302
and so its apparent magnitude
(brightness as seen from Earth)
shows how far away it is.
A type 1a supernova is a little
different from a standard supernova,
which forms when large stars run
out of fuel and explode. A type 1a
forms in a binary star system, in
which a pair of stars orbit each
other. One is a giant star, the other
is a white dwarf. The white dwarf’s
gravitational pull hauls stellar
material over from the giant. The
material accretes on the surface of
the white dwarf until it has grown
to 1.38 solar masses. At this point,
the temperature and pressure are
such that a runaway nuclear fusion
explosion ignites the star, creating
an object billions of times brighter
than the sun.Distance and motion
Both surveys used the Cerro Tololo
Inter-American Observatory in
Chile to find type 1a supernovae.
The plan was not simply to plot the
positions of the supernovae. They
used the Keck Telescope in Hawaii
to take spectra of each explosion,
giving its redshift (the lengthening
the spectra have undergone).DARK ENERGY
The brightness, or magnitude, of
each star gave the distance—often
billions of light-years—while its
redshift indicated its speed relative
to Earth, caused by the expansion
of the universe. The teams were
aiming to measure the rate at
which the expansion was changing.
The rate of expansion, as indicated
by more distant objects, was
expected to be tailing off. Exactly
how fast it was doing this would
show if the universe was “heavy”
or “light.” However, when the teams
looked beyond about 5 billion light-
years (meaning that they were
looking 5 billion years into the
past), they found that the opposite
was happening—the expansion
of the universe was not slowing
down but speeding up.Dark energy
This result was first thought to
be an error, but successive checks
showed it was not—and both
teams found the same thing. In
1998, Perlmutter and Schmidt wentDark Energy Survey In 2013, the Dark Energy Survey
began a five-year project to map
the expansion of the universe
in detail. The project uses the
Dark Energy Camera (left) at
Cerro Tololo Inter-American
Observatory, Chile. The camera
has one of the widest fields of
view in the world. In addition to
searching for type 1a supernovae,
the project is looking for baryon
acoustic oscillations. These are
regular ripples in the distribution
of normal matter about 490 million
light-years apart, which can be
used as a “standard ruler” to
show up cosmic expansion.A computer simulation shows a
white dwarf star exploding in a type 1a
supernova. A flame bubble forms inside
the star (left), rises above the surface
(center), and envelops the star (right).