Lives of stars
A star's mass decides its fate. Sun-like stars swell into red giants and leave white dwarfs. Stars above about 8 solar masses end in core-collapse supernovae, leaving neutron stars or black holes. Heavy elements forged in stars and explosions seed the next generations — the iron in our blood was made in stars.
Compact objects
White dwarfs are supported by electron degeneracy pressure and cannot exceed about 1.4 solar masses (the Chandrasekhar limit). Neutron stars pack more than a solar mass into about 20 km. Black holes have event horizons at the Schwarzschild radius r = 2GM/c². Gravitational waves from merging black holes were first detected by LIGO in 2015.
The expanding universe
Galaxies recede from us at speeds proportional to their distance (Hubble–Lemaître law). Tracing the expansion back gives an age of about 13.8 billion years. The cosmic microwave background is the afterglow of the hot early universe. Observations indicate the expansion is accelerating, attributed to dark energy.