|Walter Baade, Fritz Zwicky astrophysics|
In 1572, Tycho Brahe saw a supernova. In 1933, Baade and Zwicky explained that such an event could be the collapse and explosion of a massive star that leaves a dense spinning core of neutrons.
It could become a pulsar. It could become a magnetar. It could become a protoneutron, quark, electoweak, or preon star. It could become an X-ray binary pulsar or a radio-quiet supernova remnant.
The spinning of a neutron star, some say, is more accurate than a quartz watch; however, glitches can occur that speed it up or slow it down. A love even as true as that may sometimes settle into something finer.
Many were observed before we knew what they were.
See also in The book of science:
Readings in wikipedia: