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So the intention of this thread is for everyone to share some tidbits of trivia about astronomy. Anything that you know which you think the others may find interesting. Also, ask your questions here, chances are there might be someone who knows.

I'll start with some info of my favourite star, Betelgeuse or Alpha Orionis, the left shoulder of Orion. I've heard various pronunciations of the name Betelgeuse but the most common one, and certainly my favourite, is "beetlejuice".


The reasons I like this star so much is because:

1. It is a fairly bright star. In fact, it is the 9th brightest star in the sky. This is partly because it is reasonably close by (something between 400 and 600 light years) and partly because it is a particularly luminous star.

2. It is a monstrously big star. In fact, we don't know of many other stars that are bigger. It is estimated that it has a diameter 1000 times that of the sun which means that, if it was at the center of our solar system, it would have engulfed Mercury, Venus, Earth and Mars with its surface extending to somewhere between the orbits of Mars and Jupiter. This is on BIG puppy!

3. It is a relatively cold star (in stellar terms of course) and estimated to have only about 20 times the mass of the sun, despite having such an enormous diameter.

This is where things become particularly interesting because, Betelgeuse is nearing the end of its lifetime. In fact, it may have died already but it will take us somewhere between 400 and 600 years to find out about it.

Seeing as Betelgeuse is a red supergiant (relatively cold, very big but not overly heavy) it satisfies just about all the criteria to meet its final fate in a spectacular supernova of note. Its mass of 20 solar masses puts it comfortably over the Chandrasekhar limit which means that it will die in a supernova but not far enough to likely end in a black hole.

When Betelgeuse explodes, it will most likely be significantly brighter than the full moon, making it the brightest object by far in the skies (apart from the sun of course).

Also, the rotational axis of Betelgeuse does not point towards earth so, after the supernova, if it ends up as a neutron star, we will not be bombarded with gamma rays (which could make life on earth rather nasty).

So it seems the makings are there for a truly spectacular stellar event and here's holding thumbs that it happens in my lifetime.

Thanx, Deefstes!
That is very interesting, more please?

Cool facts, deefstes!

OK, anybody else? Spill it. We're all ears.

Very Interesting!

That's not what I was asking for, I was asking for more trivia.

You weasel!

Eish, Deefstes…There is really not much I can contribute to this topic …..My knowledge about stars only goes as far as that I know they are holes in Heaven’s floor.
I will however be an avid reader of this topic….especially with such interesting info shared

Aag okay then. Let's see, anyone keen for some introductory astrophysics? Let's have a look at star classification.

With all the gazillions of stars out there it helps to have some sort of a classification system, I suppose this is true for just about any other subject. We have a binomial nomenclature system for living organisms which forms part of a larger taxonomic ranking system. So to better make sense of all the different types of stars there is also a classification system for stars. Here's the rub.

All stars can be sorted according to two attributes:
1. Spectral Type - This is basically the colour of the star and it is a direct function of the temperature of the star.
2. Luminosity - This is basically how bright the star is and is mostly a function of the star's size. Take note that this is not how bright the star appears as seen from earth but rather its absolute brightness.

Unfortunately neither of these two attributes are particularly easy to measure as they are both affected by distance and particularly the latter. The colour of a star is affected by distance because all objects in space are moving away from a certain point (the point where the Big Bang is assumed to have taken place) and very distant objects are moving faster than objects closer by. This means that distant objects' colour is affected by "red shift" more so than objects closer by. I could write up a little something about red shift in another post if anyone is interested. The brightness of a star is affected by distance for obvious reasons.

In order to classify a star it is necessary to know the distance to the star. There are various ways to determine the distance to stars and this is also something that might make for an interesting post if anyone is interested.

Now you may have guessed that hotter stars are also brighter stars while cooler stars are dimmer. This is true for the most part but it is not always the case and there are actually a large number of exceptions. If you where to plot all stars on a graph of which the horizontal axis gives their temperature or colour while the vertical axis represents the luminosity, you get what is called the Hertzsprung–Russell diagram (or HR Diagram for short). Here is a HR Diagram plotted for some 23,000 stars, grabbed from Wikipedia.


Each one of those specks represent a different star and you can clearly see that the vast majority of stars falls in the band called the "main sequence". The HR Diagram holds lots and lots and lots of information and I won't be able to explain all of it here (mostly because I don't understand all of it anyway).

Another very interesting topic is the development and aging of stars. I won't be able to go into that in too much detail either but what I can say at this point is that stars typically begin their life at the bottom right of the main sequence and move towards the upper left as they age. At some stage in the star's life, depending on it's size and composition, it may branch off from the main sequence to become a giant or supergiant. Alternatively, if it does not have enough fuel to burn it can shed its outer layers as a planetary nebula, become a white dwarf and slowly fizzle out, sliding along the white dwarf curve from top left to bottom right (this is in all likelihood how the sun will meet its final demise).

It means the HR Diagram can be used to make predictions as to the future of a star, the age of the star, the relationship to other stars (it becomes an interesting exercise to plot the HR Diagram for all the stars in a specific open cluster galaxy for instance - but that's yet another story).

Just to put our sun in perspective, it can be noted that it falls pretty close to the center of the diagram. It sits on the main sequence at a luminosity of 1 and a colour index of 0.66 (temperature of 5780 Kelvin). This means our sun is a pretty ordinary star. It is still fairly young, has a pretty average temperature and pretty average size.

Betelgeuse on the other hand sits on the very top right of this diagram. It's pretty cold, very big, very old and about to go whallop.

Hi deefstes, thanks for this great thread.

The facts surrounding the HR Diagram are fascinating.

My knowledge regarding Astrophysics is next to nothing but I will sure follow your posts and perhaps get a lot wiser!!

So by the way, are you the same deefstes as the deefstes on the SB forums?

Deefstes, another brilliant post!
Please stop asking if we want to know 'cause we want to! Tell us!

Oh and b.t.w., thanx for speaking English and not some higher form of Astrophysical mumbo jumbo.

Hi deefstes,

sorry it seems you are all on your own....
fascinating, i am all ears to learn..
keep going! if i can find something out then i will post it

Great post once again..........Theres so much out there to be learnt!