Differences

This shows you the differences between two versions of the page.

--- courses:phy100:10 [2023/04/02 11:03] – [3. Galactic center] asad
+++ courses:phy100:10 [2023/05/20 12:29] (current) – asad
@@ Line 1: / Line 1: @@
-====== 10. The Milky Way ======
+====== 10. Universe from the Big Bang ======
-===== - The Galaxy and us =====
+===== - Big bang and beyond =====
-Our galaxy has a center with a supermassive black hole.
-{{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}}
+===== - The first three minutes =====
-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.
+===== - Birth of atoms =====
-{{youtube>3VKl3-vtXaU?large}} \\
+===== - Dark ages and cosmic dawn =====
-In this video, you see an animation of how Milky Way looks like from Dhaka during different months. Historically, only this shadowy band has been called the Milky Way, in Bangla আকাশগঙ্গা or ছায়াপথ, but the whole galaxy is also called the Milky Way now. Sometimes 'Galaxy' with an uppercase 'G' also refers to our galaxy, the Milky Way.
+===== - Dark energy domination =====
-===== - Nebula =====
-Nebula is a cloud of gas and dust bright enough emit radiation. But their mechanism of radiation is very different from the radiation of a star. Let us go back to our good old Orion Nebula once again and try to understand what exactly we are seeing in the picture.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Orion_composite1.jpg/400px-Orion_composite1.jpg?nolink}}
-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 nebulas typically look a bit red. Why?
-==== - How a nebula shines ====
-Moons shine by reflecting the light of their parent stars. But nebulas shine by not reflecting but absorbing the light of their native and neighboring stars. Let me make the point more clear.
-Massive stars emit ultraviolet radiation and the atoms in a nebula absorb that light. Let us see what happens to a hydrogen atom after such an absorption.
-{{:courses:phy100:emission-line.png?nolink&600|}}
-Simplistically speaking, a hydrogen atom has a proton at the center and an electron around the proton. The electron has some specific allowed energy levels where it can stay. After absorbing a packet of ultraviolet light, it can jump from the 1st level ($n=1$) to the 3rd energy level ($n=3$). After a while, this electron can jump back to either the 2nd level ($n=2$) or its original home, the 1st level.
-If the electron jumps to the second level, it emits a packet of light called the **H-alpha photon** and the wavelength of this light makes it red. Obviously then, inside a nebula there are a lot of 1-3-2 transitions go on giving rise to red H-alpha light. Do not ask me why it is called H-alpha.
-===== - Galactic center =====
-{{:courses:phy100:galactic-center.jpg?nolink|}}
-===== - Spiral arms =====