Class Notes - Astro & Cosmo PDF

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The lecture notes are from Dr. Sheehan Ahmed's Astronomy and Cosmology class. Only Chapters 1 - 7.2 are included....

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Dr. Sheehan Ahmed ([email protected]) 01/19/2021 Week 1 What is Astronomy & Cosmology? ● Cosmology is a branch of astronomy that deals with the origin and evolution of the universe on large scales Why the Emphasis on Physics? ● The basis of both these sciences in Physics - the study of physical phenomena all around us; the study of matter, energy, motion, etc. ○ Kinematics and dynamics: how why do things move? (plants) ○ Thermodynamics: how heat behaves (the surface of the Sun) ○ Optics: how light behaves ● Electricity and magnetism: charges, fields, and the detailed behavior of light ● Nuclear physics: how the nuclei of atoms break and form: the core of the Sun ● Relativity: how things behave close to the speed of light ● Astrophysics: all these subfields applied to celestial objects Physical Quantities & Scientific Notation ● A physical quantity is a property of a system that can be quantified by measurements ● All measurements of physical quantities have TWO parts: ○ Magnitude (the size or the number ) ■ 20 feet ○ The units ■ 20 feet ● Examples of physical quantities: ○ Length (measures in meters, feet, kilometers, miles, etc.) ○ Time (measured in seconds, hours, days, etc) ○ Mass - the amount of something (kilograms, etc) ○ Temperature (Kelvin, degrees Fahrenheit, degrees Celsius) ○ Many more ● The SI (international system) system of units are the standard units used in scientific contexts ○ length: meters (m) ○ Mass: kilograms (kg) ○ Time: seconds (s) ○ temperature: Kelvins (K) ● Combined units ○ Spee: miles/hour, km/hr, meters/second ○ area: square ft (ft^2)

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It is important to have a rough idea of what these units represent in the real world. How bug is a meter? How long is one second? How much “stuff” is one kilogram? (around 2 pounds of something)

Physical Quantities & Scientific Notation ● A lot of the numbers used in science are either incredibly large (in the incredibly small) examples: convert big or small numbers into scientific notation - 52,314 = 5.2314 x 10^4 - 3.2 = 3.2 x 1 = 3.2 x 10^0 - .0000428 = 4.28 x 10^-5 - 602,000,000 6.02 x 10^8 Physical Quantities & Scientific Notation ● Apart from scientific notation certain powers of 10 also have special names called prefixes ● We can add these prefixes before the unit to indicate these powers of 10 ● E.g. 1,000 m can be 1 x 10^3 m or 1 kilometer ● In most cases, people only focus on prefixes for 10 ^(something divisible by 3) ● Can we figure out the prefixes for some common large number English words? What is a Light-Year? ● Is a light-year a unit of time? Is it a unit of distance? ● Example: flashlight - how long does it take for the light to hit the wall? ○ It feels instantaneous, ○ Called a speed of light © ○ Light is faster than sound ○ 3 x 10^8 m/s (meters/second) ● What is a car-hour? (your friend lives 2 hours away by car… is that a measure of distance or a measure of time) measures distance by the speed of your car ● Can we figure out how big a light-year is in units that we are more common terms How Big is a Light Year? ● Speed of light ● 3 x 10^8 m/s ● 365 x 24 x 60 x 60 s = 3.1536 x 10^7 s ● Distance = speed x times = ● 3 x 10^8 m/s x 3.1536 x 10^7 s ● 9.46 x 10^15 m How Many Light-Years Away? Earth is approximately… ● 8.3 light-minutes from the sun ● 320 light-years from the North Star, Polaris

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2.5 million light-years from Andromeda, our closest neighboring galaxy 4.3 light-years away from Proxima Centauri, our closest neighboring star 26,000 light-years away from the center of our galaxy, the Milky Way 13.4 billion light-years away from one of the oldest galaxies ever found, called GN-z11

Why is Studying Light Important? ● Celestial objects usually too far away to study physically (by touching, taking samples, etc) ● The only way to study them is to study the light generated by them (distance stars) or by studying the light reflected  off them (plants in the solar system reflect the light of our Sun) ● We can break down the light into its component color to make a spectrum ● The structure of the spectrum can give us a lot of information about the object ○ Chemical component Timescales in Astronomy ● The universe formed 14 billion years ago ● 4.5 billion years ago the earth and Sun was formed ● Plants were formed 2 billion years ago ● Dinosaurs were formed 0.065 billion years ago ● 200,000/4.5 billion years x 100% = 0.04% of the life of the Earth ● ●

