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  • News

    Essays on Gaia Gaia is a satellite mission of the European Space Agency, launched in 2013, and which should be operational until about 2023. It is measuring the distances and motions of more than two billion stars in our Galaxy and beyond. It represents an enormous advance in the understanding of our Universe. As of early 2021, several thousand scientific papers have been written on its findings. In these short 'essays', I have picked out some scientific highlights as they are emerging, or as they caught my attention. They make no attempt at a complete review of a given topic, and many will quickly become superseded by new results. But they offer a snapshot of the exciting discoveries that Gaia is making across all areas of astronomy. ​ Only a few references are included, and these are 'discreetly' hyperlinked for those who want to read more... where references appear in the form (Einstein 1908) or www.gaia.com , clicking on the text (even though not highlighted) should lead to the relevant online article. ​ Click on the "access PDF" icon to access the file, and on the audio file to listen to a short interview with one of the scientists involved. select category 178. Bifurcation in the white dwarf HRD New evolutionary models inspired by Gaia's colour-magnitude diagram The prominent bifurcation in the white dwarf colour-magnitude diagram, seen for the first time in Gaia DR2, was tentatively attributed to different evolutionary tracks for hydrogen- and helium-dominated atmospheres. Subsequent studies have focussed on trace amounts of `dredged-up' carbon being the cause. 27 May 2024 access PDF Interview 00:00 / 01:04 177. An intermediate black hole in M4? Hints of an intermediate-mass black hole in our nearest globular cluster Intermediate-mass black holes lie between the masses of stellar mass black holes, formed by single star collapse, and supermassive black holes, formed in the high-density environment of galaxy centres. Plausible formation mechanisms, but no definitive candidates, are known. I describe some insights from Gaia in the case of our nearest globular cluster, M4. 20 May 2024 access PDF Interview 00:00 / 01:04 176. Black holes in stellar streams Stellar mass black holes influence the morphology of stellar streams I look at how the existence of stellar mass black holes might affect the morphology and kinematics of the stellar streams that are now known to exist in the inner and outer halo of our Galaxy, and to what extent Gaia can help to distinguish between those that are rich in, or devoid of, stellar mass black holes. 13 May 2024 access PDF Interview 00:00 / 01:04 175. Black holes in open clusters How we can infer that an open clusters hosts stellar mass black holes The existence of stellar mass black holes has observable consequences on the dynamics of open clusters, which can in turn place useful constraints on their formation. I look at some early results for the Hyades open cluster, which suggest that the cluster should contain 2-3 stellar mass black holes in long-period binary systems. 6 May 2024 access PDF Interview 00:00 / 01:04 174. Proper motion anomalies A fascinating subset of astrometric binaries A subset of astrometric binaries evaded identification by Hipparcos, but they become recognisable from the difference in the proper motion determined by Hipparcos and that measured by Gaia. Recent work on these `proper motion anomalies' has underlined their ubiquity, and their scientific importance. 29 April 2024 access PDF Interview 00:00 / 01:04 173. The breathing motion of spiral arms New insights into the dynamics and origin of spiral arms In essay 114, I looked at advances being made in understanding our Galaxy's spiral arm structure. Here I will look at some remarkable insights into their kinematics. Gaia is providing confirmation of their `breathing modes' which, in turn, support some of the theoretical and numerical models being developed to understand their origin. 22 April 2024 access PDF Interview 00:00 / 01:04 172. The basic angle Why was the basic angle 58 degrees for Hipparcos, and 106.5 degrees for Gaia? For Hipparcos and Gaia, the `basic angle' is the angle between the instrument's two viewing directions on the sky. For Hipparcos, the basic angle was 58 degrees. For Gaia it is 106.5 degrees. What is the reason for the two fields of view in the first place? How is the angle between them chosen? And why is it so different for Hipparcos and Gaia? 15 April 2024 access PDF Interview 00:00 / 01:04 171. The Small Magellanic Cloud A radically new view of one of our nearest neighbours The Large and Small Magellanic Clouds are two of our nearest galaxy neighbours. The line-of-sight structure of the SMC is particularly complex, and the wealth of observational data has proven difficult to interpret. A recent study using Gaia DR3 suggests a radically new picture: that the SMC is composed of two distinct superimposed structures. 8 April 2024 access PDF Interview 00:00 / 01:04 170. Metrication in the UK Some thoughts on the UK's move to the metric system Metrication concerns the move from the historical use of feet and inches, of pounds and ounces, and of pints and gallons, to the coherent metric system in which units are inter-related. The UK's resistance to full adoption of the metric system, used by almost all other countries, has left society confused, and at an economic disadvantage. 1 April 2024 access PDF Interview 00:00 / 01:04 169. A billion radial velocities A new catalogue of 125 million radial velocities Gaia Data Release 3 includes 33 million radial velocities, obtained from its Radial Velocity Spectrometer, ten times as many as previously known. DR4, in 2025, should include some 100 million. A recent study has derived 125 million from the low-resolution BP/RP spectra, albeit of much lower accuracy, with perhaps 1–2 billion coming available in the future. 25 March 2024 access PDF Interview 00:00 / 01:04 168. S stars in the Wesenheit diagram A powerful way of classifying AGB stars S stars are cool luminous giants, lying between the O-rich M-type giants and the C-rich carbon stars on the AGB. With atmospheres enriched with s-process elements brought to the surface through the `third dredge-up', they have created around half the atomic nuclei heavier than iron. Gaia astrometry and photometry is advancing their understanding. 18 March 2024 access PDF Interview 00:00 / 01:04 167. Carbon stars New insights into this important class of complex star Carbon stars, which reside on the asymptotic giant branch, are an important phase of stellar evolution. Their very high luminosities makes them important for integrated light studies of galaxies, and they are being considered as potential standard distance indicators. Their large distances and complex physics provide numerous challenges for theoretical modelling. 11 March 2024 access PDF Interview 00:00 / 01:04 166. Hypervelocity stars: part 2 An update on hypervelocity stars In essay 22 (May 2021) I described some of the early insights that Gaia was providing in the field of hypervelocity stars, a rare and exotic type of star, racing through our Galaxy with velocities of 500-1000 km per second or more. Here, I bring the Gaia results up-to-date, describing the latest searches, and more recent insights into their origin. 4 March 2024 access PDF Interview 00:00 / 01:04 165. Runaway stars How these high velocity stars originate Runaway stars are stars with such high space velocities that they must have been imparted by a particular formation process. Many insights into the favoured scenarios - binary-supernova and dynamical ejection - are being made as Gaia's astrometry yields their accurate distances and space motions. 26 February 2024 access PDF Interview 00:00 / 01:04 164. Sub-subgiants and (tiny) black holes A new class of star, and a link with primordial black holes I look at some recent advances in the numbers of blue stragglers identified by Gaia, and Gaia's contribution to the identification and understanding of the new class of sub-subgiant star. I discuss whether these offer a particularly interesting search sample for stars hosting primordial black holes in their centres. 