Westerhout 40

W 40

Young stars in W40 are revealed in the X-ray (blue), while infrared images (red and green) show both stars and gas.
Object type	HII region, star cluster, nebula
Other designations	W40, Sh2-64, RCW 174, "Aquila" star-forming region
Observation data (Epoch J2000)
Constellation	Serpens Cauda
Right ascension	18 31 29
Declination	-02 05.4
Distance	1420±30 ly / 436±9 pc
In visual light (V)
Size	8 arcminutes
Estimated age	0.8–1.5 Myr
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Westerhout 40 or W40 (also designated Sharpless 64, Sh2-64, or RCW 174) is a star-forming region in our galaxy located in the constellation Serpens Cauda. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars. The distance to W40 is 436±9 pc (1420±30 light-years), making it one of the closest sites of formation of high-mass O- and B-type stars. The Ionizing radiation from the massive OB stars has created an H II region, which has an hour-glass morphology.

Dust from the molecular cloud in which W40 formed obscures the nebula, rendering W40 difficult to observe at visible wavelengths of light. Thus, X-ray, infrared, and radio observations have been used to see through the molecular cloud to study the star-formation processes going on within.

W40 appears near to several other star-forming regions in the sky, including an infrared dark cloud designated Serpens South and a young stellar cluster designated the Serpens Main Cluster. Similar distances measured for these three star-forming regions suggests that they are near to each other and part of the same larger-scale collection of clouds known as the Serpens Molecular Cloud.

The W40 star-forming region is projected on the sky in the direction of the Serpens-Aquila Rift, a mass of dark clouds above the Galactic plane in the constellations Aquila, Serpens, and eastern Ophiuchus. The high extinction from interstellar clouds means that this region looks unimpressive in the optical, despite being one of the nearest sites of massive star formation.