10^7 m is about the sax of the earth 10^9 m is the earth and moon

Constellations ● The night sky appears as a dome around us with stars everywhere ○ We call this apparent dome the celestial sphere ○ Is it important to note that the stars only appear to be on a dome, all equidistant from us? But in reality, they are distances away The Changing Night Sky ● As the year goes on, the “night” part of the Earth (the side facing away from the Sun), points to different groups of distant stars ● This means over the year the stars and constellations that are visible in the night sky change

01/26/2021 Chapter 2 Light and the EM Spectrum The Scientific Method and an Ongoing Process ● What is a hypothesis? ○ Any idea that explains an observation ○ What are the general causes of the phenomenon I am wondering about? formulate hypotheses ● What is a theory? ○ The best working idea that explains a certain thing or concept ○ What is a law? ■ Same thing as a theory ● What are experiments and observations? Light, Its Properties and the Electromagnetic Spectrum The Spectrum of Light ● 300 years ago Sir Issac Newton passes sunlight through a prism ○ Other have definitely done it before but newton tried to quantify what he observed ● A band of rainbow colors comes out the other side ● White light is composed of all other colors ○ This is called a spectrum or a continuous spectrum (no color missing) ● A graph of brightness vs color is also called a spectrum Spectrum of White Light ● The colors of the optical spectrum (for what our eyes can see) ● ROY G BIV ● But if they are all light why do they have different colors? ○ To understand that we need to learn about waves What is a Wave ● A wave is a regular disturbance that travels at a constant speed through a medium or material ○ The animation shows a wave traveling through a string ● There are many kinds of waves; waves on a guitar string, sound waves, water waves, light waved, waves on a rope ● Main properties: wavelength (人), frequency (f), wave speed (v), amplitude (A) ●

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Frequency ƒ, is how often the wave “waves.” How often the source of the wave produces a bump. Measured in numbers per second or Herts (Hz) ○ 3G Hz = 3,000,000,000 per second (the speed of the phone) ○ If the frequency increases the wavelength decreases and vise versa Wavelength , is the distance between each subsequent bump. This is measured in meters (m) ○ How many bums per second, the distance of each bump?

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v is the speed of the wave or how fast the wave travels forward v = ƒ (multiplied by) 人 ○ Why is the product of ƒ and 人? Can you explain it in words? ○ V is fixed light in a particular material. So if you increase frequency (wiggle faster), what happens

Light as a Wave ● Light is also a wave. ● But what is “waving”?! (what if) there isn’t a rope? What about water? ● Light is unusual in that it does not need medium or material to travel ● It is a combination of strengthening and weakening electric and magnetic fields, we call it an electromagnetic wave ● Imagine electric and magnetic fields as the strengths of a magnet’s pull of a static charge (like a balloon rubbed on a rug). They can exist “invisibly”, but their are felt ● The strengths and directions of their “fields” go up and down just like the rope. And these bumps propagate forward, and this is what our eyes detect as light!!! ● Light has a fixed speed (v) through a vacuum: c = 3.0 x 10^8 m/s ● Higher frequency(f) light (bluer) has shorter wavelengths ● Lower frequency (f) light (redder) has longer wavelengths ● A larger amplitude (A) (how high/bright the light is) means brighter light ● A smaller amplitude (A) means a dimmer light ● The lower the frequency is the higher the amplitude ● Nanometers - is a billionth of a meter (thinner than a human hair) - tells us that the frequency of light is very fast -- [examples of optical or visible light] Electromagnetic Spectrum ● Our eyes only see wavelengths of about 400nm (purple) to about 700 nm (red) ○ These are incredibly ting lengths (what is nm? How many nm makes a mn?) ● However, wavelengths can be of any length, high or lower than that. Wavelengths can even be multiple cms long. Or even the size of football fields ● All of these different wavelengths of light together make the electromagnetic spectrum ● Our eyes can see the optical part of the spectrum (from a wavelength of 400nm to 700nm) ● Other creatures can see different wavelengths

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Wi-fi = 5 gHz = f - 5 x 10^9 Hz If you through sodium (cooking) salt on a burner it becomes yellow - similarly is you see a star with yellow on it then their atmosphere has sodium in it

01/28/2021 What particle do microwaves you - the electro/magnetic field Gas particles move as soundwaves If all EM waves are the Same why cab radio waves pass through Light, Its Properties and the Electromagnetic Spectrum Looking into the Past ● When we look at an object far away, light from it takes a non-negligible amount of time to reach us ● We are looking at the object as it was in the past ● So, for astronomical distance, looking farther away = looking more into the past ● 1 light year away means light takes 1 year to reach us ● Sun is 8 light minutes away

