COMETS MASQUERADING AS ASTEROIDS
RAS
In recent years several Solar-System bodies that have at first been
thought to be asteroids have begun to show signs of outgassing in the
form of a coma or slight tail and are now thought to be comets. A
recent paper by some Colombian astronomers describes how some of the
objects, probably inactive for millions of years, have returned to
'life'. Comets are typically a few kilometres across and composed of
a mixture of rock and 'ices' -- a generic term used by astronomers to
refer to substances that we think of as gases but which far from the
Sun are in a frozen state. If such icy bodies come close to the Sun,
then some of the ices turn to gas, which is swept away in a direction
roughly radially away from the Sun by 'radiation pressure' and the
solar wind to form a characteristic tail of gas and dust. There are
many long-period comets that have highly elliptical orbits that bring
them into the inner Solar System only at intervals of thousands of
years; there are presumed to be far more of them than the number that
we know about individually, since nobody has kept systematic records
covering thousands of years, let alone watched for so long systematic-
ally with telescopes to identify comets that are other than obvious
naked-eye spectacles. There is also a population of about 500
short-period comets, created when long-period comets pass near Jupiter
and are deflected into orbits that may have periods between 3 and 200
years. Comets have also collided with the Earth (though nothing of
any size in historical times) and may have helped originally to bring
water to it. The new work looks at the region of the Solar System
that includes the main belt of asteroids between the orbits of Mars
and Jupiter. That volume of space contains more than a million
objects ranging in size from 1 metre to 800 km. The traditional
explanation for asteroids is that they are the building blocks of a
planet that never formed, as the assembly of the pieces was prevented
by the strong gravitational field of Jupiter.
In the last decade 12 active comets have been discovered in the
main-belt region. That was something of a surprise, and the team set
out to investigate their origin. It proposes that some apparently
asteroidal bodies that show no hint of activity are not really dead
rocks but are dormant comets that may yet come back to life if the
energy that they receive from the Sun increases by a few per cent.
That can happen fairly readily, as orbits in asteroid belt can be
changed by the gravity of Jupiter, and a decrease in the perihelion
distance leads to an increase in the average temperature. According
to that view, ages ago the main asteroid belt was populated by
thousands of active comets. The population aged and the activity
subsided. What we see today is the residual activity of that active
past. Twelve supposed asteroids have proved recently to be comets
that were rejuvenated after their periheilion distances were reduced a
little. The little extra energy they received from the Sun was then
sufficient to revive them.
STARS FOUND TO HAVE LARGE OVER-ABUNDANCES OF LEAD
RAS
Two stars have been discovered to have extraordinary concentrations of
lead in their atmospheres. The small hot stars, known as helium-rich
subdwarfs, are already known to be peculiar because they contain much
less hydrogen and much more helium than normal. Three years ago, some
astronomers discovered one with a very high surface concentration of
zirconium. Now studying a group of nominally similar stars, they have
discovered two whose surfaces show abundances of lead that are 10,000
times greater than is seen in the Sun. The two stars are known as
HE 2359-2844, 250 parsecs distant in the constellation Sculptor, and
HE 1256-2738, 300 parsecs away in Hydra. The astronomers used
observations already made by others, from the archives of the VLT in
Chile. The spectra of both stars showed a few features which did not
match any atoms expected to be present, but which were eventually
identifed as arising from lead atoms three times ionised at the high
temperatures (about 38,000C) of the stars' atmospheres. With atomic
number 82, lead is one of the heaviest naturally occurring atoms;
in the Sun there is less than one lead atom for every ten billion (ten
to the power ten) hydrogen atoms. One of the stars, HE 2359-2844, was
also found to show 10,000 times more yttrium and zirconium than the
Sun. Along with the zirconium star, LS IV-14 116, the stars now form
a new group of 'heavy-metal subdwarfs'.
The team believes that those stars are a link between bright red
giants (stars thirty or forty times the size of the Sun) and faint
blue subdwarfs, stars one-fifth the size of the Sun but seven times
hotter and seventy times brighter. A few red giants lose their
thick hydrogen envelopes and shrink to become hot subdwarfs, or
'nearly-naked helium stars'. As they shrink, conditions may become
favourable for the radiation pressure in the helium stars to act on
individual atoms to sort the elements into separate layers, where they
may be concentrated by a factor of ten thousand or more. A high
concentration of an element at a height in the stellar atmosphere
where the line spectrum is being formed can give a spectroscopic
signature that we can observe and recognise.
'QUENCHED' GALAXIES DO NOT GROW
ESA
Some galaxies reach a time when their star-formation ceases and they
become 'quenched'. Quenched galaxies in the distant past appear to be
much smaller than the quenched galaxies in the Universe today. People
have wondered how galaxies can grow if they are no longer forming
stars. It is said to have been supposed that the small old quenched
galaxies grew into the larger quenched ones that we see 'nearby'
today, presumably by mergers with others, but there was a question
as to whether there were enough small galaxies ready to be merged.
