Hubble's Universe Unfiltered

  • April 19, 2013

    A Horse of a Different Color

    The expression “a horse of a different color” indicates something unusual or remarkable – something you don’t see every day. To celebrate the 23rd anniversary of the Hubble Space Telescope, we have released a new view of the Horsehead Nebula that provides an intriguing astronomical variation on that phrase.

    The Horsehead Nebula is a dark nebula (also called an absorption nebula) of dense gas and dust in the constellation of Orion. It can be found just below the star Alnitak on the eastern (left) side of Orion’s belt. The visible-light view shows a black silhouette that resembles a horse’s head. It looks a lot like the horse figure often used for a knight in chess.

    In contrast to the darkness of the Horsehead Nebula, the gas around it glows a bright pink. Ultraviolet radiation from the nearby massive star Sigma Orionis A ionizes the gas. The dominant emission from the heated gas is what we call hydrogen-alpha emission. At a wavelength of 656.28 nanometers, it is given off by an electron in a hydrogen atom dropping from the third excitation level to the second. That pink is the characteristic color of emission nebulae in the universe (not the vibrant colors seen in some Hubble images).


    Horsehead Nebula in optical from NOAO and infrared from Hubble

    The Horsehead Nebula: Visible-light view from NOAO (left) and infrared view from Hubble (right).
    Visible-light image - Credit: Nigel Sharp/NOAO   Copyright AURA Inc./NOAO
    Infrared image - Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

    To see more details of dark nebulae, astronomers use infrared light. Infrared light has longer wavelengths and lower energy than visible light. The longer wavelengths can penetrate deeper into a cloud. The lower energies mean that infrared light is emitted by cooler gas. Both qualities help see into dark nebulae.

    Hubble’s infrared image transforms the dark nebula into a softly glowing landscape. It reveals much more structure and detail in the clouds. As in the visible-light image, those regions exposed to the light of Sigma Orionis glow brightly. However, fainter infrared emission comes from across the entire nebula. In addition, the opaque cloud and the glowing gas have become more transparent. Many more background stars, and even some distant galaxies, can be seen through the thinner parts of the nebula.

    At the top of the nebula, infrared reveals a bright star surrounded by bright gas. Gravitational collapse within the cloud created this star, and now it illuminates the gas from which it formed. It is just one of many examples of star birth in and around the nebula.

    Indeed, the long-term story of this dark nebula is about the action and reaction of a gas cloud to star formation. Stars form within dark nebulae. Hot newborn stars emit intense radiation that heats the gas and makes it glow. As deeper and deeper layers are heated, the bright cloud boundary, called an ionization front, slowly advances through the dark gas. Young stars eventually destroy the nursery in which they were born.


    Animated GIF image of the Horsehead Nebula in optical from NOAO and in infrared from Hubble

    Animated GIF of the Horsehead Nebula cycling between visible and infrared light.
    Credt: Frank Summers, STScI
    Visible light image - Credit: Nigel Sharp/NOAO   Copyright AURA Inc./NOAO
    Infrared image - Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

    The Horsehead Nebula is really a temporary structure that will be eroded away in about five million years. While it lasts, astronomers will study it using not only visible and infrared light, but also all the other types of light including ultraviolet, X-ray, and radio. In astronomy, we use all the colors of the rainbow and all the “colors” across the entire electromagnetic spectrum of light.

    Beyond a new beautiful image, Hubble’s infrared insight helps us learn more about the Horesehead Nebula and other stellar nurseries. This astronomical “horse of a different color” provides new details and helps piece together a more complete scientific story.