|The Sun—our closest star|
|Nevermind all the lines, the apple is falling|
on this dude's head due to gravity!
Gravity always acts radially. So, if you have a box full of particles that experience no other force aside from gravity, they will eventually collapse into a sphere (or sphere-like object, depending on what that initial distribution is). Gravity doesn't make boxes or pyramids, nor does it build buildings or cars. It just pulls things 1 and 2 together. Gravity is love! I digress...
The gas that makes up the physical objects that we know of as stars acts much like that hypothetical box of particles that I mentioned before. However, where one might imagine that box of particles with the length of a side comparable to our own size, stars themselves (like our own Sun) are balls of gas that span sizes that are almost beyond our comprehension! And this is AFTER the gas has already collapsed from a far-larger reservoir, called a Giant Molecular Cloud (which I may get to in the future). Though this may prove to not actually help very much for visualizing this scale, for a sense of scale regarding the size of the Sun (our nearest star), the United States is roughly 2,500 miles wide from east to west. According to Google Maps, driving from Sacramento, CA to Washington, D.C. covers 2,731 miles and would take...1.83 days?! An average speed of ~62 mph? That seems...a little ambitious...I'm digressing again. Anyway, in contrast, the average diameter of the Sun is *very roughly* 865,000 (really ~864,950) miles! That amounts to 346 cross-country road trips (or close to 2 years of constant driving) in order to travel from one end of the Sun to the other, going through the center. Its circumference on the other hand is *again very roughly* 2.7 million miles. It's crazy huge.
|A very artistic and barely-accurate|
rendering of a hydrogen atom
So, what do we have so far? Star's are physically large, physically massive, and are made of gas that is gravitationally bound to itself. Awesome, we have progress. However, out of these facts come an interesting question: with all of this mass experiencing all of this gravitational force pulling everything inward, how is it that all of this material doesn't just collapse indefinitely until it becomes just an extremely massive point in space? An initial guess might be that the atoms that comprise this gas act like solid billiard balls, where the gas ensemble will compact down in volume until each atomic "sphere" is physically touching the next one. Let's do a quick and dirty calculation of what that might be like (Christ, there will be so many approximations....).
Let's first approximate the Sun as a hypothetical star being entirely comprised of hydrogen atoms. This comes out to roughly ~10^57 atoms (this symbol "^" means "the numbers following this are the exponent of the preceding number") if you divide the mass of the Sun by the mass of an individual hydrogen atom. Each of these atoms has a radius of roughly 5.3 x 10^(-11) meters, resulting in a volume for every atom of ~6.2 x 10^(-31) cubic meters. I'm going to do something really stupid and highly incorrect and invert this, giving a number density (# of atoms per unit volume) of one atom per 6.2x10^(-31) cubic meters, or 1.6 x 10^30 atoms per cubic meter of space. Given that we have ~10^57 atoms in this hypothetical pure hydrogen star, we divide the total number of atoms by the number density of atoms, giving ~6 x 10^26 cubic meters for the volume of our pure-hydrogen star. Now, if you look up the radius of the Sun and do the volume calculation, you get something like 1.4 x 10^27 cubic meters for the Sun's volume. Then, you can look at the two numbers side by side and say, considering all the approximations we made along the way, that this could possibly work.
HOWEVER, remember that the Sun is not purely hydrogen, and instead has roughly 1 helium atom per 12 hydrogen atoms, with the size of an individual helium atom being roughly 60% of a hydrogen atom. Thus if we were to make our hypothetical star a little more realistic and include helium, as well as some of the heavier elements present in the Sun like oxygen, nitrogen, carbon, neon, and even iron, our hypothetical star with atoms bumping up against one another like billiard balls begins to shrink drastically in size, and the volume may decrease by an order of magnitude (meaning a factor of 10, and no I do not have a calculation for that.). But, the Sun is puffed up to a much greater volume than that. On top of that, the Sun also shines, which an inert ball of hydrogen billiard balls cannot do. What gives? What is it that's holding the Sun up against that ever-present collapse of gravity? The answer: radiative pressure.
|The proton-proton chain of|
So now what do we have? We've discussed that stars are giant balls of gas. We've covered that these giant balls of gas are bound together by gravity, but are held up against collapse by radiative pressure. We've also covered that this radiative pressure is due to nuclear fusion occurring in their cores. And so, stars live dynamically, balancing radiative pressure against gravity! It's dynamic because there's constant action in this stand-off. If the radiative pressure relents just a bit, gravity acts right away to contract the star. If gravity contracts the star just a bit, the core will heat up from this contraction, and the radiative output will increase, puffing the star back up. Now that I've gotten through all that, where do we go from here? There's just so much to cover. I think that next time, I'll start talking about all of the different types of stars and their physical and observational properties, moving up through their evolution, and going over how different stars die. I'd like to go into detail about every stellar population, just so that we (read: I) have a good idea of what makes each population special. I'd then like to go into how they're distributed throughout the galaxy, whether they're in clusters or are cruising along in the galactic field, and how their locations and compositions tell us more about the galaxy that we live in. After that, I think it might be a good time to cut the star stuff, because there's so much more of the natural world to explore, and I'm interested to see what I can dig up.
*Physical quantities like the mass of hydrogen and diameter of the Sun were obtained from the textbook that I used to TA for Astronomy 101 last quarter, whatever that is. I don't feel like looking for it. It's got stars on the front. Unit conversions from km to miles, as well as all calculations were done by me using anything from IDL to my Macbook's Spotlight calculator to my iPhone, and may be a little (or a lot) off due to rounding errors and general laziness. This is certainly not an ultimately-reliable source for facts and figures, just an astronomer rambling about stuff that gets him excited. Enjoy for your own pleasure, but cite at your own peril!