HYDOR (Lambda Aquarii). Some stars are known for their brilliance, others for the odd qualities, some even for their ordinariness, yet others for location. Hydor (Lambda Aquarii), of Aquarius (the Water Bearer), highlights the last of these traits, and carries an unusual attribute at the same time. At the bend of a stream that falls down from Aquarius's Water Jar (appropriate to the name, which means simply "water"), Hydor is an "ecliptic star" that sits practically astride the ecliptic, less than half a degree from the apparent solar path. Of stars of its fourth magnitude (3.74) brightness or brighter, it is in league with (in order) 1 Geminorum, Wasat (Delta Gem), Delta Cancri, Regulus (Alpha Leonis), Rho Leonis, Zubenelgenubi (Alpha Librae), and Omega-1 Scorpii. As such, it is regularly occulted (passed over) by the Moon. From the time it takes the star to disappear in back of the orbiting Moon, we can determine the angular diameter, which comes out to be 0.0082 seconds of arc (3600 seconds in a degree). From the measured distance of 370 light years, we then find a true diameter of 100 times that of the Sun, perfectly consistent with its status as a somewhat unusual class M (M2.5) red giant. The star is thus large enough for interferometer measure (wherein we derive angular diameter from the interference of light waves from the different parts of the stellar surface), which yields 0.0089 seconds, the two giving an average of 104 solar radii (about half an Astronomical Unit). The radius can also be found from the temperature of 3555 Kelvin and the luminosity of 2210 times that of the Sun (most of which comes out in the infrared), the result 123 solar, not bad agreement given the inevitable measurement errors. Temperature and luminosity tell of a 3 solar mass star that is on its last legs, and is brightening as a giant star for the second time with a core that has already burned its helium into carbon and oxygen (the first giant brightening is with a dead helium core). Though we see many such stars as a result of their large luminosities, they are actually -- on a per-volume-of-space basis, quite rare. Consistent its status, Lambda Aqr is also variable. Classed as an "irregular giant" (technically, an "Lb" star), it changes by about a tenth of a magnitude, from 3.70 to 3.80, with no particular apparent rhythm. When such stars are observed long enough, we often find that they do indeed display some regularity. No one has concentrated on Lambda long enough to find out, however. Having begun life as a blue-white class B hydrogen-fusing dwarf 440 million years ago, the star will soon lose most of its outer layers, and expire as a tiny white dwarf about the size of Earth. (Thanks to Diana K. Rosenberg, who pointed out the proper name.)
Written by Jim Kaler. Return to STARS.