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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 They make no attempt at a complete review of a given topic, and many will quickly become superseded by new results.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.

186. Moving groups and traceback ages

New clues about cluster formation and gas dispersal processes

An independent method of estimating the ages of nearby stellar associations makes use of their expanding space motions to yield what are referred to as dynamical or traceback ages. The difference between various methods is providing important clues about the cluster formation and gas dispersal processes.

22 July 2024

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185. Black holes: an update

15 July 2024

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184. Stellar streams and sub-halos

An important text of Cold Dark Matter cosmology with Gaia

A specific application of the study of stellar streams is in placing constraints on the existence and nature of the numerous dark matter sub-halos (halos within halos) that are predicted, in standard Cold Dark Matter cosmology, to exist surrounding the Milky Way. The stream GD-1 shows some tantalising prospects.

8 July 2024

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183. CCDs and CTE

1 July 2024

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182. The YORP effect

A second-order Yarkovesky-like force

The YORP effect is a second-order Yarkovsky-like effect, influencing the spin rate and spin axis orientation of sub-km sized irregular asteroids. Like the Yarkovsky force, its effects accumulate over long periods, resulting in some rather remarkable changes in the properties of the asteroid population, seen clearly in DR3.

24 June 2024

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181. The Yarkovsky effect

A curious but important phenomenon in solar system dynamics

The Yarkovsky effect is important in solar system dynamics. For a small rotating body illuminated by the Sun, re-radiated thermal emission lags behind the incident radiation, contributing a component of force in the direction of its orbital motion. I explain why it is important, and how Gaia is contributing to its understanding.

17 June 2024

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180. The spectra of solar system objects

What sunlight reflected from astroids tells us about their history

Photometry of minor solar system bodies provides information about their shape and rotation. Reflected sunlight encodes information about their composition and taxonomic classification, from which information about their origin and evolution can be deduced. DR3 gives reflectance spectra for more than 60,000 such bodies.

10 June 2024

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179. Stellar masses from SB2 binaries

Gaia's contribution to fundamental stellar masses

Masses are one of the most fundamental stellar properties, crucial in determining their structure and evolution. Yet ways of determining accurate masses are strictly limited and, even today, only a couple of hundred are known to better than 1-2 per cent. I summarise how Gaia is contributing.

3 June 2024

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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