NGC 772 (also called Arp 78) is an unbarred spiral galaxy over 100 thousand light-years across (about the same size as our Milky Way Galaxy), located some 130 million light-years away in the constellation Aries. It is moving away from us at 2472 kilometers per second.
It has with a small, very bright, diffuse nucleus, probably a H II nucleus powered by young, massive stars.
Most notable is, however, a prominent elongated outer spiral arm, which has likely arisen due to tidal interactions with the nearby dwarf elliptical galaxy NGC 770 (seen toward the upper right). This unusual long arm shows lots of young blue star clusters. But NGC 772 also possesses many weak, tightly coiled arms which, although well formed, are relatively smooth, indicating only a small current rate of star formation. The relatively smooth multiple arms on the opposite side from the prominent arm are defined primarily by spiral dust lanes.
NGC 772 is surrounded by several satellite galaxies. Faint streams of material seem to connect NGC 772 with these nearby companions. The interacting galaxies NGC 772 and NGC 770 are together cataloged as nr. 78 in Arp’s Atlas of Peculiar Galaxies, a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.
Two supernovae (SN 2003hl and SN 2003iq) have been observed in NGC 772, both of them a Type II supernova.
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.
This image includes numerous faint galaxies in addition to NGC 772 and NGC 770.
Image Credit: Stephen Leshin