Thread: 3 probably dumb questions

I have been watching the series, The Universe with great interest and there are a few of things that i am curious about, well more than a few but lets start with just 3 questions.
1st. From what i can gather, gravity and the effect it has on stars and planets is heat. As something is more compressed by gravity it creates friction and this creates heat, hence volcanoes on Neptune and so on. with a star, the gravity creates so much heat that it starts to effect elements such as hydrogen and helium and they create fusion and then eventually, gradually, they create higher based elements such as calcium and iron. By this reasoning, i was wondering how much heat a black hole creates. In all the discussions on black holes they are described literally as a hole that nothing can escape from, not even light. But what about heat? the only way to recognize a black hole is by the gravitational effect it has on stars around it but what if they were looked at in the infra red spectrum? Do black holes give off heat? How hot do they get and what elements are they creating?

2nd. It has been said that the universe is 156 billion light years across. Now, if nothing can go faster than the speed of light then i would think the furtherest thing we could see away from us would be about 13.5/14 billion light years. Even if we looked at the furtherest object to our south and the furtherest from our north that would still only be about 27/28 billion light years. How can they tell that the Universe is 156 billion light years across? I mean they cant possibly see anything in this universe that is say, 25 billion light years away so how do they know it is 156 billion light years across?
3rd. As a star starts to burn up hydrogen and helium and create heavier elements, does it become more massive (heavier) and will this effect happen to our sun and increase its gravitational strength. Or, does the act of fusion cancel out the weight difference and the sun will stay relatively the same weight until it dies.

1) Heat is mainly transferred by either conduction (molecules bumping into each other, transferring kinetic energy), convection and radiation (Infra Red Light ). IR is light, so it still travels at C and so can't cross the event horizon, so IR telescopes won’t be able to see an isolated black hole. Another mechanism whereby a black hole can lose energy is Hawing Radiation, but it is such a slow process (the bigger the black hole, the slower it happens) that I think not enough can escape for a telescope of any kind to be able to pick it up.

2) The universe has undergone periods of varying rates of inflation since the big bang. Inflation is the expanding of space itself, so a particle can expand with it at speeds exceeding C relative to another particle in another part of space. That means that space-time could have expanded to dimensions much bigger than one governed by C. As far as I know, the current theory of the speed and duration of inflationary periods predict the dimensions of space-time that you quoted.

3) The energy released by a star is actually matter being converted into energy according to the famous equation E=mC². Basically, the constituents of the product have more mass than the product of fusion and the left over mass is converted into energy. So actually a star loses mass as it ages.

1. So is a Black hole incredibly hot or not and is there any way to measure its temperature.
2. I still dont get how they can tell that the universe is 156 billion light years across. What are they measuring to make that assessment? If they are measuring how fast things are moving away from each other or how fast the visible universe is expanding, how do they work out that it is at least 10 times larger than we can see or will ever be able to see. I dont see how we can detect anything beyond 14 billion light years from earth, so i am having trouble working out what methods they use to believe it is so much bigger.
3. So the gravitational strength of the sun will weaken over time and earths orbit will become larger? As the sun gets hotter and hotter, earth will get further and further away rite?

1. Heat is one form of electromagnetic radiation, like light. Since light cannot escape a black hole, then heat can’t either, at least not directly. However, virtual particle pairs are created near a black hole’s event horizon from the energy in the accretion disk. If one of the virtual particles falls in, then the other particle gains enough energy from the black hole to become a real particle. This is the process known as Hawking Radiation. This is a method for calculating the temperature of a black hole’s Hawking Radiation: http://library.thinkquest.org/C00757...ance/core6.htm
2. When we see a galaxy that is billions of light years away, we are actually seeing the light that galaxy emitted billions of years ago. However, the galaxy is now even farther away, because the universe has been expanding for billions of years. So we use the rate of this expansion to calculate its likely present distance.
3. Actually the opposite. As the sun ages and loses mass, its outer layers will expand almost to the orbit of the Earth. This may cause the Earth’s orbit to decay in such a way that it will pass into the sun. The outer planets will most likely move away from the sun, though.

Originally Posted by Arch2008

1. Heat is one form of electromagnetic radiation, like light. Since light cannot escape a black hole, then heat can’t either, at least not directly. However, virtual particle pairs are created near a black hole’s event horizon from the energy in the accretion disk. If one of the virtual particles falls in, then the other particle gains enough energy from the black hole to become a real particle. This is the process known as Hawking Radiation. This is a method for calculating the temperature of a black hole’s Hawking Radiation: http://library.thinkquest.org/C00757...ance/core6.htm

Thank you, this would seem to suggest that black holes do get very, very hot and the greater the mass, the hotter they get.

