Instruct: Given a question, retrieve documents that answer the question.
Query: What is the most common type of star?
Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are fifty of the sixty nearest stars.
According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.
Vega, also designated α Lyrae (Latinised to Alpha Lyrae, abbreviated Alpha Lyr or α Lyr), is the brightest star in the constellation of Lyra, the fifth-brightest star in the night sky, and the second-brightest star in the northern celestial hemisphere, after Arcturus. It is relatively close at only 25 light-years from the Sun, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood.
The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally. The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.
Even though Delta Muscae is the closest star to Earth in the Musca constellation, nearly 3800 stars are closer in proximity to the Earth as stated by the Gliese Catalogue of Nearby Stars, which includes stars within twenty-five parsecs of the Sun.
Water has been detected in interstellar clouds within our galaxy, the Milky Way. Water probably exists in abundance in other galaxies, too, because its components, hydrogen and oxygen, are among the most abundant elements in the universe. Based on models of the formation and evolution of the Solar System and that of other star systems, most other planetary systems are likely to have similar ingredients.
Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun, a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at formula_1, or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the CHZ concept is more complex.
Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain spectral types. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration. If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU.
The common starfish feeds on a variety of benthic organisms. These include bivalve molluscs, polychaete worms, barnacles, gastropod molluscs, other echinoderms and carrion. When feeding on a mollusc such as a mussel, it attaches its tube feet to each shell valve and exerts force to separate them slightly. Even a gap of only is sufficient for the starfish to insert a fold of its stomach, secrete enzymes and start digesting the mollusc body. When the contents is sufficiently liquid, it brings its stomach back to its rightful position with the food inside. The common starfish has a well-developed sense of smell and can detect the odour of prey species such as the common mussel ("Mytilus edulis") and crawl towards it. It can also detect the odour of the predatory common sunstar ("Crossaster papposus"), which eats other starfish, and take evasive action.
The common starfish is dioecious, which means that each individual is either male or female. In the spring, the females release their eggs into the sea. A moderate sized starfish is estimated to be able to produce 2.5 million eggs. The males shed their sperm and fertilisation takes place in the water column. The larvae are planktonic and drift for about 87 days before settling on the seabed and undergoing metamorphosis into juveniles. Common starfish are believed to live for about seven to eight years. When well fed, the juveniles can increase their radius at the rate of slightly more than per month during the summer and autumn and slightly less than per month in the winter. An adult common starfish can survive starvation for several months although it loses weight in the process. One specimen shrank from a radius of to a radius of after starvation for five months.
The largest unequivocally C-type asteroid is 10 Hygiea, although the SMASS classification places the largest asteroid, 1 Ceres, here as well, because that scheme lacks a G-type.
Since superflares can occur on stars apparently equivalent in every way to the Sun, it is natural to ask if they can occur on the Sun itself. An estimate based on the original Kepler photometric studies suggested a frequency on solar-type stars (early G-type and rotation period more than 10 days) of once every 800 years for an energy of 10 erg and every 5000 years at 10 erg. One-minute sampling provided statistics for less energetic flares and gave a frequency of one flare of energy 10 erg every 5–600 years for a star rotating as slowly as the Sun; this would be rated as X100 on the solar flare scale. This is based on a straightforward comparison of the number of stars studied with the number of flares observed. An extrapolation of the empirical statistics for solar flares to an energy of 10 erg suggests a frequency of one in 10,000 years.
Whether RGB or RC, these stars all have an extended convective envelope favorable to the excitation of solar-like oscillations. A major success of CoRoT has been the discovery of radial and long-lived non-radial oscillations in thousands of red giants in the exo field. For each of them, the frequency at maximum power ν in the frequency spectrum as well as the large frequency separation between consecutive modes Δν could be measured, defining a sort of individual seismic passport.Massive variable main sequence stars have frequency spectra dominated by acoustic modes excited by the κ mechanism at work in layers where partial ionization of iron group elements produce a peak in opacity. In addition the most advanced of these stars present mixed modes i.e. modes with a g-character in deep layers and p-character in the envelope. Hydrogen burning takes place in a convective core surrounded by a region of varying chemical composition and an envelope mostly radiative except for tiny convective layers related to partial ionization of helium and/or iron group elements. As in lower mass stars the extent of the fully or partially mixed region located just above the convective core "(extra-mixed zone)" is one of the main uncertainties affecting theoretical modeling.
A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, containing a mass of the order of hundreds of thousands, to billions times, the mass of the Sun (). This is a class of astronomical objects that has undergone gravitational collapse, leaving behind a spheroidal region of space from which nothing can escape; not even light. Observational evidence indicates that all, or nearly all, massive galaxies contain a supermassive black hole, located at the galaxy's center. In the case of the Milky Way, the supermassive black hole corresponds to the location of Sagittarius A* at the Galactic Core. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering quasars and other types of active galactic nuclei.