19 February 2024 access PDF Interview 00:00 / 01:04 163. Dual active galactic nuclei A remarkable contribution to Lambda CDM cosmology Dual active galactic nuclei, we now know, provide an important probe of the physical processes that drive the in-spiralling of supermassive black hole pairs inside a single merged galaxy. Difficult to detect and characterise, Gaia is making a significant contribution to their discovery. 12 February 2024 access PDF Interview 00:00 / 01:04 162. Cosmology with Gaia's quasars Two topical tests of Lambda CDM cosmology The large number of quasars measured by Gaia contributes fundamentally to the determination of the quasi-inertial reference frame. Today, Gaia's quasar survey is also being applied to two topical observational questions in Lambda CDM cosmology: the kinematic dipole anomaly, and the S8 tension. 5 February 2024 access PDF Interview 00:00 / 01:04 161. Strongly lensed quasars A new search for gravitational lenses in the Gaia data In this last of five essays on ESA's October 2023 'Focused Product Release' topics, I look at a new search for strongly lensed quasars in the data interval used for Data Release 3. Exploiting Gaia's 0.18 arcsec angular resolution, this new analysis found 450 previously known systems but with 86 newly detected components, and 381 new lensed candidates. 29 January 2024 access PDF Interview 00:00 / 01:04 160. More on diffuse interstellar bands Spatial distribution of these absorption features out to 4000 parsec In this fourth of five essays on ESA's October 2023 'Focused Product Release' topics, I look at improved modelling of the two diffuse interstellar bands (DIBs) present within the wavelength range of Gaia's Radial Velocity Spectrometer instrument (845-872 nm). Six million RVS spectra have been used to map their spatial distribution out to 4000 parsec. 22 January 2024 access PDF Interview 00:00 / 01:04 159. Improved solar system astrometry A dramatic improvement in orbits for 157,000 asteroids In this third of five essays on ESA's October 2023 'Focused Product Release' topics, I look at the significant improvements in orbit determination for the 157,000 asteroids provided in DR3, but now exploiting the 66-month time interval that will form the basis of Data Release 4 in 2025. This is mainly due to the observations now extending beyond a typical orbital period. 15 January 2024 access PDF Interview 00:00 / 01:04 158. Radial velocity time-series of LPVs New insights into long period variables and ellipsoidal variables In this second of five essays on ESA's October 2023 'Focused Product Release' topics, I look at the application of the newly available radial velocity time-series measurements (from Gaia itself) to the class of long-period variables. The time series data provide a powerful complement to the epoch photometry in identifying and characterising LPVs. 8 January 2024 access PDF Interview 00:00 / 01:04 157. Many more Gaia sources in Omega Cen More than half a million new sources in this globular cluster In essay 40 I described the first Gaia-based estimates of the distance to the globular cluster Omega Cen. Gaia DR3 contained 321,698 cluster sources. A series of special observations, and part of ESA's October 2023 'Focused Product Release', results in a further 526,587 sources in its core region. I also summarise other results on Omega Cen to date. 1 January 2024 access PDF Interview 00:00 / 01:04 156. Update on stellar streams Nearly 100 stellar streams now know in the Galaxy halo A growing number of accreted stellar 'streams' are being identified in our Galaxy's halo, from 20 in 2016 to nearly a hundred today, now mostly coming from Gaia. Some are attributed to captured dwarf galaxies, others to disrupted globular clusters. I described some of the early discoveries in essays 15 and 71, and bring the subject more up-to-date here. 25 December 2023 access PDF Interview 00:00 / 01:04 155. Gaia satellite operations An insight into the complexities involved Ten years on from the launch of Gaia, on 19 December 2013, I provide a picture of the tasks involved in the operations of the Gaia satellite. I will start with a brief background to some of the top-level requirements that influenced the operational design and implementation, and continue with a first-hand account by the Gaia spacecraft operations manager, David Milligan. 18 December 2023 access PDF Interview 00:00 / 01:04 154. Hipparcos: from concept to launch Some more history of space astrometry In essay 4 (25 January 2021), I gave a historical perspective on the early days of the Hipparcos mission and the 'push to space'. Here, I resume this historical and somewhat personal account describing the journey from the early concepts of Hipparcos to its launch in 1989. I describe how the various teams were organised, and some of the challenges that we faced. 11 December 2023 access PDF Interview 00:00 / 01:04 153. The solar motion New approaches to measuring this fundamental quantity Gaia represents a major advance in understanding our Galaxy's disk and halo kinematics. Here, I will look at the specific problem of determining the 'solar motion'. I explain what it is, why it is important, and how it is determined. As well as the more classical approach, methods are being developed to measure all three components with respect to the halo. 4 December 2023 access PDF Interview 00:00 / 01:04 152. M dwarfs and the Jao gap Subtle but important clues in the HR diagram In essay 42, I described some new features in the Hertzsprung-Russell diagram, including a narrow gap in the M dwarf sequence, first reported in the DR2 data. Here I look at further details of this interesting feature. As Baraffe & Chabrier wrote in 2018: "Just a small gap in a colour-magnitude diagram could provide a deep insight into the interior structure of low-mass stars." 27 November 2023 access PDF Interview 00:00 / 01:04 151. The Hyades main sequence How Gaia is tightening our knowledge of the Hyades main sequence The Hyades is the nearest open cluster. Even so, distance uncertainties have limited the definition of its main sequence, and hence its ability to constrain evolutionary models. It has nevertheless been used as the basic observational material for various fundamental relationships in astrophysics. Gaia DR3 astrometry and photometry is transforming its understanding. 20 November 2023 access PDF Interview 00:00 / 01:04 150. Convection - and the mixing length How Gaia is contributing to the understanding of convection inside stars Convection represents one of the dominant sources of uncertainty in current stellar evolutionary models, propagating through to substantial uncertainties in ages and, in turn, understanding of the chemical evolution of the Galaxy. I outline the `mixing length theory' of convection, and Gaia's contribution to characterising the associated `mixing length parameter'. 13 November 2023 access PDF Interview 00:00 / 01:04 149. Gravito-inertial asteroseismology Gaia compares favourably with Kepler I look at some other recent results on the non-radial pulsators in the SPB and Gamma Dor classes. These stars show periodic variations as a result of the usual restoring force of gravity (or buoyancy), as well as due to Coriolis forces resulting from stellar rotation. Many astrophysical results from Gaia compare favourably with the much denser sampling from Kepler. 6 November 2023 access PDF Interview 00:00 / 01:04 148. Non-radial pulsators More than 10,000 SPB and Gamma Dor pulsators I have discussed Gaia variables in several essays, focusing on Cepheids (43), on RR~Lyrae (45), their detection and classification (61), their distribution across the Hertzsprung--Russell diagram (62), and in the context of citizen science (132). Here I look at some new insights into the non-radial g-mode pulsators in the SPB and Gamma Dor class. 30 October 2023 access PDF Interview 00:00 / 01:04 147. Videos and visualisations: part 2 Some of the latest video animations for Gaia In essay 54, in January 2022, I gave links to some of the videos, animations, visualisations, and `fly-throughs' made to illustrate the science that Gaia is addressing. DR3 was released in June 2022, and many new explanatory videos have been made since. This is an introduction to some of the many video animations and illustrations that now exist. 