Astronomers often have to infer the presence and properties of objects they can’t see directly. Sometimes, as with black holes, we have a lot of data, thanks to matter swirling around and falling in. Sometimes, however, only the gentle tug of gravity reveals a hidden object. That’s the case for a companion body to a known pulsar, but in this case the fact that astronomers can’t see anything tells them a lot about it. In particular, it matches the description of a white dwarf with such low temperature that it has solidified — almost, but not quite, like a diamond in the sky. My latest column in The Daily Beast tells the tale:

This binary system is right next door to Earth, in cosmic terms—it’s only about 900 light-years away. So the pulsar’s companion can’t be a star, since it would be visible in telescopes. It might be another pulsar, but it would have to be very low-mass for that type of object, and the orbit looks wrong compared with other two-pulsar binaries astronomers have studied. That leaves one other improbable but ultimately most likely culprit: a high-mass, low-temperature white dwarf. [Read more…]