Answer to Question 1
If you were to leap into a black hole of a few solar masses from a distance of an astronomical unit, the gravitational pull would not be very large, and you would fall slowly at first. Of course, the longer you fell and the closer you came to the center, the faster you would travel. Your wristwatch would tell you that you fell for about two months before you reached the event horizon.Your friends who stayed behind would see something different. They would see you falling more slowly as you came closer to the event horizon because, as described by general relativity, time slows down in curved space-time. This is known as time dilation. In fact, your friends would never actually see you cross the event horizon. To them you would fall more and more slowly until you seemed hardly to move. Generations later, your descendants could focus their telescopes on you and see you still inching closer to the event horizon. You, however, would have sensed no slowdown and would conclude that you had crossed the event horizon after about two months.
Answer to Question 2
In studying supernovae in other galaxies, astronomers have noticed that there are a number of different types. Type I supernovae have no hydrogen lines in their spectra, and astronomers have thought of at least two ways a supernova could occur without involving much hydrogen. Type II supernovae, in contrast, have spectra containing hydrogen lines and appear to be produced by the collapse and explosion of a massive star.A type Ia supernova is thought to occur when a white dwarf in a binary system receives enough mass to exceed the Chandrasekhar limit and collapse. The collapse of a white dwarf is different from the collapse of a massive star because the core of the white dwarf contains usable fuel. As the collapse begins, the temperature and density shoot up, and the carbon-oxygen core begins to fuse in violent nuclear reactions. In a few seconds, the carbon-oxygen interior is entirely consumed, and the outermost layers are blasted away in a violent explosion that, at its brightest, is about six times more luminous than a type II supernova. The white dwarf is entirely destroyed; no neutron star or black hole is left behind. This explains why no hydrogen lines are seen in the spectrum of a type Ia supernova explosion-white dwarfs contain very little hydrogen.The less common type Ib supernova is understood to occur when a massive star in a binary system loses its hydrogen-rich outer layers to its companion star. The remains of the massive star could develop an iron core and collapse, producing a supernova explosion that lacked hydrogen lines in its spectrum.
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