23 October 2023 access PDF Interview 00:00 / 01:04 146. Benford's law - and astrometry A curious mathematical property A curious property of many collections of numbers, including naturally occurring data, is that the leading digits are not uniformly distributed, but are skewed toward smaller values. As Benford's law, it has been considered in many contexts, including identifying suspicious accounting. I consider it here in the context of the Gaia DR2 parallaxes. 16 October 2023 access PDF Interview 00:00 / 01:04 145. Spectroscopic binaries New insights in orbit circularisation Included in the DR3 data release are some 180,000 single-lined spectroscopic binaries, a colossal increase in numbers enabling the study of many aspects of short-period binaries. I look here at some new insights into orbit circularisation, where the Gaia data point to the process operating most efficiently in the pre-main sequence evolutionary phase. 9 October 2023 access PDF Interview 00:00 / 01:04 144. How many open clusters? An explosion of new discoveries with Gaia Gaia is transforming the study of open clusters. Pre-2016, some 3000 clusters had been identified. But Gaia has shown that more than half of these are unreal, being simply asterisms. In their place, from nearly 25,000 new discoveries reported with Gaia (some duplicated), there are today nearly 14,000 unique clusters known in our Galaxy. 2 October 2023 access PDF Interview 00:00 / 01:04 143. Gaia's maps of the Milky Way Different views of our Galaxy seen with Gaia The ESA-Gaia `Image of the Week', on the first anniversary of the 34-month Data Release~3 (DR3), 13 June 2023, was a remarkable multi-dimensional map of the Milky Way, produced by the many members of the Data Processing and Analysis Consortium (DPAC). I'm showing these 10 splendid sky plots again here. 25 September 2023 access PDF Interview 00:00 / 01:04 142. Gaia and the search for axions Constraints on "axionic" dark matter Gaia's contribution to understanding the distribution of dark matter follows from studies of the stellar halo population, Galactic kinematics, and stellar streams. Studies have also begun to place specific constraints on the properties of dark matter, for example if it is comprised of axions. I provide an overview of the various approaches. 18 September 2023 access PDF Interview 00:00 / 01:04 141. White dwarf pulsars A remarkable new class of white dwarf White dwarfs have featured in a number of my previous essays. Gaia is providing well-defined samples in the solar neighbourhood, with more than 260,000 from Gaia DR2. I look here at the recent discovery of the new type of `white dwarf pulsars'. With just two members of this exclusive class, Gaia is contributing to their understanding, 11 September 2023 access PDF Interview 00:00 / 01:04 140. Cataclysmic variables Binary star evolution and accretion disks Cataclysmic variables are interacting binaries containing a white dwarf accreting from a donor star. Their space density places strong constraints on models of their formation and evolution, and has revealed major discrepancies between observations and theory. New and improved space densities are becoming available with Gaia. 4 September 2023 access PDF Interview 00:00 / 01:04 139. Quadruple star systems Some very special objects How common are quadruple star systems? How is Gaia discovering and characterising new systems, and what do they tell us about the star-formation process? One specific quadruple system is providing insights into the cooling and crystallisation of white dwarf interiors, and another is providing hints about the origin of type Ia supernovae. 28 August 2023 access PDF Interview 00:00 / 01:04 138. Twin binaries - and our Sun Might our own Sun have been one? Gaia has confirmed the existence of 'twin binaries' with components of equal brightness and presumably equal mass. This has stimulated new investigations into their enigmatic origin, the possible implications for models of star formation, and perhaps some consequences for the origin of our own solar system 21 August 2023 access PDF Interview 00:00 / 01:04 137. Occultations and stellar diameters A new approach to measuring star diameters Gaia is revolutionising the study of stellar occultations because its dense grid of accurate star positions allows greatly improved prospects of predicting such occultation events. I mention some recent results on outer solar system bodies, and explain how the technique is also being used to measure the angular diameters of stars 14 August 2023 access PDF Interview 00:00 / 01:04 136. Gaia and gravitational waves How can Gaia contribute? I describe three areas in which Gaia is contributing to the field of gravitational wave research: in helping to identify `verification binaries' for the LISA gravitational wave space mission, in searches for the electromagnetic counterparts of gravitational wave sources, and in providing constraints on the stochastic gravitational wave background. 7 August 2023 access PDF Interview 00:00 / 01:04 135. Triple star systems Why are triple systems important? How common are triple star systems? Why are they important to study, why they are so difficult to discover, and what can they tell us about the star formation process? Gaia is providing an unprecedented census of wide triple systems by virtue of its discovery depth and uniformity. Amongst them is the first resolved triple white dwarf system. 31 July 2023 access PDF Interview 00:00 / 01:04 134. Resolved binaries within 1 kpc A huge sample of resolved binaries Binary and multiple stars occur in a bewildering variety of configurations, and range from the very compact to wide, long-period systems. Here I look at a uniform sample of 1.3 million resolved binaries constructed from Gaia EDR3. It includes white dwarf–white dwarf binaries, main sequence–white dwarf binaries, and many `twin' binaries. 24 July 2023 access PDF Interview 00:00 / 01:04 133. Ellipsoidal variables and black holes More searches for black holes Ellipsoidal variables are close binaries with orbit inclinations too small to yield eclipses, but with components nonetheless distorted by their mutual gravitation as revealed by their light curves. More than 60,000 have been identified in Gaia DR3. Amongst them, various searches for dormant black hole or neutron star secondaries are ongoing. 17 July 2023 access PDF Interview 00:00 / 01:04 132. Variability, DR3, and Citizen Science The content of DR3, and the role of Citizen Science In essay 61, I looked at the identification and classification of variable stars from Gaia DR2, and some of the statistics of the resulting 550,000 variable stars. Here, I update the statistics for the 10 million variable stars identified in DR3. And I describe Gaia-Vari, the first Citizen Science contributions to Gaia which are assisting with the classification process. 10 July 2023 access PDF Interview 00:00 / 01:04 131. Double white dwarf mergers Gaia is identifying some merger products Gaia is making some fascinating contributions to the known occurrence of white dwarf--white dwarf binary systems, and on the observational consequences of the eventual merger of the two white dwarfs. The story involves gravitational waves, the unusual properties of `ultra-massive' white dwarfs, and the precursors of (some) type~Ia supernovae. 3 July 2023 access PDF Interview 00:00 / 01:04 130. The initial mass function What is the mass distribution when stars are born The 'present-day mass function' describes the distribution of actual masses in any given stellar population. From it can be inferred the more fundamental 'initial mass function', intimately related to the original star formation processes. Its precise form, and whether it is universal, have been difficult to establish in the absence of accurate distances. 26 June 2023 access PDF Interview 00:00 / 01:04 129. Stars within 25 pc: the CNS5 A leap in understanding our neighbourhood In essay 33 I described Gaia's census of stars within 100 pc, the GCNS. A recent result, also from EDR3, is the Fifth Catalogue of Nearby Stars, the CNS3. With 5931 objects out to 25 pc, it covers a smaller volume than GCNS, but it is much more complete. It comprises 4946 main-sequence, 20 red giants, 264 white dwarfs, and 701 brown dwarfs. 19 June 2023 access PDF Interview 00:00 / 01:04 Show More

