Gazing outward, growing wiser

Lynn Savage, lynn.savage@photonics.com

In December, NASA’s Wide-field Infrared Survey Explorer (WISE) celebrated one full year of exploring the universe using four different bands of IR detection. The image here represents just a small fraction of the sky catalogued by the satellite during its operational run.

The image shows three nebulae that are part of the Orion molecular cloud: the Flame nebula, the Horsehead nebula and NGC 2023. The Flame nebula appears to be kindled by the bright blue star to the right of the central cloud. This star, dubbed Alnitak, is the easternmost star in the belt of the constellation Orion. The radiance captured by the satellite’s imagers originates from dust that has been heated by Alnitak.

Image courtesy of NASA.

NGC 2023 can be seen as a bright circle in the lower part of the image. It is a reflection nebula, meaning that the dust is reflecting the visible light of nearby stars. WISE, however, sees the IR glow of the warmed dust itself.

Between NGC 2023 and the Flame, the famous Horsehead nebula appears as a ghostly bump on the lower right of the vertical dust ridge. In visible wavelengths, the Horsehead is easily discerned; again, the IR detectors aboard WISE peer into the cloud to see the glow of the interstellar dust.

WISE incorporates four sensors, each with 1024 x 1024 pixels, recording near-IR bands at 3.4 and 4.6 μm and mid-IR bands at 12 and 22 µm. The near-IR sensors are composed of HgCdTe; the mid-IR ones, Si:As. Blue represents light emitted at 3.4 μm, which comes mainly from hot stars. Relatively cooler objects, such as the dust of the nebulae, are shown as green (4.6 μm) and red (12 μm). The image was made from data collected after WISE (as expected) began to run out of its supply of solid hydrogen cryogen almost nine months into its mission. From August to October 2010, while the cryogen was depleting, the satellite had only three detectors operational, and the 12-μm detector was less sensitive. This turned out to be a good thing, because the less sensitive detector reduced the glare of the Flame portion of the nebula enough to bring out the details in the rest of the image.