Topic: Late April Astronomy Bulletin

[Clive]

SURPRISE ASTEROID FLYBY
Spaceweather.com

With little warning, on April 15 a 'Tunguska-class' asteroid flew through
the Earth--Moon system.  2018 GE3 was discovered just the day before as it
plunged towards the Sun from the asteroid belt.  The asteroid is estimated
to be 48 to 120 metres in diameter; in all of observational history it is
the largest known asteroid to pass so close to the Earth.


RADIO TELESCOPE LISTENS TO INTERSTELLAR VISITOR
International Centre for Radio Astronomy Research

A telescope in the Western Australia outback has been used to listen to a
mysterious cigar-shaped object that entered our Solar System late last year.
The unusual object -- known as 'Oumuamua -- came from another solar system,
prompting speculation it could be an alien spacecraft.  So astronomers went
back through observations from the Murchison Widefield Array (MWA) telescope
to check for radio transmissions coming from the object between the
frequencies of 72 and 102 MHz -- similar to the frequency range in which FM
radio is broadcast.  While they did not find any signs of intelligent life,
the research helped to expand the search for extra-terrestrial intelligence
(SETI) from distant stars to objects closer to home.  When 'Oumuamua was
first discovered, astronomers thought it was a comet or an asteroid from
within the Solar System.  But after studying its orbit and discovering its
long, cylindrical shape, they realised 'Oumuamua was neither, and had come
from interstellar space.  Telescopes around the world observed the visitor
in an effort to learn as much as possible about it before it headed back out
of the Solar System, becoming too faint to observe in detail.  The MWA is
located in Western Australia's remote Murchison region, one of the most
radio-quiet areas on the planet and far from human activity and radio
interference caused by technology.  It is made up of thousands of antennae
attached to hundreds of 'tiles' that dot the ancient landscape, relentlessly
observing the heavens day after day, night after night.  The research team
was able to look back through all of the MWA's observations from November,
December and early January, when 'Oumuamua was between 95 million and 590
million kilometres from the Earth.  They found nothing, but as the first
object of its class to be discovered, `Oumuamua has given us an interesting
opportunity to expand the search for extra-terrestrial intelligence from
traditional targets such as stars and galaxies to objects that are much
closer.  That also allows for searches for transmitters that are many orders
of magnitude less powerful than those that would be needed to be detectable
from a planet orbiting even the nearest stars.

'Oumuamua was first discovered by the Pan-STARRS project at the University
of Hawaii in October.  Its name loosely means 'a messenger that reaches out
from the distant past' in Hawaiian, and is the first object known to be
interstellar to pass through our Solar System.  Combining observations from
a host of telescopes, scientists have determined that `Oumuamua is most
likely a cometary fragment that has lost much of its surface water because
it was bombarded by cosmic rays on its long journey through interstellar
space.  Researchers have now suggested there could be more than 46 million
similar interstellar objects crossing the Solar System every year.  While
most of such objects must be too far away to study with current technology,
future telescopes such as the Square Kilometre Array (SKA) should enable
scientists to understand more about such interstellar interlopers.


THOUSANDS OF BLACK HOLES IN MILKY WAY CENTRE (?)
Columbia University
 
A team of astrophysicists has discovered a dozen black holes gathered round
Sagittarius A* (Sgr A*), the supermassive black hole at the centre of the
Milky Way Galaxy.  The finding is the first to support a decades-old
prediction, opening up opportunities to understand the Universe better. 
For more than twenty years, researchers have searched unsuccessfully for
evidence to support a theory that thousands of black holes surround super-
massive black holes (SMBHs) at the centres of large galaxies.  There are
only about five dozen known black holes in our entire Galaxy -- 100,000
light-years across -- and there are supposed to be 10,000 to 20,000 that no
one has been able to find, in a region just six light-years wide.  Sgr A*
is surrounded by a halo of gas and dust that provides the perfect breeding
ground for the birth of massive stars, which live, die and could turn into
black holes there.  Additionally, black holes from outside the halo are
believed to fall under the influence of the SMBH as they lose their energy,
causing them to be pulled into the vicinity of the SMBH, where they are held
captive by its force.  While most of the trapped black holes remain
isolated, some capture and bind to a passing star, forming a stellar binary.
Researchers believe that there is a heavy concentration of such isolated and
mated black holes in the Galactic Centre, forming a density cusp which gets
more crowded as distance to the SMBH decreases.  In the past, failed
attempts to find evidence of such a cusp have focused on looking for the
bright burst of X-ray glow that sometimes occurs in black-hole binaries.
Astronomers turned to archival data from the Chandra X-ray Observatory to
test their technique.  They searched for X-ray signatures of black-hole--
low-mass-binaries in their inactive state and were able to find 12 within
three light-years of Sgr A*.  The researchers then analyzed the properties
and spatial distribution of the identified binary systems and extrapolated
from their observations that there must be anywhere from 300 to 500 black-
hole--low-mass-binaries and about 10,000 isolated black holes in the region
surrounding Sgr A*.

