- 2908 - MILKY WAY GALAXY - learning so much more. Our galaxy is old, nearly as old as the universe itself. But it didn’t start as a spiral of stars around a peanut-shape middle. It grew over time, both accumulating stars from collisions with other galaxies and forming stars itself from inflowing gas.
--------------- 2908 - MILKY WAY GALAXY - learning so much more.
- We need to learn. We came out of Africa walking to Europe then crossed the ocean to America. Now we have gone to the Moon and sent robots to Mars. We humans are bound to follow those robots very soon. Then the same will occur to the moons of Jupiter and Saturn. Asttonomy is how we learn?
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- To piece together the past astronomers examine globular clusters. These ancient balls of densely packed stars were pawns of history, accompanying satellite galaxies as they were subsumed into the Milky Way. Now they largely orbit outside our galaxy’s disk in the stellar halo. Astronomers know of at least 150 of them.
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- Many globular clusters formed right where they are, but the orbits of a subset of them suggest they are strangers in a strange land. These clusters don’t have as many elements heavier than hydrogen and helium, known to astronomers as metals, indicating their origin in smaller galaxies rather than the larger and metal-enriched Milky Way.
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- The Milky Way's merger history is a history of collisions. First came a galaxy named “the Kraken,” which collided with our galaxy around 11 billion years ago. Though not the most massive satellite that the Milky Way encountered, it was the most significant with respect to our galaxy’s mass at the time. It contributed at least 13 globular clusters to the Milky Way.
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- A billion years later came a smaller galaxy whose remains are seen as a rivulset of stars called the “Helmi streams“; it brought along at least five globular clusters. Another two small galaxies joined our own in quick succession, nicknamed “Sequoia” and “Gaia-Enceladus,” accompanied by at least three and at least 20 globular clusters, respectively.
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- The most recent acquisition was the “Sagittarius dwarf“, which joined us 7 billion years ago. In addition to the seven globular clusters it brought with it, the galaxy’s remains are also visible as strung-out loops of stars circling our galaxy on nearly polar orbits.
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- “Gaia-Enceladus” was previously thought to be the biggest collision. However, the merger with Kraken took place 11 billion years ago, when the Milky Way was four times less massive. The collision with Kraken must have truly transformed what the Milky Way looked like at the time.
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- It’s possible that many other, smaller galactic collisions took place that did not contribute globular clusters. Although the galaxy collisions represent the most major events of the Milky Way’s history, they only contributed about a billion stars, about the mass of the stellar halo but a drop in the bucket compared to the spiral-imprinted disk. Most of our stars formed within our galaxy.
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- Data from “Gaia” space telescope in September 2016 and April 2018 has truly revolutionized the study of the Milky Way. It ushered in the golden age of galactic archaeology, a discipline that searches for evidence of past galactic events in the characteristics and behavior of stars and stellar populations that we see today.
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- Gaia does not only reveal details of the Galaxy’s structure. The mission creates an awe-inspiring astronomical movie reconstructing the Milky Way’s evolution to the past and future over billions of years.
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- The Milky Way is a galactic cannibal. Astronomers have suspected this since the 1990s that the Milky Way was born out of collisions between smaller galaxies that had taken place over the billions of years of its history.
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- Earth-based telescopes, such as the “Sloan Digital Sky Survey“, provided first hints of the galaxy’s violent past, but it wasn’t until Gaia that astronomers could really deconstruct the processes that led to the creation of the Universe that surrounds us.
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- In 2018 astronomers discovered that a group of 30,000 stars moves in a synchronized way through the neighborhood of the Sun in the opposite direction to the rest of their sample of seven million stars. This atypical motion pattern matched what the scientists had previously observed in computer simulations modeling galactic collisions and mergers.
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- These stars also stood out in the so-called “Hertzsprung-Russell diagram“, which compares the color and brightness of stars, indicating that they come from a different stellar population, that is, from another galaxy.
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- Further analysis confirmed that the stars, which now form part of the Milky Way’s so-called inner halo and the outer layer of the galactic disc, must have originated from another galaxy.
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- This galaxy, since nicknamed “Gaia-Enceladus“, must have collided with the Milky Way some 10 billion years ago. About the size of one of the Magellanic Clouds (two satellite galaxies roughly ten times smaller than the current size of the Milky Way), Gaia-Enceladus smashed into and was gradually devoured by the Milky Way, which was at that time only four times bigger than Gaia-Enceladus. The collision must have profoundly shaken the Milky Way.
