Between 300 million and 900 million years ago, our Milky Way galaxy nearly collided with the Sagittarius dwarf galaxy. Data from the ESA’s Gaia mission shows the ongoing effect of this event, with stars moving like ripples on the surface of a pond. The galactic collision is part of an ongoing cannibalization of the dwarf galaxy by the much-larger Milky Way.
In the 1920s, Edwin Hubble made the groundbreaking discovery that the Universe was in a state of expansion. Originally predicted as a consequence of Einstein’s Theory of General Relativity, measurements of this expansion came to be known as Hubble’s Constant. Today, and with the help of next-generation telescopes – like the aptly-named Hubble Space Telescope (HST) – astronomers have remeasured and revised this law many times.
These measurements confirmed that the rate of expansion has increased over time, though scientists are still unsure why. The latest measurements were conducted by an international team using Hubble, who then compared their results with data obtained by the European Space Agency’s (ESA) Gaia observatory. This has led to the most precise measurements of the Hubble Constant to date, though questions about cosmic acceleration remain.
Since 2005, Adam Riess – a Nobel Laureate Professor with the Space Telescope Science Institute and the Johns Hopkins University – has been working to refine the Hubble Constant value by streamlining and strengthening the “cosmic distance ladder”. Along with his team, known as Supernova H0 for the Equation of State (SH0ES), they have successfully reduced the uncertainty associated with the rate of cosmic expansion to just 2.2%
To break it down, astronomers have traditionally used the “cosmic distance ladder” to measure distances in the Universe. This consists of relying on distance markers like Cepheid variables in distant galaxies – pulsating stars whose distances can be inferred by comparing their intrinsic brightness with their apparent brightness. These measurements are then compared to the way light from distant galaxies is redshifted to determine how fast the space between galaxies is expanding.
From this, the Hubble Constant is derived. Another method that is used is to observe the Cosmic Microwave Background (CMB) to trace the expansion of the cosmos during the early Universe – circa. 378,000 years after the Big Bang – and then using physics to extrapolate that to the present expansion rate. Together, the measurements should provide an end-to-end measurement of how the Universe has expanded over time.
However, astronomers have known for some time that the two measurements don’t match up. In a previous study, Riess and his team conducted measurements using Hubble to obtain a Hubble Constant value of 73 km/s (45.36 mps) per megaparsec (3.3 million light-years). Meanwhile, results based on the ESA’ Planck observatory (which observed the CMB between 2009 and 2013) predicted that the Hubble constant value should now be 67 km/s (41.63 mps) per megaparsec and no higher than 69 km/s (42.87 mps) – which represents a discrepancy of 9%.
“The tension seems to have grown into a full-blown incompatibility between our views of the early and late time universe. At this point, clearly it’s not simply some gross error in any one measurement. It’s as though you predicted how tall a child would become from a growth chart and then found the adult he or she became greatly exceeded the prediction. We are very perplexed.”
In this case, Riess and his colleagues used Hubble to gauge the brightness of distant Cepheid variables while Gaia provided the parallax information – the apparent change in an objects position based on different points of view – needed to determine the distance. Gaia also added to the study by measuring the distance to 50 Cepheid variables in the Milky Way, which were combined with brightness measurements from Hubble.
This allowed the astronomers to more accurately calibrate the Cepheids and then use those seen outside the Milky Way as milepost markers. Using both the Hubble measurements and newly released data from Gaia, Riess and his colleagues were able to refine their measurements on the present rate of expansion to 73.5 kilometers (45.6 miles) per second per megaparsec.
As Stefano Casertano, of the Space Telescope Science Institute and a member of the SHOES team, added:
“Hubble is really amazing as a general-purpose observatory, but Gaia is the new gold standard for calibrating distance. It is purpose-built for measuring parallax—this is what it was designed to do. Gaia brings a new ability to recalibrate all past distance measures, and it seems to confirm our previous work. We get the same answer for the Hubble constant if we replace all previous calibrations of the distance ladder with just the Gaia parallaxes. It’s a crosscheck between two very powerful and precise observatories.”
