M3: Inconstant Star Cluster . ... Credit Copyright: J. Hartman K. Stanek ( Harvard CfA ) Explanation: Star clusters appear constant because photographs of them are frozen in time. In reality, though, cluster stars swarm the center and frequently fluctuate in brightness. ... Most of the variable stars visible above are RR Lyrae stars , stars that can quickly double their brightness while becoming noticeably bluer. ... NASA Web Site Statements, Warnings, and Disclaimers . ...