♞ Kepler's Three Fundamental Laws of Planetary Motion

Tycho Brahe’s accurate observations of planetary positions provided the data used by Johannes Kepler to derive his three fundamental laws of planetary motion.
Kepler’s laws describe the behavior of planets in their orbits as follows:

(1) planetary orbits are ellipses with the Sun at one focus;

(2) in equal intervals, a planet’s orbit sweeps out equal areas; and

(3) the relationship between the orbital period (P) and the semimajor axis (a) of an orbit is given by 

P² = a³ 

(when a is in units of AU and P is in units of Earth years).

☀️ The Stars: A Celestial Census - Key Terms

 

binary stars

two stars that revolve about each other

brown dwarf

an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 1/12 the mass of the Sun

eclipsing binary

a binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it

H–R diagram

(Hertzsprung–Russell diagram) a plot of luminosity against surface temperature (or spectral type) for a group of stars

main sequence

a sequence of stars on the Hertzsprung–Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right

mass-luminosity relation

the observed relation between the masses and luminosities of many (90% of all) stars

selection effect

the selection of sample data in a nonrandom way, causing the sample data to be unrepresentative of the entire data set

spectroscopic binary

a binary star in which the components are not resolved but whose binary nature is indicated by periodic variations in radial velocity, indicating orbital motion

visual binary

a binary star in which the two components are telescopically resolved

white dwarf

a low-mass star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final state of life

👕 Life in the Universe. Key Terms

 

organic compounds that are the molecular building blocks of proteins	amino acids
the multidisciplinary study of life in the universe: its origin, evolution, distribution, and fate; similar terms are exobiology and bioastronomy	astrobiology
evidence of the presence of life, especially a global indication of life on a planet that could be detected remotely (such as an unusual atmospheric composition)	biomarker
a molecule that stores information about how to replicate a cell and its chemical and structural components	DNA (deoxyribonucleic acid)
a formula for estimating the number of intelligent, technological civilizations in our Galaxy, first suggested by Frank Drake	Drake equation
an organism (usually a microbe) that tolerates or even thrives under conditions that most of the life around us would consider hostile, such as very high or low temperature or acidity	extremophile
the basic functional unit that carries the genetic (hereditary) material contained in a cell	gene
an environment capable of hosting life	habitable environment
the region around a star in which liquid water could exist on the surface of terrestrial-sized planets, hence the most probable place to look for life in a star’s planetary system	habitable zone
a compound containing carbon, especially a complex carbon compound; not necessarily produced by life	organic compound
a combination of carbon and other atoms—primarily hydrogen, oxygen, nitrogen, phosphorus, and sulfur—some of which serve as the basis for our biochemistry	organic molecule
a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product	photosynthesis
a key biological molecule that provides the structure and function of the body’s tissues and organs, and essentially carries out the chemical work of the cell	protein
a molecule that aids in the flow of genetic information from DNA to proteins	RNA (ribonucleic acid)
the search for extraterrestrial intelligence; usually applied to searches for radio signals from other civilizations	SETI
solid, layered rock formations that are thought to be the fossils of oxygen-producing photosynthetic bacteria in rocks that are 3.5 billion years old	stromatolites
an organism that can tolerate high temperatures	thermophile