Institute of Astronomy

Astronomy News

Why Hawking’s PhD thesis is now an internet-breaking inspiration

3 November 2017 - 8:57am

Millions rushed to freely access Stephen Hawking's early musings when they went online. More of the same would help ignite young minds everywhere, says Geraint Lewis

Launch your design with CHEOPS

3 November 2017 - 8:55am

ESA is offering graphic designers and artists a unique opportunity to feature their work on the rocket carrying the CHEOPS satellite.

A gravitational-wave standard siren measurement of the Hubble constant

2 November 2017 - 9:22am

A gravitational-wave standard siren measurement of the Hubble constant

Nature 551, 7678 (2017). doi:10.1038/nature24471


On 17 August 2017, the Advanced LIGO and Virgo detectors observed the gravitational-wave event GW170817—a strong signal from the merger of a binary neutron-star system. Less than two seconds after the merger, a γ-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO–Virgo-derived location of the gravitational-wave source. This sky region was subsequently observed by optical astronomy facilities, resulting in the identification of an optical transient signal within about ten arcseconds of the galaxy NGC 4993. This detection of GW170817 in both gravitational waves and electromagnetic waves represents the first ‘multi-messenger’ astronomical observation. Such observations enable GW170817 to be used as a ‘standard siren’ (meaning that the absolute distance to the source can be determined directly from the gravitational-wave measurements) to measure the Hubble constant. This quantity represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Here we report a measurement of the Hubble constant that combines the distance to the source inferred purely from the gravitational-wave signal with the recession velocity inferred from measurements of the redshift using the electromagnetic data. In contrast to previous measurements, ours does not require the use of a cosmic ‘distance ladder’: the gravitational-wave analysis can be used to estimate the luminosity distance out to cosmological scales directly, without the use of intermediate astronomical distance measurements. We determine the Hubble constant to be about 70 kilometres per second per megaparsec. This value is consistent with existing measurements, while being completely independent of them. Additional standard siren measurements from future gravitational-wave sources will enable the Hubble constant to be constrained to high precision.

Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event

2 November 2017 - 9:22am

Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event

Nature 551, 7678 (2017). doi:10.1038/nature24453

Authors: Daniel Kasen, Brian Metzger, Jennifer Barnes, Eliot Quataert & Enrico Ramirez-Ruiz

The cosmic origin of elements heavier than iron has long been uncertain. Theoretical modelling shows that the matter that is expelled in the violent merger of two neutron stars can assemble into heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted to power a distinctive thermal glow (a ‘kilonova’). The discovery of an electromagnetic counterpart to the gravitational-wave source GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements. Here we report models that predict the electromagnetic emission of kilonovae in detail and enable the mass, velocity and composition of ejecta to be derived from observations. We compare the models to the optical and infrared radiation associated with the GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. The ejected mass and a merger rate inferred from GW170817 imply that such mergers are a dominant mode of r-process production in the Universe.

A kilonova as the electromagnetic counterpart to a gravitational-wave source

2 November 2017 - 9:22am

A kilonova as the electromagnetic counterpart to a gravitational-wave source

Nature 551, 7678 (2017). doi:10.1038/nature24303

Authors: S. J. Smartt, T.-W. Chen, A. Jerkstrand, M. Coughlin, E. Kankare, S. A. Sim, M. Fraser, C. Inserra, K. Maguire, K. C. Chambers, M. E. Huber, T. Krühler, G. Leloudas, M. Magee, L. J. Shingles, K. W. Smith, D. R. Young, J. Tonry, R. Kotak, A. Gal-Yam, J. D. Lyman, D. S. Homan, C. Agliozzo, J. P. Anderson, C. R. Angus, C. Ashall, C. Barbarino, F. E. Bauer, M. Berton, M. T. Botticella, M. Bulla, J. Bulger, G. Cannizzaro, Z. Cano, R. Cartier, A. Cikota, P. Clark, A. De Cia, M. Della Valle, L. Denneau, M. Dennefeld, L. Dessart, G. Dimitriadis, N. Elias-Rosa, R. E. Firth, H. Flewelling, A. Flörs, A. Franckowiak, C. Frohmaier, L. Galbany, S. González-Gaitán, J. Greiner, M. Gromadzki, A. Nicuesa Guelbenzu, C. P. Gutiérrez, A. Hamanowicz, L. Hanlon, J. Harmanen, K. E. Heintz, A. Heinze, M.-S. Hernandez, S. T. Hodgkin, I. M. Hook, L. Izzo, P. A. James, P. G. Jonker, W. E. Kerzendorf, S. Klose, Z. Kostrzewa-Rutkowska, M. Kowalski, M. Kromer, H. Kuncarayakti, A. Lawrence, T. B. Lowe, E. A. Magnier, I. Manulis, A. Martin-Carrillo, S. Mattila, O. McBrien, A. Müller, J. Nordin, D. O’Neill, F. Onori, J. T. Palmerio, A. Pastorello, F. Patat, G. Pignata, Ph. Podsiadlowski, M. L. Pumo, S. J. Prentice, A. Rau, A. Razza, A. Rest, T. Reynolds, R. Roy, A. J. Ruiter, K. A. Rybicki, L. Salmon, P. Schady, A. S. B. Schultz, T. Schweyer, I. R. Seitenzahl, M. Smith, J. Sollerman, B. Stalder, C. W. Stubbs, M. Sullivan, H. Szegedi, F. Taddia, S. Taubenberger, G. Terreran, B. van Soelen, J. Vos, R. J. Wainscoat, N. A. Walton, C. Waters, H. Weiland, M. Willman, P. Wiseman, D. E. Wright, Ł. Wyrzykowski & O. Yaron

Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of −1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90–140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

The X-ray counterpart to the gravitational-wave event GW170817

2 November 2017 - 9:22am

The X-ray counterpart to the gravitational-wave event GW170817

Nature 551, 7678 (2017). doi:10.1038/nature24290

Authors: E. Troja, L. Piro, H. van Eerten, R. T. Wollaeger, M. Im, O. D. Fox, N. R. Butler, S. B. Cenko, T. Sakamoto, C. L. Fryer, R. Ricci, A. Lien, R. E. Ryan, O. Korobkin, S.-K. Lee, J. M. Burgess, W. H. Lee, A. M. Watson, C. Choi, S. Covino, P. D’Avanzo, C. J. Fontes, J. Becerra González, H. G. Khandrika, J. Kim, S.-L. Kim, C.-U. Lee, H. M. Lee, A. Kutyrev, G. Lim, R. Sánchez-Ramírez, S. Veilleux, M. H. Wieringa & Y. Yoon

A long-standing paradigm in astrophysics is that collisions—or mergers—of two neutron stars form highly relativistic and collimated outflows (jets) that power γ-ray bursts of short (less than two seconds) duration. The observational support for this model, however, is only indirect. A hitherto outstanding prediction is that gravitational-wave events from such mergers should be associated with γ-ray bursts, and that a majority of these bursts should be seen off-axis, that is, they should point away from Earth. Here we report the discovery observations of the X-ray counterpart associated with the gravitational-wave event GW170817. Although the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow (known as a ‘kilonova’) from freshly synthesized rapid neutron capture (r-process) material in the merger ejecta, observations at X-ray and, later, radio frequencies are consistent with a short γ-ray burst viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short γ-ray bursts and gravitational waves from neutron-star mergers, and gives independent confirmation of the collimated nature of the γ-ray-burst emission.

Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger

2 November 2017 - 9:21am

Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger

Nature 551, 7678 (2017). doi:10.1038/nature24298

Authors: E. Pian, P. D’Avanzo, S. Benetti, M. Branchesi, E. Brocato, S. Campana, E. Cappellaro, S. Covino, V. D’Elia, J. P. U. Fynbo, F. Getman, G. Ghirlanda, G. Ghisellini, A. Grado, G. Greco, J. Hjorth, C. Kouveliotou, A. Levan, L. Limatola, D. Malesani, P. A. Mazzali, A. Melandri, P. Møller, L. Nicastro, E. Palazzi, S. Piranomonte, A. Rossi, O. S. Salafia, J. Selsing, G. Stratta, M. Tanaka, N. R. Tanvir, L. Tomasella, D. Watson, S. Yang, L. Amati, L. A. Antonelli, S. Ascenzi, M. G. Bernardini, M. Boër, F. Bufano, A. Bulgarelli, M. Capaccioli, P. Casella, A. J. Castro-Tirado, E. Chassande-Mottin, R. Ciolfi, C. M. Copperwheat, M. Dadina, G. De Cesare, A. Di Paola, Y. Z. Fan, B. Gendre, G. Giuffrida, A. Giunta, L. K. Hunt, G. L. Israel, Z.-P. Jin, M. M. Kasliwal, S. Klose, M. Lisi, F. Longo, E. Maiorano, M. Mapelli, N. Masetti, L. Nava, B. Patricelli, D. Perley, A. Pescalli, T. Piran, A. Possenti, L. Pulone, M. Razzano, R. Salvaterra, P. Schipani, M. Spera, A. Stamerra, L. Stella, G. Tagliaferri, V. Testa, E. Troja, M. Turatto, S. D. Vergani & D. Vergani

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named ‘macronovae’ or ‘kilonovae’, are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.

Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger

2 November 2017 - 9:20am

Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger

Nature 551, 7678 (2017). doi:10.1038/nature24291

Authors: Iair Arcavi, Griffin Hosseinzadeh, D. Andrew Howell, Curtis McCully, Dovi Poznanski, Daniel Kasen, Jennifer Barnes, Michael Zaltzman, Sergiy Vasylyev, Dan Maoz & Stefano Valenti

The merger of two neutron stars has been predicted to produce an optical–infrared transient (lasting a few days) known as a ‘kilonova’, powered by the radioactive decay of neutron-rich species synthesized in the merger. Evidence that short γ-ray bursts also arise from neutron-star mergers has been accumulating. In models of such mergers, a small amount of mass (10−4–10−2 solar masses) with a low electron fraction is ejected at high velocities (0.1–0.3 times light speed) or carried out by winds from an accretion disk formed around the newly merged object. This mass is expected to undergo rapid neutron capture (r-process) nucleosynthesis, leading to the formation of radioactive elements that release energy as they decay, powering an electromagnetic transient. A large uncertainty in the composition of the newly synthesized material leads to various expected colours, durations and luminosities for such transients. Observational evidence for kilonovae has so far been inconclusive because it was based on cases of moderate excess emission detected in the afterglows of γ-ray bursts. Here we report optical to near-infrared observations of a transient coincident with the detection of the gravitational-wave signature of a binary neutron-star merger and with a low-luminosity short-duration γ-ray burst. Our observations, taken roughly every eight hours over a few days following the gravitational-wave trigger, reveal an initial blue excess, with fast optical fading and reddening. Using numerical models, we conclude that our data are broadly consistent with a light curve powered by a few hundredths of a solar mass of low-opacity material corresponding to lanthanide-poor (a fraction of 10−4.5 by mass) ejecta.

Gravitational waves: A golden binary

2 November 2017 - 9:20am

Gravitational waves: A golden binary

Nature 551, 7678 (2017). doi:10.1038/nature24153

Authors: M. Coleman Miller

The discovery of gravitational waves from a neutron-star merger and the detection of the event across the electromagnetic spectrum give insight into many aspects of gravity and astrophysics. See Letter p.64, p.67, p.71, p.75 & p.80

Astrophysics: Chasing ghosts in Antarctica

2 November 2017 - 9:18am

Astrophysics: Chasing ghosts in Antarctica

Nature 551, 7678 (2017). doi:10.1038/551030a

Author: Alexandra Witze

Alexandra Witze welcomes a history of IceCube, an ambitious neutrino observatory.

NuSTAR Probes Black Hole Jet Mystery

2 November 2017 - 9:16am

Black holes are famous for being ravenous eaters, but they do not eat everything that falls toward them. A small portion of material gets shot back out in powerful jets of hot gas, called plasma, that can wreak havoc on their surroundings.

News Article Type: Homepage ArticlesPublished: Wednesday, November 1, 2017 - 10:34

We may have found 20 habitable worlds hiding in plain sight

1 November 2017 - 9:13am

After taking another look at data from the Kepler space telescope’s original mission we have spotted 20 possible Earth-like worlds that could host life

Asteroid impact plunged dinosaurs into catastrophic 'winter'

1 November 2017 - 9:12am

Scientists are now clearer on the freezing climate conditions that forced dinosaurs from the Earth.

We may have found 20 habitable worlds hiding in plain sight

31 October 2017 - 9:19am

After taking another look at data from the Kepler space telescope’s original mission we have spotted 20 possible Earth-like worlds that could host life

Surprisingly erratic X-ray auroras discovered at Jupiter

31 October 2017 - 9:18am

ESA and NASA space telescopes have revealed that, unlike Earth's polar lights, the intense auroras seen at Jupiter's poles unexpectedly behave independently of one another.

Oldest recorded solar eclipse helps date the Egyptian pharaohs

30 October 2017 - 9:15am

Using a combination of the biblical text and an ancient Egyptian text, the researchers were then able to refine the dates of the Egyptian pharaohs, in particular the dates of the reign of Ramesses the Great. The results are published in the Royal Astronomical Society journal Astronomy & Geophysics.

The biblical text in question comes from the Old Testament book of Joshua and has puzzled biblical scholars for centuries. It records that after Joshua led the people of Israel into Canaan – a region of the ancient Near East that covered modern-day Israel and Palestine – he prayed: “Sun, stand still at Gibeon, and Moon, in the Valley of Aijalon. And the Sun stood still, and the Moon stopped, until the nation took vengeance on their enemies.”

