Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth; the techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, an astronomical object that has a known luminosity; the ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, so on; each rung of the ladder provides information that can be used to determine the distances at the next higher rung. At the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question.
The precise measurement of stellar positions is part of the discipline of astrometry. Direct distance measurements are based upon the astronomical unit, the distance between the Earth and the Sun. Kepler's laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, but provides no measurement of the overall scale of the orbit system. Radar is used to measure the distance of a second body. From that measurement and the ratio of the two orbit sizes, the size of Earth's orbit is calculated; the Earth's orbit is known with an absolute precision of a few meters and a relative precision of a few 1×10−11. Observations of transits of Venus were crucial in determining the AU. Presently the orbit of Earth is determined with high precision using radar measurements of distances to Venus and other nearby planets and asteroids, by tracking interplanetary spacecraft in their orbits around the Sun through the Solar System; the most important fundamental distance measurements come from trigonometric parallax.
As the Earth orbits the Sun, the position of nearby stars will appear to shift against the more distant background. These shifts are angles in an isosceles triangle, with 2 AU making the base leg of the triangle and the distance to the star being the long equal length legs; the amount of shift is quite small, measuring 1 arcsecond for an object at 1 parsec's distance of the nearest stars, thereafter decreasing in angular amount as the distance increases. Astronomers express distances in units of parsecs; because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars which are near enough to have a parallax larger than a few times the precision of the measurement. Parallax measurements have an accuracy measured in milliarcseconds. In the 1990s, for example, the Hipparcos mission obtained parallaxes for over a hundred thousand stars with a precision of about a milliarcsecond, providing useful distances for stars out to a few hundred parsecs; the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars.
In 2018, Data Release 2 from the Gaia space mission provides accurate distances to most stars brighter than 15th magnitude. Stars have a velocity relative to the Sun that causes radial velocity; the former is determined by plotting the changing position of the stars over many years, while the latter comes from measuring the Doppler shift of the star's spectrum caused by motion along the line of sight. For a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities; this statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables. The motion of the Sun through space provides a longer baseline that will increase the accuracy of parallax measurements, known as secular parallax. For stars in the Milky Way disk, this corresponds to a mean baseline of 4 AU per year, while for halo stars the baseline is 40 AU per year.
After several decades, the baseline can be orders of magnitude greater than the Earth–Sun baseline used for traditional parallax. However, secular parallax introduces a higher level of uncertainty because the relative velocity of observed stars is an additional unknown; when applied to samples of multiple stars, the uncertainty can be reduced. Moving cluster parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster. Only open clusters are near enough for this technique to be useful. In particular the distance obtained for the Hyades has been an important step in the distance ladder. Other individual objects can have fundamental distance estimates made for them under special circumstances. If the expansion of a gas cloud, like a supernova remnant or planetary nebula, can be observed over time an expansion parallax distance to that cloud can be estimated; those measurements however suf
Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point above the earth in question. When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system. An old term, right ascension refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle, it contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle. Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction on an equator. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is located in the constellation Pisces.
Right ascension is measured continuously in a full circle from that alignment of Earth and Sun in space, that equinox, the measurement increasing towards the east. As seen from Earth, objects noted to have 12h RA are longest visible at the March equinox. On those dates at midnight, such objects will reach their highest point. How high depends on their declination. Any units of angular measure could have been chosen for right ascension, but it is customarily measured in hours and seconds, with 24h being equivalent to a full circle. Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the Earth rotates; the line which passes through the highest point in the sky, called the meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360°, 1/24 of a circle is measured as 1h of right ascension, or 15°. A full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86400s, or 24 × 60 = 1440m, or 24h.
Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 1h 30m 00s is at its meridian a star with RA = 20h 00m 00s will be on the/at its meridian 18.5 sidereal hours later. Sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Measured in degrees, it is the complement of right ascension with respect to 24h, it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earth's axis rotates westward about the poles of the ecliptic, completing one cycle in about 26,000 years; this movement, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, astronomers specify them with reference to a particular year, known as an epoch.
Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. Right ascension for "fixed stars" near the ecliptic and equator increases by about 3.05 seconds per year on average, or 5.1 minutes per century, but for fixed stars further from the ecliptic the rate of change can be anything from negative infinity to positive infinity. The right ascension of Polaris is increasing quickly; the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. The used standard epoch is J2000.0, January 1, 2000 at 12:00 TT. The prefix "J" indicates. Prior to J2000.0, astronomers used the successive Besselian epochs B1875.0, B1900.0, B1950.0. The concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC, but Hipparchus and his successors made their star catalogs in ecliptic coordinates, the use of RA was limited to special cases.
