X PER (X Persei). And now for a star with a serious difference, "X" in Perseus, the Roman letter telling us that it is a variable star. It's also a double that falls into a very special class of "high mass X-ray binary" or "HMXB," which is a high mass star that has a neutron star companion, the latter the end product of a supernova. To the eye, X Per is already unusual as a hot class O (O9.5) star. Though at a very large distance of some 2700 light years, it is still visible to the naked eye, albeit at (at brightest, as it is variable) sixth (6.1) magnitude, falling at times to seventh (7.0). It is so far away that we really do not know the distance well: estimates range from 2300 to 4200 light years. So let's adopt 2700 from the uncertain parallax. Then there is uncertain dimming by interstellar dust in the Milky Way to contend with. Without it, the star would be between fourth and fifth magnitude. Then we need to deal with temperature, which is also uncertain, since X Per is a "B-emission (Be) star" (like Gamma Cas or Dschubba, Delta Sco, with bright hydrogen spectral emissions) that is sometimes surrounded by a bright disk of its own making, in part as a result of its incredible spin speed of as much as 360 km per second, 180 times that of the Sun. The best estimate for the star alone without the disk (when it fades to 7th magnitude)
The "historic light curve" of X Persei from 1888 to 1974 shows it fading twice from magnitude 6 (the scale on the left labelled "V" for "apparent visual magnitude") to 7 around the turn of the 20th century. It then more or less stabilized near its brighter state (that with the surrounding disk). The observations from 1910 to 1960 show such scatter that considerable activity could be hidden. The disk disappeared and the star faded again between 1989 and 1991. From an article in the Publications of the Astronomical Society of the Pacific by D. Mook, F. Boley, C. Foltz, and D. Westpfahl).
is 29,500 Kelvin, which would make it class B0, which then yields a luminosity of 34,000 times that of the Sun, a radius 6 times solar, a rotation period under a day, a mass of around 17 solar, and an age under 10 million years. Coincidentally presaged by its name, X Per is also a source of X-rays. Buried in the X-ray emission are sharp periodic pulses every 835 seconds. Analysis of slight shifts in the pulse train reveal an orbiting binary with a period of 250 days and a separation of roughly two Astronomical Units. The companion to the massive star is an "X-ray pulsar," in this case a neutron star with a relatively weak (for a neutron star) tilted magnetic field. Matter flowing onto it from the wind of the massive star is heated to produce X-rays. Affected by the wobbling magnetic field, mass infall leads to the pulses (X Per one of the longest-period X-ray pulsars, HMXBs, known). A neutron star is a ball of neutrons some 20 kilometers across with an amazing density of a 100 million metric tons per cubic centimeter, the density of nuclear matter itself, rather like taking the Earth and putting it into a sports stadium. It is what is left when the iron core of a massive star (which results from aeons of nuclear fusion) collapses and thereby explodes the rest of the star away in a grand supernova. To have developed already into a neutron star, the companion must once have been the more massive of the pair, perhaps as high as 25 solar. Tidal flow from the more massive star to the less massive probably cut it down well under 10, making the less massive the more massive (like a big Algol). The current massive star, the one we see as X Persei, will shortly (on an astronomical time scale) develop an iron core and explode as well. For unknown reasons supernova explosions are commonly off-center, which gives the resulting neutron star an intense kick. To have remained a binary, the first explosion must have been well-centered. If the second one is centered as well, we will wind up with a double neutron star; if it is not, the stars will free themselves of each other, and two more single neutron stars will wander the cosmos. An 11th magnitude red giant nearly half a minute of arc away is sometimes taken as a third companion to the system, but such an orbit is unlikely to have survived the devastating blast.

Note: The second Hipparcos reduction gives a smaller distance of 1400 light years give or take 300, which yields a mass for the hot star of 12 Suns. The true distance remains uncertain.
Written by Jim Kaler 10/08/04; amended 1/12/11. Return to STARS.