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--- courses:phy100:9 [2023/04/15 09:15] – asad
+++ courses:phy100:9 [2023/12/08 08:04] (current) – asad
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-====== 9. Stars and their life ======
+====== 9. Galaxies and Black Holes ======
-Stars are nothing but spheres of hot gas, so hot that their centers work like nuclear reactors. Almost 90% of a star is hydrogen, the remaining 10% is helium. There are heavier elements (carbon, nitrogen, oxygen, iron) but in such small quantities that they do not even add up to a fraction worth mentioning here.
-There could be as many as 1 septillion (24 zeros after 1) stars in the universe inside all the galaxies.
+===== - Galaxy types =====
+A galaxy typically has 100 billion stars spread over 100 kly (kilo/thousand light years).
-===== - Star types =====
+Most galaxies are shaped liked footballs and are called elliptical galaxies. Some are more elongated than others. Some very big ellipticals can harbor as much as 1 trillion stars, but they are a minority. Elliptical galaxies do not have much interstellar gas, therefore they cannot form that many new stars and they are old.
-All stars have the same shape: spherical. So they cannot be classified based on their shape. Stars are classified based on their mass. The mass, radius, temperature and brightness of a star are all interrelated. Bigger stars have more mass and temperature and vice versa.
-{{:courses:phy100:star-types.jpg?nolink|}}
+{{ https://upload.wikimedia.org/wikipedia/commons/thumb/8/85/Hubble_-_de_Vaucouleurs_Galaxy_Morphology_Diagram.png/1024px-Hubble_-_de_Vaucouleurs_Galaxy_Morphology_Diagram.png?nolink }}
-Stars are labelled by the nice and simple letters O, B, A, F, G, K and M depending on their temperature, mass and size. The biggest, heaviest and hottest star is a O-type star. As you go from O to M, radius, mass and temperature decreases.
+Spiral galaxies, on the other hand, have a lot of interstellar gas and many internal structures, spiral arms being the most prominent. They can form new stars continually. They are not necessarily younger than the ellipticals, but they still have enough gas to remain active. Spiral galaxies are divided into three types depending on the shape of the central region and the compactness of the spiral arms, as shown in the diagram.
-The temperature mentioned here is only the surface temperature; temperatures at the core of stars can be millions of degrees.
+There is a third main class of galaxies called the irregulars. Most galaxies in the universe are irregular. They are smaller than spirals and ellipticals and, hence, are sometimes also called dwarf galaxies. They typically has enough interstellar gas to form stars, but the gas is nor organized in any definite shape like the spirals.
-The mass is shown relative to the sun. The sun is a G-type star, so that type has given a mass of 1, all other masses are relative to this. So O-type stars are 50 times heavier than sun. The lifetime of a star also varies with mass and size. Lifetime is shown in billion and million years. O-type stars are the heaviest and biggest, so they have the shortest lifespan of only 10 million (10M) years. On the other hand M-type stars can live as much as 100 billion (100B) years.
+Many irregular or dwarf galaxies are found close to big spiral and elliptical galaxies, sometimes orbiting around the bigger galaxies like satellites. Streams of gas bridging them with their parents suggest they might be leftovers from the formation of the parent galaxies or from violent mergers and close encounters of multiple galaxies.
-O-type stars can have surface temperatures of 40 thousand (40k) degrees celsius. They are more than 7 times bigger than the sun. The sun has surface temperature of only around 5k degrees as shown above. The types are applicable to a range of temperatures and sizes, not just a specific temperature and size. All stars having a temperature of more than 30k are actually called O-type; I have specified 40k just as an example because we are not interested in memorizing exact numbers in this course.
+Our Milky Way has many companion dwarf irregular galaxies, the most prominent among them are the Large and the Small Magellanic Clouds, named after the Portuguese explorer Ferdinand Magellan.
-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. Sun is an ordinary average star.
+===== - Active galaxies =====
-===== - Interior of stars =====
+Before delving into the birth and evolution of galaxies, we have to familiarize ourselves with one last type of galaxies which is very special, the active galaxies. Almost every galaxy has a supermassive black hole (millions of times heavier than the sun) at its center. In case of active galaxies, the central black hole swallows (accretes) gas and stars from its surroundings along a circular flat disk called the accretion disk.
-We only know the sun intimately. 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}}
+The black hole takes in material from the equatorial disk, uses some of it in order to get bigger and heavier and throw up the excess material via two jets ejected from its two poles. The artist's impression below shows how such an active galaxy would look like with its equatorial accretion disk and polar jets if they could be visualized at all.
