Many generations of high-mass stars energizing through lifecycles
of millions of years
, gradually fashioned the elaborate shining gaseous shells in Holmberg II: stars formed in dense regions, expelled stellar winds that blew away surrounding gas and material, shined for millions of years, then died in supernovae explosions that blasted heated gas through less dense regions, ScienceDaily reported
The result, according to NASA and the ESA
: An unusual patchwork appearance and intriguing unconventional features for an otherwise unremarkable dwarf irregular
galaxy that lacks spiral arms, found in the Milky Way and other spiral galaxies
in low-density regions of the universe, and also lacks the dense nucleus found in elliptical galaxies
within dense galaxy clusters. Because of the galaxy's lack of arms and nucleus, the many blasted-out, thousands of light-years-wide caverns became isolated gravitational havens within Holmberg II, where delicate galactic bubbles could hold their shapes.
Holmberg II's strange appearance has been noted by Halton Arp’s Atlas of Peculiar Galaxies
and the galaxy is home to an unusual ultraluminous X-ray source
, shown in the image towards the top right, among three gas bubbles.
This image combines near-infrared and visible light exposures captured by the Wide Field Channel of Hubble's Advanced Camera for Surveys
while it panned across