Radiation astronomy/Astrometry
"Gaia [NSSDC/COSPAR ID: 2013-074A] is a European Space Agency astronomy mission whose primary goals are to: (1) measure the positions and velocity of approximately one billion stars; (2) determine the brightness, temperature, composition, and motion through space of those stars; and, (3) create a three-dimensional map of the Milky Way galaxy."[1]
This is an exploded diagram of the European Space Agency's Gaia spacecraft for astrometry. Credit: ESA.
This image of the Milky Way galaxy has depicted onto it the various targets and experimental regions for the ESA Gaia spacecraft. Credit: ESA.
An image from an animation has the Gaia spacecraft spinning slowly (four revolutions per day) to sweep its two telescopes across the entire celestial sphere. Credit: ESA - C. Carreau.
At left is an image of the Milky Way galaxy depicted onto it the various targets and experimental regions for the ESA Gaia spacecraft.
"Repeatedly scanning the sky, Gaia will observe each of the billion stars an average of 70 times each over the five years. It will measure the position and key physical properties of each star, including its brightness, temperature and chemical composition."[2]
At lower left is an image from an animation that has the Gaia spacecraft spinning slowly (four revolutions per day) to sweep its two telescopes across the entire celestial sphere.
The Hipparcos satellite is being tested in the Large Solar Simulator at ESTEC. Credit: Michael Perryman.
Hipparcos is the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects.
These measurements allow the accurate determination of proper motions and parallaxes of stars, their distance and tangential velocity.
Hubble Space Telescope
Hubble Space Telescope - Spatial scanning precisely measures distances up to 10,000 light-years away (10 April 2014). Credit: NASA/ESA, A.Feild/STScI.
"Using NASA's Hubble Space Telescope, astronomers now can precisely measure the distance of stars up to 10,000 light-years away -- 10 times farther than previously possible."[3]
"Astronomers have developed yet another novel way to use the 24-year-old space telescope by employing a technique called spatial scanning, which dramatically improves Hubble's accuracy for making angular measurements. The technique, when applied to the age-old method for gauging distances called astronomical parallax, extends Hubble's tape measure 10 times farther into space."[3]
"By applying a technique [illustrated in the image at the right] called spatial scanning to an age-old method for gauging distances called astronomical parallax, scientists now can use NASA’s Hubble Space Telescope to make precision distance measurements 10 times farther into our galaxy than previously possible."[3]
"This new capability is expected to yield new insight into the nature of dark energy, a mysterious component of space that is pushing the universe apart at an ever-faster rate."[4]
"Parallax, a trigonometric technique, is the most reliable method for making astronomical distance measurements, and a practice long employed by land surveyors here on Earth. The diameter of Earth's orbit is the base of a triangle and the star is the apex where the triangle's sides meet. The lengths of the sides are calculated by accurately measuring the three angles of the resulting triangle."[3]
"Astronomical parallax works reliably well for stars within a few hundred light-years of Earth. For example, measurements of the distance to Alpha Centauri, the star system closest to our sun, vary only by one arc second. This variance in distance is equal to the apparent width of a dime seen from two miles away."[3]
"Stars farther out have much smaller angles of apparent back-and-forth motion that are extremely difficult to measure. Astronomers have pushed to extend the parallax yardstick ever deeper into our galaxy by measuring smaller angles more accurately."[3]
"This new long-range precision was proven when scientists successfully used Hubble to measure the distance of a special class of bright stars called Cepheid variables, approximately 7,500 light-years away in the northern constellation Auriga. The technique worked so well, they are now using Hubble to measure the distances of other far-flung Cepheids."[3]
"Such measurements will be used to provide firmer footing for the so-called cosmic "distance ladder." This ladder's "bottom rung" is built on measurements to Cepheid variable stars that, because of their known brightness, have been used for more than a century to gauge the size of the observable universe. They are the first step in calibrating far more distant extra-galactic milepost markers such as Type Ia supernovae."[3]
"To make a distance measurement, two exposures of the target Cepheid star were taken six months apart, when Earth was on opposite sides of the sun. A very subtle shift in the star's position was measured to an accuracy of 1/1,000 the width of a single image pixel in Hubble's Wide Field Camera 3, which has 16.8 megapixels total. A third exposure was taken after another six months to allow for the team to subtract the effects of the subtle space motion of stars, with additional exposures used to remove other sources of error."[3]
Nano-JASMINE is the Nano-Japan Astrometry Satellite Mission for INfrared Exploration.
"Nano-JASMINE is a 50cm class micro satellite that has space astrometry mission for the first time in Japan. Making a map of many stars, Nano-JASMINE will take us a knowledge of our Galaxy, and techniques of observation. Intelligent Space Systems Laboratory (the University of Tokyo) that took two CubeSats and one Micro-Satellite into orbit covers bus system, and National Astronomical Observatory of Japan (NAOJ) that plans more precise missions by larger satellites covers mission telescope."[5]
The diagram describes Stellar parallax motion.
Distance measurement by parallax is a special case of the principle of triangulation, which states that one can solve for all the sides and angles in a network of triangles if, in addition to all the angles in the network, the length of at least one side has been measured. Thus, the careful measurement of the length of one baseline can fix the scale of an entire triangulation network. In parallax, the triangle is extremely long and narrow, and by measuring both its shortest side (the motion of the observer) and the small top angle (always less than 1 arcsecond,[6] leaving the other two close to 90 degrees), the length of the long sides (in practice considered to be equal) can be determined.
