DTW GED PREP

The Age of Earth

Earth is estimated to have formed about 4.6 billion years ago. Scientists use three major types of evidence to determine the age of Earth and the objects on it:

1. Landforms

2. Fossils

3. Radiometric Dating

Landforms

The processes that build up and wear down landforms today are the same ones that have operated throughout Earth’s history. By understanding these processes, scientists can learn about Earth’s past by studying the structure and composition of landforms. One important process is the formation of sedimentary rock:

• Over time, weathered material (sediment) settles in layers.
• These layers compact to form sedimentary rock.
• Newer layers form on top of older layers.

Result: The youngest layers are at the top, and the oldest are at the bottom.

This principle helps scientists determine the relative age of rock layers in a landform.

Fossils

Fossils are: Remains (like shells or bones)Or evidence (like imprints) left behind by dead organisms. Fossils are usually found in sedimentary rock.

Key ideas: The age of a fossil can be estimated by the layer it is found in. Conversely, knowing the age of a fossil can help determine the age of the rock layer it is in. By comparing rock layers and fossils, scientists can piece together relative ages of objects on Earth.

Radiometric Dating

Unlike relative dating, radiometric (or radioactive) dating provides the exact, or absolute, age of rocks and objects.

Key concepts: Elements have specific numbers of protons in their nuclei. Some elements have radioactive isotopes that decay into other elements over time by losing protons. Examples: Carbon-14 and Uranium-235 (U-235) are isotopes used in dating.

Half-life: The time it takes for half of the isotope’s atoms to decay into another element. For example, U-235 has a half-life of 1 billion years: the After 1 billion years → half becomes lead (Pb). After 2 billion years → only one-quarter of the original U-235 remains. This predictable decay allows scientists to calculate the rock’s age.

 The Planets

The planets in our solar system are often grouped into two major categories:

1. Terrestrial Planets: The four smaller planets closest to the Sun.  Have rocky surfaces. Include Mercury, Venus, Earth, and Mars.

2. Gas Giants: The four larger planets farther from the Sun. Have gaseous surfaces. Include Jupiter, Saturn, Uranus, and Neptune.

3. Dwarf Planets: A newer category for certain small celestial objects. Have enough mass to be rounded by their own gravity but have not cleared their orbital paths. Example: Pluto.

Moons

A moon is any natural object in space that orbits a planet. Earth has one moon. Mercury has no moons. Jupiter has dozens of moons, including large ones like Ganymede and Europa. Planets may have no moons, one moon, or many moons, depending on their location and formation history.

Asteroids

Asteroids are large, irregularly shaped chunks of rock in space. Most asteroids are found in a region called the asteroid belt, which separates the terrestrial planets from the gas giants. Though asteroids exist throughout the solar system, this belt is the main concentration of them.

Comets

Comets are made of frozen gases and dust particles. Usually smaller than asteroids. Some orbit the Sun continuously. Others pass by the Sun once before traveling off into space. Example: Halley’s Comet: Orbits the Sun and passes Earth every 76 years.

Interactions Between Earth and the Solar System

Earth’s Movements

• Revolution: Earth orbits (revolves) around the Sun. Completes one revolution every 365¼ days (a year).
• Rotation: Earth spins (rotates) on its axis. Completes one rotation every 24 hours (a day).

(These movements are typically illustrated in a diagram showing Earth’s orbit and rotation.)

Eclipses and Tides

Interactions among Earth, the Moon, and the Sun cause: Eclipses, and Tides

• Eclipses happen when one object in space blocks light from reaching another.

Solar Eclipse: The Moon blocks sunlight from reaching Earth.

Lunar Eclipse: Earth blocks sunlight from reaching the Moon.

(Diagrams usually accompany this section to show the alignment during eclipses.)

• Tides: The daily rise and fall of ocean levels is known as tides. High tide: Ocean reaches its highest point on the shore. Low tide: Ocean reaches its lowest point on the shore.
• Most shorelines experience two high tides and two low tides every day.

Cause of Tides

• Moon’s Gravity: As the Moon orbits Earth, its gravity pulls on the ocean, creating bulges of water. Shores in these bulges experience high tide. Shores outside the bulges experience low tide. As Earth rotates, different places move through these bulges, causing changing tides throughout the day.
• Sun’s Gravity: The Sun’s gravity also affects tides. When the Sun and Moon are in line with Earth, their gravitational pulls combine, creating extra high and extra low tides, called spring tides. When the Sun and Moon are at right angles to each other relative to Earth, they pull in different directions, causing milder differences between high and low tides, known as neap tides.

(Illustrative diagrams usually accompany this topic to show spring and neap tide formation.)

The Universe

The total of all matter and energy that exists is called the universe . The different objects that exist in our solar system are found throughout the universe. The universe contains many other solar systems, as well as other types of objects.

Galaxies

A galaxy is a massive collection of stars, gas, dust, and dark matter held together by gravity. Our solar system is part of the Milky Way Galaxy, along with about 200 billion other stars. The universe is estimated to contain at least a billion other galaxies. Galaxies come in different shapes, including spiral, elliptical, and irregular forms. The Milky Way is a spiral galaxy with arms that curve outward from its dense central core.

(Illustrations often show the variety of galaxy shapes.)

Stars

A star is a ball of gas (primarily hydrogen and helium) that produces its own light and heat through nuclear fusion. Our Sun is a star.

Constellations: A constellation is a recognized pattern of stars in the night sky. Famous constellations include: Ursa Major (the Big Dipper) Orion

Because Earth orbits the Sun, different constellations are visible at different times of the year. Scientists use constellations to help identify regions of space visible from Earth.

Age and Development of the Universe

Scientists estimate that the universe formed 10 to 20 billion years ago. The most widely accepted explanation for the universe’s origin is the Big Bang theory: According to this model, all the matter and energy in the universe was once concentrated in an incredibly small, dense point—about the size of an atom. Around 13.8 billion years ago, a massive explosion—the Big Bang—caused the universe to begin rapidly expanding. Scientists believe the universe has continued to expand ever since.

(Diagrams often show the timeline of the Big Bang and the universe’s expansion.)

Age and Development of Stars

Stars vary in composition, size, and age, but all stars follow the same basic life cycle:

Formation: Stars begin in a cloud of gas and dust called a nebula. Gravity pulls gas and dust inward to form a protostar.

Main Sequence: The protostar becomes a main sequence star. Stars spend most of their life in this phase. In main sequence stars, hydrogen atoms fuse into helium, releasing heat and light through nuclear fusion.

Star’s Death

A star dies when it runs out of hydrogen fuel. A small or medium star will stop glowing and become a black dwarf after first becoming a white dwarf (a hot, dense remnant that slowly cools).

Supergiant Stars: Very massive protostars become supergiant stars. When a supergiant dies, it ends in a supernova, a powerful explosion that can briefly outshine an entire galaxy. The remains of a supernova can collapse into a black hole, an object with gravity so strong that not even light can escape it.

Key Points

•  Galaxies are vast collections of stars, gas, dust, and dark matter.
• Our solar system is in the Milky Way, a spiral galaxy.
•  Stars are glowing spheres of gas that produce their own heat and light.
• Constellations are patterns of stars used for navigation and identification.
•  The Big Bang Theory explains the universe’s origin and expansion.

Stars form in nebulas and go through life cycles that end in dwarfs, supernovae, or black holes