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SYFY WIRE Bad Astronomy

Eat lasers, Carina Nebula!

By Phil Plait
Four powerful lasers fire from the Very Large Telescope to create artificial guide stars in the sky near the Carina Nebula. Credit: ESO/G. Hüdepohl

Every now and again a photo will cross my path that shows an astronomical object in a new way, something unexpected. I love being surprised, so these images always excite me.

But sometimes the photos are exciting not because of the science in them, but the engineering. And sometimes what's actually excited is... sodium.

So, Carina Nebula, witness the firepower of this fully armed and operational observatory!

Four powerful lasers fire from the Very Large Telescope to create artificial guide stars in the sky near the Carina Nebula. Credit: ESO/G. Hüdepohl

WHOA. And, may I add: pew pew!

Everything about this photo is fantastic. The glowing nebula you see is indeed the Carina Nebula, a sprawling and ridiculously photogenic star forming gas cloud about 6,500 light years away. You can read all about it in an article I wrote that features more dazzling images.

But what are those laser-looking beams sleeting across the sky?

Well, they're laser beams. Yes, seriously.

A big problem with astronomy done from the ground is that you have to look through a hundred kilometers or more of atmosphere over our heads. The air is on constant motion, and sometimes turbulent motion. Light bends a little bit as it moves through air — this is called refraction — and the amount it bends depends on the different in density between one layer of air and the next.

light from spoon bent through glass

As all that air moves around it bends the light this way and that. From the ground, a point source (or in human terms, a dot) like a star looks like it's constantly and rapidly jumping around. When you're taking an image of the star it's doing this dance, so the final product is not a dot but a smeared out disk. Astronomers confusingly call this "seeing" (a very old term that we're stuck with). It's a pain because first of all it blurs the image, lowering the resolution (for example, two stars close together may be blurred into a single smear), but also because it spreads the starlight out, dimming it. That makes faint objects be harder to spot.

One way to avoid this is to launch your telescope into space. This can be prohibitively expensive.

Another way is to use what's called adaptive optics. By carefully measuring how seeing is distorting the star's shape (and assuming it should be a dimensionless dot), you can change the shape of the telescope's mirror to compensate for it.

This is fiercely advanced tech, and it always amazes me that we can do it. Literally, there are actuators (small pistons) on the back side of a thin mirror that can be adjusted hundreds of times per second to deform the mirror's shape, changing it on a scale of microns (millionths of a meter). The incoming starlight is analyzed by computer extremely rapidly, and the actuators are, um, actuated to change the mirror shape such that the light is undistorted, collected up into a small dot. This can dramatically reduce the effect of seeing.

The Carina Nebula seen by the VLT Survey Telescope. Credit: ESO. Acknowledgement: VPHAS+ Consortium/Cambridge Astronomical Survey Unit

The problem is you need a relatively bright star near your target to do this, and that doesn't happen often.

So what's an industrious astronomer to do? Why make their own star, of course.

Enter the lasers. Most lasers used for pointers and such have helium and neon in them, which creates a red beam. Green ones use a more complicated process, but the point is different chemicals produce different colors.

In 2009, engineers were able to create lasers that produce a yellow color at a wavelength of 589 nanometers. This is a big deal! That's because high above the Earth, in the mesosphere roughly 90–100 kilometers above the ground, there's a thin layer of sodium floating around from material that's blown off meteorites as they burn up in our atmosphere. When hit by the laser, these sodium atoms glow brightly. Seen from the ground, a laser beam appears to make a very small dot in the sky.

Aha! So if you have a powerful sodium laser you can make an artificial star anywhere you want! And that's just what's going on with the European Southern Observatory's 4 Laser Guide Star Facility. It can make several laser stars in the sky around an object, allowing a telescopes equipped with adaptive optics to compensate for Earth's roiling ocean of air and make phenomenally sharp images.

The mighty star Eta Carinae (actually a binary star), one of the most massive and luminous stars in the galaxy. Credit: NASA, ESA, N. Smith (University of Arizona, Tucson), and J. Morse (BoldlyGo Institute, New York)

And that's what you're seeing in that incredible photo! You can see the beams themselves as well, since their light scatters off dust and small particles in the air. From the laser placement, it looks to me that they were observing Eta Carinae at the time, a hugely massive and luminous star that will go supernova sometime in the next, oh, million years or so. Probably less.

Using these laser stars, big telescopes on the ground can have resolution that rivals or even exceeds Hubble's. This is fantastic because these telescopes are far larger than Hubble, and can sometimes also have much wider fields of view on the sky. Hubble still has lots of advantages over ground-based 'scopes (it can see in wavelengths of ultraviolet and infrared that get blocked by our air; and it also has a much darker sky background allowing it to see fainter objects), but this technique gives 'scopes on Earth a vast improvement.

Incidentally, the power in these lasers is jaw dropping: They fire at 22 Watts, thousands of times more powerful than your typical laser pointer. I have a 1W laser that can pop balloons from many meters away and set fire to paper, and it terrifies me. These are much stronger. That's why the laser facility is equipped with an automated aircraft detection system that shuts them off if a plane gets in the vicinity. I don't think they could slice a wing off, but of course the biggest danger is blinding the pilots.

I do love to write about fantastic and complex cosmic objects that grace our skies and delight our brains. It's nice, too, to let you know in part how we get those images. Astronomy is an amazingly complex science, from the objects we study to the ways we study them.