 
DEAD STAR CIRCLED BY LIGHT
ESO
 
New data from the MUSE instrument on ESO's Very Large Telescope in Chile
have revealed a remarkable ring of gas in a system called 1E 0102.2-7219,
expanding slowly within the depths of numerous other fast-moving filaments
of gas and dust left behind after a supernova explosion that took place 2000
years ago in the Small Magellanic Cloud.  That discovery allowed astronomers
to identify for the first time an isolated neutron star with low magnetic
field located beyond our own Milky Way galaxy.  The team noticed that the
ring was centred on an X-ray source that had been noted years before and
designated p1. The nature of that source had remained a mystery.  In
particular, it was not clear whether p1 actually lies inside the remnant or
behind it.  It was only when the ring of gas -- which includes both neon and
oxygen -- was observed with MUSE that the scientific team noticed that it
perfectly circled p1.  The coincidence was too great, and they realised that
p1 must lie within the supernova remnant itself.  Once p1's location was
known, the team used existing X-ray observations of it from the Chandra
X-ray Observatory to determine that it must be an isolated neutron star,
with a low magnetic field.  When massive stars explode as supernovae, they
leave behind curdled webs of hot gas and dust, known as supernova remnants.
Those turbulent structures are key to the redistribution of the heavier
elements -- which are cooked up by massive stars as they live and die --
into the interstellar medium, where they eventually form new stars and
planets.  Typically barely ten kilometres across, yet with masses more than
our Sun's, isolated neutron stars with low magnetic fields are thought to be
abundant across the Universe, but they are very hard to find because they
shine only at X-ray wavelengths.  The fact that the confirmation of p1 as an
isolated neutron star was enabled by optical observations is thus
particularly exciting.


FIRST PRECISE DISTANCE TO ANCIENT GLOBULAR CLUSTER
NASA/Goddard Space Flight Center

Astronomers using the Hubble Space Telescope have for the first time
measured the distance to one of the oldest objects in the Universe, a
collection of stars born shortly after the Big Bang.  That stellar assembly,
a globular star cluster called NGC 6397, is one of the closest such clusters
to the Earth.  The new measurement sets the cluster's distance at 7,800
light-years, with just a 3% margin of error.  That refined distance
yardstick provides an independent estimate for the age of the Universe. 
The new measurement may also help astronomers to improve models of stellar
evolution.  Star clusters are the key ingredient in stellar models, because
the stars in each grouping are at the same distance and have the same age
and the same chemical composition.  They therefore constitute a single
stellar population to study.  Until now, astronomers have estimated the
distances to our Galaxy's globular clusters by comparing the luminosities
and colours of stars to theoretical models, and to the luminosities and
colours of similar stars in the solar neighbourhood.  But the accuracy of
those estimates varies, with uncertainties between 10 and 20%.  However,
the new measurement relies on straightforward trigonometry.  Using a novel
observational technique to measure tiny angles on the sky, astronomers
managed to stretch Hubble's yardstick beyond the disc of our Milky Way
galaxy.

The research team calculated NGC 6397's age at 13.4 billion years old.
The globular clusters are so old that if their ages and distances deduced
from models are off by a little bit, they can seem to be older than the age
of the Universe.  Accurate distances to globular clusters are used as
references in stellar models to study the characteristics of young and old
stellar populations.  A model that agrees with the measurements inspires
some faith in its application to more distant stars.  The nearby star
clusters serve as anchors for the stellar models.  Until now, we only had
accurate distances to the much younger open clusters inside our Galaxy,
because they are closer to the Earth.  By contrast, about 150 globular
clusters orbit outside our Galaxy's comparatively younger starry disc.

The Hubble astronomers used the trigonometrical parallax method to obtain
the cluster's distance.  To obtain the distance to NGC 6397, the team
employed a method to measure accurate distances to pulsating stars called
Cepheid variables, which serve as reliable distance markers for astronomers
to calculate accurately the expansion rate of the Universe.  With that
technique, called 'spatial scanning', Hubble's Wide-Field Camera 3 gauged
the parallaxes of 40 NGC 6397 cluster stars, making measurements every six
months for two years.  The researchers then combined the results to obtain
the precise distance measurement.  They say that they could reach an
accuracy of 1% if they combine the Hubble distance measurement of NGC 6397
with the results that are hoped to be obtained from ESA's Gaia space
observatory, which is measuring the positions and distances of stars with
unprecedented precision.  The data release for the second batch of stars in
that survey is scheduled for late April.
 