“If these words are describing a real observation, then a major astronomical event was taking place - the question for us to figure out is what the text actually means,” said paper co-author Professor Sir Colin Humphreys from the University of Cambridge’s Department of Materials Science & Metallurgy, who is also interested in relating scientific knowledge to the Bible.

“Modern English translations, which follow the King James translation of 1611, usually interpret this text to mean that the sun and moon stopped moving,” said Humphreys, who is also a Fellow of Selwyn College. “But going back to the original Hebrew text, we determined that an alternative meaning could be that the sun and moon just stopped doing what they normally do: they stopped shining. In this context, the Hebrew words could be referring to a solar eclipse, when the moon passes between the earth and the sun, and the sun appears to stop shining. This interpretation is supported by the fact that the Hebrew word translated ‘stand still’ has the same root as a Babylonian word used in ancient astronomical texts to describe eclipses.”

Humphreys and his co-author, Graeme Waddington, are not the first to suggest that the biblical text may refer to an eclipse, however, earlier historians claimed that it was not possible to investigate this possibility further due to the laborious calculations that would have been required.

Independent evidence that the Israelites were in Canaan between 1500 and 1050 BC can be found in the Merneptah Stele, an Egyptian text dating from the reign of the Pharaoh Merneptah, son of the well-known Ramesses the Great. The large granite block, held in the Egyptian Museum in Cairo, says that it was carved in the fifth year of Merneptah’s reign and mentions a campaign in Canaan in which he defeated the people of Israel.

Earlier historians have used these two texts to try to date the possible eclipse, but were not successful as they were only looking at total eclipses, in which the disc of the sun appears to be completely covered by the moon as the moon passes directly between the earth and the sun. What the earlier historians failed to consider was that it was instead an annular eclipse, in which the moon passes directly in front of the sun, but is too far away to cover the disc completely, leading to the characteristic ‘ring of fire’ appearance. In the ancient world, the same word was used for both total and annular eclipses.

The researchers developed a new eclipse code, which takes into account variations in the Earth’s rotation over time. From their calculations, they determined that the only annular eclipse visible from Canaan between 1500 and 1050 BC was on 30 October 1207 BC, in the afternoon. If their arguments are accepted, it would not only be the oldest solar eclipse yet recorded, it would also enable researchers to date the reigns of Ramesses the Great and his son Merneptah to within a year.

“Solar eclipses are often used as a fixed point to date events in the ancient world,” said Humphreys. Using these new calculations, the reign of Merneptah began in 1210 or 1209 BC. As it is known from Egyptian texts how long he and his father reigned for, it would mean that Ramesses the Great reigned from 1276-1210 BC, with a precision of plus or minus one year, the most accurate dates available. The precise dates of the pharaohs have been subject to some uncertainty among Egyptologists, but this new calculation, if accepted, could lead to an adjustment in the dates of several of their reigns and enable us to date them precisely.

Colin Humphreys and Graeme Waddington. ‘Solar eclipse of 1207 BC helps to date pharaohs.’ Astronomy & Geophysics (2017). DOI: 10.1093/astrogeo/atx178.

Researchers have pinpointed the date of what could be the oldest solar eclipse yet recorded. The event, which occurred on 30 October 1207 BC, is mentioned in the Bible and could have consequences for the chronology of the ancient world. 

If these words are describing a real observation, then a major astronomical event was taking place - the question for us to figure out is what the text actually means.Colin HumphreysKevin BairdAnnular eclipse photographed at sunset in eastern New Mexico.

The text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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We now know more on the origins of weird duck-shaped comet 67P

30 October 2017 - 9:14am

One of the weirdest comets we’ve seen formed from tiny pebbles that trace back to the start of our solar system – which may tell us more about how planets are made

Small Asteroid or Comet 'Visits' from Beyond the Solar System

30 October 2017 - 9:14am

A small, recently discovered asteroid -- or perhaps a comet -- appears to have originated from outside the solar system, coming from somewhere else in our galaxy. If so, it would be the first "interstellar object" to be observed and confirmed by astronomers.

News Article Type: Homepage ArticlesPublished: Friday, October 27, 2017 - 10:36

Professor Stephen Hawking's PhD viewed two million times

30 October 2017 - 9:13am

Cambridge University say the online repository has "never seen numbers like this before".

Stephen Hawking gives talk on black holes at Oxford University

30 October 2017 - 9:13am

World-renowned theoretical physicist Stephen Hawking thrilled fans with a talk on black holes.