-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 here. The envelope is nothing but everything between the core and the corona.
+{{ https://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Artist%E2%80%99s_impression_of_stars_born_in_winds_from_supermassive_black_holes.jpg/640px-Artist%E2%80%99s_impression_of_stars_born_in_winds_from_supermassive_black_holes.jpg?nolink&600px }}-
-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 the purpose of this course. 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.
+Unfortunately, we cannot see active galaxies in such details, mainly because all of them are extremely far away, but also because they do not emit so brightly at our visible wavelengths. Remember that if a galaxy is a billion light years away, we see it as it was roughly (although not exactly) a billion years ago. So we see active galaxies as they looked like a long time ago; many of them might have become inactive in the meantime.
-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 heat 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 following diagram shows almost all the known quasars, another name for the brightest active galaxies. Each dot is a quasar here. Our location is at the very center of the diagram and from this vantage point we have been able to observe only toward the directions indicated by the two cones; it does not mean there are no galaxies in other directions---there are quasars in every direction.
-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
+{{ :courses:phy100:2dfwedge.jpg?nolink&550 |}}
-$$ \text{Inward pull of gravity} = \text{Outward gas pressure} + \text{Outward nuclear push}. $$
+Now the distance of the quasars are indicated along the top border of the cones. Notice the density of the dots and you will realize that most quasars are around 12 billion ly away and as you travel closer to earth, the total number of quasars decreases. This tells us that all galaxies formed around 10--13 billion years ago and no new galaxy formed in the last 10 billion years. The quasars that are, for example, 5 billion ly away did not really form 5 billion years ago; they formed much earlier during the heyday of galaxies, the galactic age. This diagram lets us assign a time-range for the galactic age.
-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
+Look carefully toward the center of the diagram and you will notice a gap near the center where we do not see any quasar. Most inactive galaxies are actually located within that gap, meaning most of them are very close to us. Inactive galaxies are closer and more recent whereas active galaxies are distant and ancient. This immediately tells us that the ancient universe was much more violent, turbulent and chaotic than our current local universe. You need a lot of chaos and violence for getting so many active galaxies as you see around 12 billion years ago in the diagram. And this will nicely lead us to the next section where the formation and evolution of galaxies are discussed.
-$$ G = P + N. $$
+===== - Birth in the cosmic web =====
+How did the galaxies form in the first few billion years of our history? The answer lies in the image of the universe around a million years after the big bang as shown below. During that time the universe was nothing but a homogeneous and almost isotropic conglomeration of hydrogen and helium gas. But if it was completely isotropic (the same in all directions), its picture would not look like what you see below.
-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. If $P+N$ combined becomes greater than $G$, the star will expand.
+{{https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_cmb/12583930-4-eng-GB/Planck_CMB_pillars.jpg}}
-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 lifetime or lifespan of the sun is 10 billion years. This is the same 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.
+You see a lot anisotropies, differences in density and temperature. Here the color indicates temperature, bluer regions are colder and denser whereas the redder ones are more hot and dilute. Pay attention to the blue dots because these tiny temperature anisotropies or overdensities gave rise to all the galaxies we know. But exactly, how?
-===== - Birth =====
+Gravity is the answer. The regions where hydrogen gas had higher density were more susceptible to inward gravitational pull. Starting from a few million years after the big bang, these dense regions or clouds began to collapse under their own gravity. The rotating and collapsing clouds were gradually turned into galaxies. Compare this with the atmosphere of earth. The atmosphere is made of air and it contains a lot of clouds. Sometimes the clouds, wind and air are turned into hurricanes which look similar to spiral galaxies.
-But before death, let us talk about the birth of a star, any star. How do stars form. They form from huge molecular clouds, also called interstellar clouds because they are located in the space between stars. We have already discussed the collapse of a cloud briefly while talking about the formation of planets.
-During the formation of a planetary system, the rotating molecular cloud of gas and dust contracts and becomes more and more flat like a disk. The planets are created by fragmenting the disk into many rings. But most of the materials in the whole cloud is concentrated at the center in a gigantic sphere and a star is born from this sphere. Let us see how.
+Let us follow the metamorphosis of an overdense hydrogen patch into a galaxy step by step:
+  - An overdense hydrogen patch of the universe develops into a dense cloud millions of light years across in a hundred million years.
+  - Through chance, the cloud becomes just enough dense that it begins to collapse toward its center because its inward gravitational pull wins against the universal pressure of outward expansion.
