This beautiful, blushing nebula is unique amongst its counterparts. While many of the nebulae visible in the night sky are emission nebulae — clouds of dust and gas that are hot enough to emit their own radiation and light — Caldwell 4, otherwise known as the Iris Nebula or NGC 7023, is a reflection nebula. This means that its color comes from the scattered light of its central star, which lies nestled in the abundant star fields of the constellation Cepheus. Located some 1,400 light-years away from Earth, the Iris Nebula’s glowing gaseous petals stretch roughly 6 light-years across. The Iris Nebula was discovered by German-British astronomer Sir William Herschel on October 18, 1794. (Herschel has an impressive track record, having also discovered the planet Uranus and having been knighted in 1816.) His Iris Nebula has an apparent magnitude of 7. Located near the North Celestial Pole and the North Star (Polaris), Caldwell 4 is nearly impossible to see from the Southern Hemisphere. For best viewing in the Northern Hemisphere, use a telescope in early autumn to star hop from the two brightest stars in the constellation Cepheus. Since the Iris Nebula is faint and diffuse, dark skies away from city lights are recommended for viewing it. This nebula is of particular interest to scientists because of its colors. Reflection nebulae glow because they are made up of extremely tiny particles of solid matter, up to 10 or even 100 times smaller than dust particles on Earth. These particles diffuse the light around them, giving the nebula a second-hand glow that’s typically bluish (like our sky). While the Iris Nebula appears predominantly blue, it includes large filaments of deep red, indicating the presence of an unknown chemical compound likely based on hydrocarbons. Studying nebulae like this one helps astronomers learn more about the ingredients that combine to make stars.
Caldwell 4 also called the Iris Nebula, or NGC 7023
Credit: NASA & ESA
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