Astrophysics & Meteorology Lecture 4: Stars (Part 1) PDF

Astrophysics & Meteorology Lecture 4: Stars (Part1) Physics Department- Faculty of Education Ain Shams University 4th Fundamental Science Dr. Nora Samy Outlines of Lecture: ✓ Astronomical Objects ✓ What are Nebulae? ✓ The most...

Astrophysics & Meteorology Lecture 4: Stars (Part1) Physics Department- Faculty of Education Ain Shams University 4th Fundamental Science Dr. Nora Samy Outlines of Lecture: ✓ Astronomical Objects ✓ What are Nebulae? ✓ The most famous nebulae in space ✓ Types of nebulae ✓ Stars & Stellar evolution and its life ✓ Nuclear fusion reactions & conditions ✓ Types of nuclear reactions in the core of a star  Astronomical objects A celestial body or astronomical body is every body in the outer space outside the atmosphere of planet Earth Objects of the solar system Distant space objects ✓ The Sun It is located outside the solar system ✓ Planets (inner and outer) ✓ Dwarf Planets ✓ Nebulae ✓ Moons ✓ Stars ✓ Small solar system bodies ✓ Galaxies The universe contains billions of galaxies, some smaller than ours and some larger, all containing billions of stars.  Nebulae ✓ Nebulae are the most beautiful objects in the sky. ✓ Nebulae are interstellar and irregular clouds of gas (H and He) and cosmic dust. (also medium between galaxies). ✓ Nebulae are often also regions where new stars are born. ✓ Many nebulae are formed as a result of a star exploding at the end of its life. The cosmic dust is small, but it is important: Why cosmic dust? When the gases in the nebulae are not ionized, this dust reflects the light coming from the stars in the nebulae or near them, so reflection nebulae appear.  The most famous nebulae in space Helix Nebula Eagle Nebula ▪ The planetary nebula closest to Earth. ▪ It resembles the human eye, so it is ▪ The most beautiful nebulae in space. (Pillars of Creation) called the Eye of the Universe. ▪ It was given this name due to its similarity to that of a bird ▪ It is approximately 700 light years away. ▪ The nebula is approximately 7,000 light-years away from Earth ▪ It is the result of a star exploding. ▪ This emission nebula contains a number of active stars that help generate gases and dust  Types of nebulae (3 types) Emission Nebulae ▪ Emission nebulae are produced when a hot star excites the gas near it to produce an emission spectrum. ▪ The star must be hotter than about (25,000 K). Cooler stars do not emit enough ultraviolet radiation to ionize the gas. ▪ Emission nebulae have a distinctive pink color produced by the blending of the red, blue, and violet Balmer lines. ▪ Emission nebulae are also called HII regions. HI is neutral hydrogen, and HII is ionized. ▪ In an HII region, the ionized nuclei and free electrons are mixed. When a nucleus captures an electron, the electron falls down through the atomic energy levels, emitting photons at specific wavelengths.  Types of nebulae (3 types) Reflection Nebulae ▪ A reflection nebula is produced when starlight scatters from a dusty nebula. ▪ Reflection nebulae look blue for the same reason the sky looks blue. Short wavelengths scatter more easily than long wavelengths. ▪ The dust particles must be very small in order to preferentially scatter the blue photons. ▪ Interstellar dust grains must have diameters ranging from 0.01 mm down to 100 nm or so. Dark Nebulae ▪ Dark nebulae are dense clouds of gas and dust that obstruct the view of more distant stars. ▪ Some are generally round, but others are twisted and distorted, suggesting that even when there are no nearby stars to ionize the gas or produce a reflection nebula, there are breezes and currents pushing through the interstellar medium. There are Planetary Nebulae These nebulae are emission nebulae, which are the remains of the scattering of the outer layer of a small star when it comes to the end of its life. Its outer layer scatters, and it remains like this as a scattered gas that gradually increases in size to thousands of astronomical units. A planetary nebula, so called because through a small telescope some of them look like the greenish-blue disk of a planet like Uranus or Neptune. In fact, a planetary nebula has nothing to do with a planet. It is composed of ionized gases expelled by a dying star.  