The story so far: The third dataset released by the European Space Agency’s star-mapping Gaia probe, covering almost 1.8 billion stars in the Milky Way galaxy, has revealed an unexpected phenomena called ‘starquakes’, which are similar to earthquake-like movements but on the surface of stars.
The data has also revealed the largest chemical map of the entire Milky Way, showing the DNA of millions of stars, which includes their age, mass, chemical composition, colour, temperature, and metal content. Besides, the new data has also made discoveries about binary star systems, quasars, asteroids, and exoplanets.
Scientists, in the years to follow, will interpret several terabytes of data to make discoveries about astronomical phenomena. The second dataset in 2018 allowed astronomers to show that the Milky Way merged with another galaxy in a violent collision around 10 billion years ago.
Anatomy of the Milky Way
The Milky Way, our home galaxy, is just one of the estimated one to two hundred billion galaxies of varied shapes and sizes in the universe. Roughly 13 billion years old, the Milky Way is a barred spiral galaxy consisting of 100 to 200 billion stars, with the sun as its local star.
The galaxy is shaped like a flattened disc spread across 100,000 light-years with spiral arms. The majority of the stars are located in the disc, scattered around with a mixture of gas and cosmic dust. The Milky Way has a central bulge where about 10 billion of its oldest stars are concentrated. The sun is positioned between the centre and the periphery of the galaxy.
Outside the bulge and the disc exists a halo of isolated stars and ancient clusters of stars, and further beyond this is an even bigger halo of invisible dark matter.
The Milky Way is part of a local group of galaxies, including Andromeda—its nearest large galaxy, and nearly 60 smaller galaxies.
What is the Gaia mission?
Gaia is an ongoing astronomical observatory mission launched in December 2013 by the European Space Agency (ESA) with the objective of creating the most accurate and complete 3D map of the Milky Way by surveying 1% or one billion of the galaxy’s 100 billion stars. According to the ESA, “this information allows astronomers to reconstruct the galaxy’s past and future evolution over billions of years.”
The Gaia spacecraft lifted off from Kourou, French Guiana in 2013 by a Soyuz rocket and started scientific screening in 2014. The almost hat-shaped spacecraft is fitted with two optical telescopes and the largest digital camera in space or the billion-pixel camera, with over 100 electronic detectors. This camera is precise enough to capture the diameter of a human hair from as far away as 1,000 km.
The spacecraft is strategically stationed 1.5 million km from the Earth, in the opposite direction of the sun, in an orbit around the gravitational parking point in space called the Lagrange 2 or L2.
It is from here that Gaia charts the accurate positions, motions, velocity, brightness, temperature, and composition of stars in the galaxy and other celestial objects beyond it.
In order to study the stars and objects, Gaia is also equipped with screening and imaging systems that employ three techniques—astrometry, photometry, and spectroscopy. Astrometry is the science of measuring the movement and position of stars within the plane of the sky, photometry studies the colour, brightness and other derived properties of stars, and spectroscopy measures the radial velocity or the movement of stars towards or away from us by studying stellar spectra or the fingerprints of stars. Spectra are obtained by passing the light from a star through a prism which splits it into a spectrum of colours.
Gaia’s predecessor, ESA’s Hipparcos mission, had mapped the precise positions of 100,000 stars and could gaze 1,600 lightyears away. Gaia, meanwhile, can chart starts and objects as far as 30,000 lightyears.
Gaia makes millions of observations in a single day and this raw data observed by the spacecraft is processed into meaningful scientific information by the Gaia Data Processing and Analysis Consortium (DPAC), a group of 400 scientists.
Gaia’s data release was planned in four tranches—the first two datasets were released in 2016 and 2018 respectively, the third set is the current one, and the final data will be released in 2030 after Gaia is done mapping the skies in 2025.
What has the new data revealed?
The new set of data released was collected from the Gaia probe between 2014 and 2017 and contains improved information on nearly two billion celestial objects- including stars in the Milky Way, objects outside the galaxy and those inside our solar system. It contains the largest-ever three-dimensional map of stars in the Milky Way. The map shows the chemical compositions as well as past and future movements of stars.
Starquakes: One of the most surprising discoveries, which the ESA said Gaia was not built to observe , was that of more than 100,000 ‘starquakes’- which can be likened to tsunamis or massive vibrations on the surface of stars that can change their shape. Such starquakes were also detected on the surface of stars that should not have any such large-scale vibrations as per existing astronomical theories.
“Starquakes teach us a lot about stars, notably their internal workings. Gaia is opening a goldmine for ‘asteroseismology' of massive stars,” said Belgium-based scientist Conny Aerts, who is a part of the Gaia collaboration.
Quasars: Quasars are extraordinarily active and bright cores of galaxies powered by supermassive black holes. These are the most luminous objects in the universe and are visible along its distant edges . The new Gaia data has revealed the measurements of over a million confirmed quasars and data related to 6.6 million quasar candidates. This is an increase from the previous data set which gave positions of half a million confirmed quasars. The discovery of new quasars is significant as it helps to measure the most distant stretches of our universe.
Stellar DNA: The data shows chemical components, colours, brightness, velocity, temperatures, and positions of millions of stars. The chemical composition of the stars also reveals what elements they contain. Only light elements (hydrogen and helium) were formed during the Big Bang. Heavier elements, which scientists call metals, are built inside the stars, and when they die, newer stars are formed with the metals dispersed by dead stars. Chemical composition can thus be used to determine which stars were born in another galaxy and then migrated to the Milky Way. According to the ESA, astronomers hope to use the data to understand better how stars are born and die, and how the Milky Way evolved over billions of years.
Previous Gaia data had revealed the proper positions or 2D motions of stars (how they move vertically and horizontally on the sky’s plane), but the new set shows the radial velocity of 33 million stars, i.e. how fast are these stars moving towards, or away from us. . This can not only help scientists figure out the trajectory of the stars, but also their location of origin.
Binary Stars: Binary star systems are two stars that orbit each other around a shared centre. The new data has revealed the position, distance, orbits, and masses of over 8,00,000 binary star systems. Scientists can extract from this data, the mass of individual stars of the binary system, how stars work, and even discover if one of the two stars is an exoplanet—a planet that existsoutside our solar system and generally orbits a star.
Asteroids and galaxies: The data has provided a catalogue of more than 156,000 asteroids in our Solar System, which includes their mass, composition, colour, and brightness (from which details of their shape and rotation can be derived). Besides mapping the Milky Way, the data has also found magnitudes of 2.9 million other galaxies, including how their stars were formed, how far they are and when they came into existence.
(With inputs from agencies)
