MIRA, mi'ra or mem or STELLA MIRA. (the wonderful star), in astronomy, the star Omicron Ceti, a remarkable variable, situated in the neck of ((the whale." Its variability wail discovered in 1596 by Fabricus. The period is somewhat irregular, but averages about 331 days. During the greater part of this time the star remains of about the 9th magnitude, but during about 100 days it rises to a maximum which may vary from the 2d to the 5th magni tude, remains for a week or 10 days there, and then sinks to its minimum again. When shin ing with a brightness of the 2d magnitude, it is giving out more than 600 times as much light as when at the 9th magnitude. Its greatest recorded variation occurred between the years 1779 and 1783, when, after attaining a bright ness equal to that of the first magnitude star, Aldebaran, it sunk so low that it was invisible, even in telescopes showing stars of the 10th magnitude. The period of its changes is also
irregular, sometimes varying to the extent of two months. Thus, neither the times of great est brightness nor the amount of the brighten ing can be accurately predicted.
As to what occurs in this and similar long period variables to occasion so great an out burst of light at the approximately constant interval of about 11 months, we are ignorant. As the time of greatest brightness of Mira ap proaches the spectrum gives evidence of power ful disturbance, and the outburst is from the depths of the sun, the outlying strata of vapors remaining comparatively cool. A surging of the material of the ball of the sun is thus suggested, and this so widespread as to amount to a real bodily distortion rather than to a mere tide, but the cause of this disturb ance we do not know.