 
LACK OF PHOSPHORUS IN UNIVERSE
RAS
 
Phosphorus is an essential element for life -- but new findings suggest that
it might just have been a matter of luck that there was enough of it for
life to start on Earth.  According to new observations of the Crab Nebula
(the remnant of a supernova recorded by Chinese astronomers in 1054), the
abundance and distribution of phosphorus in the Milky Way galaxy may be more
random than scientists previously thought.  As such, some places in the
galaxy may not have enough phosphorus to support life, even if they are home
to otherwise hospitable exoplanets.  Most of the Universe's phosphorus was
created during the last gasps of dying massive stars or in supernovae.
Phosphorus is difficult to observe, and only in 2013 did astronomers make
the first measurements of the element in a stellar explosion, in the wispy
remains of the supernova Cassiopeia A.  Surprisingly, they found a relative
abundance of phosphorus up to 100 times greater than is observed in the rest
of the Milky Way.

But that might have been an outlier.  Recently, astronomers pointed the
William Herschel Telescope in the Canary Islands towards the Crab Nebula,
located about 6,500 light-years away.  Preliminary data show an amount of
phosphorus more similar to the values found in the interstellar gas and dust
of the Milky Way -- a pittance compared with the abundance in Cassiopeia A.
Phosphorus abundances are remarkably variable from one site to another.  The
star that created Cassiopeia A is roughly twice as massive as the one that
made the Crab Nebula.  A more massive star could have generated different
reactions that produced more phosphorus.  The researchers said that if the
production of phosphorus varies widely across the Galaxy, so might the like-
lihood of life on other planets.  Even if a planet had every other condition
needed for habitability, it might still be bereft of life because it formed
where there was a dearth of phosphorus.  But the observations are still
preliminary.  The astronomers were able to measure only parts of the nebula
before clouds and a snowstorm curtailed their observing run.  Still, the
data they do have show significantly less phosphorus in the Crab Nebula than
in Cassiopeia A.  Ultimately, astronomers will need to measure phosphorus in
other supernova remnants.  They really want to look at how it is spreading
out from supernova remnants and falling back into the interstellar medium.


TINY DISTORTIONS IN UNIVERSE'S OLDEST LIGHT
DOE/Lawrence Berkeley National Laboratory
 
Scientists have decoded faint distortions in the patterns of the Universe's
earliest light to map huge tube-like structures known as filaments --
invisible to our eyes -- that serve as highways for delivering matter to
dense hubs such as galaxy clusters.  The international scientific team
analyzed data from past sky surveys using sophisticated image-recognition
technology to home in on the gravity-based effects that identify the shapes
of the filaments.  They also used models and theories about the filaments
to help guide and interpret their analysis.  The detailed exploration of
filaments will help researchers to understand better the formation and
evolution of the cosmic web -- the large-scale structure of matter in the
Universe -- including the unseen stuff known as dark matter that seems to
make up about 85% of the total mass of the Universe. Dark matter constitutes
the filaments -- which researchers learned typically stretch across hundreds
of millions of light-years -- and the so-called haloes that host clusters of
galaxies are fed by the universal network of filaments.  More studies of
filaments might provide new insights about dark energy, another problematic
entity that seems to drive the accelerating expansion of the Universe.
Filament properties could also put gravity theories to the test, including
Einstein's theory of general relativity, and lend important clues to help
solve an apparent mismatch in the amount of visible matter predicted to
exist in the Universe -- the 'missing baryon' problem.

The study used data from the Baryon Oscillation Spectroscopic Survey, or
BOSS, an Earth-based sky survey that captured light from about 1.5 million
galaxies to study the Universe's expansion and the patterned distribution of
matter in the Universe set in motion by the propagation of sound waves, or
'baryonic acoustic oscillations', rippling in the early Universe.  The BOSS
survey team produced a catalogue of probable filament structures that
connected clusters of matter that researchers identified in the latest
study.  Researchers also relied on precise, space-based measurements of the
cosmic microwave background, or CMB, which is the nearly uniform remnant
signal from the first light of the Universe.  While that light signature is
very similar across the Universe, there are regular fluctuations that have
been mapped in previous surveys.  In the latest study, researchers focused
on patterned fluctuations in the CMB.  They used sophisticated computer
algorithms to seek out the imprint of filaments from gravity-based
distortions in the CMB, known as weak lensing effects, that are caused by
the CMB light passing through matter.  Since galaxies live in the densest
regions of the Universe, the weak lensing signal from the deflection of CMB
light is strongest from those parts.  Dark matter resides in the haloes
around those galaxies, and was also known to spread from those denser areas
in filaments.  New data from existing experiments, and next-generation sky
surveys such as the Dark Energy Spectroscopic Instrument (DESI) now under
construction at Kitt Peak, should provide even more detailed data about the
filaments.  Researchers noted that this important step in sleuthing the
shapes and locations of filaments should also be useful for focused studies
that seek to identify what types of gases inhabit the filaments, the
temperatures of those gases, and the mechanisms for how particles enter and
move around in the filaments.  The study also allows them to determine the
lengths of filaments.