This video is sponsored by Magellan TV.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood astronomer.
Since 2013, ESA's Gaia satellite has been making the most precise measurements yet of
nearly every star in the entire sky.
In December 2020, the Gaia mission issued its third data release, and here's what it
looks like. This isn't a photograph; it's a map of 1.8 billion stars!
The map is dominated by the plane of our Milky Way galaxy. It's actually a flattened disk
seen edge-on. The disk contains most of our Galaxy’s stars. In the middle is the Galactic
center bulging with older, redder stars.
In the foreground, darker clouds of interstellar gas and dust hide more distant stars from
Gaia's view. Many of these clouds conceal stellar nurseries where new generations of
stars are forming.
Surrounding the Galaxy are the many globular star clusters forming a giant halo, and dwarf
galaxies that lay just beyond. The two bright objects in the lower right of the image are
the Large and Small Magellanic Clouds; two satellite galaxies that orbit the Milky Way.
Gaia's map of our Galaxy is the most detailed of its kind ever made. But what sets it apart
from an ordinary image is Gaia's ability to measure the distance and motions of each of
these stars. The result is a three-dimensional map of our Milky Way Galaxy.
So today we're gonna talk about the Gaia satellite, how it works, and what it's revealed about
our Galaxy so far. But first, I'd like to thank Magellan TV, who are very kindly sponsoring
today's video.
Magellan is a documentary streaming service featuring content on History, Art, Nature,
and of course, Space and Science.
New programs are added weekly, including "Is Anyone Out There?", which talks about search
for extraterrestrial intelligence, beginning with the famous WOW! Signal of 1977, through
the detection of possible Earthlike exoplanets with the Kepler Space Telescope, to the modern
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and iOS.
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who have a very special offer for my viewers. By signing up at the link below, you’ll
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the description of this video.
Gaia specializes in a sub-branch of astronomy called "astrometry". It's the science of precisely
measuring the positions, parallax, and motions of stars.
Astrometry began in ancient Greece around 190 BCE when Hipparchus of Nicea made a detailed
catalog of every star in the sky. When he compared his measurements to those of his
predecessors, he discovered that the sky had shifted. Hipparchus discovered that Earth
is precessing about its rotational axis, like a wobbling toy top.
Over the years, the positions of more and more stars were measured and catalogued with
increasingly sophisticated instruments, including the 24-inch Sproul refractor at Swarthmore
College where I worked in high school. It was a better gig than KFC.
But no matter the quality of the instrumentation, making such measurements from the ground is
ultimately limited by the atmosphere distorting starlight.
So, in 1989, the European Space Agency launched the Hipparcos mission. From its perch at geostationary
orbit, Hipparcos measured the positions, distances, and motions of about 120,000 stars.
But since its launch in 2013, Gaia annihilates Hipparcos' record by surveying over 1.8 billion
stars! And, it can measure each star's position down to an accuracy of one-one hundred (correction:
one-hundred thousandth) of an arc second! That's the apparent width of a single human
hair as seen from 1000 kilometers away.
Gaia orbits the Sun at the Sun-Earth second Lagrangian point, or L2. This is a gravitational
stable point that gives Gaia an unobstructed view of the entire sky while maintaining a
steady line of sight communication with Earth.
As it orbits the Sun, Gaia detects the tiny back and forth wobbles of stars due to parallax.
This is the same reason your finger appears to shift back and forth in position as you
alternately look through one eye and then the other.
The amount of the star's parallax is determined by two distances: the baseline separating
two observations - which in Gaia's case, is a teensy bit wider than Earth's orbit around
the Sun - and the distance from the Sun to the star itself. Thanks to its high-precision
measurements, Gaia can detect parallaxes of stars all the way out to about 9 kiloparsecs.
That's roughly 30,000 light-years - enough to map all of the stars from the solar system
to the Galactic center!
Knowing the distances to stars is essential for understanding their other properties.
For example, you can compare a star's distance to its apparent brightness to determine its
true luminosity.
That's why Gaia also measures the brightness of stars extremely accurately, using a technique
called photometry. Gaia makes two photometric measurements for each star in blue and red
color bands. The difference in brightness between these two bands reveals the star's
color, which in turn tells us the star's temperature.