Stars There are nuclear reactions that occur in the heart of the star, resulting in a lot of energy. This energy gradually moves to the surface and emerges from the surface of the star in the form of radiation that is usually visible between red and blue (close to violet), so we see it in its different colors. A star is a huge ball of plasma made up of hydrogen and helium. The huge amount of hydrogen in it undergoes nuclear reactions. These nuclear reactions lead to the release of enormous energy in the form of electromagnetic radiation that can be monitored using radio telescopes.. - The planet does not radiate itself, but rather derives its light from the star. - The reason for this is that the mass of the planet is very small compared to the star, so its gravity inside is not very high and the pressure inside it is not very high. It does not produce energy inside itself (radio nuclear), unlike a star. The solar planets appear luminous in telescopes or pictures because they reflect the sunlight falling on them  Stellar Evolution Nebulae consist of a large group of collected gases, making them the most suitable place for 1- the birth of stars. The star formation These gases are not uniformly distributed in the nebula. Some areas have low density and and birth others have high density, so their mass is greater, which causes them to attract more gases towards them, forming a ball of gases with a higher density until it reaches a sufficient mass such that the pressure inside it is very high. The temperature in its center to more than about 10 million degrees Kelvin, after which these atoms or their nuclei can melt and fuse with each other in what we call fusion nuclear reactions, thus producing energy. The start of energy production in this gas ball is considered an announcement of the birth of a new star The period between the accumulation of gases and the birth of a star takes between 1-3 million years  Stellar Evolution 2- ▪ When nuclear reactions erupt in the center of this star and it is able to The main produce energy, this energy forms pressure outward, and this opposes the sequence internal pressure formed by gravity stage of a star ▪ (after a period, an equilibrium occurs), after which this star stabilizes and becomes like the sun or any star in the sky. The stabilization process takes about 10-20 million years This star must maintain itself (i.e. constantly resist the inward pressure of gravity) because gravity never stops from the beginning of the formation of this ball until the star dies. Nuclear energy is important for the survival of the star ✓ This energy is initially produced through a series of nuclear reactions in which hydrogen is converted into helium and produces energy. ✓ 90% of the star's mass is hydrogen ✓ The transformation of this hydrogen in the core of the star takes about 10 billion years for a star like the Sun, and less than that for stars with greater mass. Why? Because the larger the mass, the higher and stronger the gravitational pressure is, and the more resistance it needs, and therefore it needs to produce much higher energy. The production of this large energy that must resist this gravity leads to the exhaustion of this hydrogen fuel in a shorter time, for example, a star that is 3 times larger than the mass of the Sun... does not take more than half a billion years, compared to 10 billion years for the Sun  When the star's core runs out of hydrogen, the nuclear reactions begin to stop, so it cannot resist gravity, and thus the star begins to collapse under the pressure of gravity. 3- Why can't helium atoms interact with each other and produce energy? The death of the Helium is possible, but its electrical charge is higher, so it resists and scatters together, and star it needs higher pressure and higher temperature, it needs 100 million degrees. It needs this additional pressure, but when this pressure occurs, after the temporary collapse of the star's core, the temperature reaches higher degrees and the helium begins to fuse, which creates a powerful energy boost, which causes this to swell all at once. When it swells like this, it turns into what is called a red giant This largest giant has a diameter 100 times larger than the diameter of a normal star, and because its energy is distributed over its large size, the temperature on its surface decreases, giving it a red color. This helium fusion process only takes a billion years. After this billion years, we exhaust the helium. What happens? The same process collapses the pressure in the core of the star and it dies Nuclear fusion reactions in stars (stellar fusion) These are atomic reactions that provide the star with energy by merging several nuclei together to form a new, larger nucleus of a completely different element. This reaction produces a large amount of energy. Conditions for nuclear fusion reactions to occur in stars 1- High Temperature: The high temperature gives the hydrogen atoms enough kinetic energy to overcome the mutual electrostatic repulsion (Coulomb repulsion) between the protons. At very high temperatures, hydrogen is a plasma rather than a gas, and a plasma is a high-energy state of matter in which all electrons are stripped from atoms and move freely. 