Why astronomers make fake stars with huge lasers

For the first time in its history, the Very Large Telescope in Chile is operating with up to seven lasers that fire up into the atmosphere. But why do astronomers need them?

If you happened to be wandering Chile's Atacama Desert in the middle of the night, you might encounter an extraordinary scene: the sight of enormous lasers piercing the sky. From a distance, you'd see them emerging from a sci-fi structure on a mountain top, like the lair of an evil villain.  

Closer up, though, you'd discover the source is actually a telescope – a particularly big one – made up of several buildings. Inside, teams of astronomers on the night shift would be positioning the lasers to create an "artificial star" at the edge of Earth's atmosphere.

If you were passing by this evening, you might catch them firing four lasers from four separate structures on the mountain-top at the same time – which would be a first. 

This is the Very Large Telescope (VLT) at Paranal – operated by the European Southern Observatory (Eso) – which the BBC recently visited. In late 2025, engineers there finished installing the last of a series of powerful new lasers to help astronomers peer into the cosmos with greater fidelity, taking the total number to seven. But why do telescopes need such lasers in the first place? 

The idea for using lasers in astronomy can be traced back to the Cold War, when the US military developed technology to track Soviet satellites. Astronomers eventually realised that similar techniques would help improve their observations from the ground.

Why? Telescopes on Earth's surface face a barrier that space telescopes do not: the atmosphere. The gases clinging to our planet distort starlight before it reaches a ground telescope's mirror. Atmospheric conditions continually change too.  

"We are protected by our atmosphere, which is fantastic. But as an astronomer it is horrible, because the atmosphere basically messes with the sharpness of the light coming from stars and galaxies," says astronomer Amelia Bayo, a project scientist with Eso who advises on how to get the most from the VLT's instruments. "If you don't compensate for the atmosphere, your star is going to be jumping in different places of your detector and you will end up with a blurred image."

To get around this problem, astronomers apply an approach called "adaptive optics". "The implementation is hard, but the idea is super simple," explains Bayo. After the distorted light strikes the telescope's primary mirror, it reflects to another smaller mirror that can deform to essentially match, and correct, the atmospheric distortion. 

In the VLT, one of these adaptive mirrors contains piezoelectric materials – ones which can generate an electric charge – that can change shape at "crazy speeds", says Bayo. The mirror is also suspended inside a magnetic field. "It's stupidly cool. It is basically floating," she adds.

For adaptive optics to work, though, astronomers need a star as a reference point – a fixed point in the sky. And crucially, it needs to be bright. Why? "Imagine that you're listening to a song, and you want to get the melody," explains Bayo. "If the volume is low; if there's more noise, or you only hear high or low pitch, then you may not be able to tell which song it is." It's the same with stars; a bright star provides more information to help correct the atmospheric distortion.

Sometimes it's possible to use natural stars, but when astronomers look at a part of the sky without any suitably bright ones, they turn to lasers instead. 

Firing lasers in the direction they are looking excites a sodium-rich layer in the atmosphere, around 90km (56 miles) above the ground. These sodium atoms will then emit photons that the mirrors back at the telescope can catch. In short, an artificial star. "Our atmosphere is sodium-rich. It offers a canvas where we can put the star anywhere we want," says Bayo.

The lasers now installed at the VLT are state-of-the-art. Bayo remembers the previous generation were difficult to calibrate, requiring toxic liquid dyes, and days of engineering work to ensure stability. "Before it almost looked like you had a laser pointer you were holding in your shaking hand, and now you have a perfect stable device that launches a laser that is way more powerful." Today, the astronomers simply press a button to launch their laser, and it stays fixed in place all night.

After upgrades in 2016, one of the four 8.2m-long (27ft) telescopes of the VLT (Yepun) gained the ability to fire four lasers simultaneously, which allows astronomers to correct atmospheric disturbance over a wider field of view. Each laser delivers 22 watts of power – about 4,000 times the maximum allowed for a laser pointer – in a 30cm-wide (12in) diameter beam.

As of 2026, the four main telescopes of the VLT (Antu, Kueyen, Melipal and Yepun) can now also combine forces – in an act of astronomical Power Rangers – to fire lasers at the same time, assembling into a single, giant "virtual" telescope with a wider field of view. By working together, the four big mirrors in each of those four telescopes can capture details many times sharper than one alone. 

The VLT has been responsible for some of the most significant astronomical discoveries of the 21st Century, including the first image of an extrasolar planet (one that lives outside of our Solar System), and the positions of individual stars moving around the supermassive black hole at the Milky Way's centre.

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One final question you might wonder: what happens if a plane passes over? After all, even the smallest laser pointers pose a risk of dazzling pilots. 

"Planes can of course pass above the observatory, but we have a safety mechanism," explains astronomer Itziar de Gregorio Monsalvo, Eso's representative in Chile. "If the planes cross near where we are observing, automatically the lasers switch off."

So, that means if you wanted to see a very large telescope firing its very large lasers in the Atacama, you'd probably have to approach on the ground. But if you did, you'd encounter a scene rarely encountered in day-to-day life: a telescope lighting up an artificial star on the edge of our planet's atmosphere. 

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