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Courtesy: ISRO - Government of IndiaThis month, APOM presents ultraviolet images of the globular cluster NGC 288, located at a distance of around 30,000 light years in the constellation Sculptor. This cluster was first described by John Dreyer in 1888. This is the second globular cluster in the APOM series, the first one being NGC 1851, presented a year ago. A globular cluster is a very large group of stars formed from the same cloud of gas and dust at nearly the same time. They are called globular because of the spherical distribution of stars, and each cluster is held together as a single entity by the gravity of the star members. Globular clusters are few of the oldest known objects in our galaxy. The stars in NGC 288 are believed to be nearly 12.6 billion years old.
The Ultraviolet Imaging Telescope (UVIT) on AstroSat has been used to image the stars in NGC 288 in the ultraviolet light, by a group of researchers from the Indian Institute of Astrophysics, Bangalore, and the National Research Council of Canada. The number of stars seen in the far-ultraviolet light are fewer than those seen in the near-ultraviolet light, and the reason for this is that only the hottest stars are seen in the far-ultraviolet.
Since globular clusters have very old stars, many of the heavier ones have already evolved to later phases of stellar lives (eg. red giant stars, horizontal branch stars). About 115 hot horizontal branch stars having surface temperature nearly twice that of the surface of Sun have been detected in NGC 288 using the near and far-ultraviolet filters of UVIT. A couple of very hot stars (extreme horizontal branch stars) whose whose surface temperatures are nearly five times that of the Solar surface have also been found. Using a combination of ultraviolet and optical light, these researchers have also identified 68 blue stragglers in this cluster. A blue straggler is a star formed when two stars either merge or transfer mass between them. Such stars have been mostly found in globular clusters where the star density is high. The capabilities of UVIT have made it easy for the researchers to see and inspect hot stars towards the cluster individually, allowing them to study the properties of each star, such as the mass and temperature.
The paper describing their results can be downloaded here.
Click here for the press story from India Science Wire and here for the entire APOM archive.
When seeing images from AstroSat, have you ever wondered where exactly is the satellite, how does it move, and how do astronomers get their hands on the data? This month's APOM is here to answer those questions for you.
AstroSat was launched by ISRO on 28 September, 2015 from Sriharikota https://www.isro.gov.in/about-isro/satish-dhawan-space-centre-sdsc-shar, on board the PSLV-C30 into its current orbit. This is a low-earth equatorial orbit, at a height of 650 km above the Earth. This orbit is not exactly over the equator, but is inclined at angle of about 6 degree to it. In the top left image, the green line marks the equator and the yellow line marks the orbit of AstroSat and the top right image is a view from over the north pole. But why was this orbit chosen?
Our Earth has a magnetic field (https://en.wikipedia.org/wiki/Earth's_magnetic_field), which behaves overall like a bar magnet (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/MagEarth.html), with its poles a few degrees from the poles defined by our rotation. These magnetic fields trap charged particles within them, which form the Van Allen belts (https://www.space.com/33948-van-allen-radiation-belts.html). These belts are much closer to the Earth over the southern Atlantic Ocean. An equatorial orbit reduces the effect of this South Atlantic Anomaly https://en.wikipedia.org/wiki/South_Atlantic_Anomaly, on AstroSat which carries very sensitive instruments. Making the inclination exactly zero requires more resources and hence a 6 degree inclination was chosen.
AstroSat takes about 97 minutes to orbit the Earth once. Hence, it will not pass directly overhead the same point in successive orbits. Each orbit, therefore, will be slightly shifted with respect to the previous one. The individual orbits shown in red in the bottom image mark orbits. One orbit per week is plotted for clarity. Data is beamed down from an antenna on the satellite once every orbit, when it passes over India. The data is received by ISRO's dedicated Indian Deep Space Network http://isrohq.vssc.gov.in/VSSC_V4/index.php/ground-segment/82-chandrayaa... antenna in Byalalu https://en.wikipedia.org/wiki/Indian_Deep_Space_Networknear Bengaluru (marked as BLR). All orbits of AstroSat fall within the visibility of this antenna, which is marked by a circle centred at Byalalu. ISRO can also use an antenna in Indonesia, marked BIK, to monitor the satellite when needed. All the command, control and tracking of AstroSat is done by ISTRAC https://www.isro.gov.in/about-isro/isro-telemetry-tracking-and-command-n... in Bengaluru.
