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Subject
- x-ray sources63
- survey source40
- surveys13
- galaxy clusters10
- pre-main sequence stars8
- open star clusters7
- quasars7
- radio astronomy7
- radio sources7
- active galactic nuclei6
- galaxies6
- photometry6
- optical counterpart5
- late-type stars4
- magellanic clouds4
- observation4
- redshifted / redshift4
- star4
- astronomical object identification3
- catalogs3