Stars vs. planets?

Question

A Star is formed when gases our pulled together by gravity. As the gases get more and more condensed they start to heat up. Gravity is still during this process continually making it more and more dense, thus the mass of gas becomes hotter and hotter until it finally gets hot enough to produce nuclear fusion. At this point the nuclear fusion helps counteract the natural force of gravity by burning up it's helium that is produced by the process, thus causing the star to shine and also and more importantly pushing against the force of gravity not letting it further condense it any smaller. If not for the stars nuclear fusion and it burning up it's helium in the battle against gravity it would collapse and gravity would win. In which case once it uses up all it's helium, gravity will win taking it to the next phase of it's life.

My question is this, if gravity is a universal law affecting all mass the same, then why aren't planets..........ran out of characters one sec

....then why aren't planets any smaller than they are being that they have no nuclear fusion to counteract the force of gravity. Shouldn't they become more densely packed than they are especially the gases planets like Jupiter and Saturn etc...?

Answer 1

It takes a certain amount of gravity ot make an object contract and pack itself in more densly. Atoms, and more specifically electrons, push off of each other to reverse the effects of gravity. Neutron stars have so much gravity that the electrons are pushed so far in that they collide with protons to create atoms made up of only densly packed neutrons. The outward force of neutrons keeps the gravity from causing the neutrons to collapse. Black holes are different. When there is enough mass the gravity is so strong that even the outward force of the neutron is not enough to hold it. So the neutrons are crushed under the pressure in to what remains a mystery. So pretty much to answer your question...it is the electrons in an atom bouncing off of electrons in other atoms that keep planets from shrinking anymore. Obviously the greater the gravity the more the electrons will be pushed closer and closer to the nucleus of an atom. I hope this helps.

I heard this from a former student. If electrons didn't exist then you would sink through a chair like a ghost if you tried to sit in it.

Answer 2

Gravity has a number of battlefields.

The first one is when the mass of an object is small (like Earth or Jupiter) gravity is defeated from compressing the mass further by the simple contact of the material in the object. Atom and molecule up against other atoms and molecules. The bigger the object, the more compressed it becomes and the empty space in most matter on Earth becomes squeezed out until the atoms are in virtual contact.
This gives us moons and planets.

To create a star, you need an object with at least 75 times Jupiter's mass to have enough gravitational force and therefore heat at the core to cause hydrogen fusion (stars don't start out fusing helium, they start fusing hydrogen, and switch to helium fusion when the hydrogen runs outs).

The next battlefield is when a star of under 3 solar masses collapses - the speed and gravity of the collapse forces the electrons in the outer shell of atoms and molecules to be pushed into the nucleus and combine with the protons to form neutrons. What we end up with is a neutron star - unable to collapse further because the sea of neutrons (called degenerate matter) resists compression.

The next battlefield is where gravity wins.
Stars that have more than 3 solar masses when they collapse are so massive that gravity can smash the neutrons and basically pulverize everything so there is nothing left to resist the collapse - and you get a black hole.

Answer 3

Gravity does indeed act the same. The difference is that the planets do not have enough mass to contract to the point of self ignition. Jupiter (largest of Sol’s planets) has contracted to the point where it is believed to have a metallic hydrogen core with pressures of about 4 million bars. This is what is believed to be the source of it’s intense magnetic fields. If it where twenty times as massive it would indeed become a brown dwarf star. Even at it’s current size, Jupiter radiates about three degrees more heat than it receives from the sun.

The smaller rocky planets (Earth included) formed out of silicates and other materials rather than hydrogen and helium gasses. They have far to little mass to compress to a density where nuclear reactions could start as well as the wrong material.

And by the way, it is Hydrogen that is the primary source of the nuclear reactions in the Sun. Helium will fuse in larger stars, then Oxygen down to Iron where the reactions end.

Answer 4

Yes, gravity affects all *matter* the same, but it's force depends on the mass:

F = Gm1m2/r^2
If we consider m2 to be a small particle, say an atom, then we can safely assume a mass of 0:
F = m1/r^2
where m1 is the mass from the rest of the particles that form the planet or star.

A planet is obviously less massive than a star, therefore it has a weaker force acting on a single atom than a star does.

And actually, Jupiter can be thought of as a star that failed ignition due to lack of mass, so that's why it is a lot smaller than the Sun(you would need 1000 jupiters to fill the Sun up). When they mention a collapsed star, it does not mean it becomes really really small, but about the size of a Gas giant like Jupiter.

Answer 5

You understand the formation of a star very well. The inner planets of our solar system accreted from rocky material, gravity can only compress the so far. The outer planets formed from gas clouds that were limited in size, enough mass was not available for compression to raise the temperature to the point of fusion.

Answer 6

Size matters.

Terrestrial planets formed much like gas giants and stars, the difference being that their total mass was insufficient to contain the lighter elements, so they ended up balls of mud, covered with No Vacancy signs and evolved primates with a bad attitude.

Jovian planets formed the same way, but due to their size and temperature, the light elements didn't blow off.

Your description of the ingnition of a young star isn't much different than the formation of a planet, but the difference is significant. Get enough mass together and you can have Hyrdogen Ignition!

Burning Helium is really kinda hard... usually a star won't do that until it has to. The core will collapse and there is a Helium Flash. (and if we weren't dead by then we soon would be!)

Answer 7

This is an interesting question. The force that keeps planets like Jupiter from collapsing into something smaller is the interatomic kinetic energy that keeps all gases from condensing. This force is higher as the temperature goes higher, which explains why certain extra-solar planets that have recently been discovered are so large in diameter. They are referred to as "hot Jupiters" and one of them was determined to be roughly the same mass as Jupiter, but it was ~2.5 times larger, because it orbits its parent star well inside the orbit of Mercury, making it ~2,000 degrees, even at its cloudtops. Jupiter's cloudtops are only ~minus 100 or so by comparison. If Jupiter were hotter, it too would be bigger, and as it cools down slowly, over the next several billion years, it should grow smaller in size, though not in mass. Interestingly, a planet that is twice or three times as massive as Jupiter, but at the same temperature, will be almost the same size as Jupiter, and not proportionally larger, as you might expect. This is because the added mass causes the gasses to become more compressed. A planet that is 10 times Jupiter's mass will still be roughly Jupiter sized, believe it or not. Likewise, our planet Saturn is only slightly smaller in size than Jupiter, but it is only about 40% as massive. Its density is less than that of water, and would float. If there was no fusion going on in our sun, it would be substantially smaller, as well. But it would not collapse into a black hole or neutron star, because it is still too small in mass. I believe the minimum mass of a black hole that forms from ordinary gavitational collapse has been calculated to be ~ 3.1 solar masses, (although smaller sized black holes can also be formed if they are smashed to extreme density by external pressures.) I hope this helps.

Answer 8

What would be smaller - a ton of granite - or a ton of helium ?
Gravity depends on the mass.
And fusion doesn't counteract gravity.

Answer 9

Planets are usually formed around stars, they orbit around stars and they are usually round in shape. Planets are usually smaller, stars burn fuel into energy and stars have a life expectancy in which planets generally don't.

Answer 10

their made out of things like carbon and iron and their arent gases. so a gas can be compressed to almost as small as you want it. but a solid cant be. so it gets as dense as it can and doesnt get any more dense. and the core of the earths creates pressure to counteract gravity.