To make a long story short, a survery was made looking into deep space, and back in time up to 11 billion years, of a pretty good proxy of star formation (the emission lines produced by emission nebulae that are lit up like neon lights by the ultraviolet light of freshly-born huge stars). After doing some fancy math to correct for the expansion of space and the like the conclusions are striking—the modern average rate of star formation in the universe is less than 1⁄30 the peak rate of 11 billion years ago, and half of the stars in the universe are over 9 billion years old. To make matters even more interesting, the empirically-derived relationship between time and star formation actually converges to a finite number of stars when you project it into the future, and at infinity reaches a total number of stars born only 5% more than currently exist today.
95% of stars that will ever exist already exist.
This makes sense and has some interesting implications when you think in terms of galactic evolution and the history of the universe. Grand spirals like our galaxy are nearly the only places in the universe where star formation happens for billions of years on end and produces generations of stars rich in heavy elements. Dwarf galaxies go through one burst of star formation and the supernovas from the big stars blow all the gas out of their weak gravity, stopping star formation. Big elliptical galaxies form with nearly no angular momentum, so all their gas falls to the center, most of it becomes stars in one huge burst of low-metal stars, and the rest gets blown out of the galaxy when the central black hole gets activated. Grand spirals only wind down slowly with time as most of their gas stays away from the center due to angular momentum but their gravity makes sure that (almost) all the gas stays bound, getting more and more enriched in heavy elements with time.
That is, until they collide with each other, which as most galaxies are part of clusters does eventually happen. It will happen to us in another four to five billion years, with andromeda. When this happens, the colliding galaxies go through one burst of star formation and then settle down into another elliptical galaxy. So over time, the number of star forming galaxies only decreases.
So, consider our position in space and time. We are in a grand spiral galaxy, which is the only sort of place that really produces high-metallicity stars. Our star system formed about 1⁄3 of the way through our galaxy’s productive life before it collides with Andromeda (probably more like halfway through its compliment of stars, seeing as even spirals settle down with age), and we find ourselves currently about 2⁄3 of the way through its productive lifetime. I would call this an utterly typical position for an origin of life as we know it.
More reading on my part makes me say that rather than being the ONLY place with high-metallicity stars, spirals are just by far the most common site for them. But the point still stands.
The stelliferous era refers to the time in which stars live, not the time in which they are forming. The longest-lived tiny stars will still be here a hundred trillion years from now, but there probably will have been almost no NEW stars formed for almost the entirety of their lifetimes.
If life either forms within a few gigayears of the formation of a star or not at all (something I find likely given the history of life on Earth and the state of current research on the origins of life, in fact it’s probably that it happens IMMEDIATELY after the formation of a star or not at all) then we should indeed expect to find ourselves near the start of the universe, when stars are still being born.
EDIT: You also just shouldn’t expect isotropy in time, there is no reason for it and indeed reasons against it. The universe is expanding. High-density, very interesting stuff happens near the start (after just enough expansion has occurred to provide some room between actual entropy and possible entropy in which structure can exist) and low-density, boring stuff happens the further in time you go forward.
Star formation rates in the universe solve this problem.
That helps, but you also need an assumption that civilisations won’t expand and colonise the universe.
If there are (or will be) such colonisers, we’re still atypical, regardless of star formation rates.
Star formation rates in the universe solve this problem.
See my old post on our position in space and time and how typical it likely actually is.
Partially quoted below:
More reading on my part makes me say that rather than being the ONLY place with high-metallicity stars, spirals are just by far the most common site for them. But the point still stands.
The stelliferous era refers to the time in which stars live, not the time in which they are forming. The longest-lived tiny stars will still be here a hundred trillion years from now, but there probably will have been almost no NEW stars formed for almost the entirety of their lifetimes.
If life either forms within a few gigayears of the formation of a star or not at all (something I find likely given the history of life on Earth and the state of current research on the origins of life, in fact it’s probably that it happens IMMEDIATELY after the formation of a star or not at all) then we should indeed expect to find ourselves near the start of the universe, when stars are still being born.
EDIT: You also just shouldn’t expect isotropy in time, there is no reason for it and indeed reasons against it. The universe is expanding. High-density, very interesting stuff happens near the start (after just enough expansion has occurred to provide some room between actual entropy and possible entropy in which structure can exist) and low-density, boring stuff happens the further in time you go forward.
That helps, but you also need an assumption that civilisations won’t expand and colonise the universe. If there are (or will be) such colonisers, we’re still atypical, regardless of star formation rates.