Galactic Central Discoveries and Cosmic Phenomena
Brightest star: Alpha Centauri. Retrograde motion: when Earth’s orbit crosses another planet such as Mars and then it seems as though the planet is going backward then forward again.
Galileo discovers: 4 moons of Jupiter, rings of Saturn, craters of the moon, and phases of Venus. 1st law of motion: objects change if force is applied. 2nd law: acceleration is caused by mass. 3rd law: for every reaction, equal opposite. Refracting telescope: chromatic aberration. Reflecting telescope: modern telescope, provide better resolution. Measured in arc seconds. Change in position of a nearby star (relative to distant stars) as the Earth moves in its orbit is called the ‘parallax’. Definition: apparent magnitude of an object at a distance of 10 parsecs
In star-forming regions of space, there exist diffuse clouds of gas and dust called nebulae. In general, nebulae have some small drift velocity – they are very slowly rotating. Nebulae can begin to collapse under their gravity, or this process can be initiated by a shockwave from a nearby supernova. As they collapse, conservation of angular momentum increases the rotation speed. Image credit: Pearson Education/Addison Wesley
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In 4 billion years, the Sun will turn red giant and then white dwarf. In star-forming, the inner planets are solid due to forming nearer the Sun and they captured more space dust, whereas gas giants captured more hydrogen and helium.
Structure of Sun: 71% hydrogen, 27% helium, and 2% etc. Nuclear fusion between hydrogen atoms to turn into helium converts mass into energy. Core: Hottest region where fusion takes place. Radiative zone: high-energy photons repeatedly absorbed and re-emitted by the plasma. Convective zone: Cool enough that absorbed photons are not readily re-emitted so plasma heats up and rises like a bubble. ‘Atmosphere’ L2
Physics of atmospheres: escape velocity increases with planet’s mass, the velocity of gas molecules increases with temperature. Radiation emitted by the Sun has short wavelengths; some radiation will reflect off the surface of the Earth back into the atmosphere. The greenhouse effect: when the gas density is high in the atmosphere. 
Life requires a significant atmosphere for a number of reasons:
1. Maintain sufficient temperature for liquid water
2. Maintain sufficient pressure for liquid water
3. Provide a shield against solar radiation
4. Provide breathable air or readily available CO2 for photosynthesis
Genesis impact formed Earth when Earth collided with Thea, evidence is found on the moon rock, Ar and K.
The Moon exerts tidal forces on Earth, 1/r^2. P-waves (primary) and s-waves (secondary) Earth’s magnetic field is generated by electric currents inside the outer core which causes the dynamo effect that creates a magnetic field. Gravitational collapse of nebula creates a protosun surrounded by a rotating protoplanetary disk. The inner region is very hot, so only metals/rocks with high boiling points are able to solidify (e.g., iron, silicon)
Venus, Earth, and Mars are in the habitable zone. We study planets’ interior through mass distribution and seismology. Volcanism indicates the presence of partially melted mantle beneath a thin crust.
• Seismology – P and S waves can be refracted or blocked
• Compositions of meteorites yield information about chemical abundances in the protoplanetary disk
& more – there are many pieces to the puzzle! Tidal heating: elliptical-shaped orbit results in tidal heating due to stretching and flexing, causing volcanism. The Roche limit: the distance an object orbiting a gas giant exceeds and is then disintegrated 
Sun: corona, radiative zone, convective zone
Triton is Neptune’s largest moon, retrograde orbit. Uranus has 13 rings; these rings are maintained by shepherd moons. A planet must orbit the Sun, a planet must have a spherical shape, a planet must dominate the gravitational. A dwarf planet is orbiting the Sun, has a spherical shape, doesn’t dominate gravitational neighborhood. CERES is a dwarf planet in the asteroid belt; all other dwarf planets are trans-Neptunian. Pluto and Charon are tidally locked. Dwarf planets’ orbits are much larger than the 8 planets inside.
Asteroids are minor planets found in the asteroid belt (Mars-Jupiter) made of rock and metal. They can have small moons. Some asteroids lay outside the belt in the Trojan, Greek, and Hildas zones. 433 Eros could hit Earth. Comets are orbiting the Sun and made out of ice, gas, and dust; when they get close to the Sun, they make a visible tail. Halley’s comet is 15.8 km. Centaurs are similar to comets and asteroids but no photo. Meteoroid in space, meteor when in Earth’s atmosphere, meteorite in the ground.
Exoplanets detection transit method, dimming of star measurement. Microlensing: light from distant stars curves due to space-time. Radial velocity: Stars will be slightly pulled by their planets orbiting and create a wobble effect, and then we use redshifted or blueshifted characteristics of light to see how much it has moved. Astrometry: When the star wobbles, we can notice the distance changed. Pulsars are rotating neutron stars. Direct imaging: by drowning out the starlight, we can see them. 
Spectra can be measured when a photon causes an electron to become excited and change energy levels. n=1-6 is Lyman, n=2-6 is Balmer, n=3-6 is the Paschen series. Main sequence star > red giant > asymptotic giant branch > white dwarf 
Four protons fuse, using carbon, nitrogen, and oxygen isotopes as catalysts, each of which is consumed at one step of the CNO cycle but regenerated in a later step. The end product is one alpha particle (a stable helium nucleus), two positrons, and two electron neutrinos.