As of 30 January 2019, AstroSat has completed more than 18,000 orbits around the Earth, acting as our high energy eye, uncovering the nature of neutron stars, black holes, hot star, and many strange celestial objects. May it continue to do so for many more orbits!
The Sun is a constant presence in our lives and is about 5 billion years old. But will the Sun itself change in the millions of years to come? Any such change will occur so far into the future, that astronomers need to look to alternate places in the sky to understand this. Globular clustersare the best laboratories to study the fate of stars. This month, APOM brings forth a globular cluster called NGC 2808 located at a distance of about 47,000 light years in the constellation Carina. This is the third globular cluster in APOM, after NGC 1851 and NGC 288.
NGC 2808 is one of the most massive globular clusters that we know, with a stellar membership of more than a million stars. Being nearly 11 billion years old, stars like the Sun and heavier stars have evolved to later stages of evolution. Due to the large number of stars present in globular clusters, stars with different masses, and in different evolutionary stagescan be studied together. This is because it is believed that all stars in the cluster formed from the same material at approximately the same time. NGC 2808 is unique because very recent optical studies have shown that it houses many distinct populations of stars (five in this case) within it, the maximum found in any globular cluster till date. Stars at the same evolutionary stage but having similar masses in this cluster seem to have other properties (eg. brightness, material from which it is made) that are slightly different. These are then said to belong to different populations.
The stars that are bright in ultraviolet in this globular cluster have been studied using UVIT on-board AstroSat by a group of researchers from the Indian Institute of Space science and Technology (IIST), Trivandrum and Tata Institute of Fundamental Research, Mumbai. Using ultraviolet light from different wavebands (filters), these authors have identified stars belonging to later stages of stellar evolution, eg. Horizontal Branch stars, hot stars that have passed through the Asymptotic Giant Branch phase. They have also established the presence of four different populations of stars that are seen in the UV, including a new population for the first time. These UV populations of stars are related to the five groups of optical stars mentioned above. Earlier studies had shown the presence of a certain group of UV stars called the Red Horizontal Branch stars in the cluster. The current study has utilized the capabilities of UVIT to report that it is not one group, but rather a mixture of two different populations. This study of the UV populations in the cluster would help in refining our understanding of the formation of multiple populations in globular clusters.
The paper describing the results is accepted for publication by the Monthly Notices of Royal Astronomical Society and can be found here.
Accompanying science story is here.
This month we bring you yet another Globular Cluster, NGC 5466, located around 52000 light years from us in the constellation Bootes. However, we are going to turn our attention away from the cluster itself, and look at one particular star. This star, called NH 84, is a very special kind of star, and is what astronomers call a Blue Straggler Star, or BSS. Why are these special and how does it relive its youth?
If you have read our previous APOMs on Globular Clusters (here, here and here), you may remember that almost all stars in a cluster are born together at the same time. You may also recall that stars are born, live sedately for a long time, and then die in various spectacular ways. The more massive a star is, the faster it will evolve, and the faster it will die. More massive stars are also usually bluer and hotter, whereas less massive stars are redder and cooler. If we start with a bunch of stars that are born at the same time, like in a Globular Cluster, then as time goes by, we expect to see less and less hot blue stars, since they would have died already. Instead, we would only see the cooler, redder and older stars. Which is why astronomers were very surprised when, in 1953, Allan Sandage found young hot blue starsin old star clusters. How did these stars retain their youth in the face of time? The answer was very surprising indeed, and involved two stars instead of one.
