Figure 10-2 Apparent Magnitude Scale (a) Several stars in and around the constellation Orion, labeled with their names and apparent magnitudes. (b) Astronomers denote the brightnesses of objects in the sky by their apparent magnitudes. Stars visible to the naked eye have magnitudes between m = −1.44 (Sirius) and about m = +6.0. However, CCD (charge-coupled device) photography through the Hubble Space Telescope or a large Earth-based telescope can reveal stars and other objects nearly as faint as magnitude m = +30. Note that Pluto’s apparent magnitude varies from +13.7 to +16.3, depending on its distance from Earth. Its average apparent magnitude is +15.1.