Observation and measurement of starlight through telescopes is the basis for many fields of astronomy,[2] including photometry and stellar spectroscopy.[3] Hipparchus did not have a telescope or any instrument that could measure apparent brightness accurately, so he simply made estimates with his eyes. He sorted the stars into six brightness categories, which he called magnitudes.[4] He referred to the brightest stars in his catalog as first-magnitudes stars, which were the brightest stars and those so faint he could barely see them were sixth-magnitude stars.[4]