Assuming the angle is small (see derivation below), the distance to an object (measured in parsecs) is the reciprocal of the parallax (measured in arcseconds):
For example, the distance to Proxima Centauri is 1/0.7687=1.3009 parsecs (4.243 ly).[7]
Cosmic rays
Main article: Radiation/Cosmic rays
"Evidences of non-thermal X-ray emission and TeV gamma-rays from the supernova remnants (SNRs) has strengthened the hypothesis that primary Galactic cosmic-ray electrons are accelerated in SNRs."[8]
"So far, the canonical distance to the Vela SNR has been taken to be 500pc, a value which was derived from the analysis of its angular diameter in comparison with the Cygnus Loop and IC443 (Milne 1968), and pulsar dispersion determination (Taylor & Cordes 1993). However, recent parallax measurements clearly indicate that the distance of 500pc is too large. Cha et al. (1999) obtained high resolution Ca-II absorption line toward 68 OB stars in the direction of the Vela SNR. The distances to these stars were determined by trigonometric parallax measurements with the Hipparcos satellite and spectroscopic parallaxes based upon photometric colors and spectral types. The distance to the Vela SNR is constrained to be 250 ± 30pc due to the presence of the Doppler spread Ca-II absorption line attributable to the remnant along some lines of sight. Caraveo et al. (2001) also applied high-resolution astrometry to the Vela pulsar (PSR B0833-45) V ∼ 23.6 optical counterpart. Using Hubble Space Telescope observations, they obtained the first optical measurement of the annual parallax of the Vela pulsar, yielding a distance of 294+76 −50 pc. Therefore, we calculate the electron flux adopting a distance of 300 pc to the Vela SNR."[8]
Main article: Radiation astronomy/X-rays
Def. a theory of the science of the biological, chemical, physical, and logical laws (or principles) with respect to any natural X-ray source in the sky especially at night is called theoretical X-ray astronomy.
An individual science such as physics (astrophysics) is theoretical X-ray astrophysics.
"Theoretical X-ray astronomy is a branch of theoretical astronomy that deals with the theoretical astrophysics and theoretical astrochemistry of X-ray generation, emission, and detection as applied to astronomical objects."[9]
"Like theoretical astrophysics, theoretical X-ray astronomy uses a wide variety of tools which include analytical models to approximate the behavior of a possible X-ray source and computational numerical simulations to approximate the observational data. Once potential observational consequences are available they can be compared with experimental observations. Observers can look for data that refutes a model or helps in choosing between several alternate or conflicting models."[9]
"Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model."[9]
"Most of the topics in astrophysics, astrochemistry, astrometry, and other fields that are branches of astronomy studied by theoreticians involve X-rays and X-ray sources. Many of the beginnings for a theory can be found in an Earth-based laboratory where an X-ray source is built and studied."[9]
"From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed. For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism.[10] During more than a decade of observations of X-ray emission from the Sun, evidence of the existence of an isotropic X-ray background flux was obtained in 1956.[11] In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux.[10] The plasma could be a coronal cloud of a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.[10]"[9]
"In the Crab Nebula X-ray spectrum there are three features that differ greatly from Scorpius X-1: its spectrum is much harder, its source diameter is in light-years (ly)s, not astronomical units (AU), and its radio and optical synchrotron emission are strong.[10] Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma. However, the Crab Nebula appears as an X-ray source that is a central freely expanding ball of dilute plasma, where the energy content is 100 times the total energy content of the large visible and radio portion, obtained from the unknown source.[10]"[9]
The "Dividing Line" as giant stars evolve to become red giants also coincides with the Wind and Coronal Dividing Lines.[12] To explain the drop in X-ray emission across these dividing lines, a number of models have been proposed:
  1. low transition region densities, leading to low emission in coronae,
  2. high-density wind extinction of coronal emission,
  3. only cool coronal loops become stable,
  4. changes in a magnetic field structure to that of an open topology, leading to a decrease of magnetically confined plasma, or
  5. changes in the magnetic dynamo character, leading to the disappearance of stellar fields leaving only small-scale, turbulence-generated fields among red giants.[12]
Main article: Radiation astronomy/Radars
This image shows the early planetary radar at Pluton, USSR, 1960. Credit: Rumlin.
"Asteroid radar astronomy began on 14 June 1968, with the detection of 1566 Icarus from Goldstone (Goldstein 1969) and Haystack (Pettengill et al. 1969)."[13]
"Radar measurements of echo Doppler frequencies and time delays permit significant refinements of orbital elements and commensurate improvements in the accuracy of prediction ephemerides because these measurements have fine fractional precision and are orthogonal to optical, angular-position measurements."[13]
"Yeomans et al. (1987) used numerical experiments to explore the extent to which delay/ Doppler astrometry can refine orbit estimates for NEAs. They concluded that radar measurements can reduce ephemeris uncertainties dramatically for asteroids having short optical-data histories. They noted that a few radar observations of a newly discovered NEA could mean the difference between successfully recovering the object during its next close approach and losing it entirely. Even for asteroids with very long astrometric histories and secure orbits, radar measurements can significantly shrink their positional error ellipsoids for at least a decade."[13]
"A typical transmit/receive cycle, or run, consists of signal transmission for a duration close to the roundtrip light time between the radar and the target, i.e., until the first echoes are about to come back, followed by reception of echoes for a similar duration. In continuous wave (cw) observations, one transmits a nearly monochromatic waveform and measures the distribution of echo power as a function of frequency. The resultant echo spectra can be thought of as one-dimensional images, or brightness scans across the target through a slit parallel to the asteroid’s apparent spin vector. In ranging observations, time coding of the waveform permits measurement of the distribution of echo power in time delay (range) as well."[13]
"An asteroid’s apparent radial motion introduces a continuously changing Doppler shift into the echoes. One avoids spectral smear by tuning the receiver according to an ephemeris based on an orbit determined from astrometric asteroid observations."[13]
"In cw experiments, voltage samples of the received signal are Fourier transformed and the results are squared to obtain an estimate of the power spectrum, with the frequency resolution equal to the reciprocal of the time series length, i.e., of the coherence time. The sampling rate is chosen to provide an unaliased bandwidth many times larger than both the a priori Doppler uncertainty and the echo bandwidth, so fest can be determined unambiguously from the received power spectrum. Normally, a number of these "single-look" spectra are averaged to improve the spectral estimates."[13]
"In principle, range resolution can be obtained by using a coherent pulsed cw waveform—the transmitter’s carrier-frequency oscillator operates continuously but radio-frequency power is radiated only during intervals that are one delay resolution cell long and occur at intervals called the pulse repetition period (PRP). The PRP is normally much greater than the target’s intrinsic delay dispersion, thereby ensuring that the echo will consist of successive, nonoverlapping range profiles. Fourier transformation of N time samples taken at the same position (i.e., the same delay relative to τ0) within each of N successive range profiles yields the echo power spectrum for the corresponding range cell on the target. This spectrum has an unaliased bandwidth B - l/[(PRP)(NCOH)] and a frequency resolution B/N, where NCOH is the number of code cycles for which voltage samples have been coherently summed prior to Fourier transformation."[13]
The maximum range of astronomy by radar is very limited, and is confined to the solar system. This is because the signal strength drops off very steeply with distance to the target, the small fraction of incident flux that is reflected by the target, and the limited strength of transmitters.[14] It is also necessary to have a relatively good ephemeris of the target before observing it.
