Oppositely charged matter. Electrons have a positive charge and protons have a negative charge.


When a planet or celestial body reaches the furthest point in its orbit around the Sun.

Apparent Magnitude:

The brightness of an object in the sky seen from Earth. The brighter the object, the higher the apparent magnitude.


A small planetary body orbiting the sun. Its size is smaller than a planet, but larger than a meteoroid, and often irregularly shaped. The orbits of asteroids can bring them within close proximately to planets. They are typically located in the asteroid belt between the orbits of Mars and Jupiter.


A branch of astronomy focused on the study of chemical substances found in stars and interstellar space.

Astronomical Unit (AU):

A common unit of measurement in astronomy equal to the average distance between the Earth and the Sun, nearly 93 million miles.


Layers of gases surrounding a planetary body held together by the body’s gravity.


Colorful bands of light in the sky at high latitudes produced when the solar wind ionizes particles in the atmosphere.




A double star system that revolves around a shared gravitational center.

Black Hole:

A region in space where gravity is so strong not even light can escape, making black holes impossible to see. They are formed when a dying star collapses in on itself and can be detected by observing how they influence their surroundings.



Celestial Equator:

A theoretical line dividing the celestial sphere into the northern and southern hemisphere.

Celestial Poles:

The celestial sphere’s North and South Poles

Celestial Sphere:

A large, imaginary sphere surrounding the Earth commonly used to reference stellar and planetary coordinates in the sky.


A layer of the Sun’s outer atmosphere directly above the photosphere.  Hydrogen heated to 20,000°C emits the reddish colored light seen at the chromosphere.

Circumpolar Star:

A star that appears to always stay above the horizon, a phenomena dependent on the location of the observer on Earth. Polaris-or the North Star- is an example of a circumpolar star in the northern hemisphere.


The area of gas and dust surrounding a comet’s nucleus formed when the comet approaches the Sun and warms during its elliptical orbit.


A small, icy body within the solar system which orbits the Sun in an extremely elliptical path.  They are often seen in the sky as long streaks of light. This is because by the time the comet passes Earth, the Sun’s has heated up the trapped ice, dust, and gases within the comet’s nucleus-or center, forming a coma and extremely long tail.


When two or more celestial bodies appear very close together in the sky. A New Moon is a common example of a conjunction. The moon is positioned between the Sun and the Earth so that it is hidden from the night sky.


A grouping of stars commonly referenced and connected together by the creation of an imaginary picture in the sky.


The outer layer of the Sun. It can be seen during a total solar eclipse as a glowing halo.


A branch of astronomy specifically focused on studying the Solar System, stars, galaxies, and galactic clusters.


A branch of astronomy specifically focused on studying the formation and nature of the universe. Today’s popular cosmologists include Stephan Hawking, Lawrence M. Krauss, Neil deGrasse Tyson, and Michio Kaku.



Dark matter:

Mysterious matter within the universe which cannot be see.  Its existence is confirmed by observing its gravitational effects on other astronomical bodies.


The amount of matter packed within a certain volume.

Double star:

When two stars appear grouped together in the sky, or are physically grouped such as in a binary star system.

Dwarf planet:

A celestial body orbiting a star large enough to have its own gravity-which is strong enough to round it- but not massive enough to have cleared the area surrounding its orbit of excessive debris or other large masses. Pluto is an example of a dwarf planet.




When one celestial body blocks another from the point of view of the observer.

Electromagnetic spectrum:

Light seen in the full range of frequencies, between and including radio waves to gamma waves.


The oval shape which an astronomical body orbits a star.

Elliptical galaxy:

A galaxy shaped like an ellipse. It is prominently defined by its absence of structure, specifically spiral arms.


The angle between the sun and a planet measured from an observer on Earth.


Refers to something outside of our own galaxy, the Milky Way.


Refers to anything not originally from Earth.




A very bright meteor that may even produce a sonic boom.




A system of millions or billions of stars, along with gas, dust, and other particles, all held together by gravity.

Gamma ray:

A form of electromagnetic radiation with the highest energy and the shortest wavelength on the electromagnetic spectrum.

Globular cluster:

Old stars compacted into a spherical star cluster, typically located in a galaxy’s outermost regions.


A force of nature which causes the attraction of two physical bodies with mass.




Considered the outermost border of the solar system, the heliopause is the boundary between our Sun’s solar wind and the interstellar medium.


A bubble shaped region in space before the heliopause surrounding our solar system. It extends far beyond Pluto.



Interstellar medium (ISM):

Matter existing in between the influences of star systems in a galaxy.


The region of charged particles located in the upper atmosphere of a planet. Earth’s ionosphere ranges from altitudes of 60 km to 1,000 km.




Gas or particles emitted in in a linear manner from a star or black hole’s axis.



Kuiper Belt:

An immense ring like structured composed of the original material that formed the solar system. The region is located beyond the orbit of Neptune.



Light year:

An astronomical unit of measurement equivalent to the distance light travels in a year, which is approximately 9.461 x km.

Local Group:

A group of more than 54 galaxies, including the Milky Way, surrounding a gravitational center located in a region between the Milky Way and the Andromeda Galaxy.

