Astronomy is the study of everything beyond the Earth’s atmosphere. It is one of the oldest natural sciences, with roots in the systematic observation of the night sky by civilizations across every inhabited continent. Modern astronomy uses physics, chemistry, and mathematics to understand the nature, composition, and behavior of celestial objects ranging from nearby moons to the most distant observable galaxies.

Many introductory astronomy courses begin at the Sun and work outward through the solar system before expanding to galactic and intergalactic scales. Planetariums often follow the same pedagogical structure. This article adopts that approach. It introduces the major bodies of the solar system, describes the large-scale structure of the Milky Way and beyond, surveys broad qualitative concepts in astronomy, and collects the mathematical formulas most commonly encountered in an introductory course.

Software Versions

# Date (UTC)
$ date -u "+%Y-%m-%d %H:%M:%S +0000"
2026-02-12 07:15:52 +0000

# OS and Version
$ uname -vm
Darwin Kernel Version 23.6.0: Mon Jul 29 21:14:30 PDT 2024; root:xnu-10063.141.2~1/RELEASE_ARM64_T6000 arm64

$ sw_vers
ProductName:		macOS
ProductVersion:		14.6.1
BuildVersion:		23G93

# Hardware Information
$ system_profiler SPHardwareDataType | sed -n '8,10p'
      Chip: Apple M1 Max
      Total Number of Cores: 10 (8 performance and 2 efficiency)
      Memory: 32 GB

# Shell and Version
$ echo "${SHELL}"
/bin/bash

$ "${SHELL}" --version | head -n 1
GNU bash, version 3.2.57(1)-release (arm64-apple-darwin23)

# Claude Code Installation Versions
$ claude --version
2.1.37 (Claude Code)

The Sun

The Sun is a G-type main-sequence star at the center of the solar system. It contains approximately 99.86% of the total mass of the solar system. The Sun’s diameter is roughly 1.4 million kilometers, about 109 times the diameter of Earth. Its surface temperature is approximately 5,500 degrees Celsius, while the core temperature reaches about 15 million degrees Celsius.

The Sun generates energy through nuclear fusion, converting hydrogen into helium in a process that releases enormous quantities of energy. This energy radiates outward as electromagnetic radiation across the full spectrum, from radio waves through visible light to X-rays and gamma rays.

The Sun’s activity follows an approximately 11-year solar cycle, marked by variations in the number of sunspots, solar flares, and coronal mass ejections. These events can affect space weather and have measurable effects on Earth’s magnetosphere and upper atmosphere.

Mercury

Mercury is the smallest planet in the solar system and the closest to the Sun. Its orbital period is approximately 88 Earth days. Mercury has no atmosphere to speak of, only a thin exosphere composed of atoms blasted off its surface by solar wind and micrometeorite impacts.

The surface of Mercury is heavily cratered and closely resembles the surface of the Moon. Temperatures on Mercury range from about 430 degrees Celsius on the sunlit side to minus 180 degrees Celsius on the dark side, one of the largest temperature swings of any body in the solar system.

Mercury has no moons.

Venus

Venus is the second planet from the Sun and the closest planet to Earth in size and mass. It is often called Earth’s “sister planet,” though the two worlds are radically different in surface conditions.

Venus has a thick atmosphere composed primarily of carbon dioxide with clouds of sulfuric acid. The atmospheric pressure at the surface is about 90 times that of Earth. A runaway greenhouse effect makes Venus the hottest planet in the solar system, with a surface temperature of approximately 465 degrees Celsius, hotter than Mercury despite being farther from the Sun.

Venus rotates in the opposite direction from most planets, a phenomenon called retrograde rotation. A single Venusian day (one full rotation) takes about 243 Earth days, longer than the planet’s orbital period of 225 Earth days.

Venus has no moons.

Earth

Earth is the third planet from the Sun and the only known body in the solar system that currently supports life. Its distance from the Sun, approximately 150 million kilometers, defines the astronomical unit (AU), a standard unit of measurement in astronomy.