We can then plot the stars' luminosities against their temperatures to construct the Hertzsprung-Russell
diagram. These diagrams are powerful tools that can tell us about stars' masses, sizes,
lifetimes, and even their evolution. Stars spend the majority of their lives along the
diagonal main sequence, fusing hydrogen into helium in their cores, and producing energy
in the process.
When their hydrogen fuel is exhausted, stars leave the main sequence to become red giants.
And, as long as they're not too massive, eventually eject their outer layers and leave behind
small hot white dwarf stars.
Such diagrams have been around since the early 1900's, but they've always been incomplete.
Consider this HR diagram made with Hipparcos. If we didn't know any better, we might think
that most of the stars out there are at least as luminous as the Sun, or even brighter.
But thanks to Gaia's ability to measure far more stars, it becomes clear that most of
the stars on the main sequence are in fact smaller, red dwarfs.
Because there are so many more stars in the Gaia data, new structures within the diagram
become visible. In the lower left, we see a rich sequence of white dwarfs. But on closer
inspection, there appears to be two branches of white dwarf evolution; those with cores
that are mostly hydrogen and those with cores that are mostly helium.
On the left of the diagram are newly-forming white dwarfs that are still ejecting the last
remnants of their stellar envelopes.
Because Gaia observes each star multiple times, variable stars appear to move about the diagram
as they evolve, changing their temperatures and luminosities as they convulse during their
red giant phases. It's like watching a movie of stellar evolution!
Having an HR diagram with such fine details gives astronomers new insights into the evolution
of stars and literally allows us to connect the dots from one stage of a star's life to
another.
So how does Gaia capture so much information for so many stars? The answer has to do with
its design and its motion. Rather than observe stars individually, Gaia spins while scanning
the sky with a pair of telescopes. The telescopes are separated by a wide angle, 105 (correction:106.5)
degrees, to be exact. This allows each star to be scanned twice in a given rotation.
Light from the two telescope mirrors are combined onto a single billion-pixel detector. Stars
appearing in the first telescope are registered in the first column of CCDs in the star mapper
on the right. The star mapper then selects the stars that will be followed and measured.
Stars from the second telescope are registered in the second column of the star mapper to
be measured.
As Gaia spins, the stars from both telescopes move across the detector. First, they pass
over the large Astrometric Field. As the star moves across the array of CCD's, its brightness
and position are measured.
The stars then pass into the Blue and Red photometric CCD's. These are prisms that record
the stars' brightness in their respective color bands. These are later compared to determine
each star's color and temperature.
Finally, some of the stars pass into the radial velocity spectrometers. These spectra are
used to determine how fast the stars are moving toward or away from us.
All of the information for each star is stored onboard Gaia's computers, and the process
is repeated again and again for each star. Gaia measures an average of 2 million stars
every hour, generating about 50 Gb of data every day. This gets transmitted to the ground
as Gaia continues to scan.
As Gaia spins, it also precesses. These motions, combined with its orbit around the Sun, allow
Gaia to survey every star in the entire sky repeatedly over time. After a nominal 5-year
observing campaign, every object in the sky is measured 70 times on average.
These repeated observations allow Gaia to track the stars as they move across the sky.
This is called the stars' "proper motion". We can extrapolate these proper motions back
and forth through time to watch our entire Galaxy buzz with the motions of stars.
We can even fast forward through time to watch the constellations change before our eyes!
The apparent motions of stars due in part to their true motions through space, but also
due to our Sun's motion throughout the Galaxy. When we average the motions of the stars within
1,000 parsecs of the Sun, our solar system's motion around the Galaxy is revealed.
It gets better - when Gaia's distance and radial velocity data are added to the proper
motions, 3D visualizations of the true motions of stars throughout our Galaxy are possible.
Here, we see 74,000 of the nearby stars that lie within 100 parsecs of the Sun. As we fast
forward through time, we see that the stars rapidly disperse; some initially moving inwards
and others moving outward toward the outer regions. Most stars have roughly circular
orbits close to the galactic plane. These stars are shown in purple. But some stars
fan out above and below the disk and are shown in yellow and red.
In fact, Gaia's observations not only confirm that our Galaxy's disk is warped, but it also
shows how it precesses over time. Interestingly, the disk's precession is faster than expected.
This suggests that the warp may have been caused by a highly energetic event, such as
a collision with another galaxy.
