Many books published by Penguin follow a standardized format designed by typographer Jan Tschichold. In this format, asterisks are used to note references or footnotes in the text. As there is no footnote on the page, we can assume it is a reference. Based on your edition, this information is most likely available at the very back of the book as a list of "Works Cited" or "References." Many contemporary works, by Bede and otherwise, are mentioned in this book as a way of offering greater context. Take note that these asterisks occur at the end of sentences or paragraphs dealing with other works of literature, like the Life of Germanus, or even law codes of the time. With so many references being made by name, it may have been easier for the publisher to simply denote a reference by an asterisk, with the full citation elsewhere, than to include an MLA or APA style in-text citation.