  • Gaia science essays

    Gaia: science essays Read more In these short weekly 'essays', I have picked out some of the scientific highlights of the Gaia mission as they are emerging, or as they caught my attention. They are not necessarily the most important. They do not follow any specific sequence. They are not a complete review of a given topic. Many will be quickly superseded by new results. But they offer a snapshot of some of the discoveries that Gaia is making across all of astronomy. I've also included some essays on related topics, including the history of astrometry, and some more technical, managerial, or developmental aspects of both the Hipparcos and Gaia missions. In each, I have included a footnote DR1, DR2, EDR3, DR3, etc to indicate which of the (latest) data releases the essay refers to (described in essays #10 and #76), with DR0 signifying technical or historical material not connected with any specific data release. Who are they written for? Anyone who might have a general interest in science and astronomy, including amateur astronomers, young scientists starting out on their careers, mid-career scientists looking in on Gaia for the first time to get a feeling of what is possible, and specialists looking in from different areas of astronomy, or physics more generally. My thanks go to many people: to all those I worked with on the Hipparcos and Gaia projects over almost 30 years, to those now dedicating huge reserves of their time, energy, and skill to the ongoing data processing, and to those who have entered into the Gaia catalogue and published the results described here. Click on the access PDF icon to access the file. Only a few references are included, and these are 'discreetly' hyperlinked for those who want to read more... where references appear in the form (Einstein 1908) or www.gaia.com, clicking on the text (even though generally not highlighted!) should lead to the relevant online article. In a few cases, I've recorded an interview on the subject (see science interview page). New : a subscribe page: to receive an email (usually Monday morning) when each new essay is published ​ New : a table page which lists all essays through to the end of 2023 (1–156 inclusive) in tabular form All essays (or select category) 17 164. Sub-subgiants and (tiny) black holes A new class of star, and a link with primordial black holes I look at some recent advances in the numbers of blue stragglers identified by Gaia, and Gaia's contribution to the identification and understanding of the new class of sub-subgiant star. I discuss whether these offer a particularly interesting search sample for stars hosting primordial black holes in their centres. 19 February 2024 access PDF 163. Dual active galactic nuclei A remarkable contribution to Lambda CDM cosmology Dual active galactic nuclei, we now know, provide an important probe of the physical processes that drive the in-spiralling of supermassive black hole pairs inside a single merged galaxy. Difficult to detect and characterise, Gaia is making a significant contribution to their discovery. 12 February 2024 access PDF 162. Cosmology with Gaia's quasars Two topical tests of Lambda CDM cosmology The large number of quasars measured by Gaia contributes fundamentally to the determination of the quasi-inertial reference frame. Today, Gaia's quasar survey is also being applied to two topical observational questions in Lambda CDM cosmology: the kinematic dipole anomaly, and the S8 tension. 5 February 2024 access PDF 161. Strongly lensed quasars A new search for gravitational lenses in the Gaia data In this last of five essays on ESA's October 2023 'Focused Product Release' topics, I look at a new search for strongly lensed quasars in the data interval used for Data Release 3. Exploiting Gaia's 0.18 arcsec angular resolution, this new analysis found 450 previously known systems but with 86 newly detected components, and 381 new lensed candidates. 29 January 2024 access PDF 160. More on diffuse interstellar bands Spatial distribution of these absorption features out to 4000 parsec In this fourth of five essays on ESA's October 2023 'Focused Product Release' topics, I look at improved modelling of the two diffuse interstellar bands (DIBs) present within the wavelength range of Gaia's Radial Velocity Spectrometer instrument (845-872 nm). Six million RVS spectra have been used to map their spatial distribution out to 4000 parsec. 22 January 2024 access PDF 159. Improved solar system astrometry A dramatic improvement in orbits for 157,000 asteroids In this third of five essays on ESA's October 2023 'Focused Product Release' topics, I look at the significant improvements in orbit determination for the 157,000 asteroids provided in DR3, but now exploiting the 66-month time interval that will form the basis of Data Release 4 in 2025. This is mainly due to the observations now extending beyond a typical orbital period. 15 January 2024 access PDF 158. Radial velocity time-series of LPVs New insights into long period variables and ellipsoidal variables In this second of five essays on ESA's October 2023 'Focused Product Release' topics, I look at the application of the newly available radial velocity time-series measurements (from Gaia itself) to the class of long-period variables. The time series data provide a powerful complement to the epoch photometry in identifying and characterising LPVs. 8 January 2024 access PDF 157. Many more Gaia sources in Omega Cen More than half a million new sources in this globular cluster In essay 40 I described the first Gaia-based estimates of the distance to the globular cluster Omega Cen. Gaia DR3 contained 321,698 cluster sources. A series of special observations, and part of ESA's October 2023 'Focused Product Release', results in a further 526,587 sources in its core region. I also summarise other results on Omega Cen to date. 1 January 2024 access PDF 156. Update on stellar streams Nearly 100 stellar streams now know in the Galaxy halo A growing number of accreted stellar 'streams' are being identified in our Galaxy's halo, from 20 in 2016 to nearly a hundred today, now mostly coming from Gaia. Some are attributed to captured dwarf galaxies, others to disrupted globular clusters. I described some of the early discoveries in essays 15 and 71, and bring the subject more up-to-date here. 25 December 2023 access PDF 155. Gaia satellite operations An insight into the complexities involved Ten years on from the launch of Gaia, on 19 December 2013, I provide a picture of the tasks involved in the operations of the Gaia satellite. I will start with a brief background to some of the top-level requirements that influenced the operational design and implementation, and continue with a first-hand account by the Gaia spacecraft operations manager, David Milligan. 18 December 2023 access PDF 17

