|
Hubble–Reynolds Law
The Hubble–Reynolds law models the surface brightness of elliptical galaxy, elliptical galaxies as :I(R) = \frac Where I(R) is the surface brightness at radius R, I_0 is the central brightness, and R_H is the radius at which the surface brightness is diminished by a factor of 1/4. It is asymptotically similar to the De Vaucouleurs' law which is a special case of the Sersic profile for elliptical galaxies. The law is named for the astronomers Edwin Hubble and John Reynolds (astronomer), John Henry Reynolds. It was first formulated by Reynolds in 1913 from his observations of galaxies (then still known as nebulae). It was later re-derived by Hubble in 1930 specifically in observations of elliptical galaxies. References Astrophysics Equations of astronomy {{astronomy-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hubble's Law
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum. Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model. The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is described by the equation , with ''H''0 the constant of proportionality—the Hubble constant—between the "proper distance" ''D'' to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation ''v'', i.e. the derivative of proper distance with respect ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Surface Brightness
In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on its surface luminosity density, i.e., its luminosity emitted per unit surface area. In visible and infrared astronomy, surface brightness is often quoted on a magnitude scale, in magnitudes per square arcsecond (MPSAS) in a particular filter band or photometric system. Measurement of the surface brightnesses of celestial objects is called surface photometry. General description The total magnitude is a measure of the brightness of an extended object such as a nebula, cluster, galaxy or comet. It can be obtained by summing up the luminosity over the area of the object. Alternatively, a photometer can be used by applying apertures or slits of different sizes of diameter. The background light is then subtracted from the measurement to obtain t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Elliptical Galaxy
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 Realm of the Nebulae'', with their intermediate scale disks, a subset of the "early-type" galaxy population. Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of globular clusters. Elliptical galaxies are believed to make up approximately 10–15% of galaxies in the Virgo Supercluster, and they are not the dominant type of galaxy in the universe overall. They are preferentially found close to the centers of galaxy clusters. Elliptical galaxies range in size from dwarf ellipticals with tens of millions of stars, to supergiants of over one hundred trillion stars that dominate their galaxy clusters. Original ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
De Vaucouleurs' Law
de Vaucouleurs's law, also known as the de Vaucouleurs profile or de Vaucouleurs model, describes how the surface brightness I of an elliptical galaxy varies as a function of apparent distance R from the center of the galaxy: : \ln I(R) = \ln I_ - k R^. By defining ''Re'' as the radius of the isophote containing half of the total luminosity of the galaxy, the half-light radius, de Vaucouleurs profile may be expressed as: : \ln I(R) = \ln I_ + 7.669 \left 1 - \left( \frac \right)^ \right or : I(R) = I_ e^ where ''Ie'' is the surface brightness at ''Re''. This can be confirmed by noting : \int^_0 I(r)2\pi r dr = \frac \int^_0 I(r)2\pi r dr . de Vaucouleurs model is a special case of Sersic's model, with a Sersic index of n=4. A number of (internal) density profiles that approximately reproduce de Vaucouleurs's law after projection onto the plane of the sky include Jaffe's model and Dehnen's model. The model is named after Gérard de Vaucouleurs Gérard Henri de Vauco ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Edwin Hubble
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually Galaxy, galaxies beyond the Milky Way. He used the strong direct period-luminosity relation, relationship between a classical Cepheid variable's luminosity and periodic function, pulsation period (discovered in 1908 by Henrietta Swan Leavitt) for scaling cosmic distance ladder, galactic and extragalactic distances. Hubble provided evidence that the recessional velocity of a galaxy increases with its distance from the Earth, a property now known as "Hubble's law", although it had been proposed two years earlier by Georges Lemaître. The Hubble law implies that the universe is expanding. A decade before, the American astronomer Vesto Slipher had provid ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
John Reynolds (astronomer)
John Henry Reynolds (1874–1949) was a British astronomer who served as the president of the Royal Astronomical Society between 1935 and 1937 and is known for his work on the classification of stellar bodies. An amateur, he was the son of Alfred John Reynolds, the Lord Mayor of Birmingham, who owned a company which cut nails. In 1899, at age 25, he was elected a Fellow of the Royal Astronomical Society and in 1907, he financed the construction of a 30-inch reflecting telescope in Helwan, Egypt, the first large telescope to study objects in the such southerly skies. He also hand-constructed a 28-inch telescope in Harborne. Images from the Reynolds telescope was later used by Gérard de Vaucouleurs in his system of classifying galaxies; Reynolds also published his own classification for spiral galaxies in 1920. Edwin Hubble frequently corresponded with Reynolds, and some of his findings into the classification of stellar bodies seems at least inspired by his work. The Hubble–Reyno ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Astrophysics
Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline said, Astrophysics "seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space–''what'' they are, rather than ''where'' they are." Among the subjects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Emissions from these objects are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, ''astrophysicists'' apply concepts and methods from many disciplines of physics, including classical mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
