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Encyclopedia > Galaxy morphological classification

Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). The system of galaxy classification is called the Hubble "tuning fork" diagram, and is the Hubble sequence. Image File history File links Wiki_letter_w. ... A giant Hubble mosaic of the Crab Nebula, a supernova remnant. ... This article is about a celestial body. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ... NGC 1300, viewed nearly face-on. ...

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Hubble sequence

The Hubble sequence is a classification of galaxy types developed by Edwin Hubble in 1925. It is also called the tuning-fork diagram due to the shape of its graphical representation. NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light-years in diameter and approximately 60 million light-years distant. ... Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer, noted for his discovery of galaxies beyond the Milky Way and the cosmological redshift. ... 1925 (MCMXXV) was a common year starting on Thursday (link will take you to calendar). ...

Tuning-fork style diagram of the Hubble sequence
Tuning-fork style diagram of the Hubble sequence

The Hubble "tuning fork" diagram starts from the left with elliptical galaxies as its base. Elliptical galaxies can be named from E0 to E7. E stands for elliptical while the number indicates how oval-shaped the ellipse is with 0 being ball shape (in other words, a giant globular cluster) to 7 being discus shape. Technically speaking, the number is ten times the ellipticity. For example, an E7 galaxy has an ellipticity of 0.7. A tuning-fork diagram of the Hubble galaxy classification scheme. ... A tuning-fork diagram of the Hubble galaxy classification scheme. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ...


After the elliptical galaxies the diagram splits into two branches. The upper branch covers spiral galaxies. It starts off with S0, also called lenticular galaxies. The "S" means spiral, the "0" means no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc. On the same branch are the next 3 types which all have spiral arms. The "S" here also means spiral, but the lower case letter after it tell how wound up the arms are. They range from "a" to "d" having the following meanings: A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ...

  • Sa - tightly-wound, smooth arms, and a bright central disc
  • Sb - better defined spiral arms than Sa
  • Sc - much more loosely wound spiral arms than Sb
  • Sd - very loose arms, most of the luminosity is in the arms and not the disc

The lower branch of the diagram covers barred spiral galaxies given the symbol "SB". This branch starts with SBO galaxies which is followed by a subscript number that indicates how heavily defined the bar is. After that the branch continues with the SB galaxies which have lower case letters after them that indicates how heavily defined the bar is. They range from "a" to "c" having the following meanings: NGC 1300, viewed nearly face-on. ...

  • SBa - a bright center and tight spirals
  • SBb - better defined arms than SBa galaxy and are more loosely wound
  • SBc - even looser arms, and a much dimmer central portion of the galaxy

The Milky Way Galaxy is currently believed to be an SBb galaxy. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos; or simply the Galaxy) is a barred spiral galaxy in the Local Group, and has special significance to humanity as the location of the solar system, which is located near the Orion...


Contrary to popular opinion, the galactic tuning fork has nothing to do with the evolution of galaxies. For example, S0 galaxies do not split into two groups, one which turns into regular spirals and one which becomes barred. Likewise, spiral or barred-spiral galaxies do not evolve into ellipticals. However, there are reasons to believe that elliptical galaxies in general are older than spiral galaxies. For instance, elliptical E galaxies appear redder than S galaxies, which indicates that they consist of older, redder stars and stellar clusters. The fact that S galaxies usually seem bluer and brighter hints at star formation. Since stellar formation requires dust clouds to collapse gravitationally, we may think S galaxies to be younger than E galaxies where all necessary ingredients for star formation has already 'been used up'. Yet it needs to be mentioned that also through intergalactical interaction star formation (Balmer lines) is frequently observed. The all-encompassing evolutionary diagram of galaxies remains one of the unresolved challenges of astronomy today.


Galaxy types are divided as follows:

  • An elliptical galaxy (E0-7) has an ellipsoidal form, with a fairly even distribution of stars throughout. The number is related to eccentricity but is defined by ten times the galaxy's ellipticity, which is mainly used in astronomy, i.e. where b is the short axis and a is the long axis. E0 galaxies are nearly round, while E7 are greatly flattened. The number indicates only how the galaxy appears on the sky, not its true geometry.
  • A lenticular galaxy (S0 and SB0) appears to have a disk-like structure with a central spherical bulge projecting from it, and does not show any spiral structure.
  • A spiral galaxy (Sa-d) has a central bulge and an outlying disk containing spiral arms. The arms are centered around the bulge, and vary from tightly wound (Sa) to very loose (Sc and Sd). The latter also have less pronounced central bulges.
  • A barred spiral galaxy (SBa-d) has a similar sort of spiral structure to spiral galaxies, but instead of emanating from the bulge, the arms project out from the ends of a "bar" running through the bulge, like ribbons on either end of a baton. Again, SBa to SBd refer to how "tightly wound" these arms are.
  • An irregular galaxy (Irr) can be of type Irr-I, which shows spiral structure but is deformed in some way, and Irr-II for any other galaxy that does not fit into another category.
Known Properties of Galaxies
Galaxy Type Mass (Solar Masses) Luminosity (Solar Luminosity) Diameter (kpc) Stellar Populations Percentage of Observed Galaxies
Spiral /
Barred Spiral
109 to 1011 108 to 1010 5-250 disk: Population I
halo:Population II
77%
Elliptical 105 to 1013 105 to 1011 1-205 Population II 20%
Irregular 108 to 1010 107 to 109 1-10 Population I 3%

