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Secrets of Orion, the brightest constellation

If there is just one constellation that everyone ought to know, it is Orion, the brightest of the 88 constellations that wallpaper the sky. And now is the season when Orion stands highest in the evening.

Its brightest stars shine right through most city sky glow. These seven form a distinctive pattern like nothing else. Orion is supposed to be a hunter or warrior holding an upraised club in one hand and a shield (or lion skin) in the other. The club and shield are faint, waiting for your next excur­sion to a dark-sky location or for you to pick out, bit by bit, with binoculars. But Orion’s brightest stars by themselves suggest a human figure in an abstract way, with broad shoulders, wide-set feet, and a diagonal, three-star belt slung in the middle.

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This winter he is especially easy to find, with Jupiter shining even brighter, high to ­Orion’s upper right. From ­Jupiter, look lower left for him, by roughly two fist-widths at arm’s length.

Orion is also a gateway into all sorts of things to know about the universe.

For instance: Everyone knows (we may hope) that the stars are suns, just much farther away than the one that lights our days. Fewer people realize how wildly varied these other suns are. Some are more than 100,000 times brighter than the middling-average one we call the sun. Ours in turn is about a million times more lumi­nous than the dimmest.

So, you shouldn’t be surprised to learn that nearly all stars you see when you look up from your back porch are much more luminous and powerful than ours. That is the main reason they stand out as they do.

Orion’s stars are grand ­examples. The seven making up his main pattern range from about 1,200 to 40,000 times as bright as our sun. If one of them replaced the sun in our daytime sky, everything below would flash-fry in a fraction of a second as if under a hydrogen bomb.

And yet, because most stars are proving to have planets, it is quite possible that worlds exist near these supergiants, made of boiling seas of yellow-hot lava whipped into waves by rock-
vapor winds.

Somewhat farther out from such stars will be a temperature zone that is nice and clement by our standards, but there is a catch. Brilliant stars like Orion’s are short-lived. They burn so hard that they use up their ­nuclear fuel in only millions to tens of millions of years, compared with our more prudent sun, which has been burning relatively gently for 4.6 billion years, with billions left to go. There may be worlds at safe distances from Betelgeuse or Rigel with shirt-sleeve temperatures and pleasant beaches for sunning and swimming, but they will be barren of life. Planets in solar systems so young won’t have had time for biology and evolution to get going properly, nor will they before their stars burn out and explode.

So, does Orion hold any nice, sunny stars? Well, yes. Look where Orion’s outstretched arm joins his shield. That star, known as Pi-3 Orionis, is only three times as bright as our sun, being just a little hotter and bigger. It is only 26 light-years away, compared with 250 to 1,500 light-years for Orion’s main dazzlers.

A better match is the faint star at the top right corner of Orion’s club. That’s Chi-1 ­Orionis, 28 light-years away and almost the sun’s twin.

And as for much dimmer stars? Those are the red dwarfs, and they vastly outnumber ­every other kind. They provide most of our closest neighbors in space. But so dim are they that you need binoculars or a telescope (and excellent star maps) to detect even the closest of them.

Notice that Betelgeuse is colored, unlike Orion’s other bright stars. It is pale reddish orange, the color of a campfire, compared with the blue-white of the others, the color of an arc welder’s work. That is because stars have different temperatures. Our yellow-white sun is, once again, in the middle of the range.

Hanging down below ­Orion’s three-star belt is a dim, straight line traditionally called Orion’s Sword. Its three or four stars are a little too faint to be plotted here, but you can often see them through moderate suburban light pollution.

Binoculars show a small, ghostly glow around one of these. That is the Great Orion Nebula, spectacular in a good telescope and one of the most-studied regions in the sky. At a distance of 1,350 light-years, it is the nearest place where new supergiant stars are being minted from inter­stellar gas and dust.

Lastly, to Orion’s lower left you can’t miss Sirius, the Dog Star. It always follows faithfully behind Orion’s feet. Two reasons combine to make Sirius the brightest star in the night sky. It is only 8.6 light-years away, making it the closest star (beyond the sun) that is ever visible to the naked eye from the latitudes of New England. And it outshines the sun by 22 times. Enjoy.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine (SkyandTelescope.com). His Star Watch column ­appears the first Saturday of every month.
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