NGC 488 is a face-on spiral galaxy in the constellation Pisces. It is at a distance of about 90 million light-years away from Earth, its diameter is estimated to be 52,6 Kpc. The galaxy has a large central bulge, is considered a prototype galaxy with multiple spiral arms, its arms are wound. Star forming activity has been traced within the arms; the nucleus of NGC 488 has been found to be chemically decoupled, being twice as metal rich as the central bulge of the galaxy. NGC 488, with the exception of its smaller companions, that form NGC 488 group, is an isolated galaxy; the galaxy was discovered by William Herschel on 13 December 1784. Two supernovae have been observed in NGC 488, SN 2010eb Ia with peak magnitude 14.7, SN 1976G, with magnitude 15. NGC 488 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted or diffracted by some material in order to characterize that object. Polarimetry of thin films and surfaces is known as ellipsometry. Polarimetry is used in remote sensing applications, such as planetary science and weather radar. Polarimetry can be included in computational analysis of waves. For example, radars consider wave polarization in post-processing to improve the characterization of the targets. In this case, polarimetry can be used to estimate the fine texture of a material, help resolve the orientation of small structures in the target, when circularly-polarized antennas are used, resolve the number of bounces of the received signal. In 2003, a visible-near IR Spectropolarimetric Imager with an acousto-optic tunable filter was reported.
These hyperspectral and spectropolarimetric imager functioned in radiation regions spanning from ultraviolet to long-wave infrared. In AOTFs a piezoelectric transducer converts a radio frequency signal into an ultrasonic wave; this wave travels through a crystal attached to the transducer and upon entering an acoustic absorber is diffracted. The wavelength of the resulting light beams can be modified by altering the initial RF signal. VNIR and LWIR hyperspectral imaging perform better as hyperspectral imagers; this technology was developed at the U. S. Army Research Laboratory; the researchers reported visible near infrared system data which required an RF signal below 1 W power. The reported experimental data indicates that polarimetric signatures are unique to manmade items and are not found in natural objects; the researchers state that a dual system, collecting both hyperspectral and spectropolarimetric information, is an advantage in image production for target tracking. A polarimeter is the basic scientific instrument used to make these measurements, although this term is used to describe a polarimetry process performed by a computer, such as is done in polarimetric synthetic aperture radar.
Polarimetry can be used to measure various optical properties of a material, including linear birefringence, circular birefringence, linear dichroism, circular dichroism and scattering. To measure these various properties, there have been many designs of polarimeters, some archaic and some in current use; the most sensitive are based on interferometers, while more conventional polarimeters are based on arrangements of polarising filters, wave plates or other devices. Polarimetry is used in many areas of astronomy to study physical characteristics of sources including active galactic nuclei and blazars, exoplanets and dust in the interstellar medium, gamma-ray bursts, stellar rotation, stellar magnetic fields, debris disks, reflection in binary stars and the cosmic microwave background radiation. Astronomical polarimetry observations are carried out either as imaging polarimetry, where polarization is measured as a function of position in imaging data, or spectropolarimetry, where polarization is measured as a function of wavelength of light, or broad-band aperture polarimetry.
Optically active samples, such as solutions of chiral molecules exhibit circular birefringence. Circular birefringence causes rotation of the polarization of plane polarized light as it passes through the sample. In ordinary light, the vibrations occur in all planes perpendicular to the direction of propagation; when light passes through a Nicol prism its vibrations in all directions except the direction of axis of the prism are cut off. The light emerging from the prism is said to be plane polarised because its vibration is in one direction. If two Nicol prisms are placed with their polarization planes parallel to each other the light rays emerging out of the first prism will enter the second prism; as a result, no loss of light is observed. However, if the second prism is rotated by an angle of 90°, the light emerging from the first prism is stopped by the second prism and no light emerges; the first prism is called the polarizer and the second prism is called the analyser. A simple polarimeter to measure this rotation consists of a long tube with flat glass ends, into which the sample is placed.
