The Horsehead nebula in Orion
One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. A blue reflection nebula dubbed NGC 2023 surrounds the bright star at the lower left. 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 in the process of forming. Light takes about 1500 years to reach us from the Horsehead Nebula.
Image credit & copyright: Adam Block, Mt. Lemmon SkyCenter, U. Arizona
Explanation: This sharp cosmic portrait features NGC 891. The spiral galaxy spans about 100 thousand light-years and is seen almost exactly edge-on from our perspective. In fact, about 30 million light-years distant in the constellation Andromeda, NGC 891 looks a lot like our Milky Way. At first glance, it has a flat, thin, galactic disk and a central bulge cut along the middle by regions of dark obscuring dust. The combined image data also reveal the galaxy’s young blue star clusters and telltale pinkish star forming regions. And remarkably apparent in NGC 891’s edge-on presentation are filaments of dust that extend hundreds of light-years above and below the center line. The dust has likely been blown out of the disk by supernova explosions or intense star formation activity. Faint neighboring galaxies can also be seen near this galaxy’s disk.
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The image, captured by the NASA/ESA Hubble Space Telescope, shows part of NGC 3621, an unusual spiral galaxy located over 20 million light-years away in the constellation of Hydra (The Water Snake).
The small, bright nucleus on the right of the image does not have the significant bulge of older stars that is common in spiral galaxies, marking NGC 3621 as a “pure-disc” galaxy. Many luminous clumps of blue young stars are scattered along the loose spiral arms, which are partially obscured by the dark dust lanes snaking across the frame. This galaxy is very useful for astronomers; some of its brightest stars can be used to estimate extragalactic distances, allowing us to measure the vast scale of the Universe.
Image credit: ESA/Hubble & NASA; Acknowledgement: Claude Cornen