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Episode 20: A Horse of a Different Color

5 Sep 2013, 04:00 UTC
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A Horse of a Different Color

The Horsehead Nebula is a striking, dark gas cloud just below Orion's belt. It is a favorite of both professional and amateur astronomers. However, as a dark nebula, most of its true structure is hidden from visible light observations. To celebrate the 23rd anniversary of the Hubble Space Telescope, we revealed the considerable detail of that unseen nebular structure via an infrared portrait. The result is even more striking, and something one just doesn't see very often: a veritable astronomical horse of a different color.


Hubble press release:

Hubble Sees a Horsehead of a Different Color



When I introduce the image of the Orion constellation, including the Hydrogen Alpha filter, I mistakenly imply that the image is only in Hydrogen Alpha. It is not. It is an image that includes the standard visible light plus the Hydrogen Alpha light to augment it. Astronomical images are often taken repeatedly through a series of filters which isolate different wavelength regions of light. Hubble's images are always done one filter at a time and then added together as needed with image processing software. In fact, everyday digital cameras that take "color" pictures are really processing the red, green, and blue light separately before combining them into a color image. Similarly, computer monitors display separate tiny red, green, and blue dots that your eye sees as a single color. Filters and color science are not just for geeks, they are part of everyday life.

Hydrogen Alpha is a friendlier term for the wavelength of light emitted when the electron in a hydrogen atom drops from the third to the second orbital level. Its wavelength is 656.28 nanometers and it provides a characteristic pinkish glow. Because hydrogen is by far the most abundant element in the universe, and hydrogen alpha is the dominant hydrogen emission in visible light, pink is the standard color of nebulae. By using filters, we can select out and showcase other colors, but the human eye would see most nebulae as pink.

The VISTA telescope is an acronym for the "Visible and Infrared Survey Telescope for Astronomy." It is run by the European Southern Obsevatory and located at Cerro Paranal in Chile. As a wide-field telescope, it can cover much larger swaths of the sky than Hubble and obtain complete astronomical portraits. By contrast, Hubble just studies the details, albeit with much higher resolution. A beautiful collection of VISTA images shows off its diverse capabilities and includes several nice visible vs. infrared comparisons.

The flight into the Horsehead Nebula video shown in this episode is the second half of a two-shot sequence. The first part is a visible light zoom into the Horsehead region on the sky and a cross-fade from visible to infrared. The full zoom and fly sequence can be downloaded from this HubbleSite page. Indeed, it was the joining of the two shots as a seamless sequence that motivated us to start the fly-in shot as a compressed, flat model and expand it to an extended, sculpted model during the first five seconds.

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