- 3418 - MILKY WAY GALAXY - the more we learn? At the beginning of the 20th century, astronomers thought the Milky Way encompassed the entire universe. Astronomical understanding has raced ahead to decipher the evolution of stars, other galaxies, and the universe itself.
--------------------- 3418 - MILKY WAY GALAXY - the more we learn?
- Because the Milky Way galaxy surrounds us, understanding its structure entails observing a large fraction of the sky. In the past, this required decades of data collection and analysis. Now, thanks to advances in detector technology, analysis software, and computing power, it has become feasible to complete such surveys in a few years.
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- On August 25, 2003, NASA launched the Spitzer Space Telescope on a mission to study the universe in infrared wavelengths.
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- What do the distribution of stars and infrared-bright star-formation regions tell us about the inner galaxy’s structure, including the disk, molecular ring, number and location of spiral arms, and central bar? What are the physics of
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- When it comes to determining the distribution of stars in the Milky Way, the most straightforward technique is simply to count stars. English astronomer William Herschel (1738–1822) introduced this technique in 1765. Historically, however, star counting has led to some of the most famous wrong answers in astronomy.
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- Imagine we live in a galaxy comprising a disk of stars , and that we are about halfway from the center to the edge . As we scan its plane, we should see the most stars when we look toward the galaxy’s center. By looking at how quickly the number of stars declines as we scan away from the galaxy’s center, astronomers should be able to determine how the stellar density decreases . When we look in directions for which our line of sight skims tangent to a spiral arm, we should see an excess of stars compared to nearby directions.
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- We don’t see this? What’s wrong? The answer is one of the unpleasant four-letter words of astronomy: dust. Although the density of dust and gas in interstellar space is nearly nothing, space is vast, and a whole lot of nearly nothing adds up to something.
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- What effect does dust have? Imagine watching a terrestrial sunset. The Sun reddens and dims as it approaches the horizon. This impairs your ability to measure the Sun’s true color and brightness.
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- In interstellar space, the Milky Way’s dust is not evenly distributed but occurs in clumps and clouds. A few “holes” exist through which astronomers can observe stars to great distances.
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- One such area, called “Baade’s Window“, named for German-born American astronomer Walter Baade (1893–1960), allows a view of the Milky Way’s central bulge. But for most of the galaxy’s inner disk, dust is a barrier to understanding the distribution of stars there.
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- The barrier began to crumble with the advent of sensitive “infrared detectors“. Because infrared light penetrates dust more readily than visible light, an infrared view of the galaxy reveals more stars.
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- The “Diffuse Infrared Background Experiment” aboard the “Cosmic Background Explorer” (COBE), launched in 1989, could not resolve individual stars. However, several groups analyzed the light distribution and found evidence for a central stellar bar in our galaxy.
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- Now, using the unprecedented sensitivity of the “Spitzer Space Telescope“, astronomers can study the galaxy at the same wavelengths but with enough angular resolution to observe individual stars
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- Using the “Infrared Array Camera“, the GLIMPSE Legacy team surveyed a 130°-long strip stretching 1° above and below the galactic plane. This strip contains most of the galaxy’s stars. Unfortunately, it also contains most of the dust. The project’s goals were to take a stellar census of the galaxy and study star formation.
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- The team’s stellar census produced a catalog of more than 40 million sources in four different wavelengths. Scientists expect more than 90 percent of these sources to be red giant stars. Because red giants are so luminous, they can be seen from large distances across the galaxy.
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- By counting stars as a function of direction and brightness, GLIMPSE found the long-expected result that the number of stars increases all the way to the galactic center. While the visible-light image shows a dark sky speckled with stars, infrared images, reveal a never-before-seen bundle of stars, called a “globular cluster“.
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- This enables astronomers to measure the radial “scale length” of the galaxy, the distance one has to go from the center for the stellar density to drop by a factor of nearly two. By determining this number and studying how the galaxy’s rotation speed varies at different radii, astronomers can infer how much of the inner galaxy is in the form of dark matter, that neither emits nor reflects enough radiation to be detected.
