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stitt29
2009-Jul-02, 12:20 PM
A question about supernova. I assumed when a star became a supernova it exploded and it threw out all its matter for miles. Then I read the star doesn't explode it collapses into a denser body all that is ejected is a shockwave and heat and light. Which is correct?

Also because I am assuming that stars collapse into a denser body, how does the theory that there were no heavy elements until the first generation of stars exploded sending heavy elements across the universe work?

To escape a planets gravitational pull requires the projectile to be moving at X m/s if the star collapses into a denser body the gravitational pull will increase requiring an even faster speed for any projectile. If the star collapses and becomes a dark hole not even light can escape this gravitational pull. Just wondering how heavy elements are supposed to have spread throughout the universe taking all this into account

Cougar
2009-Jul-02, 01:02 PM
Stars come in a very, very wide range of sizes, and different sized stars proceed through very different stages in their stellar evolution (http://en.wikipedia.org/wiki/Stellar_evolution).

JohnBStone
2009-Jul-02, 01:17 PM
A question about supernova. I assumed when a star became a supernova it exploded and it threw out all its matter for miles. Then I read the star doesn't explode it collapses into a denser body all that is ejected is a shockwave and heat and light. Which is correct?
Both. There are several types of supernova.

The first is probably an oversimplified view of the death of a large star, which throws off its outer layers as it collapses - a core "star" always remains however. Other types of supernova repeat by accumulating mass from a companion star and then throwing off a surface layer each nova as fusion temporarily restarts.


Also because I am assuming that stars collapse into a denser body, how does the theory that there were no heavy elements until the first generation of stars exploded sending heavy elements across the universe work?
Stars make heavier elements as they burn by fusion. So when they die they have accumulated a lot of heavier elements.


To escape a planets gravitational pull requires the projectile to be moving at X m/s if the star collapses into a denser body the gravitational pull will increase requiring an even faster speed for any projectile.
Gravitational pull is only proportional to mass and distance. So gravitational pull at the same distance is exactly the same. It is only surface gravity that is affected.


If the star collapses and becomes a dark hole not even light can escape this gravitational pull. Just wondering how heavy elements are supposed to have spread throughout the universe taking all this into account
When large stars supernova a lot of matter is thrown off.
Only very large stars can collapse into black holes when they supernova, and even they often avoid the fate by throwing off so much matter in the supernova that they are no longer masive enough to gravitationally collapse.

But yes, there is an awful lot of matter locked up in black holes.

PraedSt
2009-Jul-02, 01:35 PM
A question about supernova. I assumed when a star became a supernova it exploded and it threw out all its matter for miles. Then I read the star doesn't explode it collapses into a denser body all that is ejected is a shockwave and heat and light. Which is correct?
(My bold). The shockwave is ejected material impinging on the interstellar medium, so you just have to adjust that last sentence and you've got it.

The star implodes, then explodes. You can look at this as imploding material "rebounding" off of itself, or you can look at this as the gravitational energy released from the collapse being transformed into kinetic energy of the material.

Elements up-to Iron are produced under normal stellar fusion. Elements above iron are produced during the explosion. So much energy is released that temperatures get high enough for this to happen.

Cougar
2009-Jul-02, 02:37 PM
Only very large stars can collapse into black holes when they supernova, and even they often avoid the fate by throwing off so much matter in the supernova that they are no longer masive enough to gravitationally collapse.

Right. And even before they collapse, very large stars are 'burning' so hot, their outward radiation pressure is so extreme that it is constantly blowing a huge amount of the star's mass out into space.

[Edited to add: I was a little quick to agree with a generally correct statement by JohnB so I could make my point about stars with very high radiation pressure. As Tim really clarifies at Post #7, a collapse into a black hole does not produce a supernova. ]

chornedsnorkack
2009-Jul-02, 05:11 PM
A question about supernova. I assumed when a star became a supernova it exploded and it threw out all its matter for miles. Then I read the star doesn't explode it collapses into a denser body all that is ejected is a shockwave and heat and light. Which is correct?

Both.

There are at least two types of supernovae where runaway nuclear fusion produces so much energy in a short time that the whole star is unbound and explodes leaving no remnant. Type I collapsing white dwarf (slightly less massive than Chandrasekhar limit), and a pair instability supernova.



To escape a planets gravitational pull requires the projectile to be moving at X m/s if the star collapses into a denser body the gravitational pull will increase requiring an even faster speed for any projectile. If the star collapses and becomes a dark hole not even light can escape this gravitational pull. Just wondering how heavy elements are supposed to have spread throughout the universe taking all this into account

While a star might collapse all into a black hole, there are several reasons why it might not. One is the rotation of a star. If a star is large, even though it rotates slowly, the outer layers would have to rotate faster than light to fall on the black hole, which is impossible - so the only thing they can do is get heated and thrown out.

