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Glossary H

Half Life
The half-life of a radioactive isotope
is the time it takes for half of an isotope to decay into another
element. It is a measure of the stability of an isotope; the shorter
the half life, the less stable the atom. [more]
Hertzsprung-Russell diagram
The H-R (Hertzsprung-Russell) diagram
is a diagram used in stellar astronomy to plot the properties
of stars. There are two equivalent forms. One is the observer’s
form which plots the color of the star on one axis and the absolute
magnitude on the other axis. [more]
Holography
Holography (from the Greek, holos whole
+ graphe writing) is the science of producing holograms, an advanced
form of photography that allows an image to be recorded in three
dimensions. The difference between holography and photography
is that photography only captures light intensity whereas holography
record the phase of the light. [more][Holoworld]
Hubble Constant
Hubble’s law is the statement in astronomy
that galaxies move away from each other, and that the velocity
with which they recede is proportional to their distance. It leads
to the picture of an expanding universe and, by extrapolating
back in time, to the Big Bang theory. The law was first formulated
by Edwin Hubble in 1929. Hubble compared the distances to nearby
galaxies to their redshift, found a linear relationship, and interpreted
the redshift as caused by the receding velocity. His estimate
of the proportionality constant, now known as Hubble’s constant,
was however off by a factor of about 10. [more]
Hysteresis
The term that means, literally, to be
late. It describes systems that do not directly follow the forces
applied to them, but react slowly, or don’t return completely
to their original state. For instance if you push on a piece of
putty it will assume a new shape, and when you remove your hand
it will not return to its original shape, or at least not entirely.
The term is actually used almost entirely to describe an effect
seen in magnetism, specifically in ferromagnetic materials. [more]
[Hyperphysics]
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