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Why stars are ‘disappearing’ from skies?

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A child who was born in a place that had 250 visible stars would only be able to see about 100 stars by the time they turn 18. Astronomers blame it on light pollution.

Stars are disappearing right in front of our eyes at a startling rate and a large number of stars that were once visible to us are no longer visible to us. Astronomers, in a startling discovery, have found that this light pollution is growing at a far more rapid pace than we had expected earlier.

The change is so fast that for a child who was born in a place that had 250 visible stars would only be able to see about 100 stars by the time they turn 18, according to a the analysis from Globe at Night - a citizen science program run by NSF's NOIRLab.

Why stars are ‘disappearing’ from skies?

Artificial light robbing us of stars

Human eye should be able to perceive several thousand stars on a clear, dark night. But the growing brightness at night has robbed people of the night-time starry view. The study finds that about 30 percent of people around the globe and almost 80 percent of those in the United States can no longer view the galaxy.

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The new research sheds light on the problem of 'skyglow' - the diffuse illumination of the night sky that is a form of light pollution.

As the sky brightens up with artificial light, the fainter objects are not visible to the unaided human eye and disappear. According to the new study, over the past ten years, the sky has brightened by 9.6 per cent. This is much more than the around 2 percent per annual global increase measured by satellites.

The problems that come with light pollution are not only limited to astronomy or for those interested in studying stars. The study warns that it also has an impact on human health and wildlife, since it disrupts the cyclical transition from sunlight to starlight that biological systems have evolved alongside.

Stars and their visibility have been entwined in human cultural heritage too. From the myths that were passed over through generations to the various structures built in alignment with celestial bodies, humans and night stars are beyond poetry, science or culture. It encompasses all of this, and much more.

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Satellites aren't enough for starry details

The paper suggests that the existing satellites are not well suited to measure skyglow as it appears to humans, because the satellites are not capable of monitoring wavelengths shorter than 500 nanometres. Meanwhile, shorter wavelengths, such as white LEDs, contribute significantly to the skyglow.

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"Since human eyes are more sensitive to these shorter wavelengths at night-time, LED lights have a strong effect on our perception of sky brightness," said Christopher Kyba, a researcher at the German Research Centre for Geosciences and lead author of the paper detailing these results. "This could be one of the reasons behind the discrepancy between satellite measurements and the sky conditions reported by Globe at Night participants."

Beyond wavelength differences, space-based instruments do not measure light emitted horizontally very well, such as from illuminated signs or windows, but these sources are significant contributors to skyglow as seen from the ground.

More about the study

The authors of the paper analysed more than 50,000 observations submitted to Globe at Night between 2011 and 2022. They focused on data from Europe and North America, since these regions had a sufficient distribution of observations across the land area as well as throughout the decade studied.

The data for this study came from crowd-sourced observations collected from around the world as part of Globe at Night, a program run by NSF's NOIRLab and developed by NRAO astronomer Connie Walker.

Anyone could submit observations through the Globe at Night web application on a desktop or smartphone. After entering the relevant date, time and location, participants were shown a number of star maps. They then recorded which one best matched what they could see in the sky without any telescopes or other instruments.

This gave an estimate of what is called the naked eye limiting magnitude, which is a measure of how bright an object must be in order to be seen. This can be used to estimate the brightness of skyglow, because as the sky brightens, the fainter objects disappear from sight.

The findings of the study were published in an article 'Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022', in the journal 'Science'.

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