Category: astrofisica

This is the brightest so-called milli-second pulsar known. This old pulsar has been spun up by the accretion of material from a binary companion star as it expands in its red giant phase. The accretion process results in orbital angular momentum of the companion star being converted to rotational angular momentum of the neutron star, which is now rotating about 174 times a second. It spins so fast that the signal sounds like an overactive bumble-bee. This recording has been made with the Parkes radio telescope in Australia. source

This pulsar lies near the centre of the Vela supernova remnant, which is the debris of the explosion of a massive star about 10,000 years ago. The pulsar (a so-called neutron star) is the collapsed core of this star, rotating with a period of 89 milliseconds or about 11 times a second. This recording has been made with the Parkes radio telescope in Australia.
click here to see the animation / source

A New Signal for a Neutron Star Collision Discovered

  • A neutron star merger without an observed gamma-ray burst has been discovered using NASA’s Chandra X-ray Observatory.
  • This result gives astronomers another way to track down neutron star mergers as well new information about their interiors.
  • This source, called XT2, is located in the Chandra Deep Field-South, the deepest X-ray image ever obtained.
  • By studying how XT2 changed in X-ray brightness, astronomers were able to identify it as two neutron stars that merged into a larger one. read more

Symbiotic R Aquarii

Image Credit: Hubble, NASA, ESA; Processing : Judy Schmidt

While testing a new subsystem on the SPHERE planet-hunting instrument on ESO’s Very Large telescope, astronomers were able to capture dramatic details of the turbulent stellar relationship in the binary star R Aquarii with unprecedented clarity — even compared to observations from the NASA/ESA Hubble Space Telescope.

This image is from the SPHERE/ZIMPOL observations of R Aquarii, and shows the binary star itself, as well as the jets of material spewing from the stellar couple.

Credit: ESO/Schmid et al.

Astronomers Capture First Image of a Black Hole!

Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity. Credit: Event Horizon Telescope Collaboration (read more).

Astronomers Capture First Image of a Black Hole!

Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity. Credit: Event Horizon Telescope Collaboration (read more).

The ESO La Silla Observatory at Sunset. The three major telescopes still in operations are visible on this image: 2.2-m (foreground), NTT (middle), and 3.6-m.

Credit: ESO/H.H.Heyer

Stellar winds are fast moving flows of material (protons, electrons and atoms of heavier metals) that are ejected from stars. These winds are characterised by a continuous outflow of material moving at speeds anywhere between 20 and 2,000 km/s.

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In the case of the Sun, the wind ‘blows’ at a speed of 200 to 300 km/s from quiet regions, and 700 km/s from coronal holes and active regions.

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The causes, ejection rates and speeds of stellar winds vary with the mass of the star. In relatively cool, low-mass stars such as the Sun, the wind is caused by the extremely high temperature (millions of degrees Kelvin) of the corona.

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his high temperature is thought to be the result of interactions between magnetic fields at the star’s surface, and gives the coronal gas sufficient energy to escape the gravitational attraction of the star as a wind. Stars of this type eject only a tiny fraction of their mass per year as a stellar wind (for example, only 1 part in 1014 of the Sun’s mass is ejected in this way each year), but this still represents losses of millions of tonnes of material each second. Even over their entire lifetime, stars like our Sun lose only a tiny fraction of 1% of their mass through stellar winds.

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In contrast, hot, massive stars can produce stellar winds a billion times stronger than those of low-mass stars. Over their short lifetimes, they can eject many solar masses (perhaps up to 50% of their initial mass) of material in the form of 2,000 km/sec winds.

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These stellar winds are driven directly by the radiation pressure from photons escaping the star. In some cases, high-mass stars can eject virtually all of their outer envelopes in winds. The result is a Wolf-Rayet star.

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Stellar winds play an important part in the chemical evolution of the Universe, as they carry dust and metals back into the interstellar medium where they will be incorporated into the next generation of stars. 

source (read more) +

Wolf–Rayet star

Shredded Star Leads to Important Black Hole Discovery

This artist’s illustration shows the region around a supermassive black hole after a star wandered too close and was ripped apart by extreme gravitational forces. Some of the remains of the star are pulled into an X-ray-bright disk where they circle the black hole before passing over the “event horizon,” the boundary beyond which nothing, including light, can escape. The elongated spot depicts a bright region in the disk, which causes a regular variation in the X-ray brightness of the source, allowing the spin rate of the black hole to be estimated. The curved region in the upper left shows where light from the other side of the disk has been curved over the top of the black hole.

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This event was first detected by a network of optical telescopes called the All-Sky Automated Survey for Supernovae (ASASSN) in November 2014. Astronomers dubbed the new source ASASSN14-li and traced the bright flash of light to a galaxy about 290 million light years from Earth. They also identified it as a “tidal disruption” event, where one cosmic object is shredded by another through gravity.

Astronomers then used other telescopes including a flotilla of high-energy telescopes in space — NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton and NASA’s Neil Gehrels Swift observatory — to study the X-rays emitted as the remains of a star swirled toward the black hole at the center of the galaxy.

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The tidal disruption in ASASSN-14li is intriguing because it allowed astronomers to measure the spin rate of the black hole. A black hole has two fundamental properties: mass and spin. While it has been relatively easy for astronomers to determine the mass of black holes, it has been much more difficult to get accurate measurements of their spins.

This debris from the shredded star gave astronomers an avenue to get a direct measure of the black hole’s spin in ASASSN-14li. They found that the event horizon around this black hole is about 300 times the diameter of the Earth, yet rotates once every two minutes (compared to the 24 hours it takes to complete one rotation). This means that the black hole is spinning at least half as fast as the speed of light.

Scientists have determined spin rates for some stellar-mass black holes (those that typically weigh between 5 and 30 solar masses) in our Milky Way galaxy by observing rapid and regular variations in their X-ray brightness. A few supermassive black holes have shown similar variations, but they were only observed to repeat over a few cycles, rather than the 300,000 cycles seen for ASASSN-14li. With only a few cycles, the association of the variations with the spin of the black hole is not secure.

These results will likely encourage astronomers to observe future tidal disruption events for long durations to look for similar, regular variations in their X-ray brightness. source