Based on a recessional velocity of 5075 km/sec, IC 5100 is about 235 million light years away, in good agreement with redshift-independent distance estimates of 220 to 315 million light years. Given that and its 1.9 by 0.4 arcmin apparent size, it is about 130 thousand light years across. Since IC 5100 and 5101 may be at the same distance (given the uncertainties of any distance estimate), they may be as little as 400 thousand light years apart, in which case they would almost certainly be a gravitationally bound pair.

Based on a recessional velocity of 5140 km/sec, IC 5101 is about 240 million light years away. Given that and its 1.3 by 0.9 arcmin apparent size, it is about 90 thousand light years across. Since IC 5100 and 5101 may be at the same distance (given the uncertainties of any distance estimate), they may be as little as 400 thousand light years apart, in which case they would almost certainly be a gravitationally bound pair.

Sometimes mislisted as NGC 7096A

Per Corwin, this may be NGC 7102; but it is listed as "not found" by Steinicke, and although Corwin's arguments (to be posted in the next iteration of this page) are reasonable, it is not at all certain that they are correct. Still, as a result of those arguments, many references treat the two listings as identical.

Per Dreyer, IC 5146 is "pretty bright, very large, irregular figure, 9.5 magnitude star in middle". The listing is for an open cluster and emission nebula about 12 arcmin in diameter, lying about 4000 light years from Earth. (Some references state that IC 5146 refers only to the cluster, but Dreyer's description obviously refers to the nebula as well.) At that distance the Cocoon's apparent size corresponds to about 15 light years. The "cluster" actually consists of at least two compact groups surrounding massive bright stars, of which one is the bright central star referred to by Dreyer. That star, a spectral type B0 Main Sequence star, is primarily responsible for lighting up the nebula. The Cocoon is actually a "blister" on the front edge of a large molecular complex which has been exposed to our view by the ionization and outflow of gas heated by the B0V star. Most references suggest that this star is only about 100 thousand years old, but the two or three hundred hot bright young stars in the area have a range of ages averaging a million or so years, suggesting that several episodes of star formation took place in the region, continuing to the present day. The bluish regions are caused by the visible light of the star reflecting off grains of dust, while the pinkish regions are hydrogen emissions (primarily Hα) caused by the ultraviolet radiation of the star. Surrounding the bright nebula is the end of a dark (absorption) nebula, Barnard 168, which separates the emission nebula from the surrounding starry background, enhancing its dramatic appearance.