Stellar and Galactic Astronomy Vocabulary Flashcards

Vocabulary flashcards covering key astronomical concepts, units, scales, and particle physics terms from the introductory lectures and midterm review.

Cosmic Web

A filamentary large-scale structure of the Universe consisting of galaxies, galaxy clusters, and massive nearly empty regions known as voids.

Hubble Ultra-Deep Field (HUDF)

An image of roughly 10,000 galaxies covering 1/100 of the full Moon area, created from 800 exposures taken over 400 Hubble Space Telescope orbits with a total exposure time of 1 million seconds (11.3 days).

Observable Universe

A spherical sub-volume of the Universe centered on an observer's position, bounded by the cosmological horizon, containing all objects whose light has had enough time to reach us (current radius of approximately $46.5 \times 10^9\,\text{l.y.}$).

Cosmological Horizon

The spherical boundary encompassing the observable universe beyond which radiation cannot be detected even in principle, determined by the speed of light, the age of the Universe, and its expansion rate.

Light Year (l.y.)

A unit of distance equal to the distance light travels in vacuum within one year, corresponding to approximately $10 \times 10^{12}\,\text{km}$ ($10^{13}\,\text{km}$), $9.46 \times 10^{12}\,\text{km}$, or roughly $63,000\,\text{AU}$.

Astronomical Unit (AU)

A unit of distance defined as exactly $149,597,870,700\,\text{m}$ (approximately $150 \times 10^6\,\text{km}$ or $93 \times 10^6\,\text{miles}$), which closely corresponds to the average Sun-Earth separation.

Parsec (pc)

A unit of distance equal to approximately $3.26\,\text{l.y.}$ (or $200,000\,\text{AU}$), defined as the distance from which the Sun-Earth separation is subtended by an angle of 1 second of arc.

Nucleons

Protons and neutrons, the subatomic composite particles that form atomic nuclei, possessing characteristic diameters of approximately $10^{-15}\,\text{m}$ ($1\,\text{femtometer}$ or $1\,\text{fermi}$).

Baryonic Matter

Ordinary matter composed of baryons (protons and neutrons) and electrons, making up approximately $4.9\%$ (or $5\%$) of the total mass-energy budget of the Universe.

Dark Matter

A non-baryonic form of matter that exerts attractive gravitational force but does not emit or absorb electromagnetic radiation, accounting for approximately $26.8\%$ (or $27\%$) of the mass-energy content of the Universe.

Dark Energy

An enigmatic non-baryonic component of the Universe exerting repulsive gravity that causes the expansion of the Universe to accelerate over time, making up approximately $68.3\%$ (or $68\%$) of the Universe.

Proton

A stable subatomic baryon composed of 2 up quarks and 1 down quark, with a mass of $1.67 \times 10^{-27}\,\text{kg}$, an electric charge of $+1$ elementary charge ($+1.602 \times 10^{-19}\,\text{C}$), and a spin of 1/2.

Neutron

A subatomic baryon composed of 1 up quark and 2 down quarks with zero net electric charge, a spin of 1/2, and a mean lifetime of approximately 15 minutes when outside an atomic nucleus.

Oort Cloud

A hypothetical spherical reservoir of comets surrounding the periphery of the Solar system, extending from approximately $10,000\,\text{AU}$ to $100,000\,\text{AU}$.

Milky Way Galaxy

Our home spiral galaxy containing roughly 200 billion stars, featuring a stellar disk diameter of 100,000 to $200,000\,\text{l.y.}$ embedded within a massive spherical dark matter halo.

Local Group

A gravitationally bound cluster of nearly 100 galaxies spanning about $10 \times 10^6\,\text{l.y.}$ in diameter, dominated by three large spiral galaxies: Andromeda (M31), the Milky Way Galaxy, and Triangulum (M33).

Laniakea Supercluster

A large galactic supercluster defined in 2014 spanning roughly $500 \times 10^6\,\text{l.y.}$, containing around 100,000 galaxies distributed across 500 galaxy clusters and groups.

Cosmic Microwave Background (CMB)

The oldest detected electromagnetic radiation, released approximately 380,000 years after the Big Bang, representing an overall background radiation field with a present average temperature of $2.73\,\text{K}$.

Apparent Magnitude

A logarithmic numerical scale expressing the locally measured radiative flux (apparent brightness) of a celestial object, where a difference of 5 magnitudes corresponds to a brightness ratio of exactly 100.

Galactic Year

The duration required for the Sun and Solar system to complete one full revolution around the center of the Milky Way Galaxy, equal to approximately 230 million years.

Refracting Telescope

An optical telescope that utilizes a transparent glass lens as its primary optical element to gather and focus light.

Reflecting Telescope

An optical telescope that utilizes a curved mirror as its primary optical element to collect and focus incoming light.

