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#017 February 2019
#018 March 2019
#019 May 2019
#020 July 2019
#021 July 2019
Finished....
New population of Ultraviolet stars in the Globular Cluster NGC 2808
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 clusters are 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 stages can 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.
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 clusters are 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 stages can 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.
#018 March 2019
A star relives its youth while dancing with its wizened companion in NGC 5466
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 stars in 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 results described here have been published in this paper .
The paper describing the results is accepted for publication by the Astrophysical Journal 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 stars in 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 results described here have been published in this paper .
The paper describing the results is accepted for publication by the Astrophysical Journal and can be found here.
Accompanying science story is here.
#019 May 2019
Peering into the heart of the Jellyfish Galaxy
This month, we bring back our friend, the Jellyfish galaxy JO201, that we had introduced to you in an earlier APOM, where we described how this galaxy acquires its tentacles-like shape. However, this time, we do not talk about its outer shape but rather study its interior to understand its anatomy. This galaxy was imaged in near and far UV bands from UVIT on AstroSat and the zoomed-in image of the centre is shown in the picture. When we see the the galaxy closely, we notice that the UV emission is bright at the centre, surrounded by a region where UV light is faint. Thereafter, it is again bright in a broken ring-like structure.
Why is there less ultraviolet light from the region just around the centre? By comparing the UV images from AstroSat with images from other telescopes at different wavelengths, astronomers from India and abroad have shown that the ultraviolet light from the central region of this galaxy is related to the working of Active Galactic Nuclei (AGN) at the centre of this galaxy. Most large galaxies are found to have central regions, called AGN, that are very bright at certain wavelengths. They are believed to be very bright because of the presence of supermassive black holes, whose mass could be hundreds of thousands to billions of times the mass of Sun. The light doesn't come from the black-holes themselves but from gas and dust surrounding the black-hole swirling into its recesses. AGN are considered to have important roles to play in the formation of stars in galaxies, and consequently in their appearance and evolution.
In the case of JO201, the UV from regions in the outer ring and beyond is thought to be due to young stars formed in the last 200 million years, unlike the centre which is bright in UV because of the AGN. Therefore, the cavity region surrounding the centre, that has low UV light, has very little star formation occurring. What halted the star formation here? These researchers suggest that energy is released from the AGN into its surroundings, heating the surrounding cold gas clouds and disrupting the process of star formation here. They also show that the effect of stripping of gas giving the jellyfish galaxy its name, is responsible for the lack of young stars in the broken ring structure. This is evident from the fact that both the broken ring structure and tentacles are seen on the left side of the galaxy image. More details of this work can be found in their paper, accepted for publication in the Monthly Notices of Royal Astronomical Society.
The paper describing the results is accepted for publication by the Monthly Notices of the Royal Astronomical Society and can be found here. The accompanying science story, through India Science Wire, is here.
This month, we bring back our friend, the Jellyfish galaxy JO201, that we had introduced to you in an earlier APOM, where we described how this galaxy acquires its tentacles-like shape. However, this time, we do not talk about its outer shape but rather study its interior to understand its anatomy. This galaxy was imaged in near and far UV bands from UVIT on AstroSat and the zoomed-in image of the centre is shown in the picture. When we see the the galaxy closely, we notice that the UV emission is bright at the centre, surrounded by a region where UV light is faint. Thereafter, it is again bright in a broken ring-like structure.
Why is there less ultraviolet light from the region just around the centre? By comparing the UV images from AstroSat with images from other telescopes at different wavelengths, astronomers from India and abroad have shown that the ultraviolet light from the central region of this galaxy is related to the working of Active Galactic Nuclei (AGN) at the centre of this galaxy. Most large galaxies are found to have central regions, called AGN, that are very bright at certain wavelengths. They are believed to be very bright because of the presence of supermassive black holes, whose mass could be hundreds of thousands to billions of times the mass of Sun. The light doesn't come from the black-holes themselves but from gas and dust surrounding the black-hole swirling into its recesses. AGN are considered to have important roles to play in the formation of stars in galaxies, and consequently in their appearance and evolution.