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- The Milky Way’s violent history didn’t end with Gaia-Enceladus. In 2019, astronomers discovered in the Gaia data a signature of another collision with a smaller galaxy, since then nicknamed “Sequoia“, which had hit the Milky Way’s galactic disc not long after Gaia-Enceladus.
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- Gaia has also shed light on the interactions with the dwarf galaxy Sagittarius, which has been orbiting around the Milky Way’s core for billions of years. Discovered in the 1990s, Sagittarius contains only a few tens of millions of stars (compared to the Milky Way’s hundreds of billions), which makes it 10,000 times less massive than the Milky Way.
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- As the Milky Way’s gravity pulled Sagittarius closer, the smaller galaxy started smashing through the Milky Way’s disc. That happened at least three times in the past: some five or six billion years ago, two billion years ago, and one billion years ago.
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- With each collision, the Milky Way stripped stars from Sagittarius, leaving the dwarf galaxy smaller and smaller. Eventually, Sagittarius will be completely devoured by the Milky Way. Yet, the dwarf has a profound effect on its larger cannibal.
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- The perhaps most curious aspect of Sagittarius’ interaction with the Milky Way was found that in the wake of each Sagittarius’ crash through the Milky Way’s disc, stars formation in the galaxy accelerated. One of those periods roughly coincided with the formation of the Sun and the Solar System some 4.7 billion years ago. That’s us!
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- The researchers believe that each collision caused ripples in the interstellar medium, like a rock thrown into water. As a result, the concentration of gas and dust in some areas of the Milky Way increased to the level that triggered star formation.
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- After an initial violent epoch of star formation, partly triggered by an earlier merger, the Milky Way had reached a balanced state in which stars were forming steadily. The galaxy was relatively quiet. Suddenly, Sagittarius fell in and disrupted the equilibrium, causing all the previously still gas and dust inside the larger galaxy to slosh around like ripples in water.
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- The research builds on a 2019 study which calculated the true size and brightness of millions of stars observed by Gaia. From the information about the size and brightness, astronomers could deduce the age of individual stars. The study concluded that star formation in the Milky Was had been declining since its formation until about five billion years ago when it suddenly ramped up.
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- Up to half of the total mass of all the stars ever created in the Milky Way’s thin disk, which contains most of the Galaxy’s stars, was produced during this period.
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- Before Gaia, astronomers were limited in their attempts to study the structure of the Milky Way. They knew that the Milky Way was a so-called spiral galaxy, a pancake-like disc of stars with a pattern of spiral arms rotating around a much denser core.
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- The spiral arms are areas of densely packed gas and stellar matter that appear, when observed in other galaxies, in bluer shades than the rest of the disc, which indicates the hotter temperature of the stars within them. Since hot stars are massive stars and since massive stars are young stars, astronomers can tell that spiral arms are areas where stars are being formed.
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- The Sun is located in one of the two smaller arms, named the “Orion arm“, some
26,000 light years away from the Milky Way’s centre, completing one rotation around the center in about 230,000,000 years.
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- Many astronomers believe that spiral arms are short-lived structures caused by some sort of gravitational instability and that they disappear within a couple of rotations and then re-emerge with some different pattern.
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- Spiral arms do not contain the same stars throughout their billions years of existence. Spiral arms are like traffic jams, areas where stars concentrate while waiting to squeeze through some sort of a bottleneck. Stars are moving out at the front but the traffic jam stays because stars are piling up at the back.
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- The Milky Way is constantly being disturbed by other bodies, dwarf galaxies and stellar clusters, that orbit around it. These interactions leave lasting imprints on the Milky Way, which can be observed hundreds of millions of years later.
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- For example, in 2018, astronomers found a group of millions of stars following a snail-shape position and motion pattern in the Milky Way’s disc. Their calculations revealed that this ripple was most probably a result of one of the past collisions with the Sagittarius dwarf galaxy.
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- The periodic collisions with Sagittarius are believed to have had a profound effect on how stars move in the Milky Way. Some even claim that the 10,000 times more massive Milky Way’s trademark spiral structure might be a result of the crashes with Sagittarius.
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- The Milky Way is also constantly stripping stars from dwarf galaxies and stellar clusters with which it interacts. Based on Gaia data, astronomers have identified streams of stars ripped from these bodies that frequently stretch over distances of thousands of light-years, covering a large portion of the sky above our heads.
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- Data about such streams can help astronomers assess the gravitational force and therefore the mass distribution of the Milky Way and hence reveal how galaxies acquire stars.
