Category: astrophysics

NASA’s Fermi Satellite Celebrates 10 Years of …

On June 11, NASA’s Fermi Gamma-ray Space Telescope celebrates a decade of using gamma rays, the highest-energy form of light in the cosmos, to study black holes, neutron stars, and other extreme cosmic objects and events.

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Left: A STEREO B image of the far side of the sun during the Sept. 1, 2014, solar eruption. Right: The Earth-facing side of the sun at the same time as seen by NASA’s Solar Dynamics Observatory. The view includes the area from which NASA’s Fermi detected high-energy gamma rays. Includes animated gif. Credit: NASA/STEREO and NASA/SDO

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A rupture in the crust of a highly magnetized neutron star, shown here in an artist’s rendering, can trigger high-energy eruptions. Fermi observations of these blasts include information on how the star’s surface twists and vibrates, providing new insights into what lies beneath. Credits: NASA’s Goddard Space Flight Center/S. Wiessinger 

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Fermi finds the first extragalactic gamma-ray pulsar. NASA’s Fermi Gamma-ray Space Telescope has detected the first extragalactic gamma-ray pulsar, PSR J0540-6919, near the Tarantula Nebula (top center) star-forming region in the Large Magellanic Cloud, a satellite galaxy that orbits our own Milky Way. Fermi detects a second pulsar (right) as well but not its pulses. PSR J0540-6919 now holds the record as the highest-luminosity gamma-ray pulsar. The angular distance between the pulsars corresponds to about half the apparent size of a full moon. Background: An image of the Tarantula Nebula and its surroundings in visible light. Credit: NASA’s Goddard Space Flight Center; background: ESO/R. Fosbury (ST-ECF)

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These maps, both centered on the north galactic pole, show how the sky looks at gamma-ray energies above 100 million electron volts (MeV). Left: The sky during a three-hour interval prior to the detection of GRB 130427A. Right: A three-hour interval starting 2.5 hours before the burst and ending 30 minutes into the event, illustrating its brightness relative to the rest of the gamma-ray sky. GRB 130427A was located in the constellation Leo near its border with Ursa Major, whose brightest stars form the familiar Big Dipper. For reference, this image includes the stars and outlines of both constellations. Labeled. Credit: NASA/DOE/Fermi LAT Collaboration.

Novae typically originate in binary systems containing sun-like stars, as shown in this artist’s rendering. A nova in a system like this likely produces gamma rays (magenta) through collisions among multiple shock waves in the rapidly expanding shell of debris. Credit: NASA’s Goddard Space Flight Center/S. Wiessinger

Gamma Rays in Active Galactic Nuclei

Gamma-ray Burst Photon Delay as Expected by Quantum Gravity. Print resolution still. In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect, eliminating some approaches to a new theory of gravity. Credit: NASA/Sonoma State University/Aurore Simonnet


“Fermi’s first 10 years have produced numerous scientific discoveries that have revolutionized our understanding of the gamma-ray universe,” said Paul Hertz, Astrophysics Division director at NASA Headquarters in Washington. 

By scanning the sky every three hours, Fermi’s main instrument, the Large Area Telescope (LAT), has observed more than 5,000 individual gamma-ray sources, including an explosion called GRB 130427A, the most powerful gamma-ray burst scientists have detected.

In 1949, Enrico Fermi — an Italian-American pioneer in high-energy physics and Nobel laureate for whom the mission was named — suggested that cosmic rays, particles traveling at nearly the speed of light, could be propelled by supernova shock waves. In 2013, Fermi’s LAT used gamma rays to prove these stellar remnants are at least one source of the speedy particles.

Fermi’s all-sky map, produced by the LAT, has revealed two massive structures extending above and below the plane of the Milky Way. These two “bubbles” span 50,000 light-years and were probably produced by the supermassive black hole at the center of the galaxy only a few million years ago.

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The highly distorted supernova remnant shown i…

The highly distorted supernova remnant shown in this image may contain the most recent black hole formed in the Milky Way galaxy. The image combines X-rays from NASA’s Chandra X-ray Observatory in blue and green, radio data from the NSF’s Very Large Array in pink, and infrared data from Caltech’s Palomar Observatory in yellow.

