A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. Moreover, satellite galaxies are not the only astronomical objects that are gravitationally bound to larger host galaxies. For this reason, astronomers have defined galaxies as gravitationally bound collections of stars that exhibit properties that cannot be explained by a combination of baryonic matter and Newton's laws of gravity. For example, measurements of the orbital speed of stars and gas within spiral galaxies result in a velocity curve that deviates from the theoretical prediction; this observation has motivated various explanations such as the theory of dark matter and modifications to Newtonian dynamics.
Therefore, despite being satellites of host galaxies, globular clusters should not be mistaken for satellite galaxies. Satellite galaxies are not only more extended and diffuse compared to globular clusters, but are enshrouded in massive dark matter halos that are thought to have been endowed to them during the formation process. Satellite galaxies lead tumultuous lives due to their chaotic interactions with both the larger host galaxy and other satellites. For example, the host galaxy is capable of disrupting the orbiting satellites via tidal and ram pressure stripping; these environmental effects can remove large amounts of cold gas from satellites, this can result in satellites becoming quiescent in the sense that they have ceased to form stars. Moreover, satellites can collide with their host galaxy resulting in a minor merger. On the other hand, satellites can merge with one another resulting in a major merger. Galaxies are composed of empty space, therefore galaxy mergers do not involve collisions between objects from one galaxy and objects from the other, these events result in much more massive galaxies.
Astronomers seek to constrain the rate at which both minor and major mergers occur to better understand the formation of gigantic structures of gravitationally bound conglomerations of galaxies such as galactic groups and clusters. Prior to the 20th century, the notion that galaxies existed beyond our Milky Way was not well established. In fact, the idea was so controversial at the time that it led to what is now heralded as the "Shapley-Curtis Great Debate" aptly named after the astronomers Harlow Shapley and Heber Doust Curtis that debated the nature of "nebulae" and the size of the Milky Way at the National Academy of Sciences on April 26, 1920. Shapley argued that the Milky Way was the entire universe and that all of the observed "nebulae" resided within this region. On the other hand, Curtis argued that the Milky way was much smaller and that the observed nebulae were in fact galaxies similar to our own Milky Way; this debate was not settled until late 1923 when the astronomer Edwin Hubble measured the distance to M31 using Cepheid Variable stars.
By measuring the period of these stars, Hubble was able to estimate their intrinsic luminosity and upon combining this with their measured apparent magnitude he estimated a distance of 300 kpc, an order-of-magnitude larger than the estimated size of the universe made by Shapley. This measurement verified that not only was the universe much larger than expected, but it demonstrated that the observed nebulae were distant galaxies with a wide range of morphologies. Despite Hubble's discovery that the universe was teeming with galaxies, a majority of the satellite galaxies of the Milky Way and the Local Group remained undetected until the advent of modern astronomical surveys such as the Sloan Digital Sky Survey and the Dark Energy Survey. In particular, the Milky Way is known to host 59 satellite galaxies, however two of these satellites known as the Large Magellanic Cloud and Small Magellanic Cloud have been observable in the Southern Hemisphere with the unaided eye since ancient times. Modern cosmological theories of galaxy formation and evolution predict a much larger number of satellite galaxies than what is observed.
However, more recent high resolution simulations have demonstrated that the current number of observed satellites pose no threat to the prevalent theory of galaxy formation. Spectroscopic and kinematic observations of satellite galaxies have yielded a wealth of information, used to study, among other things, the formation and evolution of galaxies, the environmental effects that enhance and diminish the rate of star formation within galaxies and the distribution of dark matter within the dark matter halo; as a result, satellite galaxies serve as a testing ground for prediction made by cosmological models. As mentioned above, satellite galaxies are categorized as dwarf galaxies and therefore follow a similar Hubble classification scheme as their host with the minor addition of a lowercase "d" in front of the various standard types to designate the dwarf galaxy status; these types include dwarf irre
The angular diameter, angular size, apparent diameter, or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, in optics, it is the angular aperture; the angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side. Angular radius equals half the angular diameter; the angular diameter of a circle whose plane is perpendicular to the displacement vector between the point of view and the centre of said circle can be calculated using the formula δ = 2 arctan , in which δ is the angular diameter, d is the actual diameter of the object, D is the distance to the object. When D ≫ d, we have δ ≈ d / D, the result obtained is in radians. For a spherical object whose actual diameter equals d a c t, where D is the distance to the centre of the sphere, the angular diameter can be found by the formula δ = 2 arcsin The difference is due to the fact that the apparent edges of a sphere are its tangent points, which are closer to the observer than the centre of the sphere.