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- Gaia has also found stars in the Milky Way’s disc that are traveling at such high speeds that they might be able to escape the galaxy’s gravitational pull or might have been expelled from other galaxies and subsequently captured by the Milky Way.
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- Of the seven million Gaia stars with full 3D velocity measurements, astronomers found twenty that could be traveling fast enough to eventually escape from the Milky Way. But rather than flying away from the galactic center, most of the high-velocity stars we spotted seem to be racing towards it.
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- It is possible that these intergalactic interlopers come from the “Large Magellanic Cloud“, a relatively small galaxy orbiting the Milky Way, or they may originate from a galaxy even further away.
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- Stars can be accelerated to high velocities when they interact with a supermassive black hole. The presence of these stars might be a sign of such black holes in nearby galaxies. But the stars may also have once been part of a double-star system in another galaxy, flung towards the Milky Way when their companion star exploded as a supernova.
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- If these stars do come from other galaxies, they would carry the imprint of their place of origin. They could provide unique insight into distant universes that otherwise would be much more difficult to study.
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- The astronomers admit that these sprinting stars could be native to our Galaxy’s halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way during its build-up history.
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- A star from the Milky Way halo is likely to be fairly old and mostly made of hydrogen, whereas stars from other galaxies could contain lots of heavier elements. Looking at the colors of star’s elements tells us more about what they are made of.
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- The Sun is surfing a mysterious wave of gas in the Milky Way. In 2019, scientists discovered that interstellar gas clouds in the Sun’s galactic neighborhood form a 9,000 light-years long wave that undulates about 500 light-years above and below the galactic disc. The 400 light-years wide wave is part of what astronomers describe as the “Local Arm“, a small spiral arm of the Milky Way close to the Sun.
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- Interstellar gas clouds interest researchers because they give birth to stars when they collapse. Prior to the 2019 discovery, it was believed that such clouds in the solar neighborhood are concentrated in the so-called “Gould Belt“, which is a ring of young stars, gas, and dust which arches above and below the galactic plane.
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- Instead, this is the largest coherent gas structure we know of in the galaxy, organized not in a ring but in a massive, undulating, narrow and straight filament. The Sun lies only 500 light-years from the wave at its closest and almost appears as if it is surfing it. In fact, according to existing models, the Sun crossed the wave only some 13 million years ago and will cross it again and again in the future.
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- The Wave’s existence is forcing us to rethink our understanding of the Milky Way’s 3D structure. The scientists do not know what caused this unexpected, undulating shape. A past collision with a massive body, for example a dwarf galaxy, could be a possible explanation but further studies and more Gaia data are needed to really get a good understanding.
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- Early this year, 2020, scientists released a data-based picture of our solar system floating at the edge of a towering wave of dazzling molecular clouds and stellar nurseries. The results smashed a venerable model of our spiral galactic arm.
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- Stellar nurseries reveal how stars are born, while black holes represent their final annihilation. Astronomers are ushering in a new way of seeing our Milky Way galaxy.
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- The Milky Way’s center is home to more than 100,000 supernovae, hinting that the region must have undergone an intense period of star formation in its past. Today, this area is still packed with stars that formed early in our galaxy’s lifetime.
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- Supermassive black holes, found at the center of most galaxies, are nature’s portals to cosmic extinction. Einstein reluctantly predicted them.
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- Acquiring any photo of a black hole has long been considered nearly impossible. To do so required a telescope as big as Earth itself. That telescope was the “Event Horizon Telescope” (EHT), named for the one-way boundary of a black hole that separates two realities: ours and the unknown.
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- Within the event horizon, light cannot escape. This means we can never see what’s hiding at the center of the black hole. But we can see the “shadow” of the black hole, a dark central void of light, surrounded by a brilliant ring of gravitationally lensed photons, which are emitted by the hot gas swirling around the black hole.
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- To see that silhouette, the EHT used a network of eight radio observatories around the globe in 2017 to create a virtual Earth-size antenna. The image EHT produced was groundbreaking. The center of M87 is big enough to envelop our entire solar system.
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- But M87 is far enough away that its black hole matches the apparent size in the sky of our own, smaller black hole, Sagittarius A*. 1995 doctoral thesis at MIT focused on Sgr A*, which, at that point, had not yet been definitively identified as a black hole. In the ensuing decade, near-infrared imagery detected several stars moving in tight orbits around Sgr A* at speeds up to 11 million mph, or 1/60 the speed of light. Only the incredibly dense mass of a black hole could explain their motion.
