HEIC: A Vapour of Stars (DDO 82, UGC 5692)

[bystander]

A Vapour of Stars (DDO 82, UGC 5692)
ESA/HEIC Hubble Picture of the Week | 2012 Jun 25

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Relatively few galaxies possess the sweeping, luminous spiral arms or brightly glowing centre of our home galaxy the Milky Way. In fact, most of the Universe's galaxies look like small, amorphous clouds of vapour. One of these galaxies is DDO 82, captured here in an image from the NASA/ESA Hubble Space Telescope. Though tiny compared to the Milky Way, such dwarf galaxies still contain between a few million and a few billion stars.

DDO 82, also known by the designation UGC 5692, is not without a hint of structure, however. Astronomers classify it as an Sm galaxy, or Magellanic spiral galaxy, named after the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way. That galaxy, like DDO 82, is said to have one spiral arm.

In the case of DDO 82, gravitational interactions over its history seem to have discombobulated it so that this structure is not as evident as in the Large Magellanic Cloud. Accordingly, astronomers also refer to DDO 82 and others of a similar unshapely nature as dwarf irregular galaxies.

DDO 82 can be found in the constellation of Ursa Major (the Great Bear) approximately 13 million light-years away. The object is considered part of the M81 Group of around three dozen galaxies. DDO 82 gets its name from its entry number in the David Dunlap Observatory Catalogue. Canadian astronomer Sidney van den Bergh originally compiled this list of dwarf galaxies in 1959.

The image is made up of exposures taken in visible and infrared light by Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.3 by 3.3 arcminutes.

Credit: NASA/ESAHubble

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[Ann]

This is an unusually fine image, thanks to the fact that it was taken through three different filters: one blue, one clear optical and one infrared one. The use of the blue filter makes it possible to look for young stars in this very small galaxy.

Indeed there are young stars here, but they are all limited to the "upper right quadrant" of the galaxy, to the upper right of the bright foreground star. It is remarkable how the "lower" part of the galaxy is totally devoid of blue stars, even though there is a good population of old yellowish and reddish stars.

It is not unusual that the star formation of a galaxy is concentrated in a few small parts of it. Here, there is one "extended region" that sees all of the action. The rest of the galaxy is "dead".

The star formation that actually goes on is noteworthy but not remarkable. This is most certainly not a starburst galaxy. The B-V index of this galaxy is +0.80, which is in fact moderately red. All the old stars of UGC 5692 are making their impact here. But the U-B index of the galaxy is +0.07, which is blue enough that it clearly speaks of the presence of some hot blue stars.

Ann