Expert: Jayendra Upadhye - 3/4/2006

Question
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Followup To
Question -
Hi sir,
My question is about the black holes. I read, that the stars, after they have released all their energy, 'sink' inside themselves and have an immense power of suck everything around them. My question is, of which substance are the stars made of and how grows their power to pull, after their energy ends? Thank you.
Answer -
Hi,
Stars are made up of enormous quantity of gas.
So much of it, (99%hydrogen), that even though they are "all gas", their mass to give an example is many million times that of the earth!
The sun which is an average small star, has a diameter in excess of 1,000,000 km!
compare around 12,000 km for earth.
[no my figures are not exact, representative enough].

The "pull" has nothing to do with energy at all!
It is the result of the natural inverse squares law for gravitational pull between any two bodies.

When the star looses its heat source, it cools and shrinks.
As a result, all its matter which was a few million miles out from the center gets compacted into a very small volume.
The larger the initial size of the star, the more is the gravitational tendency to shrink to a smaller denser size.

Beyond a "particular" size, this tendency is so powerful, that in the new shrunken state, the inverse square law dictates a spherical volume of radius named "schwarzchild's radius" be formed, which becomes the "event horizon" of the new compacted body.

Anything inside the "event horizon" can not be seen from outside, as light from the inside falls back on the body and cannnot cross the even horizon. (due to warping of the space-time arund the body).

This results in a "black hole".

far away from the black hole life goes on as usual.

if the sun were to "somehow" (because our sun is incapable of becoming one) be turned into a black hole, all the planets and their satellites would be totally unaffected!
As its event horizon would be a few 10's of km only in radius.

Hope that clarifies things.
Jayen

Hi again,
Thank you for the explanation you gave, but i have to ask again. You say:
'Stars are made up of enormous quantity of gas'. And you also say: 'As a result, all its matter which was a few million miles out from the center gets compacted into a very small volume'...I understand from this, that the stars are made up of matter. Am I right? If so, what is this matter?,My second question would be:
You say, that if our sun would turn into a black hole, our planets would be unaffected of this. Could you tell more about that? Thank you..


Answer
Hi!
Gas falls in the category of "matter" too!
As i pointed out all stars are almost 100% hydrogen to start with.
And they age, they "burn" hydrogen to form "helium".
But again helium is also a gas.
Only very large stars continue further to "burn" (fuse) helium and form carbon, and other heavier elements.
these non gaseous (meaning not gases at room temperature) elements are in gas phase anyway as the temperatures are very high..
These are also concentrated towards the core of the star.
All "burning" stops once an iron core is formed.
The "iron" is NOT solid iron as we know it, but is in embedded in a chaotic plasma form, hardened beyond normal values by the pressure in the stellar core.

The core is so dense that energy liberated by fusion as gamma rays, can only come out by diffusing thru the core, by successive absorption and re-radiation by atoms constituting the "sold" core. It is dark as night inside, as all energy is radiant gamma radiation, unseen to the eye!
As it is re-radiated, it looses a little energy in each step till a million years later it emerges as visible light of lower frequency.

ref:- Carl sagan in Cosmos.

Iron cannot be "burned"..meaning nuclear fusion reactions even if they occur in iron, actually "sap" the star's energy ..they are endothermic.
The star looses all "pressure" and collapses under gravity.

Now hope you are clear what i meant by matter.

It is virtually impossible to put all in writing in a simple way on sites such as this.
you could use the book "birth and death of the sun" written by George Gamov way back many decades ago, if you can find a copy in your local library.


I had the good fortune of reading it in my school days.
Also "Frontiers of astronomy" by Fred Hoyle is another good book, nut it is more general.
jayen