At right is an image of the Pluton radar complex used for radar astronomy since 1960.
Def. the point to which the Sun appears to be moving with respect to the local stars is called the solar apex.
An antapex is a point that an astronomical object's total motion is directed away from. It is opposite to the apex.
The local standard of rest or LSR follows the mean motion of material in the Milky Way in the neighborhood of the Sun.[15] The path of this material is not precisely circular.[16] The Sun follows the solar circle (eccentricity e < 0.1 ) at a speed of about 220 km/s in a clockwise direction when viewed from the galactic north pole at a radius of ≈ 8 kpc about the center of the galaxy near Sgr A*, and has only a slight motion, towards the solar apex, relative to the LSR.[17] The Sun's peculiar motion relative to the LSR is 13.4 km/s.[18][19] The LSR velocity is anywhere from 202–241 km/s.[20]
Main article: Rocks/Rocky objects/Asteroids
This is a Goldstone radar image of asteroid 4179 Toustatis. Credit: Steve Ostro, JPL.
The image at the top of the page is of asteroid 2012 LZ1.
"On Sunday, June 10, a potentially hazardous asteroid thought to have been 500 meters (0.31 miles) wide was discovered by Siding Spring Observatory in New South Wales, Australia. Fortunately for us, asteroid 2012 LZ1 drifted safely by, coming within 14 lunar distances from Earth on Thursday, June 14."[21]
"Asteroid 2012 LZ1 is actually bigger than thought… in fact, it is quite a lot bigger. 2012 LZ1 is one kilometer wide (0.62 miles), double the initial estimate."[21]
Asteroid "2012 LZ1′s surface is really dark, reflecting only 2-4 percent of the light that hits it — this contributed to the underestimated initial optical observations. Looking for an asteroid the shade of charcoal isn’t easy."[21]
“This object turned out to be quite a bit bigger than we expected, which shows how important radar observations can be, because we’re still learning a lot about the population of asteroids”.[22]
“The sensitivity of our radar has permitted us to measure this asteroid’s properties and determine that it will not impact the Earth at least in the next 750 years”.[23]
The extremely accurate astrometry provided by radar is critical in long-term predictions of asteroid-Earth impacts, as illustrated by the object 99942 Apophis.
At right is a Goldstone radar image of the asteroid 4179 Toutatis on November 26, 1996.
The "images were recorded at NASA's Deep Space Network 70-meter and 34-meter radio/radar antennas in Goldstone, CA, and the 305-meter Arecibo Radio Telescope in Puerto Rico."[24]
"It's amazing that the shape of Toutatis can be determined so accurately from ground-based observations".[25]
"This technology will provide us with startling, close-up views of thousands of asteroids that orbit near the Earth."[25]
"We used the computer to mathematically create a three- dimensional model of the surface and rotation of Toutatis".[26]
"It's as though we put a clay model in space and molded it until it matched the appearance of the actual asteroid."[26]
"The video is of particular interest as Toutatis nears Earth and makes its closest approach on Friday, Nov. 29, when it will pass by at a distance of 3.3 million miles (5.3 million kilometers), or about 14 times the distance from the Earth to the Moon. In 2004, Toutatis will pass only four lunar distances from Earth, closer than any known Earth- approaching object expected to pass by in the next 60 years."[24]
"Toutatis poses no significant threat to Earth, at least for a few hundred years".[27]
"The discovery that we live in an asteroid swarm is important for the future of humanity".[27]
"These leftover debris from planetary formation can teach us a good deal about the formation of our Solar System. Asteroids also contain valuable minerals and many are the cheapest possible destinations for space missions."[27]
"[V]alues for the masses of Saturn and its major satellites, the zonal harmonics in the spherical harmonic expansion of Saturn's gravitational potential, and the orientation of the pole of Saturn [are] determined [...] using an extensive data set: satellite astrometry from Earth-based observatories and the Hubble Space Telescope; Earth-based, Voyager 1, and Voyager 2 ring occultation measurements; Doppler tracking data from Pioneer 11; and Doppler tracking, radiometric range, and imaging data from Voyager 1, Voyager 2, and Cassini."[28]
Neptune trojans
Neptune's L4 trojans with plutinos for reference. Credit: Eurocommuter.
  Neptune trojans (selection)
  · 2001 QR322
  · 2005 TN53
  · 2007 VL305
  · Pluto
  · Orcus
  · Ixion
Neptune trojans are bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, have approximately the same orbital period as Neptune and follow roughly the same orbital path. 22 Neptune trojans are currently known, of which 19 orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune[29] and three orbit near Neptune's L5 region 60° behind Neptune.[29]
The discovery of 2005 TN53 in a high-inclination (>25°) orbit was significant, because it suggested a "thick" cloud of trojans[30] (Jupiter trojans have inclinations up to 40°[31]), which is indicative of freeze-in capture instead of in situ or collisional formation.[30] It is suspected that large (radius ≈ 100 km) Neptune trojans could outnumber Jupiter trojans by an order of magnitude.[32][33]
In 2010, the discovery of the first known L5 Neptune trojan, 2008 LC32218}}, was announced.[34] Neptune's trailing L5 region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars.
It would have been possible for the New Horizons spacecraft to investigate 2011 HM102, the only L5 Neptune trojan discovered by 2014 detectable by New Horizons, when it passed through this region of space en route to Pluto.[33] However, New Horizons may not have had sufficient downlink bandwidth, so it was decided to give precedence to the preparations for the Pluto flyby.[35][36]
In 2001, the first Neptune trojan was discovered, 2001 QR322, near Neptune's L4 region, and with it the fifth (After the asteroid belt, the Jupiter trojans, the trans-Neptunian objects and the Mars trojans.) known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN53 has indicated that the Neptune trojans populate thick clouds, which has constrained their possible origins.
