Lesson Summary: Asteroids, Meteors, and Comets

Comets

• Comets are broken into two categories: long-period comets (orbit longer than 200 years and originate from the Oort Cloud) and short-period comets (orbit less than 200 years and originate from the Kuiper Belt).

Inner and Outer Planets

• The four planets closest to the Sun are known as the inner planets.
• The four planets farthest away from the Sun are known as the outer planets.
• The outer planets are Jupiter, Saturn, Uranus, and Neptune.
• All four outer planets share similarities in their formation from icy and gas materials.
• They all contain rings and many moons that orbit them.
• Both Jupiter and Neptune also contain storms on their surfaces.

Jovian Planets

• Jupiter has 79 known moons.
• The word Jovian literally means like Jupiter.
• Jovian planets are gas giants or ice giants that share commonalities with Jupiter.
• The other Jovian planets are: Saturn, with 82 known moons, Neptune, with 14 known moons, and Uranus, with 27 known moons.
• Moon numbers are always qualified with the word known because moons are continually being discovered.
• Jupiter's four largest moons, known as the large Jovian moons or the Galilean moons, are Calisto, Io, Europa, and Ganymede.
• Ganymede is the largest moon in the solar system and Jupiter's largest moon.
• It is larger than the planet Mercury and is the only known moon to have its own magnetic field.
• It also contains a significant amount of liquid water, perhaps as much as what is contained on Earth.
• The second-largest moon in the solar system is Titan, which is a satellite of Neptune.

Scale Model of the Solar System

• A scale model of the solar system should preserve all of the relative sizes of the planets, moons, and the sun, including the distances between them.
• On a scale of 1:90,000,000, the sun would be roughly the size of a typical house.
• The eight planets would have roughly the following sizes:
  • Mercury would be a billiard ball.
  • Venus would be a shot put.
  • Earth would be a cereal bowl.
  • Mars would be a baseball.
  • Jupiter would be a kitchen table.
  • Saturn would be a blue kiddie pool.
  • Uranus would be a beach ball.
  • Neptune would be a large pizza.
• The distances at this scale would be immense.
• Earth would have to be placed roughly 1 mile from the sun, while Neptune, the most distant planet, would be over 30 miles away.

Geocentric and Heliocentric Models

• A geocentric model of the universe places the Earth at a fixed position in the center of the universe, with heavenly bodies orbiting it.
• A heliocentric model of the universe places the Sun at the center with the planets orbiting it.
• The geocentric model of the universe can be traced back to Ancient Greece.
• The Ptolemaic model was questioned heavily by Arabic astronomers for centuries, but it was not completely abandoned until a millennia later.
• Scientists such as Nicholas Copernicus, Galileo Galilei, Johannes Kepler, and Isaac Newton all contributed to moving the scientific community toward adopting the accurate heliocentric model of the solar system.

Newton's Law of Gravitation

• Newton formulated his universal law of gravitation after he watched an apple fall to the ground under the action of Earth's gravity.
• According to Newton's law of gravitation, every object in the universe attracts every other object by virtue of its mass.
• The law of gravitation states that for two bodies with masses m1 and m2 at a distance r from each other, the attractive force of gravity is:
  • Directly proportional to m1 × m2.
  • Inversely proportional to r².
• The corresponding gravitational force is given by the formula F = G × (m1 × m2) / r².
• Here, G = 6.67 × 10^(-11) Nm²kg⁻² is the universal gravitational constant.
• The force of gravitation is large for massive objects, such as the Earth and the Sun, and it is very small for subatomic particles.

Gravity

• Gravity is a fundamental force that attracts everything with mass to everything else with mass.
• Radial gravitational attraction occurs because each object has a gravitational field that points from all directions toward the object's center of mass.
• Gravity follows Newton's universal law of gravitation: F = G × (m1 × m2) / r².
• Gravity is directly proportional to mass and inversely proportional to the square of the distance.
• The force of gravity is very weak for light objects like tennis balls and distant objects like Mars.
• Gravity is responsible for keeping objects anchored to Earth's surface, for keeping planets and moons in their orbits as they move in free fall around more massive objects, and for shaping planets and stars.

Star Formation

• Stars are formed in stellar nebulas, or large gas clouds made from hydrogen.
• When hydrogen gas collapses into a sphere, a protostar is formed.
• Protostars are the first stage in star formation.
• The goal of a protostar is to accumulate enough material to compress its core and obtain nuclear fusion of hydrogen.
• When this process fails, a brown dwarf is created.
• When the process succeeds, a new star is born, and the star enters the main sequence of its life cycle.
• The main sequence of a star can last from 10 million to 1 trillion years, depending on the mass of a star.
• A star's life after the main sequence also depends on the star's mass.
• A low-mass star will swell into a red giant, then become a planetary nebula.
• A high-mass star will become a red supergiant before ultimately undergoing a supernova.

