Our is a cosmic neighborhood centered around the . Planets, moons, asteroids, and comets orbit this fiery star, each with unique characteristics. , our home, sits in the "Goldilocks zone," perfect for life as we know it.
The , our galactic home, is a vast spiral of stars, gas, and dust. Our Sun is just one of billions of stars in this cosmic pinwheel, orbiting the galactic center where a supermassive lurks.
The Solar System
Structure of the solar system
Sun at the center with planets, dwarf planets, moons, asteroids, and comets orbiting it
Terrestrial (rocky) planets: , , Earth,
Jovian (gas giant) planets: , , ,
Dwarf planets orbit the Sun but have not cleared their orbital path (, , )
Moons are natural satellites that orbit planets or dwarf planets
Asteroids are rocky objects mostly found in the between Mars and Jupiter
Comets are icy objects with long, elliptical orbits around the Sun
Sun is the largest object in the solar system with a diameter of approximately 1.39 million km (109 times Earth's diameter)
Earth, the third planet from the Sun, has a diameter of about 12,742 km
Orbits the Sun at an average distance of 149.6 million km (1 or AU)
Moon, Earth's only natural satellite, has a diameter of approximately 3,474 km (about 1/4 of Earth's diameter)
Orbits Earth at an average distance of 384,400 km (about 30 times Earth's diameter)
plays a crucial role in maintaining the structure and dynamics of the solar system
The Milky Way Galaxy
Characteristics of Milky Way galaxy
consisting of a flat disk with spiral arms, a central bulge, and a halo
Disk is approximately 100,000 light-years in diameter and about 1,000 light-years thick
Central bulge is a densely packed region of older stars with a diameter of about 10,000 light-years
Halo is a spherical region surrounding the disk and bulge containing older stars and globular clusters
Contains an estimated 100-400 billion stars of various sizes, colors, and ages
Sun is located in the about 26,500 light-years from the galactic center
Composed of stars, gas (mostly hydrogen and helium), and dust
Interstellar gas and dust make up about 10-15% of the 's total mass
Dust is composed of small solid particles such as silicates and carbon compounds
Has a supermassive at its center known as with a mass of about 4 million solar masses
Contains regions of active star formation in nebulae, where new stars are born from collapsing clouds of gas and dust
Stars
Features and life cycles of stars
Form from the gravitational collapse of dense regions within molecular clouds of gas and dust
Protostars undergo further collapse until nuclear fusion begins in their cores marking the birth of a
Main-sequence stars fuse hydrogen into helium in their cores generating energy that counteracts gravitational collapse
A star's mass determines its size, color, luminosity, and lifetime on the main sequence
When a star exhausts its hydrogen fuel, it evolves off the main sequence and its ultimate fate depends on its initial mass:
Low-mass stars (< 8 solar masses) become red giants, shed outer layers to form planetary nebulae, leaving behind remnants
High-mass stars (> 8 solar masses) become red supergiants, explode as supernovae, leaving behind neutron stars or black holes
Many stars exist in multiple star systems where two or more stars orbit a common center of mass
Binary stars are the most common type of multiple star system
Play a crucial role in producing elements found on Earth through nuclear fusion and explosions
Main-sequence stars fuse hydrogen into helium, later helium into carbon and oxygen
Massive stars can fuse elements up to iron in their cores
Supernova explosions create and disperse elements heavier than iron enriching the
The Universe and Beyond
Cosmic structure and components
The is the entirety of space, time, matter, and energy
Galaxies are the building blocks of the universe, containing billions of stars, gas, and dust
The radiation provides evidence for the Big Bang theory
Telescopes are essential tools for observing and studying celestial objects across various wavelengths
, though invisible, plays a significant role in the structure and evolution of the universe
Key Terms to Review (51)
Asteroid Belt: The asteroid belt is a region in the Solar System located between the orbits of Mars and Jupiter, where a large number of small rocky objects, known as asteroids, orbit the Sun. This belt is a key feature of the Solar System and plays an important role in understanding the formation and evolution of the planets.