  • Exoplanets | My Site

    Exoplanets: some contributions What is expected from Gaia? In 2000, and as part of the scientific case for Gaia, I made the first estimates of the number of giant exoplanets that the satellite should discover through the positional 'wobble' of the host star, and putting the number at somewhere between 10,000 - 50,000, depending on details of their orbital distribution. Several refinements of these estimates have been made since. My study in 2014 predicted the number of astrometric discoveries at around 20,000 for a 5-year mission duration, and perhaps up to 70,000 for a 10-year mission. Others have also estimated the additional numbers of discoveries expected from Gaia's photometric data, through the detection of their photometric transits. ​ As of mid-2021, the Gaia satellite has acquired around seven years of data. It may continue to operate until around 2023. It is an enormous number-crunching problem, currently ongoing, to process the vast amounts of data coming from the satellite. Any exoplanet discoveries (including the 'final' discovery numbers) can only be announced towards the very end of the processing chain. If some reasonably secure results will be available by around 2025, it will have been a quarter century between my first experimental predictions, and the actual discoveries. I'm sure it will be worth the wait! The Exoplanet Handbook Soon after the discovery of the first exoplanets in 1995, I looked at whether the reflex motion of the host stars of the first three discoveries was seen in the Hipparcos data (it wasn't). But this research paper led to an invitation to review the embryonic field of exoplanet science, which was published in the journal Reports on Progress in Physics in 2000 . This later developed into my book-length review of the field published by Cambridge University Press in 2011 , with a second edition in 2018 . The 'Perryman Tree' I created this diagram for my review in 2000, and have updated it regularly since (mostly as of 1 January each year). It shows the total number of exoplanets known at the time, sub-divided according to discovery method. ​ This gallery has the latest version, 1 January 2024 , and some earlier versions, giving an idea of how the field is developing with time. These gallery images (navigate with > or < button) are all jpg format. A vector pdf of the latest version is available h ere . Exoplanet discoveries Status as of 1 January 2024 Exoplanet discoveries Status as of 1 January 2022 Exoplanet discoveries Status as of mid-2000 Exoplanet discoveries Status as of 1 January 2024 1/12 Exoplanet Handbook (second edition 2018): Appendixes and Updates The following files are available at the Cambridge University Press www site (under Resources/Appendixes) and included here also. ExoplanetHandbook2-AppendixC-F-end2017.pdf contains the four Appendixes C-F, which together provide a compilation of the exoplanets discovered by the stated method (radial velocity, transits, lensing and imaging respectively), and largely following the content of the NASA Exoplanet Archive as of the end of 2017 (and corresponding to 3572 known exoplanets in total). Other discoveries, notably by timing methods and astrometry, are listed in the relevant chapters of the main text. This version corresponds to the published edition, viz. listing the known planets as of the end of 2017, and the main bibliography (according to the literature survey by the author) also as of the end of 2017. In order to maintain full congruence with the published edition, the References included in this pdf version correspond to the complete bibliography for the published edition (and not only to the content of the Appendixes). Since this version has been superseded, it will be of little or no interest to most users. ExoplanetHandbook2-AppendixC-F-end2018.pdf contains the four Appendixes C-F, with system and planet content updated to the end of 2018 (according to the NASA Exoplanet Archive, and corresponding to 3869 known exoplanets in total). The bibliography has been correspondingly updated for all systems. Since this version has been superseded, it will be of little or no interest to most users. ExoplanetHandbook2-AppendixC-F-end2019.pdf contains the four Appendixes C-F, with system and planet content updated to the end of 2019 (according to the NASA Exoplanet Archive, and corresponding to 4104 known exoplanets in total). The bibliography has been correspondingly updated for all systems. Exoplanets included: Content corresponds essentially to that of the NASA Exoplanet Archive at the given dates. Some additional systems of note are also included. Primary identifiers correspond to those given in the NASA Exoplanet Archive. Bibliography included: The bibliography provides a concise overview and narrative of the currently perceived importance and scientific developments associated with each system. Ordering is by year, and then by first author. Bibliographic entries are a result of my own literature survey, and claim no completeness: the CDS SIMBAD facility can be consulted for a more extensive bibliography for each system. For the version updated to the end of 2018, and because of the many new references included, the reference keys are not (necessarily) congruent with those given in the published Second Edition. Hyperlinks: These pdf versions are extensively hyperlinked as follows: * host star names (bold) are hyperlinked to the relevant NASA Exoplanet Archive 'Planet Host Overview' page * citations are hyperlinked to the relevant page of the References * each small triangular symbol following the citation is hyperlinked to the associated SAO/NASA ADS Abstract page

