5 October 2014

Constellation Canis Major

What does Canis Major look like?

Canis Major is the easier of the two dogs to find. It is below (southwest of) Canis Minor, and its brightest star, Sirius (the Dog Star) can be found by following the line of Orion's belt westward.

This image from Pachamama Trust shows Canis Major and Canis Minor
in relation to Orion's belt and to each other.

The hunting dogs are most often shown as sturdy creatures, but a couple of images (see the two at the bottom left) show Canis Major as a skinny specimen:

Here is the skinny canine along with adjacent constellation, Lupus the rabbit, who he's often thought to be chasing. In this map depiction, it appears this rabbit is likely to escape : 


Main stars of Canis Major


Here's a reminder from StarWatch, how to find Sirius (also called Alpha Canis Majoris, α CMa, or the Dog Star), the brightest star in Canis Major, by following the stars from Orion's Belt:

I'm not sure why Sirius Black in the Harry Potter books was given his surname, apart from ironically. (Although I have seen it is after Sirius B, and supposed to represent the darker, hidden companion star, rather than the dominant Sirius A) His namesake star, Sirius, on the great dog's chest, is not only the brightest star in Canis Major, but is also the brightest star in the night sky. Only the sun, the moon, Jupiter and venus are brighter than it, so it's in the top five brightest of any object in the sky! Here's that excellent 3-D model from Closest Stars again:

This shows that, even though it isn't the brightest star in actual luminosity, because it is so close it looks like it. This is shown better in the image below. The bigger the stars, the brighter (more luminous) they actually are.
In the left circle the stars are shown as bright as they appear to us here on Earth (the bigger the spot the brighter they appear), but in the circle on the right, the stars appear as bright as if each one was put the same distance away from us (standardised by astronomers to be at 32.6 ly). As you can see, many of the stars in the tail end (the rear) of Canis Major are in reality a lot brighter than Sirius, even though is looks brightest to us, because of its proximity to Earth:

(Image from Sky and Telescope)

So, the reason Sirius (meaning 'scorcher' or 'glowing') is so bright is not because it's particularly hot (its temperature will be between 7500-9900K or about 7200 - 9600 degrees centrigrade, not the hottest around!) but because it is nearby. Our sun, though it looks white to us is actually a yellow star, with a temperature of about 6000K.

A bit about star temperatures and colours from Universe Today:

The color of a star is a function of its temperature. If a star looks red,
that means its surface temperature is approximately 2,500 Kelvin.
Just for comparison, our Sun, which actually looks white from space,
measures about 6,000 Kelvin. The hotter the star, the further up the
spectrum you go. The hottest stars are the blue stars. A star appears
blue once its surface temperature gets above 10,000 Kelvin,
or so, a star will appear blue to our eyes.

This chart (above) from Dr. Jamie Love's Principals of Astronomy beautifully shows the relationship between a star's surface temperature and luminosity, and the colour of those stars. One thing to notice is that, in this type of diagram, astronomers tend to put higher temperatures near the axis and cooler temperatures to the right. Though, to us, the sun is super-hot, and it's hard to imagine any hotter, here you can see it is actually towards the cooler end of the range, whereas Sirius is towards the hotter end of the range (but is far from the hottest).

Another thing to note is that, just because a star might have a similar surface temperature to another star, its luminosity might be different depending on such things as size :

If the surface temperature, at any point, is the same (eg, in this image, represented by the flame of a single candle) then, if you are a star with a small surface, you might have room for only a few candles,  whereas if you are a big star, having the candles the same distance apart (the same temperature at any one point) you might have hundreds of candles all over. So, although the small star has the same surface temperature as the big star, because it only has a few candles on it, it will be a lot less luminous than the big star.

This brings us to our next surprise (or not such a big surprise considering how common this is): Sirius is actually a binary star system.

(image: Windows to the Universe)

Once thought to be a single star (Sirius A), it turns out the star's luminosity hid a secret, less bright, companion...Sirius B or the Pup, discovered in 1862 by Alvan G. Clark eighteen years after its presense was predicted by Friedrich Bessel to explain the mains star's wobble.

