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💫Sombrero Galaxy, A Great Observatories View

The Sombrero, also known as M104, is one of the largest galaxies in the nearby Virgo cluster, about 28 million light years from Earth. This Great Observatories view of the famous Sombrero galaxy was made using NASA's Chandra X-ray Observatory, Hubble Space Telescope and Spitzer Space Telescope. The main figure shows the combined image from the three telescopes, while the three inset images show the separate observatory views. The Chandra X-ray image (in blue) shows hot gas in the galaxy and point sources that are a mixture of objects within the Sombrero as well as quasars in the background. The Chandra observations show that diffuse X-ray emission extends over 60,000 light years from the center of the Sombrero. (The galaxy itself spans 50,000 light years across.) Scientists think this extended X-ray glow may be the result of a wind from the galaxy, primarily being driven by supernovas that have exploded within its bulge and disk. The Hubble optical image (green) shows a bulge of starlight partially blocked by a rim of dust, as this spiral galaxy is being observed edge on. That same rim of dust appears bright in Spitzer's infrared image, which also reveals that Sombrero's central bulge of stars.

Credit:
X-ray: NASA / UMass / Q.D.Wang; Optical: NASA / STScI / AURA / Hubble Heritage; Infrared: NASA / JPL-Caltech / Univ. AZ / R.Kennicutt / SINGS Team


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💫A Tantalising Veil

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This delicate Hubble Space Telescope image shows a tiny portion of the Cygnus loop, a supernova remnant in the constellation of Cygnus, the Swan. Measurements on this super-detailed image of a cosmic veil shows that the original supernova explosion took place only 5000 years ago. This delicate Hubble Space Telescope image shows a tiny portion of the Cygnus loop, a supernova remnant in the constellation of Cygnus, the Swan. Measurements on this super-detailed image of a cosmic veil shows that the original supernova explosion took place only 5000 years ago. A gauzy remnant of an ancient supernova flies through space and is captured by cameras aboard the NASA/ESA Hubble Space Telescope, showing part of the famous Cygnus Loop in unprecedented detail. The fascinating smoke-like wisps of gas in this Hubble image are a record of the enormous amounts of energy released as the fast-moving supernova explosion ploughs into its surroundings and creates a shock front. The rapid release of energy in the shock of the collision, when the supernova material smashes into the gas of the interstellar medium at a speed of more than 600,000 kilometres per hour, makes the gas glow. In this image, the motion of the shock front is upwards.


The Cygnus loop, also known as the Veil Nebula, is well-known to amateur astronomers as a challenging target for larger telescopes. It has a diameter of about 3 degrees (corresponding to 6 full moons). This spectacular nebula was created when a massive star ended its days in an immense supernova explosion. A bubble of dust and gas was expelled into space and has continued to expand outwards ever since. The Cygnus Loop consists of two main arcs, designated NGC 6992/95 for the Eastern arc (to the left in the background image) and NGC 6960 for the Western arc (also called the Witch's Broom Nebula, to the right, close to the bright, magnitude 4, star 52 Cygni). The tiny area of the Hubble image is seen in the upper left-hand corner of the nebula, at the outer edge of one of the large filaments, just where the blast wave rams into the surrounding interstellar gas. We are seeing the shock front almost exactly edge-on, thus explaining its wispy, sheet-like appearance.

The image is a striking example of how processes that take place hundreds of light years away can sometimes resemble effects we see around us in our daily life. The image has similarities with the pattern formed by the interplay of light and shadow on the bottom of a swimming pool (known as a caustic network), rising smoke or a ragged cirrus cloud seen in a summer sky. By comparing this Hubble image with an old ground-based photograph from 1953, scientists have measured how far the shock front has actually moved. The result shows that the supernova remnant lies much closer to Earth than previously thought, at a distance of merely 1500 light years. This also implies that the explosion may have occurred only 5000 years ago and must then have been an awesome sight for early civilisations, even visible at daytime (with an estimated brightness of at least magnitude -8, corresponding to the crescent Moon).

Credit: ESA & Digitized Sky Survey (Caltech)


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💫Mysterious Arms Revealed


A combination of space and ground-based observations, including X-ray data from Chandra, has helped reveal the nature of the so-called anomalous arms in the spiral galaxy NGC 4258 (also known as M106). These arms have been known for decades, but their origin remained mysterious to astronomers. In visible (shown in gold) and infrared (red) light, two prominent arms emanate from the bright nucleus and spiral outward. These arms are dominated by young, bright stars, which light up the gas within the arms. But in radio (purple) and Chandra's X-ray (blue) images, two additional spiral arms are seen. By analyzing data from XMM-Newton, Spitzer, and Chandra, scientists have confirmed earlier suspicions that the ghostly arms represent regions of gas that are being violently heated by shock waves. Previously, some astronomers had suggested that the anomalous arms are jets of particles being ejected by a supermassive black hole in nucleus of NGC 4258. But radio observations at the Very Large Array later identified another pair of jets originating in the core. However, the jets do heat the gas in their line of travel, forming an expanding cocoon. Because the jets lie close to M106's disk, the cocoon generates shock waves and heat the gas in the disk to millions of degrees, causing it to radiate brightly in X-rays and other wavelengths.

