Origin of name and discoverer
The Butterfly Nebula, also known as IC 1318, the Gamma Cygni Nebula, and the Sadr Region, is a celestial object that lies in the heart of the Cygnus constellation. The nebula seems to be around the star Gamma Cygni, or Sadr (meaning “chest” in Arabic), at the center of the Swan (or Cygnus) constellation. It was on a night between 1892 and 1895, at the Lick Observatory, that the American astronomer Edward Emerson Barnard discovered it (or at least a part of it) during a photographic survey of the Milky Way Galaxy.
The Butterfly Nebula is a name that has been given to more than one celestial object. In fact, NGC 6302 (Scorpius), NGC 2346 (Monoceros), M2-9 (Ophiucus) are named the Butterfly Nebula as well, because they all resemble a butterfly, even though they do not look the same.
Position and dimensions
We find the Butterfly Nebula at the heart of the Swan, exactly at 20h 16m 48.0s right ascension and +41° 57′ 24″ declination.
IC 1318 is over 100 light-years across. Sadr is a bright supergiant that has an apparent magnitude of 2.23. It is 12 times more massive than the Sun, 150 times larger in radius and 33,000 times brighter. Although the Butterfly Nebula seems to wrap the star, they are unrelated. Sadr is about 1800 light-years away, while the nebula is at 4900 light-years distance. So the “Sadr region” name can be confusing.
IC 1318 is an emission nebula, as it receives radiation from the neighboring stars that heats and ionizes the cloud, that therefore itself reemits it as visible radiation.
Besides being an emission nebula, IC 1318 has a 20-light-year thick cloud that obscures the light coming from the stars behind, forming a large dark nebula and giving the whole nebula its butterfly shape. We can clearly see this dark lane in optical images such as this one and this one.
These are part of an enormous complex of star-forming regions and stellar clusters in the Swan constellation.
The Butterfly Nebula has three distinct regions, A (the part that Barnard discovered is his Milky Way survey), B, and C. As these optical images show, parts B and C are (optically but not physically) separated by the dark nebula. This image was taken using a ZWO Kamera ASI 1600 MM-Cool V3 Mono Camera, with Baader f/2 Highspeed-Filters (H-alpha, O-III, S-II), mounted on a Celestron RASA telescope. The filters used are appropriate to show ionized regions and to enhance the contrast between the dark nebula and the rest, for an exposure time greater than 5 hours.
The image also shows the young open cluster NGC 6910 that covers about 10 arcminutes of the sky, as it appears to us. Lying at about 0.5 degrees east-northeast of Sadr, scientists think that the cluster is physically related to the Butterfly nebula.
Besides, for 8 hours of exposure and with the use of an additional H-alpha filter, we took this image that shows more delicate details in the nebula. It also shows the H II emission region that extends over 100 light-years.
Infrared imagery allows us to see through the dark nebula and reveals large clouds of dust in the nebula where new stars are born.
The Sadr region presents a rich and large complex the plane of the Milky Way. This region holds a large number of nebulae and star clusters such as IC 1311, Collinder 419, Collinder 421, and NGC 6910. Stars from these clusters appear like gems on a delicate cloud. They are among the best gifts that the sky offers to us every night.