At each end of the tube is other polarizer. Light is shone through the tube, the prism at the other end, attached to an eye-piece, is rotated to arrive at the region of complete brightness or that of half-dark, half-bright or that of complete darkness; the angle of rotation is read from a scale. The same phenomenon is observed after an angle of 180°; the specific rotation of the sample may be calculated. Temperature can affect the rotation of light. Λ T = 100 α / l ρ where: λT is the specific rotation. T is the temperature. Λ is the wavelength of light. Α is the angle of rotation. L is the length of the polarimeter tube. Ρ is the mass concentration of solution. Ellipsometry Polariscope - Gemstone Buzz instrument to measure optical properties. EU Project NanoCharM nanocharm.org
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
Spitzer Space Telescope
The Spitzer Space Telescope the Space Infrared Telescope Facility, is an infrared space telescope launched in 2003 and still operating as of 2019. The planned mission period was to be 2.5 years with a pre-launch expectation that the mission could extend to five or more years until the onboard liquid helium supply was exhausted. This occurred on 15 May 2009. Without liquid helium to cool the telescope to the low temperatures needed to operate, most of the instruments are no longer usable. However, the two shortest-wavelength modules of the IRAC camera are still operable with the same sensitivity as before the cryogen was exhausted, have continued to be used to the present in the Spitzer Warm Mission. All Spitzer data, from both the primary and warm phases, are archived at the Infrared Science Archive. In keeping with NASA tradition, the telescope was renamed after its successful demonstration of operation, on 18 December 2003. Unlike most telescopes that are named after famous deceased astronomers by a board of scientists, the new name for SIRTF was obtained from a contest open to the general public.
The contest led to the telescope being named in honor of astronomer Lyman Spitzer, who had promoted the concept of space telescopes in the 1940s. Spitzer wrote a 1946 report for RAND Corporation describing the advantages of an extraterrestrial observatory and how it could be realized with available or upcoming technology, he has been cited for his pioneering contributions to rocketry and astronomy, as well as "his vision and leadership in articulating the advantages and benefits to be realized from the Space Telescope Program."The US$720 million Spitzer was launched on 25 August 2003 at 05:35:39 UTC from Cape Canaveral SLC-17B aboard a Delta II 7920H rocket. It follows a heliocentric instead of geocentric orbit and drifting away from Earth's orbit at 0.1 astronomical units per year. The primary mirror is 85 centimeters in diameter, f/12, made of beryllium and was cooled to 5.5 K. The satellite contains three instruments that allow it to perform astronomical imaging and photometry from 3.6 to 160 micrometers, spectroscopy from 5.2 to 38 micrometers, spectrophotometry from 5 to 100 micrometers.
By the early 1970s, astronomers began to consider the possibility of placing an infrared telescope above the obscuring effects of Earth's atmosphere. In 1979, a report from the National Research Council of the National Academy of Sciences, A Strategy for Space Astronomy and Astrophysics for the 1980s, identified a Space Infrared Telescope Facility as "one of two major astrophysics facilities for Spacelab", a Shuttle-borne platform. Anticipating the major results from an upcoming Explorer satellite and from the Shuttle mission, the report favored the "study and development of... long-duration spaceflights of infrared telescopes cooled to cryogenic temperatures." The launch in January 1983 of the Infrared Astronomical Satellite, jointly developed by the United States, the Netherlands, the United Kingdom, to conduct the first infrared survey of the sky, whetted the appetites of scientists worldwide for follow-up space missions capitalizing on the rapid improvements in infrared detector technology.
Earlier infrared observations had been made by both ground-based observatories. Ground-based observatories have the drawback that at infrared wavelengths or frequencies, both the Earth's atmosphere and the telescope itself will radiate strongly. Additionally, the atmosphere is opaque at most infrared wavelengths; this necessitates lengthy exposure times and decreases the ability to detect faint objects. It could be compared to trying to observe the stars at noon. Previous space observatories were launched during the 1980s and 1990s and great advances in astronomical technology have been made since then. Most of the early concepts envisioned repeated flights aboard the NASA Space Shuttle; this approach was developed in an era when the Shuttle program was expected to support weekly flights of up to 30 days duration. A May 1983 NASA proposal described SIRTF as a Shuttle-attached mission, with an evolving scientific instrument payload. Several flights were anticipated with a probable transition into a more extended mode of operation in association with a future space platform or space station.
