Institute of Astronomy

Astronomy News

Rosetta:Rosetta's comet 'sweats' two glasses of water a second

30 June 2014 - 2:16pm
ESA's Rosetta spacecraft has found that comet 67P/Churyumov–Gerasimenko is releasing the equivalent of two small glasses of water into space every second, even at a cold 583 million kilometres from the Sun.

Pollution on other worlds may show advanced alien life

27 June 2014 - 5:00pm
A NASA telescope should be able to sniff the atmospheres of Earth-sized worlds for industrial gases like CFCs – a sign of civilisation

Commissioning the Radial Velocity Spectrometer

27 June 2014 - 1:27pm

Guest blog post by George Seabroke, RVS Payload Expert, on behalf of the team commissioning the RVS instrument.

The Radial Velocity Spectrometer (RVS) is one of three instruments onboard Gaia (see Figure 1). It is designed to measure the line-of-sight velocity component of Gaia stars (radial velocity, RV) to complement Gaia astrometry, which measures the transverse velocity component (parallax converts proper motions to transverse velocity). Combining the radial and transverse velocities gives the 3D space velocity of Gaia stars, allowing Gaia to produce not only a map of where Gaia stars are but how they are moving.

Figure 1: Photographs of RVS components (insets) overlaid on a photograph of the Gaia Payload Module at Astrium in Toulouse. Credits: Astrium, except top left (ESA/Gaia/DPAC/Airbus DS), bottom left (ESA) and top right (Selex Galileo, Italy). Composite designed by George Seabroke, MSSL.

The accuracy that Gaia astrometry is aiming for can only be achieved above the Earth’s atmosphere. The RV accuracy that RVS is aiming for (a few km/s for bright stars) can be achieved from the ground: however, the ground-based RV follow-up of the 118,200 Hipparcos stars managed about 20,000 stars in 15 years (a remarkable effort at the time!). Therefore it was decided to include an RV instrument onboard Gaia from the beginning and thus RVS was born.

RVS is an optical module located between the last mirror (M6) and the 12 RVS CCDs at the right edge of the Gaia focal plane (see Figure 1). RVS consists of six optical elements: a transmission grating (see bottom left in Figure 1), which disperses all the light entering RVS into medium-resolution (λ/Δλ ~ 11,500) spectra; a band-pass filter, which limits the spectra to 845–872 nm (wavelengths visible to the human eye are 400-700 nm); and four lenses/prisms (see top right in Figure 1) to correct the main aberrations of the telescope.

The RVS wavelength range is chosen to include a set of three absorption lines called the calcium triplet (see top left in Figure 1; click here for enlarged version of the spectra). Calcium is made by nuclear fusion in the centre of massive stars. When they explode as supernovae, the calcium can end up in the atmospheres of the next generation of stars (or in our teeth and bones!). If the star is moving away or towards Gaia, the calcium triplet (and all spectral) lines will be shifted by the Doppler effect, compared to their rest wavelengths. The RV of each star can be determined from RVS spectra by measuring these shifts.

The RVS Payload Experts (PEs) are the team, in DPAC, supporting the commissioning of the RVS instrument. The team, geographical locations and responsibilities are mainly split between Observatoire Paris Meudon (OPM) and Mullard Space Science Laboratory (MSSL, part of University College London).

Unfortunately, at least from my personal perspective, we are PEs, rather than Payload Specialists. Payload Specialists were astronauts who flew on the Shuttle with specific payloads. Even if we wanted to visit Gaia, we would not be able to because at the second Sun–Earth Lagrangian point (L2), Gaia is much further than any astronaut has ever been, over 1 million km beyond the Moon’s orbit! Having not been selected to become an ESA astronaut in 2009 (one of my competitors, Alexander Gerst, is on the International Space Station right now!), working on commissioning the RVS is probably the closest I have come to being in space!

One recent commissioning activity that I led was when the RVS CCDs had charge injected into them to check for and calibrate any radiation damage.

The most critical issue for the RVS PEs was to check that the six optical elements of RVS are producing spectra that include the calcium triplet absorption lines, allowing us to derive RVs. If we had not seen the calcium lines, we would have had a bone to pick with RVS (English idiom!). Figure 2 shows what a 2D RVS spectrum looks like on a RVS CCD. RVS spectra are summed in the ACross Scan (AC) direction (y-axis in Figure 2) to produce 1D spectra, like the one in the top left inset of Figure 1 (see blog article “Gaia takes science measurements” for more details on this RVS spectrum – the first to go public). The calcium lines are clearly visible (so ironically no bone to pick!).

