Essays on Gaia

At a distance of about 4 kpc, the young compact star cluster Westerlund 1 contains many rare, evolved, high-mass stars, including red supergiants, yellow hypergiants, one of the largest known stars, and 24 Wolf-Rayet stars. Gaia is pinpointing its distance and its members, and resolving the puzzling question of its age.

Wolf-Rayet stars are the final He-burning phase in the evolution of massive O stars, the last observable stage before core collapse. They are rare, but have an enormous influence on their environment. Distances were only poorly constrained before Gaia, which is now having a significant impact on their detailed understanding.

Gravitational light-bending by the Sun during the solar eclipse of 1919, around 1.7 arcsec at the solar limb, was the first observational confirmation of general relativity. Hipparcos measured light bending over the entire celestial sphere. Gaia has now measured light bending due to Jupiter, at around 10 milli-arcsec.

The study of stellar rotation has been transformed within the past decade. Twenty years ago, the number of stars with measured rotation periods stood at 11,000. Kepler made a major advance by characterising photometric modulation for some 60,000. With Gaia DR3, more than three million are now known.

Compared to recent advances in understanding the details of our Milky Way's halo, the inner Galaxy has proven more elusive, complicated by the large distances involved. Amongst several advances being made with the Gaia data is the identification of the most ancient `proto-Galactic' component of our Galaxy.

Gaia is enabling the discovery of two important classes of stellar-mass black holes in our Galaxy: isolated black holes, and those in binary systems which lack significant mass transfer. Amongst the latter, recently reported, is the nearest known black hole, at a distance of just 480 pc.

Following my last three essays, which looked at the historical debate about the existence of life on other worlds, and the search for anomalous stars which may be the first steps along the path for discovering alien civilisations, I look in more details at the Fermi paradox: if such civilisations exist, where are they?

I look at one specific search for alien civilisations which is being assisted by Gaia: the search for so-called Dyson spheres. These are hypothetical megastructures that might encompass a star, capturing a significant fraction of its emitted energy, and satisfying that civilisation's continuously growing energy needs.

Amongst Kepler's exoplanet discoveries was the curious KIC 8462852 (Boyajian's star), which displays an unusual lightcurve whose nature is still under debate, even in the context of being an interesting SETI target. I describe a search for other similar stars, and the surprising spatial clustering of some of the new candidates.

This week, I will review the philosophical debates that have raged since antiquity on whether ‘populated’ worlds exist beyond our own. While deviating from the topic of Gaia directly, it nonetheless provides an interesting historical background to the search for exoplanets, and life on other worlds, which are being assisted by Gaia.

Why do some people, in the face of overwhelming evidence to the contrary, cling vigorously to the idea that the Earth is flat? If it is because of the way today's big science is conducted and communicated, the question may hold an important message for scientists and educators.

I look at two large-scale dynamical phenomena which are, today, believed to affect the bulk motion of our Galaxy's disk with respect to its dark matter halo. The first is a tumbling motion tied to their primordial origin, while the other is related to the orbit of our neighbouring Large Magellanic Cloud.

I look at the latest estimates of the total mass of our Local Group of galaxies. These take into account increasingly subtle effects related to their detailed dynamics, many being clarified by Gaia. And they must also be consistent with the increasingly detailed predictions of numerical models of the formation and evolution of structure in the Universe.

What is the mass of our Milky Way galaxy? Why is it important to know? Why is it so difficult to measure? And what is Gaia contributing to our knowledge? Here, I take a look at the Gaia-related papers that have been trying to tackle this problem, and see why the Galaxy halo still resists our attempts to fully characterise it.

Diffuse interstellar bands, or DIBs, comprise some 600 known absorption features widely observed in stellar spectra. Known for more than a century, only one has been securely identified. Nearly half a million sources show the 862 nm DIB in their Gaia spectra, providing new insight into interstellar absorption.

As one of the developments in Galactic archaeology being enabled by Gaia, I will explain what cerium is, why it is relevant to astronomy, how it is measured by Gaia, and what its occurrence tells us about the formation, and in particular the complex infall history, of our Galaxy.

I continue with the subject of my previous essay, and give some examples of the way in which the derived astrophysical data are providing new insights in understanding the structure, formation, and evolution of huge numbers of stars in our Galaxy. They give just a flavour of what Gaia is providing.

