  A fundamental quest of modern astronomy is to locate the earliest galaxies
and study how they influenced the intergalactic medium a few hundred million
years after the Big Bang. The abundance of star-forming galaxies is known to
decline from redshifts of about 6 to 10, but a key question is the extent of
star formation at even earlier times, corresponding to the period when the
first galaxies might have emerged. Here we present spectroscopic observations
of MACS1149-JD1, a gravitationally lensed galaxy observed when the Universe was
less than four per cent of its present age. We detect an emission line of
doubly ionized oxygen at a redshift of $9.1096\pm0.0006$, with an uncertainty
of one standard deviation. This precisely determined redshift indicates that
the red rest-frame optical colour arises from a dominant stellar component that
formed about 250 million years after the Big Bang, corresponding to a redshift
of about 15. Our results indicate the it may be possible to detect such early
episodes of star formation in similar galaxies with future telescopes.