Earth’s atmosphere is composed primarily of nitrogen (78%) and oxygen (21%), with trace amounts of argon, carbon dioxide, and water vapor. Liquid water covers approximately 71% of the surface. Earth has a strong magnetic field generated by the convection of molten iron in its outer core, which shields the surface from the majority of the solar wind.

The Moon

Earth has one natural satellite, the Moon. The Moon is the fifth largest moon in the solar system, with a diameter of approximately 3,474 kilometers. It is tidally locked to Earth, meaning the same hemisphere always faces our planet.

The Moon has no atmosphere and no magnetic field. Its surface is divided into two types of terrain: the bright, heavily cratered highlands and the darker, smoother maria (Latin for “seas”), which are ancient basaltic lava flows. The Moon is believed to have formed from debris ejected during a giant impact between the early Earth and a Mars-sized body approximately 4.5 billion years ago.

Mars

Mars is the fourth planet from the Sun, often called the Red Planet because of the iron oxide (rust) that gives its surface a reddish appearance. Mars has a thin atmosphere composed primarily of carbon dioxide.

Mars hosts the tallest known mountain in the solar system, Olympus Mons, a shield volcano approximately 21.9 kilometers high. It also has the largest canyon system, Valles Marineris, which stretches over 4,000 kilometers along the Martian equator. Evidence of ancient riverbeds, lake beds, and polar ice caps suggests that liquid water once existed on the surface.

Moons of Mars

Mars has two small moons, Phobos and Deimos. Both are irregularly shaped and are believed to be captured asteroids. Phobos, the larger of the two, orbits so close to Mars that it completes an orbit in less than eight hours. Tidal forces are gradually pulling Phobos closer to Mars, and it is expected to either crash into the planet or break apart into a ring system within the next 50 million years.

The Asteroid Belt

The asteroid belt occupies the region of space between the orbits of Mars and Jupiter, roughly 2.2 to 3.2 AU from the Sun. It contains millions of rocky bodies ranging from small boulders to objects hundreds of kilometers in diameter. Despite its large population, the total mass of the asteroid belt is estimated to be only about 4% of the Moon’s mass.

Notable Asteroids

Ceres is the largest object in the asteroid belt, with a diameter of approximately 940 kilometers. It was reclassified as a dwarf planet by the International Astronomical Union (IAU) in 2006. Ceres comprises roughly 35% of the asteroid belt’s total mass. NASA’s Dawn mission revealed evidence of water ice at high latitudes and a differentiated internal structure with a water-rich crust overlying a rocky mantle.

Vesta is the second most massive body in the asteroid belt, with a diameter of approximately 525 kilometers. Unlike most asteroids, Vesta is differentiated, with a crust of solidified basaltic lava, a rocky mantle, and a nickel-iron core.

Pallas is the third largest asteroid, with a diameter of approximately 510 kilometers. Its orbit is highly inclined to the ecliptic plane, making it unusually difficult to reach with spacecraft.

Hygiea is the fourth largest asteroid. Its nearly spherical shape has led to discussions about whether it should be classified as a dwarf planet.

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet in the solar system. Its mass is approximately 318 times that of Earth, and it is more massive than all other planets combined. Jupiter is a gas giant composed primarily of hydrogen and helium.

The most recognizable feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm larger than Earth that has been observed for at least 350 years.

Jupiter has a powerful magnetic field, the strongest of any planet in the solar system. It also has a faint ring system, discovered by the Voyager 1 spacecraft in 1979.

Moons of Jupiter

Jupiter has over 95 confirmed moons. The four largest, known as the Galilean moons, were discovered by Galileo Galilei in 1610 and are among the most scientifically interesting objects in the solar system.

Io is the most volcanically active body in the solar system. Tidal heating from Jupiter’s immense gravity drives hundreds of active volcanoes on its surface.

Europa has a smooth ice shell believed to cover a subsurface ocean of liquid water. Europa is considered one of the most promising locations in the solar system to search for extraterrestrial life.

Ganymede is the largest moon in the solar system, larger than the planet Mercury. It is the only moon known to have its own magnetic field.

Callisto is the most heavily cratered body in the solar system. Its surface has remained largely unchanged for billions of years.

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the solar system. It is best known for its extensive and prominent ring system, composed primarily of particles of water ice ranging in size from dust grains to house-sized boulders.