A team of astronomers has now used a lot of observations from the
Hubble COSMOS survey covering nearly two square degrees of the sky,
plus some other observations from the CFHT and Subaru telescopes in
Hawaii, to count the numbers of apparently quenched galaxies over
the last eight billion years of cosmic history. They found that the
answer to the question was quite simple. The small quenched
galaxies stayed small, but as time went on larger galaxies became
quenched, giving the false impression that some of the small ones
had grown.
SUPPORT FOR MERGER THEORY OF GAMMA-RAY BURSTS
Space Telescope Science Institute (STScI).
The Hubble telescope has provided the strongest evidence yet that
short-duration gamma-ray bursts are triggered by the merger of two
small, super-dense stellar objects, such as a pair of neutron stars or
a neutron star and a black hole. The evidence came from observations
in near-infrared light of the fading fireball produced in the
aftermath of a short gamma-ray burst (GRB). The afterglow appears to
be from a new kind of stellar explosion that has been called a
kilonova, related to a short-duration GRB. A kilonova is about 1,000
times brighter than a nova, which is caused by an eruption on a white
dwarf; it is, however, only 1/10th to 1/100th the brightness of a
typical supernova, the self-detonation of a massive star. GRBs are
flashes of intense high-energy radiation that appear from random
directions in space. Short-duration ones last at most a few seconds,
but they sometimes generate faint afterglows in visible and near-
infrared light for several hours or days. The afterglows have helped
astronomers determine that GRBs lie in distant galaxies. The recent
observation supports a hypothesis that was already popular, that short
GRBs are caused by the merger of two compact objects. (There is
already evidence that long-duration GRBs (those lasting more than two
seconds) are produced by the collapse of massive stars.)
Astrophysicists have theorised that short-duration GRBs are created
when super-dense neutron stars in a binary system spiral together.
The system is supposed to emit gravitational radiation, tiny ripples
in the fabric of space-time. The energy dissipated by the waves
causes the two objects to come closer and closer together. In the
final milliseconds, as the two objects merge, a lot of highly
radioactive material is formed, which heats up and expands, emitting
as much visible and near-infrared light every second as the Sun does
in a few years. Such a 'kilonova' lasts about a week. American
theoreticians have predicted that the hot plasma that produces the
radiation will also act to block visible light, causing the energy
from the kilonova to flood out in near-infrared light over several
days. An opportunity to test that idea came on June 3, when the Swift
space telescope picked up an extremely bright GRB, catalogued as GRB
130603B, in a galaxy located about a billion parsecs away. Although
the initial blast of gamma-rays lasted only a tenth of a second, it
was roughly 100 billion times brighter than the subsequent kilonova
flash. The visible-light afterglow was detected at the William
Herschel Telescope and its distance was determined with the Gran
Telescopio Canarias, both located in the Canary Islands. The team
quickly realised this was a chance to hunt for a kilonova in
near-infrared light. The researchers needed to act quickly before the
light faded, so they requested Director's discretionary time on
Hubble's Wide-Field Camera 3. On June 12-13 Hubble observed a faint
red object at the site of the initial burst. Another observation
three weeks later, on July 3, showed that the source had faded away,
more or less confirming that it was the fireball from an explosive
event. Previously, astronomers had been looking at the aftermath of
short bursts largely in optical light, and were not really finding
anything besides the light of the gamma-ray burst itself. But the new
theory predicts that when you compare near-infrared and optical
images of a short gamma-ray burst about a week after the event, the
kilonova should appear in the infrared, and that is what seemed to
happen.
The ideas on the nature of short GRBs have two other implications.
First, the origin of many heavy chemical elements, including gold and
platinum, has long been a puzzle, but kilonovae are predicted to form
such elements, ejecting them into space where they can become
incorporated into future generations of stars. Secondly, the mergers
of compact objects are also expected to emit gravitational waves,
first predicted by Einstein. Such waves have not yet been observed,
but new instruments under development *may* detect them.