2- High Pressure: It compresses hydrogen atoms together into a space that is very small compared to their size. Types of nuclear fusion in stars: 1. Proton-Proton Chain It occurs in small stars with a small mass (the Sun or smaller) and hydrogen nuclei fuse to produce helium gas. 2. Carbon, Oxygen and Nitrogen Cycle (CNO) They occur in massive stars. These stars synthesize carbon, oxygen, and nitrogen as catalysts to release helium. 3. Triple alpha process High-mass stars transform helium atoms into carbon and oxygen, followed by the fusion of carbon and oxygen into neon, sodium, magnesium, sulfur, and silicon. Subsequent reactions transform these elements into calcium, iron, nickel, chromium, copper, and others. Proton-Proton Chain PP Chain Hydrogen is converted into Helium in stars (Sun or smaller) 1. At temperature 4 million Kelvin: light nuclei fusion takes place, where the nuclei of two hydrogen atoms fuse to form a deuterium nucleus. 2. Then the deuterium nucleus merges with another hydrogen nucleus to form the helium-3 isotope. 3. In the last stage, the two atoms of the helium(III) isotope combine to produce a helium atom (He). This type releases a huge amount of energy (MeV) in the form of heat and radiation, which in reality represents a small amount of energy. But huge numbers of these interactions are constantly occurring, producing all the energy needed to keep the star radiating.  Stars with a mass greater than 1.1 solar masses are hot enough to fuse hydrogen into helium using the CNO cycle, a hydrogen fusion process that uses carbon, nitrogen, and oxygen as a starting point. These reactions pass through 6 steps: 1. The cycle begins with a carbon-12 nucleus absorbing a proton and becoming nitrogen-13. 2. Nitrogen-13 (not stable) decays to become carbon-13 and gamma rays 3. Carbon-13 nucleus absorbs a second proton and becomes nitrogen-14. 4. Nitrogen-14 absorbs a third proton and becomes oxygen-15. 5. Oxygen-15 decays to become nitrogen-15. 6. Nitrogen-15 absorbs a fourth proton, ejects a helium )CNO Cycle) nucleus, and becomes carbon-12. The cycle returns to its starting point Very Important: Notice also that, along the way, four protons combine to make a helium nucleus. This CNO cycle has the same outcome as the proton–proton chain, but it is different in an important way. The CNO cycle begins with a carbon nucleus combining with a proton, a hydrogen nucleus. Because a carbon nucleus has a positive charge six times higher than a proton, the Coulomb barrier is higher for this reaction than for the proton– proton chain. Temperatures higher than 16 million Kelvin are required so that significant numbers of protons are able to penetrate the Coulomb barrier of a carbon nucleus. In comparison, the proton– proton chain can make energy at temperatures as low as 4 million K. The CNO cycle therefore needs much hotter gas to operate than does the proton–proton chain. The temperature at the very center of the sun is estimated to be 15 million K, which is why the sun makes nearly all of its energy from the proton–proton chain and only a little from the CNO cycle.  Triple-alpha process Three helium nuclei (alpha particles) fuse to form a carbon-12 nucleus, releasing energy. Two helium nuclei fuse to form an unstable isotope of beryllium. Under conditions of sufficient temperature and pressure, it fuses with a third helium nucleus to form carbon. Triple alpha processes occur in stars that have accumulated large amounts of helium as a product of proton-proton chain and carbon cycle reactions.  Astrophysics & Meteorology Lecture 5: Stars (Part 2) Physics Department- Faculty of Education Ain Shams University 4th Fundamental Science Dr. Nora Samy Outlines of Lecture: ✓ Death of a star ✓ The life cycle of stars ✓ Stellar classification ✓ The H–R Diagram ✓ Stellar luminosity and Stellar Lifetime ✓ Sun star ✓ The temperature of the sun's surface  Nuclear fusion reactions in stars Balance What happens if one of these forces is greater than the other? Death of Star After the fuel is exhausted, the nuclear reactions in the star's core stop, and then the star dies A star collapses in different ways depending on its mass A star of medium mass (the Sun) It turns into a red giant at the end of its life, and when the nuclear reactions stop, it dies and is unable to maintain itself: ▪ The outer layers begin to gradually disperse and spread over millions of years, forming a beautiful shape called the planetary nebula. ▪ The core will die because it cannot resist gravity and will collapse in on itself, forming a very small mass the size of a planet It is called a white dwarf star: ▪ Mass is approximately equal to the mass of a star or half a star. ▪ Size is approximately equal to the size of a planet. ▪ Density is a million times greater than the density of an ordinary star Why is it hot even though it does not produce energy ? Because these are the remains of that heat star, so that it continues to cool gradually until it disappears and turns into a black dwarf, and this is the final death of this small star.  