The
Hubble sequence is a morphological classification scheme for galaxies invented by
Edwin HubbleEdwin Powell Hubble was an American astronomer. He profoundly changed our understanding of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the...
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 -
ellipticalsAn 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 flattened and in size from hundreds of millions to over one trillion stars....
,
lenticularsA 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 disc galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
and
spiralsA spiral galaxy is a galaxy belonging to one of the three main classes 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...
- based on their visual appearance (originally on
photographic platePhotographic plates preceded photographic film as a mean 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 astronomyAmateur astronomy, also called backyard astronomy, 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
On the left (in the sense that the sequence is usually drawn) lie the
ellipticalsAn 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 flattened 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
ellipsoidalAn ellipsoid is a type of quadric surface that is a higher dimensional analogue of an ellipse. The equation of a standard axis-aligned ellipsoid body in an xyz-Cartesian coordinate system is...
structures rather than disks viewed at a range of angles.
Examples of elliptical galaxies:
M49Messier 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:...
,
M59Messier 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...
,
M60Messier 60 is an elliptical galaxy approximately 55 million light-years away in the constellation Virgo.-History:...
,
M87Messier 87 is a giant elliptical galaxy. The galaxy is the largest and brightest galaxy within the northern Virgo Cluster, located about 55 million light years away. The galaxy also contains a notable active galactic nucleus that is a strong source of multiwavelength radiation, particularly radio...
,
NGC 4125NGC 4125 is a elliptical galaxy in the constellation Draco....
.
Spirals
On the right of the Hubble sequence diagram are two parallel branches encompassing the
spiral galaxiesA spiral galaxy is a galaxy belonging to one of the three main classes 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...
. A spiral galaxy consists of a flattened disk, with
starsA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
forming a (usually two-armed)
spiralIn mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Three-dimensional spirals:...
structure, and a central concentration of stars known as the
bulgeIn astronomy, a bulge is a tightly packed group of stars. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happen to have a disk of stars around them...
. 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 VaucouleursGérard Henri de Vaucouleurs was a French astronomer.-Biography:Born in Paris, he had an early interest in amateur astronomy and received his undergraduate degree in 1939 at the Sorbonne in that city...
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 example 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:
M31The Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
(Andromeda Galaxy),
M74Messier 74 is a face-on spiral galaxy in the constellation Pisces. The galaxy contains two clearly-defined spiral arms and is therefore used as an archetypal example of a Grand Design Spiral Galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur...
,
M81Messier 81 is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. M81 is one of the most striking examples of a grand design spiral galaxy, with near perfect arms spiraling into the very center...
,
M104The Sombrero Galaxy is an unbarred spiral galaxy in the constellation Virgo. 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...
(Sombrero Galaxy),
M51aThe Whirlpool Galaxy is an interacting grand-design spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici...
(Whirlpool Galaxy),
NGC 300NGC 300 is a spiral galaxy in the constellation Sculptor. It is the brightest of the five main spirals in the direction of the sculptor group...
,
NGC 772NGC 772 is an unbarred spiral galaxy approximately 130 million light-years away in the constellation Aries. NGC 772 has a satellite galaxy named NGC 770. Two supernovae were observed in NGC 772....
.
Examples of barred spiral galaxies:
M91Messier 91 is a barred spiral galaxy about 63 million light-years away in the constellation Coma Berenices. It was most probably discovered by Charles Messier in 1781 and independently rediscovered by William Herschel on April 8, 1784. M91 is a member of the Virgo Cluster of galaxies....
,
M95Messier 95 is a barred spiral galaxy about 33 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 1097NGC 1097 is a barred spiral galaxy about 45 million light-years away in the constellation Fornax. Three supernovae have been observed in NGC 1097 ....
,
NGC 1300NGC 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 is part of the Eridanus Cluster...
, NGC1672,
NGC 2536NGC 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 2903NGC 2903 is a barred spiral galaxy about 20.5 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
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
lenticularsA 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 disc 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
bulgeIn astronomy, a bulge is a tightly packed group of stars. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happen to have a disk of stars around them...
, similar in appearance to an
elliptical galaxyAn 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 flattened and in size from hundreds of millions to over one trillion stars....
, surrounded by an extended, disk-like structure. Unlike
spiral galaxiesA spiral galaxy is a galaxy belonging to one of the three main classes 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...
, 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
absorptionIn 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...
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
observationsObservational 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 SandageAllan Rex Sandage is an American astronomer.-Career:Allan R. Sandage is one of the most influential astronomers of the 20th century. Sandage graduated from the University of Illinois in 1948. By 1953 he earned his Ph.D. from the California Institute of Technology...
.
Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.
Examples of lenticular galaxies:
M85Messier 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....
,
M86Messier 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 1316NGC 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 2787NGC 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 5866NGC 5866 is a relatively bright lenticular galaxy in the constellation Draco. Some astronomers believe that the NGC 5866 may be a candidate for Messier 102....
(Spindle Galaxy),
Centaurus ACentaurus A is a lenticular galaxy about 14 million light-years away in the constellation Centaurus. It is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers...
.
Irregulars
Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed
irregular galaxiesAn irregular galaxy is a galaxy that does not have a 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 nuclear...
. 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 CloudsThe two Magellanic Clouds are irregular dwarf galaxies, which are members of our Local Group of galaxies. Once they were thought to be orbiting our Milky Way galaxy. However, new research seems to indicate that this is not the case...
- two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the
Large Magellanic CloudThe Large Magellanic Cloud is a nearby irregular galaxy, once thought to be a satellite of our own. 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...
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 CloudThe Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years. and contains several hundred million stars....
(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:
M82Messier 82 is the prototype nearby starburst galaxy about 12 million light-years away in the constellation Ursa Major. The starburst galaxy is five times as bright as the whole Milky Way and one hundred times as bright as our galaxy's center.In 2005, the Hubble revealed 197 young massive clusters...
,
NGC 1427ANGC 1427A is an irregular galaxy in the constellation Eridanus. Its 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 CloudThe Large Magellanic Cloud is a nearby irregular galaxy, once thought to be a satellite of our own. 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...
,
Small Magellanic CloudThe Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years. and contains several hundred million stars....
.
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 is a term with many meanings. For instance, Merriam-Webster lists biological evolution as one meaning out of a total of six.Evolution is not exclusively a term of biology...
sequence, from
elliptical galaxiesAn 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 flattened and in size from hundreds of millions to over one trillion stars....
through
lenticularsA 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 disc galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
to either
barredA barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately half of all spiral galaxies...
or
regular spiralsA spiral galaxy is a galaxy belonging to one of the three main classes 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...
. 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
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
s and regions of active
star formationStar 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
UniverseThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
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 orientation 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
wavelengthIn 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 astronomyExtragalactic 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....
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 Hubble
Edwin Powell Hubble was an American astronomer. He profoundly changed our understanding of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the...
- Gérard de Vaucouleurs
Gérard Henri de Vaucouleurs was a French astronomer.-Biography:Born in Paris, he had an early interest in amateur astronomy and received his undergraduate degree in 1939 at the Sorbonne in that city...
- Galaxy color-magnitude diagram
The 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