VAR! Credit: E. Hubble, NASA, ESA, R. Gendler, Z. Levay and the Hubble Heritage Team Explanation: In the 1920s, examining photographic plates from the Mt. Wilson Observatory’s 100 inch telescope, Edwin Hubble determined the distance to theAndromeda Nebula, decisively demonstrating the existence of other galaxies far beyond the Milky Way. His notations are evident on the historic plate image inset at the lower right, shown in context with ground based and Hubble Space Telescope images of the region made nearly 90 years later. By intercomparing different plates, Hubble searched for novae, stars which underwent a sudden increase in brightness. He found several on this plate and marked them with an “N”. Later, discovering that the one near the upper right corner (marked by lines) was actually a type of variable star known as a cepheid, he crossed out the “N” and wrote “VAR!”. Thanks to the work of Harvard astronomer Henrietta Leavitt, cepheids, regularly varying pulsating stars, could be used as standard candle distance indicators. Identifying such a star allowed Hubble to show that Andromeda was not a small cluster of stars and gas within our own galaxy, but a large galaxy in its own right at a substantial distance from the Milky Way. Hubble’s discovery is responsible for establishing our modern concept of a Universe filled with galaxies.
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VAR! 
Credit: E. Hubble, NASA, ESA, R. Gendler, Z. Levay and the Hubble Heritage Team

Explanation: In the 1920s, examining photographic plates from the Mt. Wilson Observatory’s 100 inch telescope, Edwin Hubble determined the distance to theAndromeda Nebula, decisively demonstrating the existence of other galaxies far beyond the Milky Way. His notations are evident on the historic plate image inset at the lower right, shown in context with ground based and Hubble Space Telescope images of the region made nearly 90 years later. By intercomparing different plates, Hubble searched for novae, stars which underwent a sudden increase in brightness. He found several on this plate and marked them with an “N”. Later, discovering that the one near the upper right corner (marked by lines) was actually a type of variable star known as a cepheid, he crossed out the “N” and wrote “VAR!”. Thanks to the work of Harvard astronomer Henrietta Leavitt, cepheids, regularly varying pulsating stars, could be used as standard candle distance indicators. Identifying such a star allowed Hubble to show that Andromeda was not a small cluster of stars and gas within our own galaxy, but a large galaxy in its own right at a substantial distance from the Milky Way. Hubble’s discovery is responsible for establishing our modern concept of a Universe filled with galaxies.

Source: apod.nasa.gov

Star Factory Messier 17 Credit: ESO, INAF-VST, OmegaCAMAcknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute Explanation: Sculpted by stellar winds and radiation, the star factory known as Messier 17 lies some 5,500 light-years away in the nebula-rich constellationSagittarius. At that distance, this degree wide field of view spans almost 100 light-years, courtesy of ESO’s new VLT Survey Telescope and OmegaCAM. The sharp, false color image includes both optical and infrared data, following faint details of the region’s gas and dust clouds against a backdrop of central Milky Waystars. Stellar winds and energetic light from hot, massive stars formed from M17’s stock of cosmic gas and dust have slowly carved away at the remaining interstellar material producing the cavernous appearance and undulating shapes. M17 is also known as the Omega Nebula or the Swan Nebula.
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Star Factory Messier 17 
Credit: ESO, INAF-VST, OmegaCAM
Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute

Explanation: Sculpted by stellar winds and radiation, the star factory known as Messier 17 lies some 5,500 light-years away in the nebula-rich constellationSagittarius. At that distance, this degree wide field of view spans almost 100 light-years, courtesy of ESO’s new VLT Survey Telescope and OmegaCAM. The sharp, false color image includes both optical and infrared data, following faint details of the region’s gas and dust clouds against a backdrop of central Milky Waystars. Stellar winds and energetic light from hot, massive stars formed from M17’s stock of cosmic gas and dust have slowly carved away at the remaining interstellar material producing the cavernous appearance and undulating shapes. M17 is also known as the Omega Nebula or the Swan Nebula.