SIRTF would be a 1-meter class, cryogenically cooled, multi-user facility consisting of a telescope and associated focal plane instruments. It would be launched on the Space Shuttle and remain attached to the Shuttle as a Spacelab payload during astronomical observations, after which it would be returned to Earth for refurbishment prior to re-flight; the first flight was expected to occur about 1990, with the succeeding flights anticipated beginning one year later. However, the Spacelab-2 flight aboard STS-51-F showed that the Shuttle environment was poorly suited to an onboard infrared telescope due to contamination from the "dirty" vacuum associated with the orbiters. By September 1983 NASA was considering the "possibility of a long duration SIRTF mission". Spitzer is the only one of the Great Observatories not launched by the Space Shuttle, as was intended. However, after the 1986 Challenger disaster, the Centaur LH2–LOX upper stage, which would have been required to place it in its final orbit, was banned from Shuttle use.
The mission underwent a series of redesigns during the 1990s due to budget considerations. This resulted in a much smaller but still capable mission that could use the smaller Delta II expendable launch vehicle. One of the most important
Galaxy morphological classification
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and expanded by Gérard de Vaucouleurs and Allan Sandage; the Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubble's scheme divides galaxies into three broad classes based on their visual appearance: Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images, they are denoted by the letter "E", followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a flattened disk, with stars forming a spiral structure, a central concentration of stars known as the bulge, similar in appearance to an elliptical galaxy.
They are given the symbol "S". Half of all spirals are observed to have a bar-like structure, extending from the central bulge; these barred spirals are given the symbol "SB". Lenticular galaxies consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not forming stars in any significant quantity; these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left and the barred and unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”. To this day, the Hubble sequence is the most used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.
The de Vaucouleurs system for classifying galaxies is a used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies—based on the tightness of the spiral arms and the presence or absence of a bar—did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses are important structural components of spiral galaxies; the de Vaucouleurs system retains Hubble's basic division of galaxies into ellipticals, lenticulars and irregulars. To complement Hubble's scheme, de Vaucouleurs introduced a more elaborate classification system for spiral galaxies, based on three morphological characteristics: The different elements of the classification scheme are combined — in the order in which they are listed — to give the complete classification of a galaxy. For example, a weakly barred spiral galaxy with loosely wound arms and a ring is denoted SABc.
Visually, the de Vaucouleurs system can be represented as a three-dimensional version of Hubble's tuning fork, with stage on the x-axis, family on the y-axis, variety on the z-axis. De Vaucouleurs assigned numerical values to each class of galaxy in his scheme. Values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three'stages': compact ellipticals, normal ellipticals and late types. Lenticulars are subdivided into early and late types. Irregular galaxies can be of type magellanic irregulars or'compact'; the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan. Together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra; the Yerkes scheme uses the spectra of stars in the galaxy. Thus, for example, the Andromeda Galaxy is classified as kS5.
Morphological Catalogue of Galaxies Galaxy color–magnitude diagram Galaxy Zoo William Wilson Morgan Fritz Zwicky Galaxies and the Universe - an introduction to galaxy classification Near-Infrared Galaxy Morphology Atlas, T. H. Jarrett The Spitzer Infrared Nearby Galaxies Survey Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project Go to GalaxyZoo.org to try your hand at classifying galaxies as part of an Oxford University open community project
Suzaku was an X-ray astronomy satellite developed jointly by the Institute of Space and Aeronautical Science at JAXA and NASA's Goddard Space Flight Center to probe high energy X-ray sources, such as supernova explosions, black holes and galactic clusters. It was launched on 10 July 2005 aboard the M-V-6 rocket. After its successful launch, the satellite was renamed Suzaku after the mythical Vermilion bird of the South. Just weeks after launch, on 29 July 2005 the first of a series of cooling system malfunctions occurred; these caused the entire reservoir of liquid helium to boil off into space by 8 August 2005. This shut down the X-ray Spectrometer, the spacecraft's primary instrument; the two other instruments, the X-ray Imaging Spectrometer and the Hard X-ray Detector, were unaffected by the malfunction. As a result, another XRS was integrated into the Hitomi X-ray satellite, launched in 2016. On 26 August 2015, JAXA announced that communications with Suzaku had been intermittent since 1 June, that the resumption of scientific operations would be difficult to accomplish given the spacecraft's condition.
