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Fantastic Cosmic Beasts and Where to Find Them – Thanks to Gaia

Fantastic Cosmic Beasts and Where to Find Them – Thanks to Gaia

Gaia's sky in colour. Caption and credit: ESA/Gaia/DPAC

On April 25, astronomers around the world were aflutter. The European Space Agency was poised to release the second batch of data one of its space missions, called Gaia, that day.

Gaia is an ambitious project to measure the positions and velocities of over a billion stars in the Milky Way galaxy. This information was crucial for numerous studies of the galaxy, and the universe as a whole. The last mission of this kind, called Hipparcos, had measured only about a million stars and with one-hundredth the precision of Gaia.

The latest data also included the distances to stars that are significantly farther away – at least a few thousand lightyears – than what had been accessible to its predecessors.

All together, Gaia has been an incredibly impactful mission for scientists studying the cosmos. For some deeper perspective, here are three highlights from the dataset from the POV of an astrophysicist.

The precursors of bright explosions

Ken Shen from the University of California, Berkeley, and his collaborators measured the velocities of white dwarfs, a class of stars that are about a million-times as dense as the Sun. For example, a white dwarf as big as Earth could weigh as much as the Sun.

One reason these cosmic objects have piqued the interest of physicists for decades now is that they’re responsible for some of the most powerful explosions in the universe, commonly known as Type Ia supernovae.

But white dwarfs are by themselves generally inert, so in order for an explosion to be set off, they need to interact with another star nearby.

The process that kickstarts this explosion remains unknown. One of the leading theories says that the white dwarf star siphons helium from its companion star, and the gas stars exploding when it touches the surface. The study by Shen et al looked for signs of this scenario in the Gaia data.

They found seven “super fast” white dwarfs in the galaxy, of which three were spinning really fast. Some of their properties matched important characteristics of the explosion scenario: lack hydrogen and helium, since the gases would’ve blown up, and be larger and fluffier than usual thanks to the explosions.

The team was able to trace one of these white dwarfs to a supernova remnant in the Pegasus constellation. While more studies will have to be conducted to validate the mechanism (e.g. by searching for the explosion’s remains in the supernova), this discovery opens exciting avenues to understand how these peculiar stars die.

Measuring the expansion of the universe

The quest to evaluate the universe’s expansion rate has excited astronomers for about a century (if not longer). Recently, astronomers used two different techniques to measure this rate and came up with two different answers, and have been scratching their heads over it since.

A class of stars called the Cepheids are of great interest when measuring distances in the universe. Their brightness fluctuates and at each moment is strongly correlated with the period of their variation. Since we already know how much light dims by when viewed from a certain distance, we can get their distance from us by measuring their period. This is particularly true for the Cepheids with long periods of variation, which are found farther away from us in the galaxy.

By using these stars to measure distances to galaxies in which Type Ia supernovae have exploded, Adam Riess at the Johns Hopkins University and his colleagues have measured the universe’s expansion rate to better than 3% precision.

Gaia allowed Riess et al to understand the relationship between the period and the brightness better: its data included distances to five times as many Cepheids of long periods as we’ve known before.

Black holes that grow very fast

While Gaia’s usefulness is usually limited to the study of stars in our own galaxy, the new batch of data was crucial in discovery of the fastest growing black hole found till date.

Astronomers at the Australian National University used the SkyMapper telescope, used to precisely map the southern sky, to discover this black hole 12 billion lightyears away. They used Gaia data to confirm that its properties indeed matched that of a black hole.

They also found that it was ‘eating’ 1,000 billion billion billion kilograms’ worth of matter every 24 hours. That means it could gobble up our Sun in two days flat, and all the Solar System planets within two hours.

Such large black holes are thought to have been present in greater numbers in the early universe, and are as such useful in understanding how it evolved over time.

These are simply some of the more striking results from scores of scientific papers that have been published on the back of the Gaia data. They are testimony to extent to which Gaia has profoundly advanced our knowledge, and so the wait for the rest of the data is now on!

Suhail Dhawan is a postdoctoral research associate at Stockholm University working on cosmology and supernovae. He has a PhD from the Technical University in Munich, conducted at the European Southern Observatory.

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