219 million stars: Astronomers release most detailed catalog ever made of the visible Milky Way

The unaided human eye struggles to distinguish individual objects in this crowded region of the sky, but the 2.5-m mirror of the INT enabled the scientists to resolve and chart 219 million separate stars. The INT programme charted all the stars brighter than 20th magnitude -- or 1 million times fainter than can be seen with the human eye.

Using the catalogue, the scientists have put together an extraordinarily detailed map of the disk of the Galaxy that shows how the density of stars varies, giving them a new and vivid insight into the structure of this vast system of stars, gas and dust.

The image included here, a cut-out from a stellar density map mined directly from the released catalogue, illustrates the new view obtained. The Turner-like brush strokes of dust shadows would grace the wall of any art gallery. Maps like these also stand as useful tests of new-generation models for the Milky Way.

The production of the catalogue, IPHAS DR2 (the second release from the survey programme The INT Photometric H-alpha Survey of the Northern Galactic Plane or IPHAS), is an example of modern astronomy's exploitation of 'big data' -- it contains information on the 219 million detected objects, each of which is summarised in 99 attributes.

With this catalogue release, the team are offering the world community free access to measurements taken through two broad band filters capturing light at the red end of the visible spectrum, and in a narrowband capturing the brightest hydrogen emission line, H-alpha. The inclusion of H-alpha also enables exquisite imaging of the nebulae (glowing clouds of gas) found in greatest number within the disk of the Milky Way. The stellar density map illustrated here is derived from the longest (reddest) wavelength band in which the darkening effect of the dust is moderated in a way that brings out more of its structural detail, compared to maps built at shorter (bluer) wavelengths.