+  - The collapsing cloud is fragmented into a lot of smaller clouds and an irregular galaxy is formed from each of the smaller clouds.
+  - The irregular dwarf galaxies gradually collide and merge with each other to form bigger elliptical and spiral galaxies.
-{{ :courses:phy100:formation.png?nolink |}}
+{{ :courses:phy100:5.1.png?nolink |}}
-This diagram shows the birth of stars from such clouds. Usually many stars form from a single cloud. A large rotating cloud almost 250 **light-years (ly)** across is shown in the first panel of the diagram. Inside this huge cloud, many smaller clouds begin to rotate and collapse on their own. The second panel in the middle shows just one of those smaller clouds.
+This scenario is called the **hierarchical clustering** theory (pictorially shown above). It has one big problem. Formation of stars is very easy to explain using the collapse and fragmentation scenario, but it does not work very well at such large scales as galaxies. Two solutions can be sought. First, maybe the extremely turbulent early period of the universe helped by creating swirling eddies akin to the eddies created by water falling into a sink. Second, an altogether different type of matter (called dark matter) might have worked as a kind of gravitational scaffolding to sustain the collapsing clouds and steer them toward the path of becoming galaxies.
-Initially the small cloud was 1 ly across, but in around 1 lakh year it contracted to a size of half a ly as shown in the middle panel. By this time the cloud has also created a core and a disk. The disk is flat because of rotation as we discussed in previous lectures. The star will form from the core. But how?
+{{:courses:phy100:galseq_d_063.jpg?nolink|}}
-In a few million years, the core of the cloud contracts enough to create a **protostar**, a sphere of gas that is extremely hot, but not hot enough to become a star. The protostar eats gas and dust from the disk, uses some of it to contract and heat up even more, and ejects the rest of the material via two jets ejected from its two poles. These jets are shown as bipolar outflows in the diagram. The disk of the protostar is now a few hundred au (astronomical unit, distance from earth to sun) in diameter.
+Whichever scenario is true, trillions of galaxies were created somehow and they are currently organized into a humongous **cosmic web** a part of which can be seen above in a snapshot from a computer simulation. Here each dot is a galaxy, the nodes are clusters of thousands of galaxies, and the filaments harbor galaxies in lesser number as well. The voids between the filaments are devoid of galaxies; they do not have enough gas to initiate gravitational collapse.
-The formation of a star from the protostar is not shown in the diagram, but you can guess what happens next. The protostar keeps contracting, shrinking. As it contracts, its temperature increases. At some point, the temperature becomes so high that nuclear reaction begins at the very center of the protostar. Then the outward push of hot gas and nuclear explosion exactly balances the inward pull of gravity. The protostar does not contract or expand anymore, begins to shine bright, looses all its jets (bipolar outflows) and becomes a star.
+{{youtube>pP-C_B8nSmw?large}}
-A real photograph of a protostar and its disk is shown on the inset of this diagram. The diagram is imaginary but the photo on the inset is real. We have been able to capture a star during its birth.
+This video gives you a glimpse of how such a simulation is created.
-===== - Life and death =====
+===== - Life and collisions =====
-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?
+However the galaxies originally formed, they must have gone through a lot of changes. We have already seen that the first galaxies were very turbulent and active, they ate material from a plate and ejected their waste via polar jets. This action was powered by black holes. The question is, did the black holes formed first or the galaxies? A chicken-and-egg problem.
-{{https://cdn.britannica.com/50/62750-050-C12B4D5F/evolution.jpg?nolink}}
+Black holes are dead stars (as will be explained in the stellar age). If stars formed after and within the galaxies, then black holes must have formed after the formation of galaxies as well. This is the 'outside-in' scenario where the galaxies form first and inside them dead stars keep eating each other until they become supermassive and occupy the central place of the galaxies. This might be true as there is a tight relation between the mass of the central bulge (central region of gas) of a galaxy and its supermassive black hole.
-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.
+What about the ellipticals and spirals? How did they form from the primordial irregulars (if this really was the case)?
-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.
+Look at the solar system and you will see that smaller objects are irregular in shape and larger objects are nicely spherical. The same could be true for galaxies. Maybe the first galaxies were small and irregular because they did not have enough mass for gravity to sculpt a spherical shape. But as more and more irregular galaxies merged, the mass increased and the giant galaxy gradually became elliptical in shape. Or, even better, maybe the irregulars became loose spirals first and then the spirals became more and more tight before turning into ellipticals.