Looking to the future, Riess and his team hope to continue to work with Gaia so they can reduce the uncertainty associated with the value of the Hubble Constant to just 1% by the early 2020s. In the meantime, the discrepancy between modern rates of expansion and those based on the CMB will continue to be a puzzle to astronomers.
In the end, this may be an indication that other physics are at work in our Universe, that dark matter interacts with normal matter in a way that is different than what scientists suspect, or that dark energy could be even more exotic than previously thought. Whatever the cause, it is clear the Universe still has some surprises in store for us!
The night sky, is the night sky, is the night sky. The constellations you learned as a child are the same constellations that you see today. Ancient people recognized these same constellations. Oh sure, they might not have had the same name for it, but essentially, we see what they saw.
But when you see animations of galaxies, especially as they come together and collide, you see the stars buzzing around like angry bees. We know that the stars can have motions, and yet, we don’t see them moving?
How fast are they moving, and will we ever be able to tell?
Stars, of course, do move. It’s just that the distances are so great that it’s very difficult to tell. But astronomers have been studying their position for thousands of years. Tracking the position and movements of the stars is known as astrometry.
We trace the history of astrometry back to 190 BC, when the ancient Greek astronomer Hipparchus first created a catalog of the 850 brightest stars in the sky and their position. His student Ptolemy followed up with his own observations of the night sky, creating his important document: the Almagest.
In the Almagest, Ptolemy laid out his theory for an Earth-centric Universe, with the Moon, Sun, planets and stars in concentric crystal spheres that rotated around the planet. He was wrong about the Universe, of course, but his charts and tables were incredibly accurate, measuring the brightness and location of more than 1,000 stars.
A thousand years later, the Arabic astronomer Abd al-Rahman al-Sufi completed an even more detailed measurement of the sky using an astrolabe.
One of the most famous astronomers in history was the Danish Tycho Brahe. He was renowned for his ability to measure the position of stars, and built incredibly precise instruments for the time to do the job. He measured the positions of stars to within 15 to 35 arcseconds of accuracy. Just for comparison, a human hair, held 10 meters away is an arcsecond wide.
Also, I’m required to inform you that Brahe had a fake nose. He lost his in a duel, but had a brass replacement made.
In 1807, Friedrich Bessel was the first astronomer to measure the distance to a nearby star 61 Cygni. He used the technique of parallax, by measuring the angle to the star when the Earth was on one side of the Sun, and then measuring it again 6 months later when the Earth was on the other side.
Over the course of this period, this relatively closer star moves slightly back and forth against the more distant background of the galaxy.
And over the next two centuries, other astronomers further refined this technique, getting better and better at figuring out the distance and motions of stars.
But to really track the positions and motions of stars, we needed to go to space. In 1989, the European Space Agency launched their Hipparcos mission, named after the Greek astronomer we talked about earlier. Its job was to measure the position and motion of the nearby stars in the Milky Way. Over the course of its mission, Hipparcos accurately measured 118,000 stars, and provided rough calculations for another 2 million stars.
That was useful, and astronomers have relied on it ever since, but something better has arrived, and its name is Gaia.
Launched in December 2013, the European Space Agency’s Gaia in is in the process of mapping out a billion stars in the Milky Way. That’s billion, with a B, and accounts for about 1% of the stars in the galaxy. The spacecraft will track the motion of 150 million stars, telling us where everything is going over time. It will be a mind bending accomplishment. Hipparchus would be proud.
With the most precise measurements, taken year after year, the motions of the stars can indeed be calculated. Although they’re not enough to see with the unaided eye, over thousands and tens of thousands of years, the positions of the stars change dramatically in the sky.
The familiar stars in the Big Dipper, for example, look how they do today. But if you go forward or backward in time, the positions of the stars look very different, and eventually completely unrecognizable.
When a star is moving sideways across the sky, astronomers call this “proper motion”. The speed a star moves is typically about 0.1 arc second per year. This is almost imperceptible, but over the course of 2000 years, for example, a typical star would have moved across the sky by about half a degree, or the width of the Moon in the sky.
