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
230. The Oosterhoff dichotomy
Find out how Gaia is advancing this long-standing problem
The Oosterhoff dichotomy is a long-standing problem in the understanding of Galactic globular clusters. It refers to the fact that clusters divide into two distinct groups according to the average period of their RR Lyrae stars pulsating in the fundamental mode. I outline the problem, and show how Gaia is advancing our knowledge, relating it to the ancient merger debris in our Galaxy's halo.
26 May 2025
229. RR Lyrae variables: an update
An update on this important class of variable star
I described Gaia's early contributions to the study of RR Lyrae stars in an earlier essay at the end of 2021. Here I bring the topic up-to-date with the results from DR3. Based on its 270,000 classified objects, applications include further constraints on the period-luminosity relation, various studies of Galactic structure, and improved inputs to stellar evolutionary models.
19 May 2025
228. Cepheid variables: an update
Bringing you up-to-date with Gaia studies of these important variables
I described Gaia's early contributions to the study of Cepheids in an earlier essay at the end of 2021. Here I bring the topic up-to-date with the results from DR3. Based on its 15,000 classified Cepheids, important applications have been improved assignment to their membership of open clusters, updates of the period-luminosity relation, and various studies of Galactic structure.
12 May 2025
227. Essays, podcasts and NotebookLM
Listen to my essays as a discussion-type audio dialogue
I am in the process of converting my written Gaia essays into audio format using the powerful technology of generative AI. I am using Google's NotebookLM to create an audio discussion-type dialogue between two avatars. The results, placed on my YouTube channel (www.youtube.com/@GaiaEssays) are comprehensive, accurate, didactic, and engaging.
5 May 2025
226. Heartbeat stars
An introduction to this curious phenomenon
A remarkable subset of the binary star ellipsoidal variables are the so-called heartbeat stars, named in view the resemblance of their light curves to an electrocardiogram. I outline the discovery journey, the physics underlying the phenomenon, and the role that Gaia can play in their understanding.
28 April 2025
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