For massive stars (for ex., 10 times larger than the sun) It passed through the stage of the red super giant. Gravity does not stop and this star collapses in on itself. Its parts scatter over large areas of space in a very large explosion. This explosion is called a Supernova ❑ Scattered parts: form nebulae, which later contribute to the formation of new stars and planets. ❑ The core of the star: its fate depends on its mass: ❑ The core of the star: its fate depends on its mass: If the remnants are 2 to 3 solar masses: Its core is compressed strongly until its diameter becomes only a few ten kilometers, and it forms what is called a neutron star ▪ It revolves around itself at a terrible speed. ▪ It has a magnetic region around it in which interactions occur that produce radio radiation that comes to us like pulses. ▪ So it is called a pulsar star. If the final mass of the star exceeds 2 or 3 solar masses: ▪ It will turn into a black hole...its gravity will be too great to allow anything to escape from it. ▪ Even the photons (light) does not escape this attraction. The life cycle of stars  Stellar Classification Stars are different in the mass Most of them have a mass close to that of the Sun. ▪ Small Stars (smaller than the Sun): with a mass of about 0.1 solar masses. ▪ Massive Stars: with a mass of about 200 solar masses. Therefore, they differ from each other in terms of temperature, degree of brightness, color, the nuclear processes that occur within them and the fusion of elements, as well as the age of each of them. There are no stars smaller than a tenth of the mass of the Sun or larger than 200 times the mass of the Sun: Not less than a tenth of the mass of the Sun: because this mass does not create a very high pressure, so it does not raise the temperature in the center, and therefore nuclear reactions do not take place, so energy is not released inside this star. Not greater than 200 times the mass of the Sun: Because then it becomes unstable. ❖ Stars have different colors (red, yellow, blue and white). These colors directly indicate the temperature on the surface of this star. Morgan–Keenan (MK) system. Most stars are currently classified under the Morgan–Keenan (MK) system using ✓ The letters: O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). ✓ Each letter is subdivided into numbers, The colors and effective temperature ranges are: with 0 being the hottest and 9 the coolest. O – greater than 30,000 Kelvin – color description: blue B – 10,000 to 30,000 Kelvin – blue white A – 7,500 to 10,000 Kelvin – white For example, the Sun is a G star in this F – 6,000 to 7,500 Kelvin – yellow white classification and has an effective temperature of G – 5,200 to 6,000 Kelvin – yellow 5,778 Kelvin. K – 3,700 to 5,200 Kelvin – orange M – 2,400 to 3,700 Kelvin – red The H–R Diagram The Hertzsprung–Russell (H–R) diagram is a graph that separates the effects of temperature and surface area on stellar luminosities and enables astronomers to sort stars according to their diameters. ▪ A star is represented by a dot that shows the luminosity and temperature of the star. ▪ The background color in this diagram indicates the temperature of the stars. ▪ The sun is a yellow-white G2 star. ✓ The main sequence is the region of the H–R diagram running from hot luminous stars upper left to cool low-luminosity stars at lower right. ✓ It includes roughly 90 percent of all normal stars. ✓ The main sequence is represented by a curved line with dots for stars plotted along it. ✓ As you might expect, the hot main-sequence stars are more luminous than the cool main-sequence stars. ✓ Notice in the H–R diagram that some cool stars lie above the main sequence. Although they are cool, they are luminous, and that must mean they are larger and have more surface area than main sequence stars of the same temperature. These are called giant stars, and they are roughly 10 to 100 times larger than the sun. ✓ There are even supergiant stars at the top of the H–R diagram that are over a thousand times the sun’s diameter. ✓ At the bottom of the H–R diagram, stars that are very low in luminosity because they are very small. ✓ At the bottom end of the main sequence, the red dwarfs are not only small, but they are also cool, and that gives them low luminosities. ✓ In contrast, the white