An Earth-sized diamond in the sky might just be 900 light-years away from us. But it technically isn’t a diamond; it’s a star. A companion to the neutron star PSR J2222-0137, the white dwarf is so cold that its carbon has crystallized, Space.com reports. What’s more, astronomers suspect at 11 billion years old, it’s the same age as the Milky Way. The white dwarf, which is the coldest star ever observed, is described in The Astrophysical Journal.