The most common way this happens is in binary star systems, i.e., two stars orbiting each other. Snehalata Sahu of the Indian Institute of Astrophysics and her colleagues imaged the cluster NGC 5466 using the UltraViolet Imaging Telescope on AstroSat and identified many Blue Straggler Stars. In particular, they looked at one of them, NH 84, carefully and discovered that it had to be such a binary system. The bright star was a BSS which had swallowed up material from its companion star, and become more massive and bluer, reliving its youth. The poor companion, though, continued on to become a very hot and dense White Dwarf. How did these astronomers know that the companion is a White Dwarf? They deduced this based on the brightness of NH 84 that they measured in the ultraviolet wavelengths, which is where the White Dwarf shines the most. The BSS itself has a surface temperature of 8000 Kelvin, is about as massive as our Sun, and about 45% bigger. The White Dwarf, on the other hand, is 32000 Kelvin, is about half as massive as our Sun, but only 2% of its size!
This is only the second such BSS–White Dwarf pair that astronomers have found in Globular Clusters. Recently, another team led by Subramaniam had discovered, using the UVIT, another binary system where a BSS was orbiting an evolved aged star whose youth it had stolen. This latest discovery was possible because of the superior resolution and sensitivity of AstroSat in the ultraviolet. The authors are now chasing after the other Blue Straggler Stars in this cluster. Let us wait and see what discoveries await them.
The paper describing the results is accepted for publication by the Astrophysical Journal and can be found here. The accompanying science story, through India Science Wire, is here.
In a breakthrough, a team of astronomers from the Physical Research Laboratory (PRL), Ahmedabad used the data from Ultra-Violet imaging telescope (UVIT) onboard India’s Astrosat space observatory to resolve the nature of a peculiar star named SU Lyn. They have utilized, for the first time, the UV spectroscopy capability of UVIT and in the process, have thrown new light on the class of stellar objects known as symbiotic stars.
SU Lyn had long been known as an ostensibly unremarkable red giant star – a class of very large and cool stars, which form at the final stages of stellar evolution. However, it was noticed in 2016 that hard X-ray emission was emanating from SU Lyn. This raised the suspicion that the star harboured a hidden, hot companion assumed to be a white dwarf – an end-product when stars of intermediate-mass die. White dwarfs can be as massive as the Sun, yet they have a size similar to the size of the Earth.
The suggestion that SU Lyn could likely host a white dwarf posed a challenge for our understanding of such systems. Binary stellar systems consisting of a white dwarf and a red giant are known as symbiotic systems. In a symbiotic system, the white dwarf and red giant's interaction gives rise to several complex physical phenomena such as an accretion disk, jets, ionized symbiotic nebula, interaction of stellar winds to name a few. Due to this, Symbiotics are considered as one of the most intriguing astrophysical laboratories. A schematic picture of a typical symbiotic system and its various constituents are shown in Figure-1. These symbiotic systems have traditionally been identified and characterized by the presence of intense emission lines of several high ionization species observed in their optical spectra using ground-based telescopes. However, the optical spectrum of SU Lyn was devoid of these lines, raising a question mark on its symbiotic nature.
A more definite way to establish the presence of a white dwarf is through ultra-violet (UV) observations since white dwarfs are hot and emit radiation mostly in the UV range. UV radiation, however, cannot penetrate the Earth’s atmosphere and can only be detected using space-based UV telescopes and instruments. But at present, there are few UV telescopes in space and UV telescopes with spectroscopic capability are even rarer.
This is where India’s Astrosat space observatory and one of its payloads UVIT – the Ultra-Violet Imaging Telescope – played a crucial role. Instruments onboard the observatory are capable of recording the UV spectrum of stars, a feature that proved extremely useful. The PRL team had been observing SU Lyn since 2016 with various Indian observing facilities and a suite of instruments, most notably with the UVIT. From the ground, the star was observed with the HESP instrument on the IIA-HCT telescope, with the indigenous in-house developed MFOSC-P spectrograph and with the Near-Infrared Camera and Spectrometer on the PRL 1.2 m telescope at Mount Abu.