Astronomical unit: An astronomical unit (AU) is a unit of distance defined as the average distance between Earth and the Sun, approximately 149.6 million kilometers (93 million miles). It is commonly used to describe distances within our solar system.
Astronomical Unit: The astronomical unit (AU) is the standard unit of length used to measure distances within our solar system. It is defined as the average distance between the Earth and the Sun, which is approximately 150 million kilometers or 93 million miles.
Barred spiral galaxy: A barred spiral galaxy is a type of spiral galaxy with a central bar-shaped structure composed of stars. The Milky Way is an example of a barred spiral galaxy.
Barred Spiral Galaxy: A barred spiral galaxy is a type of spiral galaxy characterized by the presence of a central bar-shaped structure of stars. This bar-like feature is a distinct component that extends from the galactic center and connects to the spiral arms, giving the galaxy a distinctive appearance.
Binary Star: A binary star is a system of two stars that orbit a common center of mass. These stars are gravitationally bound and revolve around each other, forming a single astronomical object that can be observed and studied as a unit.
Binary star system: A binary star system consists of two stars that orbit around a common center of mass. These systems can significantly influence the evolution and eventual death of both stars involved.
Black hole: A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. They are formed from the remnants of massive stars after they undergo supernova explosions.
Black Hole: A black hole is an extremely dense and massive object in space from which nothing, not even light, can escape due to its immensely strong gravitational pull. Black holes are formed when a massive star collapses in on itself at the end of its life cycle, creating a singularity with an event horizon that marks the point of no return.
Ceres: Ceres is the largest object in the main asteroid belt between the orbits of Mars and Jupiter. It is classified as a dwarf planet, the only dwarf planet located in the inner solar system. Ceres plays a significant role in our understanding of the formation and evolution of the solar system, as well as the study of asteroids and planetary defense.
Cold dark matter: Cold dark matter (CDM) consists of slow-moving particles that do not emit, absorb, or reflect light, making them invisible and detectable only through gravitational effects. It plays a crucial role in the formation and clustering of galaxies in the universe.
Cosmic Microwave Background: The cosmic microwave background (CMB) is the oldest light in the universe, a faint glow that permeates all of space and is a remnant of the early stages of the universe's formation. It provides crucial information about the origins and evolution of the universe, as well as its large-scale structure and composition.
Dark Matter: Dark matter is a hypothetical form of matter that cannot be seen directly but accounts for the majority of the matter in the universe. It is believed to interact gravitationally with itself and with ordinary matter, but does not emit, reflect, or absorb light, making it invisible to traditional astronomical observations.
Earth: Earth is the third planet from the Sun and the only known planet in the universe to harbor life. It is the largest and densest of the inner planets, with a diverse range of geological features, a dynamic atmosphere, and a unique position in the Solar System that has enabled the development of complex life forms.
Eris: Eris is a dwarf planet in the outer Solar System, and the most massive and second-largest known dwarf planet in the Solar System after Pluto. It is a significant object in the context of understanding the formation and evolution of our planetary system.
Galaxy: A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way is the galaxy that contains our Solar System, and is just one of the hundreds of billions of galaxies in the observable universe.
Gravity: Gravity is a fundamental force of nature that attracts two bodies with mass towards each other. It governs the motion of planets, stars, and galaxies and is described by both Newton's Universal Law of Gravitation and Einstein's theory of General Relativity.
Gravity: Gravity is a fundamental force of nature that attracts objects with mass towards one another. It is the force that keeps planets in orbit around stars, and it is also responsible for the motion of stars and galaxies throughout the universe.
Interstellar medium: Interstellar medium (ISM) is the matter that exists in the space between star systems within a galaxy. It consists of gas (both ionized and neutral) and dust, playing a crucial role in the life cycle of cosmic material.
Interstellar Medium: The interstellar medium refers to the vast expanse of gas and dust that fills the space between stars within a galaxy. It is the material that exists in the space between solar systems and plays a crucial role in the formation and evolution of stars, as well as the overall structure and dynamics of galaxies.
