AstroSat Picture of the Month (November 2017)
NGC 1851: Two star families in one Globular Cluster
Near-UV (left) and Far-UV (right) images of the Globular Cluster NGC 1851, taken by UVIT onboard AstroSat.
The FUV image shows only the hottest stars in the cluster. All colours are artificial.
Image credit: Annapurni Subramaniam et al.
NGC 1851 is a Globular Cluster which is almost 40000 light years away from us, in the southern constellation of Columba, near Canis Major. A Globular Cluster is a group of hundreds of thousands of stars tightly bound together by their own gravity. All the stars in these spherical clusters orbit around the centre of our galaxy together. NGC 1851, or Caldwell 73, is one such cluster, visible in a moderate telescope at a magnitude of 7.3, with a size that is a third of the full moon. It was discovered by James Dunlop from Australia in 1826.
The stars in a Globular Cluster are usually born together, and hence share similar properties. However, NGC 1851 is one of the few clusters where two distinct types of stars with different properties seem to co-exist! Many individual stars in this object have been studied before with the Hubble Space Telescope (HST), but good ultraviolet images were needed to understand this mystery better. This prompted a group of 18 astronomers, including 12 from India, to use the UVIT on board the AstroSat. They imaged this cluster in the Near and Far ultraviolet wavebands far better than earlier attempts with other telescopes.
The superior resolution of AstroSat allowed them, for the first time, to measure the ultraviolet properties of individual stars in the inner crowded region of the cluster. Using this data, they could show that NGC 1851 does indeed have two distinct families of stars within it, which still retain their separate histories. This tells us that NGC 1851 was probably formed when two smaller clusters merged together some time in the past!
The paper describing these results can be downloaded from
https://arxiv.org/abs/1710.03730
AstroSat Picture of the Month (December 2017)
NGC 40: A Planetary Nebula with an Ultra-Violet Halo
UV image of NGC 40 taken by UVIT onboard AstroSat.
The red gas cloud is the Bow-Tie nebula, which is being illuminated by the central hot star.The gold coloured diffuse light surrounding the nebula is the newly discovered far ultra-violet halo.
Image credit: Kameswara Rao et.al.
NGC 40, or the Bow-Tie Nebula, is a Planetary Nebula about 3500 light years away from us, in the northern constellation of Cepheus. Discovered by William Herschel in 1788, it can be seen in a moderate sized telescope by amateur astronomers. Earlier optical images of NGC 40 show a central star as hot as about 70000 K surrounded by expanding gas that gives it its characteristic shape. The central hot star is blowing a fast hot wind into this surrounding gas at 1700 km/s, and heating it up.
Kameswara Rao and his colleagues used the AstroSat to image this object in many regions of the ultra-violet. First, however, let us look at what a planetary nebula is. Some old Red Giant stars throw out their outer layers of gas, which expands away from the star. This exposes the hot inner part of the star, whose radiation makes the outer gas layers shine brightly as a planetary nebula. Our Sun too, will meet this same fate. With time, the inner star will evolve to become a White Dwarf, a very strange object indeed. And the heavier elements cooked inside the stars, thrown out into space, go back into forming newer stars and planets like ours.
The astronomers who looked at the ultra-violet images of NGC 40 using AstroSat, were not only able to study the central hot star and the surrounding gas, but also made a new discovery. As the image shows, they discovered, for the first time, a large halo of ultra-violet radiation in the far UV band surrounding the entire nebula. This halo,they figure out, is due to molecules energized due to the light from the central star.
The paper describing these results can be downloaded from
https://arxiv.org/abs/1711.07698
NASA Study: First Direct Proof of Ozone Hole Recovery Due to Chemicals Ban
For the first time, scientists have shown through direct satellite observations of the ozone hole that levels of ozone-destroying chlorine are declining, resulting in less ozone depletion.
Using measurements from NASA's Aura satellite, scientists studied chlorine within the Antarctic ozone hole over the last several years, watching as the amount slowly decreased.
Credits: NASA's Goddard Space Flight Center/Katy Mersmann
Please download more visuals at NASA's Scientific Visualization Studio
Measurements show that the decline in chlorine, resulting from an international ban on chlorine-containing manmade chemicals called chlorofluorocarbons (CFCs), has resulted in about 20 percent less ozone depletion during the Antarctic winter than there was in 2005 — the first year that measurements of chlorine and ozone during the Antarctic winter were made by NASA’s Aura satellite.