Credit:
X-ray: NASA / CXC / Univ. of Maryland / A.S. Wilson et al.; Optical: Pal.Obs. DSS; IR: NASA / JPL-Caltech; VLA: NRAO / AUI / NSF


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💫First close look at gamma-ray burst host galaxy ESO 184-G82

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A group of European astronomers have obtained the first detailed images of a galaxy in which a gamma-ray burst has occurred. The image was taken with one of the NASA/ESA Hubble Space Telescope's high-resolution cameras and reveals a barred spiral galaxy with numerous star-forming regions. The gamma-ray burst has been located in one such actively star-forming region. This is a very important step forward in our understanding of gamma-ray bursts and their immediate surroundings and offers possible clues to their progenitors. A special event took place on 25 April 1998. A gamma-ray burst occurred in a galaxy a mere 125 million light years away - cosmically speaking, our 'backyard'. Gamma-ray bursts emit very intense gamma-ray radiation for short periods and are observed a few times per day by special gamma-ray detectors on observatories in space. International spacecraft have collected information on about 3,000 of these bursts since their discovery in 1967 and today we know that they originate in galaxies - often at large distances. However, the exact nature of these mysterious objects has eluded scientists. The many different theories explaining their occurrence all involve violent and highly energetic phenomena and range from colliding neutron stars to collapsing massive stars, the so-called hypernovae.


The story of this burst, named GRB 980425 by the astronomers, is an unusual one. It was far closer than any other gamma-ray burst for which a distance has been determined and thus must have been 1000 to 1,000,000 times fainter than normal bursts. Moreover an unusually bright supernova (SN 1998bw) - it is the current record-holder at radio wavelengths - was seen in exactly the same location merely a day after the burst. Altogether a most unusual event! Several groups of astronomers have followed the development of this event closely over the last two years. The fading light from the powerful exotic supernova has been monitored carefully. On 12 June this year a group of European astronomers obtained very detailed observations of the host galaxy ESO 184-G82 for this gamma-ray burst - the first time such a galaxy has been observed in such fine detail. The observations are of extremely high quality and were taken with one of the NASA/ESA Hubble Space Telescope's high-resolution cameras, the Space Telescope Imaging Spectrograph (STIS). Nearly 20 host galaxies have been observed since the first galaxies hosting gamma-ray bursts were detected about three years ago. However, these are usually thousands of million light years away and hence much too small and faint to be observed in detail - even with Hubble's high resolution. The new observations allow scientists to investigate the phenomenon in much greater detail.

They show that the host galaxy is actively star-forming and contains numerous clouds of hydrogen and regions teeming with activity from newly born hot stars. The galaxy is a spiral with loosely wound spiral arms and a large bar of gas and dust running through the centre. The faintness of the burst may be more important than it seems at first glance. Either the gamma-ray burst was unusual or, as a member of the observing team, Stephen Holland (Danish Centre for Astrophysics with the HST) explains, it may belong to a previously unnoticed, fainter, but common class of bursts: 'In 1998 when this faint burst was first seen, we knew that we would almost certainly not have seen it had it occurred at the same distance as other previously observed bursts. This opened our eyes to the fact that gamma-ray bursts exist over a range of intensities. We believe that such faint bursts had just not been detected before.' Most astronomers today believe that the 980425 gamma-ray burst and the almost simultaneous supernova event (less than 24 hours apart) did indeed arise from the same source. The probability of two such events occurring at the same place and at nearly the same time is extremely low - perhaps as little as 1 in 10,000. Two other events have indicated a link between gamma-ray bursts and supernova explosions, but the case of SN 1998bw and GRB 980425 has made a physical connection between these two most energetic events in the Universe highly probable.

