ASTRONOMY - North Star: Polaris and Surrounding Dust

 2021 April 28

North Star: Polaris and Surrounding Dust
Image Credit & Copyright: Bray Falls

Explanation: Why is Polaris called the North Star? First, Polaris is the nearest bright star toward the north spin axis of the Earth. Therefore, as the Earth turns, stars appear to revolve around Polaris, but Polaris itself always stays in the same northerly direction -- making it the North Star. Since no bright star is near the south spin axis of the Earth, there is currently no South Star. Thousands of years ago, Earth's spin axis pointed in a slightly different direction so that Vega was the North Star. Although Polaris is not the brightest star on the sky, it is easily located because it is nearly aligned with two stars in the cup of the Big Dipper. Polaris is near the center of the eight-degree wide featured image, an image that has been digitally manipulated to suppress surrounding dim stars but accentuate the faint gas and dust of the Intergalactic Flux Nebula (IFN). The surface of Cepheid Polaris slowly pulsates, causing the star to change its brightness by a few percent over the course of a few days.

ASTRONOMY - Animation: Black Hole Destroys Star

 2021 April 27

Animation: Black Hole Destroys Star
Video Illustration Credit: DESY, Science Communication Lab

Explanation: What happens if a star gets too close to a black hole? The black hole can rip it apart -- but how? It's not the high gravitational attraction itself that's the problem -- it's the difference in gravitational pull across the star that creates the destruction. In the featured animated video illustrating this disintegration, you first see a star approaching the black hole. Increasing in orbital speed, the star's outer atmosphere is ripped away during closest approach. Much of the star's atmosphere disperses into deep space, but some continues to orbit the black hole and forms an accretion disk. The animation then takes you into the accretion disk while looking toward the black hole. Including the strange visual effects of gravitational lensing, you can even see the far side of the disk. Finally, you look along one of the jets being expelled along the spin axis. Theoretical models indicate that these jets not only expel energetic gas, but create energetic neutrinos -- one of which may have been seen recently on Earth.

PRATIQUE/POURQUOI - Pourquoi l'électricité peut-elle tuer ?

Une décharge électrique peut tuer de trois façons: brûlure, asphyxie et arrêt cardiaque. Bien que notre corps produise lui-même naturellement de l'électricité pour commander ses fonctions vitales, une trop forte intensité de courant perturbe le système. Dès 20 milliampères les muscles se contractent, risquant ainsi à bloquer la respiration: c'est l'asphyxie. Entre 100 et 300 milliampères le coeur s'emballe, de petites contractions rapides apparaissent: c'est la fibrillation, aux effets souvent irréversibles et quelquefois fatals. Soumis à des courants plus forts, le coeur ne fibrille pas mais la résistance électrique du corps s'opposant à la circulation électrique échauffe les tissus et brûle les organes.

MINERAUX - La marcassite, une cousine de la pyrite

La marcassite est un minéral composé de disulfure de fer (FeS₂). Elle fut longtemps confondue avec la pyrite jusqu'à ce que le minéralogiste français René Just Haüy la distingue en 1814. Couleur : jaune de bronze pâle. © Leon Hupperichs, Wikimedia Commons, CC by-sa 3.0

 

ASTRONOMY - Threads of NGC 1947

 2021 April 26

Threads of NGC 1947
Image Credit: ESA/Hubble & NASA, D. Rosario; Acknowledgment: L. Shatz

Explanation: Found in far southern skies, deep within the boundaries of the constellation Dorado, NGC 1947 is some 40 million light-years away. In silhouette against starlight, obscuring lanes of cosmic dust thread across the peculiar galaxy's bright central regions. Unlike the rotation of stars, gas, and dust tracing the arms of spiral galaxies, the motions of dust and gas don't follow the motions of stars in NGC 1947 though. Their more complicated disconnected motion suggest this galaxy's visible threads of dust and gas may have come from a donor galaxy, accreted by NGC 1947 during the last 3 billion years or so of the peculiar galaxy's evolution. With spiky foreground Milky Way stars and even more distant background galaxies scattered through the frame, this sharp Hubble image spans about 25,000 light-years near the center of NGC 1947.