For practical use, the distinction is only significant for spherical objects that are close, since the small-angle approximation holds for x ≪ 1: arcsin x ≈ arctan x ≈ x. Estimates of angular diameter may be obtained by holding the hand at right angles to a extended arm, as shown in the figure. In astronomy, the sizes of celestial objects are given in terms of their angular diameter as seen from Earth, rather than their actual sizes. Since these angular diameters are small, it is common to present them in arcseconds. An arcsecond is 1/3600th of one degree, a radian is 180/ π degrees, so one radian equals 3,600*180/ π arcseconds, about 206,265 arcseconds. Therefore, the angular diameter of an object with physical diameter d at a distance D, expressed in arcseconds, is given by: δ = d / D arcseconds; these objects have an angular diameter of 1″: an object of diameter 1 cm at a distance of 2.06 km an object of diameter 725.27 km at a distance of 1 astronomical unit an object of diameter 45 866 916 km at 1 light-year an object of diameter 1 AU at a distance of 1 parsec Thus, the angular diameter of Earth's orbit around the Sun as viewed from a distance of 1 pc is 2″, as 1 AU is the mean radius of Earth's orbit.
The angular diameter of the Sun, from a distance of one light-year, is 0.03″, that of Earth 0.0003″. The angular diameter 0.03″ of the Sun given above is the same as that of a person at a distance of the diameter of Earth. This table shows the angular sizes of noteworthy celestial bodies as seen from Earth: The table shows that the angular diameter of Sun, when seen from Earth is 32′, as illustrated above, thus the angular diameter of the Sun is about 250,000 times that of Sirius. The angular diameter of the Sun is about 250,000 times that of Alpha Centauri A; the angular diameter of the Sun is about the same as that of the Moon. Though Pluto is physically larger than Ceres, when viewed from Earth Ceres has a much larger apparent size. Angular sizes measured in degrees are useful for larger patches of sky. However, much finer units are needed to measure the angular sizes of galaxies, nebulae, or other objects of the night sky. Degrees, are subdivided as follows: 360 degrees in a full circle 60 arc-minutes in one degree 60 arc-seconds in one arc-minuteTo put this in perspective, the full Moon as viewed from Earth is about 1⁄2°, or 30′.
The Moon's motion across the sky can be measured in angular size: 15° every hour, or 15″ per second. A one-mile-long line painte
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
Andromeda IV is an isolated irregular dwarf galaxy. The moderate surface brightness, a blue color, low current star formation rate and low metallicity are consistent with it being a small dwarf irregular galaxy similar to Local Group dwarfs such as IC 1613 and Sextans A. Arguments based on the observed radial velocity and the tentative detection of the RGB tip suggest that it lies well outside the confines of the Local Group. Further study using the Hubble space telescope has shown it to be a solitary irregular dwarf galaxy; the galaxy is between 22 and 24 million light years from Earth, so is not close to the Andromeda Galaxy at all. The galaxy is isolated; the Holmberg diameter is 1880 parsecs, but neutral atomic hydrogen gas extends more than eight times further out in a disk. The galaxy is dark, the baryonic mass to dark matter ratio is 0.11. It was discovered by Sidney van den Bergh in 1972. List of Andromeda's satellite galaxies Andromeda IV on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Andromeda II is a dwarf spheroidal galaxy about 2.22 Mly away in the constellation Andromeda. While part of the Local Group, it is not quite clear if it is a satellite of the Andromeda Galaxy or the Triangulum Galaxy, it was discovered by Sidney Van den Bergh in a survey of photographic plates taken with the Palomar 48-inch Schmidt telescope in 1970 and 1971, together with Andromeda I, Andromeda III, the presumable non- or background galaxy Andromeda IV. Using the Keck telescope, Côté et al. 1999 observed spectra for seven stars inside Andromeda II. From this data, they found an average velocity Vr of -188 ± 3 km/s and velocity dispersion of 9.2 ± 2.6 km/s. This gives a mass to light ratio of M/Lv of 21+14−10 solar units which implies that And II contains a significant amount of dark matter. In 1999, Côté, Oke, & Cohen used the Keck to measure the spectra of 42 red giants. From this, they deduced an average metallicity of <> = -1.47 ± 0.19 and a dispersion of 0.35 ± 0.10 dex. In 1999, Da Costa et al. studied the color-magnitude diagram of And II and discovered that most of stars in And II have ages between 6 and 9 Gyr.
