5 SER (5 Serpentis). Near the southwest corner of Serpens Caput (the Serpent's Head, the constellation divided in two by Ophiuchus) lies seemingly anonymous fifth magnitude (5.09) 5 Serpentis (as numbered west to east in Flamsteed's catalogue). It's best known as that star next to the great globular cluster Messier 5, the two just a third of a degree apart. A close examination of the star, however, almost reverses the order, making M5 the guide to 5 Ser! Somewhat similar to Procyon, 5 Serpentis lists as a class F (F8) giant- subgiant (but see below). At a distance of 92.8 light years (give or take 0.6) and a well-determined temperature of 6115 Kelvin, the star shines with the light of 5.0 Suns, from which devolves a radius of 2.0 times solar. Theory gives a mass of about 1.3 Suns, and shows the star to be a subgiant (or even old dwarf) about to embark on a career as a giant about 4 billion years old (spectral classes not always quite agreeing with physical natures). With a projected equatorial rotation speed of 4.8 kilometers per second, 5 Ser rotates in under 21 days (the true value depending on the star's axial tilt). Five Ser appears to have a debris disk, which implies planets, though none has been found. Were there residents, they would have to put up with the star's magnetic activity. Flares on the Sun caused by the collapse of magnetic fields are visible only because we see the localized regions where they occur. As powerful as they are, someone seeing the Sun as a point would be unable to sense any brightening. Five Ser, however, along with a handful of others that include Omicron Aquilae and Pi-1 Ursae Majoris, blast violent superflares. Five Ser has been observed to flare on three separate occasions. One in 1979-80 brightened the star visually by up to nine percent, the event lasting for as long as 25 days. Such energetic outbursts could not be good for any exposed life. Irregular variations suggest starspots going in and out of the field of view. While flaring is common among red dwarfs (such as Proxima Centauri), this kind of superflaring is highly unusual in solar type stars. The phenomenon makes us look a bit more suspiciously at our own Sun. A tenth magnitude neighbor 11.5 seconds of arc away watches the action. Maintaining a similar separation over the past 200 years, it is doubtless a real companion. At least 290 Astronomical Units from 5 Ser A, from its brightness 5 Ser B is probably a K7 dwarf with a mass of about 0.6 Suns that takes more than 3600 years to make an orbit. Much farther away, faint 5 Ser C and D are just in the line of sight. But there's more! Clipping along at 0.63 seconds of arc per year, 5 Ser falls into the half second of arc" club. Its velocity across the line of sight (relative to us) of 76 kilometers per second coupled to a line of sight velocity of 54 km/s gives a full speed of 94 km/s, 6 times normal. Yet though the star must be a visitor to the neighborhood, the iron abundance is closely as solar. We can't leave without looking again at the magnificent cluster. Some 27,000 light years away, with perhaps a million stars, M5 is significantly exceeded only by 47 Tucanae and Omega Centauri, both of which are in the southern celestial hemisphere.

Written byJim Kaler 7/04/14. Return to STARS.