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Journal 3/28/17 What do you think is beyond the Orion Spur on our map? Take a guess. Objective Tonight’s Homework To learn about the shape and structure of the Milky Way Read section 23.3 Do p 600: 9, 10 Milky Way There’s a more complete view of our local Orion Spur. Milky Way Since we live in the Milky Way, we only see part of it edge on and it becomes hard to tell its structure. From Earth, we see it only as a bright band. Even so, we can see a few features: Milky Way Since we live in the Milky Way, we only see part of it edge on and it becomes hard to tell its structure. From Earth, we see it only as a bright band. Even so, we can see a few features: spiral arm dust lanes nuclear bulge Milky Way If we could fly above the galaxy, we’d get a view more like this (make a rough sketch): Milky Way If we could fly above the galaxy, we’d get a view more like this (make a rough sketch): nuclear bulge: the bulge is composed of old population II stars. Recent evidence has led astronomers to believe that our bulge is shaped like a bar. Milky Way If we could fly above the galaxy, we’d get a view more like this (make a rough sketch): nuclear bulge: the bulge is composed of old population II stars. Recent evidence has led astronomers to believe that our bulge is shaped like a bar. spiral arms: the arms are full of young population I stars and star forming dust clouds. Milky Way If we could fly above the galaxy, we’d get a view more like this (make a rough sketch): nuclear bulge: the bulge is composed of old population II stars. Recent evidence has led astronomers to believe that our bulge is shaped like a bar. spiral arms: the arms are full of young population I stars and star forming dust clouds. The Orion Spur: this is our local section of the galaxy. A small fraction of an arm sitting between two larger arms. 99% of the stars we see in the sky are in this spur. Milky Way The Milky Way also has a “halo”. A rough sphere around it of globular clusters and occasional stars. While these clusters don’t make up even a small percent of the galaxy, we have to account for them. Milky Way One of the weird things about the Milky Way is how it spins with its spiral arms. Most things that spin in this kind of way eventually get all “wound up” because the middle spins faster than the outside edge. But our galaxy hasn’t become wound up for some reason. In fact, pretty much no galaxies seem to have wound up. Milky Way But the solution to this is even more confusing. If we make a graph of how fast stars in the arms move compared to distance, we get line “B”. The fact that all the stars go the same speed explains why the spiral arms don’t twist up, but it doesn’t make sense. According to physics, the speeds should be following line “A”. Something is making the stars move too fast and we don’t know what it is! Milky Way The only thing we can think of is that there’s a ton of mass beyond the edge of the galaxy that’s pulling on everything and keeping it moving faster than normal. Since we don’t see anything like this, we call this stuff “dark matter”. At present, we think dark matter is mostly tons of subatomic particles. Maybe planets that don’t have stars, so they look dark. Stuff like that. Milky Way It’s hard to estimate the age of the galaxy, but our best estimates are around 9 billion years. To compare, our estimates for the Universe are that it is around 13 billion years old. So the Milky Way has been around for most of the life of the Universe. Milky Way The Milky Way is the largest galaxy nearby. It has a number of smaller satellite galaxies that orbit around it, like moons around a planet. ursa minor sagittarius dwarf magellanic clouds sculptor Milky Way But this isn’t peaceful. Nearly all our satellite galaxies were almost certainly once galaxies of their own that got torn apart by the Milky Way. The Large and Small Magellanic clouds in particular likely used to be decent galaxies. All that’s left, however, are wispy collections of stars. Worktime You have the rest of class to finish your map of the local galaxy. Exit Question What kinds of stars do we find in the middle area of the galaxy? Old ones New ones Red ones Blue ones All of the above None of the above