The proximity of the host galaxy and the supernova's brilliant light has helped European scientists pinpoint the location of the gamma-ray burst and given them the opportunity to observe and study the environment around gamma-ray bursts. The team has used the sharpness of the Hubble Space Telescope's vision to discover that the burst and the supernova occurred in an active region in one of the galaxy's spiral arms. Here an underlying hydrogen gas complex is overlaid with several bright red giant stars. At the exact position of the gamma-ray burst a very compact source of emission is seen. Most of this emission is probably the last remnant of the fading light from the supernova itself, but the scientists suspect that a faint underlying star cluster may contribute as well. SN 1998bw was a special type of supernova - type Ic. Theories predict that the progenitor stars for this type of supernova are very heavy - in this case about 40 solar masses. These stars only live for a few million years, which give them no time to leave the area of active star formation where they were born. Theorists have proposed an interesting model that can explain the event. Massive stars that lose their hydrogen and helium envelopes then explode very violently as hypernovae and emit large quantities of gamma-rays in the process. Johan Fynbo from the University of Copenhagen explains: 'This barred spiral is typical of its type in that it clearly supports a lot of star forming activity.

The presence of a bar stirs up the gas in the whole disc of the galaxy and thereby promotes star formation. This process was first suggested by the Swedish astronomer Bertil Lindblad in the forties.' Jens Hjorth from the University of Copenhagen continues: 'In our opinion we now have solid evidence that at least some gamma-ray bursts originate in star forming regions. This has very interesting cosmological implications. Gamma-ray bursts are so powerful that they may serve as direct indicators of star formation taking place at the other end of the Universe.' Although this discovery gives us crucial details about the surroundings of a gamma-ray burst for the first time, the rare combination of a very faint burst with the simultaneous occurrence of an extremely bright and unusual type of supernova still needs to be clarified. Meanwhile the group continues its investigation of the stars in this region of the nearby spiral galaxy to reveal their age, colour and other important properties. The European collaboration consists of: Stephen Holland (Danish Centre for Astrophysics with the HST), Jens Hjorth & Johan Fynbo (University of Copenhagen), Bjarne Thomsen (University of Aarhus), Michael Andersen (University of Oulu), Gunnlaugur Bjvrnsson (University of Iceland), Andreas Jaunsen (University of Oslo), Priya Natarajan (University of Cambridge & Yale), and Nial Tanvir (University of Hertfordshire).

Credit: ESA, Stephen Holland (Danish Centre for Astrophysics with the HST), Jens Hjorth, Johan Fynbo (University of Copenhagen)


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💫Galaxy Collision Causes Role Reversal


Astronomers think that there are enormous black holes at the centers of most, if not all, galaxies. These black holes, which can be millions or even billions of times more massive than the Sun, can greatly affect the galaxy and the environments around them. One way such black holes shape their surroundings is by generating powerful jets of high-energy particles. The jets, which are bright in radio waves, have been seen to push around the hot gas that envelops the galaxy. When this happens, astronomers can detect huge cavities and powerful shock fronts in the hot, X-ray emitting gas. However, the opposite scenario is apparently unfolding in the galaxy known as 3C442A. X-ray data from NASA's Chandra X-ray Observatory and radio observations from the NSF's Very Large Array show that the hot gas (blue) in the middle of 3C442A is pushing apart the radio-bright gas (orange). The inner sections of the radio structure are sharp and concave, which is consistent with the idea that the X-ray bright gas is sweeping the radio-emitting gas aside.

This is the first convincing evidence for such a role reversal. A team of scientists, led by Diana Worrall of University of Bristol, UK, has studied this system and determined why the dynamics in 3C442A seem to be topsy-turvy. First, there are two galaxies near the middle of 3C442A which are in the process of merging. These two galaxies are on their second pass toward a collision, having already experienced a close encounter. The energy generated from this impending merger is heating the combined atmospheres from these two galaxies, causing them to shine brightly in X-rays and expand. The researchers determined that the jets that had produced the lobes of radio-emitting gas are no longer active. The jets may have ceased at the time of, and possibly as a result of, the galaxy collision. Since the radio-emitting gas no longer has a power source, it is then at the mercy of the expanding hot gas and has been pushed aside.

Credit:
NASA / JPL-Caltech


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💫Hubble Left Alone in Space

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The Hubble Space Telescope (HST) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, and is well known as both a vital research tool and a public relations boon for astronomy. The HST is named after the astronomer Edwin Hubble, and is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.[6] With a 2.4-meter (7.9 ft) mirror, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely high-resolution images, with substantially lower background light than ground-based telescopes. Hubble has recorded some of the most detailed visible light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.



The HST was built by the United States space agency NASA, with contributions from the European Space Agency. The Space Telescope Science Institute (STScI) selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft.[7] Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster (1986). When finally launched in 1990, Hubble's main mirror was found to have been ground incorrectly, compromising the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission in 1993.

Hubble is the only telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, and replaced systems on the telescope, including all five of the main instruments. The fifth mission was initially canceled on safety grounds following the Columbia disaster (2003). However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, completed in 2009. The telescope is operating as of 2018, and could last until 2030–2040.[3] Its scientific successor, the James Webb Space Telescope (JWST), is scheduled for launch in May 2020.

Credit: NASA / ESA


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