Originally Posted by Arch2008

2. When we see a galaxy that is billions of light years away, we are actually seeing the light that galaxy emitted billions of years ago. However, the galaxy is now even farther away, because the universe has been expanding for billions of years. So we use the rate of this expansion to calculate its likely present distance.

Thank you again

Originally Posted by Arch2008

3. Actually the opposite. As the sun ages and loses mass, its outer layers will expand almost to the orbit of the Earth. This may cause the Earth’s orbit to decay in such a way that it will pass into the sun. The outer planets will most likely move away from the sun, though.

And, thank you again. Oh, i was just wondering, if stars can turn hydrogen into greater mass elements such as oxygen and so on, is there any process that can do the reverse?

Free neutrons can decay into hydrogen:
http://en.wikipedia.org/wiki/Free_neutron
Other heavy elements decay into lighter elements, like uranium gradually changing to lead.

Originally Posted by GrowlingDog

I have been watching the series, The Universe with great interest and there are a few of things that i am curious about, well more than a few but lets start with just 3 questions.
1st. From what i can gather, gravity and the effect it has on stars and planets is heat. As something is more compressed by gravity it creates friction and this creates heat, hence volcanoes on Neptune and so on. with a star, the gravity creates so much heat that it starts to effect elements such as hydrogen and helium and they create fusion and then eventually, gradually, they create higher based elements such as calcium and iron. By this reasoning, i was wondering how much heat a black hole creates. In all the discussions on black holes they are described literally as a hole that nothing can escape from, not even light. But what about heat? the only way to recognize a black hole is by the gravitational effect it has on stars around it but what if they were looked at in the infra red spectrum? Do black holes give off heat? How hot do they get and what elements are they creating?

2nd. It has been said that the universe is 156 billion light years across. Now, if nothing can go faster than the speed of light then i would think the furtherest thing we could see away from us would be about 13.5/14 billion light years. Even if we looked at the furtherest object to our south and the furtherest from our north that would still only be about 27/28 billion light years. How can they tell that the Universe is 156 billion light years across? I mean they cant possibly see anything in this universe that is say, 25 billion light years away so how do they know it is 156 billion light years across?
3rd. As a star starts to burn up hydrogen and helium and create heavier elements, does it become more massive (heavier) and will this effect happen to our sun and increase its gravitational strength. Or, does the act of fusion cancel out the weight difference and the sun will stay relatively the same weight until it dies.

Sorry GrowlingDog, as my post has nothing to do with your profound questions but I must point out that I love the way you write. Your choice of words is perfect and quite relaxing to read.

1. heat if infrared radiation. so all forms of light cant exscape including infrared.

3. fussion makes the hydrogen particle collide and make helium. but that whould make the star lighter because it is making energy appear by lossing matter. so it would not make matter apear out of nowhere.

Dog have pretty smart questions!
Much explanation is done by Kalster & Arch, but I think the fact universe is only 14b year and it is 150b year wide is not properly worked out with the rate of it’s expansion.

Originally Posted by unoscooter

3. fussion makes ... ... it would not make matter apear out of nowhere.

True, but gravity is a property of mass and hence a star can gain or at least maintain its present pull.

GrowlingDog. 1) Stars work by nuclear fusion under the force of gravity and mass, making light elements into heavier elements. Hydrogen is a proton and an electron. Helium is two neutrons, two protons and two electrons. For convenience, a neutron is a proton and an electron. So in the sun, four hydrogen atoms become one helium atom and the waste products are heat and light.

All black holes spin. As a singularity is a dimensionless point, it cannot spin so they don't exist. A neutron star has upto 2/3 light speed escape velocity, and at that pressure, neutrons still exist. A black hole is over light speed escape velocity so not really much more. Neutrons have of course broken down. Protons have almost certainly broken down. That leaves a spinning ball of quarks (from protons) and electrons at their centre. At such densities, they could be trillions of degrees centigrade. Black holes were recognised by the hard gamma radiation from material falling into them. we can now detect accretion disks around them, of matter waiting to fall into them.

2) The idea is that all space is expanding and everything is moving away from everything else, though no part of it is expanding faster than light. Basically, the bigger it gets, the faster it can expand, like dots on a balloon being blown up. The reason galaxies don't drift apart is said that local gravity is sufficient to overcome this. However there are some problems with the idea to the extent that some scientists have recently given some thought to the old steady state Universe.

3) Our sun is currently losing four million tons of mass per second but since it masses about 2x10^27 tons, it will keep burning for billions of years yet. As it loses mass, so it's gravitation pull gradually diminishes over cosmic time.