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💫Distant Spiral Galaxy NGC 4603, Home to Variable Stars

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Hubble Space Telescope (HST) view of the magnificent spiral galaxy NGC 4603, the most distant galaxy in which a special class of pulsating stars called Cepheid variables have been found. It is associated with the Centaurus cluster, one of the most massive assemblages of galaxies in the nearby universe. The Local Group of galaxies, of which the Milky Way is a member, is moving in the direction of Centaurus at a speed of more than a million miles an hour under the influence of the gravitational pull of the matter in that direction. Clusters of young bright blue stars highlight the galaxy's spiral arms. In contrast, red giant stars in the process of dying are also found. Only the very brightest stars in NGC 4603 can be seen individually, even with the unmatched ability of the Hubble Space Telescope to obtain detailed images of distant objects. Much of the diffuse glow comes from fainter stars that cannot be individually distinguished by Hubble. The reddish filaments are regions where clouds of dust obscure blue light from the stars behind them.




This galaxy was observed by a team affiliated with the HST Key Project on the Extragalactic Distance Scale. Because NGC 4603 is much farther away than the other galaxies studied with Hubble by the Key Project team, 108 million light-years, its stars appear very faint from the Earth, and so accurately measuring their brightness, as is required for distinguishing the characteristic variations of Cepheids, is extremely difficult. At this distance some non-variable stars may by chance appear to grow brighter and fainter in the same fashion as Cepheids due to the physical impossibility of perfect measurements of such dim objects. Determining the distance to the galaxy required an unprecedented statistical analysis based on extensive computer simulations.

Researchers found 36-50 Cepheids and used their observed properties to securely determine the distance to NGC 4603. These measurements indicate that when the expansion of the universe and the motion of the Local Group are accounted for, the Centaurus cluster is very nearly at rest compared with the surrounding regions. It is part of the cause of the rapid motions in the nearby universe, rather than being strongly pulled by other concentrations of matter. Observations of distant Cepheids such as those in NGC 4603 also help astronomers to precisely measure the expansion rate of the universe.


Credit: Jeffrey Newman (Univ. of California at Berkeley) and NASA


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💫The field around HD 87643


This image, showing a very rich field of stars towards the Carina arm of the Milky Way, is centred on the star HD 87643, a member of the class of B[e] stars. The amazing image is set ablaze by a flurry of stars of all colours and brightnesses, some of which are seen against a backdrop of clouds of dust and gas. The red emission nebula is RCW47 and was catalogued in 1960. HD 87643 has been extensively studied with a full range of ESO telescopes, including the Very Large Telescope Interferometer (VLTI). Surrounded by a complex, extended nebula that is the result of previous violent ejections, the star is shown to have a companion. Interactions in this double system, surrounded by a dusty disc, may be the engine fuelling the star's remarkable nebula. The image, taken with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla, is based on data obtained through different filters: B, V and R. The field of view is 33 x 32 arc-minutes.

Credit:
ESO/F. Millour


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💫Stellar Fireworks Are Ablaze in Galaxy NGC 4449

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Fireworks blaze over the skies of American cities in the annual Independence Day celebrations. But nearly 12.5 million light-years away in the dwarf galaxy NGC 4449 stellar "fireworks" are going off all the time. Hundreds of thousands of vibrant blue and red stars blaze in this image taken by NASA's Hubble Space Telescope. Hot bluish-white clusters of massive stars are scattered throughout the galaxy, interspersed with numerous dustier, reddish regions of current star formation. Massive dark clouds of gas and dust are silhouetted against the starlight. NGC 4449 has been forming stars for several billion years, but currently it is experiencing a star formation event at a much higher rate than in the past. This unusually explosive and intense star formation activity qualifies as a starburst. At the current rate, the gas supply that feeds the stellar production would only last for another billion years or so.




Starbursts usually occur in the central regions of galaxies, but NGC 4449 has more widespread star formation activity, since the very youngest stars are observed both in the nucleus and in streams surrounding the galaxy. A "global" starburst like NGC 4449 resembles primordial star forming galaxies, which grew by merging with and accreting smaller stellar systems. Since NGC 4449 is close enough to be observed in great detail, it is the ideal laboratory for the investigation of what may have occurred during galactic formation and evolution in the early universe. It's likely that the current widespread starburst was triggered by interaction or merging with a smaller companion. NGC 4449 belongs to a group of galaxies in the constellation Canes Venatici. Astronomers think that NGC 4449's star formation has been influenced by interactions with several of its neighbors.


Credit: NASA, ESA, A. Aloisi (STScI/ESA), and The Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration


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💫The Southern Cross


Shining in the southern sky, the four bright stars portrayed in the left part of this image represent a useful orientation mark which helps identifying the South Celestial Pole. Because of the rather distinctive asterism they form, these stars have been officially classified as the Crux constellation, or the Southern Cross. The vast dark cloud, visible in the lower part of the image, is usually referred to as the Coalsack Nebula; it also has a preeminent role in the Australian Aboriginal culture, representing the head of an emu in the traditional constellation of the "Emu in the Sky". The ruddy object glowing in the upper part of the image owes its colour to the blaze of hydrogen gas. Catalogued as IC 2948, this emission nebula hosts a sparkling cluster of young stars.

Credit:
ESO/Y. Beletsky


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💫Aristarchus Plateau on the Moon

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The Hubble Space Telescope Advanced Camera for Surveys imaged Aristarchus crater and nearby Schroter's Valley rille on Aug. 21, 2005. The Hubble images reveal fine-scale details of the crater's interior and exterior in ultraviolet and visible wavelengths at a scale of approximately 165 to 330 feet (50 to 100 meters) per picture element. Aristarchus crater is 26 miles (42 kilometers) in diameter and approximately 2 miles (3.2 kilometers) in depth, and sits at the southeastern edge of the Aristarchus plateau. The plateau is noted for its rich array of geologic features, including a dense concentration of lunar volcanic rilles (river-valley-like landforms that resulted from the collapse of lunar lava tubes), source vents, and volcanic materials that erupted in giant explosive events.




Aristarchus is one of the youngest large craters on the Moon. It probably formed between 100 and 900 million years ago. This composite image shows that many of the crater's fresh impact features are still well preserved, including a sharp rim, inner crater wall terraces, central uplift structures, and a pattern of excavated and ejected materials, known as ejecta, which blankets the nearby surroundings. This spectacular Hubble image reveals the diversity of ultraviolet- and visible-light signatures of crater ejecta, interior wall deposits, central peak materials, and those of the surrounding plateau.


Credit: NASA, ESA and J. Garvin (NASA/GSFC)


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💫A cosmic flame


Sparkling at the edge of a giant cloud of gas and dust, the Flame Nebula, also referred to as NGC 2024, is in fact the hideout of a cluster of young, blue, massive stars, whose light sets the gas ablaze. Located 1,300 light-years away towards the constellation of Orion, the nebula owes its typical colour to the glow of hydrogen atoms, heated by the stars. The latter are obscured by a dark, forked dusty structure in the centre of the image and are only revealed by infrared observations. This image is based on data acquired with the 1.5-metre Danish telescope at ESO’s La Silla Observatory in Chile, combining three exposures in the filters B (40 seconds), V (80 seconds) and R (40 seconds).

Credit:
ESO/IDA/Danish 1.5 m/R. Gendler, J.-E. Ovaldsen, C. Thöne and C. Féron


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