So, going back to the earlier diagram above, showing temperature and luminosity, we now know it is not entirely accurate. It showed only Sirius A.

In the following diagram, the size of the stars (measured compared to our sun = 1R) are smallest in the bottom left corner (dwarf stars) and increase in size (diagonally) into the top right corner, where we have the largest stars (giants and super-giants). We can see that Alpha star's dimmer companion, Sirius B, whilst actually hotter, is much smaller - about one hundedth of the size of Sirius, and that is why it's so much dimmer. In fact, even though the smaller star has the mass of the Sun, it is only the same size as Earth! It is really hot, but really small - a white dwarf.:

Image:Institute of Astronomy and National Astronomical Observatory of Bulgaria

Sirius A is in the 'throwing off its outer layers' stage of life, whilst the Pup has passed this point. It is the remnant of a star that has lost much of its mass in earlier stages...expanding and contracting, and the throwing off of its out layers and, when there was not enough matter left to defy gravity, and so gravity pulled on what was left of the mass to make it collapse in on itself. This one was not heavy enough to go supernova, nor did it have enough internal heat left to fuse elements such as carbon in its core.

So it sits there cooling down. The heat it emits is not of an active sun, but the heat of a slowly cooling one.  BUT, if matter coming off of Sirius A were to build up around it beyond a critical mass of about 1.4 times the mass of the Sun, it could still go Supernova! Not in our lifetime though. Not for millions of years.

This type of supernova is shown between about 1.36 and 2.05 mins in the Hubble video:

Significant since ancient times, Sirius has been recorded by humanity around the world. Being one of the most prominent star in the sky, it has been given all sorts of significance [too many to list here, so I'll list some of the many significant records and occurrances at the end of this post], in the way humans do - they look for patterns out of chaos to make that chaos more easy to deal with (hence the asterisms of constellations in the first place...though, nowadays its important that these areas of the sky are more clearly defined in order to be able to specifically locate various astronomical bodies):

One thing worth mentioning here is how some think it's significant that the Sun lines up in front of Sirius around the 4th Independence Day. As it happens, they line up (are in conjunction) on the 5th July now. As the author of the following image states, when the first Independence Day was declared, over 200 years ago, the Sun lined up with Sirius on a different day, three days earlier.


This has changed due to something called as precession, the movement (wobble) of the earth as it spins (like this gyroscope on Wiki):



The beta (β) star in Canis Major is Murzim (also called Mirza or Mirzam), a blue-white giant:

(Image: slide from slideshow, Kentwood Amateaur Astronomy league)

A long time ago, in Arabia, it was paired up with the star, Gomeisa (in Canis Minor), and the pair of stars together was called The Herald, or The Announcers, their arrival in the sky announcing the coming of the much brighter Sirius, which rose abover the horizon soon after they did. Murzim is a Cepheid Variable, which pulses brighter and dimmer (though the difference can't be seen with the eye). At a temperature of over 21000 degrees centrigrade, it is hotter than Sirius, but, as we saw earlier, further away, so appears dimmer.


(Alongside Sirius, Muliphen, Adhara and WezenMurzim only just found a place on the Brazilian Flag, below Procyon from Canis Minor:


Strangely, though it is the second brightest star in the constellation, Adhara (meaning 'virgins') is given the fifth letter of the Greek alphabet (see sidebar), Epsilon (ε) Canis Majoris. It sometimes also called Prima Virginum, Thanih al Adzari (also called Menkelb Posterior) being the Secunda Virginum.

In the old Chinese star charts, Adhara was the seventh star of a constellation called Bow and Arrow (don't learn this, as we don't use it today) shooting at Sirius:

(image: Mathisen Corollory)

Adhara is a binary star (surprise! :-) ) that is moving away from our sun. A million years ago, when it was only 34 light years (ly) away, it surpassed Sirius as being the brightest star in the sky.


The third virgin (Tertia Virginum) in the Arabic sky, and the First star of Bow and Arrow (see image of the Bow and Arrow in Adhara, above), Canis Major's delta (δ) star, Wezen, is a supergiant a little cooler than our sun. Within the next 100 000 years it will likely go supernova.