In the case of JO201, the UV from regions in the outer ring and beyond is thought to be due to young stars formed in the last 200 million years, unlike the centre which is bright in UV because of the AGN. Therefore, the cavity region surrounding the centre, that has low UV light, has very little star formation occurring. What halted the star formation here? These researchers suggest that energy is released from the AGN into its surroundings, heating the surrounding cold gas clouds and disrupting the process of star formation here. They also show that the effect of stripping of gas giving the jellyfish galaxy its name, is responsible for the lack of young stars in the broken ring structure. This is evident from the fact that both the broken ring structure and tentacles are seen on the left side of the galaxy image. More details of this work can be found in their paper, accepted for publication in the Monthly Notices of Royal Astronomical Society.
The paper describing the results is accepted for publication by the Monthly Notices of the Royal Astronomical Society and can be found here. The accompanying science story, through India Science Wire, is here.
#020 July 2019
Star formation around bubbles in the Dwarf galaxy IC2574
In the APOM this month, we bring you the dwarf galaxy IC2574. As the name suggests, dwarf galaxies are smaller, fainter and less massive than galaxies such as the Milky Way or Andromeda. While dwarf galaxies host hundreds of millions to a few billion stars, the larger galaxies can have nearly hundred to thousand times more number of stars. It has been found that these small dwarf galaxies are more in number as compared to larger galaxies like Milky Way in the Universe.
IC2574 is also known as Coddington's nebula, after Edwin Foster Coddington who discovered it in 1898. It is located at a distance of approximately 12 million light years away and can be found in the Ursa Major constellation in the north. It is irregular in shape like the Wolf-Lundmark-Melotte (WLM) galaxy, presented in our very first APOM. IC2574 was imaged in ultraviolet by Chayan Mondal and collaborators using the Ultraviolet Imaging Telescope (UVIT) on-board AstroSat. They wanted to study the regions where young massive stars in this galaxy are located. Hot massive stars are those stars which weigh more than 10 times the mass of Sun and they emit significant fraction of their light in ultraviolet.
These researchers have identified sites of most massive stars in IC2574 using the ultraviolet images. They find that about one-third of them could have been formed as a result of previous supernova explosions. When massive stars die in supernova explosions, the outer layers are blown outwards into the surrounding gas, and form spherical shells. These expanding bubbles compress the gas between them which could initiate star formation in the rims of the bubble structures, leading to a new generation of stars. The authors suggest that in IC2574, one of the mechanisms to form young stars is gas compression following shell expansion. Unlike spiral galaxies like Milky Way, where stars are formed mostly in the spiral arms, it is possible that formation of later generations of stars in dwarf galaxies could be because of these bubble structures.
More details of this work can be found in their paper, which has been submitted to the Astrophysical Journal.
In the APOM this month, we bring you the dwarf galaxy IC2574. As the name suggests, dwarf galaxies are smaller, fainter and less massive than galaxies such as the Milky Way or Andromeda. While dwarf galaxies host hundreds of millions to a few billion stars, the larger galaxies can have nearly hundred to thousand times more number of stars. It has been found that these small dwarf galaxies are more in number as compared to larger galaxies like Milky Way in the Universe.
IC2574 is also known as Coddington's nebula, after Edwin Foster Coddington who discovered it in 1898. It is located at a distance of approximately 12 million light years away and can be found in the Ursa Major constellation in the north. It is irregular in shape like the Wolf-Lundmark-Melotte (WLM) galaxy, presented in our very first APOM. IC2574 was imaged in ultraviolet by Chayan Mondal and collaborators using the Ultraviolet Imaging Telescope (UVIT) on-board AstroSat. They wanted to study the regions where young massive stars in this galaxy are located. Hot massive stars are those stars which weigh more than 10 times the mass of Sun and they emit significant fraction of their light in ultraviolet.
These researchers have identified sites of most massive stars in IC2574 using the ultraviolet images. They find that about one-third of them could have been formed as a result of previous supernova explosions. When massive stars die in supernova explosions, the outer layers are blown outwards into the surrounding gas, and form spherical shells. These expanding bubbles compress the gas between them which could initiate star formation in the rims of the bubble structures, leading to a new generation of stars. The authors suggest that in IC2574, one of the mechanisms to form young stars is gas compression following shell expansion. Unlike spiral galaxies like Milky Way, where stars are formed mostly in the spiral arms, it is possible that formation of later generations of stars in dwarf galaxies could be because of these bubble structures.
More details of this work can be found in their paper, which has been submitted to the Astrophysical Journal.