  • Michael Perryman | Gaia | Astronomy

    Michael Perryman Some contributions related to star positions, exoplanets, and recreational mathematics My career has been as a scientist with the European Space Agency, leading the development of astrometry (the measurement of accurate star positions) from space. I was Project Scientist for the pioneering Hipparcos space astrometry mission, from its adoption by ESA in 1980 to its completion and catalogue publication in 1997, including overall responsibility for the project after launch. As one of the co-originators of the Gaia mission in 1993, I was the mission's study scientist until its acceptance by ESA in 2000, and subsequently its Project Scientist until my retirement from ESA in 2008. ​ ​ Today, I am taking great pleasure in seeing the spectacular progress of the Gaia mission as it unfolds, and reading about its remarkable scientific results. Starting in 2021, I have been writing these summaries of some of the advances in astronomy flowing from it. They are a look back at what this long journey of space astrometry has achieved, and written in a form that I hope will be reasonably accessible to those not so deeply involved. And I have been interviewing some of the scientists and project leaders involved in these two space missions... ​ Since 2013 I have held a position as adjunct professor in the School of Physics, University College Dublin ( UCD-hosted CV). A more recent CV is also included here . Most recent essays (weekly) [see the Gaia essays page for the article link]: 19 Feb 2024 Gaia e ssay 164. Sub-subgiants and (tiny) black holes 12 Feb 2024 Gaia e ssay 163. Dual active galactic nuclei 05 Feb 2024 Gaia e ssay 162. Cosmology with Gaia's quasars 29 Jan 2024 Gaia e ssay 161. Strongly lensed quasars Most recent interviews (irregular) [see the project/science interview pages for the audio link]: 12 Dec 2023 Orlagh Creevey & Coryn Bailer-Jones talk about "DPAC Coordination Unit 8" 02 Nov 2023 Ulrich Bastian talks about "PPM, Hipparcos-Tycho, and Gaia" 12 Jul 2023 Frédéric Faye talks about "Gaia: industrial system engineering" [in two parts] 13 Oct 2022 Lennart Lindegren talks about "The Hipparcos mission"

  • Gaia project interviews | My Site

    Gaia/Hipparcos: project interviews Hipparcos is the first time since Sputnik in 1957 that a major new development in space science has come from outside the United States. ​ Freeman Dyson (Infinite in All Directions, 1988) Starting in 2021, I have been recording interviews with some of the people that played a part in shaping the developments of space astrometry. ​ These include some of the engineers and managers from the European Space Agency and its industrial teams involved in the Hipparcos and Gaia projects, some of the leading scientists, and just a few of the individuals at the forefront of ground-based astrometry in the years leading up to the move to space. Several others are planned. Amongst these, it is interesting to hear that two senior people, with key roles in the development of Gaia, both initially considered that the measurement goals targeted by the mission were not achievable! My thanks to all those who have taken part, and to Moby for the use of his unreleased track Morning Span in their introduction (https://mobygratis.com ). The order below follows the date of the interview. Enjoy! 3 28. Coordination Unit 8 The Gaia Data Processing and Analysis Consortium, DPAC, works with ESA to convert the raw satellite data into the catalogues used by the world-wide scientific community. The Consortium is divided into nine “coordination units” with various responsibilities. Joining me today are the two scientists who have been leading the work of the classification effort: Coryn Bailer-Jones of the MPIA, Heidelberg (Ithaca, 1972) and Orlagh Creevey of the Observatoire de la Côte d'Azur, Nice (Dublin, 1977). Orlagh Creevey & Coryn Bailer-Jones 12 Dec 2023 Interview: 00:00 / 1:05:18 27. PPM, Hipparcos-Tycho, and Gaia Ulrich Bastian (Haardt, Palatinate, Germany, 1951) started his long career in astrometry with his involvement in the ground-based activities of the Astronomisches Rechen-Institut (ARI, Heidelberg) in the early 1980s. He made leading contributions to the Hipparcos-Tycho project in the 1980s and 1990s and, since the earliest days of the Gaia mission 30 years ago, has played a central role in a number of critical activities, which continue to this day. We look back over this period of major advances in astrometry. Ulrich Bastian 2 Nov 2023 Interview: 00:00 / 1:05:14 26. Gaia: industrial system engineering: part 2 Frédéric Faye (Dijon, 1963) joins me for a second time. In an earlier conversation we talked about the earliest phases of the Gaia project in industry, 10 years or so before launch, and looked at the complexities that were lying ahead when he took over responsibility for the systems engineering in 2005. In this second part, Frédéric describes the organisation of his system engineering group, and some of the management and organisational challenges that his team had to deal with. Frédéric Faye 12 Jul 2023 Interview: 00:00 / 48:40 25. Gaia: industrial system engineering: part 1 Frédéric Faye (Dijon, 1963) was the System Engineering Manager for the Gaia satellite. Within the industrial prime contractor team, Airbus Defence & Space, he was in charge of the satellite design, integration and verification, from 2005, all the way through to the in-orbit commissioning review that took place a few months after the successful spacecraft insertion around L2 in 2014. Today, we're talking about his central role in the design, manufacture, assembly, testing, and launch of Gaia. Frédéric Faye 12 Jul 2023 Interview: 00:00 / 1:00:42 24. The Hipparcos mission Lennart Lindegren (Svalöv, Sweden, 1950) joins me for a second time. In an earlier conversation we talked about the period leading up to the adoption of Hipparcos in 1980. Today (as in my second conversation with Erik Høg), we discuss the Hipparcos mission, from its acceptance in 1980 to its completion in 1997. We talk about his role in the mission development, and in the preparation for - and the analysis of - the data sent down from the satellite. We also talk about his memories of the satellite launch in 1989, and of finalising the Hipparcos Catalogue. Lennart Lindegren 13 Oct 2022 Interview: 00:00 / 1:01:15 23. Validation of the Gaia catalogue data releases In the second of our two-part interview, Claus Fabricius talks about his activities after Hipparcos. With astrometry no longer supported financially in Denmark, Claus moved to the University of Barcelona in 2005, joining the active Gaia team there to work on the initial data treatment and on the photometry. Later, he took over responsibility for the validation of the successive versions of the Gaia Data Releases, and here he talks about the many tests that are made, pre-publication, on the astrometric and photometric results. Claus Fabricius 5 Sept 2022 Interview: 00:00 / 28:54 22. Astrometry in Copenhagen pre-Hipparcos Claus Fabricius (Copenhagen, 1954) spent the first part of his career at the Copenhagen University Observatory in Denmark, where he worked on one of the main instruments contributing to ground-based astrometry in the years preceding Hipparcos. In the first of this two-part interview, Claus talks about his involvement in the development and operation of the Brorfelde transit circle, later as the Carlsberg Automatic Meridian Circle in La Palma, and in the creation of the Tycho Catalogue and its associated initiatives. Claus Fabricius 5 Sept 2022 Interview: 00:00 / 54:25 21. Gaia satellite operations: part 2 In the second of our two-part interview, David Milligan, Spacecraft Operations Manager for Gaia, describes the challenges involved in the spacecraft commissioning, including the problems of 'icing' and 'straylight'. And he talks about keeping the satellite operational and optimised over almost a decade, including optimisation to allow the downlink of significantly more data, and the various tiny effects that can be detected, including bubbles in the propulsion system, micro-meteorites, and the effects of sunspots. David Milligan 8 Jul 2022 Interview: 00:00 / 56:52 20. Gaia satellite operations: part 1 David Milligan (Blackpool, UK, 1971) joined ESA, at its operations centre, ESOC in Germany, in 2000. There, from 2008 to the end of 2020, he was the Spacecraft Operations Manager for Gaia, the leader of the team responsible for its operation, health, and safety. In the first of this two-part interview, David talks about the preparations for the satellite operations in the years leading up to launch, and the major tasks involved in sending it on its way to its intended L2 orbit, 1.5 million km from Earth. David Milligan 8 Jul 2022 Interview: 00:00 / 46:06 19. The design of the Gaia photometric system Carme Jordi (Barcelona, 1958) is a senior research scientist at the University of Barcelona. She played a key part in designing the photometric measurements that could - and should - be made with Gaia, and played a key role in the preparation for the photometric data analysis. We talk about the scientific rationale for making photometric observations from the satellite itself, the considerations underlying the design of the photometric system, and how the photometric measurements compare with the original plans. Carme Jordi 9 Jun 2022 Interview: 00:00 / 58:21 3

