Astronomy Picture of the Day

Explanation: How do stars form? Images of the star forming region W5 like those in the infrared by NASA's Wide Field Infrared Survey Explorer (WISE, later NEOWISE) satellite provide clear clues with indications that massive stars near the centre of empty cavities are older than stars near the edges. A likely reason for this is that the older stars in the centre are actually triggering the formation of the younger edge stars. The triggered star formation occurs when hot outflowing gas compresses cooler gas into knots dense enough to gravitationally contract into stars. In the featured scientifically coloured infrared image, spectacular pillars left slowly evaporating from the hot outflowing gas provide further visual clues. W5 is also known as Westerhout 5 (W5) and IC 1848. Together with IC 1805, the nebulae form a complex region of star formation popularly dubbed the Heart and Soul Nebulae. The featured image highlights a part of W5 spanning about 2,000 light years that is rich in star forming pillars. W5 lies about 6,500 light years away toward the constellation of Cassiopeia.