How does a star form? What's at the center?

Sure sounds simple - gas and dust collect under gravity and when enough pressure is built up you have a star. But here’s what I don’t get. Say you’ve got a planet such as Jupiter in the middle of a dust cloud. Over eons it gains dust and gas. At some point fusion would occur. At this exact moment, wouldn’t the outward pressure from the fusion prevent the star from keeping this reaction going? Before the star lit up, there was just barely enough pressure to start fusion, so it seems to me that as soon as it starts the star should go out. The shock wave should blow some material off the star so it shouldn’t restart either. Not until it collects more matter and then the cycle should repeat. Seems like it should only be able to light and stay lit if a lot of material was added to it at one time.

One other question. I’ve read that once a star begins to make iron that it only has a few seconds to live. How? Being that the inner planets are full of iron, wouldn’t that stand to reason that the sun already has some too? And wouldn’t this iron be at the center? I realize iron does not emit energy as it fuses to new materials (either that or no energy is made from fusing elements into iron). But the sun should be able to keep fusion going around the iron. I’d think that fusion would only stop once iron filled the entire area being used for fusion.

Jupiter and other “failed stars” like it never become stars - only the much larger collections of hydrogen manage to reach critical mass, at which point their own gravity prevents them from exploding. The star ignition stage only takes place at the birth of a solar system, and not later.

Basically, once they form, stars and planetary bodies don’t get any more massive.

First of all, the sun may have “some” iron, but it’s just trace amounts. Stars (only the largest, incidentally) collapse when a significant portion of their core - many, many times the mass of our entire solar system - turns into iron.

Second of all, iron causes the star to collapse because of pressure. Once the inward pressure cause by the star’s gravity is no longer balanced by the outward pressure of its fusion - kaboom. It’s no that there’s no fusion; it’s just that there isn’t enough.

In fact, fusion does keep going on around the exhausted core; that’s one of the things that makes old stars expand into red giants. The zone where fusion is occurring begins to move outward from the core as the core begins to collapse; and heavier elements begin to fuse, and their fusion zones begin to move outward too.

I think the fusion starts pretty gradually, and it is not violent in the sense that atom bombs are. Consider that small stars last billions of years. That means that the average wait for any particular hydrogen atom to get fused with another is also billions of years.

>Basically, once they form, stars and planetary bodies don’t get any more massive.
Alessan, this isn’t right. For one thing, stars and planetary bodies don’t form in a moment, they gradually grow, so the smaller earlier star or planetary body must have continued to get more massive. There is a limit for stars, the Eddington limit, that prevents very massive ones from accumulating mass - their power is so great that the resulting wind overcomes gravitational infalling. IIRC, Eta Carinae and the so-called Pistol Star are examples of stars somewhere around the Eddington limit, which is a few hundred solar masses, and fairly recent numerical modeling work has demonstrated that stars around this limit become nonsymmetric in such a way that the limit is probably higher than previously thought. Please, anybody more up on this correct me and expand on this!

You obviously know more about the subject than I do, so I’ll concede the point. Still, it seems to be that in established solar systems, like ours, the sun and planets rarely acquire much more mass, as there isn’t that much raw material floating around to acquire. I don’t think our sun will ever get more massive (it’ll get bigger in terms of diameter, but that’s a different story), and Jupiter will never become a star, barring monolith activity.

A star’s fusion is no where near as fast as that of a Hydrogen bomb. It’s not like suddenly the whole core fuses at once.

Here’s a relevant fun fact: Pound for pound, you produce more heat than the Sun. By roughly a factor of ten. The Sun “igniting” is just it starting to generate some heat from fusion, not like a bomb going off.

Let’s not get hung up on our Jupiter becoming a star now. The OP wasn’t suggesting that.

He(?) posited a proto-star that had grown to roughly Jupiter-sized (i.e. well below minimum size for fusion ignition) while still surrounded by plenty of loose dust & gas. He then asked about what happens next as it accrets more matter and eventually does grow to the ignition mass.

I don’t have an answer for the OP. But I’m sure somebody does and I too would sure like to read it here.

First, a proto-star goes on American Idol…

ZenBeam, can you do me a favor and give me an online cite for that?

It’s not that I don’t believe you - it’s that this is such a neat fact that I’d like to bookmark it for future reference.

Well, here’s a ballpark estimate just using the Wikipedia page on the sun.