The star with the fastest proper motion that we know of is Barnard’s star, zipping through the sky at 10.25 arcseconds a year. In that same 2000 year period, it would have moved 5.5 degrees, or about 11 times the width of your hand. Very fast.
When a star is moving toward or away from us, astronomers call that radial velocity. They measure this by calculating the doppler shift. The light from stars moving towards us is shifted towards the blue side of the spectrum, while stars moving away from us are red-shifted.
Between the proper motion and redshift, you can get a precise calculation for the exact path a star is moving in the sky.
We know, for example, that the dwarf star Hipparcos 85605 is moving rapidly towards us. It’s 16 light-years away right now, but in the next few hundred thousand years, it’s going to get as close as .13 light-years away, or about 8,200 times the distance from the Earth to the Sun. This won’t cause us any direct effect, but the gravitational interaction from the star could kick a bunch of comets out of the Oort cloud and send them down towards the inner Solar System.
The motions of the stars is fairly gentle, jostling through gravitational interactions as they orbit around the center of the Milky Way. But there are other, more catastrophic events that can make stars move much more quickly through space.
When a binary pair of stars gets too close to the supermassive black hole at the center of the Milky Way, one can be consumed by the black hole. The other now has the velocity, without the added mass of its companion. This gives it a high-velocity kick. About once every 100,000 years, a star is kicked right out of the Milky Way from the galactic center.
Another situation can happen where a smaller star is orbiting around a supermassive companion. Over time, the massive star bloats up as supergiant and then detonates as a supernova. Like a stone released from a sling, the smaller star is no longer held in place by gravity, and it hurtles out into space at incredible speeds.
Astronomers have detected these hypervelocity stars moving at 1.1 million kilometers per hour relative to the center of the Milky Way.
All of the methods of stellar motion that I talked about so far are natural. But can you imagine a future civilization that becomes so powerful it could move the stars themselves?
In 1987, the Russian astrophysicist Leonid Shkadov presented a technique that could move a star over vast lengths of time. By building a huge mirror and positioning it on one side of a star, the star itself could act like a thruster.
Photons from the star would reflect off the mirror, imparting momentum like a solar sail. The mirror itself would be massive enough that its gravity would attract the star, but the light pressure from the star would keep it from falling in. This would create a slow but steady pressure on the other side of the star, accelerating it in whatever direction the civilization wanted.
Over the course of a few billion years, a star could be relocated pretty much anywhere a civilization wanted within its host galaxy.
This would be a true Type III Civilization. A vast empire with such power and capability that they can rearrange the stars in their entire galaxy into a configuration that they find more useful. Maybe they arrange all the stars into a vast sphere, or some kind of geometric object, to minimize transit and communication times. Or maybe it makes more sense to push them all into a clean flat disk.
Amazingly, astronomers have actually gone looking for galaxies like this. In theory, a galaxy under control by a Type III Civilization should be obvious by the wavelength of light they give off. But so far, none have turned up. It’s all normal, natural galaxies as far as we can see in all directions.
For our short lifetimes, it appears as if the sky is frozen. The stars remain in their exact positions forever, but if you could speed up time, you’d see that everything is in motion, all the time, with stars moving back and forth, like airplanes across the sky. You just need to be patient to see it.
Gaia is a space observatory parked at the L2 Lagrange Point, a stable place in space a million miles behind Earth as viewed from the Sun. Its mission is astrometry: measuring the precise positions, distances and motion of 1 billion astronomical objects (primarily stars) to create a three-dimensional map of the Milky Way galaxy. Gaia’s radial velocity measurements — the motion of stars toward or away from us — will provide astronomers with a stereoscopic and moving-parts picture of about 1% of the galaxy’s stars.
Think about how slowly stars move from the human perspective. Generations of people have lived and died since the days of ancient Greece and yet the constellations outlines and naked eye stars appear nearly identical today as they did then. Only a few stars — Arcturus, Sirius, Aldebaran — have moved enough for a sharp-eyed observer of yore to perceive their motion.
