"A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity."
Types of galaxies, including spiral, elliptical, and irregular galaxies. Formation and evolution of galaxies.
Cosmology: This topic is the study of the origin, evolution, and structure of the universe as a whole.
Gravity and general relativity: The set of physical laws that describes the behavior of massive objects in space.
Dark Matter: The hypothetical matter that is believed to exist but cannot be detected by any known scientific means.
Dark Energy: The mysterious force that is believed to be responsible for the accelerating expansion of the universe.
Stellar Evolution: The study of the life cycle of stars, their birth, evolution, and death.
Black Holes: A region of space-time with a gravitational force so strong that nothing, including light, can escape from it.
Galaxy Formation: The process by which galaxies are formed from the collapse of gas and dust clouds.
Galactic Dynamics: The study of the motions and interactions of stars, gas, and dust within galaxies.
Exoplanets: Planets that orbit a star other than our sun.
Intergalactic Medium: The diffuse gas and plasma that fills the spaces between galaxies.
Galactic Magnetic Fields: The magnetic fields that permeate galaxies, affecting the motions of charged particles within them.
Nuclear Astrophysics: The study of the formation and behavior of atomic nuclei in stars and other astrophysical environments.
High-Energy Astrophysics: The study of astrophysical phenomena that produce or involve high-energy radiation.
Radio Astronomy: The study of celestial objects and phenomena by observing radio waves emitted by them.
Gravitational Waves: The ripples in space-time caused by the acceleration or deceleration of massive objects.
Supermassive Black Holes: The extremely massive black holes that are found at the centers of most galaxies.
Galactic Winds and Outflows: Powerful flows of gas and dust that are expelled from galaxies, affecting their evolution.
Galaxy Clusters: Groups of galaxies that are bound together by their mutual gravity.
Interstellar Medium: The material between the stars, composed of gas, dust, and cosmic rays.
Galaxy Morphology: The study of the shapes and structures of galaxies, such as spirals, ellipticals, and irregulars.
Spiral galaxies: These are galaxies that have a pinwheel shape, with arms that spiral outwards from the center.
Elliptical galaxies: These are galaxies that have a rounded shape, with no discernible structure.
Irregular galaxies: These are galaxies that have no regular shape or structure. They can be asymmetrical, or they can have a chaotic appearance.
Barred spiral galaxies: These are galaxies that have a central bar-like structure, with arms that spiral outwards from the ends of the bar.
Lenticular galaxies: These are galaxies that have a flat, disk-like shape, but with no spiral arms.
Dwarf galaxies: These are galaxies that are much smaller and less massive than other types of galaxies. They can be irregular or have a more structured shape.
Ring galaxies: These are galaxies that have a ring-shaped structure, with a central bulge and no visible arms.
Polar-ring galaxies: These are galaxies that have a disk-like structure that is perpendicular to the rest of the galaxy. They may have a central bulge or bar-like structure.
Compact galaxies: These are galaxies that are much smaller than other types of galaxies, but with a high mass and density.
Ultra-compact dwarf galaxies: These are the smallest and most dense type of galaxy, with a stellar population that is similar to that of globular star clusters.
"The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System."
"Galaxies, averaging an estimated 100 million stars..."
"Range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars..."
"Most of the mass in a typical galaxy is in the form of dark matter..."
"Supermassive black holes are a common feature at the centers of galaxies."
"Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular."
"It is estimated that there are roughly 200 billion galaxies (2×1011) in the observable universe."
"Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years)..."
"Most galaxies are gravitationally organized into groups, clusters and superclusters."
"The Milky Way is part of the Local Group."
"The Milky Way dominates the Local Group along with the Andromeda Galaxy."
"Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea."
"The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter."
"At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids."
"The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun."
"For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly)..."
"...the Andromeda Galaxy (with diameter of about 152,000 ly)..."
"...separated from the Andromeda Galaxy... by 780,000 parsecs (2.5 million ly.)"
"...and are separated by distances on the order of millions of parsecs (or megaparsecs)."