Saturn is a gas giant with a density lower than that of water. Its oblate shape is the most pronounced of any planet in the solar system, a consequence of its rapid rotation (one day on Saturn is approximately 10.7 hours).

Moons of Saturn

Saturn has over 270 confirmed moons, the most of any planet in the solar system.

Titan is Saturn’s largest moon and the second largest moon in the solar system. It is the only moon in the solar system with a dense atmosphere, composed primarily of nitrogen with minor amounts of methane and ethane. Titan has lakes and seas of liquid methane and ethane on its surface, making it the only body in the solar system other than Earth known to have stable bodies of surface liquid.

Enceladus is a small, icy moon that has attracted significant scientific interest because of its active geysers. Plumes of water vapor and ice particles erupt from fractures near its south pole, indicating a subsurface ocean of liquid water. Like Europa, Enceladus is considered a candidate for harboring conditions suitable for life.

Uranus

Uranus is the seventh planet from the Sun and the third largest planet in the solar system. It is classified as an ice giant, with an interior composed primarily of water, methane, and ammonia ices surrounding a small rocky core.

Uranus is unique in that it rotates on its side, with an axial tilt of approximately 98 degrees. This extreme tilt means that each pole gets around 42 years of continuous sunlight followed by 42 years of darkness during its 84-year orbit.

Moons of Uranus

Uranus has 28 confirmed moons, named after characters from the works of William Shakespeare and Alexander Pope. The five major moons are Miranda, Ariel, Umbriel, Titania, and Oberon.

Miranda has one of the most varied landscapes of any moon in the solar system, including large features called coronae that are unique among known celestial bodies.

Titania and Oberon are the largest Uranian moons, discovered by William Herschel in 1787.

Ariel has the brightest and possibly youngest surface among the moons of Uranus.

Neptune

Neptune is the eighth and most distant planet in the solar system. It is an ice giant similar in composition to Uranus. Neptune has the strongest sustained winds of any planet in the solar system, reaching speeds of over 2,000 kilometers per hour.

Neptune was the first planet discovered through mathematical prediction rather than direct observation. Irregularities in the orbit of Uranus led astronomers to predict Neptune’s existence before it was observed through a telescope in 1846.

Moons of Neptune

Neptune has 16 confirmed moons.

Triton is by far the largest moon of Neptune, comprising more than 99.5% of the mass orbiting the planet. Triton is geologically active, with geysers of nitrogen gas erupting from its surface. It orbits Neptune in a retrograde direction, suggesting that it is a captured Kuiper Belt Object rather than a moon that formed in place.

The Kuiper Belt

The Kuiper Belt is a region of the solar system extending from the orbit of Neptune (approximately 30 AU from the Sun) to roughly 50 AU. It is a vast ring of icy bodies analogous to the asteroid belt but far larger and more massive.

Pluto and Charon

Pluto was considered the ninth planet from its discovery in 1930 until 2006, when the IAU reclassified it as a dwarf planet. Pluto has a diameter of approximately 2,377 kilometers. NASA’s New Horizons mission revealed a geologically complex world with nitrogen ice plains, water ice mountains, and a thin atmosphere of nitrogen, methane, and carbon monoxide.

Charon is Pluto’s largest moon, with a diameter of approximately 1,212 kilometers, roughly half the size of Pluto itself. The two bodies are tidally locked to each other, always showing the same face to one another. Their barycenter (center of mass) lies outside Pluto, leading some astronomers to describe them as a binary dwarf planet system.

Trans-Neptunian Objects

Several other dwarf planets have been identified in the Kuiper Belt region.

Eris is the most massive known dwarf planet, slightly more massive than Pluto though slightly smaller in diameter. Its discovery in 2005 was a direct catalyst for the IAU’s decision to redefine the term “planet.”

Makemake and Haumea are additional dwarf planets in the Kuiper Belt. Haumea is notable for its elongated shape, caused by its extremely rapid rotation.

The Oort Cloud

The Oort Cloud is a hypothesized spherical shell of icy bodies surrounding the solar system at distances ranging from roughly 2,000 to 100,000 AU. No direct observations of the Oort Cloud have been made, but its existence is inferred from the orbits of long-period comets that enter the inner solar system from nearly random directions.

