3 October 2014

Constellation Gemini

Before I start to talk about Gemini, a quick side-track... One of the things that used to puzzle me about astrological star signs (which are, of course, based on constellations) was why they represented the wrong time of the year...Why, for instance, was the Gemini constellation in the sky during the winter when the astrological sign was dated May to June? And why did the astrological sign of Capricorn represent December-January, when the constellation of Capricorn is a summertime one?Although I knew the astrological signs were something to do with us going around the sun, it wasn't until I gave it some thought that I realised that we are 'in' a constellation when the sun crosses in front of it. So, if the sun is in between us and the constellation, you aren't going to see its stars, because the sun is in the way (not only as a body, but also because its extreme brightness prevents you seeing any stars in its direction).



 
It follows, when you can see the constellation of Gemini, then the sun is 'in' Capricorn (during the winter), so it is Capricorn we can't see:


 
...Makes sense when you think of it like that, doesn't it! Anyway, enough of that. Time to look at constellation Gemini!

What does Gemini look like?

Most people know Gemini is 'the twins', but when I looked to see what images were available, there were surprisingly few. Most showed the cupid-like Greek version, some showed angels and there were a couple of modern variations:




I'm going to go with this one, since I can see the constellation clearly within the image (also, in mythology, they are twin brothers, hunters that take part in the argonaut story, and have nothing to do with angels!) :

 

Where is Gemini?

If you remember, I looked at the constellation briefly when I talked about the mid-December meteor shower, The Geminids, which has it's radiant (the area where most of the meteors seem to come from)in Gemini. Remember this picture?



The front end of the Plough (or saucepan) points right at Gemini (top right corner) Or if you find it easier to see Orion, then Orion's top left shoulder and arm points towards Gemini:

 

What are Gemini's main stars?





The heads of the twins are marked by the two brightest stars, Castor (on top) and Pollux (below). The rest of the stars appear, to me, to make a kind of rectangle. "Wasat?" you say? (take a look at the star at the waist of the 'Pollux twin') You want to see the constellaton with the stars around? Ok, here it is (only the brightest stars can be seen in this picture - click to enlarge):




So, here's an image without the clues...can you find the main stars?




The most noteworthy stars in Gemini are the two 'head' stars, Castor and Pollux, at the left of the image:-


Castor:

Although this star has been given the name Alpha Gemini, it isn't actually the brightest of Gemini's stars (that award goes to the other twin's star, Pollux). Actually, it isn't one star at all! Like the star Mizar, in the big dipper, it is actually more than one star close together in the sky, which are often used by astronomers to fine-tune their instruments in order to see them seperately (they call this 'resolving' them). Zooming in, Castor can be seen as a triple star, but more surprisingly, each of those three stars is a binary double (two stars held together by gravity)! So...not one star but six! I found the picture below on Manchester University's Jodrell Bank site, which shows the stars quite clearly:





The two binaries, Castor A and Castor B (Alpha and Beta Castor) make a visual double (they look close together in the sky, but might be many light years apart in reality), whilst Castor C (Gamma Gemini) is a degree away (which doesn't look much), and is a pair of faint stars, smaller than our sun, only the twice the diameter of our Sun apart from each other. How they can be so close and not be gravitationally pulled into each other I don't know!

Pollux:

Pollux is an orange giant star about 34 light years away. The following shows the size of Pollux against three other stars:


 

Alhera - spectroscopic binary

If it wasn't for the amazing physics of splitting light into different colours (like a rainbow), and being able to see lines in the spectrum [where atmospheres of suns absorb certain wavelengths] we wouldn't be able to tell that Alhera is a binary star. How the scientists tell a binary, even when we can't see the two stars, is because, while one star moves away from us (and is red-shifted), the other star moves towards us (and is blue shifted) so each line on the spectrum is split into two...one bit moving to the left and the other bit moving to the right on the spectrum. This is what spectroscopy's all about...

Wasat  

 - virtually on ecliptic (and is therefore sometimes eclipsed (blocked from view) by the Sun, moon and other planets. White star with close binary companion (some sources say this is blue , some say it's orange), which is much cooler. The star is a subgiant...in it's early dying stages, and just about to expand into a giant.

What objects are in Gemini?

The main objects of interest, other than the stars themselves, are probably the Eskimo Nebula (also known as the 'Clown Face Nebula' (NGC 2392' ) , Medusa Nebula (NGC 4194), Messier object M35 and Geminga.


 

Eskimo Nebula

Guess what this nebula is thought to look like! Well, actually, though I don't always think space objects look like their description, I can see this one clearly (see pic below).




This is a double-shelled planetary nebula (nothing to do with planets...just a name given to this type of nebula - it might be thought to look more like a planet, because it has puffed off its outer layer, and so looks more spherical than the point of a star). It's over 2800 light years away, and not very bright, but with a small telescope, you can see lines of light coming from the middle section. These are where strong 'winds' of particals (something like our solar wind, but much stronger) from the central star are blown across space. The outer streamers (or 'filaments') are as much as a light year in length! To put that into perspective, it is about 0.0008 light years from the Sun to Pluto, so the lengths of the filaments would stretch about a thousand times that distance!! That's a long way for a sun to be pushing particals!

M35 Open Cluster (can be seen with the naked eye)









When it was first discovered, the Medusa Nebula was thought to be the remains of a supernova, but now it is thought, like the Eskimo Nebula, to be a planetary nebula.




A massive black hole eclipsing a huge star in a binary system ( I don't think you can see this, but at least you can know where it is! :-) ) This is the biggest known black hole, about 15 and 1/2 times the mass of the sun!




How far away are the stars in Gemini?

(In this pic, we are in the bottom left corner looking towards the top right)

Castor  52 ly
Pollux  34 ly
Alhena 105 ly
Wasat 59 ly
Mebsuta 900 ly
Mekbuda (double star) 1200 ly


Another interesting thing about Gemini:

The Geminids

- a meteor shower (shooting stars) which seems to eminate from the Gemini constellation. It occurs every year at the same time (around the 6th to 18th December) as the Earth passes through a particular section of its orbit around the sun. The dust and small particles that light up when they enter our atmosphere, and burn brightly with the immense heat generated by falling through the increasingly dense gases surrounding our planet, are probably the debris broken free from a passing comet when it approached the sun.







Previous Constellation: Orion

Next Constellation: Canis Minor


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