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.