On August 12, 2010, the first L5 trojan, 2008 LC18, was announced.[34] It was discovered by a dedicated survey that scanned regions where the light from the stars near the Galactic Center is obscured by dust clouds.[37] This suggests that large {{L5 trojans are as common as large L4 trojans, to within uncertainty,[37] further constraining models about their origins.
It would have been possible for the New Horizons spacecraft to investigate L5 Neptune trojans discovered by 2014, when it passed through this region of space en route to Pluto.[33] Some of the patches where the light from the Galactic Center is obscured by dust clouds are along New Horizons's flight path, allowing detection of objects that the spacecraft could image.[37] 2011 HM102, the highest-inclination Neptune trojan known, was just bright enough for New Horizons to observe it in end-2013 at a distance of 1.2 AU.[38] However, New Horizons may not have had sufficient downlink bandwidth, so it was eventually decided to give precedence to the preparations for the Pluto flyby.[35][36]
An animation showing the path of six of Neptune's L4 trojans in a rotating frame with a period equal to Neptune's orbital period. Neptune is held stationary. (Click to view.) Credit: frankuitaalst from the Gravity Simulator.
The orbits of Neptune trojans are highly stable; Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System.[30] Neptune's L5 can host stable trojans equally well as its L4.[39] Neptune trojans can librate up to 30° from their associated Lagrangian points with a 10,000-year period.[37] Neptune trojans that escape enter orbits similar to centaurs.[39] Although Neptune cannot currently capture stable trojans,[30] roughly 2.8% of the centaurs within 34 AU are predicted to be Neptune co-orbitals. Of these, 54% would be in horseshoe orbits, 10% would be quasi-satellites, and 36% would be trojans (evenly split between the L4 and L5 groups).[40]
The unexpected high-inclination trojans are the key to understanding the origin and evolution of the population as a whole.[39] The existence of high-inclination Neptune trojans points to a capture during planetary migration instead of in situ or collisional formation.[30][37] The estimated equal number of large L5 and L4 trojans indicates that there was no gas drag during capture and points to a common capture mechanism for both L4 and L5 trojans.[37] The capture of Neptune trojans during a migration of the planets occurs via process similar to the chaotic capture of Jupiter trojans in the Nice model. When Uranus and Neptune are near but not in a mean-motion resonance the locations where Uranus passes Neptune can circulate with a period that is in resonance with the libration periods of Neptune trojans. This results in repeated perturbations that increase the libration of existing trojans causing their orbits to become unstable.[41] This process is reversible allowing new trojans to be captured when the planetary migration continues.[42] For high-inclination trojans to be captured the migration must have been slow,[43] or their inclinations must have been acquired previously.[44]
The first four discovered Neptune trojans have similar colors.[30] They are modestly red, slightly redder than the gray Kuiper belt objects, but not as extremely red as the high-perihelion cold classical Kuiper belt objects.[30] This is similar to the colors of the blue lobe of the centaur color distribution, the Jupiter trojans, the irregular satellites of the gas giants, and possibly the comets, which is consistent with a similar origin of these populations of small Solar System bodies.[30]
The Neptune trojans are too faint to efficiently observe spectroscopically with current technology, which means that a large variety of surface compositions are compatible with the observed colors.[30]
In 2015, the International Astronomical Union (IAU) adopted a new naming scheme for Neptune trojans, which are to be named after Amazons, with no differentiation between objects in L4 and L5.[45] The Amazons were an all-female warrior tribe that fought in the Trojan War on the side of the Trojans against the Greeks. As of 2019, the named Neptune trojans are 385571 Otrera (after Otrera, the first Amazonian queen in Greek mythology) and Clete (an Amazon and the attendant to the Amazons queen Penthesilea, who led the Amazons in the Trojan war).[46][47]
The amount of high-inclination objects in such a small sample, in which relatively fewer high-inclination Neptune trojans are known due to observational biases,[30] implies that high-inclination trojans may significantly outnumber low-inclination trojans.[39] The ratio of high- to low-inclination Neptune trojans is estimated to be about 4:1.[30] Assuming albedos of 0.05, there are an expected 400++250
200 Neptune trojans with radii above 40 km in Neptune's L4.[30] This would indicate that large Neptune trojans are 5 to 20 times more abundant than Jupiter trojans, depending on their albedos.[30] There may be relatively fewer smaller Neptune trojans, which could be because these fragment more readily.[30] Large L5 trojans are estimated to be as common as large L4 trojans.[37]
2001 QR322 and 2008 LC18 display significant dynamical instability.[39] This means they could have been captured after planetary migration, but may as well be a long-term member that happens not to be perfectly dynamically stable.[39]
As of October 2018, 22 Neptune trojans are known, of which 19 orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune,[29] three orbit near Neptune's L5 region 60° behind Neptune, and one orbits on the opposite side of Neptune (L3) but frequently changes location relative to Neptune to L4 and L5.[29] These are listed in the following table. It is constructed from the list of Neptune trojans maintained by the International Astronomical Union (IAU) Minor Planet Center[29] and with diameters from Sheppard and Trujillo's paper on 2008 LC18,[37] unless otherwise noted.
Astronomical naming conventions (Name)Provisional designation in astronomy (Prov.)