With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do, to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive is used with an equatorial mount to cancel out the Earth's rotation; as the equatorial mount became adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could be pointed at objects with known right ascension and declination by the use of setting circles; the first star catalog to use right ascen
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
A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formation rate of the Milky Way galaxy is 3 M☉/yr, starburst galaxies can experience star formation rates that are more than a factor of 100 times greater. In a starburst galaxy, the rate of star formation is so large that the galaxy will consume all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy; as such, the starburst nature of a galaxy is a phase, one that occupies a brief period of a galaxy's evolution. The majority of starburst galaxies are in the midst of a merger or close encounter with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039, IC 10. Starburst galaxies are defined by these three interrelated factors: The rate at which the galaxy is converting gas into stars.
The available quantity of gas from which stars can be formed. A comparison of the timescale on which star formation will consume the available gas with the age or rotation period of the galaxy. Used definitions include: Continued star-formation where the current SFR would exhaust the available gas reservoir in much less than the age of the Universe. Continued star-formation where the current SFR would exhaust the available gas reservoir in much less than the dynamical timescale of the galaxy; the current SFR, normalised by the past-averaged SFR, is much greater than unity. This ratio is referred to as the "birthrate parameter". Mergers and tidal interactions between gas-rich galaxies play a large role in driving starbursts. Galaxies in the midst of a starburst show tidal tails, an indication of a close encounter with another galaxy, or are in the midst of a merger. Interactions between galaxies that do not merge can trigger unstable rotation modes, such as the bar instability, which causes gas to be funneled towards the nucleus and ignites bursts of star formation near the galactic nucleus.
It has been shown that there is a strong correlation between the lopsidedness of a galaxy and the youth of its stellar population, with more lopsided galaxies having younger central stellar populations. As lopsidedness can be caused by tidal interactions and mergers between galaxies, this result gives further evidence that mergers and tidal interactions can induce central star formation in a galaxy and drive a starburst. Classifying types of starburst galaxies is difficult since starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur in disk galaxies, irregular galaxies exhibit knots of starburst spread throughout the irregular galaxy. Astronomers classify starburst galaxies based on their most distinct observational characteristics; some of the categorizations include: Blue compact galaxies. These galaxies are low mass, low metallicity, dust-free objects; because they are dust-free and contain a large number of hot, young stars, they are blue in optical and ultraviolet colours.
It was thought that BCGs were genuinely young galaxies in the process of forming their first generation of stars, thus explaining their low metal content. However, old stellar populations have been found in most BCGs, it is thought that efficient mixing may explain the apparent lack of dust and metals. Most BCGs close interactions. Well-studied BCGs include IZw18, ESO338-IG04 and Haro11. Blue compact dwarf galaxies are small compact galaxies. Green Pea galaxies are small compact galaxies resembling primordial starbursts, they were found by citizen scientists taking part in the Galaxy Zoo project. Luminous infrared galaxies. Ultra-luminous Infrared Galaxies ULIRGs); these galaxies are extremely dusty objects. The ultraviolet radiation produced by the obscured star-formation is absorbed by the dust and reradiated in the infrared spectrum at wavelengths of around 100 micrometres; this explains the extreme red colours associated with ULIRGs. It is not known for sure that the UV radiation is produced purely by star-formation, some astronomers believe ULIRGs to be powered by active galactic nuclei.
X-ray observations of many ULIRGs that penetrate the dust suggest that many starburst galaxies are double-cored systems, lending support to the hypothesis that ULIRGs are powered by star-formation triggered by major mergers. Well-studied ULIRGs include Arp 220. Hyperluminous Infrared galaxies, sometimes called Submillimeter galaxies. Wolf-Rayet galaxies, galaxies where a large portion of the bright stars are Wolf-Rayet stars; the Wolf-Rayet phase is a short-lived phase in the life of massive stars 10% of the total life-time of these stars and as such any galaxy is to contain few of these. However, because the stars are both luminous and have distinctive spectral features, it is possible to identify these stars in the spectra of entire galaxies and doing so allows good constraints to be placed on the properties of the starbursts in these galaxies. Firstly, a starburst galaxy must have a large supply of gas available to form stars; the burst itself may be triggered by a close encounter with another galaxy, a collision with another galaxy, or by another process which forces material into the centre of the galaxy.
