when was it discovered?
In 1910.
White dwarf
 In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white. I
how does a white dwarf form? After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant. It the red giant  has insufficient mass to generate the core temperatures required to fuse carbon it forms a planetary nebula. The remaining core is the white dwarf. what metals does it contain? UNANSWERABLE
wiki:19666762
White dwarf [SEP] Discovery
The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the relatively bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C. The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783. In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white. In 1939, Russell looked back on the discovery: I was visiting my friend and generous benefactor, Prof.