ASTROPHYSICS

What is a star?

A ball of gas, primarily hydrogen and helium, that generates energy through nuclear fusion in its core, producing light and heat.

What is a planetary system?

A collection of planets, moons, asteroids, comets, and other bodies that orbit around a star or star system. Our Solar System is the most well-known example.

What is a binary star system?

A system of two stars gravitationally bound to each other, orbiting around their common center of mass. Approximately one-third of stars in the Milky Way exist in binary or multiple star systems.

What is a star cluster?

A group of stars held together by gravity.

What is a constellation?

A pattern of stars that appears to form a recognizable shape or figure as seen from Earth. The stars in a constellation are not physically related; they simply appear close together from our vantage point.

What is a nebula?

A vast cloud of gas and dust in space. Nebulae serve as the birthplace of new stars and planetary systems.

What is the interstellar medium?

The matter and radiation that exists in the space between star systems within a galaxy. It is composed of gas (primarily hydrogen and helium), dust, and cosmic rays, and plays a key role in star formation.

What is a supernova?

A stellar explosion marking the violent death of a massive star. A supernova can briefly outshine an entire galaxy and disperses heavy elements into the interstellar medium.

What is a white dwarf?

A small, dense remnant left behind when a low- to medium-mass star (like our Sun) exhausts its nuclear fuel and sheds its outer layers. It no longer undergoes fusion and slowly cools over billions of years.

What is a neutron star?

An extremely dense stellar remnant formed from the collapsed core of a massive star after a supernova explosion. Neutron stars are only about 20 km in diameter but contain more mass than the Sun, and are composed almost entirely of neutrons.

What is a black hole?

A region of space where gravity is so intense that nothing, not even light, can escape once it crosses the event horizon. Black holes form when the most massive stars collapse at the end of their lives, or through other processes.

What is a galaxy?

A vast system of stars, gas, dust, and dark matter bound together by gravity, containing anywhere from millions to hundreds of billions of stars. Our galaxy, the Milky Way, is a barred spiral galaxy containing 100–400 billion stars.

What is a galaxy cluster?

A large collection of galaxies bound together by gravity, typically containing hundreds to thousands of individual galaxies. Galaxy clusters are among the largest gravitationally bound structures in the universe.

What is dark matter?

An invisible form of matter that does not emit, absorb, or reflect light, but whose presence is inferred through its gravitational effects on galaxies and galaxy clusters. Dark matter makes up approximately 27% of the universe's total mass-energy content.

What is dark energy?

A mysterious form of energy that permeates all of space and is thought to be responsible for the accelerating expansion of the universe. Dark energy makes up approximately 68% of the universe's total mass-energy content.

What are the stages of stellar evolution?

The evolution of a star includes several stages: formation in a nebula, main sequence phase where it fuses hydrogen into helium, red giant or supergiant stage as it exhausts hydrogen, a supernova event for massive stars, leading to either a neutron star or black hole, or for smaller stars, becoming a white dwarf.