The Far-UV (1300-1800 Angstroms) spectrum of SU Lyn, obtained with the Astrosat-UVIT instrument, showed emission lines of silicon (Si IV), carbon (C IV), oxygen (OIII), and nitrogen (N III) in a spectrum typical of symbiotic stars (figure-2). The high-resolution optical spectrum also shows the weak presence of few emission lines, which are typically seen in the optical spectrum of symbiotic stars. The UV spectrum, complemented by optical and NIR spectra, thus, confirms the symbiotic nature of SU Lyn. Using a simple theoretical model to fit the UV observations, it was further shown that the white dwarf in SU Lyn is orders of magnitude less luminous (0.16 solar luminosity) compared to a white dwarf in a traditional symbiotic system (~100-1000 solar luminosity). Instead, the symbiotic phenomenon is predominantly powered by the relatively weaker UV radiation from the accretion disk (0.66 solar luminosity) around the white dwarf. This is the reason that the emission lines are weak in the optical spectrum and why the symbiotic nature of SU Lyn could not be established from ground-based observations earlier.
The resolution of the nature of SU Lyn is a significant result for stellar astronomy. There are only a few hundred symbiotic systems known in our Galaxy. This is in contrast with their predicted population of several hundred thousand. The presence of intense emission lines in low-resolution optical spectra has always been the traditional way to identify and discover symbiotic stars. However, these traditional methods would fail to detect the SU Lyn type of Symbiotics. These recent results by the PRL team have firmly established the existence of SU Lyn type symbiotic systems. It is highly probable that many more symbiotic stars like SU Lyn can exist which have so far evaded the detection by conventional methods. And this could be a reason why a smaller than expected number of symbiotic systems have been discovered so far.
Reference :It is equally important to note that these results are derived from a lesser-known spectroscopic capability of the UVIT instrument, which is preliminarily designed as an imaging instrument.
Actually it is the first INTERNATIONAL LMT. But also it is the only LMT in world today."worlds first" , wonder y media hype up things like this.
Amateur astronomers in ISRO, Fahd Bin Abdul Hasis, Kiran Mohan, and VishakSasidharan from Liquid Propulsion Systems Centre (LPSC), Valiamala have photographed SN2023ixf using a modest setup consisting of a DSLR camera and basic sky tracking equipment, despite the challenging climatic conditions.
The images below showcase the progression of SN2023ixf over time. Comparing two photographs taken on May 19, 2020 and May 22, 2023.
Renowned amateur astronomer Mr. Koichi Itagaki discovered SN2023ixf on May 19, 2023. This remarkable celestial event, classified as a Type-II supernova, is located in the Pinwheel Galaxy (M101), approximately 21 million light years away from Earth. Supernovae are awe-inspiring phenomena that result from the explosive death of massive stars.
What's truly remarkable about the photograph by the amateur astronomers from ISRO is that the team captured this celestial event using a Nikon Z6 ii camera equipped with a Samyang 135 mm lens at f2.8 and ISO 1000, along with the iOptronSkyGuider pro as their sky tracking device. This simple setup allowed them to record the supernova's evolving appearance in the night sky.
The image processing techniques employed by the team involved stacking multiple frames to enhance the details of SN2023ixf. On May 22, 2023, they stacked 107 light frames of 20 seconds each, totaling 35 minutes of exposure time (shot at 135 mm and cropped). Similarly, May 19, 2020 photo was stacked from 107 light frames of 25 seconds each, totaling 45 minutes of exposure time (shot at 300 mm).
The passion, dedication, and ingenuity of amateur astronomers demonstrating that some rare celestial events can be observed and captured even with basic equipment, given the right skills and determination is praiseworthy. The wonders of the universe are within reach for those who dare to explore and observe.
A new Jupiter size exoplanet with highest density of ~14 g/cm3 known till this date, and mass 13 times that of Jupiter has been discovered by an international team of scientists led by Prof. Abhijit Chakraborty at the Exoplanet Research Group of the Physical Research Laboratory (PRL), Ahmedabad. The team includes scientists from India, Germany, Switzerland and the USA. The discovery of this massive exoplanet was made using the indigenously made PRL Advanced Radial-velocity Abu-sky Search spectrograph (PARAS) at the 1.2 m telescope of PRL at its Gurushikhar Observatory in Mt. Abu by measuring the mass of the planet precisely.
