing their degree of ellipticity on the sky.[5]
  • Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol "S". Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol "SB".[6]
  • Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.[6][7]
The Hubble Sequence throughout the Universe's history
The Hubble sequence throughout the universe's history.[8]

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).[6][4]

The Hubble sequence 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. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”.[9]

To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.[10]

De Vaucouleurs system

Hubble - de Vaucouleurs Galaxy Morphology Diagram
Hubble - de Vaucouleurs Galaxy Morphology Diagram
Galaxy morphology
NGC 6782 I HST2002
NGC 6782: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar.
NGC 7793SpitzerFull
NGC 7793: a spiral galaxy of type SA(s)d.
Large.mc.arp.750pix
The Large Magellanic Cloud: a type SBm galaxy.

The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959.[11] 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.[12]

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 a more elaborate classification system for spiral galaxies, based on three morphological characteristics[13]:

  • Bars. Galaxies are divided on the basis of the presence or absence of a nuclear bar. De Vaucouleurs introduced the notation SA to denote spiral galaxies without bars, complementing Hubble’s use of SB for barred spirals. He also allowed for an intermediate class, denoted SAB, containing weakly barred spirals.[14] Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not (usually because they are edge-on to the line-of-sight).
  • Rings. Galaxies are divided into those possessing ring-like structures (denoted ‘(r)’) and those without rings (denoted ‘(s)’). So-called ‘transition’ galaxies are given the symbol (rs).[14]
  • Spiral arms. As in Hubble’s original scheme, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms. The de Vaucouleurs scheme extends the arms of Hubble’s tuning fork to include several additional spiral classes:
    • Sd (SBd) - diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central bulge
    • Sm (SBm) - irregular in appearance; no bulge component
    • Im - highly irregular galaxy
    Most galaxies in these three classes were classified as Irr I in Hubble’s original scheme. In addition, the Sd class contains some galaxies from Hubble’s Sc class. Galaxies in the classes Sm and Im are termed the “Magellanic” spirals and irregulars, respectively, after the Magellanic Clouds. The Large Magellanic Cloud is of type SBm, while the Small Magellanic Cloud is an irregular (Im).

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 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.[15]

Numerical Hubble stage

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).[16] 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 (S0) and late (S+) types. Irregular galaxies can be of type magellanic irregulars (T = 10) or 'compact' (T = 11).

Numerical Hubble stage
Hubble stage T −6 −5 −4 −3 −2 −1 0 1 2 3 4 5 6 7 8 9 10 11
de Vaucouleurs class[15] cE E E+ S0 S00 S0+ S0/a Sa Sab Sb Sbc Sc Scd Sd Sdm Sm Im
approximate Hubble class[17] E S0 S0/a Sa Sa-b Sb Sb-c Sc Sc-Irr Irr I

The use of numerical stages allows for more quantitative studies of galaxy morphology.

Yerkes (or Morgan) scheme

Created by American astronomer William Wilson Morgan. Together with Philip Keenan, Morgan 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.[18]

Spectral Type Explanation
a Prominent A stars
af Prominent A–F stars
f Prominent F stars
fg Prominent F–G stars
g Prominent G stars
gk Prominent G–K stars
k Prominent K stars
Galactic Shape Explanation
B Barred spiral
D Rotational symmetry without pronounced spiral or elliptical structure
E Elliptical
Ep Elliptical with dust absorption
I Irregular
L Low surface brightness
N Small bright nucleus
S Spiral
Inclination Explanation
1 Galaxy is "face-on"
2
3
4
5
6
7 Galaxy is "edge-on"

Thus, for example, the Andromeda Galaxy is classified as kS5.[19]

See also

References

  1. ^ "A remarkable galactic hybrid". www.spacetelescope.org. Retrieved 27 February 2017.
  2. ^ Hubble, E. P. (1926). "Extra-galactic nebulae". Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington. 324: 1–49. Bibcode:1926CMWCI.324....1H.
  3. ^ Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. LCCN 36018182.
  4. ^ a b information@eso.org. "The Hubble tuning fork - classification of galaxies". www.spacetelescope.org. Retrieved 2019-02-06.
  5. ^ Binney, James (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-02565-0.
  6. ^ a b c "The Hubble sequence — Astronoo". www.astronoo.com. Retrieved 2019-02-06.
  7. ^ "Lenticular Galaxies". cas.sdss.org. Retrieved 2019-02-06.
  8. ^ "Hubble explores the origins of modern galaxies". ESA/Hubble Press Release. Retrieved 20 August 2013.
  9. ^ "Galaxies". www.jb.man.ac.uk. Retrieved 2019-02-06.
  10. ^ Iafrate, G. "THE HUBBLE SEQUENCE" (PDF). uni-heidelberg.de. Retrieved 2019-02-06.
  11. ^ De Vaucouleurs, G. (1959). "Classification and Morphology of External Galaxies". Handbuch der Physik. 53: 275. Bibcode:1959HDP....53..275D.
  12. ^ Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-02565-0.
  13. ^ "Galaxy - Types of galaxies". Encyclopedia Britannica. Retrieved 2019-02-06.
  14. ^ a b de Vaucouleurs, Gérard (April 1963). "Revised Classification of 1500 Bright Galaxies". Astrophysical Journal Supplement. 8: 31. Bibcode:1963ApJS....8...31D. doi:10.1086/190084.
  15. ^ a b De Vaucouleurs, G. (1994). "Global Physical Parameters of Galaxies" (PostScript). Retrieved 2008-01-02.
  16. ^ "Qualitative and Quantitative Classifications of Galaxies". ned.ipac.caltech.edu. Retrieved 2019-02-06.
  17. ^ Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-02565-0.
  18. ^ "The Yerkes Classification". ned.ipac.caltech.edu. Retrieved 2019-02-06.
  19. ^ Darling, David. "galaxy classification". www.daviddarling.info. Retrieved 2019-02-06.

