\n<\/p>\n
So it\u2019s confession time: I\u2019ve been lying to you.<\/p>\n
I\u2019ve said on many occasions that our Milky Way galaxy has a flat disk (like in this column or this one). But it\u2019s not really flat\u2014not even for a reasonable definition of the term.<\/p>\n
Now, in my defense, I wasn\u2019t lying per se; I was simplifying. That\u2019s a perfectly acceptable and even advantageous thing to do in science. When you have some complex thing that you\u2019re trying to understand or explain, it helps to make it as simple as possible so that the math and physics are easier to crack. It\u2019s like assuming, at first, that Earth is a perfect sphere or that the sun contains all the mass in the solar system. Once you work out the basic equations that describe your simplified model, you can gradually add complexity back in\u2014but in a way that makes the problem tractable.<\/p>\n
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And to be fair, when you look at the glow of the Milky Way from a dark site, it does look flat\u2014flat-ish. And we lots of similar galaxies and their disks also appear flat.<\/p>\n
But a lot of them, maybe even most of them, aren\u2019t. They\u2019re wiggly and wavy and bendy. Our galaxy is among this warped group.<\/p>\n
First, a quick overview: the Milky Way is classified as a disk galaxy, with a broad circular collection of stars, gas and dust about 120,000 light-years across. It\u2019s a few thousand light-years thick, so \u201cflat\u201d is at least a decent adjective to use for it. In the center is a central bulge of stars, and the whole thing is surrounded by a vast halo of stars and dark matter about a million light-years wide.<\/p>\n
That last bit is important. Hang on a minute, and I\u2019ll explain why.<\/p>\n
We\u2019ve known for some time that, out toward its edges, the disk of the Milky Way is warped, flared up on one side and down on the other, rather like the brim of a fedora. Research published in the journal Science in 2019, however, refined this idea considerably. The team of astronomers behind that paper used data from Gaia, a now retired European Space Agency mission that mapped the positions, motions and distances of more than a billion stars. They specifically looked at Gaia\u2019s data for some 2,400 Cepheid variables\u2014special kinds of stars that pulsate, changing their brightness. The time it takes for a Cepheid\u2019s brightness to change is related to its luminosity, the amount of energy it gives off. By comparing a Cepheid\u2019s intrinsic luminosity with how bright the star appears in our sky, its distance can then be calculated.<\/p>\n
By mapping so many Cepheids in the galactic plane, the scientists were able to trace the overall shape of the Milky Way\u2019s disk, and the warping really stands out. Our galaxy looks a bit like a vinyl LP that\u2019s been out in the sun too long (kids, ask your grandparents).<\/p>\n
What causes such a warp? It\u2019s possible that a collision with a smaller galaxy could gravitationally affect the stars in the disk, a bit like ripples in a pond after a rock is tossed in. But a team of astronomers who published their research in Nature Astronomy in 2023 had a very different idea for what\u2019s been tugging on our galaxy\u2019s brim: dark matter.<\/p>\n
As I mentioned above, the galaxy is embedded in a halo of stars and dark matter. About a year before their paper, some of the members of this team, along with other astronomers, found that the stellar halo was not spherical, as previously assumed, but instead elongated and squished a little, a bit like a slightly flattened American football. It was also tilted with respect to the plane of the galaxy.<\/p>\n
They posited that the more diffuse and nigh-invisible dark matter halo might have the same shape as the stellar halo as well. By modeling the effects of the much more massive dark matter halo if it were structured and oriented in a similar way, they found that this naturally created a gravitational field that tugged on the disk, which would explain not just the shape and size of the warp but also its orientation in the disk. While this isn\u2019t necessarily case closed, they make a pretty solid argument.<\/p>\n
But that\u2019s not the only way our galaxy\u2019s disk is off-kilter. New research shows it\u2019s also corrugated.<\/p>\n
Just this year another team of scientists used Gaia data to look at 17,000 young stars\u2014which tend to form right in the middle of the galaxy\u2019s disk\u2014and 3,400 Cepheid variables in a region of the Milky Way tens of thousands of light-years across. What they found is that, in the main disk and well into the warped outer parts, there is an up-and-down wave, a structure much like the corrugation in thick cardboard.<\/p>\n
It\u2019s similar to the \u201cwave\u201d fans do in at sports games, where they stand up and sit back down in a wave that moves around the stadium. In our case, the stars in the galaxy move up and down relative to the plane of the disk. Because so many of the stars they measured are young, the scientists think the gas in the galaxy\u2014which forms stars\u2014moves up and down as well. So whatever this is, it\u2019s intrinsic to the structure of the disk.<\/p>\n
The cause of this wave isn\u2019t known, though the most likely culprit is, this time, a collision with a smaller galaxy. One potential, even likely, guilty party is the Sagittarius Dwarf Spheroidal galaxy, a small object with a tiny fraction of the Milky Way\u2019s mass. It orbits our galaxy in a nearly vertical loop, diving through the disk as it goes. In 2018 astronomers published a paper in Nature where they found\u2014again, using Gaia data\u2014wavelike motions in six million stars within approximately 10,000 light-years of the sun, similar to the waves found in the outer disk. They suggest that the Sagittarius galaxy may have created these structures the last time it passed through the disk, several hundred million years ago.<\/p>\n
The sun also displays this motion; careful measurements show it has a vertical velocity as it orbits the galactic center. This means our solar system bobs up and down. And every so often, it moves through the disk, reaches some distance from it, and then the gravity of the disk pulls it back, and the cycle starts again. This element of our star\u2019s motion may be a part of that greater wave.<\/p>\n
So as you can see, I wasn\u2019t lying before about our flat disk. I was simply eliding over details that aren\u2019t necessary in a discussion of the overall structure of the galaxy. Yet it\u2019s certainly worth looking at these extra effects\u2014they tell us about the history of our Milky Way and can even reveal how the sun plays its part as well.<\/p>\n","protected":false},"excerpt":{"rendered":"
So it\u2019s confession time: I\u2019ve been lying to you. I\u2019ve said on many occasions that our Milky Way galaxy has a flat disk (like in this column or this one). But it\u2019s not really flat\u2014not even for a reasonable definition of the term. Now, in my defense, I wasn\u2019t lying per se; I was simplifying.<\/p>\n","protected":false},"author":1,"featured_media":32550,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[58],"tags":[11667,7052],"class_list":{"0":"post-32549","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-milky","9":"tag-warped"},"_links":{"self":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/32549","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=32549"}],"version-history":[{"count":0,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/32549\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/media\/32550"}],"wp:attachment":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=32549"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=32549"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=32549"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}