Earth in the Universe

How Scientists Study Earth in the Universe

• The shape of Earth's orbit is close to a circle, not an oval.

• Earth's distance from the sun varies throughout the year.

• Around July 4, Earth is farthest from the sun, which is summer in the northern hemisphere.

• Around January 3, Earth is closest to the sun.

• The whole sky is 180 degrees from one horizon to the opposite horizon.

• Angular degrees can be divided into 60 minutes (') and each minute into 60 seconds (").

• Both the sun and the moon are about 0.5 degrees in apparent angular diameter.

• It would take about 360 suns or moons to span the sky from horizon to horizon.

• When the sun is closest to Earth (around January 3), its apparent diameter is 0° 32′ 35".

• Around July 4, the sun's apparent angular diameter is 0° 31′ 31".

• The difference in the sun's apparent diameter is 0° 01' 04", which is very small, so distance is not the major cause of seasons.

Vocabulary

• asteroid

• Big Bang theory

• celestial object

• comet

• Doppler effect

• eccentricity

• ellipse

• focus (pl., foci)

• galaxy

• gravitation

• impact crater

• impact event

• inertia

• Jovian planet

• luminosity (of a star)

• meteor

• Milky Way Galaxy

• moon

• nuclear fusion

• red shift

• revolution

• rotation

• solar system

• star

• terrestrial planet

• universe

Topic Overview

• Humans have always observed celestial objects and wondered about them.

• A celestial object is any object outside or above Earth's atmosphere.

• There are about 6000 celestial objects visible with the unaided eye from Earth.

• This topic focuses on our solar system, our galaxy, and the universe.

• Dark matter and dark energy may compose a major portion of the universe and the Milky Way Galaxy, but are not covered in this course.

Origin and Age of the Universe

• Various cultures have different theories about the universe's origin, evolution, and structure.

• The universe includes everything that exists—all space, matter, and energy.

• Most scientists believe the universe is extremely vast and more than 10 billion years old (possibly 13.7 billion years old).

Big Bang Theory

• The universe began with the Big Bang and has been expanding ever since.

• The Big Bang theory states that all matter and energy were initially concentrated in a small area.

• After a gigantic explosion, matter organized into subatomic particles and atoms.

• The earliest atoms were mostly hydrogen and helium.

• Within a few hundred million to a billion years, atoms organized into celestial bodies.

• Most stars became part of gravitational groupings.

• As matter organized, the universe expanded in all directions and continues to do so.

Evidence for the Big Bang

• If the Big Bang occurred, energy created by the explosion expanded with matter.

• There should be radiation from the Big Bang in all parts of the universe.

• Scientists have found long-wavelength microwave background radiation from all directions in the universe.

• This cosmic background radiation supports the Big Bang theory.

Electromagnetic Spectrum

• The various types of waves that transmit energy through space are called electromagnetic energy.

• Each element emits energy in specific wavelengths within the electromagnetic spectrum.

• Humans see different wavelengths of visible light as different colors.

• Scientists study the spectrum of electromagnetic energy from stars and celestial objects to infer their composition.

• They compare signature wavelengths produced by elements on Earth with those from celestial objects.

Doppler Effect

• The position of characteristic wavelengths or colored lines can shift to shorter (blue end) or longer (red end) wavelengths.

• This shifting of wavelengths is called the Doppler effect.

• The Doppler effect is caused by relative movement between Earth and celestial objects.

• If Earth and a celestial object are coming closer, electromagnetic waves are bunched together, resulting in a blue shift.

• If Earth and a celestial object are moving apart, electromagnetic waves are spread out, causing a red shift.

• The collective light from stars in almost all galaxies has a red shift, indicating that the universe is expanding in all directions.

• The farther away a galaxy is from Earth, the greater the red shift, indicating an increasing rate of expansion.

Structure of the Universe

• Instruments like the Hubble Space Telescope help scientists understand the universe's structure.

Galaxies

• The basic structural unit of matter in the universe appears to be the galaxy.

• A galaxy is a collection of billions of stars held together by gravity, gas, and dust.

• An average galaxy has over 100 billion stars, and there are over 100 billion galaxies.

• Galaxies are classified by shape, including elliptical, irregular, and spiral.

• Our solar system is part of the spiral-shaped Milky Way Galaxy, which has over 200 billion stars.

• The solar system is located between two spiral arms about two-thirds of the distance from the galactic center.

Stars

• Stars, along with dust and gas clouds, make up most of the visible matter in a galaxy.

• A star is usually a large ball of gas held together by gravity that produces energy and shines.

