Orion
Orion is one of the most famous and easiest to spot constellations in the whole sky. It is named after a giant hunter in ancient Greek mythology and is shaped like a tall figure with a sword and a shield. Orion is packed with bright stars, including the giant red Betelgeuse and the brilliant blue Rigel, plus three bright stars in a row that form Orion's Belt. Just below the belt sits the glowing Orion Nebula, one of the closest stellar nurseries to Earth.
- Best seen fromBoth hemispheresSits on the celestial equator
- Best viewing timeNorthern winter(Southern summer) evening sky
- Brightest starRigelBlue supergiant, 7th brightest in the night sky
- Most famous starBetelgeuseRed supergiant, could explode any time
- Distance to Orion Nebula1,344 light yearsA stellar nursery visible to the naked eye
- Stars in the belt3Mintaka, Alnilam and Alnitak in a perfect row
The myth of Orion the Hunter
In ancient Greek mythology, Orion was a giant and the greatest hunter of his age. The stories about him do not always agree. In one version he boasted he could kill any animal on Earth. The goddess Gaia sent a giant scorpion to stop him, and the two killed each other. The gods placed them both in the sky as constellations, but on opposite sides, so they could never meet again. That is why, even today, when the constellation Scorpius rises in summer, Orion is just disappearing below the western horizon, and when Orion rises in winter, Scorpius is gone.
How to find Orion in the night sky
Orion is so distinctive that once you have spotted it, you will never forget it. Look for three bright stars in a perfect short row: that is Orion's Belt. Once you have the belt, you can see the four "corners" of his body around it.
- Top left: bright red Betelgeuse, the hunter's right shoulder.
- Top right: blue-white Bellatrix, his left shoulder.
- Bottom left: bright blue Saiph, his right knee.
- Bottom right: dazzling blue Rigel, his left foot.
Below the belt hangs Orion's sword: three faint dots in a vertical line, with the middle "star" actually being the Orion Nebula. Orion is best seen in the evening from approximately November through to April in the Northern Hemisphere, or May through to October in the Southern Hemisphere.
Betelgeuse: the dying giant
Betelgeuse (pronounced "BET-el-jurz") is one of the largest stars we can see with the naked eye, and one of the most interesting. It is a red supergiant, around 700 times wider than our Sun. If you put Betelgeuse where our Sun is, it would swallow Mercury, Venus, Earth and Mars and reach out almost as far as Jupiter.
Betelgeuse is also dying. It has used up almost all the hydrogen in its core and is now burning heavier elements much less efficiently. Some time in the next 100,000 years it will run out of fuel completely and explode as a supernova. For weeks afterwards it will shine brighter than the full Moon and be visible during the day. (100,000 years sounds like ages, but in astronomy that is the blink of an eye.)
Rigel: the brightest in Orion
Rigel, the dazzling blue-white star at Orion's foot, is actually the brightest star in the constellation (despite Betelgeuse being more famous). Rigel is a blue supergiant about 860 light years away, with around 18 times the mass of the Sun. It pumps out roughly 120,000 times as much light as our Sun, which is why we see it so brightly even from so far. Rigel is also part of a system of at least three stars: a smaller pair orbits the main supergiant.
The Orion Nebula: a star nursery
Hanging below Orion's Belt as part of his sword is the Orion Nebula (also called M42), one of the most spectacular sights in the sky. It is a huge cloud of glowing gas and dust about 1,344 light years from Earth and over 24 light years across. Inside it, brand new stars are being born right now from the slow collapse of dense pockets of gas.
You can see the Orion Nebula with the naked eye on a dark night (it looks like a fuzzy patch around the middle "star" of the sword). With binoculars or a small telescope it bursts into colour: pink and green wisps surrounding a tight knot of brilliant young stars called the Trapezium.
Deeper dive: what will happen when Betelgeuse explodes?
When a massive star like Betelgeuse runs out of nuclear fuel, its core collapses in a fraction of a second and the outer layers explode outwards at speeds of up to 30,000 km/s. This core-collapse supernova releases more energy in a few weeks than the Sun will release in its entire 10-billion-year lifetime.
If Betelgeuse goes supernova in our lifetime, it will be the most spectacular astronomical event in recorded history. The exploding star, 640 light years away, will reach an apparent brightness comparable to the full Moon. It will be easily visible during the day for weeks, and it will not fully fade for over a year. The next nearest supernova was Tycho's supernova in 1572, which appeared in Cassiopeia and was visible for 16 months.
Don't worry: at 640 light years away, Betelgeuse is far too distant to harm life on Earth. The dangerous radiation from a supernova only matters within about 50 light years. The biggest effect for us would be the chance to watch a once-in-many-lifetimes light show.
Once the explosion fades, the leftover core of Betelgeuse will collapse into either a neutron star or, more likely given its mass, a black hole. The expanding cloud of gas and dust the supernova throws out will eventually mix with new gas in the Milky Way and become raw material for forming the next generation of stars and planets.
Want more famous patterns? See Ursa Major, Cassiopeia, Leo or what is a constellation.