However, the observation of RR Lyrae variables and blue horizontal-branch stars demonstrates the existence of a population segment with an age greater than 10 Gyr. And II differs from And I in that it does not show a radial gradient in horizontal-branch morphology. Additionally, the dispersion in abundance is larger in And II as compared to And I; this implies that these two dwarf spheroidal companions to the Andromeda galaxy have different evolutionary histories. This raises the question of whether there is a correlation between a radial horizontal-branch gradient and the metallicity dispersion between dwarf spheroidal galaxies, and II appears to be in the possession of a stellar stream, a feature, indicative of a merger event in the past. The characteristics of And II can best be explained by the merger of two disky dwarf galaxies, some 5 billion years ago. List of Andromeda's satellite galaxies SEDS: Dwarf Spheroidal Galaxy Andromeda II
The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There are at least 2 trillion galaxies in the observable universe. Assuming the Universe is isotropic, the distance to the edge of the observable universe is the same in every direction; that is, the observable universe has a spherical volume centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth; the word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself; because no signals can travel faster than light, any object farther away from us than light could travel in the age of the Universe cannot be detected, as the signals could not have reached us yet.
Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination —and the observable universe, which includes signals since the beginning of the cosmological expansion. According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs, while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs, about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs. The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic background light amounted to 4 × 1084 photons.
Since the expansion of the universe is known to accelerate and will become exponential in the future, the light emitted from all distant objects, past some time dependent on their current redshift, will never reach the Earth. In the future all observable objects will freeze in time while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects situated beyond a certain comoving distance will never reach Earth; some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or its scientific space-based instruments, so lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable. However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light and furthermore the expansion rate appears to be accelerating due to dark energy.
Assuming dark energy remains constant, so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach the Earth. This future visibility limit is calculated at a comoving distance of 19 billion parsecs, assuming the universe will keep expanding forever, which implies the number of galaxies that we can theoretically observe in the infinite future is only larger than the number observable by a factor of 2.36. Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history, but because of the universe's expansion, there may be some age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Bi
The parsec is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 648000/π astronomical units. One parsec is equal to 31 trillion kilometres or 19 trillion miles; the nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun; the parsec unit was first suggested in 1913 by the British astronomer Herbert Hall Turner. Named as a portmanteau of the parallax of one arcsecond, it was defined to make calculations of astronomical distances from only their raw observational data quick and easy for astronomers. For this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs for the more distant objects within and around the Milky Way, megaparsecs for mid-distance galaxies, gigaparsecs for many quasars and the most distant galaxies.
In August 2015, the IAU passed Resolution B2, which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as 648000/π astronomical units, or 3.08567758149137×1016 metres. This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references; the parsec is defined as being equal to the length of the longer leg of an elongated imaginary right triangle in space. The two dimensions on which this triangle is based are its shorter leg, of length one astronomical unit, the subtended angle of the vertex opposite that leg, measuring one arc second. Applying the rules of trigonometry to these two values, the unit length of the other leg of the triangle can be derived. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky; the first measurement is taken from the Earth on one side of the Sun, the second is taken half a year when the Earth is on the opposite side of the Sun.
The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the parallax angle, formed by lines from the Sun and Earth to the star at the distant vertex; the distance to the star could be calculated using trigonometry. The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used this approach to calculate the 3.5-parsec distance of 61 Cygni. The parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the subtended angle, from that star's perspective, of the semimajor axis of the Earth's orbit; the star, the Sun and the Earth form the corners of an imaginary right triangle in space: the right angle is the corner at the Sun, the corner at the star is the parallax angle.
The length of the opposite side to the parallax angle is the distance from the Earth to the Sun (defined as one astronomical unit, the length of the adjacent side gives the distance from the sun to the star. Therefore, given a measurement of the parallax angle, along with the rules of trigonometry, the distance from the Sun to the star can be found. A parsec is defined as the length of the side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond; the use of the parsec as a unit of distance follows from Bessel's method, because the distance in parsecs can be computed as the reciprocal of the parallax angle in arcseconds. No trigonometric functions are required in this relationship because the small angles involved mean that the approximate solution of the skinny triangle can be applied. Though it may have been used before, the term parsec was first mentioned in an astronomical publication in 1913. Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance.
He proposed the name astron, but mentioned that Carl Charlier had suggested siriometer and Herbert Hall Turner had proposed parsec. It was Turner's proposal. In the diagram above, S represents the Sun, E the Earth at one point in its orbit, thus the distance ES is one astronomical unit. The angle SDE is one arcsecond so by definition D is a point in space at a distance of one parsec from the Sun. Through trigonometry, the distance SD is calculated as follows: S D = E S tan 1 ″ S D ≈ E S 1 ″ = 1 au 1 60 × 60 × π