NGC 7635, also known as the
Bubble Nebula,
Sharpless 162, or
Caldwell 11, is an
H II region[1]emission nebula in the constellation
Cassiopeia. It lies close to the direction of the
open clusterMessier 52. The "bubble" is created by the
stellar wind from a massive hot, 8.7
[1]magnitude young central
star, SAO 20575 (
BD+60°2522).
[7] The nebula is near a giant
molecular cloud which contains the expansion of the bubble nebula while itself being excited by the hot central star, causing it to glow.
[7] It was discovered in 1787 by
William Herschel.
[5] The star BD+60°2522 is thought to have a mass of about 44 M☉
Messier 52 or
M52, also known as
NGC 7654, is an
open cluster of
stars in the highly northern
constellation of
Cassiopeia. It was discovered by
Charles Messier on 1774.
[3][a] It can be seen from
Earth under
a good night sky with
binoculars. The brightness of the cluster is influenced by
extinction, which is stronger in the southern half.
[6] Its
metallicity is somewhat below that of the
Sun, and is estimated to be [Fe/H] = −0.05 ± 0.01.
[7]R. J. Trumplerclassified the cluster appearance as II2r, indicating a rich cluster with little central concentration and a medium range in the brightness of the stars.
[8] This was later revised to I2r, denoting a dense core.
[6] The cluster has a core radius of 2.97 ± 0.46 ly (0.91 ± 0.14 pc) and a
tidal radius of 42.7 ± 7.2 ly (13.1 ± 2.2 pc).
[4] It has an estimated age of 158.5 million years
[1] and a mass of 1,200
M☉.
[4]The magnitude 8.3 supergiant star BD +60°2532 is a probable member of the cluster,
[4] so too 18 candidate
slowly pulsating B stars, one being a
Delta (δ) Scuti variable, and three candidate
Gamma Doradus (γ Dor) variables.
[9] There may also be three
Be stars.
[10] The core of the cluster shows a lack of interstellar matter, which may be due to
supernovae explosion(s) early in the cluster's history.
[6] 
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