Misplaced Pages

#768231

49-741: The visual portion of the Cygnus Loop is known as the Veil Nebula, also called the Cirrus Nebula or the Filamentary Nebula. Several components have separate names and identifiers, including the "Western Veil" or "Witch's Broom", the "Eastern Veil", and Pickering's Triangle. NGC 6960, the Western Veil , is the western part of the remnant, also known as the "Witch's Broom", located at J2000 RA 20 45 58.1 Dec +30° 35′ 43″. As

98-469: A 21' square field maintained the spacecraft pointing stability to 0.5 arcseconds . Each mirror has a corresponding astigmatism -corrected, holographically -ruled diffraction grating , each one on a curved substrate so as to produce four 1.65 m (5 ft 5 in) Rowland circle spectrographs . The dispersed ultraviolet light is detected by two microchannel plate detector intensified double delay-line detectors, whose surfaces are curved to match

147-408: A circle is measured as 1 of right ascension, or 15 minutes of arc (also written as 15′); and ⁠ 1 / 86400 ⁠ of a circle contains 1 of right ascension, or 15 seconds of arc (also written as 15″). A full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86 400 , or 24 × 60 = 1 440 , or 24 . Because right ascensions are measured in hours (of rotation of

196-440: A cloud at the northern edge of the loop, to the east of the northern edge of Pickering's Triangle. NGC 6979 was reported by William Herschel , and while the coordinates he recorded for Veil objects were somewhat imprecise, his position for this one is tolerably close to the knot at J2000 RA 20 50 27.9 Dec +32° 01′ 33″. The identifier NGC 6979 is sometimes taken to refer to Pickering's Triangle, but

245-408: A compact stellar remnant have been largely concentrated here, as the hole may have been caused by the violent ejection of a neutron star. A detailed 2012 study of the blowout region identified a possible pulsar wind nebula, as well as a point-like source within it. Although at almost exactly the same position as a known Seyfert galaxy , the slight offset combined with a lack of a radio counterpart makes

294-656: A net change of   0h. The right ascension of Polaris is increasing quickly—in AD 2000 it was 2.5h, but when it gets closest to the north celestial pole in 2100 its right ascension will be 6h. The North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18 and 6 respectively. The currently used standard epoch is J2000.0 , which is January 1, 2000 at 12:00 TT . The prefix "J" indicates that it

343-651: A primary direction (a zero point) on an equator . Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries , which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently 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,

392-581: A wide-field, far-ultraviolet nebular spectrometer, tuned to OVI emission lines , was launched aboard a Nike-Black Brant from White Sands Missile Range to observe the Cygnus Loop, the first observed galactic OVI emission line source. The X-ray source Cygnus X-5 coincides with SNR G074.0-08.6 (the Cygnus Loop), located at J2000 RA 20 51.1 Dec +30° 41′, observed by Uhuru at 4U 2046+31. This source also has catalogue numbers 1E 2049.4+3050, 1H 2050+310, and 1M 2051+309, having been observed by

441-556: Is Explorer 77. The primary objective of FUSE was to use high-resolution spectroscopy at far ultraviolet wavelengths to study the origin and evolution of the lightest elements ( hydrogen and deuterium) created shortly after the Big Bang, and the forces and processes involved in the evolution of galaxies , stars and planetary systems . The spacecraft was launched as a joint NASA-Canada–France project. Only one previous mission, Copernicus (OAO-3), has given this far-ultraviolet region of

490-417: Is a Julian epoch . Prior to J2000.0, astronomers used the successive Besselian epochs B1875.0, B1900.0, and 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 , and the use of RA was limited to special cases. With

539-441: Is dispersed by four spherical, aberration-corrected holographic diffraction gratings, and recorded by two delay-line microchannel plate detectors . Two channels with silicon carbide (SiC) coatings cover the range 90.5–110 nm and two channels with lithium fluoride (LiF) coatings cover the range 100–119.5 nm. Actuators on the mirror mountings kept the focus to 90% encircled energy within 1.5". A Fine Error Sensor (FES) with

SECTION 10

#1732870121769

588-452: Is mostly unobservable by other telescopes. Its primary mission was to characterize universal deuterium in an effort to learn about the stellar processing times of deuterium left over from the Big Bang . FUSE resides in a low Earth orbit , approximately 760 km (470 mi) in altitude , with an inclination of 24.98° and a 99.80 minutes orbital period . Its Explorer program designation

