21-Jan-2021: AstroSat’s Ultraviolet Imaging Telescope spots rare ultraviolet-bright stars in a massive intriguing cosmic dinosaur in the Milky Way
Astronomers exploring the massive intriguing globular cluster in our Galaxy called NGC 2808 that is said to have at least five generations of stars have spotted rare hot UV-bright stars in it. These stars whose inner core is almost exposed, making them very hot, exist in the late stages of evolution of a Sun-like star. It is not clear how these stars end their lives as not many of them are detected in these fast-evolving phases, making their study crucial.
Motivated by the fact that old globular clusters referred to as dinosaurs of the universe present excellent laboratories where astronomers can understand how stars evolve through various phases between their birth and death, scientists at the Indian Institute of Astrophysics (IIA) an autonomous institute of the Department of Science & Technology, Government of India, looked out for NGC 2808.
With spectacular ultraviolet images of the cluster from Ultraviolet Imaging Telescope (UVIT) onboard India’s first multi-wavelength space satellite, AstroSat, they distinguished the hot UV-bright stars from the relatively cooler red giant and main-sequence stars which appear dim in these images. The findings of this study have been accepted for publication in the journal ‘The Astrophysical Journal’.
The team of scientists comprising Deepthi S. Prabhu, Annapurni Subramaniam and Snehalata Sahu from IIA combined the UVIT data with observations made using other space missions such as the Hubble Space Telescope and the Gaia telescope along with ground-based optical observations. About 34 UV-bright stars were found to be members of the globular cluster. From the data, the team derived the properties of these stars such as their surface temperatures, luminosities and radii.
One of the UV-bright stars was found to be about 3000 times brighter than the Sun with a surface temperature of about 100,000 K. The properties of these stars were then used to place them on what astronomers call the Hertzsprung-Russel (HR) diagram along with theoretical models to throw light on the characteristics of their parent stars and to predict their future evolution. Most of the stars were found to have evolved from a solar stage called the horizontal branch stars with hardly any outer envelope. Thus they were bound to skip the last major phase of life called the asymptotic giant phase and directly become dead remnants or white dwarfs.
Such UV-bright stars are speculated to be the reason for the ultraviolet radiation coming from old stellar systems such as elliptical galaxies which are devoid of young blue stars. Hence, it is all the more important to observe more such stars to understand their properties.
1-Sep-2020: Indian Astronomers discover one of the farthest Star galaxies in the universe.
As a landmark achievement in Space missions, Indian Astronomers have discovered one of the farthest Star galaxies in the universe.
Sharing this information here today, Union Minister of State (Independent Charge), Development of North Eastern Region (DoNER), MoS PMO, Personnel, Public Grievances, Pensions, Atomic Energy and Space, Dr Jitendra Singh said, it is a matter of pride that India’s first Multi-Wavelength Space Observatory "AstroSat" has detected extreme-UV light from a galaxy located 9.3 billion light-years away from Earth. The galaxy called AUDFs01 was discovered by a team of Astronomers led by Dr Kanak Saha from the Inter-University Centre for Astronomy and Astrophysics(IUCAA) Pune.
The importance and uniqueness of this original discovery can be made out from the fact that it has been reported in the leading international journal “Nature Astronomy” published from Britain. India's AstroSat/UVIT was able to achieve this unique feat because the background noise in the UVIT detector is much less than one on the Hubble Space Telescope of US based NASA.
Dr Jitendra Singh has congratulated India’s Space Scientists for once again proving to the world that India’s capability in Space technology has risen to a distinguished level from where our scientists are now offering cues and giving leads to the Space scientists in other parts of the world. According to Professor Shyam Tandon, the excellent spatial resolution and high sensitivity is a tribute to the hard work of the UVIT core team of scientists for over a decade.
According to Director of Inter-University Centre for Astronomy and Astrophysics (IUCAA) Dr Somak Ray Chaudhury, this discovery is a very important clue to how the dark ages of the Universe ended and there was light in the Universe. We need to know when this started, but it has been very hard to find the earliest sources of light, he said.
Pertinent to mention that India’s first Space Observatory AstroSat, which has made this discovery, was launched by the Indian Space Research Organization (ISRO) on September 28, 2015 during the first term of the Modi Government. It was developed by a team led by Shyam Tandon, Ex Emeritus Professor, IUCAA with the full support of ISRO.
7-Mar-2019: AstroSat discovers new group of stars in globular cluster NGC 2808
The Indian multi-wavelength space observatory AstroSat, launched in September 2015, continues to yield exciting results. Using this observatory, astronomers from Thiruvananthapuram and Mumbai have identified a new population of ultraviolet stars in the globular cluster NGC 2808.
Globular clusters are collections of thousands to millions of stars, moving as one unit. These stars are tightly held together by gravity of the cluster itself, and are believed to have formed together at roughly the same time. Some globular clusters could be among the oldest objects in our Milky Way, which hosts over 150 of them.
