WR 25

WR 25 (HD 93162) is a binary star system in the turbulent star forming region Carina Nebula, about 7,500 light-years from Earth. It contains a Wolf-Rayet star and a hot luminous companion, and is a member of the Trumpler 16 cluster.

WR 25 was recognised as a Wolf Rayet star in the 19th century, because of its brightness and spectrum dominated by broad emission lines. The spectrum contains lines of hydrogen and is intermediate between a classical WN star and an O-type supergiant. This led to early reports that it was a binary, for example a WN7 star plus an O7 star. It has also been described as WN7 + abs and WN6ha. With the introduction of specific classifications for hot slash stars, WR 25 was assigned the spectral type O2.5If^*/WN6. This recognises the presence of nitrogen, the intrinsic weakness of many emission lines, and the presence of some helium and hydrogen absorption lines. The classification represents a fine gradation of weaker emission and stronger absorption than a WN6ha spectral type. Any contribution to the spectrum from the companion cannot be clearly detected.

The primary star of the WR 25 system is notable for being the most luminous known star in the Milky Way Galaxy, substantially brighter than its nearby neighbor Eta Carinae, although it is unclear what contribution is from the companion. It is approximately 6.3 million times brighter than the Sun and illuminates the far southern end of the Trumpler 16 cluster. The model used to derive the stellar parameters is unsuitable for use in binary systems with the authors noting that the companion contributes more than 15% of the system luminosity, so the luminosity is highly uncertain. Earlier estimates based on measurements of the ionising flux produced values around 1.5 million times the sun, with correspondingly lower estimates for other physical data.

The companion is assumed to be a young hot massive star, similar to other known WR+O or WR+WR binaries. It has been reported as an O4 supergiant, but later measurements are still uncertain about the exact spectral type. Colliding stellar winds between two such hot luminous stars produce the hard X-rays that led to suspicion about the binary status long before the 208-day orbital period was detected.

Although very luminous WR 25 is beyond naked-eye visibility due to heavy dust extinction of clouds in the nebula, and because much of the emitted radiation is in the ultraviolet. It has been observed in X-rays and infra-red.

WR 25 lies at the western limit of the Trumpler 16 star cluster, part of Carina OB1, one of the largest stellar associations in the Milky Way Galaxy. Because of its extreme luminosity it greatly affects its stellar environment, seen in the thin long arcs and filaments moving away from the star, including the Finger Nebula.