Lagrangian point (Lagrangian)
Perihelion (q) (AU)Aphelion (Q) (AU)Inclination (i) (°)Absolute magnitude (Abs. mag)Diameter
Year of
NotesMinor Planet Center (MPC)
2001 QR322L429.40431.0111.38.2~1402001First Neptune trojan discovered2001+QR322
2004 KV18L524.55335.85113.68.956[48]2011Temporary Neptune trojan2004+KV18
385571 Otrera2004 UP10L429.31830.9421.48.8~1002004First Neptune trojan numbered and named385571
2005 TN53L428.09232.16225.09.0~802005First high-inclination trojan discovered[30]2005+TN53
385695 Clete2005 TO74L428.46931.7715.38.5~1002005385695
2006 RJ103L429.07731.0148.27.5~18020062006+RJ103
(527604) 2007 VL305L428.13032.02828.18.0~16020072007+VL305
2008 LC18L527.36532.47927.68.4~1002008First L5 trojan discovered[37]2008+LC18
316179 2010 EN65L321.10940.61319.26.9~200Jumping trojan316179
2010 TS191L428.60831.2536.68.1~1202016Announced on 2016/05/312010+TS191
2010 TT191L427.91332.1894.38.0~1302016Announced on 2016/05/312010+TT191
2011 HM102L527.66232.45529.48.190–180[38]20122011+HM102
(530664) 2011 SO277L429.62230.5039.67.7~1402016Announced on 2016/05/312011+SO277
(530930) 2011 WG157L429.06430.87822.37.1~1702016Announced on 2016/05/312011+WG157
2012 UV177L427.80632.25920.89.2~80[49]2012+UV177
2013 KY18L526.59833.8736.76.8~2002016Announced on 2016/05/31, stability uncertain2013+KY18
2014 QO441L426.96133.21518.88.2~130[49]Most eccentric stable Neptune trojan[50]2014+QO441
2014 QP441L428.02232.11019.49.1~90[49]2014+QP441
2015 RW277L427.74232.23630.810.2~502018Announced on 2018/10/012015+RW277
2015 VV165L427.51332.49716.98.8~902018Announced on 2018/10/012015+VV165
2015 VW165L428.48831.4885.08.1~1302018Announced on 2018/10/012015+VW165
2015 VX165L427.61232.32717.28.9~902018Announced on 2018/10/012015+VX165
2005 TN74[51] and (309239) 2007 RW10[52] were thought to be Neptune trojans at the time of their discovery, but further observations have disconfirmed their membership. 2005 TN74 is currently thought to be in a 3:5 trans-Neptunian resonance with Neptune.[53] (309239) 2007 RW10 is currently following a quasi-satellite loop around Neptune.[54]
The first person to publish a star's parallax was Friedrich Georg Wilhelm von Struve, when he announced a value of 0.125 arcseconds (0.125″) for Vega.[55]
Struve's initial result was actually close to the currently accepted value of 0.129″,[56][57] as determined by the Hipparcos astrometry satellite.[58][59][60]
AZ Cancri
Main article: Stars/Dwarfs
This is a real visual image of AZ Cancri. Credit: SDSS Data Release 6.
At right is a close-up of the SDSS DR6 image of AZ Cancri in real (visual) color. According to SIMBAD, AZ Cancri (AZ Cnc) is a spectral type M6.0V flare star, that is also an X-ray source detected by the ROSAT satellite. AZ Cancri (AZ Cnc) is a M-type flare star in the constellation Cancer.[61] It has an apparent visual magnitude of approximately 17.59.[61]
According to SIMBAD, AZ Cnc is at a location in this sky plot that does not coincide with any star (dark spot). AZ Cnc is immediately at the red arrow tip. Just east of the top of vertical line of the cross-hair is the star NED locates as AZ Cnc. Credit: Aladin at SIMBAD.
The star is in NGC 2632 designated Haro, Chavira, and Gonzalez (HCG) 4.[62] NGC 2632 is an open cluster, also called Messier 44, and the Praesepe Cluster.
The X-ray astronomy satellite ROSAT detected AZ Cnc at RX J0840.4+1824 and 1RXS J084029.9+182417.
In the SIMBAD visual sky plot at right, AZ Cnc is immediately at the tip of the red arrow J2000.0 RA 08h 40m 29.751s Dec +18°24'09.18".
Regarding the NED image at left AZ Cnc is located at J2000.0 RA 08h 40m 30.2s Dec +18°23'55", precisely coincident with the negative visual object image at exact image center.
Astronomical visual sources
Image (negative) of the star or visual object field centered on the star AZ Cnc. Image is 5' x 5'. Credit: NASA/IPAC Extragalactic Database.
The visual star is spectral type M6e,[63] specifically M6.5Ve.[64] Visual magnitude is Mv = 16.9. log Lbol = 30.48 ergs/s (or 3.020 x 1030 ergs s-1).
This SDSS DR6 sky plot is similar in areal survey to the SIMBAD and NED sky plots. The red star at center is AZ Cancri. It corresponds to the tip of the red arrow in the SIMBAD image. Credit: Sloan Digital Sky Survey.
A comparison of images between that from SIMBAD at upper right and the NASA/IPAC Extragalactic Database (NED) (5' x 5') at upper left shows some identical visual object patterns. In the NED image at left are two objects very close together. These two are near the center of the southeast quarter of the SIMBAD image. To the east-southeast of the NED designated AZ Cnc is an apparently solitary visual object that is also in the NED image. In the north-east quadrant of the NED image are four visual objects forming a triangle with a weak source just above two of the objects. This pattern is repeated on the SIMBAD image which includes a fifth object forming a diamond with the objects of the triangle.