Lunar eclipse:

When the moon passes into the Earth’s shadow. A total lunar eclipse occurs when the moon passes into the umbra, or inner region of the Earth’s shadow. A partial lunar eclipse occurs when the moon passes into the penumbra, or the outer region of the Earth’s shadow.



Magnetic field:

Regions where an object extends a magnetic influence. That is, the attraction or repulsion of charged objects. The Earth’s magnetic field protects our atmosphere from the dangerous supercharged particles of the solar wind.


A region surrounding an astronomical body where the magnetic field is the strongest magnetic field. Like Earth’s magnetosphere, it protects from the harsh environment created by the Sun within the solar system.


 A logarithmic measurement of an object’s brightness in the night sky. Magnitude -26 (the Sun) through magnitude +2 (stars of the Big Dipper) objects can generally be seen with the naked eye. Magnitude 3 (faint stars) through magnitude 5 (the moons of Jupiter) can be viewed with binoculars. Magnitude 6 (Uranus) through magnitude 14 (Pluto at its brightest) objects need telescopes to be seen. Anything with a magnitude less than 15 is extremely difficult to see, even in dark sky spots.


The total amount of matter in a body which can be calculated with or without the force due to gravity.


Anything that has mass.


A meteoroid that enters the Earth’s atmosphere and burns away. Also referred to as a shooting star.

Meteor shower:

When a high concentrations of meteors enter the Earth’s atmosphere.  Generally, it occurs when Earth encounters debris left behind by a comet.


An object which survives its journey through the planet’s atmosphere and crashes into the ground. Meteors become meteorites if they hit the ground.


A rocky or metallic body traveling through the solar system. Meteoroids become meteors if the enter the Earth’s atmosphere.




A cloud of stellar gas and dust. They are created when a star dies. Often they function as stellar nurseries.

Neutron star:

The extremely dense core of an exploded star composed predominately of neutrons. Due to their high density, Neutron stars exert very strong gravitational forces, leading some to emit pulses of high-energy particles. This type of neutron star is known as a pulsar.



Oort cloud:

A theoretical outermost region of our solar system composed of comets.


When an astronomical object is directly opposite from the Sun as measured from Earth.  It is the best time to view the object since it is at its closest approach to Earth, and it rises just as the sun sets.


The bend of a celestial object or spacecraft traveling in space around a moon, planet, or star. Orbits are mostly elliptical in shape.




When an astronomical body reaches its closest position to the Sun in its orbit.


The Sun’s visible surface.


An astronomical body large enough to become rounded by its own gravity and has cleared the surrounding region of planetesimals. Planets are not massive enough to ignite thermonuclear fusion, a process which provides a star with its light and energy.


Objects formed from the collisions of dust grains which stick together.  Planetesimals are referred to as the building blocks that create planets.


A rapidly spinning neutron star that emits pulses of energy along its gravitational axis.




Unusually bright objects which emitted immense amounts of energy, and often considered the oldest objects in the universe.



Red giant:

When a star’s fuel begins to run out. As one of the final stages of stellar evolution, it results in the star’s expansion, decrease in temperature, and the emission of a red glow.


The spin of an astronomical body on its axis.




An object, whether natural or manmade, orbiting around a planet.

Solar eclipse:

When the Earth passes into the Moon’s shadow. When the moon is close enough to completely block all sunlight, this is referred to as a total solar eclipse. When the moon is farther away and therefore unable to block all sunlight, this is referred to as an annular solar eclipse.

Solar wind:

A flow of charged particles emitted from the sun that travels out into the solar system. Also known as stellar wind.

Spiral galaxy:

A galaxy characterized by a central bulge and luminous, spiral arms of stars, gas, and dust. The Milky Way, our own galaxy, is an example of a spiral galaxy.


A giant sphere of gas and dust that produces and emits its own radiation through nuclear fusion.


Cooler spots on the Sun relative to the surrounding surface. They are linked to disturbances in the Sun’s electromagnetic field. They appear as blotchy black spots scattered across the Sun’s surface when photographed in visible light.


Considered the most powerful known force in nature, a supernova is an explosion caused by a star when it exhausts its fuel, terminating its stellar life.




The boundary dividing the light and the dark side of an astronomical body. The moon is almost always divided by a terminator, except during a full or new moon.


Anything originally from Earth.


When a relatively small astronomical body orbits across the face of another that is larger in size. This is a common phenomena used to detect extra-solar planets by measuring a star’s dip in brightness.



Universal Time (UT):

The standard measure of time used by astronomers; the local time on the Greenwich meridian.



Visible light:

The wavelength of light on the electromagnetic spectrum detectable by human eyes.




The unit of measurement used in the electromagnetic spectrum referring to the distance between consecutive crests of a wave.

White dwarf:

A small, white star that forms when a regular sized star-like our Sun- exhausts its fuel supply and collapses.



Yellow dwarf:

A regular star such as our Sun considered to have reached a stable point in its stellar evolution.

Sources: Merriam Webster Dictionary, seasky.org, NASA