External links

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.

NGC 125

NGC 125 (also known as PGC 1772) is a lenticular galaxy located in the constellation Pisces. It is designated as subclass Sa Ring in the galaxy morphological classification scheme. It lies approximately 235 million light-years away.

NGC 5001

NGC 5001 is a barred spiral galaxy located in the constellation Ursa major. It is designated as SB in the galaxy morphological classification scheme. It was discovered by John Herschel on 1 May 1831. It is at a distance of 130 million parsecs from the earth.

NGC 6104

NGC 6104 is a barred spiral galaxy located in the constellation Corona Borealis. It is designated as S(R)Pec in the galaxy morphological classification scheme, though it is clearly a barred spiral (deserving of the SB(R)Pec designation), and was discovered by William Herschel on 16 May 1787. The galaxy is approximately 388 million light-years away.

NGC 6181

NGC 6181 is a barred spiral galaxy located in the constellation Hercules. It is designated as SB(rs)c in the galaxy morphological classification scheme and was discovered by William Herschel on 28 April 1788. The galaxy is 107 million light years away.

NGC 6207

NGC 6207 is a spiral galaxy located in the constellation Hercules. It is designated as SA(s)c in the galaxy morphological classification scheme and was discovered by William Herschel on 16 May 1787. NGC 6207 is located at about 30 million light years from earth. It is located near the globular cluster Messier 13.

NGC 6215

NGC 6215 (also known as PGC 59112) is a spiral galaxy located in the constellation Ara. It is designated as SA(s)c in the galaxy morphological classification scheme. It was discovered by astronomer John Herschel on 9 July 1836.

NGC 6229

NGC 6229 is a globular cluster located in the constellation Hercules. It is designated as GC(v)B in the galaxy morphological classification scheme and was discovered by the British astronomer William Herschel on 12 May 1787. NGC 6229 is located at about 100,000 light years away from earth.

NGC 6256

NGC 6256 is a globular cluster located in the constellation Scorpius. It is designated as GCL in the galaxy morphological classification scheme and was discovered by the Scottish astronomer James Dunlop on 24 June 1834. It is about 33,600 light years away from earth.

NGC 6284

NGC 6284 is a globular cluster located in the constellation Ophiuchus. It is designated as IX in the galaxy morphological classification scheme and was discovered by the German-born British astronomer William Herschel on 22 May 1784. It is at a distance of 49,900 light years away from earth.

NGC 6287

NGC 6287 is a globular cluster located in the constellation Ophiuchus. It is designated as VII in the galaxy morphological classification scheme and was discovered by the German-born British astronomer William Herschel on 21 May 1784. It is at a distance of 30,300 light years away from earth.

NGC 6309

NGC 6309, also known as the Box Nebula, is a quadrupolar planetary nebula located in the constellation Ophiuchus. It is designated as PN in the galaxy morphological classification scheme and was discovered by the German astronomer Wilhelm Tempel in 1876. It has a luminosity of about 1800 times of the Sun.

NGC 6326

NGC 6326 is a complex and irregularly structured planetary nebula located in the constellation Ara. It is designated as PN in the galaxy morphological classification scheme and was discovered by the Scottish astronomer James Dunlop on 26 August 1826. NGC 6326 is the result of the ejected material from the central binary star, which is nearing the end of its life. The blue and red color is due to the amount of radiation the star releases, thus causing the gasses to glow. NGC 6326 is located at about 11,000 light years away from Earth.

NGC 6342

NGC 6342 is a globular cluster located in the constellation Ophiuchus. It is designated as IV in the galaxy morphological classification scheme and was discovered by the German-born British astronomer William Herschel on 28 May 1786. It is at a distance of 28,000 light years away from earth.

NGC 6355

NGC 6355 is a globular cluster located in the constellation Ophiuchus. It is designated as GCL in the galaxy morphological classification scheme and was discovered by the German-born British astronomer William Herschel on 24 May 1784. It is at a distance of 31,000 light years away from earth.

NGC 6356

NGC 6356 is a globular cluster located in the constellation Ophiuchus. It is designated as II in the galaxy morphological classification scheme and was discovered by the German-born British astronomer William Herschel on 18 June 1784. The star cluster is more dense and bright towards the middle. 6356 is located 80' north east of the brighter NGC 6333. It is at a distance of 49,600 light years away from earth.

NGC 6394

NGC 6394 is a barred spiral galaxy located in the constellation Draco. It is designated as SBb in the galaxy morphological classification scheme and was discovered by the American astronomer Lewis A. Swift on 7 July 1885.

NGC 6400

NGC 6400 is an open cluster located in the constellation Scorpius. It is designated as II2m in the galaxy morphological classification scheme and was discovered by the Scottish astronomer James Dunlop on 13 May 1826. It is at a distance of 3,097 light years away from earth.

NGC 6426

NGC 6426 is a globular cluster located in the constellation Ophiuchus. It is designated IX in the galaxy morphological classification scheme and was discovered by the British astronomer William Herschel on 3 June 1786. It is at a distance of 67,500 light years away from earth.

Morphology
Structure
Active nuclei
Energetic galaxies
Low activity
Interaction
Lists
See also

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