Jupiter: Jupiter is the largest planet in our solar system, a gas giant with a massive, turbulent atmosphere dominated by a giant, swirling storm known as the Great Red Spot. As the fifth planet from the Sun, Jupiter's immense size and powerful gravitational field have a profound influence on the dynamics and evolution of the entire solar system.
Kepler’s Supernova: Kepler’s Supernova is a Type Ia supernova that was observed in 1604 within the Milky Way galaxy. Named after astronomer Johannes Kepler, it is one of the few supernovae visible to the naked eye in recorded history.
Light-year: A light-year is the distance that light travels in one year in a vacuum, approximately 5.88 trillion miles (9.46 trillion kilometers). It is commonly used to measure distances between stars and other celestial objects.
Light-Year: A light-year is a unit of distance used in astronomy, defined as the distance light travels in one year in a vacuum. It is a useful measure for expressing the vast distances between celestial objects in the universe.
Main-Sequence Star: A main-sequence star is a type of star that is fusing hydrogen into helium in its core, which is the longest and most stable phase of a star's life cycle. Main-sequence stars make up the majority of stars in the universe and are characterized by their relatively constant luminosity and surface temperature.
Mars: Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. It is often referred to as the 'Red Planet' due to its reddish appearance, which is caused by the iron oxide prevalent on its surface. Mars has a thin atmosphere and a diverse landscape, including volcanoes, canyons, and polar ice caps, making it a fascinating subject of study in the context of astronomy and the origin of the Solar System. The term 'Mars' is significant in the context of the topics 1.6 A Tour of the Universe, 7.4 Origin of the Solar System, 10.1 The Nearest Planets: An Overview, and 10.6 Divergent Planetary Evolution. As one of the terrestrial planets, Mars provides valuable insights into the formation and evolution of the Solar System, as well as the potential for life beyond Earth.
Mercury: Mercury is the closest planet to the Sun and the smallest of the eight planets in the Solar System. It is a terrestrial planet, meaning it has a solid surface, and is known for its dense composition, slow rotation, and extreme temperature variations.
Milky Way: The Milky Way is the galaxy in which our solar system is located, comprising hundreds of billions of stars and vast amounts of gas and dust. It is a spiral galaxy, with a central bulge and a rotating disk of stars, gas, and dust. The Milky Way is an essential component in understanding the structure, formation, and evolution of the universe, as it provides a window into the larger cosmic landscape.
Milky Way Galaxy: The Milky Way Galaxy is the spiral galaxy that includes our Solar System, characterized by its barred structure and multiple spiral arms. It is one of billions of galaxies in the universe and contains over 200 billion stars.
Nebula: A nebula is a vast, interstellar cloud of dust, hydrogen, helium, and other ionized gases. Nebulae are some of the most fascinating and diverse features of the universe, playing a crucial role in the formation and evolution of stars and galaxies.
Neptune: Neptune is the eighth and farthest known planet from the Sun in the Solar System. It is a gas giant with a dense, blue atmosphere primarily composed of hydrogen and helium, and it is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the most distant major planet from the Sun.
Neutron Star: A neutron star is an extremely dense, collapsed stellar remnant that forms when a massive star runs out of fuel and undergoes a supernova explosion, leaving behind a core so dense that the electrons are forced to combine with protons, creating a star composed almost entirely of neutrons. These incredibly dense objects have immense gravitational fields and are some of the most extreme objects in the universe.
Orion Arm: The Orion Arm is a minor spiral arm of the Milky Way galaxy, situated between two major spiral arms known as the Perseus Arm and the Sagittarius Arm. It is the solar system's location within the Milky Way, and it plays a crucial role in understanding the structure and evolution of our galaxy.
Planetary Nebula: A planetary nebula is the glowing shell of gas and dust expelled from a dying, low-mass star. It is formed during the final stages of a star's life cycle, when the star sheds its outer layers, leaving behind a hot, dense core known as a white dwarf.
Pluto: Pluto is a dwarf planet located in the outer reaches of the Solar System. It was once considered the ninth planet from the Sun but was reclassified as a dwarf planet in 2006. Pluto's unique characteristics and its place in the Solar System make it an important object of study in various astronomical topics.