We know that stars are constantly on the move around the galactic center. The Sun and stars in its vicinity orbit the core at some half-million miles an hour, but nearly all are so far away that their apparent motion has barely moved the needle over the time span of civilization as we know it.
This video shows more than 2 million stars from the TGAS sample, with the addition of 24,320 bright stars from the Hipparcos Catalogue that weren’t included in Gaia’s first data release back in September 2016. The video starts from the positions of stars as measured by Gaia between 2014 and 2015, and shows how these positions are expected to evolve in the future, based on the stars’ proper motions or direction of travel across space.
Watching the show
The frames in the video are separated by 750 years, and the overall sequence covers 5 million years. The dark stripes visible in the early frames reflect the way Gaia scans the sky (in strips) and the early, less complete database. The artifacts are gradually washed out as stars move across the sky.
Using the map above to get oriented, it’s fun to watch Orion change across the millennia. Betelgeuse departs the constellation heading north fairly quickly, but Orion’s Belt hangs in there for nearly 2 million years even if it soon develops sag! The Pleiades drift together to the left and off frame and then reappear at right.
Stars seem to move with a wide range of velocities in the video, with stars in the galactic plane moving quite slow and faster ones speeding across the view. This is a perspective effect: most of the stars we see in the plane are much farther from us, and thus seem to be moving slower than the nearby stars, which are visible across the entire sky.
Some of the stars that appear to zip in and out of view quickly are passing close to the Sun. But motion of those that trace arcs from one side of the sky to the other while passing close to the galactic poles (top and bottom of the frame) as they speed up and slow down, is spurious. These stars move with a constant velocity through space.
Stars located in the Milky Way’s halo, a roughly spherical structure centered on the galaxy’s spiral disk, also appear to move quite fast because they slice through the galactic plane with respect to the Sun. In reality, halo stars move very slowly with respect to the center of the galaxy.
Early in the the visualization, we see clouds of interstellar gas and dust that occupy vast spaces within the galaxy and block the view of more distant suns. That these dark clouds seem to disappear over time is also a spurious effect.
After a few million years, the plane of the Milky Way appears to have shifted towards the right as a consequence of the motion of the Sun with respect to that of nearby stars in the Milky Way. Regions that are depleted of stars in the video will not appear that way to future stargazers but will instead be replenished by stars not currently sampled by Gaia. So yes, there are a few things to keep in mind while watching these positional data converted into stellar motions, but the overall picture is an accurate one.
I find the video as mesmerizing as watching fireflies on a June night. The stars seem alive. Enjoy your ride in the time machine!
The European Gaia spacecraft launched about a year ago with the ambitious goal of mapping one billion years in the Milky Way. That’s 1% of all the stars in our entire galaxy, which it will monitor about 70 times over its 5-year mission. If all goes well, we’ll learn an enormous amount about the structure, movements and evolution of the stars in our galaxy. It’ll even find half a million quasars. Continue reading “Astronomy Cast Ep. 365: Gaia”
Early on the morning of Dec. 19, 2013, the pre-dawn sky above the coastal town of Kourou in French Guiana was briefly sliced by the brilliant exhaust of a Soyuz VS06 rocket as it ferried ESA’s “billion-star surveyor” Gaia into space, on its way to begin a five-year mission to map the precise locations of our galaxy’s stars. From its position in orbit around L2 Gaia will ultimately catalog the positions of over a billion stars… and in the meantime it will also locate a surprising amount of Jupiter-sized exoplanets – an estimated 21,000 by the end of its primary mission in 2019.
And, should Gaia continue observations in extended missions beyond 2019 improvements in detection methods will likely turn up even more exoplanets, anywhere from 50,000 to 90,000 over the course of a ten-year mission. Gaia could very well far surpass NASA’s Kepler spacecraft for exoplanet big game hunting!
“It is not just the number of expected exoplanet discoveries that is impressive”, said former mission project scientist Michael Perryman, lead author on a report titled Astrometric Exoplanet Detection with Gaia. “This particular measurement method will give us planet masses, a complete exoplanet survey around all types of stars in our Galaxy, and will advance our knowledge of the existence of massive planets orbiting far out from their host stars”.