Figure 2: Example of a 2D RVS spectrum, extracted by Pasquale Panuzzo (OPM). The three dark vertical lines are where calcium atoms in the atmosphere of this particular star have blocked starlight to form absorption lines in this star’s spectrum.

The biggest surprise for the RVS PEs was finding there was a scattered light problem. The Basic Angle Monitor (BAM) has a laser at the same wavelength as RVS so there was a commissioning activity in January to check the levels of the BAM laser light leaking into RVS. Because of this, we were the first to measure the Gaia instrument background and found it to be much higher than expected from the BAM laser alone. Since then, Gaia’s instrument background has become a hot topic in commissioning and astrometric and photometric PEs have found that their instrument backgrounds are also affected. Various experiments have been conducted to understand the source of the scattered light and the Gaia PEs have been analysing the results of these experiments and feeding the results back to ESA.

OPM have developed most of the RVS offline tools. This involves downloading about 31 Gb of data per day (the total is now 7 Tb), ingesting it into local databases and generating daily “First Look” reports to allow the RVS PEs to have a first look at the data. The offline tools have been used in three other very important ways. Firstly they were used to analyse quickly the RVS spectra during the best focus commissioning activity, which were independently verified by MSSL. RVS spectra do have the required sharp absorption lines, including the calcium lines, which reduce the uncertainty on RV measurements. These spectra were also used by OPM to derive the RVS resolution. In addition, OPM are verifying and optimizing the onboard Video Processing Unit parameters, which control how well the readout windows are centred around RVS spectra to make sure we capture as much dispersed starlight as possible.

MSSL has developed a local infrastructure and integrated the official on-ground RVS data processing pipeline, which includes modules by all of Co-ordination Unit 6 (“Spectroscopic Processing”). MSSL are now running it to investigate the RVS data. The pipeline consists of more than 60,000 lines of code, described in over 800 pages of documentation (Software Design Descriptions). It has involved a lot of debugging of software to ensure the pipeline can process real RVS spectra. The commissioning period included 14 days of undisturbed data obtained between 9 and 23 May. The challenge now for MSSL, having ingested all these spectra (more than 111 million!) into our databases, is to process them through the RVS pipeline to estimate how accurate the RVS RVs will be (as a function of stellar brightness). RVS was never designed to measure the RVs of all Gaia’s 1 billion stars. We will soon know the number of stars for which RVS will be able to measure an RV and what the RV accuracy will be. Whatever the answer, it should be tens of millions, making RVS the largest RV survey in history!

I was fortunate enough to be in French Guiana to see the Gaia launch but like all the RVS PEs, we have never actually seen the completed RVS instrument with our own eyes! Now we will never see it, as Gaia will not return to Earth. Nevertheless, we have been getting to know RVS in the last six months of commissioning through the data it is returning. Over the next five years (or more) of the mission, we will get to know the instrument in ever-greater detail. This will allow us continually to optimize its operation onboard Gaia and also the on-ground algorithms that process the data, ensuring RVS reaches its enormous scientific potential.

Written by George Seabroke (MSSL) on behalf of the RVS PEs: Kevin Benson (MSSL), Mark Cropper (MSSL), Chris Dolding (MSSL), Joris Gerssen (Potsdam), Alain Guéguen (OPM), Leanne Guy (Geneva), Howard Huckle (MSSL), Katja Janssen (Potsdam), David Katz (OPM), Olivier Marchal (OPM), Pasquale Panuzzo (OPM, RVS PE Co-ordinator), Paola Sartoretti (OPM), Mike Smith (MSSL).


Huge X-ray space telescope planned

27 June 2014 - 11:31am
The European Space Agency initiates the process that should lead to the biggest X-ray telescope ever built going into orbit in 2028.

Cosmic Vision:Athena to study the hot and energetic Universe

27 June 2014 - 10:37am
ESA has selected the Athena advanced telescope for high-energy astrophysics as its second 'Large-class' science mission.

Better to see the beautiful, ugly truth of the cosmos

26 June 2014 - 9:00pm
The assumption that the cosmos is symmetrical over large scales is an elegant one, but how far must evidence deviate from expectations before we rethink it?