The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
Starfish are marine invertebrates. They typically have a central disc and five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence. The Asteroidea occupy several significant ecological roles. Starfish, such as the ochre sea star ("Pisaster ochraceus") and the reef sea star ("Stichaster australis"), have become widely known as examples of the keystone species concept in ecology. The tropical crown-of-thorns starfish ("Acanthaster planci") is a voracious predator of coral throughout the Indo-Pacific region, and the northern Pacific sea star is considered to be one of the world's 100 worst invasive species.
The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope.
Another such example would be HIP 11915, which has a planetary system containing a Jupiter-like planet orbiting at a similar distance that the planet Jupiter does in the Solar System. To strengthen the similarities, the star is class G5V, has a temperature of 5750 K, has a sun-like mass and radius, and is only 500 million years younger than the Sun. As such, the habitable zone would extend in the same area as the zone in the Solar System, around 1 AU. This would allow an Earth-like planet to exist around 1 AU.
The Milky Way and Andromeda galaxies each contain a central supermassive black hole (SMBH), these being Sagittarius A* (ca. ) and an object within the P2 concentration of Andromeda's nucleus (). These black holes will converge near the centre of the newly formed galaxy over a period that may take millions of years, due to a process known as dynamical friction: As the SMBHs move relative to the surrounding cloud of much less massive stars, gravitational interactions lead to a net transfer of orbital energy from the SMBHs to the stars, causing the stars to be "slingshotted" into higher-radius orbits, and the SMBHs to "sink" toward the galactic core. When the SMBHs come within one light-year of one another, they will begin to strongly emit gravitational waves that will radiate further orbital energy until they merge completely. Gas taken up by the combined black hole could create a luminous quasar or an active galactic nucleus, releasing as much energy as 100 million supernova explosions. As of 2006, simulations indicated that the Sun might be brought near the centre of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy. Alternatively, the Sun might approach one of the black holes a bit closer and be torn apart by its gravity. Parts of the former Sun would be pulled into the black hole.
Lambda Muscae (λ Mus, λ Muscae), often catalogued HD 102249 or HIP 57363, is the fourth-brightest star in the Southern Hemisphere constellation of Musca (the Fly). Lambda Muscae visibly makes up the far end of the tail of the visual Musca constellation. It is a star in a dual system according to studies, and one of the stars catalogued in astronomer Johann Bayer's 1603 publications "Uranometria". Lambda Muscae more recently has been recorded as having a stellar classification of A7III. Thus, Lambda Muscae has a white tint and burns significantly hotter than the Sun.
After Ceres, Asteroid 433 Eros is perhaps the most-commonly mentioned asteroid, probably because it is one of the largest near-Earth asteroids.
Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms. The Antarctic "Labidiaster annulatus" can have over fifty. Having descended from bilateral organisms, starfish move in a bilateral fashion, with certain arms acting like the front of the animal.
When discovered, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined by the IAU in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and develop the appearance of asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets. Also, they are theorized to be common objects amongst the celestial bodies orbiting close to the Sun.
It has been estimated that there are around 20,000 massive O-type stars in the galaxy. The low-mass sdO and CSPNe O-type stars are probably more common, although less luminous and therefore harder to find. Despite their short lifetimes, they are thought to be normal stages in the evolution of common stars only a little more massive than the Sun.
O-type stars represent the highest masses of stars on the main sequence. The coolest of them have initial masses of around 16 times the Sun. It is unclear what the upper limit to the mass of an O-type star would be. At solar metallicity levels, stars should not be able to form with masses above 120–150 solar masses, but at lower metallicity this limit is much higher. O-type stars form only a tiny fraction of main-sequence stars and the vast majority of these are towards the lower end of the mass range. The most massive and hottest types O3 and O2 are extremely rare, were only defined in 1971 and 2002 respectively, and only a handful are known in total. Giant and supergiant stars are somewhat less massive than the most massive main sequence O-type stars due to mass loss, but are still among the most massive stars known.
Most type II supernova progenitors are thought to be red supergiants, while the less common type Ib/c supernovae are produced by hotter Wolf–Rayet stars that have completely lost more of their hydrogen atmosphere. Almost by definition, supergiants are destined to end their lives violently. Stars large enough to start fusing elements heavier than helium do not seem to have any way to lose enough mass to avoid catastrophic core collapse, although some may collapse, almost without trace, into their own central black holes.
Within any giant luminosity class, the cooler stars (spectral class K, M, S, and C) are called red giants. Red giants include stars in a number of distinct evolutionary phases of their lives: a main red-giant branch (RGB); a red horizontal branch or red clump; the asymptotic giant branch (AGB), although AGB stars are often large enough and luminous enough to get classified as supergiants; and sometimes other large cool stars such as immediate post-AGB stars. The RGB stars are by far the most common type of giant star due to their moderate mass, relatively long stable lives, and luminosity. They are the most obvious grouping of stars after the main sequence on most HR diagrams, although white dwarfs are more numerous but far less luminous.