How Gaia Changed Astronomy Forever (Script) (Patreon)

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. It’s 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 thousand stars, Gaia blows it 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 1000 km. That’s one-to-two thousand 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 3-D 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 two, tracking the Earth’s orbit but 1.5 million kilometers further from Sun. From there it stares outwards to the galaxy. As it traverses its orbit, Gaia detects the tiny shift in the positions of the stars due to this motion – a phenomenon called stellar parallax. It’s just like way your finger moves relative to the background when you wink your eyes back and forth. And the degree of motion depends on distance. Gaia’s “winks” are the size of it’s 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 at both red and blue parts of the electromagnetic spectrum; combining those gives the color of the star, which in turn gives 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 H-R diagram diagram. Location on this diagram can tell us about a star’s mass, size, fusion activity, and even its past and future evolutionFor example, stars on this diagonal band – the so-called main sequence – are in the primes of their lives, fusing hydrogen into helium, after which lower mass stars will become red giants 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 Hipparchos 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 to blackness. We see intricate details within that sequence - an unexpectedly clear separation of evolutionary paths that may reveal the composition and the 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.

Looking at the red giants, we resolve the shape of the so-called red clump in greater detail than ever before. These are stars near the ends of their lives, now burning helium in their cores. We can even watch  variable stars dance along the H-R diagram as their brightnesses change. The information Gaia provides will revolutionize our understanding of stellar formation and evolution. We are barely getting started.

OK, moving on to stellar velocities. The galaxy is a dynamic place. The stars all move in their own orbits around the galactic core. That movement is imperceptible to the human eye, even over years. It’s imperceptible to most telescopes. But Gaia’s incredible astrometry revealed the changing positions of its stars over the 5 years of its operation. This gives the velocities of the stars in the plane of the sky. A dedicated high-resolution spectrograph shows the tiny Doppler shift – the stretching or compression of the wavelengths of starlight due to motion towards or away from us. Combining  motion on the sky and Doppler shift gives the full 3-dimensional velocities for these billion stars.

Combining such complete velocity measurements with our position data, it’s now possible to model the dynamics of the Milky Way with incredible detail. On the large scale we see the distribution of stellar velocities through the spiral structure of our part of the Milky Way. We can see the rotation of the Milky Way through the red and blue Doppler shift of the stars. We can map things like stellar streams and detailed substructure that tell us about the history of our galaxy’s formation. Astronomers have already found evidence in the Gaia data that our galaxy was disturbed hundreds of millions years ago, probably by an encounter with the Sagittarius Dwarf Spheroidal Galaxy.  And mapping globular cluster and dwarf galaxy orbits  also tells us about future interactions with the Milky Way. 

Knowing the current velocities and positions of the stars, we can actually wind their clocks backwards and forwards to see where they came from and where they’re going. For example, this is the field of stars of the planet-hunting Kepler telescope. And this is how they traveled to those locations over the past half-million years. And this is where they’re going over the next half million. We can now study the kinematics of stars that have cumulatively thousands of confirmed planets. We can potentially trace the origins of these stars, potentially allowing us to find solar systems that came from the same stellar nursery.

Also by constraining the distances to stars, we can get better measurements on the sizes of those stars, and thus also get better measures of the sizes of the planets around them. Based on the new data we’ve already been able to confirm a gap in planetary radius size  at around 1.9 Earth radii. We can even potentially detect exoplanets by looking at the stars radial velocity and measuring shifts due to a planet tugging on the star. New binary stars systems can be found using these same methods.

This dynamical information will be a powerful tool in understanding the dark matter distribution in the galaxy. For example, we can study stretched-out groups of stars called stellar streams. These dynamically connected flows of stars were once bound together as globular clusters or dwarf galaxies. It’s been hypothesized that disruptions in these flows are due to clumps of dark matter – so-called subhaloes. The nature of subhaloes and other details of dark matter distribution may help us figure out what dark matter really is.

And on top of all this, Gaia doesn’t only study stars. It has tracked of over 14 thousand asteroids and other solar system objects, and found many new ones. This is useful for future asteroid mining missions and to identify potential Earth-threatening objects. Gaia has also mapped the positions and brightnesses of over half a million quasars, the cores of distant active galaxies, and will identify 100,000 supernoves Gaia even helps us with the pulsar timing array – a galactic-scale gravitational wave observatory, which we spoke about recently. It does this by providing better measurements for the distances to pulsars.

Every generation we improve our maps – first our maps of the world, and now of the galaxy – filling in unexplored regions. These maps help us to better understand our place in the universe. This new map – Gaia’s 3-D atlas of the Milky Way – fills in so many of the dark gaps of our old understanding. Every dot of light in this picture is a star, with its past and future motion known. This is our Milky Way, and we just became much more familiar with our galactic home in space time.