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- In 2007, the team went back and observed Sgr A* again, this time with an additional array in California. The “Atacama Large Millimeter/sub millimeter Array” was crucial to the success of the EHT. Its 12-meter-wide antennas are some of the most precise ever built, and the signals from many dishes can be combined to mimic the behavior of a single dish with a width equal to the distance between the farthest two antennas.
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- In 2008, with the same arrays in California, Arizona, and Hawaii. The system was a multi-site telescope based on Very Long Baseline Interferometer worked at the very short wavelengths (1.3 millimeters, rather than the traditional 3.5 mm) needed to image Sgr A*, which has an angular size of only about 20 microarcseconds on the sky.
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- In radio astronomy, observing at shorter wavelengths (higher frequencies) produces higher-resolution, sharper images. That, in turn, allows astronomers to image smaller and smaller objects.
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- In 2009, they produced conclusive evidence of a shadow-sized structure in the M87 galaxy in Virgo. The resulting 2012 paper, along with the previously published 2008 results on Sgr A*, clinched the launch of the modern EHT.
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- The elliptical galaxy M87 sits 55 million light-years away. Its central supermassive black hole is the first such object ever imaged, after years of efforts by the EHT. The black hole is also the source of the 8,000-light-year-long jet blasting from the center of the galaxy.
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- M87 is a supermassive black hole that weighs about 6.5 billion Suns, as opposed to Sgr A*, which weighs about 4.5 million Suns. But based on M87’s distance of 55 million light-years, its silhouette is comparable in visual size to Sgr A*, which has a diameter akin to Mercury’s average distance from the Sun, but is over 2,000 times closer, about 26,000 light-years away.
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- For any successful EHT campaign, all observing locations must simultaneously experience perfect weather. As Earth rotates, consecutive sites pass off duties to maximize observing time. The rotation effectively sweeps the sites across different areas of the virtual dish, enhancing the quality of collected data.
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- In April 2017, five crystalline nights out of 10 at all six sites, Hawaii, Arizona, Mexico, the South Pole, Spain, and Chile, yielded pristine data on both M87 and Sgr A*. Petabytes of data stored on hard disks from all stations were physically shipped to supercomputers in Boston and Bonn for intense processing and correlation.
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- M87’s black hole emerged. Predictions of what a black hole would look like, all based on his general theory of relativity, were remarkably similar to what the EHT produced.
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- By March 2020, there were murmurs at the BHI that a separate image of Sgr A*, based on the 2017 data, might be forthcoming. And, like old-time cameras, the EHT needs its subjects to sit still for a sharp exposure. The plasma whirling around Sgr A* can change shape in a matter of minutes. That makes it hard to photograph. “The fact that Sgr A* moves means we will have to model its motion in order to image its structure, in effect, making a movie
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- Also complicating matters is the 156 trillion miles of material in the Milky Way’s disk that’s between the black hole and Earth. The intervening gas that lies between us and Sgr A* has a blurring effect even if Sgr A* were to remain still.
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- Astronomers get around the blurring largely by observing at higher frequencies where the effects of the blurring decrease. The team is developing specialized algorithms designed to further mitigate the smearing effects to create a clear picture.
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- M87’s black hole, however, is 1,000 times larger and more stable than Sgr A*and is too huge to change profile in a single night. If all the stellar-mass black holes detected through gravitational waves were scaled to the size of gumdrops, M87’s black hole would loom next to them as a gaping mouth a mile wide.
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- Next year’s observing campaign may reveal how much M87’s black hole has changed in four years. This year’s observing run was cancelled due to COVID-19.
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- In 2006, astronomers retrofitted six of the SMA’s eight dishes for shorter wavelengths using super cooled (4 degrees Kelvin) atomic clocks and receivers to create a single virtual dish. Adapting arrays originally designed for 3.5 mm wavelengths to 1.3 mm was crucial because of the need for sharp, high-resolution images.
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- There are various absorption lines that render Earth’s atmosphere opaque to radio waves, but there are ‘windows’ in the atmosphere that you can peek through, and the windows happen to be at these preferred wavelengths of 3.5 mm and 1.3 mm.
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- By observing at the shorter 1.3 mm wavelength with a telescope one Earth diameter across, the EHT grabs the minimum resolution required to see something the size of Sgr A* in the sky, akin to spotting a softball on the face of the Moon from Earth. 1 mm wavelengths not only penetrate the water vapor rampant in Earth’s atmosphere, they also pierce galactic dust.
