[right] – Clusters of stars and a fishhook-shaped cloud of luminescent gases glow brilliantly in NGC 2363, a giant star-forming region in the Magellanic galaxy NGC 2366. Even though the nebula is 10 million light-years away, the Hubble Space Telescope resolves details comparable to such nebulae in our own galaxy. This region is as bright as the gigantic 30 Doradus nebula in the Large Magellanic Cloud, a satellite galaxy to our Milky Way.
The brightest star visible on this image (at the tip of the fishhook) is a rare class called an erupting Luminous Blue Variable (LBV). This monstrous star (30 to 60 times as massive as the Sun) is in a very unstable, eruptive phase of its life. The Hubble images are the only ones in which the star can be clearly isolated from the rest of the cluster. Only four giant LBV eruptions have been recorded in history, the most famous being those of Eta Carinae (1837-1860) and P Cygni (1600), within our own galaxy.
The LBV was discovered in Hubble pictures taken in January 1996, by comparing these images with ground-based photos. An archival search of previous ground-based images showed the star grew 40 times brighter (21.5 to 17.8 magnitude) within three years – now making it the brightest star in its galaxy.
The Hubble image, taken with the Wide Field Planetary Camera-2 (WFPC2), also shows two dense clusters of massive stars, which are at two different phases of their evolution. Stellar "winds" and supernova blasts have blown the gas away from the oldest cluster (4-5 million years old) seen at the top of the image. This has created a cavity in the nebula. In contrast, the brightest cluster (bottom center) is probably less than 2 million years old and still embedded in remnants of gas and dust out of which it condensed.
Observations of galaxies like NGC 2366 will also help astronomers better understand why faint irregular galaxies can have such "firestorms" of starbirth activity, and what processes set the limit to the size of a star-forming region in a given galactic environment. One possibility is that gas streaming around the galaxy forms a bar-like pattern where gas piles up at the ends of the bars, causing a giant cloud to form.
[left] – A black and white (visual) image of the irregular galaxy NGC 2366 obtained at the Canada-France-Hawaii (CFHT) 3.6-meter telescope on Mauna Kea on February 7, 1996. The white square shows the location of Hubble's WFPC2 image.
Left Credit: Laurent Drissen, Jean-Rene Roy and Carmelle Robert (Department de Physique and Observatoire du mont Megantic, Universite Laval) and NASA
Right Credit: Laurent Drissen and Yvan Dutil/ CFHT and NASA