A Horse of a Different Color
The Horsehead Nebula:
A dark nebula in visible light
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, NASA released a new view of the Horsehead Nebula that provides an intriguing astronomical variation on that phrase.
Orion the Hunter:
Find the Horsehead in Orion's belt
The Horsehead Nebula, also known as Barnard 33, was first recorded in 1888 by Williamina Fleming at the Harvard College Observatory. The nebula is located about 1,500 light-years away in the constellation Orion and can be found just below the star Alnitak on the left side of Orion's belt.
The visible-light view (at top of story) shows a black silhouette that resembles a horse's head. The shape looks a lot like the horse figure often used for a knight in the game of chess. Astronomers call the structure a "dark nebula," and it is made up of dense gas and dust.
Dark nebulae are cold and do not give off any visible light. Instead, they are generally noticeable because they block the light from background stars. Notice how many more stars can be seen above the Horsehead than below it in the accompanying picture. This observation shows that the Horsehead Nebula is part of a much larger dark cloud.
In contrast to the darkness of the Horsehead Nebula, the gas above it shines a bright pink. This pink glow occurs along the edge of the dark cloud and is caused by the giant bright star, Sigma Orionis, at the top of the image.
The Horsehead in infrared light
To see deeper into a dark nebula, astronomers use infrared light, which can pierce the dense nebula.
Hubble's infrared image transforms the dark nebula into a softly glowing landscape. It reveals much more structure and detail inside the cloud. As in the visible-light image, the region above the nebula, exposed to the light of Sigma Orionis, glows brightly. Hubble, however, has captured the fainter infrared glow emitted by the cooler gas within the nebula.
Notice also that the dense cloud of gas and dust has become more transparent. Many more background stars, and even some distant galaxies, can be seen through the thinner parts of the nebula.
A newborn star in the Horsehead
Many parts of the Horsehead Nebula are still opaque at infrared wavelengths. This fact shows that the gas is very dense and cold. Within such cold and dense clouds are regions where stars are born.
At the top of the nebula, as seen in infrared light, a bright star is surrounded by glowing gas. Gravity caused this star to form from some of the cold, dense gas within the nebula. The newborn star now illuminates the gas from which it formed. It is just one of many examples of star birth in and around the nebula.
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 slowly advances through the dark gas. Ultimately, young stars destroy the nursery in which they were born.
Horse of many colors
Other dark nebulae
The Horsehead Nebula is only a temporary structure that will be eroded away in about 5 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. Astronomers use all the colors of the rainbow and all the "colors" across the entire electromagnetic spectrum of light to uncover hidden details.
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 of star formation.