U 0RI (U Orionis). If you are looking for answers, "abandon hope, all ye who enter here." Perhaps it's not that bad. U Ori is a long period (or Mira Type) deep class M (M6.5) pulsating red giant variable (LPV). Don't confuse it with Upsilon Ori; "U" is a Roman letter variable star designation. Most of the time U Ori is out of sight as it reaches down to thirteenth magnitude (typical visual minimum is 12.6 but it can sink to 13.3) and a spectral class of M9.5, but for a brief time during its 372.4 day period, it can reach fifth magnitude (5.3, and at times as bright as 4.8). The star is way way north in Orion, in the Hunter's upraised arm and club, 12.8 degrees north of Betelgeuse, 0.3 degrees east and a bit south of Chi-1 Ori, and more in the territory of Taurus than Orion. Only three degrees south of the ecliptic, it's regularly occulted by the Moon.

U Ori U (not Upsilon) Orionis is a classic Mira (Long-Period) variable star that pulsates between fifth and thirteenth magnitude over a well-defined period of 372.4 days. Near Taurus, it is occasionally visible to the naked eye. Typically, the maxima and minima differ somewhat from one cycle to the next. Much of the optical variation is the result of fairly small changes in temperature. The variation in the infrared, where most of the star's energy is radiated, is much less. Courtesy of the American Association of Variable Star Observers (AAVSO).

Few of the star's properties seem definitive, including the amount of interstellar absorption. It's usually derived from how red the star appears relative to that found from its class, but U Ori appears too BLUE, perhaps from its surroundings shells. It's small in any case, and we'll adopt zero. LPVs are advanced giants that have run out of their core helium fuel, their carbon/oxygen cores now contracting and heating, with hydrogen and helium fusing in surrounding shells. As a result of the energy released, LPVs greatly expand and cool at their surfaces. The variation in the infrared, where most of the radiation is emitted, and thus of the luminosity, is much smaller than it is in the optical, where the variability is more the result of temperature changes as the star pulsates. Numerous measures of U Ori's angular diameter have been made with interferometers and by lunar occultations. It hovers around 0.015 seconds of arc, but depends sharply on wavelength or color, as the star really has no well-defined surface. To get the physical radius, you have to know the distance. The Hipparcos parallax gives 1425 light years, but the "error bar" is half that, so the formal distance could lie between 930 and 3000 light years! Assuming 1425 ly, the star has a (variable) radius close to 3.5 Astronomical Units. If in our solar system, it would swallow most of the asteroid belt and extend some two-thirds of the way to Jupiter, hundreds of times the solar diameter. A study corrected for the modern distance suggests that U Ori has a luminosity of some 10,000 times that of the Sun, with a typical "surface" temperature in the neighborhood of 2800 Kelvin.The mass might be around 1.5 Suns. U Ori is losing its outer envelope, and may eventually produce a planetary nebula that is ionized by its revealed hot core, which in turn will become a white dwarf. The star is now surrounded by its own complex dusty ejecta in an asymmetrical shroud that extends to several times the size of the star itself and that radiates water, hydroxyl (OH), and silicon masers (the maser the radio version of a laser). U Ori and its kind produce copious amounts of oxygen-based dust, which adds to the interstellar dust content that is crucial in the formation of the next stellar generation. (Thanks to S. Mondal and T. Chandrasekhar in the Astronomical Journal, vol. 348, p. 1332, 2004, to the British Astronomical Association, and to the American Association of Variable Star Observers.)

Written byJim Kaler 3/28/14. Return to STARS.