637-434: Is partially intercepted by the supernova remnant. With an estimated (but uncertain) distance of about 1860 ly away, this star seemed to support the revised estimate of 1760 ly. More recent investigations of the Cygnus Loop's distance using Gaia parallax measurements of several stars seen toward the Cygnus Loop have led to more accurate distance estimates. One of these stars, a 9.6 magnitude B8 star (BD+31 4224) located near

686-453: Is the complement of right ascension with respect to 24 . It is important not to confuse sidereal hour angle with the astronomical concept of hour angle , which measures the angular distance of an object westward from the local meridian . The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the celestial poles , completing one cycle in about 26,000 years. This movement, known as precession , causes

735-482: The Attitude Control System , the solar panels , and communications-electronics and antennas . The observatory is approximately 7.6 m (25 ft) long with a baffle fully deployed. The FUSE science instrument consists of four co-aligned telescope mirrors (~39 × 35 cm (15 × 14 in) clear aperture) based on a Rowland circle design . The light from the four optical channels

784-660: The Earth's rotation . As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles . The first star catalog to use right ascension and declination was John Flamsteed 's Historia Coelestis Britannica (1712, 1725). Far Ultraviolet Spectroscopic Explorer Far Ultraviolet Spectroscopic Explorer ( FUSE , Explorer 77 , and MIDEX-0 ) represented

833-466: The Einstein Observatory , HEAO 1 , and OSO 7 , respectively. The Cygnus Loop is a strong source of soft X-rays. The center of the supernova shell as determined from X-ray data lies at J1950 RA 20 49 45 Dec +30° 53′. A characteristic thermal temperature averaged over the loop from X-ray spectral data is T x = 2.9 ± 1.5 x 10 K. An X-ray surface brightness map of the loop

882-406: The celestial equator ) then at Earth's equator they are directly overhead (at zenith ). Any angular unit could have been chosen for right ascension, but it is customarily measured in hours ( ), minutes ( ), and seconds ( ), with 24 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

931-481: The ( hour circle of the) point in question above the Earth. When paired with declination , these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system . An old term, right ascension ( Latin : ascensio recta ) refers to the ascension , or the point on the celestial equator that rises with any celestial object as seen from Earth 's equator , where

980-468: The Cygnus Loop progenitor star's mass at 12 to 15 Solar masses , a value that puts the expected remnant firmly within neutron star boundaries. However, despite many searches, no compact stellar remnant had been confidently identified since the identification of the supernova remnant. A noted anomaly is that in X-rays, the nebula appears perfectly spherical aside from a "blowout region" to the south. Searches for

1029-444: The Earth ), they can be used to time the positions of objects in the sky. For example, if a star with RA = 1 30 00 is at its meridian, then a star with RA = 20 00 00 will be on the/at its meridian (at its apparent highest point) 18.5 sidereal hours later. Sidereal hour angle, used in celestial navigation , is similar to right ascension but increases westward rather than eastward. Usually measured in degrees (°), it

SECTION 20

#1732870121769

1078-459: The Extended Mission. Less redundancy and less access to ongoing engineering support were consistent with NASA policy for missions in their extended phase, where a higher level of risk is allowed. The FUSE satellite consists of two primary sections, the spacecraft and the science instrument. The spacecraft contains all of the elements necessary for powering and pointing the satellite, including

1127-537: The FUSE project entered an extended phase of operations which lasted until mid-2007. With funding from NASA, the FUSE satellite continued to be operated as an observatory for the broad astronomical community, with 100% of on-orbit observing time selected by NASA peer review. Some 29 million seconds of science data were obtained during the Primary Mission phase, and a total of nearly 65 million seconds of data were archived from

1176-600: The Triangle is probably not what Herschel saw or what the Catalogue intended for this entry: it was discovered only photographically, after the Catalogue was published, and long after Herschel's observation. NGC 6974 was reported by Lord Rosse , but the position he gave lies in an empty region inside the main loop. It was assumed that he recorded the position incorrectly, and the New General Catalogue gives Rosse's object as

