You absolutely must see the Winter Hexagon tonight

Check out the six bright stars that connected together form a large hexagon in the evening sky.  It’s up by 7 p.m. local time but best between 8 and midnight. Maps created with Stellarium

There’s a reason the winter sky is so full of sparkling stars. A great many are concentrated around the constellation Orion the form of a gigantic hexagon. How big is it? I made a fist and reached my gloved hand to the sky last night to measure – 6 fists high by 4 fists wide or 60 x 40 degrees. This six-sided figure of celestial real estate reaches from Sirius, low in the southern sky, all the way up to Capella, located nearly overhead from mid-northern latitudes.

What makes these stars special is how bright they are. They all shine at 1st magnitude (or brighter) and appear on the list of the Top 25 brightest stars. Joining the clan are Jupiter, more luminous than any of them, and Castor slightly fainter than the faintest.

Magnitude scale showing the limits of the eye, binoculars and telescopes. Credit: Dr. Michael Bolte, UCO/Lick Observatory

Astronomers use the magnitude scale to measure star and planet brightness. Each magnitude is 2.5 times brighter than the one below it. Aldebaran, which shines at 1st magnitude, is 2.5 times brighter than a 2nd magnitude star, which in turn is 2.5 times brighter than a 3rd magnitude star and so on.

A first magnitude star is 2.5 x 2.5 x 2.5 x 2.5 x 2.5 (about 100) times brighter than a 6th magnitude star.

The bigger the magnitude number, the fainter the star. On the other hand, if an object is really bright, it’s assigned a negative magnitude. Sirius, the brightest star sparkles at magnitude -1.4, Jupiter at -2-2 (currently) and Venus brighter yet at -4.4. The full moon reaches a magnificent -12.7, topped only by the sun at -26.7.

An object’s brightness has much to do with its distance from Earth. Small things like planets, the moon or even an asteroid can look bright if close, while a brilliant supergiant star can appear faint simply because it’s far away.

Photo taken last night Jan. 16, 2013 of the Winter Hexagon and Jupiter about 9 p.m. Photo: Bob King

To get a better appreciation of an object’s true or absolute brightness, astronomers assign it an absolute magnitude, based on how bright it would appear when moved to a distance of 10 parsecs (equal to 32.6 light years) from the sun. When stars are all placed at the same distance, absolute magnitudes show differences in true star brightness.

A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arc second – parallax second – against the background sky. Parallax, which is measured in arc seconds or tiny fractions of a degree, is the apparent shift of a nearby star against the distant background of stars as seen from either end of Earth’s orbit.

One parsec equals 3.26 light years. Click HERE for a blog I wrote explaining parallax. The main thing to remember is we’re comparing objects at the same distance of 10 parsecs from the sun.

Here are the apparent (what we see with the eye) and absolute magnitudes (in parentheses) of our featured stars::
* Sirius -1.5 (1.4)
* Procyon 0.4 (2.6)
* Pollux 1.1 (0.7)
* Capella 0.1 (0.4)
* Aldebaran 0.9 (-0.3)
* Rigel 0.1 (-8.1)
* Jupiter -2.2 (55)
* Betelgeuse 0.5 (-7.2)
* Castor 1.6 (0.5)
* Our sun -26.7 (4.8)

An illustration of how the Winter Hexagon and neighboring bright stars would appear if all moved to the same distance of 32.6 light years. We would see them at the absolute magnitudes. Notice anything missing?

Right away you’ll see some dramatic differences in intrinsic brightness. Rigel and Betelgeuse, both of which appear more than a magnitude fainter than Sirius to the eye, far outshine all the others. Seen from 10 parsecs, each puts out enough light to cast shadows at night. Why? They’re both extremely luminous supergiant stars. Jupiter, the big shot of the bunch, fades out of sight.

Sirius, only twice as big as the sun, dims to a rather meek mag. 1.4. It’s overtaken by otherwise mild-mannered Castor, a double star with suns 2.4 and 1.9 times larger than our own. How does our sun fare at 10 parsecs? Not so good. At magnitude 4.8, it would blend into the background of faint stars. Unless you looked carefully, you wouldn’t even notice it.

Knowing a star’s absolute magnitude gives us a true picture of a star’s brightness. What’s more, you can derive a star’s distance by comparing its apparent magnitude to the absolute magnitude. Want to have a little fun? Click on the Magnitude and Luminosity Calculator and play around with some of your favorite stars.

16 thoughts on “You absolutely must see the Winter Hexagon tonight

  1. Great update Bob. Last night I saw an incredibly bright Orion and Jupiter…presumably last night was also a bright one (tonight it’s cloudy here). In fact I wasn’t sure if I was seeing Sirius as it didn’t seem bright enough, but your update explains it. Looking forward to the next very clear winter night!

      • It’s a great photo and a great sky. Was there that many stars visible to the eye or is that due to the exposure. Do you happen to know your camera settings? I tried to take a photo but couldn’t get much. And it was cold!! It was amazing to see Orion so clearly and (for the first time) M42!

        • H.Bob,
          I could faintly see the winter Milky Way and stars down to about 5.6-6.0 magnitude. The photo shows many more stars because of the time exposure of 30 seconds at f/2.8 and ISO 800.

  2. We may have a trio of naked eye or binocular comets to look for this March through May. Bressi, Mar.6-12, Panstaars Mar. 12-April 23, and Lemmon, April 23 – May 20.

    • Hi Edward,
      Thanks for the update. For the northern hemisphere, L4 PANSTARRS and F6 Lemmon will either be naked eye or binocular objects. As for T5 Bressi, it will be much too close to the sun to see when at max brightness in early-mid March. By the time it emerges into a reasonably dark sky, the comet will almost certainly require a telescope.

    • Thanks Mike – I’ve been watching too :) Looks like if you’re in northern Scandinavia right now, you’re seeing northern lights. The index just dropped back to Kp=3 at 3:30 p.m. If it continues at 3 or higher, I’ll do a posting. Last night we were at “3″ but no aurora here in Duluth. The oval was well north into Canada.

  3. hi Bob, excellent discussion of stellar magnitudes. good refresher “course” for me. once knew much of this, but had forgotten a lot of it. so thanks for jolting my memory.

  4. Great hexagon pic, Bob. I can see hints of M35&36 too.

    If it’s useful for next time, note that the magnitude scheme pic has errors on Venus (which is always around -4 as you wrote) and the naked eye limit.

    Thanx for the magnitude calculator. Does it take into account extinction by interstellar gas/dust (as does latest Stellarium version)? Is that kept into account by the “luminosity distance” (which is so not the physical distance?) Thanx

  5. I really loved that diagram of absolute magnitudes, Bob! I hadn’t realized that both Betelgeuse and Rigel were so intrinsically luminous and put Sirius in the shade!

      • Indeed! A question about Jupiter at that distance – what is the magnitude 55 based on? As Jupiter shines by reflected light, it would have no luminosity at all at that distance, would it?

        Is the mag. 55 based on what Jupiter would look like if it were at the same distance from the sun as it is now, with both Sun and Jupiter removed to 10 parsecs?

        • Carol,
          You got it. It would be have no luminosity at all on its own – at least not in visual light. It would shine however in infrared since the planet radiates a fair amount of heat. The mag. is based on what you said – what Jupiter would like if it and the sun were removed to 10 parsecs.

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