All About Stars
Days and Years
Definitions and Theorys
Everything Else


An ancient Greek philosopher who lived from 384 to 322 B.C. He was the pupil of Plato and the tutor of Alexander the Great.


Stars and their Cores

Star-A large ball of hydrogen and helium gases that produces light and heat by means of nuclear reactions in its core. Stars have a wide range of brightness, colors, temperatures, and sizes.

Core-The region inside a star where the temperature and pressure are sufficient enough to cause nuclear fusion, converting atoms of hydrogen to helium and producing a great deal of energy.



A cyclical event that occurs once a year.



A cloud of gas and dust in space.


Axis and Celestial Pole

Axis-A straight line that passes through an object and around which the object turns. The axis of Earth is an imaginary line through its center, between the North and South Poles. (The side or bottom line of a graph is also called an axis.)

Celestial Pole-A point in the sky that does not appear to move. The north and south celestial poles are the imaginary extensions of Earth’s rotation axis from the North and South Poles out into space. (See also axis.)


Galilei Galileo An Italian physicist and astronomer who conducted experiments to understand gravity. When was he born and when did he die?



Dwarf Stars, Giant Stars, Supergiants, and Main Sequence Stars

Dwarf Star-A small star that is much fainter than our sun.

Giant Stars-A star with a diameter of 10 to 100 times that of the sun.

Supergiants-A star with a diameter of 100 times or more of that of the sun.

Main Sequence Stars-A star that is fusing hydrogen to helium in its core. Stars spend most of their lives in this stage.



A unit for measuring distances outside our solar system. The distance that light travels in one year; about 6 trillion miles (10 trillion kilometers).



The fourth state of matter made up of electrically charged atomic particles. Another term for plasma is “ionized gas.”


Astrology and Astronomy

Astrology-The unscientific belief that objects and their movements in space influence the behavior of people on Earth.

Astronomy-The scientific study of the sun, moon, stars, planets, and other celestial objects in space.


Isaac Newton

An English philosopher and mathematician who lived from 1642-1727. He formulated the universal law of gravity. 



When hydrogen atoms are joined in the core of a star to create helium and a great deal of energy.


Solar Day

The time from noon one day to noon the next. Earth’s solar day is 24 hours.



A group of stars forming a recognizable pattern that is traditionally named after its apparent form or identified with a mythological figure. Modern astronomers divide the sky into eighty-eight constellations with defined boundaries



The force that attracts objects toward the center of Earth or, in space, toward more massive objects, such as moons toward planets and planets toward the sun.


He is said to have developed his ideas about gravity while watching an apple fall from a tree. He realized that an invisible force, which he called gravity, pulled the apple. Among his many other accomplishments, Newton proposed three laws of motion.

Newton, Isaac. An English mathematician and physicist, 1642-1727.


Nuclear Reaction

A process in which matter is either combined or broken apart and energy is released.


Orbit and Rotate

Orbit-The path of one object in space around another. For example, Earth orbits the sun, and the moon orbits Earth.

Rotate-To turn around a central point or to spin on an axis. Earth rotates on its axis once a sidereal day. (See also sidereal day and solar day.)


Earth Spinning Theory

This is the theory that is accepted today. We have proven that the Earth spins due to seasons changing and the happenings of night and day. Copernicus helped prove this theory to be true. The Galileo SpaceCraft also helped prove this theory with providing the first video evidence of the Earth Spinning.


Milky Way

The name of the galaxy in which our sun is a medium-sized star among 100 billion other stars.



A Polish astronomer who lived from 1473-1543. He developed and promoted the now-accepted theory that the Earth and other planets move around the sun.


Hydrogen and Helium

Hydrogen-A colorless, odorless, flammable gas that when combined with oxygen creates water. It is the lightest of known elements. 

Helium-An inert gas present in the core of stars.


The Two Solstices and Equinox

Winter Solstice-The day that marks the sun’s lowest path in the sky and the start of winter. The winter solstice happens around December 21 in the northern hemisphere.

Summer Solstice-The day that marks the sun’s highest path in the sky and the start of summer. The summer solstice happens around June 21 in the northern hemisphere.

Equinox-The date twice a year (in March and September) when day and night are of equal length everywhere; when the sun is directly above the equator.


Sun Spinning Theory

The Ancient Greeks believed that the sun spun. They believed this because just by observing the sky, it appeared that the sun would move from one location to another.


Newton’s Universal Law of Gravity

A law of physics, first formulated by Isaac Newton, that states that every object in the universe gravitationally attracts every other object in the universe. The strength of this attraction is proportional to the mass of the objects and is inversely proportional to the square of the distance between them.

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