  • Gaia science interviews | My Site

    Gaia: science interviews Starting in 2021, and along with the interviews of a more historical or project nature, I have been recording occasional conversations with a few scientists involved in some of the advances being made using the Gaia data. I hope to add to these in the future. ​ My thanks to all those who have taken part so far (and to Moby for the use of his unreleased track Morning Span in their introduction, https://mobygratis.com ). Enjoy! 1 7. Solar system occultations An 'occultation' occurs when a planet, moon, or any other solar system object passes in front of a distant star. Bruno Sicardy (Sorbonne University/Observatoire de Paris) explains how Gaia is contributing to our knowledge of the solar system using these curious events. Bruno Sicardy 20 Dec 2021 Interview: 00:00 / 1:00:53 6. Nearby stars Star distances are so immense that establishing a census of even the nearest stars has been an enormous challenge for astronomy. Ricky Smart, of the Observatory of Torino, joins me to explain how Gaia is revolutionising our understanding of nearby stars. Ricky Smart 14 Oct 2021 Interview: 00:00 / 48:34 5. Surprises in the Hertzsprung-Russell Diagram Gaia is transforming our empirical picture of the stellar Hertzsprung–Russell diagram. Grenoble-based astronomer Carine Babusiaux joins me to describe some of its new and remarkable insights, on white dwarfs, red dwarfs, colliding galaxies, and public outreach. Carine Babusiaux 10 Aug 2021 Interview: 00:00 / 46:47 4. The threat of near-Earth asteroids Gaia will detect and determine the orbits of tens of thousands of asteroids in our solar system. Apollo 9 astronaut Rusty Schweickart, joins me to give his assessment of the threat to humanity of these rocks in space, and what we should be doing about them. Rusty Schweickart 5 Aug 2021 Interview: 00:00 / 1:23:49 3. The Magellanic Clouds Barcelona astronomer Xavier Luri describes the impact that Gaia is having on our understanding of two of our nearest galaxy neighbours, the Large and Small Magellanic Clouds. Hipparcos observed less than 50 stars in both systems. Gaia is observing more than 10 million. Xavier Luri 28 Jul 2021 Interview: 00:00 / 50:57 2. Solar siblings To accompany my essay on the search for solar siblings with Gaia, I am joined here by the lead authors of a paper on this topic, Jeremy Webb and Natalie Price-Jones, from the University of Toronto, for some further background. Jeremy Webb & Natalie Price-Jones 22 Jul 2021 Interview: 00:00 / 55:33 1. Stellar fly-bys One of the authors of an investigation into past and future stellar fly-bys of our Sun using the Gaia data, Coryn Bailer-Jones of the Max-Planck Institute of Astronomy in Heidelberg, joins me explain what he thinks is going on. Coryn Bailer-Jones 10 May 2021 Interview: 00:00 / 38:30 1

  • Gaia: science highlights | My Site

    Gaia: science highlights I have selected 30 topics which I consider to be some of the highlights of Gaia's scientific discoveries to date. It's more to help orient anyone hoping for an overview of the science being carried out by this space mission. Of course it reflects some personal tastes, as well as the literature that I have read. ​ I've divided the topics into four main areas (solar system; stellar structure and evolution; Galaxy content and structure; large-scale structure and cosmology ), and the numbers in braces point to (but not hyperlinked to!) my essays where I go into more details. ​ I'd be pleased to receive suggestions as to other astrophysical results or scientific highlights that could be listed. Meanwhile, here's my selection, as of June 2023 ​ Solar system: - occultations enabled by accurate star and solar system positions [24, 137] - characterisation of solar system objects [64] - light deflection by Jupiter [104] - identification of interstellar travellers [25] and stellar flybys [35] Stellar structure and evolution: - crystallisation of white dwarfs [42, 108] - radiative/convective transition in M dwarfs [42] - twin binaries [138] - stellar rotation of 3 million stars [103] - white dwarf surveys [29] - hypervelocity stars [22] - nearby black holes [101] - understanding quadruple star systems [139] - variability across the Hertzsprung-Russell diagram [62] Galaxy content and structure: - characterisation of nearby stars: the 25 pc [129] and 100 pc [33] samples - characterisation of halo streams [15, 71] - the Gaia phase-space spiral [117] - the Hercules stream [115] - a dynamical distance to the Galactic centre [111] - earliest structure in our Galaxy's "heart" [102] - deceleration of the Galaxy bar [112] - the Gould Belt and the Radcliffe Wave [127] - survey of diffuse interstellar bands [92] - measurement of Galactic aberration [32] - the distance to Omega Centauri [40] - tidal tails of open clusters [13, 20] and globular clusters [109] Large-scale structure and cosmology: - the motion of globular clusters [30] and dwarf spheroidals [31] - mapping of the Magellanic Clouds [38] - contribution to the Hubble constant from Cepheids [44] - contribution to the "plane-of-satellites" problem [118] - contribution to the "core-cusp" problem [128]

