The dwarf galaxy UGCA 281, shown here as imaged by Hubble in the visible and ultraviolet, is rapidly forming new stars. An older, background population of redder stars coexists alongside the newer, bluer stars that are superimposed atop them. The newly-formed stars are largely heavily-enriched Population I stars, while the older stars are largely metal-poor Population II stars. No pristine, metal-free, Population III stars are yet known. (Credit: NASA, ESA, LEGUS Team)
Our Sun only arose after 9.2 billion years of cosmic history: with many stars living and dying first. How many prior generations were there?
Here in our modern Universe, even in just our own Milky Way, we observe stars in all different stages of life:
molecular gas clouds that are contracting and fragmenting,
leading to protostars and young stellar objects,
becoming full-fledged stars with protoplanetary disks around them,
conventional stars burning through their fuel with their own fully-formed planetary systems,
stars evolving into subgiants, giants, and even supergiants,
stars dying in planetary nebulae, supernovae, and other life-ending events,
and stellar remnants of now-extinct stars like white dwarfs, neutron stars, and black holes.
We can trace back the history of our Universe a full 13.8 billion years, to the earliest stages of the hot Big Bang, measuring the star-formation rate all throughout our cosmic history.
It was a full 9.2 billion years after the hot Big Bang first begun, or roughly 4.6 billion years ago…
