If my opponent star crosses my trove golem in response to attaching shady spectacles to watchful sentry what does my watchful sentry become

No — Watchful Sentry does not become a copy of Trove Golem; it stays Watchful Sentry (1 Might, Deathknell draw 1) with Shady Spectacles attached.

The "choose another friendly unit" isn't done when you activate Equip — it happens as the attach event executes (a replacement effect, per FAQ #11746). Star-Crossed resolves first (LIFO), so by the time Shady Spectacles actually attaches, Trove Golem is in hand and is no longer a "unit" you can choose. If you have another friendly unit on the board, you may choose it instead and copy that; if you have none, the choice instruction is ignored and no copy effect applies (Rule 359.3.e.6). Note the attach to Watchful Sentry still succeeds — only the copy choice fails.