1225-575: The actual size of the bubble was about 40% smaller than the conventional value, leading to a distance of about 1470 ly. A larger revised value of 540 pc (1760 ly) appeared to be corroborated by Blair's later discovery, via the Far Ultraviolet Spectroscopic Explorer (FUSE), of a star seemingly behind the Veil. A UV spectrum of this star, KPD 2055+3111 of spectral type sdOB, showed absorption lines in its spectrum indicate that its light

1274-427: 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

1323-453: The coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates (including right ascension) are inherently relative to the year of their observation, and 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" on

1372-467: The curvature of the focal plane. The FUSE observatory was designed for an operational lifetime of three years, although it was hoped that it might remain operational for as long as ten years. NASA since recommended an additional two-year extension beyond the prime mission. FUSE was controlled through a primary ground station antenna located at the University of Puerto Rico at Mayagüez . FUSE brought it over

1421-439: The electromagnetic spectrum. However, FUSE will provide sensitivity some ten thousand times greater than Copernicus. The first 3.5 years of FUSE operations were dubbed the Primary Mission. During this period, the observing time on FUSE was shared roughly 50–50 between the FUSE science team and a host of Guest investigators, astronomers from around the world selected by NASA to participate in the FUSE program. Starting 1 April 2003,

1470-484: The entire frequency range 25 to 5000 MHz. The southeastern knot is located at J2000 RA 20 56 21.2 Dec +30° 23′ 59″ on the southeastern rim of the Cygnus Loop. The knot has been identified as an encounter between the blast wave from the supernova and a small isolated cloud. The knot is a prominent X-ray feature, consisting of a number of filaments correlated with visual line emission. By combining visual and X-ray data, it can be shown that

1519-534: The entire mission. The Extended Mission period presented a number of challenges, especially for satellite operations. Many procedures had to be automated, allowing the project to cut back on staffing and minimize operations costs. As one example, the Satellite Control Center was staffed around the clock during the Prime Mission but transitioned to a 16-hour per day, Monday through Friday staffing profile in

Cygnus Loop - Misplaced Pages Continue

1568-468: The equator increases by about 3.1 seconds per year or 5.1 minutes per century, but for fixed stars away from the equator the rate of change can be anything from negative infinity to positive infinity. (To this must be added the proper motion of a star.) Over a precession cycle of 26,000 years, "fixed stars" that are far from the ecliptic poles increase in right ascension by 24h, or about 5.6' per century, whereas stars within 23.5° of an ecliptic pole undergo

1617-422: The ground station for less than 10 minutes at a time (on average) for about six or seven orbits in a row, followed by roughly seven or eight orbits without contact. Hence, the satellite had to operate on its own most of the time, moving from target to target, identifying star fields, centering objects in the spectrograph apertures, and performing the observations. The scientific data, which was stored in digital form,

1666-406: 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 24 of right ascension or 360° ( degrees of arc ), ⁠ 1 / 24 ⁠ of a circle is measured as 1 of right ascension, or 15°; ⁠ 1 / 1440 ⁠ of

1715-437: 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 that is 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 often is used with an equatorial mount to cancel out

1764-489: The measurement increasing towards the east. As seen from Earth (except at the poles), objects noted to have 12 RA are longest visible (appear throughout the night) at the March equinox; those with 0 RA (apart from the sun) do so at the September equinox. On those dates at midnight, such objects will reach ("culminate" at) their highest point (their meridian). How high depends on their declination; if 0° declination (i.e. on

1813-569: The next generation, high-orbit, ultraviolet space observatory covering the wavelength range of 90.5–119.5 nanometre (nm) of the NASA operated by the Johns Hopkins University Applied Physics Laboratory . FUSE was launched on a Delta II launch vehicle on 24 June 1999, at 15:44:00 UTC , as a part of NASA's Origins Program . FUSE detected light in the far ultraviolet portion of the electromagnetic spectrum , which

1862-437: The other knot in the northern cloud, located at J2000 RA 20 51 04.3 Dec +31° 49′ 41″, one degree north of Rosse's position. (This position is farther east than NGC 6979, even though NGC objects are generally ordered by increasing RA.) These filaments in the north-central area are sometimes known as the "carrot". The spectrum at 34.5 MHz of the region associated with NGC 6974 ranges straight over

