Cosmic Microwave Background: The First Light Ever Released
The cosmic microwave background is thermal radiation released 380,000 years after the Big Bang, the oldest light observable in any direction.
Space, Astronomy and Cosmology
38 glossary terms
Space, astronomy, and cosmology ask three overlapping questions: what is out there, how did it get there, and where is it going? The terms in this section describe stars and galaxies, the instruments that observe them, and the large-scale frameworks astronomers use to interpret what they see. You will find definitions for the everyday (heliocentric model, absolute magnitude) alongside ideas still under active debate, including dark matter and the cosmic expansion. Taken together, these terms give you the footing to read a modern observatory paper or follow a mission update without losing the thread.
Fermi Paradox: Why the Universe Stays Silent
The Fermi Paradox is the tension between the apparent likelihood of extraterrestrial civilizations and the complete absence of evidence for them.
Einstein Ring: How Bent Light Reveals Hidden Mass
An Einstein ring is a circle of light that forms when a distant galaxy sits directly behind a massive foreground object, and its gravity bends the light symmetrically into a ring.
Doppler Effect: How Astronomers Measure Cosmic Motion
The Doppler effect in astronomy is the shift in wavelength or frequency of light from a celestial object caused by its motion relative to an observer, revealing whether stars and galaxies are approaching or receding.
Dark Matter: The Invisible Scaffold of the Universe
Dark matter does not emit, absorb, or reflect light, yet its gravity shapes every galaxy in the universe.
Dark Energy Theoretical Models: How Physicists Explain the Accelerating Universe
Dark energy theoretical models are the mathematical frameworks cosmologists use to explain why the expansion of the universe is accelerating, driven by a form of energy composing roughly 68% of the cosmos.
Brown Dwarf: The Objects Between Planets and Stars
A brown dwarf is a substellar object too massive to be a planet but too small to sustain hydrogen fusion, occupying the mass range between roughly 13 and 80 Jupiter masses.
Blackbody Radiation: The Light That Launched Quantum Physics
Blackbody radiation is light emitted by an object that absorbs all incoming radiation. Its spectrum depends only on temperature, making it essential for measuring stellar temperatures and understanding the cosmic microwave background.
Andromeda Paradox: When "Now" Depends on Who Is Walking
The Andromeda Paradox is a thought experiment showing that two observers moving at different velocities disagree about what is happening "now" at distant locations, with walking-speed differences producing a days-long shift at the Andromeda Galaxy.
Absolute Magnitude: How Astronomers Measure True Stellar Brightness
Absolute magnitude measures a star's intrinsic brightness from a standard distance of 10 parsecs, letting astronomers compare the true luminosities of stars regardless of their distance from Earth.