Pillars of creation built by big stellar bubble

26 June 2014 - 12:01am
Models of how the iconic towers of gas in the Eagle Nebula could have been born also reveal the complex role huge stars play in sparking stellar nurseries

Mystery galactic glow may be echo of sterile neutrinos

25 June 2014 - 6:30pm
An unexplained X-ray signal from the swirling Perseus galaxy cluster could be the death rattle of an elusive particle – and hint at the nature of dark matter

Black hole trio holds promise for gravity wave hunt

25 June 2014 - 6:05pm

An international team, including University of Cambridge scientists, led by Dr Roger Deane from the University of Cape Town, examined six systems thought to contain two supermassive black holes. The team found that one of these contained three supermassive black holes – the tightest trio of black holes detected at such a large distance – with two of them orbiting each other rather like binary stars. The finding suggests that these closely-packed supermassive black holes are far more common than previously thought.

A report of the research is published in this week’s Nature.

Dr Roger Deane from the University of Cape Town said: ‘What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein’s Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth. Not only that, but using the combined signals from radio telescopes on four continents we are able to observe this exotic system one third of the way across the Universe. It gives me great excitement as this is just scratching the surface of a long list of discoveries that will be made possible with the Square Kilometre Array (SKA).’

The team used a technique called Very Long Baseline Interferometry (VLBI) to discover the inner two black holes of the triple system. This technique combines the signals from large radio antennas separated by up to 10,000 kilometres to see detail 50 times finer than that possible with the Hubble Space Telescope. The discovery was made with the European VLBI Network, an array of European, Chinese, Russian and South African antennas, as well as the 305 metre Arecibo Observatory in Puerto Rico. Future radio telescopes such as the SKA will be able to measure the gravitational waves from such black hole systems as their orbits decrease.

At this point, very little is actually known about black hole systems that are so close to one another that they emit detectable gravitational waves. According to Prof Matt Jarvis from the Universities of Oxford and the Western Cape, ‘This discovery not only suggests that close-pair black hole systems emitting at radio wavelengths are much more common than previously expected, but also predicts that radio telescopes such as MeerKAT and the African VLBI Network (AVN, a network of antennas across the continent) will directly assist in the detection and understanding of the gravitational wave signal. Further in the future the SKA will allow us to find and study these systems in exquisite detail, and really allow us gain a much better understanding of how black holes shape galaxies over the history of the Universe.’

Dr Keith Grainge of the University of Manchester, an author of the paper, said: ‘This exciting discovery perfectly illustrates the power of the VLBI technique, whose exquisite sharpness of view allows us to see deep into the hearts of distant galaxies. The next generation radio observatory, the SKA, is being designed with VLBI capabilities very much in mind.’

While the VLBI technique was essential to discover the inner two black, the team has also shown that the binary black hole presence can be revealed by much larger scale features. The orbital motion of the black hole is imprinted onto its large jets, twisting them into a helical or corkscrew-like shape. So even though black holes may be so close together that our telescopes cannot tell them apart, their twisted jets may provide easy-to-find pointers to them, much like using a flare to mark your location at sea. This may provide sensitive future telescopes like MeerKAT and the SKA a way to find binary black holes with much greater efficiency.

The discovery of three closely orbiting supermassive black holes in a galaxy more than four billion light years away could help astronomers in the search for gravitational waves: the ‘ripples in spacetime’ predicted by Einstein.

black holegravitational wavesSquare Kilometre ArrayClare RumseyUniversity of OxfordUniversity of Cape TownUniversity of ManchesterDepartment of PhysicsSchool of the Physical SciencesCavendish LaboratoryThis exciting discovery perfectly illustrates the power of the VLBI technique, whose exquisite sharpness of view allows us to see deep into the hearts of distant galaxies.Dr Keith GraingeRoger Deane (large image); NASA Goddard (inset bottom left; modified from original)Helical jets from one supermassive black hole caused by a very closely orbiting companion (see blue dots). The third black hole is part of the system, but farther away and therefore emits relatively straight jets.

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YesNews type: News

Should the Higgs boson have caused our Universe to collapse? Talk by Robert Hog...

25 June 2014 - 9:42am
Should the Higgs boson have caused our Universe to collapse?

Talk by Robert Hogan at #nam2014, on combining the Higgs boson with the recent #BICEP2 results

Should the Higgs boson have caused our Universe to collapse?
British cosmologists are puzzled: they predict that the Universe should not have lasted for more than a second. This startling conclusion is the result of combining the latest observations of the sky with the recent discovery of the Higgs boson. Robert Hogan of King's College London (KCL) will present the new research on 24 June at the Royal Astronomical Society's National Astronomy Meeting in Portsmouth.

