R HYA (R Hydrae). The "R" is a giveaway, that we are dealing with a variable star (named by single or double Roman letters), and a prominent one at that, since "R" of any constellation starts the lettered discovery sequence. And R Hydrae lives up to its billing as one of the more extreme, and moreover, one that at its maximum brightness is well-visible to the naked eye in southern Hydra. Just follow the two bottom stars of Corvus, the Crow, to the east to Gamma Hydrae and then only a bit farther to R, which sadly spends most of its time well below naked-eye visibility, leaving you with a blank sky. But about once a year, it glows redly at you at mid-fifth magnitude, and on occasion even brighter. Then around six months later this class M (M7) red giant variable star hits bottom, hovering around 9th or 10th magnitude and requiring a small telescope to spot. R Hydrae is a classic "long-period variable, an "LPV," the class equally well-known as "Mira" variables after the prototype Mira (Omicron Ceti), but one with a remarkable past. While its standard variation takes it from 5th to 9th magnitude, it can get as bright as 4th (maybe even brighter) or as faint as 11th on a superposed, multi-year variation, while the class varies between M6 and M8.
R Hya Twenty-five hundred days in the life of the Mira variable R Hydrae shows it varying between visual magnitudes 5 and 9 over a 388-day period, the size of the variation itself subject to multi-year semi-periodic change. The Julian Date is a running number of days since January 1, 4713 BC of the Julian calendar. JD 2452856 is August 4, 2003, JD 2454732 September 22, 2008. The black dots are visual estimates, the green ones photometric data. On rare occasion the star may hit magnitude 4; catalogues suggest even magnitude 3. Courtesy of the American Association of Variable Star Observers, AAVSO, the data contributed by observers worldwide.
As such it is one cool star. Temperatures are difficult to derive for such stars as the outer envelopes are so distended and windy, but it probably hovers around 2800 Kelvin. That and an uncertain distance of 400 light years gives a luminosity of about 40,000 Suns, which with temperature yields a radius 800 times that of the Sun, or 3.4 Astronomical Units. Direct measure and angular size, though, gives half that, which suggests something to be wrong, which could be distance or temperature or both, but if nothing else that it's hard to tell where the surfaces of such distended stars even ARE. Moreover, Mira-type variables pulsate in size, so the radius depends on just when you make the observations or do the calculations. The mass is almost impossible to judge, but is probably around a couple times solar. Moving at 35 kilometers per second relative to the Sun, losing mass at a rate of 30 millionths of a solar mass per year, R Hydrae's wind hammers the local interstellar gas into a shock wave, one of the few such ever observed. The oddest thing about R Hydrae, though, is its change in period. The variation was discovered in 1662. In the early and mid 1700s, the period of pulsation was measured at 495 days. Between 1770 and 1950, it lengthened, and has now settled in at 385 days. Theoreticians suggest that around the year 1600 the star underwent an interior "thermal pulse." Deep inside the star is the old nuclear burning core, which is now a dead ball of carbon and oxygen. Surrounding it is a shell of helium, and around that is a special shell of hydrogen, all surrounded by the huge, inert hydrogen envelope. The helium and special hydrogen shells, both capable of nuclear fusion, turn on and off in sequence, the helium shell fusing to carbon and oxygen, the hydrogen shell to helium. As the latter burns, it adds helium to the helium shell, and when that is thick enough, it suddenly and explosively begins its fusion: the thermal pulse, sometimes also called a helium flash. The powerful event disrupts the whole star, and quite possibly changed the pulsation period. At a distance of 22 seconds of arc, twelfth magnitude R Hydrae B apparently watches the whole thing. Alas, it appears to be no more than yet another line-of-sight coincidence as the two are moving too quickly relative to one another compared to that expected if they actually orbited.
Written by Jim Kaler 4/02/10. Return to STARS.