Source: apod.nasa.gov

Abell 2744: Pandora’s Cluster of Galaxies Image Credit: NASA, ESA, J. Merten (ITA, AOB), & D. Coe (STScI) Explanation: Why is this cluster of galaxies so jumbled? Far from a smooth distribution, Abell 2744 not only has knots of galaxies, but the X-ray emitting hot gas (colored red) in the cluster appears distributed differently than the dark matter. The dark matter, taking up over 75 percent of the cluster mass and colored blue in the above image, was inferred by that needed to create the distortion of background galaxies by gravitational lensing. The jumble appears to result from the slow motion collision of at least four smaller galaxy clusters over the past few billion years. The above picture combines optical images from the Hubble Space Telescopeand the Very Large Telescope with X-ray images from the Chandra X-Ray Observatory. Abell 2744, dubbed Pandora’s cluster, spans over two million light years and can best be seen with a really large telescope toward the constellation of the Sculptor.
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Abell 2744: Pandora’s Cluster of Galaxies 
Image Credit: NASA, ESA, J. Merten (ITA, AOB), & D. Coe (STScI)

Explanation: Why is this cluster of galaxies so jumbled? Far from a smooth distribution, Abell 2744 not only has knots of galaxies, but the X-ray emitting hot gas (colored red) in the cluster appears distributed differently than the dark matter. The dark matter, taking up over 75 percent of the cluster mass and colored blue in the above image, was inferred by that needed to create the distortion of background galaxies by gravitational lensing. The jumble appears to result from the slow motion collision of at least four smaller galaxy clusters over the past few billion years. The above picture combines optical images from the Hubble Space Telescopeand the Very Large Telescope with X-ray images from the Chandra X-Ray Observatory. Abell 2744, dubbed Pandora’s cluster, spans over two million light years and can best be seen with a really large telescope toward the constellation of the Sculptor.

Source: apod.nasa.gov

Stereo Helene Credit: Cassini Imaging Team, ISS, JPL, ESA, NASA; Stereo Image by Roberto Beltramini Explanation: Get out your red/blue glasses and float next to Helene, small, icy moon of Saturn. Appropriately named, Helene is one of four known Trojan moons, so called because it orbits at a Lagrange point. A Lagrange point is a gravitationally stable position near two massive bodies, in this case Saturn and larger moon Dione. In fact, irregularly shaped ( about 36 by 32 by 30 kilometers) Helene orbits at Dione’s leading Lagrange point while brotherly ice moon Polydeuces follows at Dione’s trailing Lagrange point. The sharp stereo anaglyph was constructed from two Cassini images (N00172886, N00172892) captured during the recent close flyby. It shows part of the Saturn-facing hemisphere of Helene mottled with craters and gully-like features.
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Stereo Helene 
Credit: Cassini Imaging Team, ISS, JPL, ESA, NASA; Stereo Image by Roberto Beltramini

Explanation: Get out your red/blue glasses and float next to Helene, small, icy moon of Saturn. Appropriately named, Helene is one of four known Trojan moons, so called because it orbits at a Lagrange point. A Lagrange point is a gravitationally stable position near two massive bodies, in this case Saturn and larger moon Dione. In fact, irregularly shaped ( about 36 by 32 by 30 kilometers) Helene orbits at Dione’s leading Lagrange point while brotherly ice moon Polydeuces follows at Dione’s trailing Lagrange point. The sharp stereo anaglyph was constructed from two Cassini images (N00172886, N00172892) captured during the recent close flyby. It shows part of the Saturn-facing hemisphere of Helene mottled with craters and gully-like features.

Source: apod.nasa.gov

MESSENGER’s Degas View Credit: NASA/JHU APL/CIW Explanation: Now imaging inner planet Mercury from orbit, the MESSENGER spacecraft wide angle camera has returned this impressive color view of Degas Crater, with a full resolution of 90 meters per pixel. Named for the impressionist painter, the 52 kilometer diameter crater is also shown in an inset context image from the Mariner 10 flyby mission in the mid 1970s. In MESSENGER’s view, the crater floor is seen to be filled with an intricate series of cracks, formed as the molten surface resulting from the impact cooled and contracted. Starkly bright, patchy deposits, suggesting compositional differences and freshly exposed material, standout around the crater’s central peaks and walls. Details of similar bright deposits are seen in even higher resolution images from MESSENGER.
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MESSENGER’s Degas View 
Credit: NASA/JHU APL/CIW

Explanation: Now imaging inner planet Mercury from orbit, the MESSENGER spacecraft wide angle camera has returned this impressive color view of Degas Crater, with a full resolution of 90 meters per pixel. Named for the impressionist painter, the 52 kilometer diameter crater is also shown in an inset context image from the Mariner 10 flyby mission in the mid 1970s. In MESSENGER’s view, the crater floor is seen to be filled with an intricate series of cracks, formed as the molten surface resulting from the impact cooled and contracted. Starkly bright, patchy deposits, suggesting compositional differences and freshly exposed material, standout around the crater’s central peaks and walls. Details of similar bright deposits are seen in even higher resolution images from MESSENGER.

Source: apod.nasa.gov

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