Science: The Universe

Absolute Magnitude (M)

The intrinsic brightness of a celestial object, measured as the apparent brightness it would have if placed at a standard distance of 10 parsecs.

Apparent Magnitude (m)

The brightness of a star or celestial object as observed from Earth, depending on its actual energy output and distance.

Astronomical Unit (AU)

The average distance from the Earth to the Sun, approximately 1.5 x 10^8 km.

Big Bang Theory

The cosmological model stating that the universe originated about 13.8 billion years ago from an infinitely hot, dense singularity.

Black Hole

An extremely dense remnant of a massive star collapse whose gravitational field is so strong that nothing can escape.

Cosmic Microwave Background Radiation (CMBR)

Low-energy microwave radiation filling all space uniformly at 2.7 K, representing the cooled remnant thermal afterglow of the Big Bang.

Cosmological Principle

The foundational postulate that the physical laws of nature operate uniformly throughout the entire universe.

Doppler Effect / Redshift

The stretching of light waves toward longer, redder wavelengths emitted by celestial objects moving away from an observer.

Galaxy

An immense system of millions to billions of stars, gas, dust, and dark matter bound together by gravity.

Geocentric Model

An ancient astronomical model placing a stationary Earth at the center of the universe.

Heliocentric Model

The astronomical model placing the Sun at the center of the solar system, with Earth and other planets orbiting around it.

Hertzsprung-Russell (H-R) Diagram

A graph plotting star absolute magnitude (luminosity) against surface temperature (spectral class/color) to classify stars.

Light-year (ly)

A unit of distance representing the total distance light travels through a vacuum in one Julian year, approximately 9.46 x 10^12 km.

Main Sequence

A stable diagonal band across the H-R diagram containing roughly 90% of all stars, which generate energy via core hydrogen fusion.

Nebula

A cloud of interstellar gas and dust that acts as a 'star nursery' where new stars condense under gravity.

Neutron Star

An extremely dense stellar remnant composed almost entirely of neutrons, formed from the core collapse of a massive star.

Nuclear Fusion

A nuclear reaction in which light atomic nuclei combine under extreme conditions to form a heavier nucleus, releasing vast energy.

Protostar

An early, dense mass of collapsing gas and dust within a nebula before core nuclear fusion ignites.

Red Giant

An expanded, cool, highly luminous evolved star formed when a medium star exhausts its core hydrogen fuel.

Supernova

The explosive death of a high-mass star, marking its end and creating heavy elements.

White Dwarf

The small, dense, glowing core of carbon and oxygen left behind after a red giant sheds its outer layers.

Natural Satellite

The Moon, which is Earth's single natural satellite, revolving around Earth approximately once every 27.3 days.

Solar Eclipse

Occurs when the Moon passes directly between the Sun and Earth, casting its shadow on Earth's surface.

Lunar Eclipse

Occurs when Earth passes directly between the Sun and Moon, blocking sunlight from illuminating the lunar surface.

Cosmic Inflation

The rapid expansion of space faster than the speed of light during the early universe.

Nucleosynthesis

The process that occurred during the Big Bang where quarks bind to form protons and neutrons, creating atomic nuclei.

Redshift

The observed increase in the wavelength of light from objects that are moving away from the observer, supporting the expanding universe.

Spectral Class

Classification of stars based on their temperatures and the absorption lines in their spectra.

Hydrostatic Equilibrium

The balance between inward gravitational force and outward radiation pressure within a star.

Turn-Off Point

The location on the H-R diagram where stars in a cluster leave the main sequence to evolve into giants, indicating the cluster's age.

Volume of a Star

For a spherical star, the volume is given by V = (4/3)πr^3, determining how much space the star occupies.

H-R Diagram

A graphical representation of stars that plots luminosity against surface temperature, demonstrating stellar evolution.

Mass-Energy Equivalence (E=mc²)

The principle that mass can be converted into energy, and vice versa, especially relevant in nuclear fusion processes.