modern telescopes. However its color is controversial, and many earlier observers stated that they saw it as green. There seems to be no consensus about what its color really is, and no generally accepted explanation for why some observers would have seen it as green. Multiple stars There are a few stars in double or multiple star systems that appear greenish, even though they are blue or white under typical viewing conditions. This can happen if the star system contains a large red or orange star. Colour contrast effects result in objects close to the reddish star appear slightly greenish (and vice versa: complementary colors). Antares The classical example is Antares (Alpha Scorpii or Sco), the brightest star in the constellation Scorpius: The primary of the double star system is the red supergiant primary Sco A; it has a blue companion Sco B. When viewed at low-enough power so that the light of the two stars is seen combined, some observers describe the color as greenish. Almach Other examples include the star system Almach (¹ and ² Andromedae) a golden-yellow star next to a dimmer, indigo-blue star about 350 light years away. Cassiopeiae Another is the sigma Cassiopeiae ( Cas) star system, a binary star 1,500 light-years from Earth. Cas‑A is a greenish-hued magnitude 5.0 primary, next to magnitude 7.3 blue-hued secondary Cas‑B. Planetary nebulae Some planetary nebulae themselves glow with the green O emission lines, especially if the nebula is enriched oxygen, well irradiated by its central late-stage star or white dwarf. These nebulae, or the stars within them (usually bluish or white Wolf-Rayet stars) may appear to be green when viewed in combination at low power. Some examples are the planetary nebulas NGC 6572, NGC 6826, and NGC 7009. Uranus The planet Uranus occasionally appeared in ancient star catalogs, and in more recent catalogs before William Herschel discovered it is a planet – most recently as 34 Tauri in Flamsteed's catalog. It appears as a small greenish dot in binoculars. The color is from its cold methane atmosphere. The Sun The Sun can sometimes appear as a green spot for a second or two as it is rising or setting: this is known as green flash. Roughly speaking, the red light from the Sun is blocked by Earth, the blue light is scattered by the atmosphere, and the green light is refracted by the atmosphere to the observer.