How Gaia Changed Astronomy Forever

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Matt O'Dowd (1:03)

The great advances in any science tend to come in sudden leaps. April 25th of 2018 marks the beginning of just such a leap for much of astronomy.

In the early hours of the morning, the Gaia mission's second data release dropped. Our understanding of our own galaxy will never be the same again.

The Gaia satellite was launched in late 2013, entirely built and operated by the European Space Agency. Its primary goal is to map the stars of the Milky Way with a scale and precision orders of magnitude greater than ever before. Gaia's predecessor, Hipparcos, catalogued 120,000 stars. Gaia blows this out of the water, with positions, colors and brightnesses of nearly 1.7 billion stars. Gaia can see orders of magnitude fainter and further away than previous missions, but its greatest superpower is its precise astrometry. Gaia can pin down a star's position to the equivalent of a human hair's width at 1,000 kilometers. That's 1 to 2,000 times smaller than the resolution of the Hubble Space Telescope. As we'll see, this precision allows Gaia to measure true distances and true velocities for 1.3 billion of its stars. The result is a 3D dynamical atlas of our quadrant of the galaxy. An atlas that we can wind both forwards and backwards in time. Let's start with distances. How does Gaia measure these? The spacecraft orbits the sun at Lagrange Point 2, tracking the Earth's orbit but 1.5 million kilometers further from the sun. From there, it stares outwards to the galaxy. As it traverses its orbit, Gaia detects the tiny shifts in the positions of stars due to this motion, a phenomenon called stellar parallax. It's just like the way your finger moves relative to the background when you wink your eyes back and forth, and the degree of motion depends on this distance.

Gaia's winks are the size of its entire orbit. Coupled with its incredible position measurements, this enables Gaia to measure distances to stars as far away as the galactic center. Knowing the distance to a star is critical for determining its other physical properties. For example, combining distance with a star's apparent brightness gives us its true luminosity. And Gaia measures brightness with incredible accuracy. It measures brightness in both the red and blue parts of the electromagnetic spectrum. Now, combining those gives us the color of the star, which in turn gives us its surface temperature. The combination of stellar luminosity and surface temperature has incredible diagnostic power. We often plot these properties against each other in a Hertzsprung-Russell or HR diagram. Location on this diagram can tell us about a star's mass, size, fusion activity, and even its past and future evolution. For example, stars on this diagonal band, the so-called main sequence, are in the primes of their lives, fusing hydrogen into helium, after which low-mass stars will become red ions before leaving behind white dwarf remnants.

So let's take a look at the Gaia HR diagram. It's pretty incredible. For the first time, we have a complete census of the stellar population far beyond the neighborhood of the Sun. Compare it to the HR diagram of Hipparcos, and we now see the main sequence extending down to include extremely faint red dwarfs. We also see the full sequence of faint white dwarfs, showing the paths they follow as they slowly fade into blackness. We see intricate details within that sequence, an unexpectedly clear separation of evolutionary paths that may reveal the composition and past life of the white dwarf.

We see hot, newly formed white dwarfs, some of which are still embedded in the nebula of gas ejected in the death of their star.