People put out about a watt per pound.

The sun’s mass is 1.9891×10[sup]30[/sup] kg or about 4.4×10[sup]30[/sup] lb.

It releases energy of 3.846×10[sup]26[/sup] W.

So humans beat the sun by a factor of 10[sup]4[/sup] or 10,000.

AFAIK, beginning of fusion in the core of a star is not some sort of endpoint, but a step along the way towards hydrostatic equilibrium. That is, fusion may start in the core but the star continues to contract until fusion is prevalent enough that its outward pressure balances out the inward gravitational pressure.

Yes, this is exactly what I was wondering about. I’m not referring to Jupiter specifically, just a hypothetical Jupiter-like planet sitting in a cloud of dust and gas gaining mass.

The “Stephen Hawking’s Universe” program a year or so ago made it sound like the fusion reaction started fast once the mass hit a certain level. In Emeril’s words ‘add some gas, add some gas, BAM! Fusion!’. And it described a large shock wave blasting material away from the star. That’s ultimately what prompted the question. If that actually happened there shouldn’t be enough left to keep it going.

I did the calculation in another thread on this board, but I’ve been searching and haven’t been able to find it. The “factor of ten” I remember may have been relative to the Sun’s core, where the fusion actually occurs.

When a main-sequence star forms, there isn’t a compact dense object like a Jupiter sitting at the center of the gas cloud collecting matter. Rather, there is only a large, collapsing gas cloud whose density is climbing. The whole star’s worth of mass (and more) is already gravitationally destined for collapse. The initially rarefied cloud of gas gets denser and denser, heating up initially due to gravitational energy release and eventually due to fusion itself, but by then, the proto-star is plenty heavy enough to maintain fusion after ignition. You can think of the proto-star as equal to the whole gas cloud, just squished down to be dense and local enough that “star” becomes a better word than “cloud”. (This misses complexities, of course, but it is closer to the truth than the Jupiter-collecting-mass picture.)

Did anyone else think of How is babby formed??

The presence of iron in the sun is, as you surprise, irrelevant to the sun’s longevity. It’s when a massive star starts making its own iron that trouble is brewing.

Late in a massive star’s life, when the core is hot enough to fuse silicon into nickel-56 (which decays to iron-56), the reactions are proceeding rapidly. The fuel doesn’t get burned up in seconds, but maybe hours or days. The energy (and hence pressure) produced in the core is all that holds up the star, so when the fuel is exhausted, the star collapses violently. Lots of energy is released both from the gravitational potential energy of the star itself and from the now-rapid fusion of lighter isotopes. Fusion of heavier isotopes (heavier than iron-56) proceeds, but costs energy. Once this runaway collapse starts, then there are only a few seconds left. In sufficiently massive stars, densities get so high at the core that electrons and protons are forced to combine, forming neutrons (and neutrinos), releasing the immense energy of a core-collapse supernova.

All the iron on earth came from previous generations of stars that lived and died.

No. WTF is that supposed to be?

I’d tell you, but I’ve just done some casual searching to find out its actual origin. The claim is that someone posted the question, ‘How is babby formed? How girl get pregnant?’ on Yahoo Answers. Someone posted an illiterate and nearly unintelligible answer to the questions. In 2008 (?) someone made a flash animation of the Yahoo Answers page, featuring cavemen and a scroll of the ostensibly real Yahoo Answers posts.

From then on, any time I see ‘How is _____ formed?’ I can’t help but think of the animation.

But I don’t know what to believe as to its actual origin. Is it real? Ir is it a hoax? I don’t know.


Ah - I see what you’re getting at. Let’s say, stars and planets always get more massive until they nova or their Eddington limit interferes, UNLESS there is no more mass available to fall into them. But that’s not a feature of star or planet behavior. NOTHING accumulates mass when there isn’t any.

I think it’s important to stress here that only a small proiportion of stars get massive enough to go beyond helium burning. Everything on the main sequence (>90% of stars) is merrily burning hydrogen in its core, with any helium or higher elements present being non-fusible “ash” at that temperature and pressure. Over 80% of stars are Types M (and its subtypes N, R, and S) and K, which burn at a slow enough rate that they will not leave the main sequence for significant multiples of the life of the Universe to date. Only <1% of stars are types B and O that will make it up to the “iron catastrophe”.