Physical characteristics
AZ Cnc is considered a very low mass star (VLMS). Distance from the Sun is 14.0 pc.[65] The radial velocity of LHS 2034 is 64.2±0.6 km/s.[66] Its galactic motion (space velocities) are U = -60.6 km/s, V = -44.3 km/s, and W = -8.3 km/s, relative to the local standard of rest.[66] LHS 2034 belongs kinematically to the old disk.[66] Its rotational velocity is v sin(i) = 7.9±2.8 km/s.[66]
Catalog designations
CSI+18-08377 is the Catalog of Stellar Identifications.[67]
GJ 316.1 is the catalog entry from the nearby star data published between 1969-1978 for numbers 2001-2159 and incorporating the earlier Catalogue of nearby stars by Gliese.[68]
LHS 2034 is the Luyten-H-S Catalogue number.[69]
Astronomical X-ray sources
The X-ray luminosity log (Lx) = 27.40 ergs/s (or 2.512 x 1027 ergs s-1).[70] Lx/Lbol = 8.318 x 10-4 does not depend on Mv, at least for older stars like AZ Cnc.[70]
The X-ray luminosity of AZ Cnc increased by at least two orders of magnitude during a flare that lasted more than 3 h and reached a peak emission level of more than 1029 ergs/s.[70] During another long duration flare (March 14, 2002) on LHS 2034, very strong wing asymmetries occurred in all lines of the Balmer series and all strong He I lines, but not in the metal lines.[66] LHS 2034 was observed for 1.5 h on March 14, 2002, and 40 m on March 16, 2002.[66]
The flaring atmosphere of LHS 2034 has been modeled with the PHOENIX atmosphere code,[71][66] consisting of
  1. an underlying photosphere,
  2. a linear temperature rise vs. log column mass in the chromosphere, and
  3. transition region (TR) with different gradients.[66]
For the underlying photosphere, Teff = 2800 K, log g = 5.0, and a solar chemical composition was used.[66] The last spectrum taken in the series after the flare was used for the quiescent chromosphere.[66]
The line asymmetries have been attributed to downward moving material,[66] specifically a series of flare-triggered downward moving chromospheric condensations, or chromospheric downward condensations (CDC)s as on the Sun.[72] For the Sun such events can last a few minutes, but for LHS 2034 they have lasted for 1.5 h.[66]
Theory of coronal heating
The electrodynamic coupling theory of coronal heating developed in a solar context,[73] has been applied to stellar coronae.[74] A distinctive feature is the occurrence of a resonance between the convective turnover time and the crossing time for Alfvén waves in a coronal loop. The resonance attains a maximum among the early M dwarf spectral types and declines thereafter. A turnover in coronal heating efficiency, presumably manifested by a decrease in Lx/Lbol, becomes evident toward the late M spectral types when the theory is applicable. This is consistent with an apparent lack of X-ray emission among the late M dwarfs.[75] Coronal heating efficiencies do not decrease toward the presumably totally convective stars near the end of the main sequence.[70] For "saturated" M dwarfs, 0.1% of all energy is typically radiated in X-rays, while for AZ Cnc this number increases during flaring to 7%.[70] So far there is no evidence to suggest that AZ Cnc is less efficient than more massive dwarfs in creating a corona.[70] The saturation boundary in X-ray luminosity extends to late M dwarfs, with Lx/Lbol ~ 10−3 for saturated dwarfs outside flaring. No coronal dividing line exists in the Hertzsprung–Russell diagram at the low-mass end of the main sequence.[70]
AZ Cnc casts doubt on the applicability of electrodynamic coupling as there is no evidence for a sharp drop in Lx/Lbol when compared with other late M stars at least until subtype M8.[70]
AZ Cnc has a corona and this may indicate that a distributive dynamo is just as efficient in producing magnetic flux as a shell dynamo.[70] Between the generation of a magnetic field and the emission of X-rays lies the coronal heating mechanism.[70]
Protoplanetary disks
This is an artist's conception of the young massive star HD100546 and its surrounding disk. Credit: P. Marenfeld & NOAO/AURA/NSF.
"A planet forming in the disk [artist's impression at the right] has cleared the disk within 13 AU of the star, a distance comparable to that of Saturn from the sun. As gas and dust flows from the circumstellar disk to the planet, this material surrounds the planet as a circumplanetary disk (inset). These rotating disks are believed to be the birthplaces of planetary moons, such as the Galilean moons that orbit Jupiter. While they are theoretically predicted to surround giant planets at birth, there has been little observational evidence to date for circumplanetary disks outside the solar system."[76]
An "orbiting source of carbon monoxide emission [has a size] consistent with theoretical predictions for a circumplanetary disk. Observations over 10 years trace the orbit of the forming planet from behind the near side of the circumstellar disk in 2003 to the far side of the disk in 2013."[76]
The "star [is] about 335 light years from Earth."[76]
"[A]n "extra" source of gaseous emission from carbon monoxide molecules ... could not be explained by the protoplanetary disk alone."[76]
"By tracking the changes in velocity and position of this extra emission over the years of the observations [using a technique called spectro-astrometry], [the observations] show that it is orbiting around the young star. The distance from the star is somewhat larger than the distance of Saturn from the Sun."[76]
"The candidate planet would be a gas giant at least three times the mass of Jupiter."[76]
"These results provide a rare opportunity to study planet formation in action. Our analysis strongly suggests we are observing a disk of hot gas that surrounds a forming giant planet in orbit around the star. While such circumplanetary disks have been theorized to surround giant planets at birth and to control the flow of gas onto the growing planet, these findings are the first observational evidence for their existence. If our interpretation is correct, we are essentially seeing a planet caught in the act of formation."[77]
SDSS J113312.12+010824.9
HVS 7 -- hyper-velocity star 7, otherwise known as SDSS J113312.12+010824.9 is a rare star that has been accelerated to faster than our Milky Way Galaxy's escape velocity.[78][79]
"Such a surface abundance pattern is caused by atomic diffusion in a possibly magnetically stabilised, non-convective atmosphere. Hence all chemical information on the star’s place of birth and its evolution has been washed out. High precision astrometry is the only means to validate a GC origin for HVS 7."[78]
"Here we report the [...] most recently discovered HVSs: [...] SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least [...] +418+/-10 km s-1 [...]."[79]
In 2013 a team under N. Przybilla wrote that the star had a chemically peculiar photosphere, which masked its origins.[78]
The star was first cataloged during the Sloan Digital Sky Survey and was identified as a hyper-velocity star in 2006.[79]
Radial velocity (cz) = 518.6 ± 3.0 km/s, Spectral type: sdB, [BGK2006] HV 7 are aka [BGK2006] J113312.12+010824.9, EPIC 201540171, Gaia DR2 3799146650623432704, GALEX 2413439155581226272, and USNO-A2.0 0900-06954189.[80]
Red hypergiants
Wide Field and Planetary Camera 2 (WFPC2) Hubble Space Telescope (HST) image shows the asymmetric nebula surrounding VY CMa, which is the central star. Credit: Judy Schmidt.