Stars born, evolve, and then die. However, the timescales for this evolution are unimaginably large. Massive stars evolve faster, over few million years and die in a spectacular fashion. However, stars like our Sun or even less massive ones, evolve slowly over billions of years. Since a globular cluster contains stars with a variety of masses but with similar chemical composition, a snapshot of it could reveal stars of different masses at different stages of their evolution. The Sun will, after 5 billion years, expand to become a red giant star and go through some of these stages.
Stars which are more massive than the Sun will undergo different evolutionary paths, and will be much brighter in the ultraviolet range since they are hotter. Hence globular clusters are good laboratories to test theories of stellar evolution.
NGC 2808 is one of the most massive globular clusters that we know of, and is located at a distance of 47,000 light years from us. This cluster was observed by the team of researchers using the UltraViolet Imaging Telescope (UVIT) on-board AstroSat.
The team was able to identify over 2,000 individual stars in images taken through various ultraviolet filters. Using their brightness in each of the filters, scientists could estimate the temperature of these hot stars that are ultraviolet-bright, which in turn allowed them to segregate them into distinct groups of stars.
Contrary to normal assumption that all stars in such clusters are of the same age, recent studies have shown that many globular clusters may well host more than one population of stars. These seem to differ from each other in their chemical makeup. The origin of this difference is still not well understood, though there is a prevalent theory that explains much of this data. NGC2808 is special since optical observations tell us that it may have at least five different populations of stars.
Using the combination of ultraviolet filters on UVIT, researchers attempted to segregate various groups of hot stars and were able to identify stars in each evolutionary stage, as expected. However, they also found, for the first time, that a class of evolved stars called, Red Horizontal Branch, actually consists of two distinct groups. Since locations of the stars on the sky are known, they could carefully look at how these different classes of stars were located within the cluster. Their analysis points to a disagreement with the widely accepted model of how a cluster acquires multiple populations of stars.
Taking advantage of the superior resolution of UVIT combined with its multiple filters, similar studies of individual stars of other globular clusters can lead astronomers to understand how these stellar populations formed in such clusters.
29-Oct-2018: AstroSat discovers ultraviolet wings of Butterfly Nebula
Indian astrophysicists have discovered large ultraviolet lobes and jets that were hurled out from a dying star, using data from AstroSat – space observatory launched by ISRO in 2015. The discovery has been featured as the AstroSat Picture of the Month (APOM) for October.
Professor Kameswara Rao of the Indian Institute of Astrophysics (IIA) and his collaborators used the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat to stare at a planetary nebula called NGC 6302, popularly known as the Butterfly Nebula. A planetary nebula is formed when a star like our Sun, or a few times heavier, is in its dying days. The term, a misnomer now, was coined by astronomers in the 19th century since the nebula looked like planets through their telescopes.
When hydrogen and helium fuel that kept the star shining gets exhausted, the star expands in size and becomes a red giant star. Such stars shed most of their outer layers which expands outwards, and the inner core, made of carbon and oxygen, shrinks further and becomes hotter. This hot core shines brightly in the ultraviolet, and ionizes the expanding gas. This glowing ionized gas is what is seen as a planetary nebula.
Its central star is one of the hottest that we know, at 220000 degrees. The name itself comes from the shape of the two lobes of expanding gas that look like the wings of a butterfly. One might expect a Planetary Nebula to be spherical, but it actually exhibits a range of complicated structures.
Astronomers have studied the two lobes of the nebula for many years through visible light images. They expect that the more energetic ultraviolet light would be emitted closer to the central star, where the hot stellar wind hits the slowly expanding gas. However the lobes imaged with the F169M filter in ultraviolet were about three times larger than the size of the lobes imaged in visible light. After careful analysis, the study concluded that this ultraviolet emission must be due to cold molecular hydrogen gas outside the visible lobes which had gone undetected so far.
The team led by Prof Rao has recently discovered a large ultraviolet halo in yet another planetary nebula using AstroSat, and will be looking at many more such objects in the future. They hope that such discoveries may provide the answer to the age old puzzle of the 'missing mass problem in planetary nebulae'.
This discovery was made possible because of the uniqueness of UVIT. Of all the ultraviolet telescopes in space, UVIT is special in its ability to image a large field of view with a very high resolution, or detail. This ability, coupled with a novel image analysis software that ISRO had developed led us to this discovery.
The AstroSat Picture of the Month series, or APOM, is a year-old initiative of the Public Outreach and Education Committee (POEC) of the Astronomical Society, as well as the AstroSat Training and Outreach Team. The aim of APOM is to share the excitement of AstroSat science as well as the beauty of the Universe with the public.
29-Jan-2017: ASTROSAT captures 'Vampire star'
India’s first dedicated space observatory 'ASTROSAT' has captured the rare phenomenon of a small six-billion-year-old “vampire” star preying on a bigger celestial body.
Scientists say the smaller star, also called as “blue straggler,” feeds off its companion star by sucking out its mass and energy, causing its eventual death.