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Interstellar Medium

The matter that exists in the space between star systems in a galaxy. It is composed of gas (primarily hydrogen and helium) and dust, and is detected mainly through radio waves.

What is reddening?

A phenomenon observed when light from stars passes through interstellar dust, causing shorter wavelengths (blue light) to scatter more than longer wavelengths (red light). This results in the star appearing redder than it actually is.

Emission Nebulae

Luminous clouds of ionized gas, primarily hydrogen, that emit light when electrons recombine with ions. These nebulae are often associated with regions of active star formation.

Reflection Nebulae

Nebulae composed of gas and dust that reflect the light of nearby stars. They appear blue because shorter wavelengths of light are scattered more efficiently by the dust particles.

Dark Dust Clouds

Dense regions in interstellar space where the concentration of dust particles is so high that they block most of the visible light from stars behind them. They are also known as absorption nebulae.

21-centimeter radiation

A specific radio wavelength (21.12 cm) emitted by neutral hydrogen atoms when the electron's spin flips. This radiation is used to map the distribution and density of interstellar gas in galaxies.

Interstellar Cloud (Stage 1 of Star Formation)

The initial stage of star formation, characterized by a large, diffuse cloud of gas and dust in interstellar space from which stars will eventually form due to gravitational collapse.

Contracting Cloud Fragment Protostar (Stages 2 & 3)

Intermediate stages in star formation where a dense region within an interstellar cloud collapses under its own gravity, forming a core that will eventually become a star. These fragments are characterized by increasing density and temperature.

Protostellar Evolution (Stages 4 & 5)

The phase in star formation where a protostar continues to accrete mass from the surrounding cloud, increasing in temperature and luminosity but not yet undergoing nuclear fusion in its core.

Newborn Star (Stages 6 & 7)

The stage in a star's life when it has accumulated sufficient mass and temperature in its core to initiate sustained nuclear fusion of hydrogen into helium, marking the transition to the main sequence.

Zero-age main sequence

A theoretical line on the Hertzsprung-Russell (H-R) diagram representing the locations of stars that have just begun hydrogen fusion and have reached a stable state. It is the starting point for stellar evolution models.

Evolutionary Track

The path a star traces on the H-R diagram as it evolves over time, showing how its luminosity and temperature change as it ages and undergoes various nuclear fusion processes.

Brown Dwarfs

Substellar objects that form like stars but do not have enough mass to sustain hydrogen fusion in their cores. They are intermediate in size between planets and small stars.

Death of Stars

The ultimate fate of a star, which depends primarily on its mass. Low-mass stars become white dwarfs, while more massive stars can end their lives as neutron stars or black holes after a supernova explosion.

Star Clusters

Groups of stars that are gravitationally bound to each other. Open clusters are loosely packed and found in the disk of the galaxy, while globular clusters are densely packed and found in the halo.

Hertzsprung-Russell Diagram (H-R)

A graphical tool used to classify stars based on their luminosity (absolute magnitude) and temperature (spectral class). It plots luminosity against temperature, revealing patterns in stellar populations and evolution.

Main Sequence Stars

Stars that are undergoing nuclear fusion of hydrogen into helium in their cores and fall along a diagonal band on the H-R diagram. These stars are in the longest and most stable phase of their lives.

Luminosity vs. Temperature

An observed correlation on the H-R diagram, where stars with higher surface temperatures tend to have greater luminosities. This relationship reflects the fundamental properties of stellar physics.

Giants, Supergiants, and Dwarfs

Categories of stars that lie off the main sequence on the H-R diagram, distinguished by their size and luminosity. Giants and supergiants are evolved stars that have expanded, while dwarfs are small, dense stars.

Importance of the H-R Diagram

The H-R diagram is essential for understanding stellar evolution, determining distances to stars and galaxies, and studying the properties of stellar populations.

Hydrostatic Equilibrium

A state of equilibrium in a star where the inward gravitational forces are balanced by the outward pressure from nuclear fusion and radiation, maintaining the star's size and stabilitHomey.

Nuclear Fusion

The process by which light atomic nuclei, such as hydrogen, combine to form heavier nuclei, such as helium, releasing a tremendous amount of energy that powers all stars.

Stellar Parallax

The apparent shift in the position of a nearby star relative to distant background stars when viewed from different points in Earth's orbit. This effect is used to measure the distances to nearby stars.

Proper Motion

The angular rate of change in a star's position on the celestial sphere, measured in arcseconds per year. It reflects the actual motion of the star through space relative to the Sun.

Apparent Magnitude

A measure of how bright a star appears to an observer on Earth. It depends on both the star's intrinsic luminosity and its distance from Earth.

