25 ORI (25 Orionis). Like Cassiopeia and
other famed prominent constellations, Orion has so many bright stars that several
fainter ones of considerable interest are neglected or ignored.
Such is the case for 25 Orionis. While sometimes referred to as
Psi-1 Ori, it's better known by Flamsteed's number 25. It's not
just interesting in its own right, but through its surroundings as
well. Fifth magnitude (4.95), at a distance of 1110 light years,
25 Ori is dimmed a quarter magnitude by interstellar dust. One of
the hotter blue hydrogen-fusing dwarfs, this class B (B1) star
glows with a temperature (that is not all that well determined) of
nearly 25,000 Kelvin. Allowance for a lot of ultraviolet light
yields a luminosity 10,500 times that of the Sun, a radius of 6 times solar, and a mass of
10.5 times that of the Sun, just over the limit at which it should
explode as a supernova.
Not just your ordinary hot B star, the rapid minimum equatorial
rotation of 310 kilometers per second (giving a rotation period of
just under a day) has through a still-mysterious mechanism turned
"25" into a "B-emission" ("Be") star, one that also radiates from
a circumstellar disk. Moreover, it is listed as a "shell star,"
one whose disk appears more or less on edge and thus thick, making
the rotation parameters actual figures, not just limits. Like all
class B dwarfs, the star is young, under 10 million years old.
While brilliant and notable, Be stars are so bright that we see a
lot of them. 25 Ori's special claim to notoriety is the flock of
very young "pre-dwarfs" that surround it, lower mass stars that are
still in the late stages of their formation, enough of them that
the group is named after the star as the "25 Orionis association."
Orion is filled with, indeed made of, such stellar "associations,"
groups of young stars that were born about the same time, but that
are gravitationally unbound and expanding from a common center.
They are usually recognized by their luminous hot stars, hence are
called "OB associations" even though they contain many lower mass
stars. They often come in hierarchies. The 25 Ori group is a
subassociation that seems to belong to the Orion OB1a association
(stars up and to the right of the Belt), which in turn is included
Orion OB1 association, which includes
other subassociations that contain the Belt, Sword, and the Orion
Nebula and its stars, as well as the wonderful multiple,
sigmaori-p.html">Sigma Orionis.
Written by Jim Kaler 12/14/07. Return to STARS.