The newly discovered exoplanet is found around the star called TOI4603 or HD 245134. NASA’s The Transiting Exoplanet Survey Satellite (TESS) initially declared TOI4603 as a possible candidate to host a secondary body of unknown nature. Using PARAS, scientists discovered it as a planet by measuring the mass of the secondary body and hence the planet is called TOI 4603b or HD 245134b. It is located 731 light years away. It orbits a sub-giant F-type star TOI4603 every 7.24 days. What sets this discovery apart is that the planet falls into the transition mass range of massive giant planets and low-mass brown dwarfs with masses ranging from 11 to 16 times the mass of Jupiter. Only fewer than five exoplanets are currently known in this mass range so far.
Massive giant exoplanets are those having mass greater than four times that of Jupiter. The newly discovered exoplanet TOI 4603b is one of the most massive and densest giant planets that orbits very close to its host star at a distance less than 1/10th the distance between our Sun and Earth. The exoplanet with a surface temperature of 1670 K is likely undergoing high-eccentricity tidal migration with an eccentricity value of approximately 0.3 The detection of such systems provides valuable insights into the formation, migration, and evolution mechanisms of massive exoplanets.
The findings of this study is published in the journal Astronomy & Astrophysics Letters. The publication titled "Discovery of a massive giant planet with extreme density around the sub-giant star TOI-4603." provides details of the discovery and a comprehensive account of exoplanet's characteristics.Click here to access the publication.This discovery marks the third exoplanet discovery by India, and by the PRL scientists using PARAS spectrograph and the PRL 1.2m telescope, following the discoveries in 2018 (K2-236b) and 2021 (TOI-1789b).
The AstroSat Picture of the Month for October 2017 is the Near-UV image of the beautiful barred spiral galaxy NGC 2336, taken by UVIT on board the ASTROSAT. The bright spots along the spiral arms are regions of active star formation.
(Picture Credits: UVIT Team)
To download a high resolution image, see link.
The AstroSat Picture of the Month for November 2017 are the Near-UV (left) and Far-UV (right) images of the Globular Cluster NGC 1851, taken by UVIT on board the ASTROSAT. The FUV image shows only the hottest stars in the cluster. All colours are artificial.
(Picture Credits: Annapurni Subramaniam and collaborators )
To download a high resolution image, see link.
The AstroSat Picture of the Month for December 2017 are the the ultra-violet images of NGC 40 using ASTROSAT. The pink-red part is the gas cloud NGC 40, also known as the Bow-Tie nebula, which is being illuminated by its central hot star. The new discovery is that of the halo region surrounding the nebula. It is observed to be glowing in diffuse, ultra-violet light and shown as golden here in this false-colour image.
(Picture Credits: Kameswara Rao and collaborators )
To download a high resolution image, see link.
The AstroSat Picture of the Month for February 2018 is the ultra-violet image of NGC 6960 or the Witch's Broom, using ASTROSAT. The beautiful filaments are due to gas heated up because of the shocks from the Supernova explosion that happened thousands of years ago.
(Picture Credits: Firoza K. Sutaria, K.P. Singh, P. T. Rahna, J. Murthy, A.K. Ray, N.K. Rao & A. Kumar )
To download a high resolution image, see link.
The AstroSat Picture of the Month for March 2018 is the ultra-violet image of NGC 7252 or the 'Atoms for Peace' galaxy, using ASTROSAT. The two tails of gas and stars ripped out due to the merger of two galaxies can be seen in the image on the left (going upwards and also downwards and right). The central part, which hosts filaments and loops, is magnified and shown on the right.
(Picture Credits: Koshy George )
To download a high resolution image, see link.
The AstroSat Picture of the Month for April 2018 is the ultra-violet image of the Jellyfish Galaxy JO201, using ASTROSAT. This image shows hot young stars formed in the tentacles, visible to the left of the parent galaxy. These have been formed from the gas stripped from JO201 as it falls through the galaxy cluster.
(Picture Credits: This image is from the work done as part of an international collaboration (GASP))
To download a high resolution image, see link.