Interferometry

An observational technique in astronomy that combines incoming radiation from two or more distinct telescopes pointing at the same target simultaneously to dramatically enhance angular resolution.

Adaptive Optics

An optical technology used in ground-based telescopes to improve angular resolution by dynamically counteracting image blurring caused by atmospheric turbulence ('seeing'), often employing a laser-generated artificial star.

Astronomical Unit (AU)

A unit of distance equal to $149\,597\,870\,700\text{ meters}$ (approximately $150\text{ million km}$ or $93\text{ million miles}$), closely corresponding to the average Sun-Earth separation.

Light Year (l.y.)

A unit of distance equal to the distance light travels in a vacuum within one year, amounting to approximately $10\text{ trillion km}$ ($10^{13}\text{ km}$) or roughly $63\,000\text{ AU}$.

Parsec (pc)

A unit of distance equal to approximately $3.26\text{ light years}$ (or roughly $200\,000\text{ AU}$), defined as the distance from which the Sun-Earth separation is subtended by an angle of 1 second of arc.

Observable Universe

A spherical sub-volume of the Universe centered on the observer's position containing objects whose electromagnetic signals have had enough time to reach the observer, currently having a radius of about $46.5\text{ billion light years}$.

Cosmological Horizon

The spherical boundary of the observable universe beyond which radiation cannot be detected, determined by the speed of light, the age, and the expansion rate of the Universe.

Cosmic Web

The filamentary large-scale structure of the Universe composed of galaxies, galaxy clusters, and nearly empty regions called voids.

Voids

Large quasi-empty regions of space situated between the filamentary concentrations of galaxies and galaxy clusters in the cosmic web.

Speed of Light ($c$)

A fundamental constant of nature representing the speed of all electromagnetic radiation in a vacuum, equal to exactly $299\,792\,458\text{ m/s}$ (approximately $300\,000\text{ km/s}$).

Baryonic Matter

Ordinary matter composed of baryons (protons and neutrons) and electrons, making up approximately $5\%$ (or $4.9\%$) of the total mass-energy budget of the Universe.

Dark Matter

A non-baryonic form of matter that does not emit electromagnetic radiation but exerts attractive gravity, contributing approximately $27\%$ (or $26.8\%$) to the Universe's mass-energy total.

Dark Energy

A non-baryonic component contributing approximately $68\%$ (or $68.3\%$) to the total mass-energy of the Universe that exerts repulsive gravity, causing cosmic expansion to accelerate.

Oort Cloud

A hypothetical structure of comets located at the outer periphery of the Solar system, extending from roughly $10\,000\text{ AU}$ to $100\,000\text{ AU}$ ($1\text{ to }2\text{ light years}$) from the Sun.

Galactic Year

The time required for the Solar system to complete one full orbit around the center of the Milky Way Galaxy, lasting approximately $230\text{ million years}$.

Cosmic Microwave Background (CMB)

The oldest detected electromagnetic radiation in the Universe, released approximately $380\,000\text{ years}$ after the Big Bang, with a present average temperature of $2.73\text{ K}$.

Apparent Magnitude

A logarithmic scale used by astronomers to express the apparent brightness (locally measured radiative flux) of a celestial object, where a difference of 5 magnitudes corresponds to a light intensity ratio of 100.

Nucleons

Protons and neutrons, the composite particles that constitute atomic nuclei, characterized by a physical scale of approximately $10^{-15}\text{ m}$ ($1\text{ femtometer}$ or $1\text{ fermi}$).

Proton

A stable nucleon composed of two up quarks and one down quark, with a mass of $1.67 \times 10^{-27}\text{ kg}$, an electric charge of $+1$ elementary charge, and a spin of $\frac{1}{2}$.

Neutron

An unstable free nucleon composed of one up quark and two down quarks, with a mean lifetime of 15 minutes, zero electric charge, and a spin of $\frac{1}{2}$.

Refracting Telescope

An optical telescope that uses a lens as its primary (input) optical element.

Reflecting Telescope

An optical telescope that uses a curved mirror as its primary (input) optical element.

Interferometry

An observational technique using two or more telescopes simultaneously observing the same object to improve angular resolution.

Adaptive Optics

An observational technique designed to mitigate atmospheric image blurring ("seeing") to improve angular resolution, sometimes using a laser beam to create an artificial guide star.

Local Group

A gravitationally bound system of nearly 100 galaxies spanning approximately $10\text{ million light years}$ across, anchored by three large spiral galaxies: Andromeda (M31), the Milky Way, and Triangulum (M33).

Laniakea Supercluster

A supercluster defined in 2014 spanning approximately $500\text{ million light years}$ containing about $100\,000\text{ galaxies}$ across 500 galaxy clusters and groups.