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The Rich Colors of a Cosmic Seagull

September 26th 2012

Seagull Nebula, Sh 2-292 from ESO
Part of the Seagull Nebula, Sh 2-292 (credit: ESO)

Nebulae are among the most visually impressive objects in the night sky. They are interstellar clouds of dust, molecules, hydrogen, helium, and other ionised gases where new stars are being born. Although they come in different shapes and colours, many share a common characteristic: when observed for the first time, their odd and evocative shapes trigger astronomers’ imaginations and lead to curious names. This dramatic region of star formation, which has acquired the nickname of the Seagull Nebula, is no exception.

This new image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile shows the head part of the Seagull Nebula. It is just one part of the larger nebula known more formally as IC 2177, which spreads its wings with a span of over 100 light-years and resembles a seagull in flight. This cloud of gas and dust is located about 3700 light-years away from Earth. The entire bird shows up best in wide-field images.

This object has received many other names through the years; it is also known as Sh 2-292, RCW 2, and Gum 1. The name Sh 2-292 means that the object is #292 in the second Sharpless catalogue of HII regions, published in 1959. The RCW number refers to the catalogue compiled by Rodgers, Campbell, and Whiteoak and published in 1960. This object was also the first in an earlier list of southern nebulae compiled by Colin Gum, and published in 1955.

The Seagull Nebula lies just on the border between the constellations of Monoceros (The Unicorn) and Canis Major (The Great Dog) and is close to Sirius, the brightest star in the night sky. The nebula lies more than four hundred times further away than the famous star.

The complex of gas and dust that forms the head of the seagull glows brightly in the sky due to the strong ultraviolet radiation coming mostly from one brilliant young star -- HD 53367 -- that can be spotted in the centre of the image and could be taken to be the seagull’s eye. HD 53367's mass is twenty times that of our Sun. It is classified as a Be star, which are a type of B star with prominent hydrogen emission lines in its spectrum. This star has a five solar mass companion in a highly elliptical orbit.

The radiation from the young stars causes the surrounding hydrogen gas to glow with a rich red colour and become an HII region. HII regions are so named as they consist of ionised hydrogen (H) in which the electrons are no longer bound to protons. HI is the term used for un-ionised, or neutral, hydrogen. The red glow from HII regions occurs because the protons and electrons recombine and in the process emit energy at certain well-defined wavelengths (colours). One such prominent transition (called hydrogen alpha, or H-alpha) leads to the strong red colour. Light from the hot blue-white stars is also scattered off the tiny dust particles in the nebula to create a contrasting blue haze in some parts of the picture.

Although a small bright clump in the Seagull Nebula complex was observed for the first time by the German-British astronomer Sir William Herschel back in 1785, the part shown here had to await photographic discovery about a century later.

By chance this nebula lies close in the sky to the Thor’s Helmet Nebula (NGC 2359), which was the winner of ESO’s recent Choose what the VLT Observes contest. This nebula, with its distinctive shape and unusual name, was picked as the first ever object selected by members of the public to be observed by ESO’s Very Large Telescope. These observations are going to be part of the celebrations on the day of ESO’s 50th anniversary, 5 October 2012. The observations will be streamed live from the VLT on Paranal.

This article is adapted from EurekAlert.


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