Stars and Galaxies
They are massive luminous sphere of plasma held together
by its own gravity – nearestÂ starÂ to Earth is the Sun.
(Click the blue text, where itâ€™sÂ available,Â below for more information.)
AÂ star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity – nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements.
The Sun is the closestÂ starÂ to Earth.
AÂ star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion – all starsÂ are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin.
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature – they lie above the main sequence on the Hertzsprungâ€“Russell diagram and correspond to luminosity classes II and III.
TheÂ smallest starsÂ are calledÂ tinyÂ red dwarfs. Red dwarfs are the most commonly-foundÂ stars near the Earth – astronomers have determined that about 2/3 of all the starsÂ in the vicinity of Earth are red dwarfÂ stars; this includes Proxima Centauri, which is the closest starÂ to Earth.
StarsÂ have a wide range of apparentÂ brightness measured here on Earth – the variation in their brightnessÂ isÂ causedÂ by both variations in their luminosity and variations in their distance. An intrinsically faint, nearbyÂ starÂ can appear to be just as bright to us on Earth as an intrinsically luminous, distantÂ star.
The Hertzsprungâ€“Russell diagram, abbreviated as Hâ€“R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars’ absolute magnitudes or luminosities versus their stellar classifications or effective temperatures.
A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems – these systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as multiple by other means.
Variable starsÂ areÂ starsÂ that change brightness. The brightness changes of theseÂ starsÂ can range from a thousandth of a magnitude to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type ofÂ variable star.
A nova is a transient astronomical event that causes the sudden appearance of a bright, apparently “new” star, that slowly fades over several weeks or many months – causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars.
A supernova is a powerful and luminous stellar explosion – this transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion.
A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, excluding black holes and hypothetical white holes, quark stars, and strange stars.
A pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles – this radiation can be observed only when a beam of emission is pointing toward Earth, and is responsible for the pulsed appearance of emission.
Star clustersÂ are very large groups ofÂ stars. Two types of star clusters can be distinguished:Â globular clustersÂ are tight groups of hundreds to millions of old stars which areÂ gravitationallyÂ bound, whileÂ open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence ofÂ giant molecular cloudsÂ as they move through theÂ Galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound.
A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, the term was used to describe any diffuse astronomical object, including galaxies beyond the Milky Way.
A black hole is a region of spacetime where gravity is so strong that nothing – no particles or even electromagnetic radiation such as light can escape from it; the theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.
A quasar is an extremely luminous active galactic nucleus, in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk.
Some scientists think there could be as many as five hundred billion galaxiesÂ in the universe.
Our galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter – the word galaxy is derived from the Greek galaxias, literally “milky”, a reference to the Milky Way.
The Local Group is the galaxy group that includes the Milky Way – it has a total diameter of roughly 3 megaparsecs, and a total mass of the order of 2Ã—10Â¹Â² solar masses.
From the wordÂ star,Â meaningÂ luminous astronomical object seen in the night sky (the Sun is also aÂ star).
TheÂ patternsÂ ofÂ starsÂ seen in the sky are usually called constellations, although more acurately, a group ofÂ starsÂ that forms aÂ patternÂ in the sky is called an asterism. Astronomers use the term constellation to refer to an area of the sky.
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth – all objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer.
A star chart or star map, also called a sky chart or sky map, is a map of the night sky. Astronomers divide these into grids to use them more easily – they are used to identify and locate constellations and astronomical objects such as stars, nebulae, and galaxies.