The galaxy color–magnitude diagram shows the relationship between absolute magnitude (a measure of

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

) and

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar Sys ...

. A preliminary description of the three areas of this diagram was made in 2003 by Eric F. Bell et al. from the COMBO-17 survey that clarified the

bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode (i.e., more than one local peak of the distribution). These appear as distinct peaks (local maxima) in the probability density function, as shown ...

distribution of red and blue galaxies as seen in the analysis of

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

data and even in de Vaucouleurs's 1961

analyses Analysis (: analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (38 ...

of galaxy morphology.

Features

The diagram has three main features: the red sequence, the green valley, and the blue cloud. The red sequence includes most red galaxies, which are generally

elliptical galaxies An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Re ...

. The blue cloud includes most blue galaxies, which are generally

spirals In mathematics, a spiral is a curve which emanates from a point, moving further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects. Two-dimensional A two-dimension ...

. In between the two distributions is an underpopulated space known as the green valley which includes a number of red spirals. Like the comparable Hertzsprung–Russell diagram for

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s, galaxy properties are not necessarily completely determined by their location on the color–magnitude diagram. The diagram also shows considerable evolution through time. The red sequence earlier in the

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

of the

universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...

was more constant in color across magnitudes and the blue cloud was not as uniformly distributed but showed sequence progression. A Deep Look into a Dark Sky

New research suggests the green valley is actually composed of two different populations of galaxies: one of late-type galaxies, where star formation has been

quenched In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such a ...

due to their gas supplies being shut off followed by exhaustion of their gas reservoirs for several billion years, and another of early-type galaxies where both the gas supplies and gas reservoirs have been destroyed very quickly, likely because of mergers with other galaxies and/or the presence of an

active galactic nucleus An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such e ...

. The

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

and the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

are assumed to lie in the green valley due to their star formation slowing down as a result of both running out of gas.

References

{{DEFAULTSORT:Galaxy color-magnitude diagram Galaxies Diagrams

Features

See also

References