The Oort Cloud is believed to contain billions or even trillions of icy objects. Gravitational perturbations from passing stars or the galactic tide occasionally deflect objects from the Oort Cloud into orbits that carry them into the inner solar system, where they become visible as comets.

Galactic Features

The Milky Way

The solar system resides in the Milky Way, a barred spiral galaxy approximately 100,000 light-years in diameter. The Milky Way contains an estimated 100 to 400 billion stars, and the Sun is located approximately 26,000 light-years from the galactic center in one of the spiral arms.

The galactic center hosts Sagittarius A, a supermassive black hole with a mass of approximately 4 million times that of the Sun. The existence of Sagittarius A was confirmed through decades of observations of stars orbiting an invisible massive object, work that earned the 2020 Nobel Prize in Physics.

Nebulae

A nebula is a cloud of gas and dust in interstellar space. Nebulae are significant because they are the regions where new stars are born and the remnants of stars that have died.

Emission nebulae glow because the gas within them is ionized by ultraviolet radiation from nearby hot stars. The Orion Nebula is a well-known example.

Reflection nebulae do not emit their own light but are visible because they reflect light from nearby stars.

Planetary nebulae are shells of gas expelled by dying low-to-intermediate-mass stars during their transition to white dwarfs. Despite the name, they have no connection to planets.

Dark nebulae are dense clouds of gas and dust that block light from objects behind them. The Horsehead Nebula in Orion is a famous example.

Star Clusters

Stars frequently form in groups. Two types of star clusters are commonly distinguished.

Open clusters are loosely bound groups of a few hundred to a few thousand young stars. They are found in the disk of the galaxy and gradually disperse over hundreds of millions of years. The Pleiades is a well-known open cluster.

Globular clusters are tightly bound spherical collections of tens of thousands to millions of old stars. They orbit in the halo of the galaxy and are among the oldest objects in the Milky Way, with ages of 10 to 13 billion years.

Black Holes

A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape. Black holes are predicted by general relativity and have been confirmed through multiple observations.

Stellar black holes form from the collapse of massive stars at the end of their lives. They typically have masses between 3 and 100 times that of the Sun.

Supermassive black holes reside at the centers of most large galaxies and have masses ranging from millions to billions of solar masses. The Event Horizon Telescope produced the first direct image of a supermassive black hole’s shadow in 2019, observing M87* at the center of the galaxy Messier 87.

Intergalactic Features

Galaxy Types

Galaxies are classified by their morphology into three broad categories, following the Hubble sequence.

Spiral galaxies have a flat, rotating disk of stars, gas, and dust with a central bulge of older stars. Spiral arms extend outward from the center. The Milky Way is a barred spiral galaxy.

Elliptical galaxies range from nearly spherical to highly elongated shapes. They contain mostly older stars and have little gas or dust for new star formation.

Irregular galaxies lack a distinct regular shape. They are often rich in gas and dust and are frequently sites of active star formation. The Magellanic Clouds, satellite galaxies of the Milky Way, are irregular galaxies.

The Local Group

The Milky Way belongs to a small galaxy group called the Local Group, which contains more than 80 known galaxies within a volume roughly 10 million light-years across. The two largest members are the Milky Way and the Andromeda Galaxy (M31). The Andromeda Galaxy is the nearest large spiral galaxy, located approximately 2.5 million light-years from Earth. The Milky Way and Andromeda are expected to merge in approximately 4.5 billion years.

Galaxy Clusters and Superclusters

Galaxy clusters are the largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies. The Local Group is part of the Virgo Supercluster, which in turn is part of the Laniakea Supercluster, a structure spanning approximately 520 million light-years.

The Observable Universe

The observable universe has a radius of approximately 46.5 billion light-years in every direction from Earth. This is larger than the age of the universe (approximately 13.8 billion years) might suggest because space itself has been expanding since the Big Bang.

The observable universe is estimated to contain approximately 2 trillion galaxies, though recent estimates vary. Beyond the observable universe, the total extent of the universe is unknown and may be infinite.

