The cosmic material lifecycle is a grand recycling process. Stars form from collapsing gas clouds, fuse elements in their cores, and eventually return enriched material to space. This cycle drives the chemical evolution of galaxies, creating heavier elements essential for planets and life.

The interstellar medium plays a crucial role, existing in various phases from dense molecular clouds to hot ionized gas. As stars live and die, they enrich this medium with heavy elements, setting the stage for new generations of stars and planets to form.

The Life Cycle of Cosmic Material

Flow of interstellar matter

• Interstellar medium (ISM) contains gas and dust between stars primarily hydrogen and helium with traces of heavier elements in different phases
  • Molecular clouds densest regions of ISM composed of molecular hydrogen (H2) serve as stellar nurseries where new stars form through gravitational collapse
  • Neutral atomic gas less dense than molecular clouds consists of neutral hydrogen atoms (HI) can condense to form molecular clouds under the right conditions
  • Ionized gas created by high-energy radiation from hot, massive stars includes HII regions (ionized hydrogen) around young, massive stars as well as planetary nebulae and supernova remnants
  • Hot ionized gas most diffuse and hottest phase of ISM heated by supernovae and stellar winds can cool and condense contributing to the formation of new molecular clouds
• Stars form from the collapse of molecular clouds
  1. Protostellar phase involves gravitational contraction and accretion of matter
  2. Main sequence phase marked by hydrogen fusion in the core
  3. Post-main sequence phase leads to red giant, planetary nebula, or supernova depending on initial mass
• Stellar evolution enriches the ISM with heavy elements through stellar winds and supernova explosions contributing to the formation of new generations of stars and planets
  • This process of cosmic recycling drives the chemical evolution of galaxies over time

Origin of heavy elements

• Heavy elements (beyond helium) produced by nucleosynthesis in stars
  • Main sequence stars generate helium and carbon through the CNO cycle
  • Red giant stars create elements up to iron through the triple-alpha process and the s-process
  • Supernovae forge elements heavier than iron through the r-process and explosive nucleosynthesis
• Stellar winds and supernova explosions expel heavy elements into the ISM enriching it with metals (in astronomy, "metals" refer to all elements heavier than helium) leading to the formation of more metal-rich stars and planets
• Dust grains form in the cool, outer atmospheres of evolved stars and in supernova ejecta
  • Composed of silicates, graphite, and other compounds containing heavy elements
  • Serve as catalysts for the formation of molecular hydrogen (H2) in the ISM
  • Play a crucial role in the formation of planetary systems by providing surfaces for volatile elements (water, methane) to condense onto in protoplanetary disks and coagulating to form larger particles eventually leading to planetesimals and planets
• Dust grains can also absorb and scatter light affecting observations of distant objects
  • Interstellar extinction dust absorbs and scatters blue light more than red light reddening the appearance of stars
  • Interstellar reddening measures the amount of dust along the line of sight to a star

Baryon cycle in space

• The baryon cycle describes the flow of ordinary matter (protons and neutrons) throughout the universe
• Intergalactic medium (IGM) contains most of the universe's baryonic matter
  • Mostly ionized hydrogen and helium with traces of heavier elements
  • Can be enriched by outflows from galaxies (galactic winds, active galactic nuclei)
• Gas from the IGM falls into galaxies through gravitational attraction fueling star formation by providing raw material for molecular clouds
• Stars form from the collapse of molecular clouds in the ISM
  • Stellar winds and supernovae enrich the ISM with heavy elements
  • Some of the enriched material may be ejected back into the IGM through galactic winds or outflows
• Stellar remnants (white dwarfs, neutron stars, black holes) lock up a portion of the baryonic matter but can return it to the ISM through binary interactions (accretion, mergers)
• The cycle continues as new generations of stars form from the enriched ISM increasing the overall metallicity of galaxies over cosmic time due to continuous enrichment by stellar populations
  • This process of galactic chemical evolution shapes the composition of future generations of stars and planets