Credits: X-ray: NASA/CXC/MIT/L.Lopez et al; Infrared: Palomar; Radio: NSF/NRAO/VLA

Gravitational Wave Event Likely Signaled Creat…

The spectacular merger of two neutron stars that generated gravitational waves announced last fall likely did something else: birthed a black hole. This newly spawned black hole would be the lowest mass black hole ever found, as described in our latest press release.

After two separate stars underwent supernova explosions, two ultra-dense cores (that is, neutron stars) were left behind. These two neutron stars were so close that gravitational wave radiation pulled them together until they merged and collapsed into a black hole. The artist’s illustration shows a key part of the process that created this new black hole, as the two neutron stars spin around each other while merging. The purple material depicts debris from the merger.

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An additional illustration shows the black hole that resulted from the merger, along with a disk of infalling matter and a jet of high-energy particles.

A new study analyzed data from NASA’s Chandra X-ray Observatory taken in the days, weeks, and months after the detection of gravitational waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) and gamma rays by NASA’s Fermi mission on August 17, 2017.

X-rays from Chandra are critical for understanding what happened after the two neutron stars collided. The question is: did the merged neutron star form a larger, heavier neutron star or a black hole?

Chandra observed GW170817 multiple times. An observation two to three days after the event failed to detect a source, but subsequent observations 9, 15 and 16 days after the event, resulted in detections (bottom left). The source went behind the Sun soon after, but further brightening was seen in Chandra observations about 110 days after the event (bottom right), followed by comparable X-ray intensity after about 160 days.

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If the neutron stars merged and formed a heavier neutron star, then astronomers would expect it to spin rapidly and generate a very strong magnetic field. This, in turn, would have created an expanding bubble of high-energy particles that would result in bright X-ray emission. Instead, the Chandra data show levels of X-rays that are a factor of a few to several hundred times lower than expected for a rapidly spinning, merged neutron star and the associated bubble of high-energy particles, implying a black hole likely formed instead.

By comparing the Chandra observations with those by the NSF’s Karl G. Jansky Very Large Array (VLA), researchers explain the observed X-ray emission as being due entirely to the shock wave – akin to a sonic boom from a supersonic plane – from the merger smashing into surrounding gas.  There is no sign of X-rays resulting from a neutron star. Thus, the researchers in this study claim this is a strong case for the merger of two neutron stars merging to then produce bursts of radiation and form a black hole.

source: nasa.gov

Wolf–Rayet star Wolf–Rayet stars, often a…

Wolf–Rayet star

Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. Their surface temperatures range from 30,000 K to around 200,000 K, hotter than almost all other stars.

Classic (or Population I) Wolf–Rayet stars are evolved, massive stars that have completely lost their outer hydrogen and are fusing helium or heavier elements in the core. A subset of the population I WR stars show hydrogen lines in their spectra and are known as WNh stars; they are young extremely massive stars still fusing hydrogen at the core, with helium and nitrogen exposed at the surface by strong mixing and radiation-driven mass loss. A separate group of stars with WR spectra are the central stars of planetary nebulae (CSPNe), post asymptotic giant branch stars that were similar to the Sun while on the main sequence, but have now ceased fusion and shed their atmospheres to reveal a bare carbon-oxygen core.

source | images: NASA/ Judy Schmidt, Michael Miller

Planetary nebula A planetary nebula, abbrev…

Planetary nebula

A planetary nebula, abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The word “nebula” is Latin for mist or cloud, and the term “planetary nebula” is a misnomer that originated in the 1780s with astronomer William Herschel because, when viewed through his telescope, these objects resemble the rounded shapes of planets. Herschel’s name for these objects was popularly adopted and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

Most planetary nebulae form at the end of the star’s life, during the red giant phase, when the outer layers of the star are expelled by strong stellar winds. After most of the red giant’s atmosphere is dissipated, the ultraviolet radiation of the hot luminous core, called a planetary nebula nucleus (PNN), ionizes the ejected material. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.

