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--- courses:phy100:8 [2023/11/28 22:39] – asad
+++ courses:phy100:8 [2023/11/29 00:02] (current) – asad
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 ===== - Milky Way =====
+==== - Our view of the Galaxy ====
 We are inside the disc of this galaxy, so we cannot see it from outside. But we can see different parts of the disk during different months of a year.
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 {{https://cdn.britannica.com/17/94917-050-CD521B4F/views-Milky-Way-Galaxy.jpg?nolink}}
-How did we first realize that we are not at the center of our galaxy? In 1917, Harlow Shapley located a lot of globular clusters (tightly bound clusters of thousands of stars). And he saw that the globular clusters are distributed in a halo around a point which is almost 25,000 light years away from us.
+How did we first realize that we are not at the center of our galaxy? In 1917, Harlow Shapley located a lot of globular clusters (tightly bound clusters of thousands of stars). And he saw that the globular clusters are distributed in a halo around a point which is almost 25,000 light years away from us. You can see the positions of these clusters in the side view below.
-{{:courses:phy100:harlow.png?nolink|}}
+==== - Structure of the Galaxy ====
+{{:courses:phy100:mw.webp?nolink&700|}}
-Here you see the positions of those clusters. And the center is of course the center of our galaxy where there is a supermassive black hole. The globular clusters are not located in the disk of the galaxy. They are old, they are made of old stars. Some of these clusters are as old as the galaxy itself, almost 12--13 billion years.
+The Milky Way has three main parts: a **disk**, a **bulge** and a **halo**. The disk is almost 100 kly (kilo/thousand light years) in diameter. The bulge is located at the center of the disk and the spherical halo surrounds the whole disk. The bulge is a little bit elongated like a //flattened football// and, hence, the Milky Way is a **barred spiral** galaxy. Spiral because the Galaxy has multiple **spiral arms** in the disk that spiral toward the Galactic Center (GC). The halo contains the globular clusters.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/1/12/Artist%27s_impression_of_the_Milky_Way_%28updated_-_annotated%29.jpg/1024px-Artist%27s_impression_of_the_Milky_Way_%28updated_-_annotated%29.jpg?nolink}}
+Gravity would make the whole Galaxy collapse onto itself if not for the rotation of the stars and gas around the Galactic Center. Rotation opposes gravity and makes the galaxy flat similar to the solar system. The sun and its neighbors orbit the Galactic Center at a speed of **220 km per second**. At this speed it would take **220 million years** for the Sun to complete one orbit around the center of the Milky Way.
-Our galaxy has 3 main parts:
+The Sun is **26 kly** away from the center, so it revolves around the center comparatively slowly. If you go closer to the center, your orbital speed will increase. As you go nearer, both the number of stars per cubic light-year and their average orbital speed increase. In the neighborhood of the Sun on average there are only 3 stars in 1000 cubic light-years, but near the core of our galaxy there are 10 million stars within just 1 cubic light-year.
-  - The nuclear bulge
+===== - Stars and clusters =====
-  - The disk
+==== - Star populations ====
-  - The halo
+<wrap>
+{{:courses:phy100:mw-pop-1-2-edge.webp?nolink&400|}}
+{{:courses:phy100:mw-pop-1-2-face.webp?nolink&360|}}
+</wrap>
+==== - Star clusters and associations ====
+{{:courses:phy100:open-globular.webp?nolink&600|}}
+^ Type ^ Number of stars ^ Size (diameter) ^ Location ^
+| Open cluster | Tens to a few thousand | 14 -- 40 ly | Spiral arms and disk |
+| Globular cluster | Lakhs to a million | 80 -- 320 ly | Halo and bulge |
+| Associations | Five to seventy O/B type stars | 130 -- 650 ly | Spiral arm |
+===== - Interstellar medium and traffic =====
+{{:courses:phy100:ism-obstruction.webp?nolink&500|}}
+{{:courses:phy100:density-wave.webp?nolink&800|}}
 ===== - Nebula =====
 Nebula is a cloud of gas and dust bright enough to emit radiation. But their mechanism of radiation is very different from the radiation of a star. Let us talk about the Orion Nebula and try to understand what exactly we are seeing in the picture.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/4/47/Orion_Head_to_Toe.jpg/1328px-Orion_Head_to_Toe.jpg}}
 The position of the Orion Nebula is at lower part of the Orion Constellation in this picture taken using an ordinary camera.
-{{:courses:phy100:m42-ashvin1.jpg?nolink&500|}}
 Here is the image of the Orion Nebula taken by our telescope, Ashvin II.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/f/f3/Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg/768px-Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg?500}}
 And here is the Orion Nebula through the eye of the great Hubble Space Telescope. Why is it so red? Yes, the colors are not real, and this is actually a composite image that combines visible-light and infrared radiations. But, it is still true that many nebulas look red. Why?
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 There are **reflection nebulas** as well that reflect light instead of emitting or absorbing, almost like the moon. These nebulas look blue because the starlight they reflect is blue and also dust is very efficient at reflecting blue light.
-===== - Star clusters =====
-==== - Open clusters ====
-==== - Globular clusters ====