between the Sun and Pluto. It only started
glowing in the last few decades.
Observations from 1971 give a tem-
perature of 21,000 kelvin for the central
star. But in 2002, it was up to 60,000 K. So
Nicole Reindl (Eberhard Karls University
of Tübingen, Germany) and colleagues
dug through archived ultraviolet and
optical observations from a range of space-
and ground-based telescopes in order to
understand this rapidly evolving star.
The team found that between 1988 and
2002, the temperature increased from
38,000 K to 60,000 K. During this time the
star also contracted, dimmed, and reduced
its mass-loss rate, while its winds sped up
from 1,800 km/second to 2,800 km/sec (
to 6.3 million mph). Then between 2002
and 2006 it cooled down again, dropping
to 55,000 K.
The team thinks the star’s jump in
temperature and wind speed are telltale
signs of a helium-shell fl ash. It could,
however, also be the byproduct of an evolv-
ing close binary system, the team reports
in Astronomy & Astrophysics. The star is
probably less than half the Sun’s mass.
■ SHANNON HALL
COSMOLOGY I Universe in a Box 2.
Astronomers have created the most
realistic computer simulation of the uni-
verse’s large-scale evolution to date, track-
ing activity across 13 billion years of cosmic
history, Mark Vogelsberger (Massachusetts
Institute of Technology) and colleagues
report in the May 8th Nature.
Supercomputer simulations allow cos-
mologists to study how the laws of physics
worked together to build up the universe
we observe today. By tracking the behavior
of matter, researchers learn more about the
universe’s evolution as they work to match
their simulated webs of galaxies to those
observed in the real universe.
Vogelsberger’s team is the fi rst to simul-
taneously re-create both the large-scale net-
work of fi laments formed by massive galaxy
clusters and the smaller-scale gas and stel-
lar buildup within large galaxies.
The phenomenal detail of this simula-
tion required supercomputers in France,
Germany, and the U.S. The model, Illustris,
begins from initial conditions resem-
bling the very young universe 12 million
years after the Big Bang. The team then
unleashed complex physical processes —
the gravitational pull of matter, the chemi-
cal processes in diff use gas, radiation,
and magnetic fi elds, as well as the physics
of star and black hole formation — and
allowed everything to evolve for 13 billion
years, then sat back and watched.
The Illustris simulation matched theThe results of the recent Illustris cosmological simulation are so realistic that a fi eld of
galaxies taken with the Hubble Space Telescope (left) looks just like a simulated view (right).
Illustris was unable to accurately reproduce low-mass galaxies, however.observable universe remarkably well. It suc-
ceeded in producing a variety of galaxies
(41,416 of them), including spiral galaxies
like our own Milky Way. But it struggled to
produce realistic low-mass galaxies. As in
many previous simulations, these smaller
galaxies formed far too early, ending up
with prematurely aged stellar populations
— stars two to three times older than what
observations show.
Other simulations working on smaller
scales have successfully reproduced the
delay in starbirth (S&T: May 2014, p. 12). Phil
Hopkins (Caltech), who’s involved in that
work, explains that the zoomed-in studies’
successes come from their ability to follow
the nitty-gritty physics of star formation
and stellar feedback. The method enables
them to predict what these processes do to
small, growing galaxies.
The simulations by Hopkins and oth-
ers can watch gravity’s eff ects on scales
of a few light-years, whereas Illustris only
resolves gravity’s eff ects down to about
2,300 light-years. But the smaller-scale
studies only look at an individual galaxy
and its immediate neighbors, not the large-
scale cosmic structure that Illustris does.
Combining both approaches and using the
results from the small-scale simulations to
inform the large-scale ones will allow cos-
mologists to improve on Illustris’s results,
Hopkins says.
■ SHANNON HALLarcsecond per year or less.
Third, WISE J0855−0714 is literally
cool. Using images of the object taken
in diff erent fi lters, Luhman estimates its
temperature to be about 250 kelvin, or
about 10 degrees below zero in Fahrenheit.
This makes WISE J0855−0714 not only the
coldest neighbor to the Sun but also the
coldest brown dwarf ever discovered.
The discovery of WISE J0855−
points out just how important large-scale
sky surveys such as WISE really are. This
cold brown dwarf was discovered rela-
tively close to the plane of our Milky Way,
which astronomers often avoid because of
“crowding.” But as Luhman has shown,
this region of the sky might be a fertile
hunting ground for fi nding more close
companions to the Sun. The discovery
appears in the May 10th Astrophysical
Journal Letters.
■ JOHN BOCHANSKI
NASA / ILLUSTRIS COLLABORATIONSkyandTelescope.com August 2014 15