Mission operators decided to complete the mission imminently, as Suzaku had exceeded its design lifespan by eight years at this point. The mission came to an end on 2 September 2015, when JAXA commanded the radio transmitters on Suzaku to switch themselves off. Suzaku carried high spectroscopic resolution wide energy band instruments for detecting signals ranging from soft X-rays up to gamma-rays. High resolution spectroscopy and wide-band are essential factors to physically investigate high energy astronomical phenomena, such as black holes and supernovae. One such feature, the broad iron K line, may be key to more direct imaging of black holes. X-ray Telescope X-ray Spectrometer X-ray Imaging Spectrometer Hard X-ray Detector Uses Gadolinium Silicate crystal, Gd2SiO5 Uses Bismuth Germanate crystal, Bi4Ge3O12 Suzaku was a replacement for ASTRO-E, lost in a launch failure; the M-V-4 carrier rocket launched on 10 February 2000 at 01:30:00 UTC but experienced a failure 42 seconds failing to achieve orbit and crashing with its payload into the ocean.
Suzaku discovered "fossil" light from a supernova remnant. Special Issue: First Results from Suzaku. Publications of the Astronomical Society of Japan. Vol. 59, No. SP1. January 30, 2007. Retrieved 4 October 2010. X-ray Astronomy Satellite "Suzaku" JAXA/ISAS Suzaku mission overview JAXA/ISAS Suzaku Information for Researchers JAXA report presentation of failure analysis of XRS NASA Astronomy Picture of the Day: Launch of the Red Bird NASA ASTRO-EII mission description NASA/GSFC Suzaku Learning Center NASA/GSFC XRS-2 project page
Hubble Space Telescope
The Hubble Space Telescope is a space telescope, launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile and is well known as both a vital research tool and a public relations boon for astronomy; the HST is named after the astronomer Edwin Hubble and is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory and the Spitzer Space Telescope. With a 2.4-meter mirror, Hubble's four main instruments observe in the ultraviolet and near infrared regions of the electromagnetic spectrum. Hubble's orbit outside the distortion of Earth's atmosphere allows it to take high-resolution images, with lower background light than ground-based telescopes. Hubble has recorded some of the most detailed visible light images allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe.
The HST was built by the United States space agency NASA, with contributions from the European Space Agency. The Space Telescope Science Institute selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, the Challenger disaster; when launched in 1990, Hubble's main mirror was found to have been ground incorrectly, creating a spherical aberration, compromising the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the only telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired and replaced systems on the telescope, including all five of the main instruments; the fifth mission was canceled on safety grounds following the Columbia disaster.
However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, completed in 2009. The telescope is operating as of 2019, could last until 2030–2040. After numerous delays, its successor, the James Webb Space Telescope, is scheduled to be launched in March 2021. In 1923, Hermann Oberth—considered a father of modern rocketry, along with Robert H. Goddard and Konstantin Tsiolkovsky—published Die Rakete zu den Planetenräumen, which mentioned how a telescope could be propelled into Earth orbit by a rocket; the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzer's paper "Astronomical advantages of an extraterrestrial observatory". In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the angular resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as seeing.
At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for a telescope with a mirror 2.5 m in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are absorbed by the atmosphere. Spitzer devoted much of his career to pushing for the development of a space telescope. In 1962, a report by the US National Academy of Sciences recommended the development of a space telescope as part of the space program, in 1965 Spitzer was appointed as head of a committee given the task of defining scientific objectives for a large space telescope. Space-based astronomy had begun on a small scale following World War II, as scientists made use of developments that had taken place in rocket technology; the first ultraviolet spectrum of the Sun was obtained in 1946, the National Aeronautics and Space Administration launched the Orbiting Solar Observatory to obtain UV, X-ray, gamma-ray spectra in 1962.
An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, in 1966 NASA launched the first Orbiting Astronomical Observatory mission. OAO-1's battery failed after three days, it was followed by OAO-2, which carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year. The OSO and OAO missions demonstrated the important role space-based observations could play in astronomy, in 1968, NASA developed firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope, with a launch slated for 1979; these plans emphasized the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available.
The continuing success of the OAO program encouraged strong consensus within the astronomical community that the LST should be a major goal. In 1970, NASA established two committees, one to plan the engineering side of the space telescope project, the other to determine the scientific goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-bas