·        Gravity is an attractive force between masses
·        Gravity is responsible for forming the Sun, the planets, and the moons in the solar system into their spherical shapes
·        Gravity is responsible for holding the solar system together
·        Gravity is responsible for internal pressures in the Sun, Earth and other planets
·        Gravity is responsible for the atmosphere.

Universal Gravitation

·        Newton observed the moon orbiting the Earth and hypothesized that the same force that causes objects to fall to Earth causes the Moon to orbit (or fall around) the Earth
·        Newton knew that the Moon should travel in a straight line (getting farther and farther from Earth) unless a force was acting on it to change its direction into a circular path
·        He published his theory of Universal Gravitation that the force between all massive objects is directly proportional to the produ
ct of their masses and inversely proportional to the square of the distance between their centers.
·        He also used this to explain the motion of Earth and other planets about the Sun.

Formation of the Universe

Right after the formation of the universe all matter was made of light elements, primarily

·        Hydrogen,
·        Helium, and
·        Lithium

These elements were scattered out in space

·        Because of the attraction of every particle of matter for every other particle of matter these scattered atoms began to join together, eventually forming stars

We use the fundamental force gravity to explain the formation of stars


·        Stars are not uniformly distributed throughout the universe

·        Stars are clustered by the billions in galaxies

·        Some of the stars visible in the night sky were originally thought to be stars


·                    gravity also explains the formation of the clusters of stars called galaxies

·                    and it explains their motion

Gravity controls the size and shape of the universe.

·        galaxies are clusters of billions of stars

·        galaxies may have different shapes.

·        Galaxies them-selves appear to form clusters that are separated by vast expanses of empty space

·        As galaxies are discovered they are classified by their differing sizes and shapes.

Shapes of Galaxies

The most common galaxy shapes are
·           spiral,
·           elliptical, and
·           irregular.
Astronomers have inferred the existence of planets orbiting some stars.

 The Milky Way

·           the Sun is one of many stars in the Milky Way galaxy
stars may differ in
·           size,
·           temperature, and
·           color.
The Sun is a star located on the rim of a typical spiral galaxy
·                    Our galaxy is called the Milky Way
·                    Our Solar system orbits the center of the Milky Way galaxy
·                    In similar spiral galaxies this galactic center appears as a bulge of stars in the heart of the disk.
·                    The bright band of stars cutting across the night sky is the edge of the Milky Way as seen from the perspective of Earth, which lies within the disk of the galaxy.
·                    Stars vary greatly in size, temperature, and color.
·                    For the most part those variations are related to the stars’ life cycles.
·                    Light from the Sun and other stars indicates that the Sun is a fairly typical star.
·                    It has a mass of about 2 × 1030 kg and
·                    an energy output, or luminosity, of about 4 × 1026 joules/sec.
·                    The surface temperature of the Sun is approximately 5,500 degrees Celsius, and
·                    the radius of the Sun is about 700 million meters.
·                    The surface temperature determines the yellow color of the light shining from the Sun.
·                    Red stars have cooler surface temperatures, and
·                    blue stars have hotter surface temperatures.
·                    To connect the surface temperature to the color of the Sun or of other stars,
·                    teachers should obtain a “black-body” temperature spectrum chart, which is typically found in high school and college textbooks.
·                    c. Students know how to use astronomical units and light years as measures of distances between the Sun, stars, and Earth.
·                    Distances between astronomical objects are enormous.
·                    Measurement units such as centimeters, meters, and kilometers used in the laboratory or on field trips are not useful for expressing those distances.
·                    astronomers use other units to describe large distances.
·                    The astronomical unit (AU) is defined to be equal to the average distance from Earth to the Sun: 1 AU = 1.496 × 1011 meters.
·                    Distances between planets of the solar system are usually expressed in AU.
·                    For distances between stars and galaxies, even that large unit of length is not sufficient.
·                    Interstellar and intergalactic distances are expressed in terms of how far light travels in one year, the light year (ly):
·                    1 ly = 9.462 × 1015 meters = approximately 6 trillion miles
·                    The most distant objects observed in the universe are estimated to be 10 to 15 billion light years from the solar system.
·                    become familiar with AUs by expressing the distance from the Sun to the planets in AUs instead of meters or miles.
·                    A good way to become familiar with the relative distances of the planets from the Sun is to lay out the solar system to scale on a length of cash register tape.
·                    d. Students know that stars are the source of light for all bright objects in outer space
·                    the Moon and planets shine by reflected sunlight, not by their own light.
·                    The energy from the Sun and other stars, seen as visible light, is caused by nuclear fusion reactions that occur deep inside the stars’ cores
·                    By carefully analyzing the spectrum of light from stars, scientists know that most stars are composed primarily of hydrogen, a smaller amount of helium, and much smaller amounts of all the other chemical elements.
·                    Most stars are born from the gravitational compression and heating of hydrogen gas.
·                    A fusion reaction results when hydrogen nuclei combine to form helium nuclei.
·                    This event releases energy and establishes a balance between the inward pull of gravity and the outward pressure of the fusion reaction products.
·                    Ancient peoples observed that some objects in the night sky wandered about while other objects maintained fixed positions in relation to one another (i.e., the constellations).
·                    Those “wanderers” are the planets.
·                    Through careful observations of the planets’ movements, scientists found that planets travel in nearly circular (slightly elliptical) orbits about the Sun.
·                    Planets (and the Moon) do not generate the light that makes them visible
·                    This is demonstrated during eclipses of the Moon or by observation of the phases of the Moon and planets when a portion is shaded from the direct light of the Sun.
·                    Various types of exploratory missions have yielded much information about the reflectivity, structure, and composition of the Moon and the planets.
·                    Those missions have included spacecraft flying by and orbiting those bodies, the soft landing of spacecraft fitted with instruments, and, of course, the visits of astronauts to the Moon.


