astronomers had hypothesized. He was the first (1922) to divide the bright diffuse nebulae of the Milky Way into reflection and emission nebulae, giving the proper astro-physical explanations for their different spectra. Three decades after his death, the hubble space telescope (HST) was named to honour him.
Hubble classification System for classifying galaxies according to their shape on photographs; it was introduced by Edwin hubble in 1925, with extensions and revisions by several later workers, including Allan sandage, Gerard de vaucouleurs, and Sidney Van den Bergh (1929-).
elliptical galaxies are denoted E with a numeral describing the apparent shape, from E0 for a circular image, through E5 for an image with an axial ratio of 2, to the most flattened ellipticals at E7. Ordinary (nonbarred) spiral galaxies are denoted S with a letter a,b,c,d to indicate a stage along the tuning-fork diagram. Sa galaxies have large and bright central bulges, with tightly wound arms. Sc galaxies have small and faint bulges, with prominent and loosely wound arms. Intermediate types Sab and Sbc can also be used. A parallel sequence of barred spirals includes SBa, SBb and SBc. irregular galaxies are denoted Irr or simply I. Originally two types, Irr I and Irr II, were used, but Irr II galaxies appear to be temporary results of gravitational collisions and not a distinct kind of galaxy. The extension to the system by de Vaucouleurs recognized inner ring-like or spiral-shaped structures, as well as external rings. The van den Bergh classification added the use of the spiral pattern as a rough indicator of galaxy luminosity.
While the Hubble classification was originally designed to be descriptive, and based on blue-light photographs of a particular exposure range, it has retained enormous utility because these designations correlate well with physically interesting galaxy properties. Stellar and gas content and star formation rate, for example, change systematically between Hubble types.
Hubble constant Parameter in the hubble law that relates the velocity of a galaxy to its redshift. In general relativity, it is the rate at which the scale size (R) of the Universe is changing with time. It is also the rate at which distances between all 'co-moving' objects - that is, those objects with no individual motion relative to the fabric of space - are increasing. The units for the Hubble constant are such that its inverse is a time, which can very loosely be thought of as an 'age' for the Universe. More usually, however, the units employed are kilometres per second per megaparsec since, for small redshifts, galaxy recession velocities increase by this amount for each megaparsec of distance.
The Hubble constant is normally designated H0, the zero subscript specifying that it is the expansion rate at the present epoch that is meant. The relative expansion rate changes with time and the Hubble parameter and its rate of change are related to the deceleration parameter, q0, by Ho = R/R qo = R/(RHo2) both values being calculated for the present epoch.
The search for accurate values for H0 and q0 has been the driving force behind much of observational cosmology in the last 80 years. Many different methods of obtaining values for H0 have been devised. But they all have in common redshift and distance measurements of a set of similar objects carefully chosen to eliminate selection and measurement bias to the greatest extent possible. A key project for the Hubble Space Telescope was the task of deriving an accurate value for the Hubble constant. After many years of work, the best value of H0 available at present from the Space Telescope Key Project is 68 ± 6.0 km per second per megaparsec.
Hubble Deep Field (HDF) Long exposure of a small area of sky in the north of Ursa Major obtained by the hubble space telescope in 1995. Just 2'.5 across, the HDF is a composite of 342 exposures accumulated over 100 hours in four wavelength ranges in the visible, near-ultraviolet and near-infrared. In 1998 the Hubble Deep Field South (HDF-S) was imaged: a 2' sample of a region in the constellation Tucana. Both fields show over 2000 galaxies, including some of the most distant - and thus the youngest - ever imaged.
Hubble diagram Graph in which the apparent magnitude of galaxies is plotted against the redshift of their spectral lines. It is a straight line, demonstrating the linear relation between redshift and distance, as embodied in the hubble law.
Hubble law Law proposed by Edwin hubble in his landmark paper of 1929 claiming a linear relation between the distance of galaxies from us and their velocity of recession, deduced from the redshift in their spectra. The law can be stated as: v = H0d where v is the radial velocity of a galaxy, d is the distance to the galaxy and H0 is a constant now known as the hubble constant. This linear velocity-distance relation could be readily explained if the Universe as described by general relativity were expanding.
Hubble Space Telescope (HST) Telescope put into orbit by the Space Shuttle Discovery STS 31 in 1990 April. The Hubble Space Telescope (HST) is in an orbit inclined to the equator by 28°.5, which is almost circular at an altitude of about 607 km (380 mi). HST was
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