Caldwell 20

While the spiral feature in the upper portion of this Hubble image may resemble the arm of a galaxy or the heart of a cosmic storm, it is actually a small portion of Caldwell 20 (NGC 7000), or the North America Nebula. Discovered by William Herschel in 1786 and nicknamed for its resemblance to the continent of North America, Caldwell 20 is located roughly 1,800 light-years from Earth and occupies a space in the constellation Cygnus that appears more than three times larger than the full moon. However, despite an apparent magnitude of 5, the nebula does not appear very bright against the starry Milky Way. Binoculars or a small telescope will reveal it as a subtle brightening against its galactic backdrop. Observers in the Northern Hemisphere should look for Caldwell 20 in autumn, and Southern Hemisphere observers will have their best view in spring.

Caldwell 20 is a stunning example of an emission nebula. The clouds of gas that make up the nebula are being ionized by a nearby star, causing the gas to glow as it emits energy. Colors emitted by emission nebulae depend on the chemical composition of the region. The reddish color that is characteristic of hydrogen and dominates Caldwell 20 can be picked up by sensitive cameras.

This image of Caldwell 20, taken with Hubble’s Wide Field and Planetary Camera 2, is a part of a survey of T Tauri stars in nearby star-forming regions. T Tauri stars are young stars that vary in brightness over time. One reason this kind of star is important to astronomers is because a large portion of T Tauri stars have been found to host circumstellar disks. These disks of gas and dust may one day become fully fledged solar systems.

Apparent Magnitude - The brightness of an astronomical object as seen from Earth, influenced by the object's distance from Earth, its absolute magnitude, and even gas and dust that lie between the object and Earth.

Emission Nebula - A cloud of ionized gas that emits energy at wavelengths across the electromagnetic spectrum, causing it to glow with vibrant colors.

Nebula - An interstellar cloud of dust and gas; either a location where new stars are being forged or a cloud of material ejected into space by a dying star.

T Tauri Star - A young star that has not yet reached temperatures high enough to ignite nuclear fusion, and exhibits variations in its brightness.