Tim Thompson
2009-Jul-02, 06:03 PM
A question about supernova. I assumed when a star became a supernova it exploded and it threw out all its matter for miles. Then I read the star doesn't explode it collapses into a denser body all that is ejected is a shockwave and heat and light. Which is correct?
In the case of a core collapse supernova, it is the core of the star which collapses to form a neutron star, while the outer layers are blasted into space. The scenario goes something like this ...

Nuclear fusion inside a star cannot form any nuclei heavier than iron. So a massive star will build up a central core of mostly iron, "ash" from the nuclear reactions in the rest of the star. The core will have a very high temperature (in excess of 1,000,000,000 Kelvins). Once that core exceeds about 1.4 solar masses (the Chandrasekhar Limit (http://en.wikipedia.org/wiki/Chandrasekhar_limit)), and temperature about 3,000,000,000 Kelvins, it will collapse from something roughly the size of Earth down to something about the size of a city (20 km or so across). The collapse is catastrophic, and takes about 0.1 seconds to complete. From the point of view of the outer layers, the core has simply & suddenly vanished, and the outer layers respond by falling very fast under the enormous weight of the entire remaining star. The collapsing core, meanwhile, is falling so fast itself that it falls too far and rebounds. As the core rebounds outward, the fast falling stellar stuff rams into it at a significant fraction of the speed of light and the result is a powerful shock wave that propagates outward and eventually blows away most of the star. Material that falls through the shack wave lands on the core, and if it continues to grow and adds enough mass, the neutron star will collapse in turn to a black hole, which does not make an explosion.

That's the way the physics is supposed to work in theory. But the devil is in the details as they say, and so far as I recall, nobody has yet managed to build a computer model of a core collapse supernova that actually explodes. In the computer models the shock wave invariably stalls as it tries to pass through the dense & massive outer layers, so there is no explosion. Obviously we have a few things yet to learn about such things.

You can find all the physical & mathematical detail you could ever hope for in the 172 page review paper Explosion mechanism, neutrino burst and gravitational wave in core-collapse supernovae (http://adsabs.harvard.edu/abs/2006RPPh...69..971K); Kotake, Sato & Takahashi, Reports on Progress in Physics 69(4): 971-1143, April 2006, plus citations thereto and references therein. See also Theory of core-collapse supernovae (http://adsabs.harvard.edu/abs/2007PhR...442...38J); Janka, et al., Physics Reports 442(1-6): 38-74, April 2007.

There is another class of supernova, the type-Ia deflagration supernova. While core collapse supernovae start with a normal star, a type-Ia starts with a white dwarf accreting material from a companion star. A type-Ia supernova (also known as thermonuclear supernovae) leaves no compact core behind, the entire white dwarf is totally destroyed. See, i.e., Roepke, 2008 (http://adsabs.harvard.edu/abs/2008arXiv0804.2147R); Iapachino, et al., 2006 (http://adsabs.harvard.edu/abs/2006A%26A...450..655I).


Also because I am assuming that stars collapse into a denser body, how does the theory that there were no heavy elements until the first generation of stars exploded sending heavy elements across the universe work?
The first generation of stars were made of pure hydrogen & helium and are expected to be very massive compared to stars we see today (Bromm & Larson, 2004 (http://adsabs.harvard.edu/abs/2004ARA%26A..42...79B)). They will generate heavier elements inside via the usual nucleosynthesis, and can explode in the same way. Stars in excess of about 100 solar masses experience supernovae through another mechanism, the pair-production supernova (i.e., Bromm, Yoshida & Hernquist, 2003 (http://adsabs.harvard.edu/abs/2003ApJ...596L.135B); Scannapieco, et al., 2005 (http://adsabs.harvard.edu/abs/2005ApJ...633.1031S); Scannapieco, 2009 (http://adsabs.harvard.edu/abs/2006astro.ph..9208S)).


To escape a planets gravitational pull requires the projectile to be moving at X m/s if the star collapses into a denser body the gravitational pull will increase requiring an even faster speed for any projectile. If the star collapses and becomes a dark hole not even light can escape this gravitational pull. Just wondering how heavy elements are supposed to have spread throughout the universe taking all this into account
As you might guess from the accounts above, the mass that winds up trapped in black holes & neutron stars is a miniscule fraction of the total mass involved. The vast majority of the mass is blown away in the supernova explosions. The speed of the outgoing blast is far beyond the limit required to escape the massive core. Besides, as already pointed out by Cougar, massive stars can lose more than half their mass just from the stellar wind pushed out by radiation pressure, although that effect is much weaker for the first generation of stars which have low metal abundances, as the winds are driven by spectral absorption by the metals.

trinitree88
2009-Jul-02, 06:11 PM
Tim. I believe the only peer-reviwed journal article for a successful explosion was first claimed by the Russians, Leinson & Oraevskii (sp?..I'll check) in the Soviet Journal of Nuclear Physics circa 1986-8. In it they claimed a 30 % larger total cross section for neutrino scattering due to spin waves in the core (magnons). pete

see:http://adsabs.harvard.edu/abs/1989PZETF..49...65L