On an interstellar scale, parallax created by the different orbital positions of the Earth causes nearby stars to appear to move relative to more distant stars. By observing parallax, measuring angles and using geometry, one can determine the distance to various objects. When the object in question is a star, the effect is known as stellar parallax. Stellar parallax is most often measured using annual parallax, defined as the difference in position of a star as seen from the Earth and Sun, i.& e. the angle subtended at a star by the mean radius of the Earth’s orbit around the Sun. Annual parallax is normally measured by observing the position of a star at different times of the year as the Earth moves through its orbit. The angles involved in these calculations are very small and thus difficult to measure. The nearest star to the Sun (and thus the star with the largest parallax), Proxima Centauri, has a parallax of 0.7687& ±& 0.0003& arcsec. Hipparcos was able to measure parallax angles for stars up to about 1,600 light-years away, a little more than one percent of the diameter of the Milky Way Galaxy; i.e., to an accuracy of about a milliarcsecond (mas).

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Balloons have also been used to pinpoint X-ray source positions, e.g., Coma Berenices X-1. To observe weak X-ray sources at photon energies between 20 and 100 keV near the galactic center, a balloon flight was conducted on February 29, 1968, from Mildura, Australia. Lupus XR-1, Norma XR-2, Scorpius X-2, Scorpius XR-3, and Scorpius XR-4 could not be detected.

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The NOVAS library provides three levels of subroutines (functions): basic, utility, and supervisory. Basic-level subroutines supply the values of fundamental variables, such as the nutation angles and the heliocentric positions of solar system bodies for specific epoches. Utility-level subroutines perform transformations, such as those caused by precession, nutation and aberration. Supervisory-level subroutines serve as interfaces to the basic and utility subroutines to compute the coordinates of stars or solar system bodies for specific dates and times.

Adapted from the Wikipedia article Naval Observatory Vector Astrometry Subroutines, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Of the first X-ray sources discovered in each constellation (126 for 89 areas), some 63% are visibly dark. These visually dark X-ray sources can be radiative cosmic dust, hydrogen gas such as an H II region (e.g. the Orion Nebula), an H I region of hydrogen, a molecular cloud, or a coronal-type gas cloud. Pinpoint accuracy may be needed as likely sources may move in relatively short times to convert the astronomical X-ray source to an X-ray transient.

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Norma XR-1 was discovered at RA Dec by an Aerobee 150 rocket flight on April 25, 1965. It may be the same source as Lockheed 1. Its visual counterpart is V801 Ara. It is a recurrent transient, located at 2S 1636-536, and designated X336-1. Although Nor X-1 has also been designated 2S 1624-490, 4U 1626-49 has been associated with the visual counterpart G334.9-0.3.

There is a problem of nomenclature with the names Nor XR-1, Nor X-1, and Nor-1: they do not necessarily imply the same source. Nor-1 (Hill, close to 3U 1624-49) has had its position revised and is now almost exactly coincident not with 3U 1624-49, but with 3U 1636-53. Nor X-1 (Cooke), detected on two rocket flights displays too much low energy absorption to be 3U 1636-53, it is more intense than 3U 1624-49 by more than an order of magnitude, and is more than six times stronger than another Nor X-1 candidate, 3U 1630-47. Nor X-1 (Cooke) appears to have no 3U counterpart. GX 339-4 (or 3U 1658-48) is not a candidate, since it was below the horizon for one of the rocket flights. Nor XR-1 (Adelaide) cannot be identified with any 3U object as its error box is devoid of 3U sources. 3U 1624-49 has no reliable counterpart among any of the above pre-Uhuru sources.

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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In the beginning of X-ray astronomy, the first celestial object discovered to emit X-rays was the Sun (see solar X-ray astronomy). These X-rays were detected by sounding rocket. Several different types have been used to carry X-ray detectors above the Earth’s atmosphere: an Aerobee 150 first detected Scorpius X-1 (the first X-ray source detected in the constellation Scorpius), a Nike Tomahawk first detected X-rays from the Large Magellanic Cloud (Dorado X-1) and Vela X-1.

For rocket launches the attitude of the rocket can be controlled by a computer pointing program such as STRAP-IV which consists of attitude reference updates on two bright stars, a 20 s exposure of a nearby X-ray source, 8 s containing no known sources, and 150 s exposure of the desired target.

There are inherent difficulties in making X-ray/optical, X-ray/radio, and X-ray/X-ray identifications based solely on positional coincidents, especially with handicaps in making identifications, such as the large uncertainties in positional determinants made from balloons and rockets, poor source separation in the crowded region toward the galactic center, source variability, and the multiplicity of source nomenclature.

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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According to SIMBAD AT Mic has year 2000 (J2000) equatorial coordinates right ascension (RA) declination (Dec) due to Earth orbit precession from AT Mic at J1950 RA Dec .

It has a stellar parallax in milliarcseconds (mas) of 97.80, or 10.22 parsecs (33.33 ly).

Adapted from the Wikipedia article AT Microscopii, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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X‐ray source counterparts to stars can be identified by calculating the angular separation between source centroids and position of the star. The maximum allowable separation is a compromise between a larger value to identify as many real matches as possible and a smaller value to minimize the probability of spurious matches. “An adopted matching criterion of 40″ finds nearly all possible X‐ray source matches while keeping the probability of any spurious matches in the sample to 3%.”

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary. Research between the early 1800s and today suggests that many stars are part of either binary star systems or star systems with more than two stars, called ”multiple star systems”. The term ”double star” may be used synonymously with ”binary star”, but more generally, a double star may be either a binary star or an ”optical double star” which consists of two stars with no physical connection but which appear close together in the sky as seen from the Earth. A double star may be determined to be optical if its components have sufficiently different proper motions or radial velocities, or if parallax measurements reveal its two components to be at sufficiently different distances from the Earth. Most known double stars have not yet been determined to be either bound binary star systems or optical doubles.

Binary star systems are very important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated. This also determines an empirical mass-luminosity relationship (MLR) from which the masses of single stars can be estimated.

Binary stars are often detected optically, in which case they are called ”visual binaries”. Many visual binaries have long orbital periods of seve

If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these ”close binary systems” can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of which one member is probably a black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.

Adapted from the Wikipedia article Binary star, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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When satellite launching technology became available, satellites greatly increased the number of first X-ray sources in constellations: OSO 3 – Canis Minor X-1, Vela 5B – Camelopardalis X-1, Uhuru – Hercules X-1, and Einstein – Corona Australis X-1.

For example, although Scutum X-1 was initially discovered with an X-ray payload on a sounding rocket in 1973, its accurate positioning has been difficult, requiring subsequent observations from Copernicus, Ariel 5, HEAO 1, Ginga, ASCA, XMM-Newton and Rossi X-ray Timing Explorer (RXTE), especially with Uhuru and EXOSAT not detecting it unambiguously.

Adapted from the Wikipedia article Extrasolar X-ray source astrometry, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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