Hind's variable nebula — おうしの反射星雲

NGC 1555 is a small reflection nebula in Taurus, about 460 light-years away, known as 'Hind's Variable Nebula' — one of the few nebulae in the sky that noticeably varies in brightness on timescales of months. It is illuminated by T Tauri, a young, still-contracting sun-like star that is the prototype of the entire T Tauri class of pre-main-sequence stars. Because T Tauri itself is variable (due to accretion from its protoplanetary disk), and because dust clouds between the star and the nebula periodically occlude its light, the reflected brightness of NGC 1555 rises and falls — sometimes dramatically over months or years. It is a living, slowly-pulsing object.

An observational curiosity rather than a showpiece. At its brightest the nebula reaches about magnitude 9, visible in a 6-inch telescope as a small faint glow immediately west of T Tauri itself. At its faintest — which has happened for years at a stretch — it is invisible even in large amateur scopes. Always check recent variability reports before observing. The best approach is to identify T Tauri (magnitude 10-13, itself highly variable) and look for any nebulous glow to its immediate west-southwest. A 10-inch or larger telescope helps. Best observed October through February.

Discovered by English astronomer John Russell Hind on October 11, 1852, using an 7-inch refractor at the private observatory of George Bishop in London. Hind noted both the nebula and the variable star at its center. Within a decade the nebula had faded almost to invisibility and remained so for much of the late 19th and early 20th centuries. It brightened again in the mid-20th century and has continued its slow pulsation. T Tauri itself was studied extensively in the 20th century and gave its name to the entire class of young pre-main-sequence stars.