Origins: Fourteen Billion Years of Cosmic Evolution (34 page)

6:
In this image of the Coma cluster of galaxies, nearly every faint object is in fact a galaxy made of more than 100 billion stars. Located about 325 million light-years from the Milky Way, this cluster spans a diameter of several million light-years and contains many thousand individual galaxies, orbiting one another in a kind of ballet choreographed by the forces of gravity.

7:
The central region of the Virgo cluster of galaxies, a mere 60 million light-years from the Milky Way, shows dozens of galaxies of different types, including giant elliptical galaxies at the top left and top right of the image. Spiral galaxies appear throughout this image, taken with the Canada-France-Hawaii Telescope at the Mauna Kea Observatory. The Virgo cluster’s immense gravitational force, and its proximity to the Milky Way, significantly affect the motion of the Milky Way through space. Indeed, the Milky Way and the Virgo cluster form part of an even larger system of galaxies called the Virgo
super
cluster.

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This pair of interacting galaxies, named Arp 295 from their entry in Halton
Arp’s Catalog of Peculiar Galaxies
, have drawn out long filaments of their own stars and gas, stretching across a quarter-million light-years. The two galaxies lie about 270,000 light-years from the Milky Way.

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A giant spiral galaxy similar to our own dominates this photograph taken by the Very Large Telescope array in Chile. Our face-on view of this galaxy—about 100 million light-years from the Milky Way and named NGC 1232—allows us to observe the yellowish light from relatively old stars near the galaxy’s center, as well as the massive hot, young, bluish stars that dominate the surrounding pinwheel of spiral arms. Astrophysicists also detect large numbers of interstellar dust grains within these arms. A smaller companion to NGC 1232, known as a barred spiral galaxy because its central regions have a barlike shape, appears to the left of the giant spiral.

10:
This spiral galaxy, called NGC 3370 and about 100 million light-years away, closely resembles our own Milky Way in size, shape, and mass. This Hubble Space Telescope image reveals the complex spiral traced by the young, hot, highly luminous stars that outline the spiral arms. From rim to rim, the galaxy spans about 100,000 light-years.

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In March 1994, astronomers discovered Supernova 1994D in the spiral galaxy NGC 4526, one of the thousands of galaxies in the Virgo cluster, about 60 million light-years from the Milky Way. In this image obtained by the Hubble Space Telescope, the supernova appears as the bright object at the lower left, below the belt of light-absorbing dust in the galaxy’s central plane. Apart from enriching its environment with the chemical ingredients of life, Supernova 1994D is an example of the Type Ia supernovae used to discover the acceleration of the cosmic expansion.

12:
When we look at this spiral galaxy, NGC 4631, about 25 million light-years away, our line of sight lies edge-on to the galaxy’s disk, so we cannot see the galaxy’s spiral-arm structure. Instead, dust that lies within the disk obscures much of the light from the galaxy’s stars. The patch of red to the left of center marks a stellar nursery. Above NGC 4631 lies a smaller, elliptical galaxy, an orbiting companion to the giant spiral.

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In this small irregular galaxy, called NGC 1569 and only 7 million light-years away, a burst of star formation began about 25 million years ago and can still be seen, accounting for most of the galaxy’s light. Two large star clusters are visible in the left center of this Hubble Space Telescope image.

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: The Andromeda galaxy, the closest big galaxy to the Milky Way, lies about 2.4 million light-years from us and spans a region of the sky several times larger than the full moon. In this image, taken by amateur astron-omer Robert Gendler, one of the galaxy’s two elliptical satellites appears below and to the left of its center, while a fainter one appears above and to the right of that center. All the other small bright objects in this image are individual stars within the Milky Way, sitting practically on our noses at less than 1/100 of the distance to the Andromeda galaxy.

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: Relatively close to the Milky Way, at about the same distance as the Andromeda galaxy (2.4 million light-years), lies the smaller spiral galaxy M33, whose largest star-forming region appears in this Hubble Space Telescope image. The most massive stars to form in this region have already exploded as supernovae, enriching their environment with heavy elements, while other massive stars are producing intense ultraviolet radiation that blasts electrons from the atoms surrounding them.