On April 25, Teresa Antoja of the University of Barcelona was one of thousands of astronomers who downloaded and began exploring an exquisite new map of the Milky Way made by the European Space Agency’s Gaia spacecraft. Within a day, she and her colleagues reported the discovery of never-before-seen substructures throughout the galaxy: “shapes such as arches … snail shells and ridges,” they wrote — each one a clue about the Milky Way’s obscure past.
Antoja’s paper is one of a torrent following the long-awaited second data release from Gaia, which was launched in 2013 and has since charted the positions, brightnesses and colors of 1.7 billion Milky Way stars, and the velocities of 1.3 billion of those stars. (In September 2016, the Gaia team released its first map with only position and brightness measurements for 1.1 billion stars.) Astronomers, who had previously catalogued just 2.5 million of the brightest stars in the galaxy, are hailing a new era of precision astronomy. These are some of the most important discoveries to come from the Gaia data so far.
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For decades, astrophysicists have debated the origin of Type Ia supernovas — star explosions that serve as “standard candles” for gauging cosmic distances. Using the Gaia data and follow-up telescope observations, Ken Shen of the University of California, Berkeley, and collaborators found strong evidence for a theory dubbed the “dynamically driven double-degenerate double-detonation” (D6) scenario.
The D6 scenario begins with two white dwarfs — dense, planet-size cores of dead stars — in a close mutual orbit. According to the theory, the heavier white dwarf will suddenly start stripping matter away from the lighter one, so rapidly and turbulently that some of the helium in the transferred matter detonates. This detonation triggers the detonation of carbon and oxygen in the heavier dwarf, causing it to explode as a Type Ia supernova. With nothing holding on to it anymore, the other white dwarf is flung into space at high speed.
Among the Gaia data, Shen and colleagues discovered three “hypervelocity white dwarfs” hurtling through the galaxy at more than 1,000 kilometers per second, which is fast enough to escape its gravity. They claim the discoveries represent “tentative confirmation” of the D6 scenario. In an email, Shen said knowing the backstory of Type Ia supernovas will reduce uncertainties in cosmic distance measurements and models of supernova-driven chemical enrichment of galaxies.
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