1911-411: The point-like source probably unrelated to the galaxy. Whether the feature is a pulsar wind nebula, and if so whether it is related to the Cygnus Loop, is still unknown for certain. If it is indeed the compact stellar remnant of the supernova, the neutron star would have to have been ejected from the center of the nebula at a speed of roughly 1,850  km/s , depending on the precise age and distance of

1960-507: The remnant's northwestern rim shows evidence of interactions of its stellar wind with the Cygnus Loop's shock wave, thereby indicating it is located actually inside the remnant. This star's Gaia estimated distance of around 730 pc, along with two other stars both at about 740 pc which exhibit spectral features indicating they must lie behind the remnant, leads to new distance of 725 ± {\displaystyle \pm } 15 pc or around 2400 light-years. The Gaia estimated distance to

2009-481: The remnant's optical filaments to calculate a distance of 770 parsecs or 2500 light-years . However, in 1999, William Blair, assuming that the shock wave should be expanding at the same rate in all directions, compared the angular expansion along the sides of the bubble (visible in Hubble Space Telescope images) with direct line-of-sight measurements of the radial expansion towards the Earth and concluded that

Cygnus Loop - Misplaced Pages Continue

2058-445: The remnant. In the novel Mindbridge by Joe Haldeman , the Cygnus Loop is the remains of the home star of an omnipotent, immortal race that ultimately decided to destroy itself. Right ascension Right ascension (abbreviated RA ; symbol α ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to

2107-399: The satellite was incapable thereafter of the fine pointing control required to acquire useful science data, and the mission was terminated. Over 400 scientific papers have been written using data from FUSE, with subjects ranging from cool stars to the intergalactic medium . One of the primary science goals of FUSE was to study the abundance of deuterium, an isotope of hydrogen . Because of

2156-517: The sdOB star KPD 2055+3111 is 819 pc (2,670 ly). This new distance, surprisingly close to the value estimated some 60 years ago by Minkowski, means the Cygnus Loop is physically some 37 pc (120 ly) in diameter and has an age of around 20,000 years. The brightest far-ultraviolet sources of the Cygnus Loop occur in the north-east edge of the remnant. The first flight of the High Resolution Emission Line Spectrometer (HIRELS),

2205-449: The southeastern knot is an indentation on the surface of the blast wave, not a small cloud but the tip of a larger cloud. The presence of a reverse shock is evidence that the knot represents an early stage of a blast wave encountering a large cloud. Until 1999, the most often-quoted distance to the supernova remnant was a 1958 estimate made by R. Minkowski , combining his radial velocity measurements with E. Hubble 's proper motion study of

2254-729: The westernmost NGC object in the nebula (first in right ascension ), its number is sometimes used as an NGC identifier for the nebula as a whole. These three luminous areas make up the Eastern Veil . NGC 6992 is an HI shell located along the north-eastern edge of the loop at J2000 RA 20 56 19.0 Dec +31° 44′ 34″. NGC 6995 is located farther south at J2000 RA 20 57 10.7 Dec +31° 14′ 07″, and IC 1340 even farther south at J2000 RA 20 56 12.0 Dec +31° 04′ 00″. Also known as Pickering's Wedge, or Pickering's Triangular Wisp, this segment of relatively faint nebulosity

2303-483: Was discovered photographically in 1904 by Williamina Fleming at Harvard Observatory, where Edward Charles Pickering was director at the time. The Triangle is brightest along the northern side of the loop, though photographs show the nebulosity extending into the central area as well. These two objects are generally identified today (as by the NGC/IC Project and Uranometria ) with two brighter knots of nebulosity in

2352-473: Was obtained with a one-dimensional X-ray telescope flown aboard an Aerobee 170 sounding rocket launched on March 30, 1973, from the White Sands Missile Range . Most stars that produce supernovae leave behind compact stellar remnants - a neutron star or black hole , typically depending on the mass of the original star. Various techniques based on the features of the supernova remnant estimate

2401-411: Was radioed to the ground during contact with the ground station. FUSE Science Operations were on hold from 10 December 2001, to approximately the end of January 2002, due to a problem with the pointing system. The last operational reaction wheel on FUSE stopped on 12 July 2007. Attempts to restart any of the reaction wheels were unsuccessful. Although the telescope itself remained in excellent condition,

#768231