Why Do The Stars Seem To Grow At Longer Wavelengths?

The images you see in Disk Detective generally seem to get bigger and blurrier at longer wavelengths.  You might be wondering: why does this happen?   Or you might be wondering: even when you are looking at a single tiny, tiny star, the image you see in Disk Detective is never a perfect single tiny, tiny dot on your screen.  Why is that?

The answer to these questions has to do with what happens to the light from a star on its way to the detector to make the image that you see.  Before it can land on the detector, the light from a star (or galaxy or asteroid, etc.) first hits the Earth’s atmosphere, then the telescope mirrors.  When it hits those objects, the starlight gets altered in ways that affect the final image.

When the starlight hits the atmosphere, the atmosphere tends to scatter the starlight in different directions.  It’s a bit like looking into a swimming pool full of water; if there are lines on the bottom of the pool, they end up looking a little wiggly even when they are actually straight because of the waves on the surface of the pool.  Waves in between layers of the Earth’s atmosphere do the same thing to the light from our stars. This subject clearly shows the effects of atmospheric distortion–affecting multiple stars in the same way.

When the starlight hits the telescope, it scatters some more, and also diffracts. The diffraction occurs when the edge of the telescope blocks some of the incoming wave of light, and so the rest of the light wave tends to bend around that edge.  That process also affects the final image we see. Both diffraction and scattering get more severe at longer wavelengths of light. That’s because as the wavelength gets longer, the light behaves more and more like a wave (bending and dancing), and less like a particle (direct, like a bullet).

Diffraction

Diffraction

Now, the images you see in each flipbook on Disk Detective come from three, sometimes four different telescopes.  The WISE telescope is in space, so the images are blurred mainly by diffraction and scattering within the telescope. The other telescopes on the ground (DSS, 2MASS and Sloan), so the images are blurred by diffraction and scattering inside each telescope—and also scattered by the Earth’s atmosphere. As a result, each telescope yields a different angular resolution as a function of wavelength.

Angular resolution is another way of referring to the size of the smallest image a telescope can produce given diffraction and scattering—the size of a tiny, tiny star as seen by this telescope.  Here are the angular resolutions of some of the telescopes whose data you’ve been looking at:

  • DSS at 0.4 microns: 1-2 arcseconds
  • 2MASS at 1.6 microns:  2-3 arcseconds
  • WISE at 3.4 microns (W1):  6.1 arcseconds
  • WISE at 22 microns (W4): 12 arcseconds

When it comes to angular resolution, smaller numbers are better. The worst angular resolution in DiskDetective comes from WISE at 22 microns.  That’s because light at this long wavelength scatters and diffracts very readily. Also, diffraction is worse in small telescopes, and WISE is the smallest telescope of the bunch.  Here are the diameters of the primary mirrors (the big mirror that the light hits first) in the telescopes we’re using.

  • Palomar Oschin Schmidt Camera (DSS):  1.2 meters
  • 2MASS: 1.3 meters
  • WISE: 0.4 meters

Now, diffraction and scattering are not the only phenomena that affect how a perfect star looks in the Disk Detective images.  Sometimes, as we discussed a few days ago, the stars are so bright that they saturate the detector, and appear bigger than the angular resolution of the telescope would suggest.  Photon noise and detector noise are also important, especially in 2MASS data.  We’ll come back to that in another article!

So remember: some of the objects in DiskDetective may indeed be bigger at longer wavelengths.  But every object in DiskDetective will tend to look bigger at longer wavelengths anyway because of how diffraction and scattering cause the images to blur.

“They say that genius is an infinite capacity for taking pains. It’s a very bad definition, but it does apply to detective work.”

Marc Kuchner

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