DSST Astronomy key terms

A planet sweeps out equal areas in equal times as it orbits the Sun, meaning its speed increases when it is closer to the Sun and decreases when farther away.

Each planet moves on an ellipse with the Sun at one focus, so the distance between planet and Sun varies over one orbital period rather than staying constant.

A number describing how flattened an ellipse is, ranging from 0 for a perfect circle toward 1 for a highly elongated orbit; it sets how much a body's distance from its primary varies.

The minimum speed an object needs to leave a body's gravity permanently without further propulsion; it grows with the body's mass and shrinks with distance from its center.

Every pair of masses attracts each other with a force proportional to the product of the masses and inversely proportional to the square of the distance between their centers.

The difference in gravitational pull across a body's width, stretching it along the line toward the attracting mass; it causes ocean tides and can lock a moon's rotation.

A slow wobble in the orientation of a spinning body's axis, caused by torques from outside; Earth's axis traces a cone roughly every 26,000 years, shifting the celestial poles.

The apparent yearly path of the Sun against the background stars, which is also the plane of Earth's orbit projected onto the sky; planets and the Moon stay near it.

The projection of Earth's equator onto the sky, a great circle halfway between the celestial poles; it intersects the ecliptic at the equinoxes and defines declination zero.

The celestial coordinate analogous to longitude, measured eastward along the celestial equator from the vernal equinox, usually expressed in hours, minutes, and seconds.

The celestial coordinate analogous to latitude, measured in degrees north or south of the celestial equator, ranging from +90 at the north celestial pole to -90 at the south.

The time Earth takes to rotate once relative to the distant stars, about 23 hours 56 minutes; it is shorter than a solar day because Earth also moves along its orbit.

The interval between successive passages of the Sun across the same meridian, averaging 24 hours; it is longer than a sidereal day because Earth's orbital motion shifts the Sun's apparent position.

An instrument that gathers light with a lens and brings it to focus, forming an image; large versions suffer from chromatic aberration and from sagging under their own weight.

An instrument that gathers light with a curved mirror and focuses it toward a detector; mirrors avoid chromatic aberration and can be built far larger than lenses.

A lens defect in which different colors focus at slightly different distances, producing colored fringes around images; mirrors do not suffer from it because reflection is wavelength-independent.

The smallest angular separation at which a telescope can distinguish two nearby sources; it improves with larger aperture and worsens at longer observing wavelengths.

A technique that measures how Earth's atmosphere distorts incoming starlight and continuously flexes a deformable mirror to cancel that blur, sharpening ground-based images.

An instrument that spreads light into its component wavelengths so astronomers can identify chemical fingerprints, measure temperatures, and detect Doppler shifts in spectra.

A state in which a body's spin period equals its orbital period, so the same hemisphere always faces its partner; tidal forces drive most large moons into this configuration.

The distance from a massive body inside which tidal forces overwhelm a smaller object's self-gravity and tear it apart, often producing a ring of debris.

The separation of a molten body into layers by density, with dense metals sinking to form a core and lighter silicates rising; it explains layered interiors and magnetic fields.

The buildup of larger bodies from smaller ones through collisions and gravitational attraction, the process that assembled planetesimals into planets in the early solar system.

A small solid body, roughly kilometers to hundreds of kilometers across, formed from sticking dust grains in a protoplanetary disk and later colliding to build planets.

A rotating disk of gas and dust surrounding a young star, from which planets form; its inner regions are hot and rocky while outer regions stay cold enough for ices.

The long stable phase in which a star fuses hydrogen into helium in its core; a star's mass sets its position, luminosity, and how long it remains there.

A plot of stellar luminosity against temperature or spectral type that reveals distinct populations such as the main sequence, giants, and white dwarfs, and traces stellar evolution.

The exposed, Earth-sized core left after a low-mass star sheds its outer layers; it is supported by electron degeneracy pressure and slowly cools over billions of years.

A late evolutionary stage of a low- or intermediate-mass star in which the core contracts and outer layers expand and cool, giving a large, luminous, reddish appearance.

The explosive destruction of a star, either when a massive star's core collapses or when a white dwarf in a binary exceeds a critical mass; it briefly outshines an entire galaxy.

The extremely dense remnant of a massive star's collapsed core, typically about 20 kilometers across and composed mostly of neutrons, often observed as a pulsing radio source.

A region where gravity is so strong that nothing, not even light, can escape; it is bounded by an event horizon whose size depends only on the object's mass and spin.

The boundary around a black hole beyond which no signal can return to the outside universe; its radius grows in direct proportion to the black hole's mass.

A curved, denser region of gas, dust, and young stars winding through a disk galaxy, often linked to density waves that compress material and trigger star formation.

Unseen material that exerts gravity but emits no detectable light; it is inferred from galaxy rotation curves and cluster motions and outweighs ordinary matter several times over.

A graph of orbital speed versus distance from a galaxy's center; flat outer curves imply that unseen mass, interpreted as dark matter, surrounds the visible disk.

The observation that distant galaxies recede at speeds proportional to their distance, expressed as velocity equals the Hubble constant times distance; it implies an expanding universe.

The stretching of light's wavelength caused by the expansion of space during the light's journey, not by motion through space; greater distance produces greater redshift.

Faint microwave radiation filling the sky, released when the young universe cooled enough to become transparent; its tiny temperature ripples record early density variations.

A mysterious component that accelerates the universe's expansion, inferred from distant supernovae appearing fainter than expected; it currently dominates the cosmic energy budget.

The apparent shift of a nearby star against more distant background stars as Earth moves around the Sun; the shift angle directly gives distance for stars within a few thousand light-years.

The distance at which one astronomical unit subtends an angle of one arcsecond, about 3.26 light-years; it is the natural unit for stellar parallax and galactic distances.

An object whose intrinsic luminosity can be determined from observable properties, so comparing apparent brightness with that luminosity yields distance; supernovae and Cepheids are examples.

A pulsating star whose period of brightness change is tightly related to its true luminosity, allowing astronomers to use it as a distance indicator across galaxies.

The range of orbital distances around a star where a rocky planet could keep liquid water on its surface, given a suitable atmosphere and pressure.

A measurable feature, such as atmospheric oxygen alongside methane, that would be hard to explain without life and could indicate biological activity on a distant world.

A formula estimating the number of communicating civilizations in the galaxy by multiplying factors such as star formation rate, fraction with planets, and fraction where intelligence arises.