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V PUP (V Puppis).Puppis, the Stern ofancientArgo (the Ship), is so filled withbright and fascinating stars and other celestial sights that it'shard to pick one over another. But among all of them, V Puppisstands out as the only naked-eye (so the observations seem to say)black hole, thus toppingthe black hole prototype,Cygnus X-1. That leading statement requires quite a lot of qualification. Bydefinition, you can't see the black hole itself. At the "surface"of such a beast (called the "event horizon"), the escape velocity(that required to leave and not come back) equals the speed oflight, and the body disappears from view. It's there, as is itsgravity; you just can't see it. But you CAN see the effect that ithas on its environment, in this case a massivedouble star, which is the actualnaked-eye object. Fourth magnitude (4.41) V Puppis is a doublestar 1165 light years away that consists of two hot blue class Bstars (a B1 dwarf coupled to a B3 subgiant) in tight mutual orbit,the orbital period only 1.4545 days. With an orbital tilt of just11 degrees to the line of sight, each of the two stars partiallyeclipses the other, resultingin strong variations in brightness of some six tenths of amagnitude, hence thevariablestar name of "V." (The letter "vee" does not stand for"variable," but for the fifth variable star found in theconstellation, where the first is "R." Astronomical nomenclaturecan be strange indeed.) Analysis of the light curve (magnitudeplotted against time) coupled with that of the velocities fromspectroscopy give masses of 14.9Suns for the hotter (28,200 Kelvin) B1 star and 7.8 Suns for thecooler (26,600) member.

Every 1.4545 days, each of the two blue class B stars of V Puppispartially eclipses the other, as seen in the top plot made in bluelight. The left-hand eclipse is that of the larger, hotter, moreluminous star by the smaller, while the right-hand one is that ofthe smaller by the larger. Neither star can completely hide theother. Moreover, the distortion of the two caused by tides androtation renders the binary continuously variable even when thereis no actual eclipse going on, as we see different projections ofthe distorted surfaces as the stars go around each other. Themiddle plot shows only slight variations in the amount of bluelight relative to yellow (visual) light, while the bottom one showsultraviolet-to-blue color changes. The bigger dip at bottom is theresult of the eclipse of the hotter star. The stars are so closetogether that the smaller and fainter of the two loses mass to thelarger. (From an article by J. Andersen, J. V. Clausen, A.Gimenez, and B. Nordstrom inAstronomy and Astrophysics,vol. 128, p. 17, 1983.)

The radii come in at 6.2 and 4.9 solar.Kepler'slaws then tell that the two are just 15 solar radii apart, only2.5 times the radius of the brighter of the two. The stars arenearly in contact with each other. They are so close that the"radii" don't mean much, as the stars will each be tidally (androtationally) distorted. The larger star raises such huge tides inthe smaller that it fills its zero-gravity surface (the "RocheLobe"), where the gravities of the two stars in a sense cancel eachother. The smaller thus loses mass to the larger and to the systemas a whole, as also told by spectroscopic signatures ofcircumbinary matter. The system is thus rather likeAlgol, except that in Algol's case, thelarger is losing to the smaller. Allowance for 0.2 magnitudes ofdimming by interstellar dust, distance, temperatures (which tell ofhuge amounts of ultraviolet light) give respective luminositiesmasses for V Pup A and B of 16,900 and 7940 solar, smaller radii of5.5 and 4.2 solar, and masses of 13 and 11 solar. Radii are indecent agreement with those derived from the orbit and eclipses, asis the mass for the primary companion, but the other mass is not,most likely the result of non-sphericity (which also compromisestemperature), a poorly known distance, and evolutionary changescaused by the mass transfer.

The most interesting property of this amazing system, though, isthat the timing of the eclipses is not constant. The moment ofcentral eclipse first comes earlier and then later with a period of5.47 years, showing that the whole binary is being pushed back andforth by another large mass, that is, the binary is itself inmutual orbit about a mysterious third party. The tilt of thelarger orbit is not known, so all we have is a lower limit to thethird party's mass, but it has to be greater than 10.4 solarmasses. Yet there is no light, nothing to see as would beexpected. The conclusion is that Number Three is a black hole, theleavings of a more massive star within the triple that exploded asasupernova. Kepler'slaws then give an orbital size of at least 10 Astronomical Units. Weak X-ray radiation (faintly reminiscent of that from Cygnus X-1)may be coming from matter lost by the binary being accreted by theblack hole. The fate of this amazing system is uncertain. Theprimary of the binary component will probably explode as yetanother supernova, its core collapsing into aneutron star, while theless massive secondary may become awhite dwarf. If the nextexplosion does not disrupt the system, future astronomers may seeV Puppis with all three leavings of stellar evolution within onesystem. (Binary data from J. Andersen et al. in "Astronomy andAstrophysics", vol. 128, p. 17, 1983; black hole discovery by (anddata from) S.-B. Qian, W.-P. Liao, and E. Fernandez Lajus in theAstrophysical Journal, vol. 687, p. 466, 2008.)

Written byJim Kaler 1/02/09. Return toSTARS.

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