# Few Question from a Naive Astro enthusiast

1. Dec 6, 2011

### galisuman

Hi to all the astrophysicists out there i have few question I want to know the answers to so here I go

1. Why does earth rotate around the sun in a particular orbit and what factors contribute earth to stick in the orbit.

2. Why does earth has to revolve on its axis is it to maintain the magnetic field around the earth or is there any other reason.

3. How to black holes rotate and suck gases from the external surface of the near by star and due to superheated gases blackholes emit x rays is it true.

4. How can black holes rotate so fast and how to suck in the hot gases from stars as the space has no air???

2. Dec 6, 2011

### Staff: Mentor

The gas and dust that collapsed to form the solar system aquired a rotation in a particular direction during collapse. The resulting orbit of all the planets and most other objects in the solar system is a result of this initial rotation.

The Earth and all other objects are attracted to the Sun via gravity.

See here: http://en.wikipedia.org/wiki/Dynamo_theory

I can't say anything on rotation, but when a black hole or neutron star "sucks in" matter, it isn't like a vacuum cleaner. The material FALLS toward it just like a spaceship falls to Earth during reentry. The diameter of most black holes and all neutron stars is very small, only about 10-20 kilometers, and the material spirals down into it. During this process it collides with other material and gets compressed, generating EM radiation sent out as light, x-rays, etc.

As I explained above they do not suck anything up. They simply have extremely high gravity. When an object such as a star gets close to a black hole the side nearest to the black hole is attracted to it more than the other side is. If this difference in attractive force is strong enough it can rip material from the star and even rip the entire star apart.

3. Dec 6, 2011

### e^(i Pi)+1=0

Black holes rotate quickly because of conservation of momentum. You have a mass, say, the size of the earth compressing to a few miles diameter, of course it's going to rotate much quicker. Just as an ice skater rotates quicker when they tuck in their arms. Aside from matter "falling" into a BH, which it certainly does, BHs, because of their mass and rotation, "whip up" the space around them, creating something of a whirlpool of spacetime.

4. Dec 7, 2011

### rexregisanimi

Hello! )

1. Why does earth rotate around the sun in a particular orbit and what factors contribute earth to stick in the orbit.
As far as current physical theory can describe, the solar system (including the Earth) formed from a large cloud revolving around the Sun several billion years ago. The Earth was formed as a piece of this cloud was pulled together slowly by gravity. (see this video and this other video on YouTube for an OK but not perfect description of these events)

The force that caused this cloud the resulting Earth to orbit around the Sun is gravity. It pulls the Earth toward the Sun and the motion of the Earth forward keeps it from falling into the Sun. (see this video for a good description of this)

The Earth won't slow down and fall into the Sun because nothing will ever slow it down. If something doesn't slow something down, it will keep moving the same way it always has - forever. (This is called Newton's First Law of Motion.) So Earth stays "stuck" in its orbit.

2. Why does earth has to revolve on its axis is it to maintain the magnetic field around the earth or is there any other reason.
The Earth's revolution on its axis probably isn't what produces the magnetic field. Most likely, the Earth's magnetic field is created by the motion of the liquid in the Earth's outer core.

3. How to black holes rotate and suck gases from the external surface of the near by star and due to superheated gases blackholes emit x rays is it true.
4. How can black holes rotate so fast and how to suck in the hot gases from stars as the space has no air???

The first thing you really need to understand before I can answer these questions is that black holes are not really all that different from any other object in the universe. They have gravity, mass, and don't "suck" anything toward them any differently than the Earth sucks on the Moon or the Sun sucks on the Earth. You would get a black hole if you took all the matter in the Earth and mashed it into a ball the size of a marble. That's why black holes are so powerful - they just have really strong gravity in a small space.

If the sun were suddenly turned into a black hole with the same mass as the sun, the planets would continue orbiting just like they do now.

So...how do black holes rotate and suck gas from the surface of nearby stars? Black holes rotate for the same reason the Sun and the Earth rotate - the object that formed them was spinning before it turned into a black hole. They pull gas in from nearby stars because the mass of the gas is attracted to the mass of the black hole by gravity - just like the Moon is attracted to the Earth.

There is "air" in space - you just said so yourself: the "air" is the hot gas coming from the star! That "air" (the hot gas) gets pulled into the black hole by gravity just like I described above. Of course, the gas around a star isn't as dense as the air in the Earth's atmosphere but it's still there. The Earth's atmosphere is just the gas that Earth's gravity is holding down on the surface. The gas a black hole may be sucking from a nearby star is just the "atmosphere" of the star - the "air" around the star.

I hope those answers help! Let me know if you have any questions about what I said. )

5. Dec 7, 2011

### e^(i Pi)+1=0

When you say "the size of a marble" are you describing the size the Earth would have to be crushed for its own gravity to continue the collapse to a singularity? Or would that be the size of the event horizon of a black hole the mass of the Earth?

6. Dec 7, 2011

### rexregisanimi

I was estimating the Schwarzschild radius. (i.e. the radius at which the escape velocity is equal to the speed of light - the definition of a black hole)

The Earth's would be 8.8 millimeters or so. (That's about the size of a marble, right? lol)

rSchwarzchild = m$\frac{2G}{c^2}$

$\frac{2G}{c^2}$ is about 1.48 * 10-27 $\frac{m}{kg}$.

The mass of the Earth is about 5.97 * 1024 kg.

1.48 * 5.97 = 8.84
and
10-27 $\frac{m}{kg}$ * 1024 kg = 10-3 m.

The final result is 8.84 * 10-3 m.

Last edited: Dec 7, 2011
7. Dec 7, 2011

### Chronos

Objects falling into a gravitating mass do not just 'dive' straight into the higher mass body, they spiral in. During this process they aquire high velocities. If the tidal force exerted by the high mass body are sufficiently strong, the infalling body will be torn apart.