The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 384 2014 October 5
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SOME OF THE EARTH'S WATER IS PROBABLY OLDER THAN THE SUN
Water was crucial to the rise of life on Earth and is also important to evaluating the possibility of life on other planets. Identifying the original source of the Earth's water could help us to understand how life-fostering environments come into being and how likely they are to be found elsewhere. New research has found that much of our Solar System's water may have originated as ices that formed in interstellar space. Water is found throughout the Solar System, not just on Earth, but on icy comets and moons, and in the shadowed basins of Mercury. Water has been found included in mineral samples from meteorites, the Moon, and Mars. Comets and asteroids in particular, being primitive objects, provide a natural 'time capsule' of the conditions during the early days of the Solar System. Their ices can tell scientists about the ice that encircled the Sun after its birth, the origin of which was an unanswered question until now. In its youth, the Sun was surrounded by a protoplanetary disc, the so-called solar nebula, from which the planets were born. But it was not clear to researchers whether the ice in that disc originated from the interstellar molecular cloud from which the Sun was formed, or whether that interstellar water had been destroyed and was re-formed by the chemical reactions taking place in the solar nebula.
If water in the early Solar System were primarily inherited as ice from interstellar space, then it is likely that similar ices, along with the pre-biotic organic matter that they contain, are abundant in most or all proto-planetary discs around forming stars. But if the early Solar System's water were largely the result of local chemical processing during the Sun's birth, then it is possible that the abundance of water varies considerably among forming planetary systems, which would obviously have implications for the potential for the emergence of life elsewhere. In studying the history of the Solar System's ices, the team focused on hydrogen and its heavier isotope deuterium. Isotopes are atoms of the same element whose nuclei have the same number of protons but different numbers of neutrons. The difference in masses between isotopes results in subtle differences in their chemical behaviour, so the ratio of deuterium to ordinary hydrogen in water molecules can tell scientists something about the conditions under which the molecules formed. For example, interstellar water-ice has a high ratio of deuterium to hydrogen because of the very low temperatures at which it forms. Until now, it was unknown how much of the deuterium enrichment was removed by chemical processing during the Sun's birth, or how much deuterium-rich water-ice the newborn Solar System was capable of producing on its own. So the team created models that simulated a proto-planetary disc in which all the deuterium from space ice had already been eliminated by chemical processing, and the system had to start again 'from scratch' at producing ice with deuterium in it during a million-year interval. They did that in order to see if the system could reach the ratios of deuterium to hydrogen that are found in meteorite samples, but it could not do so, which told them that at least some of the water in the Solar System has an origin in interstellar space and pre-dates the birth of the Sun. The findings show that a significant fraction of the Solar System's water is older than the Sun, which indicates that interstellar ices should probably be found in all young planetary systems.
219 MILLION STARS IN MOST DETAILED CATALOGUE OF VISIBLE MILKY WAY
A new catalogue of the northern part of our Galaxy includes no fewer than 219 million stars. It represents the fruit of a ten-year programme with the Isaac Newton Telescope (INT) on La Palma. The Milky Way is the disc of our own Galaxy, and contains the majority of the stars in the Galaxy, including the Sun, and the densest concentrations of dust and gas. The INT programme charted all the stars brighter than 20th magnitude in the zone within 5 degrees of the Galactic equator. From the catalogue, scientists have made a map of the disc of the Galaxy that shows how the density of stars varies. The catalogue, IPHAS DR2 (the second release from the survey programme, 'INT Photometric H-alpha Survey of the Northern Galactic Plane' or 'IPHAS'), illustrates modern astronomy's exploitation of 'big data': information on each of the 219 million detected objects is summarised in 99 attributes. There are magnitudes measured through two broad-band filters in the red part of the visible spectrum, and in a narrow band centred on H-alpha. The H-alpha band also maps the emission nebulae that are so common in the plane of the Milky Way.
SMALLEST KNOWN GALAXY WITH SUPERMASSIVE BLACK HOLE
University of Utah
Astronomers have discovered that an ultra-compact dwarf galaxy called M60-UCD1 harbours a super-massive black hole -- the smallest galaxy known to contain one. It is also one of the most black-hole-dominated galaxies known. The team used observations from the Gemini North 8-m telescope in Hawaii and the Hubble telescope, and discovered that M60-UCD1 has a black hole with a mass of 21 million suns. The only way that such a massive hole could arise in such a small galaxy seems to be that the galaxy is the stripped remnant of an originally larger one that was torn apart in a galactic collision. Super-massive black holes, having masses of at least 1 million Suns, are thought to exist at the centres of many galaxies. The one at the centre of our own Galaxy has a mass of about 4 million Suns, but it is less than 0.01% of the Galaxy's total mass; the one at the centre of M60-UCD1 is five times more massive than the Milky Way's, and is 15% of the small galaxy's total mass of about 140 million Suns.
Some scientists are reported to believe that M60-UCD1 once was a big galaxy with perhaps 10 billion [U.S. billion, = 1000 million] stars in it, but then it passed very close to the centre of the even larger galaxy M 60, and in that process all the stars and dark matter in the outer parts were torn away and became part of M 60, which is among the largest galaxies in what some astronomers refer to as the 'local universe'. M 60 appears also to be pulling in another galaxy, named NGC 4647; M 60 is about 25 times the mass of NGC 4647. M60-UCD1 is roughly 54 million light-years away from us, but 'only' 22,000 light-years from the centre of M 60. Astronomers have debated whether such dwarf galaxies are the nuclei of larger galaxies whose outer parts were stripped away during collisions with other galaxies, or whether they formed like globular clusters -- groups of perhaps 100,000 stars, all born together. There are about 200 globular clusters in our Milky Way, but some galaxies have thousands.