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- GLIMPSE also has shed light on two of the major structural features of our Milky Way. First, astronomers counted the number of stars at equal angles to the left and right of the galactic center and examined their brightnesses.
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- These GLIMPSE data confirmed the galaxy has a long stellar bar. This bar is characterized by an excess population of stars called “red giant clump” stars. These stars, which shine with a fixed luminosity, were used as “standard candles” to pin down the bar’s angle and length.
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- The second surprise lurking in GLIMPSE star counts concerned our galaxy’s spiral structure. The survey’s outer regions contained two areas where astronomers expected to see an excess of stars because our line of sight skims along spiral arms at these points.
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- One of these arms, the “Scutum-Centaurus Arm“, shows a dramatic enhancement of stars in the direction expected from studies of star-forming regions in optical and radio wavelengths. But the second region, the “Sagittarius Arm tangency“, shows no strong evidence for such an enhancement. This is despite the fact that both tangencies are approximately the same distance from the Sun.
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- Although the Sagittarius Arm is clearly a major star-forming structure, it lacks any evidence of compression in the older stellar populations. Astronomers have noted this characteristic in other spiral galaxies as well because infrared light traces the old stellar populations and most of a galaxy’s stellar mass, while visible light traces recent star formation. It sometimes happens that spiral galaxies show secondary star-forming compressions between the major infrared arms. Is our galaxy such a case?
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- The band of light from the Milky Way makes our galaxy look like a crowded streak of stars and gas clouds. But today, scientists know its shape and structure in detail: the spiral arms wrapping around the galaxy’s center, its central bulge packed with stars and a gargantuan blackhole, and its fainter, fluffy halo of stars farther out, as if our galaxy were cocooned within a stellar cotton ball. They also know that an even more diffuse cloud of dark matter extends farther out still, revealed by the motions of the stars.
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- The latest telescopes, including the European Space Agency’s “Gaia” that launched in 2013, have dramatically opened up our view to most of the galaxy. Their data reveal faraway clumps and streams of stars that, like fossils, offer hints about the galaxy’s complex history.
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- Our galaxy’s disk, halo and wispy streams of stars bear evidence of this galactic collision and other long-ago events. The Milky Way’s past is comparatively peaceful. It assembled mostly by birthing new stars from cooling gas and from older stars, rather than by dragging other galaxies into its maw.
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- But even this relatively quiet history leaves plenty of remnants to sift through, including clues in the stars’ chemical make ups, since each generation of stars has new chemical signatures that lean toward heavier elements than the stars that came before them.
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- A “galactic archaeology” tries to reconstruct the history, the sequence of events, that led to the formation of the Milky Way. In archaeology, you use the remains, or the leftovers, or the artifacts, of different civilizations or events. In this case, the leftovers are stars, so we use stars to try to figure out how the Milky Way was put together. Stars remember where they came from, they have memory of their origins.
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- The way they move, their ages and their chemical compositions tell us about where they were born. If you find groupings of stars with distinct chemical compositions, for example with different relative amounts of oxygen, magnesium and iron, that may tell you they came from different environments, since stars born in the same cloud are bound to have the same chemical fingerprints.
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- Similarly, if you find groups of stars moving together through space, that tells you they have a similar trajectory, so they came from the same place. You can use that to reconstruct where they were born.
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- Near the sun, roughly half of the stars in what we call “the halo“, which hosts very old stars in the Milky Way, were rotating in the opposite direction to the vast majority of stars in the Milky Way, the other half belonging to a very puffy, disk-like component. So that was suspicious.
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- That could mean the halo was formed by a merger with another galaxy. But it wasn’t enough evidence, because maybe there are other ways you can produce these kinds of stars.
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- They looked at the ages and chemistry of those counter-rotating halo stars and found that they were following a different track than those of the vast majority of stars in the Milky Way. The chemical composition of those halo stars tells you they were born in a different, smaller system.
