Hubble sequence
Encyclopedia
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 diagram because of the shape in which it is traditionally represented.
Hubble’s scheme divides regular galaxies into 3 broad classes - ellipticals
, lenticulars
and spirals
- based on their visual appearance (originally on photographic plate
s). A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy
.
On the left (in the sense that the sequence is usually drawn) lie the ellipticals
. Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer
representing their degree of ellipticity on the sky. By convention, 
is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as 
for an ellipse with semi-major and semi-minor axes of lengths 
and 
respectively. The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). This is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles.
Examples of elliptical galaxies: M49
, M59
, M60
, M87
, NGC 4125
.
On the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies
. A spiral galaxy consists of a flattened disk, with star
s forming a (usually two-armed) spiral
structure, and a central concentration of stars known as the bulge
. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows:
Hubble originally described three classes of spiral galaxy. This was extended by de Vaucouleurs
to include a fourth class:
Although strictly part of the de Vaucouleurs system of classification, the Sd class is often included in the Hubble sequence. The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending 2 lower-case letters to the main galaxy type (for example56 Sbc for a galaxy that is intermediate between an Sb and an Sc).
Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms. However, this classification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer.
Examples of regular spiral galaxies: M31
(Andromeda Galaxy), M74
, M81
, M104
(Sombrero Galaxy), M51a
(Whirlpool Galaxy), NGC 300
, NGC 772
.
Examples of barred spiral galaxies: M91
, M95
, NGC 1097
, NGC 1300
, NGC1672, NGC 2536
, NGC 2903
.
At the centre of the Hubble tuning fork, where the two spiral arms meet the elliptical branch lies an intermediate class of galaxies known as lenticulars
and given the symbol S0. These galaxies consist of a bright central bulge
, similar in appearance to an elliptical galaxy
, surrounded by an extended, disk-like structure. Unlike spiral galaxies
, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. The bulge component is often the dominant source of light in a lenticular galaxy.
Face-on lenticulars are difficult to distinguish from ellipticals of type E0, making the classification of many such galaxies uncertain. When viewed edge-on, prominent dust-lanes are sometimes visible in absorption
against the light of stars in the disk.
At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly-flattened ellipticals and spirals. Later observations
(by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage
.
Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.
Examples of lenticular galaxies: M85
, M86
, NGC 1316
, NGC 2787
, NGC 5866
(Spindle Galaxy), Centaurus A
.
Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed irregular galaxies
. Hubble defined two classes of irregular galaxy:
In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds
- two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the Large Magellanic Cloud
led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud
(denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.
Examples of irregular galaxies: M82
, NGC 1427A
, Large Magellanic Cloud
, Small Magellanic Cloud
.
sequence, from elliptical galaxies
through lenticulars
to either barred
or regular spirals
. In fact, Hubble was clear from the beginning that no such interpretation was implied:
The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young star
s and regions of active star formation
, while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe
appears to be dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-day ellipticals formed as a result of mergers between these earlier building blocks. Lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation.
effects (the same galaxy looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength
of light in which they are observed. Nevertheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy
and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates.
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...
in 1926. It is often known colloquially as the Hubble tuning-fork diagram because of the shape in which it is traditionally represented.
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
, lenticulars
Lenticular galaxy
A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disk galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
and spirals
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
- based on their visual appearance (originally on photographic plate
Photographic plate
Photographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...
s). A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy
Amateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
.
Ellipticals
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
. Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer
representing their degree of ellipticity on the sky. By convention, is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as for an ellipse with semi-major and semi-minor axes of lengths and respectively. The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). This is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles.Examples of elliptical galaxies: M49
Messier 49
Messier 49 is an elliptical / lenticular galaxy about 49 million light-years away in the constellation Virgo. The galaxy was discovered by Charles Messier in 1771.-Supernovae:...
, M59
Messier 59
Messier 59 is an elliptical galaxy in the constellation Virgo.-History:Messier 59 and the nearby elliptical galaxy Messier 60 were both discovered by Johann Gottfried Koehler in April 1779 during observations of a comet in the same part of the sky...
, M60
Messier 60
Messier 60 is an elliptical galaxy approximately 55 million light-years away in the constellation Virgo.-History:...
, M87
Messier 87
Messier 87 is a supergiant elliptical galaxy. It was discovered in 1781 by the French astronomer Charles Messier, who cataloged it as a nebulous feature. The second brightest galaxy within the northern Virgo Cluster, it is located about 16.4 million parsecs from Earth...
