Ask HN: What scientific phenomenon do you wish someone would explain better?

The core of the star is the hottest and most dense part. Greater heat and density make it easier for fusion reactions to run. If suddenly the core is made mostly of iron, then the amount of energy it produces rapidly drops. Even if there are nice, easily fusible hydrogen atoms farther out from the core, they will not be fusing at a very high rate, because the temperature and pressure is lower where they are. Also, the more easily fusible atoms remaining outside the core can't diffuse into the core fast enough to refuel it. The only possible outcome is collapse.

In some sense "dark matter" and "dark energy" are just placeholder words for "whatever thing is causing all this weird stuff to happen". This is actually very analogous to how "the ether" was a placeholder term for "whatever thing that radio waves are waves in". (Now we refer to it as "the electromagnetic field". The "ether" terminology was associated with some incorrect assumptions, such as a privileged reference frame, which is why people sometimes say it was an incorrect hypothesis. But the electromagnetic field is certainly real, it just didn't turn out to work like some people thought it did.) Scientists have observed so far the dark matter seems to behave pretty much like ordinary matter, except that it just happens to ignore the electromagnetic and strong nuclear forces. Not only does it hold galaxies together, but its gravity also bends the paths of light rays, just as we expect of anything massive. So calling it "matter" isn't too much of a stretch. It's still very mysterious, though.

Radiation pressure actually does limit the mass of stars, to something on the order of 100 to 200 solar masses, see this stack exchange question: https://astronomy.stackexchange.com/questions/328/is-there-a... That doesn't stop smaller clouds of gas from collapsing to form smaller stars, though.

Imagine you have two spaceships in London. At the fire of a starting gun, they both take off, and fly around, each one taking a different path. Each ship has a clock that records the time elapsed since it took off. Eventually, they both return to the same landing pad in New York, landing at the same time. Thanks to time dilation, the readings on the clocks of the two ships might be different, even though they started and landed at the same places and times. Imagine we start both ships from a space station in deep space. One ship doesn't leave the station at all, it just stays in its docking bay, with its clock ticking along. The other flies down to the surface of Earth, sits there for a few years, and then flies back up to the space station. Thanks to the gravitational field of Earth, the ship that stayed home has more time elapsed on its clock than the ship that went to Earth.

Now suppose that each ship is carrying one end of a wormhole. The clocks on either end of the wormhole must stay synchronized. Someone sitting in the middle of the wormhole would be able to see the inside of one ship by looking to their left, and the inside of other ship by looking to their right. The clocks start out synchronized. As you point out, no matter how the ships move about, this does not change as the ships fly around. Anyone standing in the middle of the wormhole always sees that the clocks on the wall of each ship are in sync.

So: Entering the wormhole from ship A when ship A's clock reads X means you exit at ship B, at the time when ship B's clock reads X. And vice-versa for going from B to A. Now thanks to time dilation, ship B might arrive back at the station when its clock reads X, while ship A, which stayed behind, has a clock reading of (X + 20 minutes). If you are the station master, you can go into each ship to look at the clocks, and you will find that ship A's clock is always 20 minutes ahead of ship B's. But suppose that instead of walking between the ships through the station, you use the handy wormhole that connects them directly.

Suppose you enter ship B when its clock reads Y, and walk through the wormhole. You exit at ship A when its clock also reads Y. Then you step out of ship A, and walk through the station to ship B. It's clock reads (Y - 20 minutes), since according to people on the station, ship A's clock is still 20 minutes ahead of ship B's. When you originally entered ship B, its clock read Y. It now reads (Y - 20 minutes). Time travel. By retracing your path in the opposite direction, you can also travel 20 minutes into the future.