“We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it,” said lead author Susan Strahan, an atmospheric scientist from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
CFCs are long-lived chemical compounds that eventually rise into the stratosphere, where they are broken apart by the Sun’s ultraviolet radiation, releasing chlorine atoms that go on to destroy ozone molecules. Stratospheric ozone protects life on the planet by absorbing potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and damage plant life.
Two years after the discovery of the Antarctic ozone hole in 1985, nations of the world signed the Montreal Protocol on Substances that Deplete the Ozone Layer, which regulated ozone-depleting compounds. Later amendments to the Montreal Protocol completely phased out production of CFCs.
Past studies have used statistical analyses of changes in the ozone hole’s size to argue that ozone depletion is decreasing. This study is the first to use measurements of the chemical composition inside the ozone hole to confirm that not only is ozone depletion decreasing, but that the decrease is caused by the decline in CFCs.
The study was published Jan. 4 in the journal Geophysical Research Letters.
The Antarctic ozone hole forms during September in the Southern Hemisphere’s winter as the returning sun’s rays catalyze ozone destruction cycles involving chlorine and bromine that come primarily from CFCs. To determine how ozone and other chemicals have changed year to year, scientists used data from the Microwave Limb Sounder (MLS) aboard the Aura satellite, which has been making measurements continuously around the globe since mid-2004. While many satellite instruments require sunlight to measure atmospheric trace gases, MLS measures microwave emissions and, as a result, can measure trace gases over Antarctica during the key time of year: the dark southern winter, when the stratospheric weather is quiet and temperatures are low and stable.
The change in ozone levels above Antarctica from the beginning to the end of southern winter — early July to mid-September — was computed daily from MLS measurements every year from 2005 to 2016. “During this period, Antarctic temperatures are always very low, so the rate of ozone destruction depends mostly on how much chlorine there is,” Strahan said. “This is when we want to measure ozone loss.”
They found that ozone loss is decreasing, but they needed to know whether a decrease in CFCs was responsible. When ozone destruction is ongoing, chlorine is found in many molecular forms, most of which are not measured. But after chlorine has destroyed nearly all the available ozone, it reacts instead with methane to form hydrochloric acid, a gas measured by MLS. “By around mid-October, all the chlorine compounds are conveniently converted into one gas, so by measuring hydrochloric acid we have a good measurement of the total chlorine,” Strahan said.
Nitrous oxide is a long-lived gas that behaves just like CFCs in much of the stratosphere. The CFCs are declining at the surface but nitrous oxide is not. If CFCs in the stratosphere are decreasing, then over time, less chlorine should be measured for a given value of nitrous oxide. By comparing MLS measurements of hydrochloric acid and nitrous oxide each year, they determined that the total chlorine levels were declining on average by about 0.8 percent annually.
A view of Earth's atmosphere from space.
Credits: NASA
The 20 percent decrease in ozone depletion during the winter months from 2005 to 2016 as determined from MLS ozone measurements was expected. “This is very close to what our model predicts we should see for this amount of chlorine decline,” Strahan said. “This gives us confidence that the decrease in ozone depletion through mid-September shown by MLS data is due to declining levels of chlorine coming from CFCs. But we’re not yet seeing a clear decrease in the size of the ozone hole because that’s controlled mainly by temperature after mid-September, which varies a lot from year to year.”
Looking forward, the Antarctic ozone hole should continue to recover gradually as CFCs leave the atmosphere, but complete recovery will take decades. “CFCs have lifetimes from 50 to 100 years, so they linger in the atmosphere for a very long time,” said Anne Douglass, a fellow atmospheric scientist at Goddard and the study’s co-author. “As far as the ozone hole being gone, we’re looking at 2060 or 2080. And even then there might still be a small hole.”
To read the study, visit:
http://onlinelibrary.wiley.com/doi/10.1002/2017GL074830/abstract
By Samson Reiny
NASA’s Earth Science News Team
Last Updated: Jan. 4, 2018
NASA Sees Definitive Evidence of the Montreal Protocol's Success