HUBBLE FINDS SOURCE OF MAGELLANIC STREAM
Space Telescope Science Institute (STScI)
Astronomers using the Hubble telescope have identified the origin of
the Magellanic Stream, a long ribbon of gas stretching nearly halfway
around our Milky Way galaxy. The Large and Small Magellanic Clouds,
two dwarf galaxies orbiting the Milky Way, are at the head of the
gaseous stream. Since the stream's discovery by radio telescopes in
the early 1970s, astronomers have wondered whether the gas comes from
one or both of the satellite galaxies. Now the Hubble observations
show that most of the gas was stripped from the Small Magellanic Cloud
about 2 billion years ago, and a second region of the stream
originated more recently from the Large Magellanic Cloud. The team
determined the source of the gas filament by using Hubble's 'Cosmic
Origins Spectrograph' (COS) to measure the amount of heavy elements,
such as oxygen and sulphur, at six locations along the Magellanic
Stream. COS observed distant quasars whose emitted light passes
through the stream and recorded absorption lines formed by stream
material in the ultraviolet. There were low abundances of oxygen and
sulphur along most of the stream, matching the levels in the Small
Magellanic Cloud about 2 billion years ago, when the gaseous ribbon
was thought to have been formed. There is, however, a much higher
level of sulphur in a region closer to the Magellanic Clouds, where
the composition is similar to that of the Large Magellanic Cloud,
suggesting that that part of the stream was stripped out of that
galaxy more recently.
Only a telescope in space can measure such abundances because the
relevant absorption lines are in the ultraviolet, where the Earth's
atmosphere is opaque. Apart from the Magellanic Clouds, other
satellite galaxies of the Milky Way have lost their gas. The
Magellanic Clouds have been able to retain much of their gas and
continue forming stars because they are more massive than the other
satellites. However, their proximity to the Milky Way and to one
another has led to their loss of the material that we see as the
stream.
A GOOD TIME TO STUDY THE ICE GIANTS
Alan Clitherow, Director, SPA Planetary Section
Neptune comes to opposition on August 27. It will not be very high in
the sky, ranging from about 22 to 27°, depending on where in the UK
you are; however, that is comparable with the recent apparition of
Saturn, and some very successful observations were made of that planet
over the last few months. Neptune is moving slowly westwards through
Aquarius, but at magnitude 7.9 it is not visible to the naked eye.
To find it, draw a line through the star Enif (Epsilon Pegasi), at the
south-western extreme of that constellation, to Alpha Aquarii some 10°
degrees south and slightly to the east. Then simply extend the line
the same distance again and you will be looking almost directly at
Neptune; it drifts only a small amount against the background stars
over the coming couple of months.
As there are several stars of similar magnitude in the area, Neptune
can best be found by looking for its distinctive blue colour and then
ramping up the magnification to see if the object resolves into a
disc or remains a point-like source; if you resolve its 2.4 arc-
second disc then you have found the planet. Binoculars will not be
powerful enough to distinguish the planet from the stars; to see the
disc you will need a magnification of at least 100 times and the more
telescopic aperture available the better to help with increased
resolution. Well-equipped amateurs, with large-aperture telescopes
and atmospheric-dispersion correctors to combat the low elevation,
will still need very steady seeing conditions to observe more than a
featureless blue disc; however, more modest equipment will still show
the disc along with Neptune's largest moon, Triton, as it circulates
around the planet. Received wisdom has it that you will need at least
a 200-mm aperture to glimpse Triton, but some observers claim to have
seen it with smaller telescopes under excellent seeing conditions.
On the same night, look towards Pisces. Currently in that constell-
ation, below and a little to the left of the eastern corner of the
Great Square of Pegasus, is Uranus. To find it, draw a line down from
the star at the top-left of the Great Square of Pegasus (Alpheratz) to
the star at the bottom-left of the square (Algenib or Gamma Pegasi).
Next extend this line for the same amount down towards the southern
horizon, then turn it 90 degrees left and draw a line for half the
same distance towards the east; slightly above that point is Uranus.
Its altitude will be a little better than Neptune's at more than 30°,
and will improve towards dawn, reaching around 37° by sunrise. Uranus
is also brighter at magnitude 5.7, on the edge of naked-eye
visibility, depending on your viewing location and your eyes! With an
angular diameter of 3.6 arc-seconds, bigger than Neptune's, its disc
is more obvious and has a pale blue-green colour. It is interesting
that some amateurs have already been able to image distinct banding on
the disc, but only by using monochrome cameras and filters that pass
near-infrared light, specifically the Baader 685NM filter. That would
be a suitable challenge for a well-equipped UK amateur, and any such
observation would be very welcome to the Section. Finally, as dawn
approaches, look to the east. Prominent as a rising 'star' will be
Jupiter, already 20° up. Trailing it, perhaps half-way down to the
horizon, will be the tiny disc of Mars, faint but obviously red in the
pre-dawn sky. The two planets will become much more prominent in
September, with Jupiter, particularly, being well placed for UK
observation. Jupiter has been very dynamic over the last few years,
with large-scale changes in its atmosphere and visible cloud
structure; it will be fascinating to see how much it has altered since
it was last high in our skies.
Topic: Mid August Astronomy Bulletin (Read 1295 times)