2 October 2014

Constellation Orion

Being a winter constellation, Orion is most visible in the Northern
Hemisphere between September and March. So, to give time to study it,
given the wonderful weather we have in the winter, I figured it should
be given pride of place now.

Orion was said to have been a heroic hunter in Greek mythology (wikipedia), the son of the god Neptune and the nymph Euryale, who was mentioned in Homer's Odyssey as a shadow in the underworld, and who apparently shaped the straits of Italy. He was a boastful man, so Gaia, the Goddess of the Earth, sent a scorpion to kill him . He was then placed in the sky by the god Zeus, and the scorpion continues to chase Orion to this day - Orion sets in the west as the scorpion rises in the east!

For sky-watchers, the constellation of Orion is one of the brightest, most recognisable constellations in the sky, and is often one of the first constellations learnt. It is a rich area of sky, full of nebulas (areas where stars are made) and interesting objects. It is also very useful in finding other constellations...

What does Orion look like?

As usual, there are many images of Orion on the Internet, ranging from those of ancient times to more modern ones. Sometimes he is sitting, sometimes standing; sometimes he is fighting a bull (the nearby constellation of Taurus); occassionally he is holding up a shield, but usually he holds up a lion he has fought and killed:

I've always pictured him with a sword and shield, so this is the image I'd choose to share with you here:

And here are just the stars on their own:

The main things to notice about Orion:

  • Four bright stars marking the body of the constellation
  • Three stars line up in a row for his belt
  • Hanging from the belt is a sword
  • In front of him is a shield-shaped curve of stars (representing either a shield or a lion)
  • Above his head he holds up a sword/club

  • Where can you find Orion?

Unlike The Plough and Cassiopeia, Orion is not a circumpolar constellation, and so is not visible all the year around. In the northern hemisphere, the constellation is visible during the winter months, from September to March...and is one of the brightest asterisms (= star patterns) in our sky. In the southern hemisphere, he is prominent in the summer.

    (Remember, when a constellation is no longer visible
    in the northern hemisphere, it becomes visible in the
    southern hemisphere... and, interestingly, the celestial
    equator (the projection of the Earth's equator onto the sky)
    passes close enough to Mintaka, the top star of Orion's
    belt, for it to be used as a guide of zero 'declination'
    (a measurement of the celestial equivalent to 'lattitude'). So, the
    top half of Orion lies in the Earth's northern hemisphere,
    and the bottom half lies in the southern hemisphere.)

    Once you've spotted Orion, you'll probably always be able to find it easily, because of it's distinctive shape, but to first find it, we can use the two constellations we already know.

    If you imagine a line running from the Plough's handle, through the front of the saucepan shape (between stars Dubhe and Merak), and forward, and take a second line from Cassiopeia in front of the W (or from the back of the M, if it's upside down), then those two lines cross roughly in Orion to form a triangle:

    Or, from a different angle:

What are the stars that make up Orion?

    These are the main stars to learn if you want to show off to your friends:

    (Yes, yes, I know there's a couple of Harry Potter characters hiding in here! I'll look at Betelgeuse and Bellatrix shortly :-) )

    There's a few more stars labelled in this image:

    ...and this one has a couple of deep space objects marked on it, as well as giving the Greek star name in brackets (see the chart in the sidebar to remind yourself of the order):


What objects can be found within Orion?

    Some of the brightest stars in the sky:

    Two of the stars that make up Orion's main shape are nearly the brightest stars (suns) in the sky. Rigel, marking Orion's foot, and the giant Betelgeuse, at the sword/club bearing shoulder, are so enormous we'd have difficulty even imagining how big they are (our sun is only a little dot compared to them!). Imagine how big Antares (in the next image) is :-)

    Amazing, huh? It is hard to know exactly how big Betelgeuse is, but if you happened to have a big enough transporting machine, and moved it to where our sun is, some think it could stretch out past Mars, others say it's diameter would stretch from where our sun is right out to Jupiter:

      Betelgeuse is the eighth brightest star in the sky, and although it is the 'alpha' star in Orion, it is not as bright as Rigel, which is the sixth brightest star in the sky. Betelgeuse is a red supergiant, which varies in brightness at a possible distance of 650 light years away (scientists have a problem working out its distance because of its changing mass, brightness and size).

