Green’s Dictionary of Slang

star n.2

also star man
[abbr. prison jargon star prisoner; a star is affixed to their name in the prison records]

(UK prison) a first offender.

Pall Mall Mag. Nov. 454: ‘Star’ prisoners are generally, but not always, first offenders. Upon conviction the record of each prisoner is thoroughly inquired into.
[UK]Lord W. Neville Penal Serv. 66: There were only five parties in which ‘star’ men were allowed to work [OED].
[Aus]West Gippsland Gaz. (Warragul, Vic.) 28 Feb. 6/3: The ‘star men’ are kept strictly apart from old offenders [...] they also occupy a separate hall.
[UK]S. Scott Human Side of Crook and Convict Life 46: First offenders (‘star men’), particularly, were against it.
[US]M.C. Sharpe Chicago May (1929) 186: Even the ‘stars,’ who are not underworld women, despise such cattle.
[Ire]J. Phelan Tramp at Anchor 24: ‘Stars,’ as the first-offence prisoners were called.