Hello Max,
as was said before, Asel, Borsum, Sorsum are villages around Hildesheim. (there are several more villages named with -sum or - um at the end.)
The catholic church records lie with the archive of the diocese of hildesheim.
This list shows what records they have: http://www.bistum-hildesheim.de/bho/dcms/sites/bistum/bistum/generalvikariat/bistumsarchiv/dok/kirchenbuecher.pdf
For the village of Asel, these are there from 1664 to 1958, for example. For Borsum and Sorsum it is similar. I know that the archive works on digitalizing their church records, but I don�t know if they have any plans on getting these online as well.
From 1876 on, there should be civilian records as well, but I don�t know where these would be kept today.
Do you have any further information about Julius Wilhelm Burgdorf? The name Burgdorf is quite common, so to narrow it down, any information could be helpful.
good luck,
Stefan