ASTRONOMY - Planetary Nebula Mz3: The Ant Nebula

 2021 April 25

Planetary Nebula Mz3: The Ant Nebula
Image Credit: R. Sahai (JPL) et al., Hubble Heritage Team, ESA, NASA

Explanation: Why isn't this ant a big sphere? Planetary nebula Mz3 is being cast off by a star similar to our Sun that is, surely, round. Why then would the gas that is streaming away create an ant-shaped nebula that is distinctly not round? Clues might include the high 1000-kilometer per second speed of the expelled gas, the light-year long length of the structure, and the magnetism of the star featured here at the nebula's center. One possible answer is that Mz3 is hiding a second, dimmer star that orbits close in to the bright star. A competing hypothesis holds that the central star's own spin and magnetic field are channeling the gas. Since the central star appears to be so similar to our own Sun, astronomers hope that increased understanding of the history of this giant space ant can provide useful insight into the likely future of our own Sun and Earth.

ASTRONOMY - Flying Over the Earth at Night II

 2021 April 23

Flying Over the Earth at Night II
Video Credit: NASA, Gateway to Astronaut Photography, ISS Expedition 53; Music: The Low Seas (The 126ers)

Explanation: Recorded during 2017, timelapse sequences from the International Space Station are compiled in this serene video of planet Earth at Night. Fans of low Earth orbit can start by enjoying the view as green and red aurora borealis slather up the sky. The night scene tracks from northwest to southeast across North America, toward the Gulf of Mexico and the Florida coast. A second sequence follows European city lights, crosses the Mediterranean Sea, and passes over a bright Nile river in northern Africa. Seen from the orbital outpost, erratic flashes of lightning appear in thunder storms below and stars rise above the planet's curved horizon through a faint atmospheric airglow. Of course, from home you can always check out the vital signs of Planet Earth Now.

ASTRONOMY - Planet Earth at Twilight

 2021 April 22

Planet Earth at Twilight
Image Credit: ISS Expedition 2 Crew, Gateway to Astronaut Photography of Earth, NASA

Explanation: No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture was taken in June of 2001 from the International Space Station orbiting at an altitude of 211 nautical miles. But you can check out the vital signs of Planet Earth Now.

ASTRONOMY - Centaurus A's Warped Magnetic Fields

 2021 April 21

Centaurus A's Warped Magnetic Fields
Image Credit: Optical: European Southern Observatory (ESO) Wide Field Imager; Submillimeter: Max Planck Institute for Radio Astronomy/ESO/Atacama Pathfinder Experiment (APEX)/A.Weiss et al; X-ray and Infrared: NASA/Chandra/R. Kraft; JPL-Caltech/J. Keene; Text: Joan Schmelz (USRA)

Explanation: When galaxies collide -- what happens to their magnetic fields? To help find out, NASA pointed SOFIA, its flying 747, at galactic neighbor Centaurus A to observe the emission of polarized dust -- which traces magnetic fields. Cen A's unusual shape results from the clash of two galaxies with jets powered by gas accreting onto a central supermassive black hole. In the resulting featured image, SOFIA-derived magnetic streamlines are superposed on ESO (visible: white), APEX (submillimeter: orange), Chandra (X-rays: blue), and Spitzer (infrared: red) images. The magnetic fields were found to be parallel to the dust lanes on the outskirts of the galaxy but distorted near the center. Gravitational forces near the black hole accelerate ions and enhance the magnetic field. In sum, the collision not only combined the galaxies’ masses -- but amplified their magnetic fields. These results provide new insights into how magnetic fields evolved in the early universe when mergers were more common.

ASTRONOMY - Ingenuity: First Flight over Mars

 2021 April 20

Ingenuity: First Flight over Mars
Video Credit: NASA, JPL-Caltech, ASU, MSSS

Explanation: What's the best way to explore Mars? Perhaps there is no single best way, but a newly demonstrated method shows tremendous promise: flight. Powered flight has the promise to search vast regions and scout out particularly interesting areas for more detailed investigation. Yesterday, for the first time, powered flight was demonstrated on Mars by a small helicopter named Ingenuity. In the featured video, Ingenuity is first imaged by the Perseverance rover sitting quietly on the Martian surface. After a few seconds, Ingenuity's long rotors begin to spin, and a few seconds after that -- history is made as Ingenuity actually takes off, hovers for a few seconds, and then lands safely. More tests of Ingenuity's unprecedented ability are planned over the next few months. Flight may help humanity better explore not only Mars, but Saturn's moon Titan over the next few decades.