02/02/2021 Week 3 What can We do with Spectra? ● We can find the temperature of a body ● We can find the chemical composition of a body ● We can find out how fast an object is moving towards or away from us Quick Primer: Temperature ● Fahrenheit, Celsius/Centigrade and Kelvin temperature scales ● Temperature is a measure of how much the particles making up a substance”jiggle”, how much moving energy they have (kinetic energy) ● OK is lowest possible temperature (absolute zero) + -273 C or -459 F ○ Here “jiggling” stops ● Based on the speed of the particles (they faster they move the higher the temperature) Temperature Scales

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Lowest temperature is 0 K Fahrenheit is how humans feel temperature Celsius is how water “feels temperature” Kelvin is how the Universe “feels” temperature

How to Use Spectra: Finding Temperature ● The figure shows an example of a spectrum (plural: spectra) ● The y-axis is brightness (intensity) and the x-axis is wavelength ○ This graph is very limited since it only deals with the visible wavelengths. Normally we would be concerned with much wider range of wavelengths ● Blub: gives off a little bit of blue ○ Medium amount of green and yellow ○ Gives off a lot of red ○ We may mostly see white and little bits of red ○ We get all of the colors but see more red ● White light - mixture of all different colors/wavelengths ● The x-axis is wavelength ○ Graphs are very limited since it only deals with the visible wavelengths. Normally we would be concerned with much wider range of wavelengths Heating an Object Color/Wavelength of Light vs Temperature ● We want to talk about how hot an object is vs what the color of the light it gives off (or more accurately, what brightness of each color it gives off) ● In other words, we want to see how the temperature of an object affects the spectrum of light it gives off ○ Remember: spectrum isn’t only what colors of light, but also how bright each color is ● The brightest color a lightbulb can give off is infrared ● The infrared light is hitting our skin if you touch a lightbulb it is not heat ● The sun has the temperature of about 5750 6000 K (is about 10,000 F) ● We want to talk about how (continue slide) ● Trend (Sun): as the object's temperature went up the peak(brightest) wavelength got smaller ● Trend (Sirius A) Blackbody Radiation ● An LED light bulb is not a blackbody (it has different properties) ● When objects (light a piece of metal or a star) are heated, they give out a spectrum of EM radiation ● They specific “shape” of the spectrum (how bright is each wavelength) depends on the temperature

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Hot things emit more in the bluer end, (shorter end of the spectrum) cooler things more in the redder end Spectra of heated objects have a distinctive curved shape, with a peak ○ The color with the highest brightness (the peak) shifts toward smaller wavelengths as the object gets hotter (Wien’s Law) By studying the spectrum of distant hot objects, we can figure out what their surface temperature is This is how we know about the temperature of the surface of stars Wien’s Laws: the color with the highest brightness (the peak) shifts towards smaller wavelengths as the object gets hotter The higher the temperature, the smaller the peak wavelength, the bluer the blackbody is Wavelength of maximum emission in meter = 0.0029 Km / temperature of object in Kelvin

Examples of Using our Knowledge of Spectra ● Here is a common constellation called Orion (the hunter) ● On the top left you see a star named Rigel ● Can you qualitatively compare the surface temperature of these two stars? ○ Which one is hotter? Which one is cooler? ● To get an exact temperature we need to get the full spectrum Atom (Structure) - How do you explain the black light in a spectrum in the early 1800s? - Wee needed to learn what objects are made out of ● An atom is mostly empty space ● Everything is made up of tiny structures called atoms ● An atom is made up of smaller particles known as: protons and neutrons and electrons ● The nucleus contains the heavy protons and neutrons ● The electrons exist in a large cloud around the tiny nucleus ● The nucleus is about 10^-10 m (shell) → the nucleus is tiny ● The electrons shell is about 10^-10 m ● The whole atom (or the electron shells) are about 100,000 x bigger than the nucleus ● The nucleus contains 99.9% of the mass of an atom ● Electrons are a thousand times lighter than a proton or a neuton ● Protons are positively charged ● Electrons are negatively charged ● Neutrons are neutral (not charged)

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Most of an atom is empty space Speed is how much length you cover in a specific amount of time [speed = length / time]

How to Read the Periodic Table ● Each element has two numbers, the smaller number is the atomic number = number of protons ● The larger number is the mass number = number of protons + number of neutrons ● In a neutral atom the number of protons = number of electrons ● Can we figure out the number of protons, neutrons and electrons in Aluminum? (if the number is a decimal, round it to the nearest whole number) ○ Protons = 13 ○ Neutrons is 26 - 13 = 13 ● Atomic mass = # of protons + number of Neutrons ● Atomic number = # of protons