Planetary nebulae likely play a crucial role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances.

Stars greater than 8 solar masses (M) will likely end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8 M to 8.0 M.

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  • images: NASA/ESA, Hubble

The Crab Pulsar (PSR B0531+21) is a relative…

The Crab Pulsar (PSR B0531+21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant.

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The Crab Pulsar is one of very few pulsars to be identified optically. The optical pulsar is roughly 20 kilometres (12 mi) in diameter and the pulsar “beams” rotate once every 33 milliseconds, or 30 times each second.

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The outflowing relativistic wind from the neutron star generates synchrotron emission, which produces the bulk of the emission from the nebula, seen from radio wavesthrough to gamma rays. The most dynamic feature in the inner part of the nebula is the point where the pulsar’s equatorial wind slams into the surrounding nebula, forming a termination shock.

The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar into the main body of the nebula. The period of the pulsar’s rotation is slowing by 38 nanoseconds per day due to the large amounts of energy carried away in the pulsar wind.

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The Crab Nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar itself.

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A History of the Crab Nebula

images: NASA/ESA, Hubble, Cambridge University Lucky Imaging Group, 

NASA/CXC/ASU/J.Hester et al.

Fast radio burst (FRB) In radio astronomy…

Fast radio burst (FRB)

In radio astronomy, a fast radio burst (FRB) is a high-energy astrophysical phenomenon of unknown origin manifested as a transient radio pulse lasting a few milliseconds on average.

The first FRB was discovered by Duncan Lorimer and his student David Narkevic in 2007 when they were looking through archival pulsar survey data, and it is therefore commonly referred to as Lorimer Burst. Many FRBs have since been found, including a repeating FRB. Although the exact origin and cause is uncertain, they are almost definitely extragalactic, with the nearest roughly 1.6 billion light years away, and the furthest 17 billion light years away (comoving).

When the FRBs are polarized, it indicates that they are emitted from a source contained within an extremely powerful magnetic field. The origin of the FRBs has yet to be determined; proposals for its origin range from a rapidly rotating neutron star and a black hole to extraterrestrial intelligence.

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Neutron star

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Black Holes

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Sign of extraterrestrial intelligence?

The localization and characterization of the one known repeating source, FRB 121102, has revolutizoned the understanding of the source class. FRB 121102 is identified with a galaxy at a distance of approximately 3 billion light years, well outside the Milky Way Galaxy, and embedded in an extreme environment.

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Fast radio bursts are named by the date the signal was recorded, as “FRB YYMMDD”. The first fast radio burst to be described, the Lorimer Burst FRB 010724, was identified in 2007 in archived data recorded by the Parkes Observatory on 24 July 2001. Since then, most known FRBs have been found in previously recorded data. On 19 January 2015, astronomers at Australia’s national science agency (CSIRO) reported that a fast radio burst had been observed for the first time live, by the Parkes Observatory.

Parkes radio telescope

Fast radio bursts are bright, unresolved (pointsource-like), broadband (spanning a large range of radio frequencies), millisecond flashes found in parts of the sky outside the Milky Way. Unlike many radio sources the signal from a burst is detected in a short period of time with enough strength to stand out from the noise floor. 

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The burst usually appears as a single spike of energy without any change in its strength over time. The bursts last for a period of several milliseconds (thousandths of a second). The bursts come from all over the sky, and are not concentrated on the plane of the Milky Way. Known FRB locations are biased by the parts of the sky that the observatories can image.

Images captured by NASA’s Spitzer Space …

Images captured by NASA’s Spitzer Space Telescope. (Some images include data from other telescopes)

The Spitzer Space Telescope is the final mission in NASA’s Great Observatories Program – a family of four space-based observatories, each observing the Universe in a different kind of light. The other missions in the program include the visible-light Hubble Space Telescope (HST), Compton Gamma-Ray Observatory (CGRO), and the Chandra X-Ray Observatory (CXO).

