# Physics 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.

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.

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.

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.

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.

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