Protostar: A protostar is an early stage in the formation of a star, where gas and dust collapse under gravity. It is characterized by the accumulation of material from a surrounding molecular cloud.
Protostar: A protostar is an early stage of stellar evolution, where a dense cloud of gas and dust begins to collapse under its own gravitational force, marking the initial formation of a star. This process is a crucial step in the life cycle of stars, as it sets the stage for the subsequent stages of star development.
Red Giant: A red giant is a large, cool, and luminous star that has entered the later stages of its life cycle. This type of star is characterized by its expanded size, cooler surface temperature, and reddish-orange appearance, resulting from the star's evolution beyond the main sequence stage.
Red Supergiant: A red supergiant is a large, luminous, and cool star that is nearing the end of its life cycle. These stars are among the largest and most massive stars in the universe, with diameters that can be hundreds of times larger than our Sun.
Sagittarius A: Sagittarius A* (Sgr A*) is a supermassive black hole located at the center of the Milky Way Galaxy. It is approximately 4 million times the mass of the Sun and plays a crucial role in the dynamics of our galaxy.
Sagittarius A*: Sagittarius A* (Sgr A*) is a supermassive black hole located at the center of the Milky Way galaxy. It is a key feature in understanding the architecture, dynamics, and evolution of our galaxy, as well as the nature of black holes and their role in the universe.
Saturn: Saturn is the sixth planet from the Sun and the second-largest planet in the Solar System. It is known for its iconic ring system and diverse system of natural satellites. Saturn's unique features and characteristics make it a significant focus of study in various topics within astronomy.
Solar System: The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, including planets, dwarf planets, moons, asteroids, comets, and other small bodies. It formed 4.6 billion years ago from the gravitational collapse of a giant molecular cloud, and is located in the Milky Way galaxy.
Sun: The Sun is the star at the center of the solar system, providing light, heat, and energy that sustains life on Earth. As the closest star to our planet, the Sun's gravitational influence shapes the orbits of the planets and other objects in the solar system, and its nuclear fusion powers the processes that drive the evolution of the universe.
Supernova: A supernova is a powerful and luminous stellar explosion that occurs at the end of a massive star's life cycle. It is one of the most energetic and dramatic events in the universe, releasing an immense amount of energy and ejecting vast amounts of material into space.
Telescope: A telescope is an optical instrument that uses lenses, mirrors, or a combination of both to magnify and focus distant objects, allowing for the observation and study of celestial bodies. Telescopes are essential tools in the field of astronomy, enabling scientists to explore the universe and unravel its mysteries.
Universe: The universe is the totality of existence, encompassing all matter, energy, space, and time. It is the largest-scale structure in the cosmos, containing galaxies, stars, planets, and all forms of life and non-living entities. The study of the universe and its evolution is the focus of the field of cosmology.
Uranus: Uranus is the seventh planet from the Sun and the third-largest planet in the Solar System. It is a gas giant with a distinctive blue-green color and is known for its unusual tilted axis of rotation, which causes it to essentially roll on its side as it orbits the Sun. Uranus plays a significant role in several topics covered in an introductory astronomy course, including the exploration of the outer planets, the characteristics of the giant planets, and the study of ring and moon systems.
Venus: Venus is the second planet from the Sun and the closest planetary neighbor to Earth. It is often referred to as Earth's 'sister planet' due to their similar sizes and compositions. Venus has a unique and complex relationship with the topics of 1.6 A Tour of the Universe, 7.4 Origin of the Solar System, 10.1 The Nearest Planets: An Overview, 10.3 The Massive Atmosphere of Venus, and 10.6 Divergent Planetary Evolution.
White dwarf: A white dwarf is the remnant of a low to medium mass star that has exhausted its nuclear fuel and shed its outer layers. It is incredibly dense, with a mass comparable to the Sun but a volume similar to Earth.
White Dwarf: A white dwarf is the dense, compact remnant of a low-mass star that has exhausted its nuclear fuel and shed its outer layers, leaving behind a core composed primarily of degenerate matter. This stellar endpoint is a crucial component in understanding the evolution of stars and the structure of the universe.