The planets Gaia will be able to spot are expected to be anywhere from 1 to fifteen times the mass of Jupiter in orbit around Sun-like stars out to a distance of about 500 parsecs (1,630 light-years) from our own Solar System. Exoplanets orbiting smaller red dwarf stars will also be detectable, but only within about a fifth of that distance.
While other space observatories like NASA’s Kepler and CNES/ESA’s CoRoT were designed to detect exoplanets through the transit method, whereby a star’s brightness is dimmed ever-so-slightly by the silhouette of a passing planet, Gaia will detect particularly high-mass exoplanets by the gravitational wobble they impart to their host stars as they travel around them in orbit. This is known as the astrometric method.
A select few of those exoplanets will also be transiting their host stars as seen from Earth – anywhere from 25 to 50 of them – and so will be observable by Gaia as well as from many ground-based transit-detection observatories.
After some issues with stray light sneaking into its optics, Gaia was finally given the green light to begin science observations at the end of July and has since been diligently scanning the stars from L2, 1.5 million km from Earth.
With the incredible ability to measure the positions of a billion stars each to an accuracy of 24 microarcseconds – that’s like measuring the width of a human hair from 1,000 km – Gaia won’t be “just” an unprecedented galactic mapmaker but also a world-class exoplanet detector! Get more facts about the Gaia mission here.
The team’s findings have been accepted for publication in The Astrophysical Journal.
In astronomy we throw around the term “light-year” seemingly as fast as light itself travels. And yet actually measuring this distance is incredibly tricky. A star’s parallax — its tiny apparent shift once a year caused by our moving viewpoint on Earth — tells its distance more truly than any other method.
Is our Solar System normal? Or is it weird? How does the Solar System fit within the strange star systems we’ve discovered in the Milky Way so far?
With all the beautiful images that come down the pipe from Hubble, our Solar System has been left with celestial body image questions rivaling that of your average teenager. They’re questions we’re all familiar with. Is my posture crooked? Do I look pasty? Are my arms too long? Is it supposed to bulge out like this in the middle? Some of my larger asteroids are slightly asymmetrical. Can everyone tell? And of course the toughest question of all… Am I normal?
The idea that stars are suns with planets orbiting them dates back to early human history. This was generally accompanied by the idea that other planetary systems would be much like our own. It’s only in the last few decades that we’ve had real evidence of planets around other stars, known as exoplanets. The first extrasolar planet was discovered around a pulsar in 1992 and the first “hot jupiter” was discovered in 1995.
Most of the known exoplanets have been discovered by the amazing Kepler spacecraft. Kepler uses the transit method, observing stars over long periods of time to see if they dim as a planet passes in front of the star. Since then, astronomers have found more than 1700 exoplanets, and 460 stars are known to have multiple planets. Most of these stellar systems are around main sequence stars, just like the Sun. Leaving us with plenty of systems for comparison.
So, is our Solar System normal? Planets in a stellar system tend to have roughly circular orbits, just like our Solar system. They have a range of larger and smaller planets, just like ours. Most of the known systems are even around G-type stars. Just like ours.….and we are even starting to find Earth-size planets in the habitable zones of their stars. JUST LIKE OURS!
Not so fast…Other stellar systems don’t seem to have the division of small rocky planets closer to the star and larger gas planets farther away. In fact, large Jupiter-type planets are generally found close to the star. This makes our solar system rather unusual.
Computer simulations of early planetary formation shows that large planets tend to move inward toward their star as they form, due to its interaction with the material of the protoplanetary disk. This would imply that large planets are often close to the star, which is what we observe. Large planets in our own system are unusually distant from the Sun because of a gravitational dance between Jupiter and Saturn that happened when our Solar System was young.
Although our Solar System is slightly unusual, there are some planetary systems that are downright quirky. There are planetary systems where the orbits are tilted at radically different angles, like Kepler 56, and a sci-fi favorite, the planets that orbit two stars like Kepler 16 and 34. There is even a planet so close to its star that its year lasts only 18 hours, known 55 Cancri e.