Absolute Magnitude

The brightness a star would have if it were located at a standard distance of 10 parsecs from Earth. It is a measure of the star's intrinsic luminosity, independent of its distance.

Luminosity

The total amount of energy emitted by a star per unit of time. It is a measure of the star's intrinsic brightness and is usually expressed in watts or as a multiple of the Sun's luminosity.

Spectral Classification

A system for classifying stars based on the appearance of their spectra, which is determined by their surface temperature and chemical composition. The classes are O, B, A, F, G, K, and M, with subdivisions from 0 to 9.

OBAFGKM

A mnemonic used to remember the sequence of stellar spectral classes from hottest to coolest: O, B, A, F, G, K, M. Each class is associated with distinct spectral lines and temperatures.

H-R Diagram Axes

The axes of the Hertzsprung-Russell (H-R) diagram, which plot a star's luminosity (absolute magnitude) against its temperature (spectral class). The diagram is a fundamental tool for studying stellar evolution.

Main Sequence

A prominent band on the H-R diagram where most stars are found. These stars are fusing hydrogen into helium in their cores and are in the longest and most stable phase of their lives.

Red Giants

Evolved stars that have exhausted the hydrogen in their cores and have expanded and cooled, becoming larger and more luminous than main sequence stars. They appear redder in color.

White Dwarfs

The dense, hot remnants of low- to intermediate-mass stars that have exhausted their nuclear fuel and have collapsed to a very small size. They slowly cool and fade over billions of years.

Cepheid Variables

Stars that pulsate in brightness due to periodic expansions and contractions in their outer layers. Their pulsation periods are related to their luminosities, making them useful as standard candles for measuring cosmic distances.

Standard Candle

An object with a known luminosity, allowing its distance to be calculated by comparing its intrinsic brightness with its observed brightness. Cepheid variables and Type Ia supernovae are examples of standard candles.

Mass-Luminosity Relationship

An empirical relationship that describes how a star's luminosity is related to its mass. More massive main sequence stars are much more luminous than less massive ones, typically expressed as L \propto M^{3.5}.

Binary Stars

A system of two stars that are gravitationally bound and orbit around a common center of mass. Binary stars are important for determining stellar masses.

Eclipsing Binaries

Binary star systems in which the orbital plane is oriented such that one star periodically passes in front of the other, causing a dip in the system's overall brightness as observed from Earth.

Interstellar Extinction

The reduction in the brightness of starlight as it passes through the interstellar medium, caused by the absorption and scattering of photons by dust and gas.

Molecular Clouds

Cold, dense regions in interstellar space where molecules, such as hydrogen (H_2) and carbon monoxide (CO), can form. These clouds are the birthplaces of stars.

T Tauri Stars

Young, pre-main sequence stars that are still accreting material from a surrounding protoplanetary disk. They exhibit strong stellar winds and outflows.

Herbig-Haro Objects

Bright, knot-like regions of nebulosity associated with newborn stars. They are formed when jets of gas ejected from the star collide with the surrounding interstellar medium at high speeds.

CNO Cycle

A nuclear fusion process that occurs in stars more massive than the Sun, using carbon, nitrogen, and oxygen isotopes as catalysts to convert hydrogen into helium. It is the dominant energy source in massive stars.

Triple-Alpha Process

A set of nuclear fusion reactions that convert three helium-4 nuclei (alpha particles) into carbon-12. This process occurs in the cores of evolved stars like red giants.

Asymptotic Giant Branch (AGB)

A region on the H-R diagram populated by evolved, cool, and luminous stars. It is a late stage of stellar evolution for low- to intermediate-mass stars, characterized by a thermally pulsing helium-burning shell.

Planetary Nebula

A type of emission nebula formed by the expanding shell of gas ejected from an aging star during the late stages of its life. The star's hot core ionizes the gas, causing it to glow.

Chandrasekhar Limit

The maximum mass that a stable white dwarf star can have, approximately 1.44 solar masses. Beyond this limit, the white dwarf will collapse and explode as a Type Ia supernova.

Supernova

The explosive death of a star, resulting in a sudden and dramatic increase in brightness. Supernovae can occur via core collapse in massive stars or through thermonuclear explosions in white dwarfs.

Neutron Stars

Extremely dense remnants of massive stars that have collapsed under their own gravity. They are composed almost entirely of neutrons and have incredibly strong magnetic fields.

Pulsars

Highly magnetized, rotating neutron stars that emit beams of electromagnetic radiation out of their magnetic poles. These beams sweep across the sky as the star rotates, producing regular pulses of radio waves or other radiation.

