An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.

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From the course by University of Rochester

Confronting The Big Questions: Highlights of Modern Astronomy

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An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.

From the lesson

How do Stars Evolve and Die?

The Life of Stars - Star formation, fusion and stellar middle age, black holes and stellar death-

- Adam FrankProfessor

Physics and Astronomy

So in the last lecture, we talked about the idea of the distortion of space time.

Â And how that is really, in Einstein's view, equivalent to gravity.

Â Or that is what we should think about when we think about gravity.

Â Now, we want to think about what happens to a really massive star when it dies and

Â that neutron star that forms at the center is the quantum mechanical forces

Â are too strong to even allow it to support itself against its own weight.

Â What happens then?

Â And before we go there, we need one very important concept.

Â Which is on our way towards black holes and as the concept of escape velocity.

Â And the question here you want to ask yourself is what do you need in order to

Â launch something into space?

Â What do you need in order to escape a massive object?

Â To be able to travel freely out into space.

Â How fast does it have to be moving?

Â And as it turns out, there's a simple formula that we can use based on

Â conservation of energy that tells us that a massive object with some mass r and

Â some radius r that the escape velocity has a very simple form.

Â The v escape is equal to the square root of 2 times Newton's gravitational

Â constant G times the mass of the object, divided by the radius.

Â The important thing about this to see is that the escape velocity increases, for

Â example, if the mass goes up.

Â And that certainly makes sense, if I have a rocket that can produce or

Â that can launch something into space from the Earth, and then I suddenly,

Â magically, double or triple or quadruple the mass of the Earth.

Â I'd expect I'd need a more powerful rocket now to get that satellite into space.

Â But likewise, we could imagine taking the Earth and shrinking it down, making

Â the Earth half the size of its present value or a quarter or a tenth of the size.

Â What we see from this formula is that the escape velocity would still go up.

Â That smaller more compact objects, have very high escape velocities as well.

Â So the important thing to understand, is there's three possibilities here.

Â If I launch a probe or

Â I launch a rocket from the surface of a gravitational object or a massive object.

Â If v, if the velocity I'm launching with is less than the escaped velocity

Â it'll fall back to Earth.

Â It's just like when you take a ball and throw it in the air.

Â Eventually it goes up and then it comes down.

Â If I launch it with exactly the escaped velocity what

Â will happen is I will be able to get that object to go into orbit.

Â It will continually circle the gravitational object.

Â If, however, the velocity is greater than the escaped velocity I will actually be

Â able to get that probe out to infinity.

Â Of course it will take an infinite amount of time to get there, but

Â I will essentially be able to escape the gravitational object.

Â So that idea of escape velocity's going to be very important for

Â us to be able to define the all-important quantities associated, or

Â physical structures, associated with a black hole.

Â So that's what we'll do next.

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