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Subject
- x-ray sources55
- survey source40
- surveys12
- galaxy clusters10
- open star clusters7
- pre-main sequence stars7
- quasars6
- galaxies5
- optical counterpart5
- photometry5
- magellanic clouds4
- observation4
- radio astronomy4
- radio sources4
- redshifted / redshift4
- star4
- active galactic nuclei3
- astronomical object identification3
- catalogs3
- apparent magnitude2