The inside of the starburst is quite an extreme environment. The large amount
In physics, redshift is a phenomenon where electromagnetic radiation from an object undergoes an increase in wavelength. Whether or not the radiation is visible, "redshift" means an increase in wavelength, equivalent to a decrease in wave frequency and photon energy, in accordance with the wave and quantum theories of light. Neither the emitted nor perceived light is red. Examples of redshifting are a gamma ray perceived as an X-ray, or visible light perceived as radio waves; the opposite of a redshift is energy increases. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift. There are three main causes of red in astronomy and cosmology: Objects move apart in space; this is an example of the Doppler effect. Space itself expands; this is known as cosmological redshift. All sufficiently distant light sources show redshift corresponding to the rate of increase in their distance from Earth, known as Hubble's Law. Gravitational redshift is a relativistic effect observed due to strong gravitational fields, which distort spacetime and exert a force on light and other particles.
Knowledge of redshifts and blueshifts has been used to develop several terrestrial technologies such as Doppler radar and radar guns. Redshifts are seen in the spectroscopic observations of astronomical objects, its value is represented by the letter z. A special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Special relativistic and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842.
The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler predicted that the phenomenon should apply to all waves, in particular suggested that the varying colors of stars could be attributed to their motion with respect to the Earth. Before this was verified, however, it was found that stellar colors were due to a star's temperature, not motion. Only was Doppler vindicated by verified redshift observations; the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is sometimes called the "Doppler–Fizeau effect". In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, the yearly change in the Doppler shift of stars located near the ecliptic due to the orbital velocity of the Earth.
In 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The earliest occurrence of the term red-shift in print appears to be by American astronomer Walter S. Adams in 1908, in which he mentions "Two methods of investigating that nature of the nebular red-shift"; the word does not appear unhyphenated until about 1934 by Willem de Sitter indicating that up to that point its German equivalent, was more used. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies mostly thought to be spiral nebulae, had considerable redshifts. Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years he wrote a review in the journal Popular Astronomy. In it he states that "the early discovery that the great Andromeda spiral had the quite exceptional velocity of –300 km showed the means available, capable of investigating not only the spectra of the spirals but their velocities as well."
Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable "positive" velocities. Subsequently, Edwin Hubble discovered an approximate relationship between the redshifts of such "nebulae" and the distances to them with the formulation of his eponymous Hubble's law; these observations corroborated Alexander Friedmann's 1922 work, in which he derived the Friedmann-Lemaître equations. They are today considered strong evidence for the Big Bang theory; the spectrum of light that comes from a single source can be measured. To determine the redshift, one searches for features in the spectrum such as absorption lines, emission lines, or other variations in light intensity. If found, these featur
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by West Germany, the United Kingdom and the United States. It was launched on 1 June 1990, on a Delta II rocket from Cape Canaveral, on what was designed as an 18-month mission, with provision for up to five years of operation. ROSAT operated for over eight years shutting down on 12 February 1999. In February 2011, it was reported that the 2,400 kg satellite was unlikely to burn up while re-entering the Earth's atmosphere due to the large amount of ceramics and glass used in construction. Parts as heavy as 400 kg could impact the surface intact. ROSAT re-entered the Earth's atmosphere on 23 October 2011 over Bay of Bengal. According to NASA, the Roentgensatellit was a joint German, U. S. and British X-ray astrophysics project. ROSAT carried a German-built imaging X-ray Telescope with three focal plane instruments: two German Position Sensitive Proportional Counters and the US-supplied High Resolution Imager.
The X-ray mirror assembly was a grazing incidence four-fold nested Wolter I telescope with an 84-cm diameter aperture and 240-cm focal length. The angular resolution was less than 5 arcsecond at half energy width; the XRT assembly was sensitive to X-rays between 0.1 and 2 keV. In addition, a British-supplied extreme ultraviolet telescope, the Wide Field Camera, was coaligned with the XRT and covered the energy band from 0.042 to 0.21 keV. ROSAT's unique strengths were high spatial resolution, low-background, soft X-ray imaging for the study of the structure of low surface brightness features, for low-resolution spectroscopy; the ROSAT spacecraft was a three-axis stabilized satellite which can be used for pointed observations, for slewing between targets, for performing scanning observations on great circles perpendicular to the plane of the ecliptic. ROSAT was capable of fast slews which makes it possible to observe two targets on opposite hemispheres during each orbit; the pointing accuracy was 1 arcminute with stability less than 5 arcsec per sec and jitter radius of ~10 arcsec.
