Although the æ/œ spelling (known as a digraph, the combined letters representing a single sound) for forms such as œstrogen or cæsium is still in use in British English, this is now not universally the case for most other words that were traditionally written with a digraph; these words now have a single vowel that substitutes the older æ form (medieval and encyclopedia being well-known examples). Opt for the simplified spelling for all such words.