The AstroSat Picture of the Month for May 2018 is the X-ray data from the Crab Nebula, using the ASTROSAT. The left panel shows the new result. One rotation period of the Crab Pulsar, of 33 milliseconds, is represented as the phase going from 0.0 to 1.0 and is repeated once more, from 1.0 to 2.0, for clarity. The grey line is the X-ray brightness of the Crab lighthouse, observed by CZTI. The data in colour, obtained by aggregating a large number of measurements, shows what fraction of the X-ray light is polarised, i.e., can be given a specific direction. It can be seen that when the X-ray emission (grey line) is low, the polarisation fraction is high, which is unexpected. The right panel is an artist's impression of how the supernova explosion would have looked like, in the past (credit: ESA/Hubble).
The AstroSat Picture of the Month for June 2018 is the ultraviolet image of the merging galaxy cluster Abell 2256, more than 800 million light years away. This cluster contains galaxies spread over a large area, and we have zoomed in on six of these galaxies to show you their ultraviolet images. The brightest objects in the full image are actually foreground stars in our galaxy which happen to lie in the same direction as Abell 2256.
Picture Credits: UVIT team/ISRO/CSA.
The AstroSat Picture of the Month for July 2018 is an image of the central part of the merging galaxy NGC 7252. This image has been obtained by dividing the NUV by the FUV image, pixel by pixel, indicating 'ultraviolet colour'. The outer redder parts are where the stars older than 400 million years are, the blue ring is the location of younger stars which are 250 million years or younger.
Picture Credits: Koshy George et al.
http://www.iucaa.in/The AstroSat Picture of the Month for August 2018 is an X-ray image of the Tycho Supernova Remnant in the 0.8-2.0 nanometres (0.6-1.6 keV) range, made by the Soft X-ray Telescope on board AstroSat. The supernova remnant is roughly 8 arcminutes big (3.7 times smaller than the full moon in the sky) and the emission is brighter near the edge of the expanding supernova remnant.
Pic Credit: Kulinder Pal Singh (IISER Mohali) and the entire SXT Instrument and POC teams at TIFR, University of Leicester, and IUCAA
The AstroSat Picture of the Month for September 2018 is a photograph of AstroSat itself. The top panel has two photographs of the fully assembled AstroSat. The bottom panel is an artists's conception of the observatory in space. Can you identify each of the five telescopes in the top panel photographs?
Pic Credit: Top panel - ISRO; Bottom panel - ASI POEC, ISRO and Adrita Das
Click here for a full resolution image.
The AstroSat Picture of the Month for October 2018 presents the false-color ultraviolet images of NGC6302 (Butterfly Nebula) using ASTROSAT. On the left is a cartoon showing the full extent of the nebula with far-ultraviolet represented in blue. The picture on the right shows the zoomed-in view of the brightest part of the butterfly shaped planetary nebula in far-ultraviolet.
Pic Credit: Kameshwara Rao and Sriram Krishna
The AstroSat Picture of the Month for November 2018 presents false colour image of the Globular Cluster NGC 288 in the ultraviolet taken by UVIT. The near-UV emission the stars are in yellow and the far-UV emission is in white. It is clear that the UVIT image shows only the hot stars as the cooler stars become undetectable.
Picture credit: Snehalata Sahu)
The AstroSat Picture of the Month for December 2018 presents the Ultra Violet Imaging Telescope itself. A schematic of the design of the UVIT is shown on the top left, and a photograph of the two UVIT telescopes is in the top right. An image of the fully assembled UVIT, containing both telescopes sitting snugly together, wrapped in insulating foil, waiting to be integrated into AstroSat, is shown at the bottom. The Indian Institute of Astrophysics, the Inter-University Centre for Astronomy and Astrophysics, and the Canadian Space Agency developed the UVIT.
Picture credit: ISRO, UVIT Team
The AstroSat Picture of the Month for January 2019 shows the 97 minute long orbit of AstroSat around the Earth. This orbit is roughly equatorial (top right), inclined at around 6 degrees to it (top left). This results in each orbit being slightly displaced from the previous one (bottom).
Picture credit: Leo Jackson John, Operation Director - AstroSat, ISTRAC, ISRO