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Scientists use lasers to create the brightest artificial star in the sky

Late last month, astronomers with the ESO’s Very Large Telescope began operation of a new tool called the Four Laser Guide Star Facility (4LGSF). This incredible new laser rig will create the brightest artificial star ever seen, and use it to produce some of the most incredible imagery we’ve yet to collect about the heavens. But what are guide stars, and why does the Very Large Telescope need one so badly?

Astronomers have incredible tools at their disposal these days, telescopes and radio dishes and gravitational wave detectors that can observe the universe in incredible detail. But there’s a problem with all that power — you need signals that are good enough to fulfill all that potential of signal collection. Look up at the sky with the naked eye, and you won’t get thrown off by the constant motion of the stars; look at the same sky through the lens of a high-powered telescope, and the rotation of the Earth will cause your tiny field of view to slide off your target every few seconds. When you can distinguish between two points just a tiny fraction of an arcsecond apart, and when you have to to fulfill your experimental goals, tiny distortions in your signals can completely ruin the validity of your observation.

To orient themselves in space, and to calibrate their incredible machinery to offset subtle optical effects in the atmosphere, astronomers have had to turn to guide stars. These are bright, easily identifiable points of light that allow astronomers to find a target area and adjust the telescope for the best possible reading. Unfortunately, only a tiny minority of the sky lines up closely enough with a guide-capable star for this to work. Without such a guide, they cannot employ a method called adaptive optics, which allows them to compensate for tiny irregularities in the path of light through the atmosphere.

So, astronomers had only one choice: If nature wouldn’t give them the guide they needed, they would simply have to make those guides themselves. For adaptive optics, all they really need is a known, reliable light source coming at them through the atmosphere, so they can see how that atmosphere is affecting light. First proposed in the 1950s, a laser guide star is a fake light source created in the upper atmosphere that provides these reference light beams.

Facilities like the 4LGSF use lasers to excite atoms in the upper atmosphere and cause them to emit light — at the Very Large Telescope, the ESO’s new toy will user four 22-Watt lasers to excite sodium atoms, causing a very specific, characteristic glow brighter than any other man-made point in the sky. The light from this fake star will filter down to the telescope, just like the light from a real one. Though the guide light source is infinitely closer to the telescope than the target source it is associated with, it can still serve for most of astronomers’ purposes.

Now, real guide stars are often still required to account for a couple of final optical variables. But since only a few such variables still to be need collected, astronomers can pull from a much, much wider array of real stars, fainter than ever before. Opening the guide book to a dimmer array of stars greatly increases the number of targets where adaptive optics can be used — and that means a greater selection of targets astronomers can image with astounding visual fidelity.

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