Bohr Model of the atom Atom (hydrogen) ● The Bohr model of the atom is a simple description of the atom ● It says that the electrons are in “s  hells” around the nucleus ● The larger the shell, the more energy the electron has ● Each shell is given a number n=1, 2, 3, etc. ● Energy is is compared to something that will hurt you more

02/04/2021 Bohr Atom ● To go from a lower shell to a higher shell, an electron needs to absorb energy. This can come from thermal energy (heating) or from a photon (light) ● High frequency has more energy, the one that wiggled less has low energy ● When an electron drops from a higher shell to a lower shell gives off energy (usually in the form of a photon) ● The shells have fixed energy values so only specific amounts of energies are absorbed or given out, depending on the difference of energies of the shells Emission and Absorption ● Electrons jump up to a higher energy level when they absorb a photon (or are heated) ● When electrons drop down to a lower energy level they emit a photon ● A photon can only be absorbed if its energy matches the jump energy ● So specific atoms tend to give out or absorb only structural selective colors (or wavelengths) ●

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When an electron jumps up from a low energy shell to a high energy shell, it needs to absorb a photon with a particular energy (color) that matched the energy difference of those two shells when an electron falls from a high energy shell to a low energy shell, it emits a photon with a particular energy (color) that matches the energy difference of those two shells

Emission and Absorption Spectra ● Certain sources, when heated, produce a continuous spectrum (like stars) ● Hot gas (that is only made of a few elements), only produces light at a particular wavelength (that corresponds to the energy differences between the electron shell). This is called an emission spectrum ● If a continuous spectrum passes through cold gas, the cold gas will absorb light of particular colors. This creates an absorption spectrum

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Thus by studying the emission or absorption spectrum of gas cloud, we can figure out the different atoms that are within it since different atoms will emit/absorb different specific wavelengths of light

Emission Lines From Hydrogen ● This is the emission spectrum for hydrogen. You can think of it as hydrogen’s “fingerprint” ● The different colored emissions are caused be electrons jumping between different shells (from 2 to 1, from 3 to 1, or 3 to 2, from 4 to 2, etc) ● Emissions is something → 2 (example: 3→2, 4→ 2) Lyman and Belmer Series (hydrogen Atom) ● Jumps between certain shells have special names ● Lyman series/Lyman lines: a fall to or a jump from n=1 shell ○ Energies are high ○ Photons absorbed/emitted are beyond the optical range (ultraviolet and smaller wavelength) ● Balmer (or hydrogen) series: a fall to or a jomp from n=2 shells ○ Energies are such that photons absorbed/emitted are in the visible range Emission Lines from Hydrogen ● Hydrogen series: ○ n=3 to n=2 is H-alpha (Hα) ■ This involved light at 650 nm ○ n=4 to n=2 is H-beta (Hβ) ■ This involves light at 486.1 nm ○ n=5 to n=2 is called H-gamma (Hy) ■ This involves light at 434 nm ○ N=6 to n=2 is called H-delta (Hδ) ■ This involves light at 410.2 nm Why does Hydrogen give out this Emission? Shouldn’t it stop emitting once all the Electron drops down to the Lowest Level? ● When an atom gets enough hydrogen (through light, heat, or even bumping into another atom), sometimes it can completely lose an electron

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The electron jumps out of the shells and goes free. This is called ionization But this doesn’t last. The electron is still attached to the protons in the nucleus and will come back to the shells

Ionization ● Given the chance, electrons can come back and join the atom ● When they join they tend to drop step by step back down to the lowest shells ● It drops from n=3 to n=2, we get H-alpha emission. They atom gives of a very particular wavelength of red light ○ If you see this light coming from a cloud of gas, in space you know that there is hydrogen there and that hydrogen is being ionized ● Ionized hydrogen is an indicator for star formation, since stars give out a lot of energy. Enough energy to ionize the hydrogen around them Types of Spectra Orion ● The Orion nebula is not with the naked eye ● Look at the middle of Orion’s sword that is hanging from his belt Orion Nebula ● Nedula: an interstellar cloud of gasses (in our galaxy) ● All the red in the image is mostly at 656 nm wavelengths ● Usually formed by an exploding star We Can Find Out How Fast an Object is Moving Towards or Away From Us Doppler Effect ● Examples: ambulance ○ You can hear the sirens become louder as the ambulance comes closer to you ○ The wavelengths become faster ○ Vise versa when it the ambulance moves far away from you ● When an ambulance is moving towards you, you hear the sirens at a higher pitch (higher frequency. Shorter wavelength) than normal ● When an ambulance is moving away from you, you hear the siren at a lower pitch (lower frequency, l...