Tuesday, 23 October, 2018

Supernova Remnants

Posted on Friday, 30 January, 2015Sunday, 20 August, 2017 by Zheluo Cai

A supernova remnant is the structure resulting from the explosion of a star in a supernova, bounded by an expanding shock wave, and consists of ejected material expanding from the explosion and the interstellar material it sweeps up. There are two common routes to a supernova: (1) a massive star may run out of fuel, ceasing to generate fusion energy in its core, and collapsing inward under the force of its own gravity to form a neutron star or a black hole; (2) a white dwarf star may accumulate material from a companion star until it reaches a critical mass and undergoes a thermonuclear explosion. In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 10% the speed of light, that is, about 30,000 km/s. These ejecta are highly supersonic: assuming a typical temperature of the interstellar medium of 10,000 K, the Mach number can initially be > 1000. Therefore, a strong shock wave forms ahead of the ejecta, that heats the upstream plasma up to temperatures well above millions of K. The shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds of thousands of years and over tens of parsecs before its speed falls below the local sound speed.

Credit: Wikipedia

Tajik is a general designation for a wide range of native Persian-speaking people of Iranian origin, with current traditional homelands in present-day Tajikistan, Afghanistan and Uzbekistan. Not all Tajiks speak a variety of modern Persian. They may speak any one of the extant Iranian languages. For example, the Tajiks of China are actually Pamiris and speak the Eastern Iranian Pamiri languages and are distinct from more western Tajiks. They are concentrated in the Oxus Basin, the Farḡāna valley (Tajikistan and parts of Uzbekistan) and on both banks of the upper Oxus, i.e., the Pamir Mountains (Mountain Badaḵšān, in Tajikistan) and northeastern Afghanistan and western Afghanistan (Badaḵšān, Kābol, Herat, Balkh, Mazar-i-Sharif, Ghazni and other urban regions). Historically, the Tajiks were initially agriculturalists but urbanized rather early in ancient times as indicated by their role in the history of what later became Greater Khorasan. Several surviving ancient urban centres of the Tajik people include Herat, Samarkand, Bukhara, Khujand, and Kabul.

Planetary Nebula NGC 2438.
Image Credit & Copyright: Daniel López, IAC. NGC 2438 is a planetary nebula, the gaseous shroud cast off by a dying sunlike star billions of years old whose central reservoir of hydrogen fuel has been exhausted. About 3,000 light-years distant, it lies within the boundaries of the nautical constellation Puppis. This image of NGC 2438 highlights a halo of glowing atomic gas over 4.5 light-years across, extending beyond the nebula's brighter inner ring. Similar haloes have been found in deep images of other planetary nebulae, produced during the earlier active phases of their ageing central stars.

The current flag of Myanmar was adopted on 21 October 2010. The new flag was introduced along with implementing changes to the country's name, which were laid out in the 2008 Constitution. The design of the flag has three horizontal stripes of yellow, green and red with a five-pointed white star in the middle. The three colours of the stripes are meant to symbolise solidarity, peace and tranquility, and courage and decisiveness. The white star that symbolised the Union of Burma.

The Elgin Marbles are a collection of Classical Greek marble sculptures made under the supervision of Phidias and his assistants in c. 447 to 438 BC; originally part of the temple of the Parthenon. From 1801 to 1812, agents of Thomas Bruce, 7th Earl of Elgin removed about half of the surviving sculptures and transported them to Britain. The half not removed is now displayed in the Acropolis Museum. In Britain, the acquisition of the collection was not supported by some, and the Earl's actions was likened to vandalism. Following a public debate in Parliament and its subsequent exoneration of Elgin, he sold the Marbles to the British government in 1816. They were then passed to the British Museum, where they are now on display in the purpose-built Duveen Gallery.

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