, NGC 4125
NGC 4125
NGC 4125 is a elliptical galaxy in the constellation Draco....
.
Spirals
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
. A spiral galaxy consists of a flattened disk, with star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s forming a (usually two-armed) spiral
Spiral
In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...
structure, and a central concentration of stars known as the bulge
Bulge (astronomy)
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies...
. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows:
- Sa (SBa) - tightly-wound, smooth arms; large, bright central bulge
- Sb (SBb) - less tightly-wound spiral arms than Sa (SBa); somewhat fainter bulge
- Sc (SBc) - loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge
Hubble originally described three classes of spiral galaxy. This was extended by de Vaucouleurs
Gérard de Vaucouleurs
-External links:* * http://nedwww.ipac.caltech.edu/level5/normal_galaxies.html-Other resources:...
to include a fourth class:
- Sd (SBd) - very loosely-wound, fragmentary arms; most of the luminosity is in the arms and not the bulge
Although strictly part of the de Vaucouleurs system of classification, the Sd class is often included in the Hubble sequence. The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending 2 lower-case letters to the main galaxy type (for example56 Sbc for a galaxy that is intermediate between an Sb and an Sc).
Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms. However, this classification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer.
Examples of regular spiral galaxies: M31
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
(Andromeda Galaxy), M74
Messier 74
Messier 74 is a face-on spiral galaxy in the constellation Pisces. It is at a distance of about 32 million light-years away from Earth. The galaxy contains two clearly defined spiral arms and is therefore used as an archetypal example of a Grand Design Spiral Galaxy...
, M81
Messier 81
Messier 81 is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. Due to its proximity to Earth, large size and active galactic nucleus Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away in the constellation Ursa...
, M104
Sombrero Galaxy
The Sombrero Galaxy is an unbarred spiral galaxy in the constellation Virgo located 28 million light years from earth. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a...
(Sombrero Galaxy), M51a
Whirlpool Galaxy
The Whirlpool Galaxy is an interacting grand-design spiral galaxy that is estimated to be 23 ± 4 million light-years from the Milky Way Galaxy. in the constellation Canes Venatici...
(Whirlpool Galaxy), NGC 300
NGC 300
NGC 300 is a spiral galaxy in the constellation Sculptor. It is one of the closest galaxies to the Local Group, and probably lies between us and the Sculptor Group. It is the brightest of the five main spirals in the direction of the Sculptor Group...
, NGC 772
NGC 772
NGC 772 is an unbarred spiral galaxy approximately 130 million light-years away in the constellation Aries. It is notable for possessing a single elongated outer spiral arm, which has likely arisen due to tidal interactions with nearby galaxies...
.
Examples of barred spiral galaxies: M91
Messier 91
Messier 91 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...
, M95
Messier 95
Messier 95 is a barred spiral galaxy about 38 million light-years away in the constellation Leo. It was discovered by Pierre Méchain in 1781, and catalogued by Charles Messier four days later.-Nucleus:...
, NGC 1097
NGC 1097
NGC 1097 is a barred spiral galaxy about 45 million light-years away in the constellation Fornax. As of 2006, three supernovae have been observed in NGC 1097....
, NGC 1300
NGC 1300
NGC 1300 is a barred spiral galaxy about 61 million light-years away in the constellation Eridanus. The galaxy is about 110,000 light-years across; just slightly larger than our own galaxy, the Milky Way. It may be part of the Eridanus Cluster...
, NGC1672, NGC 2536
NGC 2536
NGC 2536 is a barred spiral galaxy in the constellation Cancer that is interacting with NGC 2536. The two galaxies are listed together in the Atlas of Peculiar Galaxies as an example of a spiral galaxy with a high surface brightness companion....
, NGC 2903
NGC 2903
NGC 2903 is a barred spiral galaxy about 30 million light-years away in the constellation Leo. It was discovered by William Herschel who cataloged it on November 16, 1784. NGC 2905 is a bright star cloud within this galaxy.- Additional images :...
.
Lenticulars
Lenticular galaxy
A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disk galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
and given the symbol S0. These galaxies consist of a bright central bulge
Bulge (astronomy)
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies...
, similar in appearance to an elliptical galaxy
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
, surrounded by an extended, disk-like structure. Unlike spiral galaxies
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. The bulge component is often the dominant source of light in a lenticular galaxy.
Face-on lenticulars are difficult to distinguish from ellipticals of type E0, making the classification of many such galaxies uncertain. When viewed edge-on, prominent dust-lanes are sometimes visible in absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
against the light of stars in the disk.