And so, the Kepler telescope has presented us with a wealth of exoplanets, that we can compare our beautiful Solar System to. Future telescopes such as Gaia, which was launched in 2013, TESS and PLATO slated for launch in 2017 and 2024 will likely discover even more. Perhaps even discovering the holy grail of exoplanets, a habitable planet with life…
And the who knows, maybe we’ll find another planet… just like ours.
What say you? Where should we go looking for habitable worlds in this big bad universe of ours? Tell us in the comments.
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Europe’s powerful Milky Way mapper is facing some problems as controllers ready the Gaia telescope for operations. It turns out that there is “stray light” bleeding into the telescope, which will affect how well it can see the stars around it. Also, the telescope optics are also not transmitting as efficiently as the design predicted.
Controllers emphasize the light problem would only affect the faintest visible stars, and that tests are ongoing to minimize the impact on the mission. Still, there will be some effect on how well Gaia can map the stars around it due to this issue.
“While there will likely be some loss relative to Gaia’s pre-launch performance predictions, we already know that the scientific return from the mission will still be immense, revolutionizing our understanding of the formation and evolution of our Milky Way galaxy and much else,” wrote the Gaia project team in a blog post.
Both of these problems have been known publicly since April, and the team has been working hard in recent months to pinpoint the cause. Of the two of them, it appears the team is having the most success with the optics transmission problems. They have traced the issue to water vapor in the telescope that freezes (no surprise since Gaia operates between -100 degrees Celsius and -150 Celsius, or -148 Fahrenheit and -238 Fahrenheit.)
The team turned on heaters on Gaia (on its mirrors and focal plane) to get rid of the ice before turning the temperature back down so the telescope can do its work. While some ice was anticipated (that’s why the heaters were there) there was more than expected. The spacecraft is also expected to equalize its internal pressure over time, sending out gases that again, could freeze and cause interference, so more of these “decontamination” procedures are expected.
The stray light problem is proving to be more stubborn. The light waves from sunlight and brighter sources of light in the sky are likely moving around the sunshield and bleeding into the telescope optics, which was unexpected (but the team is now trying to model and explain.)
Perhaps it was more ice. The challenge is, there were no heaters placed into the thermal tent area that could be responsible for the issue, so the team at first considered moving the position of Gaia to have sunlight strike that area and melt the ice.
Simulations showed no safety problems with the idea, but “there is currently no plan to do so,” the team wrote. That’s because some tests on ground equipment in European laboratories didn’t show any strong evidence for or against layers of ice interfering with the stray light. So there didn’t seem to be much point to doing the procedure.
So instead, the idea is to do “modified observing strategies” to collect the data and then tweaking the software on the spacecraft and on the ground to “best optimize the data we will collect,” Gaia managers wrote.
“The stray light is variable across Gaia’s focal plane and variable with time, and has a different effect on each of Gaia’s science instruments and the corresponding science goals. Thus, it is not easy to characterise its impact in a simple way,” they added. They predict, however, that a star at magnitude 20 (the limit of Gaia’s powers) would see its positional accuracy mapping reduced by about 50%, while stars that are brighter would have less impact.
“It is important to realize that for many of Gaia’s science goals, it is these relatively brighter stars and their much higher accuracy positions that are critical, and so it is good to see that they are essentially unaffected. Also, the total number of stars detected and measured will remain unchanged,” the managers added.
The team is also tracking a smaller issue with a system that is supposed to measure the angle of separation between the two telescopes of Gaia. It’s needed to measure how small changes in temperature affect the angle between the telescopes. While the system is just fine, the angle is varying more than expected, and more work will be needed to figure out what to do next.
But nevertheless, Gaia is just about ready to start a science session that will last about a month. The team expects to have a better handle on what the telescope is capable of, and how to work with these issues, after that time. Gaia operates about 1.5 million km (932,000 miles) away from Earth in a gravitationally stable point in space known as L2, so it’s a bit too far for a house call such as what we were used to with the Hubble Space Telescope.