P Cygni is the name of a variable star in the constellation Cygnus. This star was first discovered in the year 1600, when it appeared as a nova -- a previously undetected star. P Cygni is a Wolf-Rayet Star -- a hot, massive star, with a very strong stellar wind. The peculiar shapes of lines in the spectrum of this star are due to the stellar wind, and are now known as P Cygni profiles when observed in other stars.

"Nova" Cygni 1600

The occurrence of "novae stellarum" -- new stars in the sky -- was well known (if not well understood) at the turn of the sixteenth century. Supernovae and classical novae had been known since antiquity, and the occurrence of Tycho Brahe's supernova of 1572 was still fresh in the minds of astronomers in the year 1600. On August 18 of that year, cartographer Willem Janszoon Blaeu, a former student of Brahe's, discovered a new star in the constellation Cygnus. He dutifully recorded its existence for posterity, but since this was prior to the existence of even the crudest telescopes, little more could be learned.

Johannes Kepler observed it again in 1602, and found it was still a third magnitude star. This is very atypical of novae; they were normally very bright for a short time, and then faded away to invisibility after a few months. "Nova" Cygni remained visible for more than twenty-five years when it finally faded beyond naked-eye view in 1626. It returned briefly in 1655, when it was observed by Giovanni Domenico Cassini, and again in 1665. At last, by 1677, P Cygni finally settled near magnitude 5.0, just at the edge of naked-eye visibility. It has stayed within a magnitude of this brightness for over 300 years.

P Cygni -- a Luminous Blue Variable

The years-long periods of naked-eye brightness and quiescence make it clear that P Cygni is not a classical nova, nor really a nova of any kind at all. P Cygni is actually a member of a very exclusive group of stars called luminous blue variables, whose name is fully descriptive of their behavior. These objects are extremely massive stars -- some more than 50 times the mass of the Sun -- whose lives are measured in a few million years rather than the billions we expect for stars like our Sun. Because they are massive stars, they are both luminous (because they are very large), and blue (because they are very, very hot). The variability comes from the inherent instability of these stars.

Many of these "LBVs" lie in a region of the Hertzsprung-Russell Diagram where stars are inherently unstable. When stars form, they accrete matter from nearby space. As they gain more and more mass, nuclear reactions in their cores become so rapid that the heat generated by the nuclear reactions will start blowing off the outer layers of the star, placing an effective limit on how massive such stars can become. Thus P Cygni is a member of a very elite club of supermassive stars like Eta Carinae, blessed to shine brightly, but doomed to die a very young death.

During their lives, however, they can also experience large outbursts, caused by the intense radiation pressure and vigorous convection inside the star. Most of the time, these stars pulsate very slightly, resulting in very small changes in brightness. On rare occasions, they can undergo great outbursts of activity, which blow off huge amounts of matter from the star. Such an outburst created the "Homunculus" nebula around Eta Carinae, and P Cygni was probably in the midst of one such outburst when Blaeuw first observed it 400 years ago. However, these outbursts aren't the only way these stars lose mass. The great heat of these stars also generates a strong stellar wind. For P Cygni, it is this wind which gave it it's greater fame

P Cygni line profiles

During the late nineteenth century, many astronomers were engaged in a great race to catalog the universe. The telescope, the spectrograph, and the photographic plate had all opened up the universe to intense and precise scrutiny by astronomers and physicists. Much of the work of the nineteenth century was to find and observe all there was to see in the cosmos. Stellar spectra were an early and rich area of research, beginning with the discovery of dark bands in the solar spectrum by Wollaston and Fraunhofer in the early 1800s. By the latter half of the century, many observatories around the world were dedicated to measuring the spectra of thousands and millions of stars. One of the more interesting finds among the stellar bestiary was the class of Wolf-Rayet stars, stars so hot and bright that they ejected a continuous stream of gas from their surfaces. This wind could be observed as strong emission lines in the spectra. P Cygni was a member of the Wolf-Rayet class, but even among this odd class of stars, it had a very strange spectrum.

The emission lines of P Cygni's spectrum had very curious profiles. Normally, emission and absorption lines have smooth profiles, nearly Gaussian in shape (as in Figure 1 below). But P Cygni was different -- its line profiles had strong emission peaks on the red sides of its lines, but had troughs on the blue sides, indicating absorption. What would cause this sort of behavior?

 i                        .|.
 n   Figure 1             .|.
 t                        .|.
 e                       . | .
 n                       . | .
 s                      .  |  .
 i                     .   |   .
 t                   .     |     .
 y..................       |       .....................

  blue                line center                 red

Imagine that P Cygni is surrounded by an expanding shell of gas, flowing radially outwards from the star. If you are looking at the star and the gas from a great distance, what would you see? Most prominent would be the star itself, and whatever emission and absorption lines come from the star's surface. But what about the gas? Because the gas is moving, light emitted by it will be Doppler shifted by an amount proportional to its speed along our line of sight -- gas moving directly towards us will have its lines blue shifted, and gas moving away red shifted. So what happens? The gas moving towards us emits some light, but it also acts as a strong light absorber. The gas moving towards us will absorb lots of light on the blue side of the line, causing an absorption trough in the spectrum. But light emitted by the star and by the gas moving away won't be absorbed, because its light is red shifted in the rest frame of the absorbing gas. So any light emitted in our direction by the gas moving away will pass through the shell and be detectable by us. The result is a line profile with a trough on the blue side, and a peak on the red (Figure 2).

 i                         | .
 n   Figure 2              |. .

 t                         |. .
 e                         |.  .   red emission
 n                         |.   .
 s                         .     .
 i                         .       .
 t                         .          .
 y...................     .|            .................
                     .    .|
                      .  . |
     blue absorption   . . |
                        .  |

                  stellar rest frame
                      line center

It was the Canadian astronomer Carlyle Beals who correctly explained this odd behavior in papers published from 1929 to 1934. Now, we know that these P Cygni profiles are a common feature in all bright, hot objects with strong winds. They are most commonly associated with hot stars, but are also observed in the expanding shells of novae and even in some quasars.

The star P Cygni is located at α 20h 17m 47.2s, δ +38° 01' 58.5'' (2000), in the constellation Cygnus. It has a spectral type of B2, and appears faintly bluish in larger binoculars or telescopes. Although it is one of the most luminous stars in the Milky Way it is several thousand light years away, making it barely visible to the naked eye. Like the rest of Cygnus, it is best observed in the evenings of later summer to early autumn in the northern hemisphere.

Beals, C.S., "On the nature of Wolf-Rayet emission," Monthy Notices of the Royal Astronomical Society 90, 202 (1929)
Beals, C.S., "P Cygni and radial ejection", The Observatory 57, 378 (1934)
Burnham, Robert Jr., Burnham's Celestial Handbook, volume 2, Dover Books: New York 1978
Frost, Edwin P., "On The Spectrum of P Cygni," Astrophysical Journal 35, 286 (1912)
Humphreys, R. and Davidson, K., "Studies of luminous stars in nearby galaxies," Astrophysical Journal 232, 409 (1979)