At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly-flattened ellipticals and spirals. Later observations
Observational astronomy
Observational astronomy is a division of the astronomical science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical models...
(by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage
Allan Sandage
Allan Rex Sandage was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California. He is best known for determining the first reasonably accurate value for the Hubble constant and the age of the universe.-Career:Sandage was one of the most...
.
Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.
Examples of lenticular galaxies: M85
Messier 85
Messier 85 is a lenticular galaxy in the Coma Berenices constellation. It is 60 million light years away, making it the 94th most distant Messier object, and it estimated to be 125,000 light years across....
, M86
Messier 86
Messier 86 is a lenticular galaxy in the constellation Virgo. It was discovered by Charles Messier in 1781. M86 lies in the heart of the Virgo Cluster of galaxies and forms a most conspicuous group with another giant, Lenticular Galaxy M84...
, NGC 1316
NGC 1316
NGC 1316 is a lenticular galaxy about 70 million light-years away in the constellation Fornax. NGC 1316 is a radio galaxy. It is the fourth-brightest radio source in the sky .-Structure and formation:...
, NGC 2787
NGC 2787
NGC 2787 is a barred lenticular galaxy approximately 24 million light-years away in the constellation Ursa Major. In 1999, the Hubble Space Telescope took a look at NGC 2787.-LINER emission:...
, NGC 5866
NGC 5866
NGC 5866 is a relatively bright lenticular or spiral galaxy in the constellation Draco. NGC 5866 was probably discovered by Pierre Méchain or Charles Messier in 1781, and independently found by William Herschel in 1788.-Dust disk:One of the most outstanding features of NGC 5866 is the extended...
(Spindle Galaxy), Centaurus A
Centaurus A
Centaurus A is a prominent galaxy in the constellation of Centaurus. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance...
.
Irregulars
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
. Hubble defined two classes of irregular galaxy:
- Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars
- Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters
In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...
- two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
(denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.
Examples of irregular galaxies: M82
Messier 82
Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major...
, NGC 1427A
NGC 1427A
NGC 1427A is an irregular galaxy in the constellation Eridanulip. distance modulus has been estimated using the globular cluster luminosity function to be 31.01 ± 0.21 which is about 52 Mly. It is the brightest dwarf irregular member of the Fornax cluster and is in the foreground of the...
, Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
, Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
.
Physical significance
Elliptical and lenticular galaxies are commonly referred to together as “early-type” galaxies, while spirals and irregular galaxies are referred to as “late types”. This nomenclature is the source of the common, but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionaryEvolution (term)
The English noun evolution refers to any kind of gradual change.It is used in biology, of biological evolution, in economics, historical linguistics, and many other technical fields where systems develop or change gradually over time, e.g...
sequence, from elliptical galaxies
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
through lenticulars
Lenticular galaxy
A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disk galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
to either barred
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
or regular spirals
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
. In fact, Hubble was clear from the beginning that no such interpretation was implied:
The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution...
The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s and regions of active star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
, while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
appears to be dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-day ellipticals formed as a result of mergers between these earlier building blocks. Lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation.
Shortcomings
A common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type ). The different classification criteria can also be at odds with each other: for example, a more dominant bulge component does not always go hand-in-hand with more loosely-wound spiral arms. Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientationOrientation (geometry)
In geometry the orientation, angular position, or attitude of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it is in....
effects (the same galaxy looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of light in which they are observed. Nevertheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy
Extragalactic astronomy
Extragalactic astronomy is the branch of astronomy concerned with objects outside our own Milky Way Galaxy. In other words, it is the study of all astronomical objects which are not covered by galactic astronomy, the next level of galactic astronomy....
and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates.
See also
- Edwin HubbleEdwin HubbleEdwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...
- Gérard de VaucouleursGérard de Vaucouleurs-External links:* * http://nedwww.ipac.caltech.edu/level5/normal_galaxies.html-Other resources:...
- Galaxy color-magnitude diagramGalaxy color-magnitude diagramThe Galaxy color-magnitude diagram shows the relationship between absolute magnitude, luminosity, and mass of galaxies. A preliminary description of the three areas of this diagram was made in 2003 by Eric F. Bell et al...
- Galaxy morphological classification
External links
- Galaxies and the Universe - an introduction to galaxy classification
- Near-Infrared Galaxy Morphology Atlas, T.H. Jarrett
- The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project
- Galaxy Zoo - Galaxy classification participation project.