Rigel in the context of "IC 2118"

Orion is a prominent set of stars visible during winter in the northern celestial hemisphere. It is one of the 88 modern constellations; it was among the 48 constellations listed by the 2nd-century AD/CE astronomer Ptolemy. It is named after a hunter in Greek mythology.

Orion is most prominent during winter evenings in the Northern Hemisphere, as are five other constellations that have stars in the Winter Hexagon asterism. Orion's two brightest stars, Rigel (β) and Betelgeuse (α), are both among the brightest stars in the night sky; both are supergiants and slightly variable. There are a further six stars brighter than magnitude 3.0, including three making the short straight line of the Orion's Belt asterism. Orion also hosts the radiant of the annual Orionids, the strongest meteor shower associated with Halley's Comet, and the Orion Nebula, one of the brightest nebulae in the sky.

The Winter Hexagon is an asterism appearing to be in the form of a hexagon with vertices at Rigel, Aldebaran, Capella, Pollux, Procyon, and Sirius. It is mostly upon the Northern Hemisphere's celestial sphere. On most locations on Earth (except the South Island of New Zealand and the south of Chile and Argentina and further south), this asterism is visible in the evening sky at the equator from approximately December to June, and in the morning sky from July to the end of November, while in the evenings on the northern hemisphere it is less months visible between December and June, and on the southern hemisphere less months between July and November. In the tropics and southern hemisphere, this (then called "summer hexagon") can be extended with the bright star Canopus in the south.

Smaller and more regularly shaped is the Winter Triangle, an approximately equilateral triangle that shares two vertices (Sirius and Procyon) with the larger asterism. The third vertex is Betelgeuse, which lies near the center of the hexagon. These three stars are three of the ten brightest objects, as viewed from Earth, outside the Solar System. Betelgeuse is also particularly easy to locate, being a shoulder of Orion, which assists stargazers in finding the triangle. Once the triangle is located, the larger hexagon may then be found.

Betelgeuse is a red supergiant star in the equatorial constellation of Orion. It is usually the tenth-brightest star in the night sky and, after Rigel, the second brightest in its constellation. It is a distinctly reddish, semiregular variable star whose apparent magnitude, varying between +0.0 and +1.6, with a main period near 400 days, has the widest range displayed by any first-magnitude star. Betelgeuse is the brightest star in the night sky at near-infrared wavelengths. Its Bayer designation is α Orionis, Latinised to Alpha Orionis and abbreviated Alpha Ori or α Ori.

With a radius between 640 and 764 times that of the Sun, if it were at the center of the Solar System, its surface would lie beyond the asteroid belt and it would engulf the orbits of Mercury, Venus, Earth, and Mars. Calculations of Betelgeuse's mass range from slightly under ten to a little over twenty times that of the Sun. For various reasons, its distance has been quite difficult to measure; current best estimates are of the order of 400–600 light-years from the Sun – a comparatively wide uncertainty for a relatively nearby star. Its absolute magnitude is about −6. With an age of less than 10 million years, Betelgeuse has evolved rapidly because of its large mass, and is expected to end its evolution with a supernova explosion, most likely within 100,000 years. When Betelgeuse explodes, it will shine as bright as the half-Moon for more than three months; life on Earth will be unharmed. Having been ejected from its birthplace in the Orion OB1 association – which includes the stars in Orion's Belt – this runaway star has been observed to be moving through the interstellar medium at a speed of 30 km/s, creating a bow shock over four light-years wide.

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the inverse logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the whitish Sun has a B−V index of 0.656 ± 0.005, whereas the bluish Rigel has a B−V of −0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B−V = −0.03). Traditionally, the color index uses Vega as a zero point. The blue supergiant Theta Muscae has one of the lowest B−V indices at −0.41, while the red giant and carbon star R Leporis has one of the largest, at +5.74.

To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U−B or B−V color index respectively.

Deneb (/ˈdɛnɛb/) is a blue supergiant star in the constellation of Cygnus. It is the brightest star in the constellation and the 19th brightest in the night sky, with an apparent magnitude slightly varying between +1.21 and +1.29. Deneb is one of the vertices of the asterism known as the Summer Triangle and the "head" of the Northern Cross. Its Bayer designation is α Cygni, which is Latinised to Alpha Cygni, abbreviated to Alpha Cyg or α Cyg.

Deneb rivals Rigel, a closer blue supergiant, as the most luminous first-magnitude star. However, its distance, and hence luminosity, is poorly known; its luminosity is estimated to be between 55,000 and 196,000 times that of the Sun. Distance estimates range from 1,400 to 2,600 light-years; assuming its highest value, it is the farthest star with an apparent magnitude brighter than 2.50.