Free Astronomy Magazine January-February 2019

47 JANUARY-FEBRUARY 2019 SPACE CHRONICLES T he NASA/ESA Hubble Space Telescope has captured part of the wondrous Serpens Neb- ula, lit up by the star HBC 672. This young star casts a striking shadow— nicknamed the Bat Shadow — on the nebula behind it, revealing tell- tale signs of its otherwise invisible protoplanetary disc. The Serpens Nebula, located in the tail of the Serpent (Serpens Cauda) about 1300 light-years away, is a re- flection nebula that owes most of its sheen to the light emitted by stars like HBC 672 — a young star nestled in its dusty folds. In this image the NASA/ESA Hubble Space Telescope has exposed two vast cone-like shad- ows emanating from HBC 672. These colossal shadows on the Ser- pens Nebula are cast by the proto- planetary disc surrounding HBC 672. By clinging tightly to the star the disc creates an imposing shadow, much larger than the disc — approximately 200 times the di- ameter of our own Solar System. The disc’s shadow is similar to that produced by a cylindrical lamp shade. Light escapes from the top and bottom of the shade, but along its circumference, dark cones of shadow form. The disc itself is so small and far away from Earth that not even Hubble can detect it encircling its host star. However, the shadow fea- ture — nicknamed the Bat Shadow — reveals details of the disc’s shape and nature. The presence of a shadow implies that the disc is being viewed nearly edge-on. Whilst most of the shadow is com- pletely opaque, scientists can look for colour differences along its edges, where some light gets through. Using the shape and colour of the shadow, they can de- termine the size and composition of dust grains in the disc. The whole Serpens Nebula, of which this image shows only a tiny part, could host more of these shadow projections. The nebula en- velops hundreds of young stars, many of which could also be in the process of forming planets in a pro- toplanetary disc. Although shadow-casting discs are common around young stars, the combination of an edge-on view- ing angle and the surrounding nebula is rare. However, in an un- likely coincidence, a similar looking shadow phenomenon can be seen emanating from another young star, in the upper left of the image. These precious insights into proto- planetary discs around young stars allow astronomers to study our own past. The planetary system we live in once emerged from a similar protoplanetary disc when the Sun was only a few million years old. By studying these distant discs we get to uncover the formation and evo- lution of our own cosmic home. ! T his animation compares the ap- pearance of the Serpens Nebula as seen from the ground — with the Very Large Telescope of the European Southern Observatory, and from space — with the NASA/ESA Hubble Space Telescope. Both instruments used detectors and filters observing in the near-infrared to gather their data. [NASA, ESA/Hubble, ESO]