astronomers struggle to keep their heads in the stars

From Van Gogh’s paintings to Lamartine’s poems and Star Wars films, the stars have always lit up our collective imagination. But light pollution interferes with their observation. A study published Thursday, January 19, 2023 in the journal Science estimates that the number of stars observable with the naked eye could be halved in less than twenty years in some places.

Light pollution is progressing, the number of visible stars is falling

The team of four researchers behind this article relied on observations made by just over 51,000 “citizen researchers” in North America and Europe between 2011 and 2022. These contributions have enabled scientists to to establish the following observation: “a place where 250 stars are visible would see this number reduced to 100 visible stars” in the space of eighteen years.

The reason ? The increase in light pollution, caused by human lighting. According to the study, the brightness of the sky increases by 7 to 10% per year. “The more artificial light we have emitted into the atmosphere, the more we make the sky greyish and the more we reduce the contrast with faintly bright objects, which are then difficult to distinguish from the background of the sky”, explains Samuel Challéat, researcher at the CNRS and coordinator of the Night Environment Observatory.

The increase in light pollution is greater than estimated by previous studies. The reason: the latter were based on satellite observations. However, they are not sensitive to the wavelengths of the LEDs and to the lights emitted horizontally. Hence the conclusion of the study: “The use of naked eye observation by citizen researchers provides additional information to satellite data. »

Observatories flee inhabited areas

According to these new results, the number of stars observable with the naked eye could be halved in less than twenty years in some places. What harms scientific research? “This inevitably poses a problem, explains Florence Durret, astronomer emeritus at the Institute of Astrophysics in Paris. As professionals, we must go to sites where there is as little pollution as possible. The observation must have moved to more distant places, such as the Andes or Namibia.

The Paris Observatory was surrounded by fields when it was built in 1667: “Today, it no longer allows you to see much, except perhaps Jupiter”, says Florence Durret. In France, only a few observatories, such as that of the Pic du Midi de Bigorre (Hautes-Pyrénées), are still used by researchers. Astronomical research also benefits from technological innovations, such as the James-Webb telescope, which observes the stars directly from space.

Researchers also point to another form of light pollution: that of satellites, whose trails alter scientific photographs. In particular, the satellites of Starlink, the telecommunications company of Elon Musk, are targeted. They will be 12,000 in orbit by 2025.

For amateurs, “it’s becoming more and more an impossible mission”

Amateur astronomers are the first to suffer from the proliferation of light pollution. Some telescope filters can limit its effects. But they are ineffective when the brightness of the sky is too high. “We have to move away from big cities, and even villages, regrets Corine Yahia, president of the Astronomie Gironde 33 association. Last night, I even had trouble seeing the tail of the Big Dipper because it was drowned in the lights of Bordeaux. »

“Amateur astronomers are used to saying that their first observation instrument is their car, not their telescope,” jokes Samuel Challéat. They have to travel long distances to find sufficiently dark skies. “A few years ago, we could still go to the Saclay plateau to have reasonable observation conditions, but today it is no longer possible”, notes Didier Schneider, member of the astronomy club of Boulogne- Billancourt (in the Parisian suburbs). “You have to go to the Perche (about a hundred kilometers from Paris), or even further, in the Baronnies (in the South-East); it is becoming more and more an impossible mission, ”he adds.

Beyond astronomy enthusiasts, light pollution impacts anyone who sometimes likes to lift their head to admire starry summer skies. “Previously, when people went out at night, they were somehow confronted with the cosmos, says Christopher Kyba, physicist at the GFZ Center in Potsdam, Germany and one of the authors of the Science study, quoted by AFP. And now it’s like it’s become an unusual event. »

Some children grew up in the city without ever having seen completely starry skies: “There is an extinction of the experience of nature which is part of a generational environmental amnesia”, notes Samuel Challéat. With increasingly bright nights, we will have fewer and fewer stars in our eyes.

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