Astrophysics: A tight duo in a trio of black holes

25 June 2014 - 1:00am

Astrophysics: A tight duo in a trio of black holes

Nature 511, 7507 (2014). doi:10.1038/nature13511

Authors: Greg Taylor

Tight pairs of supermassive black holes are expected to emit gravitational waves that could give astronomers a new way to explore the cosmos. One relatively tight pair has been discovered within a rare triple system. See Letter p.57

A close-pair binary in a distant triple supermassive black hole system

25 June 2014 - 1:00am

A close-pair binary in a distant triple supermassive black hole system

Nature 511, 7507 (2014). doi:10.1038/nature13454

Authors: R. P. Deane, Z. Paragi, M. J. Jarvis, M. Coriat, G. Bernardi, R. P. Fender, S. Frey, I. Heywood, H.-R. Klöckner, K. Grainge & C. Rumsey

Galaxies are believed to evolve through merging, which should lead to some hosting multiple supermassive black holes. There are four known triple black hole systems, with the closest black hole pair being 2.4 kiloparsecs apart (the third component in this system is at 3 kiloparsecs), which is far from the gravitational sphere of influence (about 100 parsecs for a black hole with mass one billion times that of the Sun). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs (ref. 10). Here we report observations of a triple black hole system at redshift z = 0.39, with the closest pair separated by about 140 parsecs and significantly more distant from Earth than any other known binary of comparable orbital separation. The effect of the tight pair is to introduce a rotationally symmetric helical modulation on the structure of the large-scale radio jets, which provides a useful way to search for other tight pairs without needing extremely high resolution observations. As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments.

Astronomy: Speedy stars revealed nearby

25 June 2014 - 1:00am

Astronomy: Speedy stars revealed nearby

Nature 510, 7506 (2014). doi:10.1038/510447c

Astronomers have spotted 28 stars that are hurtling through space fast enough to escape the Milky Way's gravitational pull — the biggest set of such stars, and the nearest to Earth, so far identified.A team led by Jing Zhong of the Shanghai Astronomical Observatory,

XMM-Newton:Puzzling X-rays point to dark matter

24 June 2014 - 3:21pm
Astronomers using ESA and NASA high-energy observatories have discovered a tantalising clue that hints at an elusive ingredient of our Universe: dark matter.

Black hole made peek-a-boo galaxy go mysteriously dark

24 June 2014 - 11:49am
A bright galaxy that suddenly went dark was obscured by wind spurting from its central black hole – enhancing our understanding of these distant objects

Spectral ‘ruler’ is first standardised way to measure stars

24 June 2014 - 9:44am

Previously, as with the longitude problem 300 years earlier for fixing locations on Earth, there was no unified system of reference for calibrating the heavens.

But now, when investigating the atmospheric structure and chemical make-up of stars, astronomers can use a new stellar scale as a ‘ruler’ – making it much easier for them to classify and compare data on star discoveries.

In fact, the work is a critical first step in the Gaia satellite’s mission to map the Milky Way, as the unprecedented levels of stellar data that will result need “consistent stellar parameters”, the same way we need values to measure everything from temperature to time, say astronomers.

The guidelines are free to download and are already being used by the world’s largest astronomy projects. The work has recently been published in the journal Astronomy & Astrophysics

The team, including Dr Paula Jofre from the University of Cambridge’s Institute of Astronomy, selected 34 initial ‘benchmark’ stars to represent the different kinds of stellar populations in our galaxy, such as hot stars, cold stars, red giants and dwarfs, as well as stars that cover the different chemical patterns – or ‘metallicity’ – in their spectrum, as this is the “cosmic clock” which allows astronomers to read a star’s age.

This detailed range of information on the 34 stars form the first value set for measuring the millions of stars Gaia aims to catalogue. Many of the benchmark stars can be seen with the human eye, and have been studied for most of human history – dating back to the very first astronomical records from ancient Babylon.          

“We took stars which had been measured a lot so the parameters are very well-known, but needed to be brought to the same scale for the new benchmark - essentially, using the stars we know most about to help measure the stars we know nothing about,” said Jofre. 

“In previous galactic studies, the Sun is used as the standard to show a method is working, along with a few other well-known stars. But I choose this one because it works for my method, you choose a different one for different reasons; data may not match.

“This is the first attempt to cover a wide range of stellar classifications, and do everything from the beginning – methodically and homogenously.”    

Launched at the end of last year, Gaia will gather data on over a billion stars in the Milky Way, allowing astronomers to study for the first time in close detail its myriad stars and planetary systems. Petabytes of data will be sent back to Earth – roughly the equivalent of all the information held in all the libraries of the world today.

