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Chunky
2008-Dec-31, 05:30 PM
what would happen if you launched thousands of planets at a star?

would it turn into a black hole?

would it change into a diff type of star?

would it prolong the stars life?

Hornblower
2008-Dec-31, 05:52 PM
what would happen if you launched thousands of planets at a star?
Let's start with a simple example. Drop 1,000 Jupiter-like planets, mostly hydrogen and helium, into a star, thus increasing its mass by one solar mass. After some initial fireworks, it should stabilize as a more luminous star.


would it turn into a black hole?
Not immediately. For a very massive star, the final remnant after burning out might gravitationally collapse into a black hole when it would not have without the accretion of additional mass.


would it change into a diff type of star?The spectral type would change. For our Sun, it would change from the present G type to an A type like Sirius.


would it prolong the stars life?No, a more massive star burns out sooner.

Durakken
2008-Dec-31, 05:54 PM
depends on the type of planet and star... Throwing say Jupiter at the sun would probably cataclysmic, on the other hand throw mercury in and and i doubt there would be much change if any at all.

PetersCreek
2008-Dec-31, 05:58 PM
As I recall the process, a star can become a black hole at the end of its life cycle if it weighs in at more than 20 solar masses or so. However, it's not just the mass itself that does the trick. The star must also lack enough internal pressure to withstand gravitational collapse. So, just from a mass perspective, throwing 20 MSol at a middling star in its prime wouldn't do the trick, I think.

Additionally, I'm unsure what mass would be assimilated and what would be ejected by the impacts. I'm also not clear on what effect the addition of so many heavier elements would have on the star's processes, assuming we're talking about rocky planets and not gas giants. Perhaps it would have a quenching effect if the mass addition was rapid enough? I'd be interested in hearing more about that from the better informed here.

But if you want to carry this experiment out...preferably on a star more remote than our own...I hope you have planets to spare: 1 MSol = 332,946 MEarth.

Chunky
2008-Dec-31, 06:03 PM
what if we never stopped feeding it?

would it grow in size until it encompasses everything?

cosmocrazy
2008-Dec-31, 07:41 PM
what if we never stopped feeding it?

would it grow in size until it encompasses everything?

What do you mean by encompass everything?

It is predicted that when our sun is older it will become a red giant and be so large it may encompass the earth's orbit, so gobbling up the inner planets in the process.

If you keep adding mass to mass, after some other forms such as stars, neutron stars, super nova and the like (dependent on the nature of the mass added) eventually you end up with a black hole due to the gravitational effects that collapse the matter to a "singularity". Continue feeding and you end up with a very massive BH with a very large event horizon. Add all the mass of the universe and you end up with possibly a BB (big bang) scenario all over again.. who knows..:)

Cheap Astronomy
2009-Jan-01, 06:26 AM
It is not clear that concentrating all the mass of the universe into one spot would collapse the universe. Under current thinking dark energy would just keep accelerating the expansion of empty space.

Trying to throw all the matter into one spot would also take quite some time -you would have a very large accretion disk rather than an instant black hole.

We already have supermassive black holes (http://en.wikipedia.org/wiki/Supermassive_black_holes) of several billion solar masses in the middle of galaxies - they are yet to destroy the universe.

AonSao
2009-Jan-01, 12:45 PM
The title of your post really grabbed my attention. Ignoring black holes for a bit... would 'throwing hydrogen' at our star be a viable method for prolonging its life? Or would it just be useless since (as far as I know) those reactions take place internally in the sun.

Hornblower
2009-Jan-01, 01:54 PM
The title of your post really grabbed my attention. Ignoring black holes for a bit... would 'throwing hydrogen' at our star be a viable method for prolonging its life? Or would it just be useless since (as far as I know) those reactions take place internally in the sun.
It would be worse than useless. Not only would hydrogen piled onto the outer layers of a solar-mass star fail to reach the core, its added mass would actually shorten the lifetime of the star.

Let me expand on what I said in post #2. The internal dynamics of a more massive star are such that it is disproportionally more luminous, and is burning up its supply of hydrogen so much faster that the lifetime is actually shortened. If we were to do that with our Sun, it would scorch us into oblivion during that shortened lifetime.