How Much Do You Know About Stars?

Large Self-Illuminated Object in Space: A Summary

Stars are astronomical objects comprising a luminous spheroid of plasma held together by self-gravity.
The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make them appear as fixed points of light.
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements.
Stellar nucleosynthesis in stars or their remnants creates almost all naturally occurring chemical elements heavier than lithium.
Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars.
Historical observations of stars have been important to civilizations throughout the world, including for religious practices, celestial navigation and orientation, to mark the passage of seasons, and to define calendars.
The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC, and the earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC.
Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable.
The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi.
The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique.
Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit.
The International Astronomical Union (IAU) maintains the Working Group on Star Names (WGSN) which catalogs and standardizes proper names for stars.
The IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters.The Life Cycle of Stars
Stars form from regions of space with higher matter density, known as molecular clouds, which consist mainly of hydrogen and some heavier elements.
Most stars form in groups, and feedback effects from star formation may ultimately prevent further star formation in their region.
Stars spend the majority of their existence as main sequence stars, fueled by the nuclear fusion of hydrogen into helium within their cores.
Different mass stars have different properties and fates, and astronomers often group stars by their mass.
The formation of a star begins with gravitational instability within a molecular cloud, and when a region reaches a sufficient density, it begins to collapse under its own gravitational force.
T Tauri stars and Herbig Ae/Be stars are newly formed stars that emit jets of gas along their axis of rotation, which may help to drive away the surrounding cloud from which the star was formed.
Most stars are members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.
Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions in the core region, and the Sun is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. Very massive stars can lose significant mass each year, significantly affecting their evolution.
The time a star spends on the main sequence depends on the amount of fuel it has and the rate at which it fuses it.
As stars of at least 0.4 M☉ exhaust the supply of hydrogen at their core, they start to fuse hydrogen in a shell surrounding the helium core, and the outer layers of the star expand and cool greatly as they transition into a red giant.
In massive stars, fusion continues until the iron core has grown so large that it can no longer support its own mass, and the core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. This core collapse causes the rest of the star to explode in a supernova.Overview of Stars
Most stars are part of gravitationally bound, multiple-star systems.
Open and globular clusters are groups of stars that formed from the same giant molecular cloud.
Binary systems are common, with most newly formed stars observed in binary systems.
Stars can be much closer or farther apart in different regions of the galaxy.
Characteristics of a star, including luminosity, size, and lifespan, are determined by its initial mass.
Most stars are between 1 billion and 10 billion years old, but some can be older or younger.
The chemical composition of a star is mostly hydrogen and helium, with heavier elements indicating the likelihood of a planetary system.
Stars range in size from neutron stars to supergiants like Betelgeuse.
The motion of a star relative to the Sun can provide useful information about its origin and age, as well as the structure and evolution of the surrounding galaxy.
The magnetic field of a star is generated within regions of the interior where convective circulation occurs.
The rotation rate of stars can be determined through spectroscopic measurement or tracking their starspots.
The surface temperature of a star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.Understanding Stars: Their Luminosity, Magnitude, Classification, Variability, Structure, and Nuclear Fusion Reaction Pathways
Astronomers can determine a star's surface temperature, surface gravity, metallicity, and rotational velocity from its spectrum. Luminosity can be derived if the distance of the star is found, and from there, mass, radius, surface gravity, and rotation period can be estimated.
Luminosity is the amount of light and other radiant energy a star emits per unit of time. It's determined by the star's radius and surface temperature, and starspots can affect it. The apparent brightness of a star is expressed in terms of its apparent magnitude, which is influenced by distance, interstellar dust and gas, and Earth's atmosphere. Absolute magnitude is directly related to a star's luminosity.
Stars are classified by their spectra, ranging from type O (very hot) to M (so cool that molecules may form in their atmospheres), and by their luminosity effects found in their spectral lines. There is additional nomenclature in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum.
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. The primary types of intrinsically variable stars are pulsating, eruptive, and cataclysmic or explosive. Extrinsic factors that can affect luminosity include eclipsing binaries and rotating stars with extreme starspots.
The interior of a stable star is in hydrostatic equilibrium, with inward gravitational force and an outward force due to the pressure gradient within the star. The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, while the convective zone is where convection occurs. The photosphere is that portion of a star that is visible to an observer, while above it is the stellar atmosphere and the corona, a volume of super-heated plasma.
A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. The hydrogen fusion process is temperature-sensitive, and a moderate increase in the core temperature will result in a significant increase in the fusion rate. The core temperature of main sequence stars varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star. In the Sun, hydrogen fuses to form helium in the proton–proton chain reaction.