All posts tagged stars

Credit SiBr4 – CC-BY-SA

We live on a rocky ball about 12,750 kilometers through and 40,050 kilometers around its widest diameter. It has a mass of about six quadrillion megatonnes, which is so ridiculously large compared to things we’re used to that it doesn’t really mean anything to us. Our size compared to Earth’s size is roughly equivalent to the size of viruses compared to us.

That is not to imply that humans are like germs living on Earth. It just provides some perspective on where we are. To go a little further, the next level at the same ratio compares the size of the planet Earth to the size of the whole Solar System. Give or take a few billion kilometers. It’s not an analogy that can stretch forever. (Here’s a link to a video that goes from the very small to the very large – 10 minutes)

And it doesn’t imply that humans are merely an unimportant example of a repeating theme. After all, why is our size one of the levels? We could just as easily be included with all of life to fall between the very small and the very large. As in – subatomic particles – living things – planets and stars. But that’s another analogy that shouldn’t be pushed too hard.

We choose humans as a level because we’re human. We tend to look at things relative to what we’re used to, so we don’t think twice about our size being one of the steps on the ladder. Besides, it was necessary to use something familiar as a point of reference in such a wide array of dimensions.

The dimensions of the physical universe are measured by such huge numbers, both hugely big and hugely small, that they don’t convey much meaning on their own. The very large is measured in billions of light years. Light years are trillions of kilometers each. Even a single kilometer is big compared to us. The small is measured in fractions of meters, changing the numbers from positive to negative right about our size. The very smallest things are such small decimals of a meter that we don’t write them out in full, using special mathematical shorthand instead.

In all that vastness, all that range of realities and possibilities, what is most amazing is that part of it is conscious. A narrow band in the middle has produced something that can look out and try to understand the whole thing.

I think we can justify being a little self-centered.

Image credit - Dave Jarvis - CC-BY-SA

Image credit – Dave Jarvis – CC-BY-SA

Click images for larger versions.

How big is the biggest* star? I’m sure most people, once they learn that stars come in different sizes, will ask themselves that question. The answer, of course, is provisional. According to calculations, the biggest a star can be before the force of its radiation overcomes the strength of its gravity is 150 solar masses. (Stars are measured as multiples of our Sun.) At that size they radiate so fiercely that they blow away mass faster than they can accumulate it. However, in theory the conditions were right shortly after the Big Bang for the formation of stars of 300 solar masses. So, let’s say that the biggest star should be 150 – 300 solar masses.

Image credit - Urhixidor - Public Domain

Image credit – Urhixidor – Public Domain

That brings us naturally to the question of how small the smallest star can be. Again, we have to set the criteria. Should a star be something that glows by its own light, or should we be more strict? I prefer the requirement that a star must be heated by hydrogen fusion, as a distinct cut-off. Otherwise, really big planets heated to glowing by intense gravitational compression might be called stars, and the line would be blurred. At 93 times the mass of Jupiter, AB Doradus C is the smallest known star by that criterion. The theoretical lower limit is 75 Jupiters. Below that is the blurry area occupied by objects called brown dwarfs. Another blurry line at about 13 Jupiter masses separates brown dwarfs from gas giants like Jupiter. It would take over a thousand Jupiters to make our Sun.

For a broad picture of star sizes, here’s the Hertzsprung-Russel Diagram.

Image credit - Atlas of the Universe

Image credit – Atlas of the Universe

* for this article, bigness is defined by mass, not physical size.