• Some old stars can be the size of planets or moons, and some stars no longer emit much radiation.

• The sun is the dominant gravitational force in our solar system.

Energy Production in Stars

• Most energy produced by stars results from nuclear fusion in their cores.

• Nuclear fusion combines the nuclei of smaller elements to form larger elements, converting some mass into energy.

• The sun converts hydrogen nuclei into helium nuclei, with about 0.07 percent of the mass forming energy.

• Nuclear fusion requires extremely high temperature and pressure, found in star interiors.

• Energy from nuclear fusion is radiated into space as electromagnetic energy.

Characteristics of Stars

• Stars are grouped by surface temperature and luminosity, as shown in the Characteristics of Stars diagram.

• Luminosity is the actual brightness of a star compared to the sun, measured as absolute magnitude.

• Absolute magnitude is a star's brightness at a standard distance.

• Apparent brightness depends on absolute luminosity and distance from us.

• Star properties are not random; they are grouped by differences in luminosity and surface temperature (color).

• Star color changes from red to blue as surface temperatures increase.

Star Types

Main Sequence Stars

• About 90 percent of studied stars are located on the main sequence.

• Stars spend most of their life span as main sequence stars.

• Most are average size; as surface temperatures increase, luminosity increases.

• Luminosity increase from red to blue-white is mostly related to an increase in star size and the resulting higher temperatures.

• Our sun is a yellow main sequence star.

• The smallest and coolest stars of the main sequence are red dwarfs.

Giant Stars

• Red, orange, and yellow giant stars are rare but commonly seen due to their large size (10 times or more the diameter of the sun) and high luminosity.

• These low-temperature stars are in a late stage of evolution for medium to small-size main sequence stars, expanding greatly in size.

Super Giants

• Super giant stars can be anywhere from 100 to 1000 times the diameter of the sun.

• These highly luminous stars represent the late evolution of stars more massive than the sun.

• Super giants usually explode in a supernova.

• The brightest and hottest super giant stars are blue super giants.

White Dwarfs

• Not all white dwarf stars are white, but they are all small (around the size of Earth).

• They are hot on the surface and low in luminosity.

• They represent the last luminous stage of low- to medium-mass stars.

Black Dwarfs

• When a white dwarf cools and no longer emits much electromagnetic energy, it becomes a black dwarf.

• Black dwarfs are probably very common in the universe because many white dwarfs have stopped nuclear fusion over billions of years.

Star Origin and Evolution

• Stars have an origin, evolution, and ending.

• Stars originate from clouds of gas and dust molecules, created from the Big Bang or from mass given off by other stars.

• Gravity causes these clouds to clump up, forming larger balls of gas and dust molecules.

• When the mass of these spherical balls becomes slightly larger than Jupiter, gravitational contraction starts nuclear fusion.

• The ball begins to shine, radiating electromagnetic energy, and a star is born.

Star Evolution Stages

• Star evolution after the main sequence depends on its original mass.

• Stars with masses similar to the sun spend billions of years on the main sequence and eventually expand to become red giants.

• These stars use up most of their nuclear fuel and collapse to form a white dwarf, then slowly die, becoming a black dwarf.

• This process may take many billions of years.

• Stars with original masses greater than 1.5 times the mass of the sun evolve differently and exist for shorter periods (approximately 100,000,000 years).

• These massive stars evolve into super giants after being large main sequence stars, eventually exploding in a supernova.

• After a supernova, they rapidly collapse, forming a body much smaller than a white dwarf.

• If the density becomes so great that only neutrons can exist, a neutron star is formed.

• When even more massive stars collapse, the extreme gravity field allows no visible light or energy to escape, forming a black hole.

Solar System

• Powerful telescopes have found evidence of planets around more than 100 stars.

• "Solar system" can refer to any star or group of stars with orbiting objects.

• In this book, the solar system refers to our solar system: the sun and all objects that orbit it.

Parts of the Solar System

• Most of the solar system is space devoid of much mass.

• About 99 percent of the mass is contained in the sun.

• Satellites orbit or revolve around other objects; planets, asteroids, meteoroids, and comets are satellites of the sun; moons are satellites of planets or asteroids.

• Planets are the largest objects that independently orbit the sun and don't share their environment with other large objects; they are generally spherical.

Kuiper Belt and Oort Cloud

• The Kuiper Belt is a region beyond Neptune that includes Pluto and other dwarf planets, moons, and comets.

• Many comets that enter the inner solar system originate in the Kuiper Belt, which extends out to about 1000 times Earth's distance from the sun.

