The leading theory for the formation of our solar system, stating that it formed from the gravitational collapse of a giant molecular cloud, which then flattened into a protoplanetary disk.
This explains why planets orbit in roughly the same plane and direction.
Card 2
Terrestrial Planets
Answer
Inner planets (Mercury, Venus, Earth, Mars) characterized by their rocky composition, high density, and solid surfaces.
Their proximity to the Sun meant lighter elements were swept away, leaving heavier, rocky materials to accrete.
Card 3
Jovian Planets
Answer
Outer planets (Jupiter, Saturn, Uranus, Neptune) characterized by their gaseous composition, large size, low density, and extensive ring systems.
Their distance from the Sun allowed them to retain lighter elements like hydrogen and helium, forming massive gas giants.
Card 4
Asteroid Belt
Answer
A region between Mars and Jupiter containing a large number of irregularly shaped rocky bodies called asteroids, which are remnants from the early solar system.
Gravitational perturbations from Jupiter prevented these materials from coalescing into a full planet.
Card 5
Kuiper Belt
Answer
A region beyond Neptune's orbit, extending to about 50 AU, containing icy bodies, including dwarf planets like Pluto, and is a source of short-period comets.
It is analogous to the asteroid belt but composed primarily of icy rather than rocky material.
Card 6
Oort Cloud
Answer
A hypothetical spherical cloud of icy planetesimals surrounding the Sun at distances up to 100,000 AU, believed to be the source of long-period comets.
Its vast distance means objects are only occasionally perturbed into the inner solar system.
Card 7
Light-year
Answer
The distance light travels in one Earth year in a vacuum, approximately 9.46 trillion kilometers.
It is a unit of distance, not time, used to measure vast astronomical separations.
Card 8
Astronomical Unit (AU)
Answer
The average distance between the Earth and the Sun, approximately 150 million kilometers.
It is a convenient unit for measuring distances within our solar system.
Card 9
Apparent Magnitude
Answer
A measure of how bright a celestial object appears from Earth, with lower numbers indicating brighter objects.
This value depends on both the object's intrinsic luminosity and its distance from the observer.
Card 10
Absolute Magnitude
Answer
A measure of the intrinsic brightness of a celestial object, defined as its apparent magnitude if it were located at a standard distance of 10 parsecs from Earth.
This allows for direct comparison of the true luminosities of different stars, independent of their distance.
Card 11
Doppler Effect (astronomy)
Answer
The change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source, causing light from receding objects to be redshifted and light from approaching objects to be blueshifted.
This effect is crucial for determining the radial velocity of stars and galaxies.
Card 12
Parallax (stellar)
Answer
The apparent shift in the position of a nearby star against the background of more distant stars as the Earth orbits the Sun, used to measure stellar distances.
The larger the parallax angle, the closer the star, providing a direct method for measuring distances to nearby stars.
Card 13
Protostar
Answer
A dense core of gas and dust that is contracting under its own gravity, but has not yet begun nuclear fusion in its core.
This is the earliest stage in the formation of a star, preceding the main sequence.
Card 14
Main Sequence Star
Answer
A star that is fusing hydrogen into helium in its core, representing the longest and most stable phase of a star's life.
The Sun is currently a main sequence star, and most stars in the universe are in this phase.
Card 15
Red Giant
Answer
A star that has exhausted the hydrogen fuel in its core and has expanded significantly, cooled, and brightened as it begins fusing hydrogen in a shell around its core.
This stage occurs after a star leaves the main sequence and before it sheds its outer layers or undergoes further collapse.
Card 16
Planetary Nebula
Answer
A shell of gas and dust ejected by a low-to-intermediate mass star (like our Sun) at the end of its life, glowing due to ionization by the central hot white dwarf.
It is a transient phase, lasting only tens of thousands of years, and has nothing to do with planets.
Card 17
White Dwarf
Answer
The dense, compact remnant of a low-to-intermediate mass star, consisting primarily of degenerate matter, that has shed its outer layers and no longer undergoes fusion.
It slowly cools and fades over billions of years, prevented from further collapse by electron degeneracy pressure.
Card 18
Supernova
Answer
A powerful and luminous stellar explosion, occurring either when a massive star collapses at the end of its life or when a white dwarf in a binary system accretes too much matter.
Supernovae are crucial for dispersing heavy elements into the interstellar medium and can briefly outshine entire galaxies.
Card 19
Neutron Star
Answer
The extremely dense, compact remnant of a massive star that has undergone a supernova, composed almost entirely of neutrons supported by neutron degeneracy pressure.
These objects are typically only a few kilometers in diameter but contain more mass than the Sun.
Card 20
Black Hole
Answer
A region of spacetime where gravity is so strong that nothing, not even light, can escape, formed from the gravitational collapse of the most massive stars or by other processes.
Its boundary is called the event horizon, beyond which causality breaks down for external observers.
Card 21
Spiral Galaxy
Answer
A galaxy characterized by a flattened, rotating disk with spiral arms, a central bulge, and a halo, containing both young and old stars, gas, and dust.
Our Milky Way galaxy is a barred spiral galaxy.
Card 22
Elliptical Galaxy
Answer
A galaxy with an elliptical shape, ranging from nearly spherical to highly elongated, containing mostly old stars and very little gas and dust, with no significant ongoing star formation.
They are often found in galaxy clusters and are thought to result from galaxy mergers.
Card 23
Irregular Galaxy
Answer
A galaxy that does not have a distinct regular shape, often rich in gas and dust, and undergoing vigorous star formation, possibly due to gravitational interactions with other galaxies.
The Magellanic Clouds, satellite galaxies of the Milky Way, are examples of irregular galaxies.
Card 24
Active Galactic Nucleus (AGN)
Answer
A compact region at the center of some galaxies that emits a tremendous amount of energy across the electromagnetic spectrum, believed to be powered by a supermassive black hole accreting matter.
Quasars are a particularly luminous type of AGN.
Card 25
Big Bang Theory
Answer
The prevailing cosmological model describing the universe's earliest known periods, its subsequent large-scale evolution, and its ongoing expansion from an extremely hot, dense state.
It is supported by evidence such as the expansion of the universe, the cosmic microwave background, and the abundance of light elements.
Card 26
Cosmic Microwave Background (CMB)
Answer
Faint background radiation filling all space, interpreted as the residual heat from the Big Bang that has cooled and redshifted over billions of years.
Its discovery provided strong evidence supporting the Big Bang theory.
Card 27
Dark Matter
Answer
A hypothetical form of matter that does not interact with light or other electromagnetic radiation, but whose presence is inferred from its gravitational effects on visible matter.
It is believed to make up about 27% of the universe's mass-energy content and helps explain galaxy rotation curves and galaxy cluster dynamics.
Card 28
Dark Energy
Answer
A hypothetical form of energy that permeates all of space and is responsible for the observed accelerating expansion of the universe.
It is believed to constitute about 68% of the universe's mass-energy content, acting as a repulsive force.
Card 29
Hubble's Law
Answer
The observation that galaxies are receding from us with a velocity proportional to their distance, indicating the expansion of the universe.
The constant of proportionality is known as the Hubble Constant, which describes the rate of expansion.
Card 30
Redshift (cosmological)
Answer
The stretching of light waves to longer, redder wavelengths as they travel through expanding space from distant galaxies, indicating that the universe itself is expanding.
This is distinct from the Doppler redshift, which is due to an object's motion through space, not the expansion of space itself.