Surlethe wrote:According to Wiki, Canis Majoris is between 15 and 25 solar masses. The theoretical upper limit for a star is, IIRC, about 200 solar masses*, above which the star blows itself apart when fusion ignites.
The Eddington limit depends heavily on a star's metallicity, because massive stars with significant amounts of metals (astronomer's definition!) undergo a different, far faster fusion process than those that don't have metals in their core, and are thus much brighter. Today, the Eddington limit is much lower, about 120-150 solar masses, than it was in the past. In the early universe, it was as high as 3-400 solar masses, because there was nothing heavier than lithium anywhere.
The Spartan wrote:Okay, this is where my lower level of understanding comes into play. How can the universe be expanding faster than light? How do we know that it's doing so? How do we know the magnitude of it doing so?
Obviously, we can't observe it, so I would guess that there are calculations that require it to make sense?
adam_grif got it mostly right. Imagine a magical yardstick, and that every second, this magical yardstick will grow by one inch for every inch long it is, so that another inch-long section is inserted between every currently existing inch-long bit every second (I hope I explained that clearly). Now, before it starts expanding, I mark it in three places: both ends, and one inch from one end. It should be fairly easy to see what will happen: after one second, the two closer marks are an inch farther away from each other, but the ends are a another full 36 inches away. After two seconds, the two nearer marks are four inches away from each other, but the ends are 12 feet from each other, and so on.
This is basically what's happening with space in the universe, as more space is being created between existing space, so the universe expands like the yardstick does. If we make the yardstick cosmically long, and pull the same trick, the ends will at some point move away from each other faster than the speed of light, and the marks are carried along for the ride.
In the yardstick example, the recession velocity of an object is directly proportional to its distance from another (i.e., the rate of the yardstick/universe's expansion is constant). But what if that
wasn't true? What if the recession velocity were proportional to a power of the distance, or an exponential relationship, or something? Well, then the yardstick/universe's expansion would accelerate. So, if we observe such a relation in our universe by looking at a bunch of galaxies, getting distances via Type 1a supernovae and comparing this with their redshifts (which tell us their recession velocities), then we can say the expansion of the universe is accelerating. And indeed, this is exactly what we see today.
As for what is causing this, we have no fucking clue. We call this mysterious force dark energy, which is astronomer speak for "I got nothin'." Seriously, we have absolutely no idea what the hell dark energy is. By comparison on level of knowledge, if dark energy is that guy you met 10 or 20 years ago on the street and said "hi" to, dark matter is your annoying coworker, and the rest of physics is your SO.
