THE BRIGHTEST STARS ARE OFTEN IN BINARY SYSTEMS
ESO
An international team has used the VLT to study stars at the top end
of the main sequence -- O-type stars, which are 15 or more times the
mass of our Sun and can be up to a million times brighter, shine with
a blue-white light and have surface temperatures over 30 000 °C.
O-type stars are only a tiny fraction of the stars in the Universe,
but the violent phenomena associated with them give them a
disproportionate effect on their surroundings. Their winds and shocks
can both trigger and stop star formation, their radiation powers the
glow of bright nebulae, their supernovae enrich galaxies with heavy
elements, and they are associated with gamma-ray bursts, which are
among the most energetic phenomena in the Universe.
The astronomers studied a sample of 71 such stars in six 'nearby'
young star clusters in the Milky Way, and found that 75% of them are
members of binary systems, a higher proportion than previously
thought, and that in many cases the pairs are close enough for their
evolution to be greatly affected by interaction between them. If two
stars orbit very close to each other, matter may be transferred from
one to the other, and/or they may eventually merge. The team
estimates that mergers will be the ultimate fate of around 20-30% of
O-type stars, and mass transfer will occur for a further 40-50%.
In a close binary, the more massive component, which evolves first,
transfers mass to its companion, whose mass will accordingly increase
substantially and it will survive much longer than a single star of
the same mass would. The donor star, meanwhile, is stripped of its
envelope before it has a chance to become a luminous red supergiant;
instead, its hot, blue core is exposed. As a result, the stellar
population of a distant galaxy may appear to be much younger than it
really is: both the rejuvenated former secondary, and the stripped
former primary, are hotter, and bluer in colour, than they were
originally, mimicking the appearance of younger stars. Knowledge of
the true proportion of interacting high-mass binary stars is therefore
important to the understanding of distant galaxies.
SUPERNOVA PROGENITOR IDENTIFIED?
RAS
Type-Ia supernovae are supposed to be thermonuclear explosions of
white-dwarf stars that are members of binary systems. Mass is
gradually donated to the white dwarf by its evolving companion. When
the mass of the white dwarf reaches 1.4 times that of the Sun, the
star explodes as a type-Ia supernova. Recent studies have observed
sodium gas associated with type-Ia supernovae. The sodium might have
been ejected from the binary's donor star before the white dwarf
exploded; if so, the existence of a sodium cloud might allow the
identification of progenitor systems before any explosion takes place.
Using data from the DuPont telescope of the Las Campanas observatory
in Chile, astronomers looked for sodium signatures and identified a
binary star called QU Carinae as a possible supernova progenitor. It
contains a white dwarf which is accreting material from its companion
at a very high rate, and sodium has been detected around the system.
It seems a bit optimistic, however, to think that it will explode
while we watch it: celestial time-scales are mostly not tailored to
human convenience.
NEW WHITER LIGHTS WILL MAKE LIGHT POLLUTION WORSE
RAS
Scientists of the Freie Universität Berlin and the Leibniz Institute
of Freshwater Ecology and Inland Fisheries in Germany have used a new
instrument to study how clouds affect nocturnal sky brightness in
urban areas. Until the advent of serious light pollution, clouds made
the sky darker at night, just as they do in daytime. In areas with
artificial light the effect of clouds is now reversed, and the size of
the effect depends on colour. The researchers found that in Berlin
the blue portion of skyglow is 7, and in the red 18, times brighter on
cloudy nights than on clear. The team predicts that, with increasing
use of LED street lamps, the colour of the night sky will become
bluer. The authors say that, in the visual range used by most
animals, cloudy skies are now thousands of times brighter near cities
than they were throughout most of history. They expect that the extra
light affects predator-prey relationships where the predator uses
vision to hunt, for example between owls and mice. The sky is blue in
the daytime because the cloud-free atmosphere is very good at
scattering short-wavelength light. The scientists therefore express
concern that unless care is taken in design and implementation, a
switch to whiter LED lights could make the sky much brighter on clear
nights. They suggest that cities that have decided to change to
solid-state lighting should purchase lamps that emit no upward light,
and use 'warm white' lights with as little blue light as possible.
SIGNS CHANGING FAST FOR VOYAGER AT EDGE OF SOLAR SYSTEM
NASA
Two of three key signs of changes expected to occur at the boundary of
interstellar space have changed faster than at any other time in the
last seven years, according to new data from the Voyager 1 spacecraft,
which was launched in 1977 and is now 18 billion kilometres from the
Sun. For the last seven years, Voyager 1 has been approaching the
outer edge of the bubble of charged particles that the Sun blows
around itself. In one day, July 28, Voyager 1's cosmic-ray instrument
showed that the level of high-energy cosmic rays originating outside
our Solar System jumped by 5%. During the last half of that same day,
the level of lower-energy particles originating inside our Solar
System dropped by half. However, in three days, the levels had
recovered to near their previous values. A third key sign is the
direction of the magnetic field; a preliminary analysis of the latest
magnetic-field data is expected to be available in the next month.
Scientists expect that all three of the signs will have changed when
Voyager 1 has crossed into interstellar space. The levels of high-
energy cosmic-ray particles have been increasing for years, but more
slowly than they are now. The previous jump -- of 5% -- took one week
in May. The levels of lower-energy particles from inside the Solar
System have been slowly decreasing for the last two years. Scientists
expect that the lower-energy particles will drop close to zero when
Voyager 1 finally crosses into interstellar space. The recent
increase and decrease are sharper than had occurred before, and that
was also true of the May data.
Topic: August astronomy bulletin (Read 1363 times)