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P CYG (P Cygni) Buried in the Milky Way, in the heart ofCygnus, the Swan, just to the southwest ofSadr (which marks the core of the Northern Cross), lies fifthmagnitude (4.8) P Cygni, one the most distant stars you can seewith the naked eye, and surely one of the most remarkable. Thesimple name "P" is a holdover from Bayer's continuation of hisGreek-lettering system. After heran out of them, he used lower-case, then upper-case, Romanletters, stopping at the most extreme with Q. The Roman lettersare hardly ever seen anymore, lingering in "P Cygni," the cluster"h Persei," and a few others. From its rather astounding

The "luminous blue variable" P Cygni shines at center in a beautifulsetting in the middle of the thick Milky Way. Down and to the leftare stars heavily reddened by intervening interstellar dust, whichabsorbs and scatters blue starlight more efficiently than red. The bright "star"just up from the left center edge is the cluster Messier 29.

(and uncertain) distance of 6000 or more light years, P Cyg, a class B(B2) supergiant (if not "hypergiant), pours between 500,000 and900,000 times the power of theSun intospace from a blue-white surface of 19,000 Kelvin, making it one ofthe most luminous stars of the whole Galaxy. Dimmed a little bitby interstellar dust, if the view were clear, the star would appear4 times brighter. P Cygni belongs to an extremely rare group ofstars called "luminous blue variables." In the year 1600 it flaredto third magnitude, and still sometimes carries the name "NovaCygni 1600." (It is not a nova. True novae are caused by nucleareruptions on the surfaces of white dwarfs from hydrogen donated byclose companions). The eruption went on for six years, and thenthe star faded below naked-eye visibility, only to rise again forseveral years in 1654. Since settling in at 5th magnitude acentury later, it has slowly increased its brightness (with manysuperimposed variations) by about 15 percent per century, notintrinsically, but as a result of cooling by six percent percentury, which transfers progressively more of the star'sultraviolet light into the visible. Eruptions of luminous bluevariables are accompanied by the ejections of vast amounts ofmatter. The star is surrounded by a faint nebula that has beencreated over the past 900 years by the current eruptive mass loss,and by faint shells that tell of eruptions from 2400 and 20,000years ago. P Cyg is still losing mass at the astonishing rate ofover three hundred thousandths of a solar mass per year at a speedof 300 kilometers per second. While that might not sound likemuch, it is 300 million times the rate in the solar wind. (Technote: the wind induces odd features in P Cygni's spectrum,emission, or bright, lines that are flanked to the blue by darkabsorptions. Such features, known universally as "P Cygni lines,"granting permanent grand status to the star.) Highly evolved, andwith an huge mass of between 50 and 60 times that of the Sun, thestar will build an iron core, its only destiny someday to explodeas a great supernova, or even as a newly-recognized "hypernova,"one whose core may collapse into a black hole. When that mayhappen, however, is entirely unknown. Thanks to Luis Lopes, whosuggested this star.

Written byJim Kaler. Return toSTARS.

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