The second dataset is from H3 (Hectochelle in the Halo at High Resolution), a ground-based survey conducted at the MMT, located at the Fred Lawrence Whipple Observatory in Arizona, and a collaboration between the CfA and the University of Arizona. Gaia has continued compiling the most precise measurements of the positions, motions, and distances of millions of stars in the Milky Way, including some nearby stellar halo stars. The first set is from Gaia, a revolutionary spacecraft launched by the European Space Agency in 2013. The new study by CfA researchers and colleagues leverages two major datasets gathered in recent years that have plumbed the stellar halo as never before. The goal has since been to study the patterns of over-densities of stars - spatially appearing as bunches and streams - to sort out the ultimate origins of the stellar halo. Through such studies, astronomers have realized that halo stars are not evenly distributed. Yet over time, astronomers have succeeded in identifying many thousands of stars that populate this halo, which are distinguishable from other Milky Way stars due to their distinctive chemical makeup (gaugeable by studies of their starlight), as well as by their distances and motions across the sky. "Unlike with external galaxies, where we just look at them and measure their halos," says Han, "we lack the same sort of aerial, outside perspective of our own galaxy's halo."Ĭomplicating matters further, the stellar halo has proven to be quite diffuse, containing only about one percent of the mass of all the galaxy's stars. The stellar halo extends out several hundred thousand light years above and below the star-filled plane of our galaxy, where our Solar System resides. "In order to learn more about galactic haloes in general, and especially our own galaxy's galactic halo and history, the stellar halo is a great place to start."įathoming the shape of the Milky Way's stellar halo, though, has long challenged astrophysicists for the simple reason that we are embedded within it. "The stellar halo is a dynamic tracer of the galactic halo," says Han. To better understand how galaxies form and interact, as well as the underlying nature of dark matter, stellar haloes are accordingly valuable astrophysical targets. In turn, that visible matter forms stars and other observable galactic structure. These halos serve as a sort of scaffold upon which ordinary, visible matter hangs. Every galaxy has its own halo of dark matter. This galactic halo is dominated by invisible dark matter, whose presence is only measurable through the gravity that it exerts. The Milky Way's stellar halo is the visible portion of what is more broadly called the galactic halo. "We now know that the textbook picture of our galaxy embedded within a spherical volume of stars has to be thrown out." "For decades, the general assumption has been that the stellar halo is more or less spherical and isotropic, or the same in every direction," adds study co-author Charlie Conroy, Han's advisor, and a professor of astronomy at Harvard University and the Center for Astrophysics. "There are a lot of important implications of the stellar halo not being spherical but instead shaped like a football, rugby ball, or zeppelin - take your pick!" "The shape of the stellar halo is a very fundamental parameter that we've just measured to greater accuracy than was possible before," says study lead author Jiwon "Jesse" Han, a PhD student at the Center for Astrophysics | Harvard & Smithsonian. The results, for example, shed light on the history of our galaxy and galactic evolution, while also offering clues in the ongoing hunt for the mysterious substance known as dark matter. The findings - published this month The Astronomical Journal - offer insights into a host of astrophysical subject areas. Now a new model based on modern observations shows the stellar halo is oblong and tilted, much like a football that has just been kicked. For decades, astronomers have thought that this cloud of stars - called the stellar halo - was largely spherical, like a beach ball. – A new study has revealed the true shape of the diffuse cloud of stars surrounding the disk of our galaxy. view moreĬredit: Melissa Weiss/Center for Astrophysics | Harvard & SmithsonianĬambridge, Mass. This artist's illustration emphasizes the shape of the three-dimensional halo surrounding our galaxy. Image: Astronomers have discovered that the Milky Way galaxy's stellar halo - a cloud of diffuse stars around all galaxies - is zeppelin-shaped and tilted.
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