1 October 2014

Constellation Cassiopeia

We've previously looked at the constellation of the Plough. I hope everyone managed to get a glimpse through the snow clouds and find it. Maybe you even had the chance to name the stars (and show off a little to your friends you know something about it! :-) )
Cassiopeia will be good to learn next, because recognising it will help guide you through the sky'll soon see why!

What does it look like?

Let's start off by finding out what this asterism (remember, asterism means pattern of stars?) looks like. Easy! It's shaped like a huge, wide W

(although, because it swings around the North star as the Earth turns (i.e. it is circumpolar), sometimes it will look like it's fallen on its side, and sometimes it will look like an M

Trying to understand where the W shape comes from, I have looked at a lot of images that are supposed to represent Cassiopeia on the Internet. I haven't found most of them very helpful (I can see the stars, but can't make the W shape from the pictures). These are a few of the images of Cassiopeia I found [click on image to see better]:

After some searching, I found an old one I liked, that I could understand where in Casseopia's body the (five) main stars are, which I've marked with yellow dots. The W is on it's side (looking more like an angular 3):

This one is also clear (and appears to be as described by the ancient greeks):

The Main Stars

So, now it's time to name the main stars (there are more, but we'll leave them for a bit): Caph; Shedar (a multi-named star also called α Cas, α Cassiopeiae, Shedar, Shadar, Schedir, or Shedir); Tsi (Gamma Cassiopeia); Ruchbah (Delta Cas) and Segin (Epsilon Cas):

And see what it looks like with a few other stars surrounding it. (Try squinting...that sometimes helps.):

Can't see the stars? (Truth be told, I find it difficult to find the asterisms in photos, though I can see them clearly outside :-) ) Take a look at the left image below. It's exactly the same as the one above, but with the stars of the constellation highlighted with yellow dots; and in the right image the constellation lines are back, so you can compare with the image above (Really, it's easier to see them in the sky!)

Ok, so now we know what Cassiopeia looks like, we need to know where to find it. If we can find the Plough, we can find Cassiopeia:

Where is it?

Cassiopeia and the Plough are opposite each other with the North star (Polaris) in the middle, both rotating around it.

This means that, if the Plough is low on the horizon below the North Star, then Cassiopeia will be high in the sky above the North Star and more easily visible, and vice versa. When The Plough is high, Cassiopeia is low on the horizon. As they rotate around there will be times when both will be about level with each other and the North Star, and all three will be clear.

So, when you can't see the Plough, Cassiopeia can be used to locate the North Star (Polaris):

This is the reason I decided to do Cassiopeia before Ursa Minor (the smaller bear) - because, if you can find both the Plough and Cassiopeia, it will make finding Ursa Minor much easier, since it's tucked nicely between the two!



It's all Greek to me!

Before I go into what objects and interesting things can be found in Cassiopeia, I'm just going to flash this table at you.
Lots of star names are made from combining a greek letter and the constellation name. The nearer to the beginning of the alphabet the letter, the more likely it is that it's a brighter star eg Alpha = brightest, Beta = next brightest and so on....
So, a very quick look at the alphabet, will give you a clue to how 'important'/ prominent is a star within the constellation.
(I'll put this up in the sidebar for future reference.)

Interesting Objects in Cassiopeia

An object map (from Derekscope) shows where some of the following objects are located [as usual, click to see clearer]:

Heart and Soul Nebulae

Cassiopeia seems to be a very busy area of the sky... To start with, there's the wonderfully named Heart and Soul Nebulae [Nebulae = plural of Nebula] (guess which one's the heart!):

...and these can be found here:

Here's an image of the Soul Nebula, containing the the radio source, W5 , a massive star-forming region within the nebula (6500 light years away) (infrared):

This is the famous 'Mountains of Creation', within the soul nebula, a place where stars are born:

...and this image shows how a super massive sun in W5 is blowing off the dust discs from smaller suns (what looks like comet tails), both images taken by the Spitzer Space Telescope:

It's windy in Cassiopeia! Image credit:
NASA/JPL-Caltech/Harvard-Smithsonian CfA

Rho Cassiopeia

Let's talk about active.. There are only seven known yellow hypergiants in the Milky Way, and two of them are in Cassiopeia. The first is called Rho Cassiopeia. It is about 11,650 light-years away from Earth and yet, amazingly, it can be seen with the naked eye. It's about 550,000 times the brightness (luminosity) of our sun! You can find it just here near the end of the W:

This particular star gets brighter and dimmer (it's a variable star) as it throws off its outer layer about every 50 years, and may be close to going supernova (In fact, some scientists think it may have already gone supernova, and may now be a black hole or a neutron star, but that we're just too far away to have seen it yet!)

The other yellow hypergiant in Cassiopeia is called V509 Cassiopeia (V509 Cas), which is also a variable star, though smaller than Rho Cas.

Cassiopeia A (Cas A)

Whilst Rho Cas might have gone supernova, there are two stars in Cassiopeia that have definately exploded. The first was in 1572, which became brighter than Venus in the sky, and the second is called (unimaginatively) Cassiopeia A (or Cas A). This is the most recent (about 1667) supernova in our own Milky Way, and is the 'brightest' radio source in the sky:

 The blown off gas and dust (the remnant) can be faintly seen:

(It's thought that the neutron star in the middle
might actually have a carbon atmosphere)

Although there are many, many images in a google search for Cas A, I had great difficulty finding out where it was actually located!In the end, I estimated its position using the program Stellarium:


Apart from being a hotbed of supernova and star forming activity, Cassiopeia has another claim to fame - it's absolutely, super massive, amazingly big Alpha star, Shedir! If you look at this image, you can see how small it makes our sun (which is actually a pretty average sized star), look:

[Our sun's in the top right corner, and Shedir is marked with a cross]

Though it's so big, and although it is the Alpha star in Cassiopeia, it's not the brightest star of the constellation. That claim to fame goes to Rho Cas (above).


Another object to see if you can spot is a binary star (two stars linked by gravity to each other) in Cassiopeia, which can be found quite close to the second 'V' dip of the W, called Achird:

Distance of stars

As I did for the Plough, I'll leave you with an idea of how far the stars in the constellation are from us. I managed to find two source for this. The first is from Warren-Wilson:

The second takes a 3D look at the stars (visually clearer), but doesn't give numerical distances:

(We, on Earth, are looking up from the bottom right corner)

There is so much more information I've collected for this constellation, for instance:
..but, if I were to put everything there was about this constellation here, I would be writing this for a long time to come. So, I hope there is enough here to get you going and explore Cassiopeia yourself!

Previous Constellation: The Plough

Next Constellation: Orion

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