Astronomy

The Big Bang Theory

• What It Is:

• The leading scientific explanation for how our universe began.

• About $13.8billion$ ago, the universe expanded rapidly from an extremely hot, dense point and has been expanding and cooling ever since.

• It was not an explosion in pre-existing space; rather, space itself was created and expanded.

• Key Pieces of Evidence:

• Cosmic Microwave Background (CMB): Faint, cool glow of leftover heat filling the entire universe, detectable in every direction.

• Galactic Redshift: Light from distant galaxies is stretched out, proving that space is still expanding.

• Element Abundances: The universe is made of roughly $75\%$ Hydrogen and $25\%$ Helium, which matches calculations of what could form right after the Big Bang.

The Doppler Effect and Redshift

• The Doppler Effect:

• The apparent change in pitch or frequency of a wave when the source and listener move toward or away from each other.

• Everyday example: An ambulance siren sounds higher-pitched as it drives toward you and lower-pitched as it moves away.

• Light Waves in Space:

• Blueshift: When an object moves toward Earth, its light waves get squished to shorter wavelengths (shifting toward the blue end of the visible spectrum).

• Redshift: When an object moves away from Earth, its light waves get stretched to longer wavelengths (shifting toward the red end of the visible spectrum).

• Hubble's Discovery: Distant galaxies are redshifted, and the farther away they are, the faster they are moving away from us.

Isotopes and Nuclear Fission

• Isotopes:

• Atoms of the same element that have the same number of protons, but different numbers of neutrons.

• Because they have the same number of protons and electrons, isotopes behave identically in chemical reactions, but they have different atomic masses.

• Nuclear Fission:

• The splitting of a heavy, unstable atomic nucleus (such as Uranium-235) into two smaller nuclei.

• When struck by a neutron, the heavy nucleus breaks apart, releasing extra neutrons and a tremendous amount of energy.

• Mass is converted directly into energy according to Einstein's equation:       $E = mc^2$

• Fission powers nuclear energy plants and atomic weapons.

The Life Cycle of a Star

• Birth and Main Sequence:

• Stars form inside huge clouds of dust and gas called nebulae, which collapse under gravity.

• Main Sequence: The star shines steadily because inward gravity balances the outward pressure from nuclear fusion (turning Hydrogen into Helium in its core). Our Sun is currently in this stage.

• Path of an Average Star (Sun-sized):

• Red Giant: The core runs out of Hydrogen fuel and begins fusing Helium; outer layers swell and cool.

• Planetary Nebula: The dying star gently puffs off its outer gas layers into space.

• White Dwarf: The dense, glowing carbon core left behind slowly cools over billions of years.

• Path of a Massive Star:

• Red Supergiant: Fuses heavier elements until it builds up an Iron core.

• Supernova: Iron cannot be fused for energy, causing the core to collapse suddenly and explode with extreme brightness.

• Remnant: Leaves behind either an extremely dense Neutron Star or, if the mass is great enough, a Black Hole where gravity is so intense that nothing—not even light—can escape.

Kepler's Laws of Planetary Motion

• First Law (Law of Ellipses):

• Planetary orbits are shaped like flattened circles called ellipses, with the Sun located at one focus (off-center).

• Second Law (Law of Equal Areas):

• A planet speeds up when it is closer to the Sun and slows down when it is farther away, sweeping out equal amounts of orbital area in equal amounts of time.

• Third Law (Law of Harmonies):

• The farther a planet is from the Sun, the longer its orbital period (year):       $T^2 \propto a^3$       where $T$ is the orbital period and $a$ is the average distance from the Sun.

Newton's Laws of Motion and Gravity

• Newton's Three Laws of Motion:

• First Law (Inertia): An object at rest stays at rest, and an object in motion stays in motion at a constant speed in a straight line, unless acted on by an outside net force.

• Second Law ($F = ma$):       $F = ma$       Force equals mass times acceleration. A heavier object requires more force to accelerate.

• Third Law (Action-Reaction):       For every action force, there is an equal and opposite reaction force.

• Newton's Law of Universal Gravitation:

• Every object with mass pulls on every other object with mass:       $F = G \frac{m_1 m_2}{r^2}$

• Gravity gets stronger when masses ($m_1$, $m_2$) are larger, and gets weaker quickly as the distance ($r$) between them increases.