Two CCD star sensors were used for optical position sensing of guide stars and attitude determination of the spacecraft. The post facto attitude determination accuracy was 6 arcsec; the ROSAT mission was divided into two phases: After a two-month on-orbit calibration and verification period, an all-sky survey was performed for six months using the PSPC in the focus of XRT, in two XUV bands using the WFC. The survey was carried out in the scan mode; the second phase consists of the remainder of the mission and was devoted to pointed observations of selected astrophysical sources. In ROSAT's pointed phase, observing time was allocated to Guest Investigators from all three participating countries through peer review of submitted proposals. ROSAT was expected to operate beyond its nominal lifetime; the main assembly was a German-built imaging X-ray Telescope with three focal plane instruments: two German Position Sensitive Proportional Counters and the US-supplied High Resolution Imager. The X-ray mirror assembly was a grazing incidence four-fold nested Wolter I telescope with an 84 cm diameter aperture and 240 cm focal length.
The angular resolution was less than 5 arcsec at half energy width. The XRT assembly was sensitive to X-rays between 0.1 and 2 keV. Each Position Sensitive Proportional Counter is a thin-window gas counter; each incoming X-ray photon produces an electron cloud whose position and charge are detected using two wire grids. The photon position is determined with an accuracy of about 120 micrometers; the electron cloud's charge corresponds to the photon energy. The US supplied High Resolution Imager used a crossed grid detector with a position accuracy to 25 micrometers; the instrument was damaged by solar exposure on 20 September 1998. The Wide Field Camera was a British-supplied extreme ultraviolet telescope co-aligned with the XRT and covered the wave band between 300 and 60 angstroms. X-ray all-sky survey catalog, more than 150,000 objects XUV all-sky survey catalog Source catalogs from the pointed phase containing ~ 100,000 serendipitous sources Detailed morphology of supernova remnants and clusters of galaxies.
Detection of shadowing of diffuse X-ray emission by molecular clouds. Detection of pulsations from Geminga. Detection of isolated neutron stars. Discovery of X-ray emission from comets. Observation of X-ray emission from the collision of Comet Shoemaker-Levy with Jupiter. 1RXS - an acronym, the prefix used for the First ROSAT X-ray Survey, a catalogue of astronomical objects visible for ROSAT in the X-ray spectrum. See Category:ROSAT objects ROSAT was planned to be launched on the Space Shuttle but the Challenger disaster caused it to be moved to the Delta platform; this move made it impossible to bring it back to Earth. Designed for a five-year mission, ROSAT continued in its extended mission for a further four years before equipment failure forced an end to the mission. For some months after this, ROSAT completed its last observations before being switched off on 12 February 1999. On 25 April 1998, failure of the primary star tracker on the X-ray Telescope led to pointing errors that in turn had caused solar overheating.
A contingency plan and the necessary software had been developed t
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dark matter. The word galaxy is derived from the Greek galaxias "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers; the Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, observed as it existed just 400 million years after the Big Bang. Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. For comparison, the Milky Way has a diameter of at least 30,000 parsecs and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs; the space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups and superclusters; the Milky Way is part of the Local Group, dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are arranged into sheets and filaments surrounded by immense voids; the largest structure of galaxies yet recognised is a cluster of superclusters, named Laniakea, which contains the Virgo supercluster. The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias, or kyklos galaktikos due to its appearance as a "milky" band of light in the sky.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, it produces the faint band of light known as the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe; the English term Milky Way can be traced back to a story by Chaucer c. 1380: "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." Galaxies were discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term fell into disuse, as the word universe implied the entirety of existence. Instead, they became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, the MCG and UGC. All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, PGC 37617; the realization that we live in a galaxy, one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae. The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World, continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger was critical of this view, arguing that if the Milky Way is sublunary it should appear different at different times and places on Earth, that it should have parallax, which it does not. In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at observing and measuring the Milky Way's parallax, he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī