Origins: Fourteen Billion Years of Cosmic Evolution (35 page)

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The Milky Way has two large irregular satellite galaxies, called the Large and Small Magellanic Clouds. This image of the Large Magellanic Cloud shows a large bar of stars at the left, with many additional stars and star-forming regions to the right. The bright Tarantula Nebula, named for its shape and seen at the upper center of the photograph, is the largest star-forming region in this galaxy.

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This star-forming region, called the Papillon nebula for its resemblance to a butterfly, belongs to the Large Magellanic Cloud, the Milky Way’s largest satellite galaxy. Young stars illuminate the nebula from inside and excite hydrogen atoms so they emit a characteristic shade of red, captured in this image by the Hubble Space Telescope.

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A survey of the entire sky in infrared radiation reveals that we live inside the flattened disk of a spiral galaxy, which extends in this image to the left and right of the Milky Way’s central region. Dust particles absorb some of the light from this region, just as they do in faraway spiral galaxies. Below the plane of our galaxy we can see the Milky Way’s two irregular satellite galaxies, the Large and Small Magellanic Clouds.

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When we look toward the center of our Milky Way galaxy, about 30,000 light-years from the solar system, enormous dust-rich clouds block our view in visible light. Infrared light does a better job of penetrating the dust, so this infrared image obtained from the Two Micron All Sky Survey project reveals radiation that arises close to the galactic center, the particularly bright region in this image, where a supermassive black hole may be steadily swallowing matter.

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The Crab nebula lives about 7,000 light-years from the solar system, and was produced by an exploding star whose light reached the Earth on July 4, 1054. In this image taken by the Canada-France-Hawaii Telescope at the Mauna Kea Observatory, the reddish filaments consist primarily of hydrogen gas, expanding away from the region of the explosion at the center. The whitish glow arises from electrons moving at nearly the speed of light through intense magnetic fields. Supernova remnants such as this one add their evolved material to interstellar clouds of gas and dust. These clouds give birth to new stars that contain more “heavy” elements such as carbon, nitrogen, oxygen, and iron than older stars do.

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This expanding region of gas, named IC 443 by astronomers, is the remnant of a supernova, about 5,000 light-years from the solar system. The star exploded about 30,000 years before the supernova remnant produced the light recorded in this image, obtained with the Canada-France-Hawaii Telescope at the Mauna Kea Observatory.

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These wisps of gas in the Trifid nebula, about 5,000 light-years away, were imaged by the high-resolution optics of the Hubble Space Telescope. The gas in these pillars must be denser than their surroundings, which have been stripped away by radiation from young, hot stars nearby.

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This nebula, called NGC 2440, surrounds the fuel-exhausted but still hot core of what was once a star. This “white dwarf” appears as a bright spot of light near the center of the nebula in this Hubble Space Telescope image. Before long, the gas surrounding this object, about 3,500 light-years from the solar system, will evaporate into space, leaving the white dwarf isolated as it slowly cools and grows dimmer.

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This spectacular object, discovered by the famous astronomer William Herschel in 1787, bears the name Eskimo nebula for its resemblance to a face surrounded by the furry hood of a parka. The nebula, about 3,000 light-years away, consists of gas expelled from an aging star and illuminated by ultraviolet radiation from that star, whose surface has grown so hot that it emits more ultraviolet than visible light. Like Herschel, astronomers call objects like these “planetary nebulae” because a small telescope shows them only as featureless disks, similar to the images of planets. This Hubble Space Telescope image removes the confusion by revealing a host of detail in the gases expanding away from the central star.

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Amidst a star-forming region in our galaxy, a relatively cool and dense cloud of gas and dust absorbs starlight, creating the aptly named Horsehead nebula, photographed with the Canada-France-Hawaii Telescope at the Mauna Kea Observatory. This dust cloud, about 1,500 light-years from the solar system, forms part of a much larger dark and cool interstellar cloud, some of which creates the dark area below the horse’s head.