Helio-Eclipitic Radial Tangential Normalcoordinate system. Typically centered at a spacecraft. The X axis (radial)is set as the primary axis, and is defined as the axis pointing from thespacecraft to
the Sun. The Z axis (tangential) is set as the secondary axis,and is defined as that portion of the ecliptic rotational axis which isperpendicular to the primary axis. The Y axis (Normal) is defined as Z crossX.
A region of the Hertzsprung-Russell diagram, between the main sequence and the giant branch, occupied by very few stars. It corresponds to a very short period in stellar evolution.
A display of stellar properties using a plot of effective temperature (or instead color or spectral type) along the abscissa versus luminosity (or absolute magnitude). The temperature is plotted in th
e inverse direction, with high temperatures on the left and low temperatures on the right. On the diagram the majority of stars are concentrated in a diagonal strip running from upper left to lower right, i.e. from high temperature-high luminosity massive stars to low temperature-low luminosity low-mass stars. This feature is known as the main sequence. This is the locus of stars burning hydrogen in their cores (proton-proton chain). The lower edge of this strip, known as the zero age main sequence (ZAMS), designates the positions where stars of different mass first begin to burn hydrogen in their cores. Well below the main sequence there is a group of stars that, despite being very hot, are so small that their luminosity is very small as a consequence. These are the class of white dwarfs. These objects represent old and very evolved stars that have shed their outer layers to reveal a very small but extremely hot inner core. They are no longer generating energy but are merely emitting light as they cool (white dwarf cooling track). Stars with high luminosities but relatively low temperatures occupy a wide region above the main sequence. The majority of them have used up all the hydrogen in their cores and have expanded and cooled as a result of internal readjustment. Called red giants, they are still burning helium in their cores (helium burning, carbon burning). There are also stars with very high luminosities, resulting from their enormous outputs of energy, because they are burning their fuel at a prodigious rate. These are the supergiants. They can be hot or cool, hence blue or red in color.