(Image: students at Fresno)


Another of the star 'virgins', and the Second star of the Bow and Arrow, this super-bright super giant is a variable star (getting brighter and dimmer) and is often used by astronomers to measure the brightness/distance of other stars:



Yet another binary, but too far away to see the two stars apart visually, Farud's second star was found by recording and measuring the change of the electromagnetic spectrum as they orbit around each other - a spectroscopic binary system.


Interestingly, the name of Muliphen is used three times in naming the stars, and all three variants are 'gamma' stars: Muliphein is Gamma Canis Majoris, Muhlifain is Gamma Centauri, and Muliphen is Gamma Ophiuchi, of Ophiuchus's

Some deep sky objects in Canis Major

This pic shows the deep space objects very well (as well as the Milky way to the left of the constellation):

(image Science Centre)
(M46, M47 and M93 are in adjacent constellations)

 Messier Object 41 (M41)

[also called NGC 2287]  The main deep-space object in  Canis Major is M41, an open cluster, a group of about one hundred related stars, gravitationally bound to each other, about 2300 ly away. They are usually all formed from the same molecular cloud at around the same time.

(Image: Jodrell Bank Centre for Astrophysics)

The overall group of stars should be visible with the naked eye, but about half of the stars in it are not so bright, but can be seen in an amateur telescope. There are also a few red giants, the best of which, K3, apparently sits right in the middle and stands out against the background like a jewel.

Other deep sky objects

Other open clusters in Canis Major are NGC 2204, NGC 2360 and NGC 2362 (I think I mentioned before that the 'NGC' stands for National General Catalogue?)

NGC 2207 and NGC 2217, marked on the above image, are both 'barred spiral' galaxies. They are quite faint and would need dark sky and a good telescope to see them.

The first, NGC2207 is colliding/merging with another spiral galaxy IC 2163. Where they are merging, the stars become particularly active, with interstellar molecular gas and dust being stripped off some stars and grabbed by others, and there are four known supernovae in this mix:

The other, NGC 2217 is more obviously barred (has a bars of gas and dust stemming from the centre, with arms coming off them)... The galaxy is a little brighter than NGC 2207, and it's a little bit bigger too. The bar across the middle has a fuzziness about it:

(Image: Science Center)

To me, that looks more like the Darth Vader Tie Fighter from Star Wars :-)

Distance of stars in Canis Major

-------- Sirius Snippets -------

(From many sources)

  • When the Sirius rose with (was ‘in conjunction’ with) the Sun, Egyptians knew this heralded the coming of the hot summer – dog days – but also known as the 'Nile Star', alerting the Egyptians to the coming of the flooding of the Nile. Egyptians also called it the Star of Isis.
  • Agean inhabitants offered sacrifices to Sirius and Zeus to bring cooling breezes – its bright reappearance bringing good luck, but if it was dimmed then pestilence would follow.
  • In Ancient Vedas, Sirius (Tistrya or Tistar) was known as the Chieftan's star, or Chieftan of the East. In Hindu writings, Sirius was known as the Rain Star, or Rain God. The Babylonian's knew it as 'm'l KAKSLDI, "the arrow-star"
  • Romans made animal sacrifices to the goddess Robigo so that Sirius wouldn’t cause the crops to have wheat rust.
  • Polynesians used bright stars to navigate between the islands and atolls of the Pacific Ocean
  • For the Māori of New Zealand, Sirius appearance in the morning sky marked the beginning of winter.
  • Name of Character in the Harry Potter books; Sirius Black, like Betelgeuse and Bellatrix may have been named after stars.
  • In the film The Truman Show, a ‘star’ that fell was actually a lamp with the words ‘Sirius (9 Canis…’ [some have taken this to symbolise Truman’s awakening ]
  • Masons revere the symbolism of the Dog Star.
  • The Egyptians may have built the shafts down to the King’s and Queen’s burial chambers to align with major stars, including one that aligns with Sirius.
  • In Egypt, Sirius was associated with Anubis
  • Disney’s song, ‘When you wish upon a star’ is purportedly ‘an ode to Syrius’

Previous constellation: Canis Minor
Next constellation: Taurus

1 comment:

Anonymous said...

A very in-depth post...