dwarfs lie in the lower left of the H–R diagram and are lower in luminosity than expected, (high temperatures- they are very small). Although some white dwarfs are among the hottest stars known, they are so small they have very little surface area from which to radiate, and that limits them to low luminosities. Luminosity ▪ All stars look like points of light, , no matter how big the telescope. ▪ There is a way to find out how big stars really are. ▪ If you know their temperatures and luminosities, you can find their diameters. ▪ Two factors affect a star’s luminosity: surface area and temperature. A hot star may not be very luminous if it has a small surface area, but it could be highly luminous if it were larger and had a larger surface area from which to radiate. On the other hand, even a cool star could be luminous if it had a large surface area. The luminosity (L) of a star is the total energy the star radiates in one second. ✓ Stars are spheres, and the surface area of a sphere is 4πR2 ✓ Each square meter radiates like a blackbody, the total energy given off each second from each square meter is T4. ✓ The luminosity of the star L and its relationship to the star’s surface temperature T and its radius R: L = 4R 2  T 4 The total luminosity of the star is the surface area multiplied by the energy  : Stefen − Boltzmann constant radiated per square meter 2 4 L  R   T  =     LSun  RSun   TSun  Example : Suppose a star is 10 times the sun’s radius but only half as hot. How luminous would it be? Th e star would be 6.25 times more luminous than the sun.  How can you find the diameters of the stars? ✓ If you see a cool star that is very luminous, you know it must be very large ✓ If you see a hot star that is not very luminous, you know it must be very small. Example: Suppose that a star is 40 times the luminosity of the sun and twice as hot. ✓ The luminosity of the star L and its relationship to the mass of the star M: L=M 3.5 Example: Find the luminosity od a star of 4 solar masses? 4-solar-mass star will have a luminosity of about 128 times the luminosity of the sun. ✓ Stellar Lifetime (age) τ  The Sun ✓ The Sun is the largest and most massive object in our solar system, but it is just an average-sized star among the hundreds of billions of stars in the Milky Way. ✓ The Sun belongs to the main sequence star type G ✓ The mass of the Sun constitutes about 99.8632% of the mass of the Solar System as a whole. ✓ Its shape is almost completely spherical, with the diameter at the pole differing from the diameter at the equator by only ten kilometers ✓ The PP chain dominates the fusion in the Sun and generates 99% of its energy, and the CNO cycle accounts for 1% of the energy produced by the Sun. ✓ The CNO cycle plays a minor role in our Sun, but it is essential in the life and evolution of the most massive stars. ✓ The sun rotates around its axis. It takes about a month. - The time of the sun’s rotation around its axis at the sun’s equator takes a little less than a month (25 days). - The sun’s rotation around its axis at the poles takes a little more than a month (35 days). This difference is due to the fact that the sun is a ball of gases. ✓ The Sun takes 225 million years to rotate once around the center of the galaxy.  Sun surface’ Temperature The surface temperature of the sun is calculated using the laws of thermal radiation, where the sun is viewed as a black body, which is the body that absorbs all the electromagnetic energy falling on it and re-radiates it outward with 100% efficiency. These laws are: 1- Stefan-Boltzmann Law Energy radiated by black body in unit time per unit surface area is proportional to the fourth power of the absolute temperature of the body. 2- Wien’s Law: black body radiation has different peaks of temperature at wavelengths that are inversely proportional to temperatures. The sun has an approximate black-body spectrum with most of the energy radiated (maximum energy) at a wavelength of 5 x 10 -7 m , b= 2.90 x 10-3 Km The surface temperature of the Sun is 5780 ≈ 5800 Kelvin.  Astrophysics & Meteorology Lecture 6: Galaxies Physics Department- Faculty of Education Ain Shams University 4th Fundamental Science Dr. Nora Samy Outlines of Lecture: ✓ Galaxies ✓ Classification of galaxies ✓ Interaction between galaxies ✓ Red Shift Phenomenon ✓ Blue Shift Phenomenon  Galaxies ▪ Galaxies consist of stars, planets, and vast clouds of gas and dust, all bound together by gravity. ▪ The largest contain trillions of stars and can be more than a million light-years across. ▪ The smallest can contain a few thousand stars and span just a few hundred light-years ✓ There are many galaxies in the universe, and people used to think that our galaxy is the only galaxy in the universe. ✓ Many galaxies have been observed, millions and hundreds of millions of light years away from Earth. ✓ Hubble found that galaxies are moving away from each other, and from here it began to be verified that the universe is expanding.  