The Cryogenic Telescope Assembly, which contains the a 85 centimeter telescope and Spitzer’s three scientific instruments

The Spacecraft, which controls the telescope, provides power to the instruments, handles the scientific data and communicates with Earth

It may seem like a contradiction, but NASA’s Spitzer Space Telescope must be simultaneously warm and cold to function properly. Everything in the Cryogenic Telescope Assembly must be cooled to only a few degrees above absolute zero (-459 degrees Fahrenheit, or -273 degrees Celsius). This is achieved with an onboard tank of liquid helium, or cryogen.  Meanwhile, electronic equipment in The Spacecraft portion needs to operate near room temperature.

Spitzer’s highly sensitive instruments allow scientists to peer into cosmic regions that are hidden from optical telescopes, including dusty stellar nurseries, the centers of galaxies, and newly forming planetary systems. Spitzer’s infrared eyes also allows astronomers see cooler objects in space, like failed stars (brown dwarfs), extrasolar planets, giant molecular clouds, and organic molecules that may hold the secret to life on other planets.

Spitzer was originally built to last for a minimum of 2.5 years, but it lasted in the cold phase for over 5.5 years. On May 15, 2009 the coolant was finally depleted and the Spitzer “warm mission” began.  Operating with 2 channels from one of its instruments called IRAC, Spitzer can continue to operate until late in this decade. Check out: Fast Facts and Current Status.

Credit NASA | images: NASA/Spitzer

Galaxies: Types and morphology

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy’s center of mass.

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Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. 

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Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies.

Ellipticals

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The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions.

Spirals

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Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter that extends beyond the visible component, as demonstrated by the universal rotation curve concept.

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge.

Barred spiral galaxy

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A majority of spiral galaxies, including our own Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) that indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). 

Ring galaxy

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A ring galaxy is a galaxy with a circle-like appearance. Hoag’s Object, discovered by Art Hoag in 1950, is an example of a ring galaxy. The ring contains many massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this “collision” rarely results in any actual collisions between stars.

Lenticular galaxy

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A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. They contain large-scale discs but they do not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks.

Irregular galaxy

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An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.

Dwarf galaxy

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Despite the prominence of large elliptical and spiral galaxies, most galaxies in the Universe are dwarf galaxies. These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.

Interacting

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Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust. Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge.

Starburst

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Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst. If they continue to do so, then they would consume their reserve of gas in a time span less than the lifespan of the galaxy. Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy.

Active galaxy

A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus (AGN). A significant portion of the total energy output from the galaxy is emitted by the active galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy.

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The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region of the galaxy. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc. In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light. The mechanism for producing these jets is not well understood.

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The main known types are: Seyfert galaxies, quasars, Blazars, LINERS and Radio galaxy.

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  • images: NASA/ESA, Hubble (via wikipedia)

Twinkling stars are far more desirable to poet…

Twinkling stars are far more desirable to poets and romantics than to astronomers. Even in the near-pristine seeing conditions over Chile, home to ESO’s fleet of world-class telescopes, turbulence in Earth’s atmosphere causes stars to twinkle, blurring our view of the night sky.

These four laser beams are specially designed to combat this turbulence. The intense orange beams dominating this image originate from the 4 Laser Guide Star Facility, a state-of-the-art component of the Adaptive Optics Facility of ESO’s Very Large Telescope (VLT). Each beam is some 4000 times more powerful than a standard laser pointer! Each creates an artificial guide star by exciting sodium atoms high in the Earth’s upper atmosphere and causing them to glow.

Creating artificial guide stars allows astronomers to measure and correct for atmospheric distortion, by adjusting and calibrating the settings of their observing equipment to be as accurate as possible for that particular area of sky. This gives the VLT a crystal-clear view of the cosmos, so it can capture the wonders of the Universe in stunning detail.

The first image was made using a drone flying over the VLT by the ESO Photographic Ambassador, Gerhard Hüdepohl.

Source: eso.org

Image credit:

ESO/G. Hüdepohl, Babak Tafreshi, Mark McCaughrean