The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: ellipticals (E), spirals (S), barred spirals (SB), and irregulars (Ir). Later on a sep
arate class of lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies.
The Hubble parameter for the present epoch. It is the constant of proportionality between the recession velocities of galaxies and their distances from each other. The latest determinations using the
Hubble Space Telescope observations of Cepheids give H_0 = 72 ± 8 km s^-1 Mpc^-1, the WMAP observations yield 70.4 ± 1.3 km s^-1 Mpc^-1, and the Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1. More recently, the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy. The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.
The Hubble parameter for the present epoch. It is the constant of proportionality between the recession velocities of galaxies and their distances from each other. The latest determinations using the
Hubble Space Telescope observations of Cepheids give H_0 = 72 ± 8 km s^-1 Mpc^-1, the WMAP observations yield 70.4 ± 1.3 km s^-1 Mpc^-1, and the Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1. More recently, the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy. The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.
The Hubble parameter for the present epoch. It is the constant of proportionality between the recession velocities of galaxies and their distances from each other. The latest determinations using the
Hubble Space Telescope observations of Cepheids give H_0 = 72 ± 8 km s^-1 Mpc^-1, the WMAP observations yield 70.4 ± 1.3 km s^-1 Mpc^-1, and the Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1. More recently, the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy. The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.
The speed with which a galaxy cluster recedes from us is directly proportional to its distance. It can be stated as v = H_{0}d, where v is the recessional velocity, H_0 the Hubble-Lemaitre constant, a
nd d the distance.
A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that w
orked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.
A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that w
orked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.
A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that w
orked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.