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--- courses:phy100:7 [2023/08/05 10:54] – asad
+++ courses:phy100:7 [2023/11/25 23:08] (current) – [3.2 High-mass stars] asad
@@ Line 22: / Line 22: @@
 Most stars in the universe are small. There are more K-type and M-type stars in the universe than there are O, B, A and F-type stars. Our little sun is an ordinary average star.
-===== - Interior of stars =====
-The only star we know intimately is the sun. As sun is a typical G-type star, we can get a very good idea about the interiors of all stars by looking at the interior of the sun shown in the following illustration.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Sun_poster.svg/1024px-Sun_poster.svg.png?nolink}}
-We have not discussed the sun in such details during the class. In reality, we will just focus on 2 features of the sun, the **core** and the **envelope**. Here you see the core at the very center and the corona as streaks of light coming out of the sun. Different layers of the envelope are also shown. The envelope is nothing but everything between the core and the corona.
-The envelope has the radiative zone, the convective zone, the photosphere and the chromosphere as you go from the center to the surface. The detail of each of these layers is not important for us. If you are interested, just note that the flares and prominences stream out of the chromosphere and sunspots are seen on the photosphere. You are all probably familiar with sunspots.
-The main point I want to discuss here is this. How does a star billions of kilometers in diameter remain stable. Why doesn't it explode because of the energy of so much hot gas cooked in the central nuclear oven? And why doesn't it collapse because of its own gravity with so much mass? Why?
-The answer is simply that, the total amount of outward pressure created by the hot gas and nuclear reactions is exactly equal to the total amount of inward pull of gravity. We can write
-$$ \text{Inward pull of gravity} = \text{Outward gas pressure} + \text{Outward nuclear push}. $$
-Let us make it simpler by using **G** for gravitational pull, **P** for the outward push by gas, and **N** for the outward push by the nuclear explosions always continuing in the core. Then the simpler form would be
-$$ G = P + N. $$
-If somehow this balance is broken, the star will either expand or collapse. If $G$ becomes greater than $P+N$ combined, the star will contract because of the inward pull of gravity. If $P+N$ combined becomes greater than $G$, the star will expand because of the outward push of gas and nuclear explosions.
-Luckily, neither of these two happened inside the sun in the last 5 billion years, and will not happen in the coming 5 billion years. So the lifespan of the sun is 10 billion years. The lifespan is similar for all G-type stars. They live for around 10 billion years. After 10 billion years, $G$, $P$ and $N$ will start misbehaving and the star will begin to die.
 ===== - Birth =====
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 We are born to die. Life is nothing but a bridge between the stations of birth and death. Same is true for stars. What happens to a star as it goes from birth to death through the bridge of life?
-{{https://cdn.britannica.com/50/62750-050-C12B4D5F/evolution.jpg?nolink}}
+{{:courses:phy100:starevolution.webp?nolink&700|}}
 It depends on the mass of the star as shown above. Key events in the life of a low-mass and medium mass star (A, F, G, K and M-types) are shown above, and key events in the life of a high-mass star (O and B-types) are shown below.
@@ Line 71: / Line 49: @@
 Let us say a low and a high mass star has formed from a nebula already. The stars are now in a stable state, so they are called **main sequence star**. What happens as next are described below for the two different stars.
-==== - Low-mass stars ====
+==== - Lightweight stars ====
 Always keep in mind that the inward pull of gravity (G) must equal the combined outward push of hot gas (P) and nuclear explosion (N) in order to keep the star stable, in the main sequence. But after living for around 10 billion years a low-mass star like the sun will run out of fuel for nuclear reaction. What is this fuel?
@@ Line 82: / Line 60: @@
 After a while, the planetary nebula will disperse into space, go away and only the tiny core will remain. When the envelope was expanding into a nebula, the core was contracting. As it contracts, it heats up even more and becomes white-hot. At that point it is called a **white dwarf**. This is the final fate of a poor star like our sun.
-==== - High-mass stars ====
+==== - Heavyweight stars ====
 But if a star is more massive, the fate will be different as you see in the lower panels of the diagram above.
@@ Line 101: / Line 79: @@
 ===== - Afterlife =====
 Stars can be three different things during their afterlife: white dwarf, neutron star or black hole. There was no time in the class to discuss these in details. So we skip these for another semester and another batch of students.
+==== White dwarf ====
+{{https://chandra.harvard.edu/photo/2018/wdac/wdac.jpg?nolink}}
+{{https://upload.wikimedia.org/wikipedia/commons/d/d2/SN2018gv.gif}}
+{{https://upload.wikimedia.org/wikipedia/commons/c/c8/New_Hubble_Observations_of_Supernova_1987A_Trace_Shock_Wave_%284954621859%29.jpg?nolink}}
+==== Neutron star ====
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Neutronstarsimple.png/768px-Neutronstarsimple.png?nolink}}
+{{https://i0.wp.com/argiub.space/wp-content/uploads/2023/07/NG_Pulsar_Timing_2021.png?nolink}}
+==== Black hole ====
+{{https://www.science.org/cms/10.1126/science.364.6437.217/asset/25d98852-18b8-4757-a45e-d8aba240e00c/assets/graphic/364_217_f1.jpeg?nolink}}
+{{https://cdn.eso.org/images/screen/eso1907h.jpg?nolink}}