Deep Astronomy

  • September 30, 2013

    Episode 5: The Horsehead Nebula in the Infrared

    by Tony Darnell

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    The constellation of Orion is one of the most recognizable patterns in our night sky.  Within its boundaries lies the Orion Molecular Cloud Complex, located some 1,500 light years away.

    Inside this complex, on the farthest eastern edge of Orion's belt, is one of the most photographed nebulae in astronomy: The Horsehead Nebula.

    This nebula was first recorded in 1888 by Scottish astronomer Williamina Fleming at the Harvard College Observatory and ever since, we have been captivated by it.

    The red or pinkish glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis, which is really a young, five-star system just out of view.

    The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just beginning to form. 

    Since 1888, this has been our main view of the Horsehead Nebula and it has become an icon.

    But in April, 2013, astronomers using the Hubble Space Telescope released another view, one that only Hubble's infrared cameras in space could provide.

    Here is the Horsehead Nebula in the infrared.

    In the Hubble image, the backlit wisps along the Horsehead's upper ridge, illuminated by Sigma Orionis, can be seen.  Along the nebula's top ridge, two fledgling stars peek out from their now-exposed nurseries.

    This nebula gets its shape from powerful forces within this stellar cradle.  

    Harsh, ultraviolet radiation from one of these bright stars is slowly evaporating the nebula. Gas clouds surrounding the Horsehead have already dissipated, but the tip of the jutting pillar contains a slightly higher density of hydrogen and helium, laced with dust. This casts a shadow that protects material behind it from being stripped away, and a pillar structure forms.

    The Hubble Space Telescope has been providing us with ground-breaking science for two decades.  And every once in a while, it takes time out to give us a portrait of familiar friends in a completely new light.

    Keep Looking up.