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- The Milky Way and its components, viewed edge-on, with the spiral arms lying in the band of light stretching across the middle has the densest concentration of stars and a supermassive blackhole in its middle; the thin disk, a collection of stars extending from the bulge; and the thick disk, a less dense field of stars that lies beyond the thin disk. The dark wisps are clouds of gas and dust.
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- Further out is the stellar halo, an even more diffuse field of stars, and beyond that, the dark halo, composed of invisible dark matter. The Large and Small Magellanic Clouds, two dwarf galaxies that will eventually collide with the Milky Way.
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- That 10-billion-year-old impact with a smaller galaxy, “Gaia-Enceladus event“, perturbed the Milky Way so much that many of the stars present at the time ended up in the puffed up, or hot, thick disk.
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- The thick disk, which contains roughly one-fifth of the stars in the galaxy, probably was also formed in the event. That’s because galaxies in the past were likely gas-rich, so when you had a massive merger like that, it would pull gas clouds together, making high-density regions that trigger a lot of star formation.
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- We do see that star formation peaked at the same time as the merger, substantially growing the Milky Way’s disk. If there is a merger, dark matter makes the merger happen on a faster timescale, because there’s so much more mass in dark matter than in the stars alone.
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- Before Gaia, we had the measurements of about 2 million stars nearby, from a mission called “Hipparcos” in the ’90s. Now it’s 2 billion. The volume of space we can measure the motions in is a factor of 100 in radius larger now. And it’s a factor of 1,000 more precise. It’s a vast amount of data of exceedingly high quality.
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- We cannot consider the Milky Way as an isolated system. People used to think of galaxies as “island universes,” separated from the environment around them. That is an important change in how we approach the problem of determining the distribution of mass throughout the galaxy.
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- In the past it was often assumed that the galaxy was in equilibrium and wasn’t really changing much. Now we have the data showing us that that’s an oversimplification, since the motions of the stars near the sun are revealing the imprints of the pull of neighboring galaxies, which themselves are being pulled in by the Milky Way.
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- In a few billion years, the Milky Way will merge with the Large and Small Magellanic Clouds. Roughly a billion years after that, the galaxy will also merge with Andromeda.
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- Vast rivers of stars swirl around the Milky Way, cutting against the current of our galaxy's halo in a complex gravitational dance. A new study of these so-called stellar streams, their offbeat orbits may be the key to uncovering the troves of invisible dark matter lurking within our galaxy.
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- Observations from two telescopes to map the orbits, velocities and compositions of 12 stellar streams crisscrossing the Milky Way. “Stellar streams” are the remnants of ancient collisions between the Milky Way and smaller neighboring star clusters; when these petite neighbors come in contact with the comparatively massive Milky Way, our galaxy's gravity tugs and warps them, sometimes pulling them into spaghettified strands that orbit the fringes of our galaxy.
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- Computer models were used to unwind these stretched-out streams and determine where they originated. Based on the speed and composition of the stars in each stream, the team found that six of the streams came from nearby dwarf galaxies (small galaxies containing up to several billion stars), while the other six originated from globular clusters (much smaller gravitational-bound bodies that contain up to a few thousands stars).
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- In charting the orbital paths of these 12 stellar streams, the researchers found that the streams moved in ways that the gravity of the Milky Way alone could not explain. The streams' orbits appear to be influenced by invisible clumps of dark matter, that non-luminous substance that scientists suspect accounts for about 85% of all matter in the universe.
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- Researchers have detected more than 60 stellar streams swirling around the Milky Way to date, but they have never mapped this many of them at the same time, the researchers added. By studying the movement of multiple streams at the same time, the invisible distribution of dark matter in the Milky Way becomes easier to pinpoint.
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- Good luck with that. The more we learn the more we find we don’t know.
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January 16, 2022 MILKY WAY GALAXY - the more we learn? 3418
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