{{free media}}
Astrometric "results of phase-referencing very long baseline interferometry observations of 43 GHz SiO maser emission toward the red hypergiant VY Canis Majoris (VY CMa) [are from] using the Very Long Baseline Array (VLBA)."[81]
VY CMa is a single star with a large infrared (IR) excess, making it one of the brightest objects in the sky at wavelengths of between 5 and 20 µm and indicating a dust shell or disk heated by the star.[82][83]
VY CMa is embedded within the large molecular cloud Sharpless 310 (Sh2-310), one of largest star-forming H II regions with a diameter of 480 ' or 681 ly (209 pc).[84][85]
Since 1847, VY Canis Majoris has been described as a crimson star.[86] Visual observations in 1957 and high-resolution imaging in 1998 showed that there are no companion stars.[86][87]
VY CMa was also discovered to be a strong source of OH (1612 MHz), H
2O (22235.08 MHz), and SiO (43122 MHz) masers emission, which is typical of an OH/IR star.[88][89][90]
Many molecules, such as HCN, NaCl, PN, CH, CO, CH
3OH, TiO, and TiO
2, have also been detected.​[91]​[92]​[93]​[94]​[95]
The variation in VY CMa's brightness was first described in 1931 when it was listed (in German) as a long period variable with a photographic magnitude range of 9.5 to 11.5.[96]
It was given the variable star designation VY Canis Majoris in 1939, the 43rd variable star of the constellation Canis Major.[97]
Main article: Radiation astronomy/Galaxies
Galaxy rotation curve is of a typical spiral galaxy: predicted based on the visible matter (A) and observed (B). The distance is outward from the galactic core. Credit: PhilHibbs.
{{free media}}
"The rotation of galaxies was discovered in 1914, when Slipher (1914) detected inclined absorption lines in the nuclear spectra of M31 and the Sombrero galaxy, and Wolf (1914) detected inclined lines in the nuclear spectrum of M81."[98]
In the diagram on the right, expected (A) and observed (B) visible matter velocities as a function of distance from the galactic center are plotted.
"At the dedication of the McDonald Observatory in 1939, Oort’s (1940) comment that “...the distribution of mass [in NGC 3115] appears to bear almost no relation to that of the light” seems from the view in 2000 to have attracted little attention. His conclusion concerning the mass distribution in NGC 3115 is worth quoting, even 60 years later. “In the outer parts of the nebula the ratio f of mass density to light density is found to be very high; and this conclusion holds for whatever dynamical model we consider. The spectrum of the nebula shows the characteristics of G-type dwarfs. Since f cannot be much larger than 1 for such stars, they can account for roughly only 1/2 percent of the mass; the remainder must consist either of extremely faint dwarfs having an average ratio of mass to light of about 200 to 1 or else of interstellar gas and dust”. From a reanalysis of the (scattered) velocities for M31, Schwarzschild (1954) concluded that the approximately flat rotation curve was “not discordant with the assumption of equal mass and light distribution.”"[98]
"Rotation curves are tools for several purposes: for studying the kinematics of galaxies; for inferring the evolutionary histories and the role that interactions have played; for relating departures from the expected rotation curve Keplerian form to the amount and distribution of dark matter; for observing evolution by comparing rotation curves in distant galaxies with galaxies nearby."[98]
"Although Hα, [NII], and [SII] emission lines have traditionally been employed, the Seyfert galaxy NGC 1068 has become the first galaxy whose velocity field has been studied from the IR [Si VI] line (Tecza et al. 2000)."[98]
"For a limited number of nearby galaxies, rotation curves can be produced from velocities of individual HII regions in galactic disks (Rubin & Ford, 1970, 1983; Zaritsky et al. 1989, 1990, 1994)."[98]
"The HI line is a powerful tool to obtain kinematics of spiral galaxies, in part because its radial extent is often greater, sometimes 3 or 4 times greater, than that of the visible disk. Bosma’s thesis (1981a, b; van der Kruit & Allen 1978) played a fundamental role in establishing the flatness of spiral rotation curves."[98]
"While comparison of the inner velocity rise for NGC 3198 showed good agreement between the 21-cm and the optical velocities (van Albada et al. 1985; Hunter et al. 1986), the agreement was poor for Virgo spirals observed at low HI resolution (Guhathakurta et al. 1988; Rubin et al. 1989)."[98]
"The rotational transition lines of carbon monoxide (CO) in the millimeter wave range [e.g., 115.27 GHz for 12
CO (J = 1 − 0) line, 230.5 GHz for J = 2 − 1] are valuable in studying rotation kinematics of the inner disk and central regions of spiral galaxies, for extinction in the central dusty disks is negligible at CO wavelengths (Sofue 1996, 1997). Edge-on and high-inclination galaxies are particularly useful for rotation curve analysis in order to minimize the uncertainty arising from inclination corrections, for which extinction-free measurements are crucial, especially for central rotation curves."[98]
"CO lines are emitted from molecular clouds associated with star formation regions emitting the Hα line. Hence, CO is a good alternative to Hα and also to HI in the inner disk, while HI is often weak or absent in the central regions. The Hα, CO, and HI rotation curves agree well with each other in the intermediate region disks of spiral galaxies (Sofue 1996; Sofue et al. 1999a, b). Small displacements between Hα and CO rotation curves can arise in the inner regions from the extinction of the optical lines and the contamination of the continuum star light from central bulges."[98]
Interferometric "observations have achieved sub- or one-arcsec resolution (Sargent and Welch 1993; Scoville et al. 1993; Schinnerer et al. 2000; Sofue et al. 2000), comparable to, or sometimes higher than, the current optical measurements [...]. Another advantage of CO spectroscopy is its high velocity resolution of one to several km s−1."[98]
"Radial velocity observations of maser lines, such as SiO, OH and H2O lines, from circum-stellar shells and gas clouds allow us to measure the kinematics of stellar components in the disk and bulge of our Galaxy (Lindqvist et al. 1992a, b; Izumiura 1995, 1999; Deguchi et al. 2000). VLBI astrometry of SiO maser stars’ proper motion and parallax as well as radial velocities will reveal more unambiguous rotation of the Galaxy in the future. VLBI measurements of water masers from nuclei of galaxies reveal circumnuclear rotation on scales of 0.1 pc around massive central black holes, as was successfully observed for NGC 4258 (Miyoshi et al. 1995; see Section 4.4)."[98]
