Key Concepts in Stellar and Galactic Astronomy

Chandrasekhar Limit

The maximum mass (approximately 1.4 times the mass of the Sun) that a white dwarf star can have before collapsing into a neutron star or black hole.

Dark matter

Matter that does not emit light or energy, making it invisible, but its presence is inferred from its gravitational effects on visible matter in galaxies.

Dwarf stars (red, white, brown)

Stars smaller than the Sun, with white dwarfs being the remnants of low-mass stars, red dwarfs being small, long-lived stars, and brown dwarfs being 'failed' stars that never ignited fusion.

Edwin Hubble

Astronomer who discovered that the universe is expanding and developed Hubble's Law, which relates the velocity of galaxies to their distance from Earth.

Galaxies

Massive systems of stars, gas, dust, and dark matter bound together by gravity; come in various shapes like spiral, elliptical, and irregular.

Galaxy shapes (spiral, barred, etc.)

Galaxies come in different shapes including spiral, barred spiral, elliptical, and irregular, each defined by structure and stellar population.

Giant and Supergiant stars

Massive stars that have exhausted the hydrogen in their cores, expanding to many times their original size and later evolving into supernovae or black holes.

Globular clusters

Spherical groups of old stars orbiting the galactic core; often found in the halos of galaxies and contain some of the oldest stars in the universe.

Interstellar medium

The matter that exists in the space between stars, composed of gas (mostly hydrogen and helium) and dust, playing a key role in star formation.

Interstellar reddening

The phenomenon where starlight passing through interstellar dust appears redder due to scattering of shorter blue wavelengths.

Milky Way

The galaxy that contains our solar system, a barred spiral galaxy with an estimated 100-400 billion stars.

Neutron stars

The collapsed cores of massive stars that underwent supernovae, extremely dense with typical diameters of about 20 kilometers, and composed almost entirely of neutrons.

No-Hair theorem

The postulate in general relativity that all black hole solutions can be completely characterized by three parameters: mass, charge, and angular momentum.

Nova

A stellar explosion that occurs in a binary system where a white dwarf accretes material from its companion star, leading to a sudden brightening.

Nuclear bulge (center)

The dense, central region of a galaxy, often containing a supermassive black hole and surrounded by a high concentration of stars and interstellar matter.

Pauli Exclusion Principle

A quantum mechanical principle stating that two identical fermions (such as electrons) cannot occupy the same quantum state within a quantum system simultaneously.

Physics of falling into a black hole

Describes the extreme gravitational effects as objects approach the event horizon of a black hole, experiencing time dilation and spaghettification.

Post-Main-Sequence events

The stages in a star's life after it leaves the main sequence, including shell fusion, red giant phase, and possible collapse into a white dwarf, neutron star, or black hole.

Postulates of Special and General Relativity

Special relativity: Laws of physics are the same for all inertial observers, and the speed of light is constant. General relativity: Gravity is the result of spacetime being curved by mass.

Protostars, Pre-Main-Sequence stars, Main-Sequence stars

Protostars are early-stage forming stars; Pre-Main-Sequence stars are evolving towards stability; Main-Sequence stars fuse hydrogen in their cores.

Pulsars

Highly magnetized, rotating neutron stars that emit beams of electromagnetic radiation observed as regular pulses as they spin.

Recombination photons

Photons emitted when free electrons recombine with protons during the early universe, marking the epoch of recombination when neutral atoms formed.

Relativity

Einstein's theories of special and general relativity explain the relationships between space, time, and gravity, leading to phenomena such as time dilation and gravitational lensing.

Roche Limit

The minimum distance at which a celestial body, held together by its own gravity, can orbit another body without being torn apart by tidal forces.

Rotation curves (Keplerian, solid-body, etc.)

The velocity of objects in orbit versus their distance from the center; in galaxies, they provide evidence for dark matter due to the non-Keplerian behavior.

Roy Kerr

A mathematician who found the exact solution to Einstein's equations describing rotating black holes, known as the Kerr metric.

Schwarzschild's contributions

Karl Schwarzschild provided the first exact solution to Einstein's field equations, describing a non-rotating, spherical black hole.

Shell fusion reactions and products

In post-main-sequence stars, fusion occurs in shells around the core, producing heavier elements like carbon and oxygen in red giants.

Spin-Flip radiation of hydrogen

Occurs when the electron in a hydrogen atom reverses its spin, emitting a photon at a wavelength of 21 cm, crucial for radio astronomy.

Star characterizations based on masses

Stars are categorized by their mass, which determines their lifetime, evolution, and end states such as white dwarfs, neutron stars, or black holes.

Star formation mechanism

The process by which dense regions within molecular clouds collapse under gravity, forming protostars and eventually main-sequence stars.

Stellar lifetimes

The duration a star remains in different stages of its life cycle, with massive stars burning faster and living shorter lives than low-mass stars.

Stephen Hawking

A theoretical physicist known for his work on black holes, Hawking radiation, and contributions to cosmology and quantum gravity.

Supernova (Type II and Type Ia)

Type II supernovae result from the collapse of massive stars, while Type Ia supernovae occur in binary systems with white dwarfs.

Superstrings

A theoretical framework in which point-like particles are replaced by one-dimensional strings, aiming to reconcile quantum mechanics and general relativity.

Synchrotron radiation

Electromagnetic radiation emitted when charged particles are accelerated in magnetic fields, often observed in active galactic nuclei and pulsars.

Temperatures for fusion reactions

Nuclear fusion in stars requires extreme temperatures, typically around 10 million K for hydrogen fusion, increasing for heavier elements.

Variable stars

Stars that vary in brightness over time due to changes in their structure or external factors like eclipses in binary systems.

Wormholes

Hypothetical tunnels in spacetime connecting distant points, possibly allowing faster-than-light travel, though currently speculative in physics.

X-Ray Bursters

Binary systems where a neutron star accretes material from its companion, leading to periodic bursts of X-rays as nuclear fusion ignites on the neutron star's surface.