[Greg]

could someone tell me please exactly how a star is born. ive always wanted to know.

[Tormod]

I don't know how it is done exactly, but I can recommend a marvellous book called "The Magic Furnace" by Marcus Chown. It is about the birth and evolution of stars - a great read.

[infamous]

Originally Posted by Greg

could someone tell me please exactly how a star is born. ive always wanted to know.

Maybe we should refer to this as rebirth of stars rather than birth of a star. There is much evidence to support the view that new stars are born from the remnants of super nova explosions. These dust clouds of debris form into aggregations of mass sufficient to initiate spontaneous thermonuclear fusion resulting in the rebirth of stars.

[orbsycli]

i've always thought that over millions of years, galactic matter just combines with one another because of gravity. but that doesn't explain the birth of a star. wow i really wish i knew, too.

[Jay-qu]

i dont know about spontaneous, but wouldnt all the intense pressure from the condensing of such a large amount of gas in a small area start the fusion...

[Boerseun]

The way I have it is that large hydrogen clouds (the simplest element) initially falls together gravitationally, until fusion is achieved. The gravitational collapse of the cloud causes the flattening into a disc of the whole system, due to the conservation of angular momentum. That's why planets all lie in generally the same orbital plane.
The amount of hydrogen (the mass of the star) determines the lifespan of the particular star, although it should be noted that pure hydrogen (1st generation stars) don't live very long - it's been estimated to be generally only a few hundreds of millions of years.
However - after the star went through it's 1st cycle, the initial hydrogen has been fused into a myriad of heavier elements, and the whole thing falls together again, until the fusion of heavier elements are achieved. These stars live considerably longer.
Interestingly enough, it seems as if dual star systems are more common that single star systems, in fact, a lot of astronomers refer to Saturn and Jupiter as "Failed Stars"; they just didn't accrete enough hydrogen and other stuff in the initial phase of planet-building.
So, stars go through cycles, synthesizing heavier and heavier elements through each incarnation. All elements around you, the carbon in the wood your desk is made of, for instance, have been synthesized from hydrogen in the hearts of stars. And each time a star goes nova, it blows itself apart so violently, that not all matter falls together again - some (most, I guess) matter might even be ejected out of the system completely due to the violence and force of the stellar explosion. So - the carbon in your left hand might have been fused in a star on one side of the Milky way, and the carbon in your right hand might have been made on the other side!
Pretty amazing, I'd say!

[amt7565]

Yes Boerseun!
Just add to the conversation:

All particles fuse together due to the intense effects of gravity. The gravity is so intense it triggers a nuclear explosion- well, Nuclear Fusion. The Sun is constantly converting Hyrdogen to Helium. About 90% of the H is converted to He. The remaining 10% is converted to heat, light etc, which reaches us on Earth. The 10% loss is what E=MC2 is all about.

The Sun is expanding. In a couple of billion years, it would grow so big that it would incinerate the Earth, after having eaten up Mercury and Venus. In 5 billion years our Sun would exhaust all it's burning(energy) and collapse unto itself- a White Dwarf.

And I read, larger Suns collapse and form Nuetron stars.

I strongly recommend watching cable TV channel INHD. It's in HDTV and has a 2 hour program about the Sun known as "Solarmax". Truly astounding pictures of the Sun shown in HDTV.

Sorry for the extra information.
AMT-

[Harzburgite]

A couple of quick corrections to the otherwise useful posts from Boerseun and amt7565.

The amount of hydrogen (the mass of the star) determines the lifespan of the particular star, although it should be noted that pure hydrogen (1st generation stars) don't live very long - it's been estimated to be generally only a few hundreds of millions of years.

The early stars, as you note, were composed primarily of hydrogen and helium, the latter constituting around 10% [25% by mass]. These are known, for historical reasons, as Population II stars. Many such stars are long lived. Look, for example at the globular clusters.
It is indeed the mass of the star (not the amount of hydrogen) that primarily determine its lifespan. There are plenty of young stars (Population I), formed from the remnants of earlier stars, that are massive enough to burn out and go supernova in a few tens of millions of years, while their less massive, profligate cousins will endure for tens of billions of years.

About 90% of the H is converted to He. The remaining 10% is converted to heat, light etc, which reaches us on Earth. The 10% loss is what E=MC2 is all about.

Qualitatively correct, but quantitatively you are an order of magnitude out. 0.7% of the reacting masses is converted to energy.

[bumab]

All the posts are great- the size of the collapsing cloud is the main factor in determining the stellar lifespan, and also how it dies (of course).

The interesting conundrum in stellar formation is how to keep the collapsing gas cloud cool. As the H/He clouds collapse, they heat simple due to compaction. If that pressure gets to high to fast, the gas will be expelled before fusion pressure is achieved. Currently, several telescopes are searching for methods by which a collapsing cloud can radiate it's energy away and remain cool while undergoing star formation.

At first, the suspected culprits were water and elemental oxygen, but apparently those are not super common in star forming regions. As far as I know, the jury is still out on what keeps those clouds cool as they collapse. Anybody know if they've found an answer yet?

Here's a really basic tutorial, but pretty good for you:
http://physics.njit.edu/~dgary/202/Lecture18.html

[Harzburgite]

Originally Posted by bumab

the size of the collapsing cloud is the main factor in determining the stellar lifespan, and also how it dies (of course).

At the risk of acquiring a reputation for nitpicking I would point out that the collapsing cloud typically generates many, many star systems, most of them multiple star systems. Within these stellar nurseries it is the size of the sub-cloud eddieswhich, as you say, determine the subsequent life and death of the star.
The sun's cousins have long since separated from us, but a glance at the Pleiades (or Seven Sisters) can give one an idea of what we were once part of.
Also, I don't think anyone has noted that the collapse of these clouds is very likely triggered by shock waves from a nearby supernova.