25 ORI (25 = Psi-1 Orionis). From the story of its name, fifth magnitude (4.95) 25 Orionis (Flamsteed's number 25 in Orion, the Hunter) might seem to be an "also ran" as it follows somewhat brighter and more northerly (by about a degree) Psi Ori across the sky ("25" also 4.5 degrees due south of much brighter Bellatrix, Gamma Ori). Bayer had given only one star the Greek letter "Psi" (next to last in the Greek alphabet), whereas Flamsteed split it apart, calling his 25 Orionis "Psi-1" (the western of the two) and his 30 Ori "Psi-2." (He did not actually number them. That was done by Halley and Newton from his positional measures.) Given the possible confusion, it may be best to drop the "Psi-1" designation, and stick with the Flamsteed number. But 25 Ori hardly takes a back seat to original Psi (30) Ori. This really hot (24,100 Kelvin but with a significant uncertainty) class B (B1) dwarf shines to us from a distance of 1040 light years (plus or minus a hefty 275), enough for it to suffer around 0.3 magnitudes of dimming by intervening interstellar dust. (It's also been called a B2 dwarf, which from the color of the star would drop the dimming to 0.2 magnitudes; we'll stick with the original.) That, the distance, and allowance for a large amount of ultraviolet light, give the star a luminosity of 9300 Suns, a radius of 5.5 solar, and from theory a mass of 10.5 times that of the Sun. The borderline mass places it just above the usual limit of 9 or 10 (depending on whom you ask) solar masses, above which stars will blow up as supernovae, though there is still a possibility that once it throws off its outer layers as a supergiant, 25 Orionis could become a rare neon white dwarf preciously near the white dwarf collapse limit of 1.4 Suns. The star's future will have to wait awhile, though, as it is young and recently formed. Two other items make 25 Ori stand out even more. First, it's a "Be" star (one with hydrogen emissions in its spectrum), which tells of a circumstellar disk. And a special one at that, the disk set pretty much in the line of sight, making 25 Ori into a "shell star." That's consistent with the very high projected equatorial rotation speed of 316 kilometers per second (the rotation axis set closely perpendicular to the line of sight), which with radius gives a rotation period of 0.9 days or even a bit less. No one yet knows the origin of he "Be" phenomenon, but it seems related to high rotation and vibrational mass loss. Of even greater significance is that 25 Ori is the heart of the "25 Orionis" cluster of hundreds of faint young stars. Belonging to the vast Orion OB1 {star_intro.html#assoc">association of hot young stars, 25 Ori was long thought to be part of the Ori OB1a subdivision that consists of the stars up and to the right of Orion's Belt+ and that is a mere 2 to 5 million years old. In fact, the 25 Ori group (which centers at a distance of 1075 light years, close to that measured for 25 Ori itself) seems to be an independent set that is even younger, just 10 to 100 million years, consistent with the youth of 25 itself.

Written by Jim Kaler 12/14/07; revised 2/22/13. Return to STARS.