Star of the Month
T Tauri
Position: 04 hrs 21 min 59.4 sec 19 degrees 32 min 06 sec
Due south at 22:45 (GMT) on 15 December 2020
T Tauri and Hind’s Variable Nebula (NGC 1555)
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
As its name implies, T Tauri is a variable star in the constellation Taurus. It is an erratic variable with a magnitude which varies between 9.3 and 14, hence it will be visible in a small telescope when near maximum, but invisible when near minimum. This variable is the archetype of the T Tauri variables which are currently attracting much scientific attention, partly because of the use of the Atacama Large Millimeter Array for such studies as many of them have protoplanetary discs. T Tauri is a very young star, only a million years old (in contrast to Fomalhaut’s 440 million years), and it has started to fuse hydrogen, which means that matter is no longer falling onto the star and the material round the star is being blown away by a powerful stellar wind (the so-called T Tauri wind). This means that T Tauri, unlike many T Tauri stars, does not have a protoplanetary disc any longer. Furthermore, it has been expelled from a system of three stars of which it was a former member. T Tauri lies between Aldebaran and the Pleiades in the Hyades cluster, although it is not part of the cluster since it is 471 light years away while the Hyades are only 153 light years distant. More precisely, it lies between Epsilon Tauri (Ain) and Omega Tauri, flanked by two tenth magnitude stars and a brighter eighth magnitude star to its south-west.
T Tauri is associated with three nebulosities. The most obvious one is NGC 1555, which was discovered by John Russell Hind in October 1852. This reflection nebula has been known as Hind’s variable nebula since the late nineteenth century and may contain a very young stellar object itself. Even closer to T Tauri is the Herbig-Haro object HH25, discovered by the double star observer Sherburne Wesley Burnham in 1890 and hence called Burnham’s nebula. It must not be confused with Burnham’s nebula in Cygnus (NGC 7026) which is a planetary nebula. HH 255 is produced by the shock wave created by the stellar winds from T Tauri. The third nebula is even more mysterious. Otto Wilhelm von Struve (the son of the double star observer Wilhelm Struve) discovered a nebula near T Tauri in 1868, which was confirmed by Heinrich d'Arrest and duly became NGC 1554. But it has not been seen since and is known as Struve’s lost nebula. Burnham and Barnard were looking for the nebula when Burnham found HH255. It may have simply been a mistake, but the more recent example of McNeil’s nebula shows that such nebulae can be very variable.