Broad Qualitative Concepts

The Electromagnetic Spectrum

Astronomers observe the universe across the full electromagnetic spectrum, not just visible light. Radio astronomy reveals cold gas and dust. Infrared observations penetrate dust clouds to reveal forming stars. Ultraviolet and X-ray telescopes observe hot, energetic phenomena like stellar coronae and accretion disks around black holes. Gamma-ray telescopes detect the most violent events in the universe, including gamma-ray bursts and active galactic nuclei.

Each wavelength range reveals different physical processes and different populations of objects. Modern astronomy requires observations across the entire spectrum to build a complete understanding of celestial phenomena.

The Hertzsprung-Russell Diagram

The Hertzsprung-Russell (H-R) diagram is a fundamental tool in stellar astronomy. It plots stars by their luminosity (vertical axis) against their surface temperature or spectral type (horizontal axis). Most stars fall along a diagonal band called the main sequence, where they spend the majority of their lifetimes fusing hydrogen into helium.

The position of a star on the H-R diagram reveals its mass, luminosity, temperature, and evolutionary state. Red giants and supergiants occupy the upper right. White dwarfs occupy the lower left. The H-R diagram provides a framework for understanding stellar evolution from birth to death.

Stellar Evolution

Stars evolve through a sequence of stages determined primarily by their initial mass. Low-mass stars like the Sun burn hydrogen for billions of years on the main sequence, expand into red giants, shed their outer layers as planetary nebulae, and end as white dwarfs.

High-mass stars burn through their fuel much faster, evolving through red supergiant phases before exploding as supernovae. The remnant is either a neutron star or, for the most massive stars, a black hole. Supernovae distribute heavy elements forged in the star’s core into the interstellar medium, enriching the material from which future stars and planets form.

The Cosmic Distance Ladder

Measuring distances in astronomy is challenging because direct measurement is impossible for all but the nearest objects. Astronomers use a series of overlapping techniques, each calibrated against the previous one, known as the cosmic distance ladder.

Parallax measures the apparent shift in a star’s position as the Earth orbits the Sun. It is reliable for stars within a few thousand light-years.

Standard candles are objects of known luminosity. By comparing the known luminosity with the observed brightness, the distance can be calculated. Cepheid variable stars and Type Ia supernovae are the two most important standard candles.

Redshift measures the stretching of light from distant galaxies due to the expansion of the universe. Hubble’s Law relates a galaxy’s recession velocity to its distance.

Light as a Time Machine

Because light travels at a finite speed (approximately 300,000 kilometers per second), looking at distant objects means looking back in time. The light from the Andromeda Galaxy took approximately 2.5 million years to reach Earth, so we see it as it appeared 2.5 million years ago. The cosmic microwave background radiation, the oldest light in the universe, was emitted approximately 380,000 years after the Big Bang, about 13.8 billion years ago.

Mathematical Formulas

The following formulas are commonly introduced in an introductory astronomy course.

Kepler’s Laws of Planetary Motion

Kepler’s First Law states that the orbit of each planet is an ellipse with the Sun at one focus.

Kepler’s Second Law states that a line connecting a planet to the Sun sweeps out equal areas in equal intervals of time.

Kepler’s Third Law relates the orbital period to the semi-major axis of the orbit.

\[P^2 = a^3\]

where $P$ is the orbital period in years and $a$ is the semi-major axis in astronomical units. In its more general form using SI units,

\[P^2 = \frac{4\pi^2}{G(M_1 + M_2)} a^3\]

where $G$ is the gravitational constant, and $M_1$ and $M_2$ are the masses of the two bodies.

Newton’s Law of Universal Gravitation

Every particle of matter attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

\[F = \frac{G M_1 M_2}{r^2}\]

where $F$ is the gravitational force, $G$ is the gravitational constant ($6.674 \times 10^{-11}$ N m$^2$ kg$^{-2}$), $M_1$ and $M_2$ are the masses, and $r$ is the distance between the centers of mass.