    Rigel is the bottom right corner of Orion. It is probably a little further away than Betelgeuse at around 700 to 900 light years, and is about 17 times the size of our sun. It is a blue supergiant.

      *If you want to see more about how the different sun sizes compare (there is an amazing picture, which you can click on to enlarge, then click on again to see clearer!) see The Sun Compared to Other Stars by Patrick Mylund Nielsen

    Orion's Belt

      Orion's Belt: (apart from the very important part it played in Men in Black, this is actually three stars lined up in the sky!): Alnilam, Alnitak, Mintaka

    The belt can be used to find two other objects/constellations - The very bright star, Sirius (or the Dog Star) in the constellation Canis Major (one of the two hunting dogs at Orion's heels. Canis Major = Big Dog. The other dog is Canis Minor = Small Dog):

      ...and the alpha (brightest) star in Taurus the bull, Aldebaran: well as two of Gemini's stars, Castor and Pollux:

    The Orion Molecular Cloud:

    There is an absolutely enormous molecular cloud in Orion (strangely enough called the Orion Molecular Cloud), which can be seen below the belt - around the sword region - in the image below on the right. It is in infrared, which has a longer wavelength than visual light, so can be detected through the dust and gas material):

      The cloud covers a huge area of sky, stretching from Orion's belt right down his sword. In 2006, it was discovered that a huge slinky-shaped magnetic field holds the lower giant cloud in shape (see ) proving the theories that said this was possible.

    Many of Orion's famous nebula are found within the molecular cloud, including Orion's Nebula (on his sword), the Horsehead Nebula, Flame Nebula and M78 (all close to the bottom belt star, Alnitak)

    Orion Nebula:

      It is within this cloud the famous Orion Nebula lies, a reddish glow in the sword:

      The Orion Nebula (also called M42) is one of the most studied star forming regions in the sky, because it is so close by (only 1500 light years from Earth). It is a bright area of sky full of stars, dust and gas, and is visible to the naked eye (so you can see it without any telescopes or binoculas). Right in the middle of the nebula is a cluster of four bright new stars called the Trapezium, which make the dust and gas clouds around them glow (this is called a reflection nebula, because it reflects the light of the stars).:

      It is believed that, what looks like little balls of dust and gas are the beginnings of stars (although they call them protoplanets - huh, what's that about?). These are all images of these dusty blobs from within the Orion Nebula:


    Horsehead Nebula:

      The Horsehead Nebula (also called Barnard 33) can be found within part of Orion molecular cloud complex close to the bottom belt star, Alnitak. It is so thick with dust and gas that it blocks much of the light of the stars on the far side, and contrasts with the emission nebula within which it sits, called IC 434.

      This amazing image of the larger gaseous cloud containing
      Horsehead Nebula is from Astrophotography
      by Rainer Smaritsch & Alexander Gross
      (To the left of the horse's head, you can see what looks
      a bit like a tree growing up towards the top left corner.
      This is the Flame Nebula.)

      Close-up the darkest part of the gaseous cloud of the Horsehead Nebula looks like a....guess what?

      The dust and gas are so thick it blocks visible light from passing through, so it's hard to see stars in the darkest parts of the nebula. Yet, it is these areas that new stars are formed. Where lots of new stars are formed near to each other like this, it is called a stellar nursery

  • [There are so many objects in Orion, I'd thoroughly recommend you google it, and see what else you can come up with. I would love to cover more here, but this post is already long enough for now :-) ]

  • How far away are the stars in Orion?

    I found this image at Science is the Name, Understanding is the Game:

    ...and this one at Astro Bob:

    Campaign for Dark Skies survey: If you're looking at Orion after 7pm, between 31st January and 6th February (2011), and you'd like to support the Campaign for Dark Skies and help combat light pollution (so you can see stars better), the Campaign for Rural England (CPRE) is asking for volunteers to count the stars (excluding the four brightest corner stars) that can be seen in Orion, and fill in a short survey:

    [edit: Just discovered this brilliant tour of Orion using different wavelength's really worth doing!}

    Previous Constellation: Cassiopeia

    Next Constellation: Gemini

    1 comment:

    Anonymous said... favorite constellation!