It's unclear which galaxy it was in particular, since there are so many of them surrounding
the Milky Way. But a strong contender is the Sagittarius dwarf galaxy. This dwarf galaxy
appears to have plunged through the disk several times in the past. With each collision, more
and more stars in Sagittarius are stripped out and added to our Milky Way.
In other words, the Milky Way is a galactic cannibal, having built itself up out of mergers
with many smaller dwarf galaxies. Each merger triggers a new wave of star formation.
In fact, the Gaia data shows our Galaxy had three large epochs of star formation that
occurred 6 billion, 2 billion, and 1 billion years ago.
There's a striking coincidence between those epochs of star formation and the models for
previous collisions with Sagittarius.
These collisions would have sent ripples through the disk, concentrating dust and gas in some
areas, while leaving other areas empty. Concentrated regions would then. trigger new star formation.
In fact, Gaia found that our local arm of the Milky Way, called the Orion Spur, undulates
about 500 light-years above and below the disk. Our Sun is just 500 light-years from
the nearest cloud in the wave. Gaia showed that our Sun crossed this wave just 13 million
years ago and will cross it again in the future. In other words, our Sun is surfing this particular
Galactic wave!
This suggests that our Sun, our solar system, and ourselves may owe our collective existence
to a collision with Sagittarius 6 billion years ago.
The Galactic disk features large spiral arms that emerge from a central bar of older stars
on highly elongated orbits. While the bar has long been known to us, Gaia revealed its
three-dimensional structure for the first time.
It's still unclear how the bar formed, or if it's a permanent fixture in our Galaxy
and was always there since the beginning. Or maybe it's a transient phenomenon and comes
and goes. But as more data becomes available, more stars will be added to this map and fuller
picture will emerge.
We're learning a lot about our Galaxy thanks to Gaia, but its measurements are so precise,
it can even discern the motions of stars in galaxies beyond the Milky Way!
The Large and Small Magellanic Clouds are the Galaxy's two largest satellites. Thanks
to several years of careful observation, Gaia can track their motions. Here we see a prediction
of their motions over the next 5 million years.
Taking a closer look at the Large Magellanic Cloud, for example, we see the overall rotation
of this galaxy.
When we crank up the brightness of the most luminous stars, the Magellanic bridge between
the large and small clouds become visible, and we can see the motions of these stars
over the next 10 million years.
All of these motions reveal how our Milky Way formed through billions of years of mergers
with smaller galaxies. And those mergers are still taking place today!
But there's one big merger coming up in a few billion years with the Andromeda Galaxy.
It's currently 2.5 million light-years away, and closing fast.
Previous studies suggested that Andromeda and the Milky Way will collide more or less
head-on in about 3.9 billion years from now. But Gaia's measurements of Andromeda's motion
and its rotation suggest that it's actually going to swing out a little further than previously
thought, and delaying the collision by about 600 million years.
Closer to home, Gaia tracks more than 14,000 asteroids. Most of these are previously known
objects, but Gaia discovered several new asteroids as well. Many of these new objects have orbits
that have higher inclinations than those that are typically found in the main asteroid belt.
High-inclination asteroids aren't as well studied as those with "flatter" orbits, because
asteroid surveys typically focus on the main plane of the asteroid belt. Since Gaia scans
the entire sky, it can find new asteroids that were missed in previous surveys.
Gaia began science operations in July 2014. Over the years, the mission team issued a
series of data releases, the most recent in early December 2020. With each data release,
the precision and accuracy of previous measurements are improved, and fainter objects that were
missed previously are added to the Gaia catalog. Of course, the global pandemic really slowed
down the Gaia team's momentum, so the December data release is officially called Early Data
Release 3, and the rest of the dataset should be ready for release sometime in 2021.
The good news is that Gaia is still healthy and its mission has already been extended
through at least 2022. The more it observes, the more precise its measurements and the
more complete a picture we can build of our Galaxy's structure, its formation, and its
evolution.
One thing Gaia has already shown us is just how massive our Galaxy is due to the presence
of dark matter. Even though it's home to hundreds of billions of stars, it's just a small fraction
of our Galaxy's mass. Gaia teamed up with the Hubble Space Telescope to make this measurement,
and I made a video about that so I'll see you over there when we're done here.
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