·                    e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids originally thought to be stars are now known to be distant galaxies.
·                    Nine planets are currently known in the solar system:
·                    Mercury,
·                    Venus,
·                    Earth,
·                    Mars,
·                    Jupiter,
·                    Saturn,
·                    Uranus,
·                    Neptune, and
·                    Pluto.
·                    They vary greatly in size and appearance.
·                    For example, the mass of Earth is 6 × 1024 kg and the radius is 6.4 × 106 m.
·                    Jupiter has more than 300 times the mass of Earth, and the radius is ten times larger.
·                    The planets also drastically vary in their
·                    distance from the Sun,
·                    period of revolution about the Sun,
·                    period of rotation about their own axis,
·                    tilt of their axis,
·                    composition, and
·                    appearance.
·                    The inner planets (Mercury, Venus, Earth, and Mars) tend to be relatively small and are composed primarily of rock.
·                    The outer planets (Jupiter, Saturn, Uranus, and Neptune) are generally much larger and are composed primarily of gas.
·                    Pluto is composed primarily of rock and is the smallest planet in the solar system.
·                    All the planets are much smaller than the Sun.
·                    All objects are attracted toward one another gravitationally, and the strength of the gravitational force between them depends on their masses and the distance that separates them from one an-other and from the Sun.
·                    Before Newton formulated his laws of motion and the law of universal gravitational attraction, German astronomer Johannes Kepler deduced from astronomical observations three laws (Kepler’s laws) that describe the motions of the planets.

·      Moons   

Planets have smaller objects orbiting them called satellites or moons.
·                    Earth has one moon that completes an orbit once every 28 days (approximately).
·                    Mercury and Venus have no moons, but
·                    Jupiter and Saturn have many moons.
·                    Very small objects composed mostly of rock (asteroids) or the ice from condensed gases (comets) or both also orbit the Sun.
·                    The orbits of many asteroids are relatively circular and lie between the orbital paths of Mars and Jupiter (the asteroid belt).
·                    Some asteroids and all comets have highly elliptical orbits, causing them to range great distances from very close to the Sun to well beyond the orbit of Pluto.
·                    A visit to a planetarium would be another way of observing the sky.
·                    If feasible, teachers should have students observe the motion of Jupiter’s inner moons as well as the phases of Venus.
·                    Using resources in the library-media center, students can research related topics of interest.