Gaia: science essays
In these short (mostly 2-page) 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 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: I have converted the first of these essays into audio "discussion-type" podcasts, entirely using generative AI. I think the results are rather remarkable. They are available at my Gaia Essay YouTube channel, and I describe their construction in Essay 227. Take a listen, and let me know your reactions.
This table page lists all essays, updated to the end of December 2024 (1–209 inclusive), in tabular form
This Gaia Science Tree (v2.0, Jan 2025) presents essays 1–209 (Jan 2021–Dec 2024) as a hyperlinked "mind map"
* all end-nodes are hyperlinked to the given essay number (links are to "legacy" copies at the CERN-Zenodo site)
* catalogue content topics are at top right, background material at bottom left, otherwise moving "outwards" clockwise in the diagram
* I have prepared this as a didactic tool. Please feel free to make use of it as you wish
Please make use of this subscribe page to receive an email (usually Monday) when each new essay is published
Essays through to the end of 2023 (1–156 inclusive) also appear in a hyperlinked indexed form in the Bulletin of the American Astronomical Society (BAAS Vol. 56, Issue 1, 15 March 2024): ADS 2024BAAS...56a.008P
225. Rotational parallaxes
A method to determine distances to Local Group galaxies
Distances can be determined to Local Group disk galaxies using the `rotational parallax'. For a system in uniform bulk rotation, equating the rotation amplitude from proper motions to that from radial velocities fixes the distance. I describe the cosmological relevance, and the current status for M31 and M33.
21 April 2025
224. Strings, snakes and pearls
New and unexpected discoveries in the solar neighbourhood
A curious and unexpected discovery are the thousands of filamentary stellar structures in the solar neighbourhood, referred to as strings (or snakes), and multiple orbitally coherent clusters referred to as pearls. The strings appear to be coherent in terms of their spatial and kinematic distribution, and age.
14 April 2025
223. Associations: an update
An update of what we are learning from Gaia
Gaia results support a hierarchical picture of star formation, in which stars are formed across a continuous density distribution throughout molecular clouds, and in which OB associations are formed in situ as relatively large-scale and gravitationally-unbound structures.
7 April 2025
222. Open clusters: chemistry and models
What their place in the colour–magnitude diagram tells us
The substantial improvement in the definition of the colour–magnitude diagrams of nearby open clusters is leading to improved definition of their main sequence, the detailed structure of the extended turnoff region, the origin of blue stragglers, and the evolutionary history of white dwarfs.
31 March 2025
221. Open clusters: kinematics
How open clusters, and the stars within them, move
Gaia's improved membership and astrometry of open clusters is providing detailed insights into their Galactic orbits, their bulk rotation and expansion, the tidal tails formed by their escaping members, and their binary population evolution and mass segregation.
24 March 2025
220. Open clusters: ages
Different ways of estimating their ages
The ages of open cluster are generally obtained by comparing their colour–magnitude diagrams to theoretical isochrones. I look at our present understanding of their ages, and how other dating methods (based on lithium depletion, gyrochronology, dynamical traceback, and asteroseismology) compare.
17 March 2025
219. Open clusters: numbers
How many clusters have been found by Gaia, and where are they?
Of the 3000 open clusters catalogued pre-Gaia, nearly two thirds have now been shown to be chance overdensities, while Gaia's census today lists around 14,000. I describe how open clusters are distributed throughout our Galaxy, how they relate to the spiral arms, and the interesting class of binary clusters.
10 March 2025
218. Red supergiants: the biggest stars
Puzzling stars whose photospheres would extend beyond the orbit of Jupiter
The biggest stars have radii some 1500 times that of the Sun. Although extremely luminous, their detection poses a number of challenges, and their large distances make them difficult to characterise. I explain their importance for understanding stellar evolution, and the ways in which Gaia is contributing.
3 March 2025
217. Zero-point of the Gaia parallaxes
An explanation of why the offset arises, and how it's characterised
I explain how a global offset of the Gaia parallaxes arises. And I describe the investigations carried out to identify and characterise the nature of such a `zero-point' offset, both for Gaia DR2 and (E)DR3. In practice, the offset is a non-trivial function of magnitude, colour, and sky coordinate.
24 February 2025
216. Star and exoplanet radii: an update
Huge numbers of new star and exoplanet radii
Compared to the few hundred stellar diameters directly measured with an accuracy of 1-2 percent, Gaia's distances and stellar parameters are providing diameters, of somewhat lower accuracy, for some 500 million stars. One consequence is for the improved radius estimates for several thousand transiting exoplanets.
17 February 2025