The recently published Data Release 3 includes a wealth of `extracted' astrophysical data, of staggering extent, including stellar spectroscopic and evolutionary parameters for up to 470 million sources. A dozen refereed papers detail the underlying computations. Here, I provide a synopsis of the methods, and results.

Continuing with the study of exoplanets, I focus here on three areas where Gaia is helping to vet, and to pinpoint, these other worlds: rejecting false positives from transit searches, establishing masses from radial velocity minimum estimates, and identifying accelerating systems for imaging searches.

Gaia Data Release 3 provides radial velocities for more than 33 million stars down to about 14 mag. Here I look at some of the first scientific results from these radial velocities: their distribution across our Galaxy, their high-velocity star content, and their distribution in the globular cluster 47 Tuc.

Of the 1.8 billion sources in Gaia Data Release 3, more than 33 million have published radial velocities. More than 100 million are expected in DR4. I describe how the radial velocities are acquired on board, how they are processed, and the additional data that is being extracted from the spectra.

I look back to the period leading up to the selection of Gaia in 2000, and recall why the decision was made to acquire radial velocity observations on-board the satellite itself. And I recall the various considerations which influenced the choice of spectral range chosen for the Gaia radial velocity spectrometer.

Photometric microlensing has been responsible for the many thousands of events that have been discovered to date, including more than 350 exoplanets. Gaia is opening a new chapter in these studies, with its all-sky coverage, and its 3-colour sampling. Data Release 3 has identified 363 such events, 90 of them new.

The Gaia Andromeda Photometry Survey contains 3-colour epoch photometry for a million stars in the region of the Andromeda Galaxy. It was selected, as representative of the sky scanning and range of stellar densities, to provide a test region in advance of the full-sky epoch photometry planned for DR4.

Gaia Data Release 3 contains the first treatment of sources considered to be extended. Out of nearly one million galaxies, profile fitting yields robust parameter solutions for more than 900,000 mostly elliptical systems. Out of more than a million known quasars, a host galaxy has been detected around more than 60,000.

Continuing with the theme of neutron stars and pulsars, I look here at some well-known supernova remnants and the search for runaway stars escaping from them. I also look at Gaia's distance to the Crab Pulsar, to Gaia's photometry of its synchrotron-induced spin down, and to a class of stars that... disappear!

Apart from the Crab, pulsars are not bright enough to be observed by Gaia. But more than 20 binary pulsar companions have been identified in Gaia DR2. Astrometry provides crucial input to models of supernova core collapse and their equation of state, their orbital decay and spin-down rates, and their birth sites.

Gaia DR3 was accompanied by new insights into the nature of non-single stars in the Gaia survey. In Essay 78, I looked at the first results on exoplanet companions discovered by astrometry. Here, I look at some other results on stellar masses, brown dwarf and white dwarf companions, and some of the more exotic variables.

Pre-Gaia DR3, the NASA exoplanet archive tabulated more than 5000 exoplanets, with just one discovered from astrometry. With just over 34 months of data, Gaia DR3 is accompanied by 130,000 astrometric orbit solutions, including 1843 brown dwarf companion candidates, and 72 exoplanet candidates.

What is the total mass of our Galaxy? How far out does our Galaxy halo extend? The distribution of stellar velocities, and in particular the ‘escape’ velocity from the solar neighbourhood, holds a number of clues. Estimates from Gaia are converging on a Milky Way mass of about 10^(12) times the mass of our Sun.

Gaia is now almost seven years into a possible 10-year data collection phase. Today marks the latest data release, Gaia DR3. For the same stretch of time and the same set of observations as EDR3 (Early Data Release 3), DR3 presents a stunning wealth of new data products derived from this first 3 years of mission data.

Stars in the Galactic disk 'bounce' slowly up and down around its mid-plane as a result of the force exerted by the matter comprising the disk itself. The detailed stellar motions depend on the total disk mass, both visible and dark matter. The Gaia data are throwing new light on the disk structure and its dark matter content.

Gaia is revolutionising the study of Galactic open clusters. High-quality distances allow cluster membership to be refined, space motions convey details of their dynamics and dispersion, and its unprecedented multi-epoch multi-colour photometry further contributes to classifying membership and chemistry.

