PI-4 ORI (Pi-4 Orionis). A near-vertical string of six stars hangs
down from Taurus to the west of Orion. Representing Orion's upraised
lion's skin (or sometimes his sleeve), they are named from south to
north Pi-6 down to Pi-1 Orionis, and topped by Omicron-1 and
Omicron-2 Ori. The brightest of the "Pi"s is third magnitude Pi-3,
the faintest fifth magnitude Pi-1. In the middle lies just-barely-
fourth magnitude (3.69) Pi-4, which shines to us from a large
distance of 1250 light years. Like many stars, it is deceptive, as
it is not one star, but a pair, each of which is a very hot (21,800
Kelvin) class B (B2) star, the brighter one classed as a giant
(whose internal hydrogen fuel has run out), the fainter as a
subgiant (run out or close to it). The two are so close together
that they are not resolvable through the telescope. Instead, one
must rely on the spectrum, the array of
colors seen when the starlight is broken into its rainbow. Nestled
within the rainbow are dark absorptions, narrow cutouts of color
produced by various chemicals in the stellar gases. Each star
produces a unique set of absorptions, allowing us to tell
exactly what chemicals, and what kinds of stars, are present and
how fast they move. This particular pair swings around each other
every 9.5191 days. Little else is actually known about the system.
Allowing for considerable ultraviolet radiation, the total
luminosity is 27,000 times that of the Sun.
Assuming that the dominant "giant" is 1.5 times brighter than the
fainter "subgiant" (which would be roughly typical of these kinds
of stars), the respective luminosities are 16,200 and 10,800 times
solar, the radii 9 and 7 times solar. These are massive stars.
Depending on its exact state of evolution, the mass of the brighter
is between 10 and 11 times that of the Sun, while that of the
fainter is about 10 times solar. Given these values, the two must
orbit each other at a distance of only a quarter of an astronomical
unit, less than two-thirds Mercury's distance from the Sun. The
combination of temperature and luminosity, however, strongly
suggests that both may be hydrogen-fusing dwarfs rather than more
evolved stars, and are less than 20 million years old. The
brighter of them has a rotation velocity of at least 35 kilometers
per second, but probably much higher, as the orbital measures
suggest that the orbital and rotation poles may be nearly pointing
at Earth. The brighter also varies marginally (if at all) by only
0.003 magnitudes over a period of 0.62 days. Like many of Orion's
stars, Pi-4 is loosely connected to the others, and is a part of
the Orion OB1 association of hot young stars that include those of
the Belt and the central star of the Sword. The fates of these
stars are hard to know. They are both on the edge of becoming
massive white dwarfs,
blowing up as exploding supernovae, or each taking a
different route to the end. Or we could wind up with a double
white dwarf that would someday merge, again to produce a supernova. Thanks to Jeff Bryan,
who suggested this star.