Questions of a Newbie: Iron Cores in Stars and Planets

[stunner5000pt]

ive read on wikipedia that mercury has an iron core. I also read (in a different article) that certain kind of small stars (red dwarfs i think) will eventually cool down as they start fusing the heavier elements upto iron and the end up with iron cores. Is there a connection here?

Stars are supposed to come from large gas clouds (right?) where did the gas cloud originate? DId the cloud simply form from a 'burnt out' star? SO then other materials just happening to pass by contributed to the cloud?
WHen or why did planets form then? Wouldnt a ball of gas prefer to stay in its state rather than solidify into a terrestrial planet like Earth or venus?

 

[tony873004]

Mercury is not an ex-star. The universe isn't old enough for it to have cooled into a solid.

The gas clouds that form stars are mostly hydrogen, laced with heavier elements created in stars that exploded or shed layers.

Yes, when these other materials happen to pass by, they contribute to the cloud. The cloud needs to slow them to a reasonable velocity relative to the cloud. Initially, they would be screaming through with high velocities, which may make the process of a cloud capturing heavy elements inefficient.

When or why did planets form? You might try Googling for this since it can't sumed up in a few sentences, but I'll quickly try. A gas cloud collapses into a disk, central part of the disk formes a protostar, outer parts collided at low velocities and stuck together. Later, when large enough, the big ones had enough gravity to suck up lots of what remained, hence planets.

 

[SpaceTiger]

Stars are supposed to come from large gas clouds (right?) where did the gas cloud originate? DId the cloud simply form from a 'burnt out' star?

Soon after the Big Bang, everything was in the form of a gas/plasma, so the simple answer is that the gas cloud was just part of the matter that collapsed to form the galaxy.

 

[DaveC426913]

Wouldnt a ball of gas prefer to stay in its state rather than solidify into a terrestrial planet like Earth or venus?

There are only two major forces at work in a cloud of gas.
1] Gravity - attractive
2] Heat and/or radiation pressure - repulsive

- The gas tends to be very cold, having bled off its heat in the form of radiation.
- That means the only force acting is gravity.
- The gas contracts.
- Its temperature goes up (Boyle's Law - temperature and pressure are proportional)

So, we have a gas that is constantly attracting to a central point, but kept at bay by rising temperature. Temperature will drop as the gas cools, but gravity never, ever gives up.

Eventually, gravity overcomes the repulsive force and we have a body. It may contract into a small ball of solid, or, if large enough the heat will come into play much sooner, and push outwards, keeping it as a gas. But it is still under huge pressure - the nuclear process ignites, and we have a star.

 

[Phobos]

ive read on wikipedia that mercury has an iron core. I also read (in a different article) that certain kind of small stars (red dwarfs i think) will eventually cool down as they start fusing the heavier elements upto iron and the end up with iron cores. Is there a connection here?

Planets (at least those that had molten interiors at some time) may have iron cores because the heavier material (iron) sinks to the planet's center. Stars may have iron cores because the fusion process (creating heavier elements) occurs in the center of the star where temperatures and pressures are highest. The 'normal' fusion process can't make elements heavier than iron, so it stops there.

Stars are supposed to come from large gas clouds (right?) where did the gas cloud originate?

As noted above, hydrogen and helium were the major forms of matter that resulted from the Big Bang. Gas clouds (nebulae) are where these bits of matter gathered.

DId the cloud simply form from a 'burnt out' star?

Some did. As stars burn out/explode, they send out new & heavier particles to form/contribute to nearby gas clouds. Solar systems with terrestrial planets formed from clouds that had a good amount of heavier elements.

WHen or why did planets form then? Wouldnt a ball of gas prefer to stay in its state rather than solidify into a terrestrial planet like Earth or venus?

Some event occurs (e.g., passing star or whatever) which nudges/stirs up an otherwise static nebula. This causes the material to interact more and gravity starts pulling it together more (or differently) than before. The collapse (gravitational pulling) is not symmetrical (so you don't get a sphere) and conservation principles flatten it out into a spinning disk. The closer things are together, the stronger gravity becomes...and because of the asymmetry throughout the cloud, you end up with discrete bodies (star, planets, asteroids, etc.) that sweep up the remaining bits of the gas cloud (need not be 100% complete...the planets of our solar system are still sweeping up smaller bodies/dust/etc.