Ok Jackie, I will give it a go...

What, exactly, is altruism? Is it defined by intentions or results?
I agree with Fald, that the means define altruism; it is the intent that defines it. However, if the end is not achieved, then the effort will not be *recognised as altruistic, will fall by the wayside and might even be derided as do-gooding. And so, the results are important for the recognition of the lofty ideal behind the act.

If a person does a good deed but then trumpets about it, is he altruistic?
No...that is clearly opportunism

if someone meant to do good but unintentionally causes a disaster: is that altruism?
*This might be meddling dogooder-y!
Seriously though, it might depend on the scale of the disaster and the planning that went into the effort to help. A knee jerk reaction to help, that results in chaos, would be an interfering annoyance. But a well planned effort devoid of motives, that goes all wrong could still be called, 'a heroic but frutiless effort of altruism'.

Is it possible that someone becomes a movie star for altruistic reasons
Framed the way you ask it, I will have to say no. But, he/she could have acted in a movie with a social message that has the potential of bringing about great societal change; he/she might have acted in it gratis, for the sake of the cause and inadvertently ended up becoming a movie star. I guess then, his act of altruism paid rich dividends!!! Inadvertently, of course!