When electrons and protons run out of space in a white dwarf, this results in the creation of neutrons, and a neutron star is born. Chandrasekhar limit is 1.4 solar masses for a white dwarf to go supernova thermonuclear; for a neutron star, the white dwarf must be greater than 1.4 but less than 3. A black hole occurs if gravity wins, and it is 3 solar masses. High-mass star > super red giant > carbon-oxygen core > iron core > core collapse supernova. If Silicon is present in the core, then it will be type 1a S.
we can see how old the observable universe is by looking at the cold red giants. Elliptical galaxies tend to be older than spirals because they have a lesser amount of high-mass hot blue stars. Big bang fusion, cosmic ray fission, merging neutron stars, dying low-mass star, exploding massive stars, exploding white dwarfs. Gravitational waves are waves that are emitted that bend the space-time. First detection was in 2015 with 2 black holes detected. Planetary nebula = outer layer of red giant. Star-forming nebula. The great debate: are nebulas or other galaxies? Harlow Shapley there are more galaxies, Heber D. Curtis nebulae are galaxies. Hubble: telescope, Rhodes Scholar, Cepheid variables, distance million light years. Redshift tells us that galaxies are expanding away from us at acceleration due to forces of dark energy, while dark matter holds galaxies together. 
Hubble’s constant is 75 Kmpc, 
energy (and mass, via E=mc2) at each point in space. Not diluted by expansion / ‘vacuum energy’. Whereas the density of regular matter is decreased by expansion. Eventually, dark energy becomes the dominant component… Drives accelerated expansion (again, a prediction of Einstein’s equations). Extra, unexpected ingredient. Seems to ensure that the overall universe is at the critical density. #5
The cosmic web is the structure of the observable universe that we know; it is a web design made up of many galaxies. LSST takes images of the southern sky, can see the large-scale structure in the background. General relativity, theory of gravity. Curved space-time when a larger mass has a gravitational effect. Objects follow a straight line in space-time but curves. Einstein was right because of perihelion precession of Mercury, time dilation, bending of starlight, and the existence of black holes + gravitational waves. Mercury has a very elliptical orbit. Starlight bends as it travels through space-time around objects. Time dilation is, objects in a larger gravitational field move faster in time. Black holes were fully discovered by Schwarzschild. Alexander Friedmann said the universe was isotropic, meaning that there is no center. Parts of the universe move faster than the speed of light. If the recession velocity is greater and it can’t reach us
Here’s a structured summary of the sections mentioned regarding the Galactic Central topic covered by Prof. Richard Easther in his lecture:
1. Shape and Size of the Milky Way Galaxy
- Discovery: Initially estimated by Jacobus Kapteyn using parallax and proper motion of stars.
- Size: Approximately 200,000 light-years in diameter.
- Structure: Includes a disk, bulge, and halo.
- Location: Earth is located within the disk, not at the center.
2. Spiral Arms and Star Formation
- Nature of Spiral Arms: Waves of star formation define the spiral arms, not static formations.
- Star Formation: Concentrated in the disk where gas and dust are abundant.
- Examples: Open clusters like those visible in the Milky Way.
3. Radio Astronomy and the Galactic Center
- Invention: Developed in the 1930s by Karl Jansky, marking the birth of radio astronomy.
- Identification: Sagittarius A identified as a significant radio source from the galactic center.
- Techniques: Directional detection and timing methods used for studying cosmic radio emissions.
4. Multiwavelength Observations of the Galactic Center
- Methods: Infrared, X-ray, and radio observations provide a comprehensive view.
- Instruments: Spitzer Space Telescope, Chandra X-ray Observatory, Very Large Telescope (VLT).
- Findings: Reveals ionized gas, synchrotron radiation, and other phenomena.
5. Dynamics of the Central Star Cluster and Evidence for a Supermassive Black Hole
- Observations: Study of stars like S0-2 orbiting a central invisible object.
- Black Hole: Estimated mass of approximately 4 million solar masses.
- Implications: Central region characterized by violent activity, strong gravitational effects
Option 1: Don’t Know The Rules…
- Solar System Insights: Planets obey Kepler’s Laws derived from Newtonian mechanics.
- Deviation Discovery: Uranus and Neptune’s orbits didn’t match predictions, hinting at unseen forces.
- Einstein’s Contribution: General Relativity (1915) introduced to explain Mercury’s perihelion precession.
Option 2: Can’t See The Players
- Galactic Enigma: Early 20th-century debates on galaxies revealed billions of stars, gas, and dust.
- Key Questions: Does Newton’s framework apply universally? Does light trace all mass?
Connecting Insights:
- Dark Matter Hypothesis: Observations in galaxies show stars move differently than expected, suggesting invisible mass.
- Modified Gravity Considerations: Proposed to reinterpret galactic dynamics without invoking dark matter.
- Evidence Accumulation: Includes rotation curves, cluster dynamics, gravitational lensing, and structure formation.
Conclusion:
- Current Understanding: The universe’s structure and dynamics remain complex, with ongoing debates between dark matter and modified gravity theories.
- Continued Exploration: Further observations, like microwave background and ultra-faint galaxies, refine our understanding of cosmic phenomena.
What is Dark Matter?
- Not Ordinary Matter: Dark matter cannot be conventional matter like atoms or their components (electrons, protons, neutrons) because it would emit or absorb light, which we could detect.
- Beyond Observable: It doesn’t emit light or interact strongly with regular matter, making it invisible to most detection methods.
- Big Bang Constraints: The abundance of ordinary matter produced by Big Bang nucleosynthesis doesn’t account for dark matter.
- Black Holes Insufficient: While black holes are considered, their numbers and sizes do not explain dark matter’s gravitational effects.
- meters, influencing cosmological models and theories.