  • Gaia: science essays - table | My Site

    Gaia: science essays – table This is a table listing of my Gaia essays 1–156 (2021–23), with titles linked to the relevant essay. I will update it only occasionally: essays 157 onwards are only included in the "Gaia: science essay" page. ​ Columns can be re-ordered, and there is a simple search (on titles only).

  • Essaysonmaths

    Maths stuff 52. Discovery or invention? Are mathematics and numbers out there in the Universe, waiting to be discovered? Or are they constructs applied by us to the Universe, imposed rather than uncovered? If mathematics is a process of discovery, how can we be sure that the mathematics we think we have discovered is the right one? 51. Goldbach meets Gödel Goldbach's conjecture is that every even integer greater than 2 is the sum of two prime numbers. Proposed in 1742, it has resisted all attempts to date to prove it true or false. Thanks to Kurt Gödel, we do know that there is no way to tell, in advance, whether any given conjecture can or cannot be proved. Perhaps it's just unprovable? 50. Some brain-teasers Amongst the immense literature on recreational mathematics and logic are a vast array of associated puzzles and brain-teasers. I've picked out just a few examples that have amused or entertained me over the years. I particularly like those that appear tricky at first sight, but which yield easily to the correct approach. 49. Complex numbers The square root of -1 is an important concept in mathematics and physics, but it can give rise to some alarm when first encountered... in part because it lies outside of our own day-to-day experiences, and in part because it is labelled as `imaginary', and forms the basis of what are called `complex' numbers. 48. Sorting Sorting is the systematic rearrangement of a list into some chosen order. Commonly, we may want to rearrange a set of numbers, or a table of words or names into ascending or descending sequence. Sorting is central to many computer algorithm and database applications, but there is no unique best way of doing so. 47. The Riemann hypothesis The importance of the Riemann hypothesis lies in its intimate connection to the understanding of the distribution of prime numbers. And its fascination is compounded by the fact that, while generally assumed to be true, it remains unproven, despite being long held as one of the biggest open problem in mathematics. 46. Elliptic curves An elliptic curve is a particular form of cubic equation which turns out to be of spectacular importance. Originating in the search for an expression for the circumference of an ellipse, elliptic curves find use in public key cryptography, in the factorisation of large numbers, in the testing of primes and, famously, in the proof of Fermat's Last Theorem. 45. Ellipses Except for straight lines, the simplest algebraic curves are the 'conics': the hyperbola, the parabola, and the ellipse (the circle being just a special case of the ellipse). After circles, ellipses are probably the most familiar curves in all of mathematics. And, like circles, their applications in mathematics and physics are many and varied. 44. Phi and the Fibonacci series The 'golden ratio', or divine ratio, often referred to as phi, is the irrational number 1.618033... It appears in countless places in mathematics, most notably in the Fibonacci series, originally formulated in the context of the growth of a population of rabbits. It also appears, often alongside the Fibonacci series, in art and Nature. 16. Voronoi diagrams Place a number of objects anywhere in a plane. For each object there is a region consisting of all points of the plane closer to that object than to any other. The regions associated with each object with this proximity property are called Voronoi cells. They turn out to have many, and often surprising, real world applications. 28. Triangle centres Draw straight lines from each vertex of a triangle to the mid-point of their opposite sides. Whatever the triangle, these three lines always meet in a single point, the triangle's centroid, corresponding to its centre of gravity. There are many other 'centres' associated with a triangle's geometry, and some beautiful properties are still being found. 41. Sphere packing A classical problem in geometry relates to sphere-packing. How densely can a number of identical spheres be packed together in 3-dimensions? How curious it is that something everyone 'knows' intuitively - how to pack such spheres - should have required 400 years, and various great minds, to prove!

  • Talks | My Site

    Talks Future talks: 18 Jan 2024 Max Planck Institute for Astronomy, Heidelberg : Some Gaia insights into the H–R diagram 19 Apr 2024 Swindon Star Gazers : Gaia: Advances in our Understanding of the Galaxy Recent talks: 13 Nov 2023 Shaw Prize Laureate Forum, Hong Kong: The origin and evolution of our Milky Way Galaxy 07 Dec 2023 Nanjing University: Advances in understanding our Galaxy through space astrometry 06 Jul 2023 18th Patras Workshop on Axions, WIMPs, and WISPs: Latest insights on cosmology from Gaia 04 Jul 2023 Heidelberg Joint Astronomy Colloquium: Advances in astrometry: from Hipparchus to Gaia 13 Jun 2023 Warsaw University: Measuring star positions from space: Hipparcos and Gaia 12 Jun 2023 Toruń University: Measuring star positions from space: Hipparcos and Gaia 25 May 2023 University College Dublin: Measuring star positions: giant strides in understanding the Universe 20 Feb 2023 Copernican Congress, Toruń: Is the Earth flat? 04 Feb 2023 Eastbourne Astronomical Society: Latest results from Gaia 04 Feb 2023 Astrofest, London: Gaia: Mapping our Galaxy from space 02 Dec 2022 Bath Royal Literary and Scientific Society, BRLSI: The Fermi Paradox 01 Oct 2022 Herschel Society: A Celebration of William Herschel: Giant Strides from Herschel to Gaia 04 Aug 2022 JPL, Pasadena : Gaia: Advances in Our Understanding of the Galaxy 25 July 2022 Sagan Summer Workshop, Caltech : Exoplanet Science in the Gaia Era

  • Subscribe | My Site

    Subscribe You will receive an email when a new essay is published on this site, usually each Monday morning An "unsubscribe" option is given in the footer of each email First name Last name email Subscribe Added - thanks for your interest!

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