Astrophysics and Cosmology Vocabulary

Vocabulary flashcards covering key definitions in astrophysics, star lifecycle, spectral analysis, cosmological models, and dark matter/energy based on the lecture material.

Planet

A body that orbits around a star, has cleared its orbit, has a mass large enough for its own gravity to give it its shape, and in which no nuclear fusion occurs.

Dwarf Planet

Planets that haven't cleared their orbit of other objects.

Asteroids

Objects too small and uneven to be planets; usually in near-circular orbits round the Sun and without the ice present in comets.

Planetary Satellite

A body that orbits a planet (e.g. moon, man-made satellites).

Comet

A small, irregularly shaped ball of rock, dust and ice, with a highly elliptical orbit, all of which orbit the Sun.

Solar System

A group of planets and bodies that orbit a common star.

Galaxy

A collection of billions of stars, planets, gases and dust, held together by gravitational attraction.

Gravitational Collapse

The inward movement of material in a star due to the gravitational force caused by its own mass.

Radiation Pressure

Pressure due to the momentum of photons released in fusion reactions, which acts outwards in the direction of energy flow.

Gas Pressure

Pressure inside a star related to temperature $T$ and volume $V$ using $pV = nRT$ or $pV = \frac{1}{3} N m c^2$, acting in all directions.

Main Sequence Star

A star in the main part of its life cycle, where it is fusing hydrogen to form helium in its core.

Red Giant

A star in the later stages of its life that has nearly exhausted the hydrogen in its core and is now fusing helium nuclei, expanding and cooling its outer layers.

White Dwarf

The dense end product of a low-mass star, formed when outer layers disperse into space, characterized by high surface temperature and low luminosity.

Planetary Nebula

An expanding, glowing shell of ionised hydrogen and helium ejected from a red giant star at the end of its life.

Electron Degeneracy Pressure

The pressure caused by electrons being forced into lower energy levels that stops the gravitational collapse of a low-mass star below the Chandrasekhar limit.

Chandrasekhar Limit

The maximum possible mass for a stable white dwarf star, equal to $1.4$ times the mass of our Sun ($1.4\text{ solar masses}$).

Red Super Giant

A star that has exhausted all the hydrogen in its core and has a mass much higher than the Sun.

Supernova

A huge explosion produced when the core of a red super giant collapses.

Neutron Star

The extremely dense remains of the core of a red super giant after a supernova explosion, composed mainly of neutrons.

Black Hole

The core of a massive star that has collapsed almost to a point, creating a gravitational field so strong that light cannot escape.

Luminosity

The total energy that a star emits per second.

Hertzsprung-Russell (HR) Diagram

A scatter graph showing the position of stars based on their luminosities and their surface temperatures.

Continuous Spectrum

A spectrum that appears to contain all wavelengths over a comparatively wide range.

Emission Line Spectrum

A spectrum consisting of a series of bright lines against a dark background, emitted due to electron transitions from higher to lower energy levels within an atom.

Absorption Line Spectrum

A pattern of dark lines across a continuous spectrum caused by light passing through an absorbing medium such as a cool gas.

Transmission Diffraction Grating

A glass surface having a large number of very fine parallel grooves or slits, used to produce optical spectra by diffraction of transmitted light.

Wien's Displacement Law

A law stating that peak wavelength $\tan(\theta)$ or $\tan(p)$ is inversely proportional to temperature $T$, given by $\tan(\theta) = \frac{\text{constant}}{T}$ where the constant is $2.89 \times 10^{-3}\text{ m K}$.

Stefan's Law

Relates the luminosity $L$ of a star to its surface area and absolute temperature $T$ via $L = 4\frac{\frac{1}{2}}{1}\frac{\frac{1}{2}}{1} r^2 \text{ σ } T^4$, where $\text{σ} = 5.67 \times 10^{-8}\text{ W m}^{-2}\text{ K}^{-4}$.

Astronomical Unit (AU)

The mean distance from the centre of the Earth to the centre of the Sun, equal to $1.496 \times 10^{11}\text{ m}$.

Parsec (pc)

The distance that gives a parallax angle of $1\text{ second of arc}$ ($\frac{1}{3600}$ of a degree) using the radius of Earth's orbit ($1\text{ AU}$) as baseline, approximately equal to $3.1 \times 10^{16}\text{ m}$.

Stellar Parallax

The apparent shifting in position of a star viewed against a background of distant stars when observed from different positions of the Earth's orbit.

Light-year (ly)

The distance travelled by light in one year, approximately equal to $9.5 \times 10^{15}\text{ m}$.

Doppler Effect

The change in wavelength or frequency caused by the relative motion between the wave source and an observer.

Red Shift

The apparent increase in wavelength of electromagnetic radiation caused when the source is moving away relative to the observer.

Hubble's Law

States that the recessional velocity $v$ of a galaxy is directly proportional to its distance $d$ from Earth, expressed as $v = H_0 d$.

Cosmic Microwave Background Radiation (CMBR)

Microwave radiation received from all over the sky originating from after the Big Bang, corresponding to a peak wavelength of temperature $2.7\text{ K}$.

Big Bang Theory

The theory stating that the universe was created from a single point where all current mass was situated and has been expanding and cooling ever since.

Cosmological Principle

States that on a large scale the universe is isotropic (same in all directions) and homogeneous (uniform density), and subject everywhere to universal physical laws.

Dark Matter

Matter which cannot be seen and does not emit or absorb electromagnetic radiation, detected indirectly through its gravitational effects on galaxies.

Dark Energy

A type of energy permeating the universe that exerts a negative pressure opposing gravity, causing the acceleration of the expansion of the universe.

Critical Density

The density of the universe ($\text{ρ}_c = \frac{3 H_0^2}{8 \frac{\frac{1}{2}}{1}\frac{\frac{1}{2}}{1} G} \text{≈} 9.5 \times 10^{-27}\text{ kg m}^{-3}$) required for the universe to decrease in velocity and reach a final finite size.

WIMPs

Weakly Interacting Massive Particles; hypothetical undiscovered particles with mass between $1$ and $1000$ proton masses that are dark matter candidates.

MACHOs

Massive Astrophysical Compact Halo Objects; ordinary matter composed of protons and neutrons found in concentrated areas in dark halos around galaxies.

Composition of the Universe

The mass-energy budget of the universe comprising approximately 68.3\text{%} dark energy, 26.8\text{%} dark matter, and 4.9\text{%} ordinary matter.