The Inverse Square Law for Light

The intensity (flux) of light from a point source decreases with the square of the distance from the source.

\[F = \frac{L}{4\pi d^2}\]

where $F$ is the observed flux (energy per unit area per unit time), $L$ is the luminosity of the source, and $d$ is the distance from the source.

Stefan-Boltzmann Law

The total energy radiated per unit surface area of a blackbody per unit time is proportional to the fourth power of its temperature.

\[L = 4\pi R^2 \sigma T^4\]

where $L$ is the luminosity, $R$ is the radius of the object, $\sigma$ is the Stefan-Boltzmann constant ($5.670 \times 10^{-8}$ W m$^{-2}$ K$^{-4}$), and $T$ is the surface temperature in Kelvin.

Wien’s Displacement Law

The wavelength at which a blackbody emits the most radiation is inversely proportional to its temperature.

\[\lambda_{\max} = \frac{b}{T}\]

where $\lambda_{\max}$ is the peak wavelength, $b$ is Wien’s displacement constant ($2.898 \times 10^{-3}$ m K), and $T$ is the temperature in Kelvin.

This law explains why hot stars appear blue and cool stars appear red.

Doppler Effect and Redshift

When a light source moves relative to an observer, the observed wavelength shifts. For speeds much less than the speed of light,

\[\frac{\Delta \lambda}{\lambda_0} = \frac{v}{c}\]

where $\Delta \lambda = \lambda_{\text{obs}} - \lambda_0$ is the change in wavelength, $\lambda_0$ is the rest wavelength, $v$ is the radial velocity of the source, and $c$ is the speed of light.

A positive value indicates a redshift (source moving away). A negative value indicates a blueshift (source approaching).

Hubble’s Law relates the recession velocity of a distant galaxy to its distance.

\[v = H_0 d\]

where $v$ is the recession velocity, $H_0$ is the Hubble constant (approximately 70 km s$^{-1}$ Mpc$^{-1}$), and $d$ is the distance in megaparsecs.

Parallax and Distance

Stellar parallax provides the most direct method of measuring stellar distances.

\[d = \frac{1}{p}\]

where $d$ is the distance in parsecs and $p$ is the parallax angle in arcseconds. One parsec is the distance at which a star has a parallax angle of one arcsecond, equivalent to approximately 3.26 light-years.

Apparent and Absolute Magnitude

The distance modulus relates a star’s apparent magnitude $m$ (how bright it appears from Earth) to its absolute magnitude $M$ (how bright it would appear from a standard distance of 10 parsecs).

\[m - M = 5 \log_{10}\left(\frac{d}{10}\right)\]

where $d$ is the distance in parsecs.

Summary

The solar system extends from the Sun through eight planets, their moons, the asteroid belt, the Kuiper Belt, and the hypothesized Oort Cloud. Beyond the solar system, the Milky Way contains hundreds of billions of stars organized into spiral arms around a central bar, with a supermassive black hole at its center. Nebulae mark the birth and death of stars. Star clusters trace the history of stellar formation.

Beyond the Milky Way, the observable universe contains an estimated two trillion galaxies organized into clusters and superclusters. The Local Group, the Virgo Supercluster, and the Laniakea Supercluster represent progressively larger scales of cosmic structure.

The mathematical tools of introductory astronomy provide the foundation for quantitative reasoning about celestial phenomena. Kepler’s laws govern orbital motion. Newton’s gravitation explains why. The Stefan-Boltzmann and Wien’s laws connect a star’s temperature to its luminosity and color. The Doppler effect and Hubble’s Law reveal the expansion of the universe. Parallax and the magnitude system provide the distance and brightness scales on which all other measurements depend.

Future Reading

  • NASA Solar System Exploration, NASA’s comprehensive guide to every body in the solar system with mission data, images, and interactive tools.

  • OpenStax Astronomy 2e, a free, peer-reviewed introductory astronomy textbook covering the full scope of a two-semester course.

  • ESA Space Science, the European Space Agency’s portal for space science missions including solar system exploration, astrophysics, and cosmology.

  • IAU Minor Planet Center, the official clearinghouse for observations and orbits of minor planets, comets, and natural satellites.

  • Hubble Site, the public information portal for the Hubble Space Telescope, with images, news, and educational resources.

References