#021 July 2019
A star feasts on its companion in the King Cobra Cluster
This month, we bring you the ultraviolet image of M67 or the King Cobra cluster which is an open cluster of stars. Located roughly 2800 light years away in the constellation of Cancer, M67 is one of the oldest star clusters that we know of, and is about the same age as our Sun (about 4.5 billion years). Given its closeness to us, it is one of the best studied clusters as well. It has around 500 stars at present, with many of them being very similar to our Sun in their properties. As can be guessed from its membership in the Messier Catalogue, it can be seen visually using a small telescope.
We had talked a lot about a class of stars called Blue Straggler Stars in a previous APOM. These appear bluer than the rest of the stars in the cluster, and hence masquerade as being younger than these other stars. We think of stars as huge balls of gas living a solitary life. However, this puzzle of the apparent youth of BSS was resolved by astronomers when they realized that the evolution of these stars was intricately linked with a companion star. These Blue Stragglers get rejuvenated by either merging with another star or, more commonly, swallowing up most of the matter of its companion. This unfortunate companion, having lost most of its matter, usually evolves into a small hot White Dwarf. Such BSS-White Dwarf pairs have now be detected in a few clusters. Here the authors report the first detection of such a BSS-white dwarf pair, where the white dwarf is of a rare type.
M67 is known to host 14 Blue Straggler Stars. Astronomers knew that one of them, WOCS1007, has a low mass companion, due to its gravitational effect on the motion of the BSS. By using the ultraviolet brightness measurements of this star in the AstroSat image, a team of astronomers led by N. Sindhu were able to confirm that this companion to the BSS is indeed a White Dwarf that had donated a large part of its mass to the BSS in the past. By combining AstroSat data with those from other telescopes, they modelled how the brightness of the star changes from ultraviolet to visible to infrared wavelengths. They noticed that the star is excessively bright in ultraviolet, which had to be due to a low mass White Dwarf, which is about 20% the mass of our Sun, and about 10% its size, but as hot as 13500 degrees. Stars like our Sun end their life as a White Dwarf which are more massive that this. Such low mass White Dwarfs are rare to find, and could have been formed only because a part of their matter has been sucked out by a companion star that is very close to it. This other star, regaining its youth at the expense of our low mass White Dwarf, is the Blue Straggler we see today!
More details of this work can be found in their paper, which has been accepted for publication in the Astrophysical Journal.
This month, we bring you the ultraviolet image of M67 or the King Cobra cluster which is an open cluster of stars. Located roughly 2800 light years away in the constellation of Cancer, M67 is one of the oldest star clusters that we know of, and is about the same age as our Sun (about 4.5 billion years). Given its closeness to us, it is one of the best studied clusters as well. It has around 500 stars at present, with many of them being very similar to our Sun in their properties. As can be guessed from its membership in the Messier Catalogue, it can be seen visually using a small telescope.
We had talked a lot about a class of stars called Blue Straggler Stars in a previous APOM. These appear bluer than the rest of the stars in the cluster, and hence masquerade as being younger than these other stars. We think of stars as huge balls of gas living a solitary life. However, this puzzle of the apparent youth of BSS was resolved by astronomers when they realized that the evolution of these stars was intricately linked with a companion star. These Blue Stragglers get rejuvenated by either merging with another star or, more commonly, swallowing up most of the matter of its companion. This unfortunate companion, having lost most of its matter, usually evolves into a small hot White Dwarf. Such BSS-White Dwarf pairs have now be detected in a few clusters. Here the authors report the first detection of such a BSS-white dwarf pair, where the white dwarf is of a rare type.
M67 is known to host 14 Blue Straggler Stars. Astronomers knew that one of them, WOCS1007, has a low mass companion, due to its gravitational effect on the motion of the BSS. By using the ultraviolet brightness measurements of this star in the AstroSat image, a team of astronomers led by N. Sindhu were able to confirm that this companion to the BSS is indeed a White Dwarf that had donated a large part of its mass to the BSS in the past. By combining AstroSat data with those from other telescopes, they modelled how the brightness of the star changes from ultraviolet to visible to infrared wavelengths. They noticed that the star is excessively bright in ultraviolet, which had to be due to a low mass White Dwarf, which is about 20% the mass of our Sun, and about 10% its size, but as hot as 13500 degrees. Stars like our Sun end their life as a White Dwarf which are more massive that this. Such low mass White Dwarfs are rare to find, and could have been formed only because a part of their matter has been sucked out by a companion star that is very close to it. This other star, regaining its youth at the expense of our low mass White Dwarf, is the Blue Straggler we see today!
More details of this work can be found in their paper, which has been accepted for publication in the Astrophysical Journal.
Finished....
