Correct, since any element heavier than hydrogen or helium is created in stellar cores or from supernovas; stars that are metal-poor are either very old or formed from virgin materials from the big bang.
That's a weird distinction to try to make. Metal does mean the elements, not the spectral lines. The presence of those elements is usually inferred from the spectral lines, but if another method was used we would still say "metal poor/rich".
And there may just as well be an invisible dragon in my garage. There's no theory nor evidence that such exotic hydrogen impostors exist, so by Occam's razor we must assume that if it looks like hydrogen and quacks like hydrogen, it is indeed hydrogen.
An anti-hydrogen star - or even an entire star system - surrounded by normal dust would show annihilation events in the spectrum. So it would be very distinctive and unusual.
I guess so, if its electron/positron shell is the same. But I dont mean just antihydrogen. I'm sure there are many nucleus configurations that have the +1 charge and thus look like hydrogen from the outside.
But a larger nucleus would be unlikely to produce the same spectral signature even with the same charge. The energy transitions would be different, especially with different orbital shells and subtle quantum effects.
What particularly surprises me about this is that the star is in our own galaxy. I'm used to the idea of seeing remnants from the early universe from observations of very distant galaxies, but of course then we're seeing them as they were long ago. This is a second generation star that's almost as old as the universe, but in our neighbourhood and we're seeing it as it is "now" (or close enough).
I wonder where in the galaxy it's located? Intuitively I'd expect such an object to survive longer if it were out in the rim or halo of the galaxy, in a more rarified region, but of course intuition on these things can be very misleading.
Small stars live a very long time. A star of the mass of our sun has a lifespan of ~10 billion years which gets you almost back to the beginning of the universe (~13 billion). This star is smaller (0.8 solar masses), so it can live even longer, so we don't need to look at distant objects to see early, low mass stars.
The other issue is that we just can't see individual stars at cosmological distances except in incredibly rare cases. See for example [1] where they discovered a single star at redshift 1 (roughly 6 billion years ago). Basically, if the star gets lensed in just the right way, it can be hugely magnified. This is a strong contender for the coolest observation that I know about. Galaxies are hard to see at redshift 1, to get a single star is crazy.
On your second question, generally people think that galaxies formed "inside-out". I.e. the inner region forms first, then the outer region. See [2].
Stars almost never collide with things (except maybe at the very center of the galaxy, but still super rare there), so survival isn't really a function of position in the galaxy.
That said, this observation is in stripe 82 (a very famous section of the SDSS. They observed it to greater depth than the rest of their area and many other surveys have since also observed it) which I'm pretty sure is away from the galactic plane (so not straight toward the center).
> Based on its chemical abundance pattern, we speculate that SPLUS J2104−0049 could be a bona fide second-generation star, formed from a gas cloud polluted by a single metal-free ~ 30M⊙ star
The short answer is we don't know; it is not something we can directly observe. However, they are not expected to have them as the elements to form them would not have been present when the star was formed.
With the universe being so big, lots of highly unlikely phenomena are bound to happen here and there. Like if the terrestrial planet originated from a metal-rich star system, got kicked out by gravitational slingshot, wandered for a while, and happened to come near a metal-poor star with a suitable velocity. But this would be an extremely rare.
