Astronomy Lecture Notes: Nuclear Processes, Solar Dynamics, and Planetary Motion

Vocabulary practice flashcards covering nuclear fission and fusion, solar structure, space weather, historical models of the universe, and Kepler's laws from lecture notes.

Nuclear Fission

The process of splitting a large atomic nucleus into smaller nuclei.

Uranium-235 ($^{235}\text{U}$)

A fissile isotope that makes up $0.7\%$ of uranium on Earth and is capable of sustaining a nuclear chain reaction.

Uranium-238 ($^{238}\text{U}$)

A fissionable isotope that constitutes $99.3\%$ of uranium found naturally on Earth.

Mass Defect

The difference in mass where the mass of the products is less than the mass of the original atom, with the missing mass converted into energy according to $E = mc^2$.

Critical Mass

The minimum amount of fissile material required to sustain a nuclear chain reaction.

Uranium Enrichment

The process of increasing the relative percentage of $^{235}\text{U}$ compared to $^{238}\text{U}$.

Highly Enriched Uranium

Uranium enriched to greater than $20\%$ $^{235}\text{U}$, such as the $80\%$ enrichment used in the Hiroshima weapon.

Control Rods

Components in a nuclear reactor that absorb $\frac{2}{3}$ of emitted neutrons so that only 1 neutron remains to initiate another fission event.

Nuclear Fusion

The process of combining lighter atomic nuclei to create a larger nucleus, releasing energy from the mass defect.

Fusion Gain Factor ($Q$)

The ratio of energy output to energy input ($Q = \frac{\text{energy out}}{\text{energy in}}$), where values of $Q < 1$ represent a net loss of energy.

Inertial Confinement

A method of containing nuclear fusion reactions by containing small internal explosions.

Magnetic Confinement

A method of confining high-temperature fusion plasma using magnetic fields.

Gravitational Contraction

The early theoretical model proposing that the Sun was powered by shrinking and converting gravitational kinetic energy into thermal energy.

Solar Core

The innermost powerhouse region of the Sun where nuclear fusion takes place.

Radiative Zone

The interior layer of the Sun located outside the core through which energy transfers outward by radiation via photons.

Convective Zone

The outermost interior layer of the Sun where heat is transferred outward through fluids via convection, producing granulation.

Photosphere

The visible surface disc of the Sun with a temperature of approximately $6000\,\text{K}$, from which light takes 8 minutes to reach Earth.

Cecilia Payne

The astronomer who discovered that the Sun is composed primarily of hydrogen and helium, and the first woman to earn a PhD in astronomy.

Plasma

A state of matter consisting of superheated gas in which electrons and protons are separated.

Chromosphere

The atmospheric layer of the Sun directly above the photosphere that appears red in color.

Solar Transition Region

A thin layer in the solar atmosphere characterized by a rapid, massive increase in temperature.

Corona

The outermost layer of the solar atmosphere with temperatures reaching millions of degrees.

Solar Wind

Outflowing plasma from the Sun's outer corona boiling away into space in a spiral structure containing the Sun's magnetic field.

Differential Rotation

The phenomenon in which rotation speed varies by latitude, averaging approximately $27\,\text{days}$ on the Sun.

Sunspots

Cooler regions of the photosphere (~$4000\,\text{K}$) that appear dark due to twisted magnetic fields inhibiting convection underneath.

Umbra and Penumbra

The structural components of a sunspot, consisting of the darker inner core (umbra) and the lighter surrounding region (penumbra).

Solar Filament

A loop of cooled plasma that has lifted off the surface against the face of the Sun.

Solar Prominence

A filament of cooled plasma extending outward off the side or limb of the Sun.

Solar Flare

A sudden burst of electromagnetic radiation (UV and X-rays) that reaches Earth in 8 minutes, detected first with X-rays and seen with UV.

Coronal Mass Ejection (CME)

A massive eruption and release of plasma matter from the solar corona, observed in visible light.

Solar Cycle

An approximately 11-year cycle of solar magnetic activity alternating between periods of solar minimum and solar maximum.

Van Allen Radiation Belts

Two donut-shaped zones in Earth's magnetosphere that trap charged particles, consisting of an inner belt of mostly protons and an outer belt of mostly electrons.

Solar Radiation Storm

A space weather disturbance caused primarily by solar flares that releases high-energy protons, hazardous to astronauts and polar flights with a warning time under 1 hour.

Radio Blackout

The most common space weather event, caused by flare-generated X-rays entering the ionosphere within 8 minutes and blocking radio wave bounce with no warning time.

Geomagnetic Storm

A lethal space weather disturbance in Earth's magnetosphere caused primarily by coronal mass ejections (CMEs) that threatens satellites and electrical grids.

Hale's Polarity Law

The law stating that sunspot pairs in opposite hemispheres have opposite magnetic polarities, and all sunspots in a new cycle exhibit reversed polarity relative to the previous cycle.

Magnetosphere

The protective cavity carved out in the solar wind by Earth's electromagnetic field.

Kristian Birkeland

The scientist who determined that auroral displays are caused by energetic electrons colliding with Earth's atmosphere.

Ground-Induced Currents

Currents generated during geomagnetic storms that can disrupt power grids and destroy electrical transformers.

Geocentric Model

An Earth-centered model of the universe popularized by Ptolemy that utilized epicycles to explain the motions of planets.

Apparent Retrograde Motion

The visual phenomenon in which planets appear to move backward in the sky relative to background stars.

Stellar Parallax

The apparent shift in the position of nearby stars against the background sky as Earth orbits the Sun, historically undetected because stars are too distant.

Foucault Pendulum

A pendulum experiment that provided the first direct physical evidence of Earth's rotational motion.

Kepler's First Law

The orbital law stating that the orbits of planets are ellipses with the Sun at one focus, rather than perfect circles.

Kepler's Second Law (Law of Areas)

The orbital law stating that a line connecting a planet to the Sun sweeps out equal areas in equal times, causing a planet to orbit faster when closer to the Sun.

Four Fundamental Forces

The four fundamental interactions of physics: gravity (the weakest), electromagnetism, the weak nuclear force, and the strong nuclear force.