As far as I can tell that's only star formation theory. To my knowledge we have never actually (verifiably) observed the formation of a star...
...We have witnessed many deaths of stars, but to my knowledge there hasn't been a case where we have seen, beyond a doubt, a star forming.
We will likely never actually observe the formation of a star, as under the current most widely accepted star formation theory, it would take millions (billions?) of years for it to actually occur. The death of a star is (sometimes) very dramatic, so it is much more easily observed. Both the births and deaths of stars were once only theories, (supernovas were once only a predicted occurrence of what
should happen during the end of a large star's life, we actually had to look for them to gain concrete evidence) but we now know them to be real from photos, and even an ancient supernova observed and recorded in ancient china (although back then they didn't fully understand what they were seeing).
Star formation on the other hand, can never be photographed or observed because it would take way too long according to theory. Unless of course we had billion year life spans and the patience to watch nebulas for that long. Or perhaps a time lapse camera that took a picture every few hundred years or so, with the hopes that someone would still be around at the end to view the results.
I'm not trying to say that the theory is 100% correct, but given the evidence, its enough for me to believe it. If I only believed what I could physically see, I would lead a very empty and delusioned life.
We have but 5 senses and some various extensions of them, so who knows what exists beyond our capability of observation. Homing pigeons, for example, can detect magnetic fields with a sort of 6th sense, while we humans must use other things to translate data to suit one of our 5 senses (
watching or
feeling the pull between two magnets, for example). Otherwise we would never know that magnetic fields existed. For all we know there could be other forms of life among us physically, but existing in some way as to evade our senses. A bit far-fetched, I know, but it isn't impossible when you truely think about it. (Read Isaac Asimov's "Hostess" for an interesting fictional short story dealing with such an idea.)
As for the possibility of life elsewhere in the universe, while I'm aware of the extreme good fortune we've had on earth that has allowed life to make it thus far and even to gain a foothold in the first place, I think its fairly self centered to believe ours to be the only likely planet harboring semi-intelligent life. There are just too many possibilities.
I can't remember his name, but there is an astronomer who actually came up with a formula to determine a star system's chances of harboring an earth-like planet and thusly the chance for life as we know it here. The formula takes into account the possibility of a planet being the "right" distance from the sun, whether it has a large moon, (some argue that without our moon life would have never happened as we know it here) The presence of large Jupiter-like planets in close proximity, which would likely throw smaller nearby planets out of the solar system or swallow it before life could begin, and some other things I can't remember. In the small area of our galaxy we have so far examined, he believes there to be about 100 star systems that fall into the
possibility of sustaining earth-like life.
Of course they are just possibilities, and no one has ever
seen a planet outside our solar system. The only current way to detect other planets is to watch a star's wobble as it pulled upon by other large masses (planets). We have only been able to detect extremely large planets (like Jupiter) with current technology, so we're quite far off from hard evidence of earth-like planets of course, but the fact that they believe there to be some 100 possibilities just in our corner of our galaxy is astounding when you figure the sheer number of stars in the universe.
For a common visualization of the number of stars that we know to exist, it is said that if you had a grain of sand to represent every star, and they were all put in hopper cars on a freight train roaring past you at a railroad crossing, it would take THREE YEARS for the train to pass. Theres also the "each grain would fill all the beaches in the world" analogy. Thats a lot of fucking stars, and a lot of possibilities.
As for the notion of Aliens being hostile, I have mixed feelings about this. In our own common sense they would likely see us as competition/lab rats/potential resources, I agree. But one thing to think about is if they shared our competitive/destructive traits, would they be able to survive themselves before gaining the technology for interstellar travel?