ETA CRU (Eta Crucis). Among the most striking figures of the sky, sadly lost to much of the world's population, is Crux, the Southern Cross, which is really accessible only south of the Tropic of Cancer. But from the southern hemisphere, the constellation appears brilliant as it leads Beta and Alpha Centauri (Hadar and Rigil Kentaurus) across the sky with the southernmost portion of the spectacular Milky Way stretched between them. Like the northerner's Big Dipper, Crux is circumpolar, or nearly so, from modest temperate latitudes towards the poles, and is accessible throughout the year. Looking like a kite with the luminary Acrux at its southern point, Crux even has something of a fainter "tail" that consists of Zeta, Eta, and Theta (1 and 2) Cru, along with (Roman) j and Lambda over in neighboring Centaurus. The southern-most and third brightest of the tail (after Lambda Cen and Zeta Cru) is fourth magnitude (4.15) Eta Crucis. Such patterns, though, while fine aids to memory, hardly represent real relations. Zeta Cru is a hot class B2.5 star 358 light years away, while next- door Eta Cru is a class F (F2) "giant" (but see below) that is not only the coolest of the lot, but lies at a distance of only 64 light years, making it also the closest. Compared to the raft of class B stars in the area (that includes Zeta and Theta-2 Cru as well as Lambda Cen), Eta is downright "solar" in nature. With a well-measured temperature of 6950 Kelvin (which means nearly all the radiation is in the visual spectrum), Eta Cru shines at a rate of just 6.5 times that of the Sun (compared with Zeta's 1950 Suns), which gives the star a radius of 1.3 times solar. The projected equatorial rotation speed is problematic, two values consistently giving 63 kilometers per second, one a mere 7 km/s. Since rotation rates for warm class F stars tend to be modestly high, we adopt the 63 value, which gives a rotation period under 1.4 days. Theory then brings the mass in at 1.55 times that of the Sun and clearly shows the star to be not a giant, but a dwarf that is middle-aged compared to its 2.6 billion year hydrogen-fusing lifetime (such discrepancies between dwarf and giant status among warmer classes not uncommon). Planets? None found. But Eta Cru does have a notable surrounding, infrared-radiating debris disk in which planets, or bodies of some sort, may well be embedded.
Eta Cru The dusty disk of Eta Crucis (red image denoted by the cross) is seen resolved in this composite infrared image. The rectangle is just four minutes of arc long. The green source marked by the triangle is an advanced giant carbon star called CL Crucis, which is so enmeshed in dust formed in a strong wind (very unlike the dust in the debris disk around Eta Cru, which is presumably caused by collisions among embedded bodies) that it is completely hidden from optical view. (From an article by C. A. Beichman, G. Bryden, K. R. Stapelfeldt, et al. in the Astrophysical Journal, vol. 652, p. 1674, 2006.)
Two recognized "companions," however, do hover more or less nearby. At a last-measured separation of 46 seconds of arc (now much higher) lies 12th magnitude Eta Cru B, while 36 seconds away is slightly brighter Eta Cru C. Both are moving at much too high a rate relative to Eta proper, and are clearly just line of sight affairs, not surprising given the density of stars within the surrounding Milky Way.
Written by Jim Kaler 6/10/11. Return to STARS.