Black Holes

Regions in spacetime where gravity is so strong that nothing, not even light, can escape. They are formed from the collapse of massive stars and are characterized by an event horizon and a singularity.

Event Horizon

The boundary around a black hole beyond which events cannot affect an outside observer. It is the 'point of no return' for matter or light entering the black hole.

Schwarzschild Radius

The radius of the event horizon of a non-rotating black hole, proportional to its mass. It is defined as R_s = \frac{2GM}{c^2}, where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light.

Spacetime

A mathematical model that combines the three dimensions of space with time into a single, four-dimensional continuum, used to describe the geometry of the universe in general relativity.

Supergiant Stars

The most luminous and massive stars, located at the top of the H-R diagram. They have very short lifespans and end their lives as supernovae, leaving behind neutron stars or black holes.

Variable Stars

Stars whose brightness varies over time, either regularly or irregularly. These variations can be due to pulsations, eclipses in binary systems, or explosive events on the star's surface.

RR Lyrae Variables

Periodic variable stars with periods shorter than one day, commonly found in globular clusters. They are used as standard candles for measuring distances within our galaxy and to nearby galaxies.

Type Ia Supernova

A type of supernova that occurs in binary systems when a white dwarf accretes matter from a companion star and exceeds the Chandrasekhar limit, triggering a thermonuclear explosion. They are used as standard candles for measuring large cosmic distances.

Population I Stars

Stars that are relatively young and metal-rich (containing elements heavier than hydrogen and helium). They are commonly found in the spiral arms of galaxies and exhibit Population I characteristics.

Population II Stars

Older, metal-poor stars found in globular clusters and the halo of galaxies. They formed early in the universe when heavy elements were less abundant, demonstrating Population II characteristics.

Electromagnetic Spectrum

The range from the lowest to the highest frequency (or longest to shortest wavelength) of electromagnetic radiation.

Ultraviolet Radiation

Electromagnetic radiation with wavelengths shorter than those of visible light, but longer than those of X-rays.

Infrared Radiation

Electromagnetic radiation with wavelengths longer than those of visible light, but shorter than those of microwaves.

Power

A measure of how much energy a star radiates each second.

Solar Constant

The amount of energy passing through each square meter of a sphere with a radius equal to the Earth's distance from the Sun, with a value of approximately 1360 \frac{W}{m^2}.

Parsec

The distance at which one astronomical unit (AU) subtends an angle of one arcsecond; equivalent to 3.26 light-years.

Magnitude Scale

A system for measuring stellar brightness where brighter objects have smaller magnitudes than fainter objects.

Flux

The amount of light received from a star at Earth.

Distance Indicators

An object whose brightness is known, allowing distances to celestial objects to be inferred.

Dark Nebula

An opaque cloud of gas whose composition doesn't allow light to pass through.

Process of Protostar Formation

The process of how a star is formed, starting with a dense core in a molecular cloud becoming unstable and collapsing, forming a protostar that eventually becomes a main sequence star.

Protostar

A hot, dense core that radiates infrared light as the star forms.

Proton-Proton Chain (P-P Chain)

The set of reactions by which stars less massive than the Sun fuse hydrogen into helium.

Hydrogen Fusion

A series of reactions where four hydrogen nuclei combine to form one helium nucleus, releasing energy.

Fusion of Massive Stars

Stars with more than eight times the Sun's mass that go through additional stages of nuclear burning after hydrogen and helium are exhausted.

Late Fusion Stages

The process of a massive star fusing heavier elements such as carbon, oxygen, silicon, and iron after the hydrogen and helium in its core are exhausted.

Chandrasekhar Limit

This limit is approximately 1.4 times the mass of the Sun; exceeding this limit leads to the collapse of a white dwarf, resulting in a Type Ia supernova.

Type II Supernovae

An explosion caused by the collapse of a massive star after its core has run out of nuclear fuel, resulting in heavy elements being distributed into space.

Iron Core

The last element to be fused in the core of a massive star before it collapses into a supernova, eventually forms a neutron star or black hole.

Singularity

The remains of a star that has collapsed and whose gravity is so strong that not even light can escape, resulting in the formation of a black hole.

Cepheid Variable Stars

The most useful way of measuring the distance to remote galaxies and calibrating the expansion rate of the universe.

Stellar Evolution

The study of the evolution of stars from their formation to their eventual demise.

Event Horizon

The point in spacetime where a black hole is located; a region of infinite density.

High Mass Stars

The name given to stars with more than 8 times the mass of our sun.

Low Mass Stars

The name give to stars with less then 8 times the mass of our sun