"A simple “rotation curve” is an approximation as a function of radius to the full velocity field of a disk galaxy. As such, it can be obtained only by neglecting small scale velocity variations, and by averaging and smoothing rotation velocities from both sides of the galactic center. Because it is a sim- ple, albeit approximate, description of a spiral velocity field, it is likely to be valuable even as more complex descriptions become available for many galaxies."[98]
"An extreme case of a nuclear warp is counterrotation. Rotating nuclear disks of cold gas have been discovered in more than 100 galaxies, types E through Sc (Bertola & Galletta 1978; Galletta 1987, 1996; Bertola et al. 1990; Bertola et al. 1992; Rubin 1994b; Garcia-Burillo et al. 1998); counterrotation is not especially rare. Simulations of disk interactions and mergers which include gas and stellar particles (Hernquist & Barnes 1991; Barnes & Hernquist 1992) reveal that a kinematically distinct nuclear gas disk can form; it may be counterrotating. Simulation of galactic-shock accretion of nuclear gas disk in an oval potential, such as a nuclear bar, produces highly eccentric streaming motion toward the nucleus, some portion being counterrotating (Wada et al. 1999). Kinematically decoupled stellar nuclear disks are also observed in early type galaxies (Jedrzejewski & Schechter 1989; Franx et al. 1991). Counter rotating nuclear disks can result from merger, mass exchange and/or inflow of intergalactic clouds. In addition to forming the central disk, an inflow of counterrotating gas would also be likely to promote nuclear activity."[98]
"A disk rotation curve manifests the distribution of surface mass density in the disk, attaining a broad maximum at a radius of about twice the scale radius of the exponential disk. For massive Sb galaxies, the rotation maximum appears at a radius of 5 or 6 kpc, which is about twice the scale length of the disk. Beyond the maximum, the rotation curve is usually flat, merging with the flat portion due to the massive dark halo. Superposed on the smooth rotation curve are fluctuations of a few tens of km s−1 due to spiral arms or velocity ripples. For barred spirals, the fluctuations are larger, of order 50 km s−1, arising from non circular motions in the oval potential."[98]
"Universal rotation curves reveal the following characteristics. Most luminous galaxies show a slightly declining rotation curves in the outer part, following a broad maximum in the disk. Intermediate galaxies have nearly flat rotation from across the disk. Less luminous galaxies have monotonically increasing rotation velocities across the optical disk. While Persic et al. conclude that the dark-to-luminous mass ratio increases with decreasing luminosity, mass deconvolutions are far from unique."[98]
"Only a handful of galaxies are presently known to have counterrotating components over a large fraction of their disks (Rubin 1994b). The disk of E7/S0 NGC 4550, (Rubin et al. 1992; Kenney & Faundez 2000) contains two hospital stellar populations, one orbiting programmed, one retrograde. This discovery prompted modification of computer programs which fit only a single Gaussian to integrated absorption lines in galaxy spectra (Rix et al. 1992). In NGC 7217 (Sab), 30% of the disk stars orbit retrograde (Merrifield & Kuijen 1994). The bulge in NGC 7331 (Sbc) may (Prada et al. 1996) or may not (Mediavilla et al. 1998) counterrotate with respect to the disk. Stars in NGC 4826 (Sab; the Black Eye or Sleeping Beauty) orbit with a single sense. Gas extending from the nucleus through the broad dusty lane rotates prograde, but reverses its sense of rotation immediately beyond; radial infall motions are present where the galaxy velocities reverse (Rubin et al. 1965; Braun et al. 1994; Rubin 1994a; Walterbos et al. 1994; Rix et al. 1995; Sil’chenko 1996)."[98]
"Only recently have rotation curves been obtained for distant galaxies, using HST and large-aperture ground-based telescopes with sub-arc second seeing. We directly observe galaxy evolution by studying galaxies closer to their era of formation. Rotation velocities for moderately distant spirals, z≈ 0.2 to 0.4, (Bershady 1997, et al. 1999, Simard & Prichet 1998, Kelson et al. 2000a) have already been surpassed with Keck velocities reaching z≈1 (Vogt et al. 1993, 1996, 1997; Koo 1999), for galaxies whose diameters subtend only a few seconds of arc. The rotation properties are similar to those of nearby galaxies, with peak velocities between 100 to 200 km s−1, and flat outer disk velocities."[98]
"The maximum rotation velocities for Sa galaxies are higher than those of Sb and Sc galaxies with equivalent optical luminosities. Median values of Vmax decreases from 300 to 220 to 175 km s−1 for the Sa, Sb, and Sc types, respectively (Rubin et al. 1985)."[98]
"Large-scale rotation properties of SBb and SBc galaxies are generally similar to those of non-barred galaxies of Sb and Sc types."[98]
Barred "galaxies show velocity jumps from ± ∼ 30 − 40 km s−1 to ≥ 100 km s−1 on the leading edges of the bar, R ∼ 2 − 5 kpc, whereas some barred galaxies show flat rotation (e.g., NGC 253: Sorai et al. 2000)."[98]
"Until the last decade, observations of rotational kinematics were restricted to spirals with average or high surface brightness. Only within the past decade have low surface brightness (LSB) galaxies been found in great numbers (Schombert & Bothun 1988; Schombert et al. 1992); many are spirals. Their kinematics were first studied by de Blok et al. (1996) with HI, who found slowly rising curves which often continued rising to their last measured point. However, many of the galaxies are small in angular extent, so observations are subject to beam smearing. Recent optical rotation curves (Swaters 1999, 2001; Swaters et al. 2000; de Blok et al. 2001) reveal a steeper rise for some, but not all, of the galaxies studied previously at 21-cm."[98]
"Dwarf galaxies, galaxies of low mass, are often grouped with low surface brightness galaxies, either by design or by error. The two classes overlap in the low surface brightness/low mass region. However, some low surface brightness galaxies are large and massive; some dwarf galaxies have high surface brightness. Early observations showed dwarf galaxies to be slowly rotating, with rotation curves which rise monotonically to the last measured point (Tully et al. 1978; Carignan & Freeman 1985; Carignan & Puche 1990a,b; Carignan & Beaulieu 1989; Puche et al. 1990, 1991a, b; Lake et al. 1990; Broeils 1992)."[98]
Main article: Hypotheses
The use of satellites should provide ten times the information as sounding rockets or balloons.
A control group for a radiation satellite would contain
  1. a radiation astronomy telescope,
  2. a two-way communication system,
  3. a positional locator,
  4. an orientation propulsion system, and
  5. power supplies and energy sources for all components.
A control group for radiation astronomy satellites may include an ideal or rigorously stable orbit so that the satellite observes the radiation at or to a much higher resolution than an Earth-based ground-level observatory is capable of.
See also
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