Heavy element pollution in white dwarf atmospheres is attributed to the accretion of rocky planetesimals which have been scattered and torn apart to form a dusty debris disk that can be accreted by the white dwarf. Deep insights into the nature of the associated planetary systems are now being assembled.

Various explanations have been proposed for our Galaxy's warped structure, including infall of intergalactic material, a close encounter with a companion galaxy, or misalignment of the disk and its dark matter halo. Gaia is contributing to an improved picture of its structural complexity, but the underlying driving mechanism remains uncertain.

Ancient signatures of tidal infall, responsible for our Galaxy's stellar halo, remain evident because orbital time-scales in the outer parts of the Milky Way extend to billions of years. As a result, the halo retains kinematic evidence of the surviving remnants of accretion. A number of these are being found and characterised by Gaia.

Our Sun lies within a low-density region of the interstellar medium known as the local cavity. This is partially filled with hot, low-density gas, about 100 pc in size, and referred to as the Local Bubble. Its detailed morphology can be probed through our knowledge of stellar distances, and these are being transformed by Gaia.

Today, most astronomers would probably place their bets on the microwave background radiation providing the most secure estimate of the age of the Universe. But there are a number of nearby stars whose estimated ages push at the limits of this inferred upper bound. Accurate distances are crucial to a better understanding.

Multi-colour photometry of stars can be carried out from the ground. But there were compelling reasons to make these observations onboard Gaia, at the multiple epochs coinciding with the astrometric measurements. I explain the reasoning, and the challenges involved in the technical implementation.

Gaia observes from the Sun-Earth Lagrange point L2, about 1.5 million km from Earth, in the direction away from the Sun. Various interesting properties of the L2 orbit have made it a popular destination for science missions observing deep space. But a special observation programme is need to continuously monitor Gaia's position.

The number of known terrestrial-type planets in the `habitable zone' are increasing, based on transit measurements with the space missions Kepler and TESS. Gaia is fixing their distances, and characterising their properties, allowing the list of potentially habitable planets to be sharpened, and adding key targets to SETI searches.

Amongst exoplanets known today are the perplexing class of `hot Jupiters', gas giants orbiting close to their host star. What brought them to such bizarre orbits? Gaia shows that their existence correlates with ancient star clusters, and with other waves and ripples of star densities in space and space motion in our Galaxy.

Hipparcos observed around 100,000 stars, but just 48 asteroids. Gaia is expected to determine highly accurate orbits and reflectance spectra for 350,000 solar system objects, permitting profound studies of their dynamics, structure, and taxonomy. Their properties will shed new light on the formation and evolution of our solar system.

When making the successive Gaia data releases available to the world-wide scientific community, an important question is to what extent the positions, distances, proper motions, photometry, and radial velocities can be ‘trusted’? What sort of independent validation can be made before publication? Many tests are made, and I describe a few.

Variable stars have long been recognised as offering deep insights into stellar structure and evolution. Similarly, the Hertzsprung–Russell diagram provides a quantitative visualisation of all stages of stellar evolution. Together with its empirical analogue, the colour–magnitude diagram, it has enabled many advances in stellar astrophysics.

There are enormous numbers of known variable stars, of dozens of different types. Some variability is 'intrinsic', being due to the luminosity of the star itself changing with time. Others are 'extrinsic', meaning that something affects the stellar light on the way from the star to us. Gaia is, again, transforming the field.

In this short introduction to my own experiences, I will argue that there are skill sets that a scientific leader should possess in order to best optimise the prospects of success, and that there are lessons that can be learned from previous space projects, and indeed from wider sociological studies of how people and teams best work together.

Supernovae are dramatic and violent end-points of stellar evolution, and several thousand should be detected. But the physics of the progenitor star, and the evidence that each supernova leaves behind, are also rich in diagnostics: of neutron stars, black holes, and millisecond pulsars. We look at some of this forensic evidence being discovered by Gaia.

Amongst the 500,000 quasars being observed by Gaia, and the several hundred showing multiple gravitationally lensed images, we take a more detailed look at the curious quadruple-imaged quasars, how they arise, how they are being discovered in the Gaia data, and what they might tell us about black holes, dark matter, and the Hubble constant.

Why does ESA fund the development of space science missions? Why do some of these run over budget, and over schedule? And what steps were put in place during the Concept and Technology Study in the 1990s to ensure that the advanced technologies needed for Gaia would be available consistent with its target launch date of 2012?