Luminosity

• Luminosity is the total energy a star radiates in one second.
• Luminosity is the star's intrinsic brightness.
• This is different from the apparent brightness of a star.
• Meaning, how bright it appears to us here on Earth.

Asteroids, Meteors, and Comets

• Asteroids, meteors, and comets are three types of space debris.
• Asteroids range in size from less than one kilometer to hundreds of kilometers wide and are composed of rocky material.
• They are typically found orbiting in the asteroid belt located between Mars and Jupiter.
• They are broken into three primary types:
  • C-type – chondrite asteroids; the most common type; believed to be composed of clay and silicate rocks; identified by their dark color.
  • S-type – stony asteroids; made of nickel-iron and silicate materials; appear somewhat bright.
  • M-type – metallic asteroids; composed of nickel-iron, and some have volcanic lava on their surface; bright appearance.
• Meteors are rocks and debris from space that enter the Earth's atmosphere and burn up.
• They are commonly referred to as shooting stars.
• When a meteor is still in space, it is referred to as a meteoroid.
• If a meteor makes it through the atmosphere and falls to the ground, it is then referred to as a meteorite.
• Comets are icy objects with large orbits around the sun that release gases when they come close enough to the sun to warm up.
• These gases produce a large, bright "head" with a trailing tail that can be millions of miles long.
• Comets originate from the Oort Cloud and Kuiper Belt.

Stars and Temperature

• The temperature of stars has a great deal to do with their color.
• By using certain tools, such as a blackbody curve, it is possible to categorize stars into certain spectral classes, from as hot as 50,000 Kelvin to as cool as 2500 Kelvin.
• A blackbody curve is a graph that plots wavelength versus intensity of light.
• It allows us to understand the correlation of a star's temperature and its color.
• The greater the intensity of a star, the more energy and heat it emits.
• Thus, it produces wavelengths that are shorter.
• Because light is a form of electromagnetic radiation, in the visible light spectrum, this translates to hotter stars appearing blue, and cooler stars appearing red.
• Blackbody curves operate based on Wien's Law, a scientific law which states that the wavelength of maximum intensity that a blackbody emits is inversely proportional to its temperature.
• The higher the temperature, the shorter the wavelength, the lower the temperature, the longer the wavelength.

Size of Stars

• To find out the size of a star, you first need to know its temperature and luminosity.
• Luminosity refers to the total energy a star radiates in one second.
• Luminosity and temperature are used on the Hertzsprung-Russell (H-R) diagram.
• It is a diagram that plots a star's luminosity vs. surface temperature.
• Approximately 90% of stars lie on a band on the H-R diagram that is called the main sequence.
• A star whose characteristics place it in the main sequence is called a main-sequence star.
• Cool stars like red giants have to be huge in order to be luminous.
• This is because a cool star radiates a lot less light per unit of surface area than a hot star.
• On the other hand, small hot stars, like white dwarfs, have very low luminosities despite their high temperatures precisely because they are so small### Stellar Characteristics
• The absolute visual magnitude of a star is its apparent visual magnitude if it were 10 pc away.
• 1 pc (parsec) is a unit of distance equal to 2.06 * 10^5 AU or 3.26 light years.

Star Temperature and Color

• The temperature of a star determines its color.
• Stars can be categorized into spectral classes using a blackbody curve, ranging from 50,000 K to 2500 K.
• A blackbody curve plots wavelength vs. intensity of light, showing the correlation between a star's temperature and color.
• Hotter stars appear blue, while cooler stars appear red due to Wien's Law.
• Wien's Law states that the wavelength of maximum intensity is inversely proportional to a star's temperature.

Star Size and Luminosity

• A star's size can be determined by knowing its temperature and luminosity.
• Luminosity is the total energy a star radiates in one second.
• The Hertzsprung-Russell (H-R) diagram plots a star's luminosity vs. surface temperature.
• Approximately 90% of stars lie on the main sequence of the H-R diagram.
• Surface area and temperature affect luminosity, which determines a star's positioning on the H-R diagram.
• Cool stars like red giants need to be huge to be luminous, while small hot stars like white dwarfs have low luminosities despite their high temperatures.

Space Debris

• Asteroids are rocky objects ranging in size from <1 km to hundreds of kilometers, found in the asteroid belt between Mars and Jupiter.
• There are three primary types of asteroids: C-type (chondrite), S-type (stony), and M-type (metallic).
• Meteors are rocks/debris that enter the Earth's atmosphere and burn up, also known as shooting stars.
• Meteoroids are meteors in space, while meteorites are meteors that fall to the ground.
• Comets are icy objects with large orbits around the Sun that release gases when warmed up, producing a bright head and trailing tail.
• Comets originate from the Oort Cloud and Kuiper Belt.