A delta is a landform where the mouth of a river flows into an ocean, sea, desert, estuary, lake or another river. It builds up sediment outwards into the flat area which the river's flow encounters (
as a deltaic deposit) transported by the water and set down as the currents slow. Deltaic deposits of larger, heavily-laden rivers are characterized by the main channel dividing amongst often substantial land masses into multiple streams known as distributaries. These divide and come together again to form a maze of active and inactive channels.
The prototype of classical Cepheid variables, which is a pulsating yellow supergiant. John Goodricke was the first in 1784 to discover its variability. The star shows a quick and sharp rise from minim
um to maximum, and slowly declines to its minimum again. The changes in brightness are accompanied by and principally caused by changes in stellar temperature and also by changes in radius. δ Cephei was actually the second Cepheid variable to be discovered. The first one, Eta Aquilae, had been discovered earlier the same year by Edward Pigott. δ Cephei varies with a period of 5.366341 days (or 5 days 8 hours 37.5 minutes) from magnitude 3.48, spectral type F5 Ib in its maximum to magnitude 4.37, spectral type G2 Ib in its minimum. It lies at a distance of 1,340 light-years.
A member of a class of pulsating stars with periods less than 0.3 days, spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the H-R
diagram, δ Scuti stars form a group which lies in an instability strip which includes the classical Cepheids at its brightest end and the pulsating white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.
A member of a class of pulsating stars with periods less than 0.3 days, spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the H-R
diagram, δ Scuti stars form a group which lies in an instability strip which includes the classical Cepheids at its brightest end and the pulsating white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.
A member of a class of pulsating stars with periods less than 0.3 days, spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the H-R
diagram, δ Scuti stars form a group which lies in an instability strip which includes the classical Cepheids at its brightest end and the pulsating white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.