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Beyond Earthly Skies

Two Record-Breaking Compact Stellar Systems

20 Jul 2015, 23:00 UTC
Two Record-Breaking Compact Stellar Systems
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Following the discovery of the densest known galaxy, M60-UCD1, Sandoval et al. (2015) present the discovery of M59-UCD3 and M85-HCC1 - two record-breaking compact stellar systems. Since the density of stars in a galaxy decreases gradually away from the center, one way to express the size of a galaxy is by its half-light radius. The half-light radius is basically the size of the volume of space that is contributing to half the galaxy’s brightness and it can also apply to other stellar systems.M59-UCD3 is an ultracompact dwarf (UCD) galaxy similar in size to M60-UCD1. It has a half-light radius of roughly 70 light years, but it is 40 percent more luminous than M60-UCD1. This makes M59-UCD3 the new densest known galaxy. M59-UCD3 is estimated to contain roughly ~180 million times the Sun’s mass and this means an average density of roughly 30 solar masses per volume of space one light year across. For comparison, an observer on Earth can see ~6,000 stars with unaided eyes under “typical” dark sky conditions. However, an observer in the core of M59-UCD3 would see roughly a million stars in the sky.M85-HCC1 is an extremely compact stellar system with a half-light radius of roughly 6 ...

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