FIFTH MOON ORBITING PLUTO
NASA
Astronomers using the Hubble telescope have reported the discovery of
another moon orbiting Pluto, making five altogether. The moon is
irregular in shape and estimated to be 6 to 15 miles across. It is in
a 58,000-mile-diameter circular orbit that is assumed to be co-planar
with the orbits of the other satellites in the system. The Pluto team
is intrigued that such a small planet can have such a complex
collection of satellites. The favoured idea is that all the moons are
relics of a collision between Pluto and another large Kuiper-belt
object billions of years ago. The new detection will help scientists
navigate the New Horizons spacecraft through the Pluto system in 2015,
when it makes an historic and long-awaited high-speed fly-by.
Pluto's largest moon, Charon, was discovered in 1978 in observations
made at the US Naval Observatory in Washington, D.C. Hubble
observations in 2006 discovered two additional small moons, Nix and
Hydra. In 2011 another one, P4, was found in Hubble data.
Provisionally designated S/2012 (134340) 1, the latest moon was
detected in nine separate sets of images taken by Hubble's wide-field
camera on June 26, 27, 29, and July 7 and 9. In the years following
the New Horizons Pluto fly-by, astronomers hope to use Hubble's
planned successor, the James Webb telescope, for follow-up
observations.
4-HOUR BINARY STARS DISCOVERED
RAS
For the last five years, a team of astronomers has been using the
wide-field camera on the United Kingdom Infrared Telescope (UKIRT) on
Hawaii to monitor the brightness of hundreds of thousands of stars,
including thousands of red dwarfs, in near-infrared light. Many of
the stars in our Milky Way galaxy are in binary systems. Most likely,
the stars in such systems were formed close together and have been in
orbit around each other ever since. The team has investigated
binaries of red dwarfs, stars up to ten times smaller and a thousand
times less luminous than the Sun. Although they are the most common
type of star in the Milky Way, red dwarfs do not show up much in
normal surveys because of their dimness in visible light. The
astronomers found several red-dwarf binaries with orbital periods
significantly shorter than the 5-hour cut-off found for Sun-like
stars. (Two such stars, even if they remained spherical, would have
to be touching one another to circulate in an orbit with a period of 5
to 6 hours, so it is not surprising that there are no shorter periods
than that for stars like the Sun.) The faint red dwarfs, being much
smaller, can circulate closer together, and the new survey has
actually found four pairs that have orbital periods less than 4 hours.
SPIRAL GALAXY IN EARLY UNIVERSE
BBC News
Astronomers have observed the earliest known spiral galaxy, dating to
just three billion years after the Big Bang. Theories of galaxy
formation hold that the Universe was still too chaotic a place then to
allow a spiral to form, since it should take far longer for gravity to
bring matter into thin, neat discs. The team first noticed the galaxy
BX442 as the one and only spiral-looking object in a survey of 300
galaxies carried out with the Hubble telescope. Even though some
discs could be seen existing at that time, they are very thick and
puffy, not like the Milky Way, which has an amount of random motion
only about a tenth or so the amount of ordered rotation, giving rise
to a very thin disc. To get a better look at BX442, the team went on
to use the 'Osiris' spectrograph at the Keck observatory in Hawaii --
which can remove the effects of a lot of lines that are imposed on all
infrared astronomical spectra by the hydroxyl radical (OH) in the
Earth's atmosphere. The Keck observations confirmed a hint apparent
in the Hubble data - that BX442 was associated with a smaller galaxy,
which may have helped it to form a disc so relatively early in the
history of the Universe.
MIDDLEWEIGHT BLACK HOLES
RAS
A new model shows how an elusive type of black hole can be formed in
the gas surrounding a super-massive counterpart. The researchers
propose that intermediate-mass black holes -- with masses ranging from
hundreds to many thousands of times the mass of the Sun -- can grow in
the gas discs around super-massive black holes in the centres of
galaxies. The physical mechanism parallels the model that
astrophysicists use to describe the growth of giant planets in the gas
discs surrounding stars. We think we know about small black holes,
which have masses a few to 10 times that of our Sun, and about
super-massive ones, which are found in the centres of galaxies and
have masses millions to billions of times the mass of the Sun, but
intermediate-mass holes are much harder to find.
The birth of an intermediate black hole starts with the death of a
star that forms a stellar- or low-mass black hole. In order for such
a 'seed' to grow, it must collide with and consume other stars. But
even though there are many billions of stars in large galaxies,
there's a great proportion of empty space, making a collision a very
rare occurrence. The focus of searches for intermediate holes has
been on star clusters, but objects there move quickly and there is not
much gas. Attention is now turned instead to active galactic nuclei,
the hot and ultra-bright cores of galaxies that feed super-massive
black holes. The gas in such a system is the key, causing the stars
to slow down and conform to circularised orbits. The resulting
collisions allow a stellar black hole to swallow stars and grow. The
black hole's size and gravitational pull increase with its mass,
improving its chance of further collisions. As they increase in size,
the black holes can alter the gas discs in which they form. The
researchers' model shows that a black hole of a certain mass can
create a gap in the gas disc, a signature that might give scientists
the first glimpse of intermediate black holes.
Topic: late July Astronomy Bulletin (Read 1713 times)