-==== - Low-mass stars ====
+The problem is that this picture is not correct. If this was true, all irregular and loose spiral galaxies would be young and ellipticals would be old. But there are a lot of old irregulars and spirals and young ellipticals in our vicinity.
-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?
-In a nuclear reaction hydrogen is converted to helium at the core of a star. In 10 billion years sun will use up all its hydrogen at the core. So nuclear reaction will stop. N is gone, P alone cannot resist G. So gravity wins and the star begins to contract again. As the star contracts it heats up more than before.
+In reality, the evolution of galaxies is somewhat like the evolution of different biological species on earth. Humans and chimpanzees did not evolve from each other, but both of them evolved from a single ancestral species which no longer exists. Similarly, irregular, spiral and elliptical galaxies did not evolve from each other, but evolved from a single ancestral galaxy.
-After some contraction, the temperature at the core becomes so high that the star can now start another nuclear reaction, this time converting helium into even heavier elements. This nuclear burning (of helium) is more powerful than the previous one (of hydrogen). So the combined outward push of nuclear explosion and hot gas now becomes greater than inward gravity, and the star begins to expand. This expanding star becomes so big that it is called a **red giant** as you see in the second panel for low-mass stars above.
+{{ :courses:phy100:5.2.png?nolink |}}
-When the sun becomes a red giant, it will become so large that even the earth will be inside its surface. There is no turning back from the red giant phase. A star like the sun will keep expanding and at some point eject its whole envelope into space. At that point the envelope will look like a huge bubble surrounding the tiny core of the star. This is called a **planetary nebula** shown in the third panel.
+The picture gets even more complicated when we see that any galaxy can be turned into any other galaxy through various mechanisms. The diagram above shows how two spirals merge to become an elliptical. And how a spiral galaxy interacts with a dwarf irregular and becomes an even more enhanced spiral.
-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.
+{{ :courses:phy100:5.3.png?nolink |}}
-==== - High-mass stars ====
+And, even more interesting, we see that an elliptical galaxy can turn into a spiral because of its interaction with a dwarf galaxy that is passing it by. The dwarf displaces some gas from the center of the elliptical and this displaced gas spirals toward the galactic center. In a few billion years this process could convert the elliptical into a spiral.
-But if a star is more massive, the fate will be different as you see in the lower panels of the diagram above.
-At the end of its main sequence,a high-mass star runs out of hydrogen at the center just like a low-mass star. Nuclear reaction stops, gravity wins, star collapses and heats up. The temperature rises and nuclear burning of helium begins, helium is converted to carbon at the center, star becomes stable again. After a while, the star runs out helium, nuclear reaction stops, star collapses and heats up more. Now carbon is burned at the center, star is stable again. After a while, the star runs out of carbon, nuclear reaction stops, star collapses and heats up. Another nuclear reaction occurs.
+===== - Galaxy clusters and dark matter =====
+{{https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2007/07/the_bullet_cluster2/10084622-2-eng-GB/The_Bullet_Cluster_pillars.jpg?nolink}}
-This continues until the star can produce iron. Nothing heavier than iron is produced by a normal star. Note that when helium burned, the previous hydrogen was still there. When carbon burned, there was helium around it. Each heavier element was surrounded by a lighter one and you get many elements at the core of a high-mass star as shown below. The serial is like this:
-Hydrogen -> Helium -> Carbon -> Oxygen -> Neon -> Magnesium -> Silicon -> Iron.
-{{ :courses:phy100:staronion.jpg?nolink&500 |}}
-But when such a star runs out of silicon from which it could produce iron, nothing can stop its collapse. Now the collapse is violent that the star heats up a lot and because of the various elements burning in different shells surrounding the core and because of the hot gas, the star now expands violently into a **red supergiant**.
-Unlike a low-mass star, the red supergiant does not disperse into space and form a nebula. Instead, the mass of the supergiant is so heavy that when the burning of elements in the inner shells of the star stops, the supergiant collapses because of its own gravity. This collapse is violent. In a matter of hours the envelope of the star collapses toward the core. When the envelope encounters the solid core, the gaseous envelope bounces from the core in a huge explosion called a **supernova**.
-After a supernova explosion, the gas of the envelope disperses into space and gradually disappears. And the solid core encounters two different fates depending on its mass. If the core is moderately heavy, it becomes a **neutron star** and if the core is extremely heavy it becomes a **black hole**. What these things are we discuss next.
-===== - Afterlife =====
-As there was