Holmberg II, a dwarf irregular galaxy in Ursa Major

Holmberg II, a dwarf irregular galaxy in Ursa Major

Image Credit: NASA & ESA

Holmberg II (also known as Arp 268 and UGC 4305) is a very bright dwarf irregular galaxy located only about 9.8 million light-years away in the constellation Ursa Major. It is a member of the M81 Group of galaxies, and one of the few that isn’t distracted by gravity from other nearby galaxies.

This small galaxy is a patchwork of dense star-forming regions and extensive barren areas with less material, which can stretch across thousands of light-years.

Holmberg II is dominated by giant bubbles of glowing gas – the largest about 5,500 light-years wide – which are regions of old star formation. The cavities are blown by high-mass stars (as these stars form in dense regions of gas and dust, they expel strong stellar winds that blow away the surrounding material) and of gas by the shock waves produced in supernovae (the violent explosions that mark the end of the lives of massive stars).

As a dwarf galaxy, it has neither the spiral arms of galaxies like the Milky Way nor the dense nucleus of an elliptical galaxy of which the gravitational pull would destroy the fragile bubbles. This makes Holmberg II a gentle haven where these fragile structures can hold their shape.

New star birth is also taking place, but not in the same areas as the bubbles because these are drained now of gas or dust. The star formation regions in Holmberg II appear as massive, disorganized patches filled with hundreds of young, blue stars, that occupy a relatively large fraction of the disk. One region in particular has almost as many young stars as the famous Tarantula Nebula in the Large Magellanic Cloud.

Holmberg II is the perfect example of the “champagne” model of starbirth – where new stars create even newer ones. It works like this: when a bubble is created by stellar winds, it moves outwards until it reaches the edge of the molecular cloud that spawned it. At the exterior edge, dust and gas have been compressed and form a nodule similar to a blister. Here another new star forms.. and triggers again… and triggers again… similar to the chain reaction which happens when you open a bottle of champagne.

The galaxy also hosts an ultraluminous X-ray source in the middle of three gas bubbles in the top right of the image. There are competing theories as to what causes this powerful radiation — one intriguing possibility is an intermediate-mass black hole which is pulling in material from its surroundings.

Holmberg II enables astronomers to study star birth in an environment that isn’t disturbed by density waves (as happens in larger galaxies such as the Milky Way) or by deformation caused by the pull of another galaxy, and that is conveniently close.

This image was captured by the NASA/ESA Hubble Space Telescope. It is a composite of visible and near-infrared exposures taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys.

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