The new value system was needed to ensure that analysis of this extraordinary amount of data is done in the most effective and efficient way, a template to measure the vast stream of data against.

Jofre focused on spectroscopic data to work out metallicity: the chemical combination that makes up a star. Just as a raindrop can split sunlight into the colours of the rainbow, spectroscopes split the light from a star into its chemical elements – and the results can be read like a musical score, with high notes or low notes in the scale giving clues as to the star’s age. On average, the higher a star’s metal content the younger it is.

Jofre created a ‘spectral’ library, combining the best data on the atmospheric structure of benchmark stars to determine a uniform scale for metallicity. Together with definitive scales for the stars’ temperatures and surface gravities, produced with colleagues at the University of Uppsala and the University of Bordeaux, her work completes the measuring system that will be used to gauge data from Gaia.

“Now this set of data scales for the benchmark stars can be used as a way of making definitive measurements of others stars – invaluable to astronomers working on a wide range of projects,” Jofre said.

The benchmark stars are already being used as a standardising model by Gaia’s sister project, the Gaia-ESO survey, which is observing stellar spectra at a high resolution from the Very Large Telescope in Chile. They will also provide the basis for the thousands of reference stars needed to set the parameters for the hugely ambitious Gaia satellite once it starts mapping the entire galaxy – the “pillars for the enormous calibrators”.

The fact that the ideal benchmark stars needed to be ones we already have a lot of data on means that many are bright and relatively near to the Earth – and have been the subject of wonder across civilisations.

Aldebaran, Arcturus, Pollux, Procyon and Alpha Centauri have played a part in the culture and mythology of mankind since they were first identified thousands of years ago. Babylonian astronomers used them as a reference point to describe the positions of the moon and planets as they moved through the night sky, appearing in the Babylonian Astronomical Diaries dating back to almost 1000 years BC.

“Many people interested in astronomy know these stars, their position in constellations, and the best time of year to see them. It is amazing that there is still so much to learn about the physics of these most well-known stars," said Dr. Ulrike Heiter from the Uppsala University.

“While stars do move over millennia, for humans they are fixed points – used to navigate the Earth for centuries. We are still using them as fixed points, but this time for navigating the galaxy,” Jofre said.

UK Gaia lead Professor Gerry Gilmore added: “Advances in understanding the history and structure of our Galaxy with ambitious projects are possible only because, like Newton, we see farther by standing on the shoulders of giants. For reliably determining what chemical elements the stars are made of, those giants are the benchmark stars. All our vastly expanding knowledge depends on really understanding the few."

Inset images: Crab Nebula and graphic rendering of the Gaia satellite

A team of astronomers have created the first standardised set of measurement guidelines for analysing and cataloguing stars.

This is the first attempt to cover a wide range of stellar classifications, and do everything from the beginning – methodically and homogenouslyPaula JofreAmanda Smith/Institute of AstronomyThe first standardised way to measure stars has been developed for Gaia mission

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YesNews type: News

Clingy dark matter may slow the spin of corpse stars

23 June 2014 - 6:40pm
Dark matter with a tiny electrical charge could put the brakes on pulsars, offering a new way to look for clues to the nature of the mysterious substance

A (personal) call for a giant space telescope from Martin Barstow. He'll talk ab...

22 June 2014 - 10:40am
A (personal) call for a giant space telescope from Martin Barstow. He'll talk about this on Tuesday at the National Astronomy Meeting.

Time to think big: a call for a giant space telescope
In the nearly 25 years since the launch of the Hubble Space Telescope (HST), astronomers and the public alike have enjoyed ground-breaking views of the cosmos and the suite of scientific discoveries that followed. The successor to HST, the James Webb Space Telescope should launch in 2018 but will have a comparatively short lifetime. Now Prof Martin Barstow of the University of Leicester is looking to the future. In his talk at the National Astronomy Meeting (NAM 2014) in Portsmouth on Tuesday 24 June, he calls for governments and space agencies around the world to back the Advanced Technologies Large Aperture Space Telescope (ATLAST), an instrument that would give scientists a good chance of detecting hints of life on planets around other stars.

Mountain top exploded to make way for ghost telescope

20 June 2014 - 2:56pm
The telescope that will live on the now-flat summit of Cerro Amazones in the Atacama desert will be the largest of its kind in the world - and a ghost

Confidence drop for Big Bang signal

20 June 2014 - 12:25am
Scientists who claimed to have found a pattern in the sky left by the super-rapid expansion of space just fractions of a second after the Big Bang say they are now less confident of their result.