  The formation, evolution and death of massive stars release large quantities
of energy and momentum into the gas surrounding the sites of star formation.
This process, generically termed 'feedback', inhibits further star formation
either by removing gas from the galaxy, or by heating it to temperatures that
are too high to form new stars. Observations reveal feedback in the form of
galactic-scale outflows of gas in galaxies with high rates of star formation,
especially in the early Universe. Feedback in faint, low-mass galaxies probably
facilitated the escape of ionizing radiation from galaxies when the Universe
was about 500 million years old, so that the hydrogen between galaxies changed
from neutral to ionized--the last major phase transition in the Universe.