Hubble based his classification on photographs of the galaxies through the telescopes of the time. He originally believed that elliptical galaxies were an early form, which might have later evolved into spirals; our current understanding suggests that the situation is roughly opposite, however, this early belief left its imprint in the astronomers' jargon, who still speak of "early type" or "late type" galaxies according to whether a galaxy's type appears to the left or to the right in the diagram. An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... (This page refers to eccentricity in mathematics. ... The Spindle Galaxy (NGC 5866), a lenticular galaxy in the Draco constellation. ... A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ... NGC 1300, viewed nearly face-on. ... NGC1427A, an example of an irregular galaxy. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... The Sun is the star at the centre of the Solar System[]. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for more than 99% of the solar systems mass[]. Energy from the Sun—in the form of... Metal-poor is a term that is used to describe the chemical make up of an astronomical object. ... Metal-poor is a term that is used to describe the chemical make up of an astronomical object. ... It has been suggested that Archival digital print be merged into this article or section. ... 50 cm refracting telescope at Nice Observatory. ...


More modern observations of galaxies have given us the following information about these types:

  • Elliptical galaxies are generally fairly low in gas and dust, and are composed mostly of older stars.
  • Spiral galaxies generally have plentiful supplies of gas and dust, and have a broad mix of older and younger stars.
  • Irregular galaxies are fairly rich in gas, dust, and young stars.

From this, astronomers have constructed a theory of galaxy evolution which suggests that ellipticals are, in fact, the result of collisions between spiral and/or irregular galaxies, which strip out much of the gas and dust and randomize the orbits of the stars. See galaxy formation and evolution. In astrophysics, the questions of galaxy formation and evolution are: How, from a homogeneous universe, did we obtain the very heterogeneous one we live in? How did galaxies form? How do galaxies change over time? A spectacular head-on collision between two galaxies is seen in this NASA Hubble Space...

Elliptical Galaxies examples
Name Right Ascension Declination Hubble Type
M49 (NGC 4472) 12h 29.8m 8° 00' E4
M59 (NGC 4621) 12h 42.0m 11° 39' E3
M60 (NGC 4649) 12h 43.7m 11° 33' E1
M84 (NGC 4374) 12h 25.1m 12° 53' E1
M86 (NGC 4406) 12h 26.2m 12° 57' E3
M89 (NGC 4552) 12h 35.7m 12° 33' E0
M110 (NGC 205) 00h 40.4m 41° 41' E6

Elliptical Galaxy M49 (also known as Messier Object 49, Messier 49, M49, or NGC 4472) is an elliptical galaxy in the Virgo constellation. ... M59, courtesy of NOAO. Elliptical Galaxy M59 (also known as Messier Object 59, Messier 59, M59, or NGC 4621) is an elliptical galaxy in the constellation Virgo. ... M60, courtesy of NOAO Elliptical Galaxy M60 (also known as Messier Object 60, Messier 60, M60, or NGC 4649) is an elliptical galaxy in the constellation Virgo. ... The M84 (also known as Messier Object 84, Messier 84, or NGC 4374) is a lenticular galaxy in the constellation Virgo. ... The Lenticular Galaxy M86 (also known as Messier Object 86, Messier 86, M86, or NGC 4406) is a lenticular galaxy in the Virgo constellation. ... Elliptical Galaxy M89 (also known as Messier Object 89, Messier 89, M89, or NGC 4552) is an elliptical galaxy in the Virgo constellation. ... The Elliptical Galaxy M110 (also known as Messier Object 110, Messier 110, M110, or NGC 205) is an elliptical galaxy in the Andromeda constellation, a satellite of the Andromeda Galaxy, and a member of the Local Group galaxies. ...

The de Vaucouleurs system

There is an extension to the Hubble sequence that widely used: the de Vaucouleurs extensions. The distinction between the de Vaucouleurs and Hubble classification systems lies primarily with spiral galaxies. While the Hubble type describes spiral galaxies based upon the two criteria of tightness of spiral and barredness, de Vaucouleurs adds a third descriptor, internal ring.

  • Spiralness: galaxies range from E, through S0, through the other spirals, to Im.
  • Barredness: galaxies are described as being A (ordinary), B (barred), or AB (intermediate).
  • Ringedness: galaxies are described as being s-shaped (no ring), r-shaped (ring), or sr (intermediate).

Therefore, a galaxy may be described as being SAB(rs)c - Sc spiral, between barred and ordinary, and between ringed and no ring.


Visually, the de Vaucouleurs system is often represented in three dimensions, with spiralness on the x-axis, barredness on the y-axis, and ringedness on the z-axis. A cross-section of one spiralness (eg: Sb) will yield a representation in two dimensions with ringedness on the x-axis and barredness on the y-axis. Pictures are available here.


The Yerkes (or Morgan) scheme

The Yerkes classification involves using the spectrum of the stars in the galaxy and the shape, real and apparent, and the degree of the its central concentration.

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"

So, for example, the Andromeda Galaxy is classified as kS5. M31 in a small telescope The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Andromeda Nebula) is a spiral galaxy approximately 2. ...


See also

Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer, noted for his discovery of galaxies beyond the Milky Way and the cosmological redshift. ... Gérard Henri de Vaucouleurs (April 25, 1918–October 7, 1995) was a French-American astronomer. ... William Wilson Morgan (January 3, 1906 – June 21, 1994) was an American astronomer. ...

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