Viewers of NBC News on Sept. 22 were treated to some spectacular claims about artificial objects that might have been in orbit around the earth before Sputnik. Dr. Beatriz Villarroel repeated her now-viral claims about 'vanishing stars' - stars that appear on a red-sensitive photographic plate, but not in the blue-sensitive one. As if this were something remarkable.
Dr. V. has circled some 'missing stars' between the Red and Blue plates, which she assumes to be mysterious objects orbiting the earth before Sputnik. Does she expect the objects on the two plates to otherwise be the same? If so, why did astronomers even bother to photograph star images in two different colors?
Frankly, anybody making this claim does not understand why two different color plates are used in the Sky Survey.
It is not to add color or sparkle to the effort. Rather, it is to measure a star's relative brightness in two different colors, which gives us its color index, and hence its temperature and spectral class. Indeed the color of a star is determined by its relative brightness on each plate. That is why two different colors are used. Astronomers want to determine the "Color Temperature" of a individual star, which reveals a lot about the star's makeup. The astronomer measures the apparent brightness of a star in red and blue light. Then the calculation of B-V (blue minus red) brightness tells us what spectral class this star belongs to. The image below illustrates why astronomers took images of star fields using two different color-sensitive plates.
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| Stars have colors,which in turn tells astronomers its temperature (from Slooh). |
A star that is very red (Type M) will shine brightly on the red-sensitive plate, but may not show up at all on the blue plate, while the very blue stars (Types O, B, and A) might not be seen in the red. That's how the sky survey works.

The 18 1/2 inch Alvan Clark refractor at Dearborn Observatory, from 1860.
Way back when I took astronomy classes at Northwestern University, we learned the fine art of using glass plates for astrophotography, using this venerable 19th Century gem, the 18 1/2 inch Alvan Clark refractor at Dearborn Observatory (and yes, this class was taught by J. Allen Hynek). Nobody uses glass plates any longer, today it's all electronic imaging (which of course did not exist almost 60 years ago). Below is an image of the famous Hercules Cluster (M13) I took on a glass plate circa 1969, using the Alvan Clark refractor. What look like scratches on the plate are in fact alien space probes.
So, enough of this nonsense about 'missing stars!!'