DISTANT GALAXY CONTAINS 3 CLOSELY ORBITING BLACK HOLES
A team of scientists has discovered a trio of super-massive black holes, closely orbiting the centre of a distant galaxy more than four billion light-years away. It is the tightest trio of black holes known -- with two of them orbiting one another like binary stars. The new discovery may help astronomers in their search for gravitational waves, a phenomenon predicted by Einstein. The black holes are at the very extreme of Einstein's theory of General Relativity. Some scientists are said to believe that gravitational waves originate among merging black holes, and the current study of the tightly-packed black-hole trio may help or hinder that theory. The idea that we might be able to find more potential sources of gravitational waves is attractive, as knowing where such signals might originate could help us to try to detect those 'ripples' in space-time as they warp the Universe. Before we get too excited, however, we must notice that just recently other observers have cast doubt on even the reality of the announced trio of black holes.
MASSIVE GALAXIES GROW BY ABSORBING SMALLER NEIGHBOURS
Massive galaxies seem mostly to have stopped making their own stars and are instead absorbing nearby galaxies. Australian astronomers looked at more than 22,000 galaxies and found that while smaller galaxies are very efficient at creating stars from gas, the most massive galaxies are much less efficient, producing hardly any new stars themselves, and instead grow by eating other galaxies. The researchers said that our own Milky Way is at a tipping point and is expected to grow mainly by incorporating smaller galaxies, rather than by collecting gas. The Milky Way has not merged with another large galaxy for a long time, but we can still see remnants of all the old galaxies that it has cannibalised. It is likely to absorb two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years, and is eventually likely to merge with the 'nearby' Andromeda Galaxy. Ultimately, gravity is expected to cause all the galaxies in bound groups and clusters to merge into a few super-giant galaxies, although it will be many billions of years before that happens.
INTERSTELLAR MOLECULES FOUND AT GALACTIC CENTRE
The search for molecules in interstellar space began in the 1960s, and about 180 different molecular species have been discovered so far. Now a carbon-bearing molecule with a branched structure has for the first time been detected. The molecule, iso-propyl cyanide (i-C3H7CN), was discovered in a gas cloud called Sagittarius B2 (SgrB2), a region of star formation that is a favourite spot for molecule-hunting astronomers, about 27,000 light-years away and close to the centre of our Galaxy. The branched structure of the carbon atoms within the iso-propyl cyanide molecule is unlike the straight-chain carbon backbone of other molecules that have been detected so far, including its sister molecule, normal propyl cyanide (n-C3H7CN). It is not just the structure of the molecule that is a surprise -- it is also plentiful, at almost half the abundance of its straight-chain sister molecule, which the team had already detected with the single-dish radio telescope of the Institut de Radioastronomie Millimetrique (IRAM) a few years ago. This time it used the Atacama Large Mm/sub-mm Array (ALMA), in Chile, to investigate anew the molecular content of Sgr B2, which is rich in emission from complex interstellar molecules. As many as 50 individual features for i-propyl cyanide and even 120 for n-propyl cyanide were identified in the ALMA spectrum of Sgr B2. The two molecules, each consisting of 12 atoms, are also the joint-largest molecules yet detected in any star-forming region. The team constructed computational models that simulate the chemistry of formation of the molecules detected in SgrB2. In common with many other complex molecules, both forms of propyl cyanide were found to be efficiently formed on the surfaces of interstellar dust grains. But the models indicate that for molecules large enough to produce branched-chain structure, those may be the prevalent forms. Any detection of the next member of the alkyl cyanide series, n-butyl cyanide (n-C4H9CN), and its three branchedisomers would allow scientists to test that idea. Amino acids identified in meteorites have a composition that suggests that they originate in the interstellar medium. Although no interstellar amino acids have yet been found, interstellar chemistry may be responsible for the production of a wide range of important complex molecules that eventually find their way to planetary surfaces.
ROSETTA COMET LANDING DATE
The date has been fixed for ESA's attempt to land on a comet: Wednesday November 12. The date is actually a day later than the one that had been discussed in provisional planning in recent months. It will see the Rosetta satellite, which is currently orbiting the comet known as 67P, drop a small robot from a height of 20km. If all goes well, the lander will free-fall towards the comet, making contact with the surface somewhere in a 1km-wide zone at roughly 15:35 GMT. Because the event will be taking place so far away, radio signals will take 28 minutes and 20 seconds to arrive here, so confirmation of success or failure will not come until just after 16:00 GMT.
The chosen landing site is on the 'head' of the rubber-duck-shaped comet and is currently referred to simply as 'J', the designation it was given in a list of possible destinations in the selection process. It is far from ideal: it contains some cliffs, but is the flattest, most boulder-free location the mission team could find in its survey of the comet. Mapping of J and a back-up site known as C is ongoing. Rosetta has recently manoeuvred into an orbit just 20km from 67P, enabling its camera system to see details that are less than a metre across. For landing, such information has only limited utility, however, as the automated touchdown can only be targeted to a precision of hundreds of metres, and such an error is larger than any of the apparently smooth areas in the J zone. The whole separation, descent and landing (SDL) procedure is expected to take seven hours. Philae will take a picture of Rosetta as it leaves its 'parent'. It will also point a camera downwards so that we can see the surface as it approaches -- not that that information can change anything, as Philae has no thrusters to control or alter its descent trajectory. It will land where it will land. But the images will help controllers determine where the robot ended up after the event. If Philae gets down successfully into a stable, operable configuration, it will fire harpoons and deploy screws to try to hang on to the surface, since the gravitational force of such a small body as the comet is tiny. Already the main satellite has returned some astonishing pictures of Comet 67P and the close-quarters observations it will conduct over the next year will be very interesting..
Bulletin compiled by Clive Down
(c) 2014 the Society for Popular Astronomy
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