White dwarf stars are very densely packed stars that have passed the stage of being Red Giant Stars and are beginning to collapse. As they do so, their core becomes even more dense.
As most have a low brightness, it is difficult to detect them in the sky. In 1844, the German Astronomer Fredrich Bessel discovered the first one, Sirius B. in 1862 popular telescope builder Alvan Clark observed it, though it was not known to be a white dwarf at that time. In 1914 the American Astronomer Walter Adams observed that Sirius B was still hotter than our sun. From its characteristics it was found to be 130,000 times more dense than platinum.