• The Oort Cloud is a vast sphere of comet-like bodies surrounding the solar system, potentially extending 50,000 to 100,000 times Earth's distance from the sun.

Asteroids

• Asteroids are solid rocky and/or metallic bodies that independently orbit the sun.

• They have irregular shapes (except for larger spherical ones) and no atmosphere.

• A large percentage are located between Mars and Jupiter.

• Asteroids are smaller than planets (about 100 to 1000 kilometers in diameter) and are often called minor planets.

Moons

• Moons orbit a planet or asteroid as these objects orbit the sun.

• There are approximately 175 known moons, which vary in size.

Comets

• Comets are composed of solids that easily turn to gases when heated.

• They are largely ices of water and methane mixed with rocky or metallic solids.

• Most comets are 1 to 100 kilometers in diameter.

• When comets get near the sun, some ices turn to gases, releasing solids and forming tails.

Meteoroids, Meteors, and Meteorites

• Meteoroids are very small solid fragments that orbit the sun.

• When meteoroids burn up in Earth's atmosphere, they create visual streaks called meteors.

• If a meteoroid survives passage through Earth's atmosphere and lands on the surface, it is called a meteorite.

• Some meteorites create depressions in Earth's crust called impact craters.

Evolution of the Solar System

• Our solar system started forming approximately 5 billion years ago.

• A gas-dust cloud condensed due to gravitation, possibly aided by a shock wave from an exploding star.

• Planets, moons, and asteroids formed from these masses.

• Impact events (collisions with comets, asteroids, and meteoroids) occurred, causing craters on solid surfaces and influencing global climate changes and mass extinctions.

• Intensive scientific searches are conducted to find potentially hazardous space objects.

Planet Characteristics

• A planet's distance from the sun significantly affects its characteristics.

• When planets were forming, the sun likely radiated more energy, driving less dense elements from the inner solar system.

• The high temperatures and pressure from particles emitted by the sun drove the less dense elements and compounds away from the inner solar system.

• Outer parts of the solar system were not so hot, leading to the classification of planets into terrestrial and Jovian types.

Terrestrial Planets

• Terrestrial planets are close to the sun and mostly solid.

• They have small diameters and high densities.

• Their rocky surfaces are dotted with impact craters.

• They have few or no moons and no rings.

• Examples include Mercury, Venus, Earth, and Mars.

Jovian Planets

• Jovian planets are far from the sun and largely gaseous.

• They have large diameters and low densities.

• They have no solid surfaces (though they may have a solid core) and thus no craters.

• These planets have many moons and rings.

• Examples include Jupiter, Saturn, Uranus, and Neptune.

Motions of the Planets

• Planets move with the solar system around the Milky Way Galaxy in approximately 225 million years.

• They rotate (spin around an axis) and revolve (move around the sun in an orbit).

Planet Rotation

• Rotation is the spinning of a planet on its axis.

• The period of rotation is the time it takes for one spin, determining the length of a planet's day.

• Six of the eight planets rotate in the same direction as they revolve around the sun.

Planet Revolution

• Revolution is a planet's movement around the sun in a path called an orbit.

• Planets revolve around the sun in a counterclockwise direction (as viewed from Polaris).

• Planetary orbits are elliptical, with the sun at one focus.

Eccentricity of Planet Orbits

• Eccentricity measures the "ovalness" or flattening of an ellipse.

• The formula for eccentricity is: eccentricity = \frac{distance \, between \, foci}{length \, of \, major \, axis}

• As the foci get closer, the ellipse becomes more circular, and eccentricity decreases toward zero.

• Except for Mercury, planetary orbits look like circles to the human eye.

Varying Distance of Planets from the Sun and Apparent Solar Size

• Elliptical orbits cause planets to vary in distance from the sun.

• Earth is closest to the sun around January 3rd and farthest around July 4th.

• This difference in distance does not cause seasons.

• The sun appears largest when Earth is closest (around January 3) and smallest when farthest (around July 4).

Inertia, Gravitation, Orbital Velocity/Speed, and Planet Orbits

• Planets operate under a balance between inertia and gravitation.

• Inertia is the tendency of an object to remain at rest or in motion unless acted upon by an opposing force.

• Gravitation is the attractive force between any two objects, proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

• The orbit of a planet is a balance between inertia and gravitation.

• Since planetary orbits are elliptical, distance from the sun varies, causing orbital speed to vary.

• A planet's orbital speed and velocity is greatest when closer to the sun and slowest when farthest from the sun.

Period of Revolution

• The period of revolution is the time it takes for a planet to orbit the