The galaxy color–magnitude diagram shows the relationship between

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

(a measure of

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

) and

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, 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 System. ...

. 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 (statistics), mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, ...

distribution of red and blue galaxies as seen in 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 (plural, : analyses) is the process of breaking a complexity, complex topic or Substance theory, substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics a ...

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 four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...

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

spirals In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:Hertzsprung–Russell diagram for

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

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 change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

of the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...

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, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phas ...

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 has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not pr ...

. The

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

and the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

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