Observational Astronomy and Astrophysics Vocabulary

Vocabulary flashcards covering introductory astronomy concepts, physical units, telescopes, blackbody radiation, atomic spectra, and key equations from lecture notes.

Light-year

The distance light travels in a vacuum in one year, equal to approximately $9.5 \times 10^{15}\,\text{m}$ ($= 3 \times 10^8\,\text{m/s} \times 3.15 \times 10^7\,\text{s}$). It is a unit of distance, not time.

Astronomical Unit (AU)

The average Earth-Sun distance, equal to approximately $1.5 \times 10^{11}\,\text{m}$. It serves as the baseline used in parallax geometry ($1\,\text{parsec} = 206,265\,\text{AU}$).

Parsec (pc)

The distance at which a star has a parallax angle of $1\text{ arcsecond}$. $1\,\text{pc} = 3.26\,\text{ly} \approx 3.09 \times 10^{16}\,\text{m} = 206,265\,\text{AU}$.

Order of Magnitude

A power of ten; being $10\times$ too big or too small means an answer or estimate is off by one order of magnitude.

Zenith

The point in the sky directly overhead, corresponding to an altitude of $90^\circ$.

Horizon

The boundary between the sky and the ground, corresponding to an altitude of $0^\circ$.

Altitude

The angle of an astronomical object above the horizon measured in degrees, ranging from $0^\circ$ at the horizon to $90^\circ$ at the zenith.

Constellation

A recognizable pattern of stars and the officially defined region of sky around it ($88$ in total), whose stars are usually at very different distances and not physically related.

North Celestial Pole (NCP)

The point in the sky directly above Earth's North Pole where Earth's rotation axis points, situated very near Polaris.

South Celestial Pole (SCP)

The point in the sky directly above Earth's South Pole, opposite the North Celestial Pole, with no bright pole star nearby.

Celestial Equator

Earth's equator projected onto the sky, positioned halfway between the North Celestial Pole and the South Celestial Pole.

Circumpolar Star

A star that never sets below the horizon, remaining visible all day and night as it circles the celestial pole.

Solstice

The day when the Sun reaches its farthest northern or southern point in the sky (the longest or shortest day of the year), occurring around June 21 and December 21.

Equinox

The day when the Sun is directly over Earth's equator and day and night are about equal everywhere on Earth, occurring around March 21 and September 21–23.

Waxing Phase

The Moon phase progression during which the visible illuminated portion is growing from new moon to full moon.

Waning Phase

The Moon phase progression during which the visible illuminated portion is shrinking from full moon to new moon.

Lunar Eclipse

An astronomical event that occurs at Full Moon when the Moon passes into Earth's shadow, visible to everyone on Earth's night side.

Solar Eclipse

An astronomical event that occurs at New Moon when the Moon's shadow falls on a portion of Earth's surface.

Umbra

The darkest, central region of a shadow where the light source is completely blocked, leading to a total eclipse.

Penumbra

The lighter outer region of a shadow where the light source is only partially blocked, leading to a partial eclipse.

Precession

The slow wobble of Earth's rotation axis that traces out a circle over approximately $26,000\,\text{years}$ while maintaining a $23.5^\circ$ tilt.

Photon

One discrete packet of light energy whose energy is determined by its frequency or wavelength ($E = hf = \frac{hc}{\lambda}$).

Luminosity ($L$)

The total energy a source emits per second ($L = E/t$), measured in watts ($\text{W} = \text{J/s}$). It is an intrinsic property of the source independent of distance.

Flux ($F$)

The radiative power per unit area arriving at a location ($F = \frac{L}{4\pi d^2}$), measured in $\text{W/m}^2$.

Refracting Telescope

A telescope that uses a transparent glass lens to bend (refract) light and bring it to a focus.

Reflecting Telescope

A telescope that uses a curved mirror to bounce (reflect) light to a focal point.

Angular Resolution ($\theta$)

The smallest angular separation between two points that a telescope can distinguish as separate objects.

Diffraction Limit

The sharpest image a circular telescope can make, set by the wave nature of light and the ratio $\lambda/D$.

Seeing

The blurring of astronomical images caused by turbulence in Earth's atmosphere, typically limiting ground resolution to about $0.5'' - 1''$.

Adaptive Optics

A technique that cancels atmospheric blurring in real time using a flexible mirror and a bright reference star.

Interferometry

A technique of combining signals from multiple separated telescopes so that their effective angular resolution is determined by the largest separation baseline.

Blackbody

An idealized object whose emitted spectrum depends solely on its surface temperature.

Wien's Law

The physical relationship stating that the peak emission wavelength of a blackbody is inversely proportional to its temperature ($\lambda_{\text{max}} = \frac{2.9 \times 10^6\,\text{K}\cdot\text{nm}}{T}$).

Stefan-Boltzmann Law

The physical law stating that total luminosity is proportional to surface area and the fourth power of temperature ($L = 4\pi R^2 \sigma T^4$).

Parallax

The apparent shift in position of a nearby star relative to distant background objects, caused by Earth's orbital motion around the Sun.

Ionization

The process of supplying enough energy to completely free an electron from an atom, creating an ion.

Ground State

The lowest available energy level ($n=1$) for an electron in an atom.

Continuous Spectrum

A smooth rainbow of light across all wavelengths, produced by a hot, dense source viewed directly.

Emission Line Spectrum

A spectrum of bright lines on a dark background, produced by a hot, thin gas with no bright source behind it.

Absorption Line Spectrum

A spectrum showing dark lines at specific wavelengths superimposed on a continuous rainbow, produced when cool thin gas sits between an observer and a continuous source.

Doppler Effect

The shift in observed wavelength or frequency of light caused by relative line-of-sight motion between the light source and observer ($\frac{\Delta\lambda}{\lambda} = \frac{v}{c}$).

Balmer Series

Hydrogen electron transitions originating from or terminating at the $n=2$ level, producing lines in the visible light spectrum (e.g., $H\alpha$ at $656.3\,\text{nm}$).