Astronomy Picture of the Day

Explanation: Why is RJX J185635-375 so cool and so dim? Previously, this compact star held claim to being the closest neutron star -- only 150 light-years away. Now new observations and analysis indicate not only a cool temperature for RXJ J185635-375, pictured above, but also a larger distance: roughly 450 light-years away. Given an expected age of about one million years, this neutron star should be significantly hotter and brighter than we see it today. One hypothesized solution holds hope a RJX J185635-375 is actually a not a neutron star but a quark star -- something new. Now quark stars are truly strange -- they may have made a transition to type of matter known as strange quarks. Quark stars, were they to exist, would be intermediate between neutron stars and black holes in size and density. Quark stars should be more compact and cool faster than neutron stars. In fact, some might even be ultracompact -- so dense that light itself can orbit. Future observations will likely settle the controversial claims of RJX J185635-375's distance and radiative geometry, and hence determine if a previously undiscovered type of beast roams the sky.