PI-1 UMA (Pi-1 Ursae Majoris, with a nod to 2 UMa.) In far western
Ursa Major, about as far as you can get from
the Big Dipper, seven degrees north of Omicron UMa and near the border with Camelopardalis, lies a small curve of three
stars, from north to south fifth magnitude (5.47) 2 UMa (a Flamsteed designation), sixth
magnitude (5.64) Pi-1 UMa, and fifth magnitude (4.60) Pi-2 UMa (which are NOT Flamsteed numbers).
Pi-1 and Pi-2 have been tagged with the name "Muscida" (the "muzzle"),
which more usually is attached to Omicron. They have nothing physical
to do with one another. 2 UMa is 152 (give or take 3) light years away,
Pi-1 a neighborly 46.8 light years (with an uncertainty just 0.3 ly),
and Pi-2 the most distant at 256 (plus or minus 6) light years. At first,
the prize would seem to go to Pi-2, an otherwise ordinary class K (K1)
helium-fusing giant, but one
with a massive orbiting planet about which little else is known and
that might even qualify as a brown
dwarf, a "star" with insufficient mass to fuse its core hydrogen
to helium. Surprisingly, the real winner is the star in the middle,
Pi-2 Ursae Majoris, which appears to be a much sought-after solar
clone, a star like 18 Scorpii or 9 Ceti that is very much like our own Sun even though its dwarf classification is slightly
different (G1.5 as opposed to the G2 Sun), its temperature of 5850
Kelvin about 70 K warmer, its mass three percent higher, and its radius
five percent less. The object of studying solar mass stars is to see
how the Sun develops from youth to old age and to test theory, that
is, we would like to know both in the short and long terms what is going
to happen to us. Pi-1 UMa is on the youthful side, the star roughly
500 million years old, and belongs to the Ursa Major cluster or, "moving group," of
about the same age (whose most prominent members are the middle five
stars of the Big Dipper). Of the star's parameters, the rotation is
among the more interesting. The projected equatorial rotation speed
of 10 kilometers per second gives a rotation period of under 5 days,
But such a star should be magnetically active with star spots and
related spectral characteristics. Periodic variations of a few
hundredths of a magnitude give a true rotation period of 4.79 days,
which then shows the star to be rotating with its axis perpendicular
to us. A decade of data also suggests a long-term activity variation
of a dozen years, perhaps one similar to the solar cycle. The rapid
rotation also produces X-ray
radiation 40 times that from the Sun and violent X-ray flaring. On
the other hand, the global magnetic field seems to be about the same
as that of the Sun. In addition, though we might expect a strong stellar
wind, it appears to flow at only about half the rate of the solar wind,
showing we have a long way to go in understanding early solar evolution.
What about 2 UMa, which seems to have been left behind? As a metallic-
line Am star it's interesting on its own. Such stars alter their surface
chemical compositions as a result of gravitational settling of some
elements and radiative lofting of others. This phenomenon can give rise
to confusion of the subclass depending on the ions used for
classification. While usually given as A2m, 2 UMa might be classed
anywhere from A3 to A7. From its distance of 152 light years and
temperature of 8050 Kelvin, it radiates at a rate of 11.1 Suns. Theory
then shows 2 UMa to be a young dwarf with a mass 1.8 Suns. While the
rotation velocity is not well constrained, it's low enough to be
consistent with the odd composition, as otherwise the atmospheric
gases would be stirred back to normal. (Data on Pi-1 UMa from
J.J.Bochanski et al., Information Bulletin on Variable Stars No, 5043,
Konkoly Observatory, 2001: B.E. Wood et al., Astrophys. J. Letters
781:L33, 2014; I.G.Ribas et al., Astrophys. J., 622, 680, 2005.)
Written byJim Kaler 10/21/16. Return to STARS.