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- Chile’s Atacama Large Millimeter/sub millimeter Array (ALMA) 64 dishes into one 85-meter virtual dish. Called the “ALMA Phasing Project“, it digitally synced sine waves from all dishes into one correlated signal. This is necessary to compensate for minute differences in the time it takes radio waves to arrive at each dish, since each is at a slightly different distance from targets in the sky.
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- Banking on its success with 1.3 mm observations, the EHT team is now moving to the even shorter wavelength of 0.87 mm which will improve resolution by 50 percent.
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- In 2021, three new sites in Greenland, France, and Arizona, with increased bandwidth and capable of seeing ever shorter wavelengths, will join the existing eight to obtain sharper images. But observing such short wavelengths also pushes radio antennas into a danger zone, where they are more vulnerable to dish surface imperfections, atmospheric aberrations, and receiver limitations. It makes the observations even more challenging.
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- And even these “short” radio wavelengths are still long compared to infrared, visible, or X-ray light observed by other instruments. These are very low-energy photons of the light they are trying to capture. When the EHT observes Sgr A* for an entire day, all the radio waves we collect have a total energy equivalent to a mosquito landing on your arm.
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- In 2019, a team of scientists started looking closer to home in our own spiral arm of the Milky Way. Their prime tool was “Gaia“, a European Space Agency survey telescope capable of measuring distances and positions of stars with unprecedented accuracy.
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- Spinning in space 930,000 miles beyond Earth’s orbit, Gaia is parked at the L2 Lagrange point, where Earth and the Sun’s combined gravity balances the spacecraft’s own orbital centrifugal force.
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- With an all-sky view of the Milky Way, the SUV-sized instrument uses a collection of mirrors, diffraction gratings, and CCD sensors to measure the distance to more than 1 billion Milky Way stars, about 1 percent of our galaxy’s estimated total. It does so with high-tech precision using a low-tech method devised by the Greek astronomer Hipparchus in 189 B.C. to measure the distance to the Moon: “parallax“.
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- By measuring each star’s relative position, brightness, and color up to 70 times over the course of its multi-year mission, Gaia is generating an encyclopedic map of the Milky Way.
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- Using recent Gaia data releases, along with observations from ground-based telescopes, astronomers discovered a stream of interconnected stellar nurseries snaking in and out of the plane of our galaxy’s spiral-armed disk.
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- Called the “Radcliffe Wave“, the sinusoidal string of molecular clouds is 9,000 light-years long and 400 light-years wide, dips some 500 light-years above and below the galactic plane just beyond our solar system, within 500 light-years of the Sun.
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- Some calculations indicate we may have even “surfed” through the Radcliffe Wave 13,000,000 years ago.
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- This discovery has shattered a venerable astronomical model. Called “Gould’s Belt“, the model envisioned the familiar band of molecular clouds that runs through Orion and neighboring constellations as an expanding ellipse surrounding our solar system. In that visualization, the Sun sat within a ring of star-forming clouds.
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- Seeing through the clouds? The trouble with molecular clouds is that they are not a single, solid object, but sprawling features made of nebulous gas. That makes it hard to measure the distances to these stellar nurseries.
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- The astronomers combined Gaia data with star color information from ground-based telescopes, which hold clues about intervening material between Earth and those stars. Molecular clouds cause the stars behind them to appear both dimmer and redder in color than they naturally are called extinction and reddening, respectively.
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- The Radcliffe Wave may have left its mark on Earth when we last passed through, perhaps in the form of iron-60, a radioactive atomic isotope born in supernovae.
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- In 2021, EHT will train its giant eye on the heavens once more with a vastly improved network of radio arrays. Its three new sites will substantially multiply the effectiveness of the global array: Doubling the number of telescopes quadruples the number of measurements possible, since VLBI relies on baselines between pairs of telescopes.
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- The team is already talking about making movies of a black hole or adding a space-based radio telescope to the mix someday. Meanwhile, they are refining Sgr A* data from 2017 and 2018, ramping up for next year and preparing to stun the world again, perhaps this time with a super-sharp snapshot of our very own black hole.
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- Working with both the infinitesimal and the infinite is inherent to modern astronomy.
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- Whether astronomers are training their eyes on nearby stellar nurseries or faraway black holes, their discoveries can help us better piece together the galaxy in which we live forever on the inside, looking out.
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- We are standing o the shoulders of giants. Peering onto the unknown with curiosity and the obsession to learn more.
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- November 21, 2020 2908
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--------------------- --- Saturday, November 21, 2020 ---------------------------
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