Classification of Galaxies Edwin Hubble studied cosmic galaxies and classified them according to their external shapes into three main categories: Astronomical statistics indicate that: 78 % 18 % 4%  It is a giant 1- Elliptical Galaxies: elliptical galaxy classified E1. It is several times Elliptical galaxies are round or elliptical. larger than our Elliptical galaxies do not have a specific axis of galaxy and is surrounded by rotation. a swarm of more than 500 They contain no visible gas and dust, and have few globular or no bright stars. clusters. It contains the oldest stars and most rare. They are classified with a numerical index ranging from 1 to 7; E0s are round, and E7s are highly elliptical. The formation rate of stars in these galaxies is very low because it contains no visible gas and dust.  Spiral Galaxies ✓ Spiral galaxies contain a disk and spiral arms of stars. ✓ Their halo stars are not visible, but all spiral galaxies have halos. ✓ Spirals contain gas and dust and hot, bright O and B stars. The presence of short-lived O and B stars alerts us that star formation is occurring in these galaxies. ✓ Older stars are near from the center. ✓ They are divided into Normal spiral and Barred spiral. The shape of spiral galaxy: 1.Nucleus: contains the oldest stars. 2.Disk (has arms): contains gas, dust, and younger stars. 3.Halo: contains globular clusters.  Spiral Galaxies Classification Normal Spiral ✓ Sa galaxies have larger nuclei, less gas and dust, and fewer hot, bright stars. ✓ Sc galaxies have small nuclei, lots of gas and dust, and many hot, bright stars. ✓ Sb galaxies are intermediate.  Barred Spiral ✓ Roughly 2/3 of all spiral galaxies are barred spiral galaxies classified SBa, SBb, and SBc. ✓ They have an elongated nucleus with spiral arms springing from the ends of the bar. ✓ Our own galaxy is a barred spiral.  Irregular galaxies They are a chaotic mix of gas, dust, and stars with no obvious nuclear bulge or spiral arms. Telescopic images show that they are irregular galaxies that are interacting gravitationally with our own much larger galaxy.  Irregular Galaxy ✓ The Large and Small Magellanic Clouds are visible to the eye as hazy patches in the southern hemisphere sky. ✓ Star formation is dramatic in the Magellanic Clouds. ✓ The bright pink regions are emission nebulae excited by newborn stars. ✓ The brightest nebula in the Large Magellanic Cloud is called the Tarantula Nebula. (its diameter is larger than 1000 light year) Galaxy Interactions: Galactic interaction or galactic collision: It is the process of two galaxies merging with each other. It is more like a merger than a collision. Scientists believe that this phenomenon was prevalent among small galaxies shortly after the Big Bang, when the distances between them were still small, and from them the large galaxies arose. As for fast galaxies, their speeds cause them to pass through each other and then separate a gain. It may also happen sometimes that two galaxies just pass each other without collision or interference occurring ❑ The collision could lead to merging, assuming that neither galaxy has enough momentum to continue on its way after the collision. If one of the galaxies is much more massive than the other, it will not be greatly affected and will maintain its shape, while the smaller galaxy will be torn apart and become part of the larger galaxy. ❑ The coalescence of galaxies takes between hundreds of millions of years and 1.5 billion years, and the calming down and stabilization phase takes much longer than that.  Redshift and blueshift Edwin Hubble was able to use the Doppler effect to discover that our neighboring galaxies are shifting from the Milky Way, and through it he concluded that the universe is expanding, in what is called the universe's expansion. Redshift and blueshift describe the change in the frequency of a light wave depending on whether an object is moving toward or away from us. Blueshift phenomenon: The movement of the celestial body in space towards the observer (approaching), and the blue shift is determined by studying the spectrum of light radiated from the body, through certain telescopes or using a spectrometer. Redshift phenomenon: The celestial body moves away from the place from which it is observed, and if the body moves away from the eye or lens of the observer on Earth, the light expands and descends to the red part in the layers of light, and this phenomenon is known as redshift.