[ti:Astronomers Identify ‘White Dwarf’ Star that Survived Massive Explosion]
[by:www.21voa.com]
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[00:00.04]Scientists have identified a star
[00:04.40]that, they say, survived a massive explosion
[00:09.12]and ended up being brighter than before.
[00:13.88]The star is a kind known as a "white dwarf."
[00:18.68]These are very dense objects
[00:22.16]that have a mass about equal to the sun.
[00:26.60]But they can be as large as planets like Earth.
[00:30.80]A white dwarf is formed when the star burns off
[00:36.20]all its central nuclear fuel and loses its outer layers.
[00:42.12]Astronomers observed the white dwarf star
[00:46.92]with the Hubble Space Telescope.
[00:49.96]The telescope is operated by the American Space Agency NASA.
[00:57.04]The Hubble has discovered "the oldest burned-out stars
[01:02.08]in our Milky Way Galaxy," NASA reported.
[01:07.36]Some ancient white dwarf stars seen by Hubble
[01:12.28]are estimated to be 12 to 13 billion years old.
[01:17.80]That is nearly as old as the universe itself.
[01:22.20]The observed star is in a galaxy called NGC 1309.
[01:29.84]It is about three quarters the size of our Milky Way.
[01:35.88]The white dwarf is about 108 million light years from Earth.
[01:41.76]A light year is the distance light travels in a year.
[01:47.88]The observation team said that at some point,
[01:52.16]the white dwarf reached a mass that was 1.4 times the sun's mass.
[01:59.24]This led to thermonuclear reactions
[02:03.52]in the star's center that caused a supernova event.
[02:09.04]NASA describes a supernova as an "extremely bright,
[02:15.32]super-powerful explosion of a star."
[02:19.16]A supernova takes place at the end of a star's life.
[02:25.44]The scientists say the supernova
[02:29.04]should have killed off the white dwarf star but did not.
[02:34.36]The research was led by a team
[02:37.88]at the Las Cumbres Observatory in California.
[02:42.12]The lead writer of a study describing the finding
[02:47.00]was astronomer Curtis McCully.
[02:50.56]He told Reuters news agency, "During the explosion,
[02:56.36]radioactive material was produced.
[03:00.12]This is what powers the brightness of the supernova."
[03:05.80]He added that some of the material "was left over
[03:10.20]in the surviving remnant star" and acted as fuel to heat it.
[03:17.12]"We were quite surprised that the star itself
[03:21.12]had not been destroyed but had actually survived
[03:25.12]and is brighter than before it exploded," McCully said.
[03:31.04]There are different kinds of supernova events.
[03:35.36]Each depends on the size and chemical makeup of the star
[03:41.08]and the power of the explosion.
[03:44.20]This observed white dwarf produced an uncommon
[03:49.24]kind of supernova called "Type Iax,"
[03:53.72]the Las Cumbres Observatory said in a statement.
[03:57.96]The team said this kind is less bright
[04:02.20]and weaker than traditional "Type Ia" supernovas.
[04:07.80]Scientists believe this kind of supernova
[04:12.52]does not destroy the white dwarf.
[04:15.32]Instead, it leaves behind an "undead" remnant.
[04:21.04]This theory is confirmed by the new observations,
[04:26.28]the research team said.
[04:29.44]"We have called these objects 'zombie stars' for this very reason.
[04:35.80]They died, but not quite," McCully said.
[04:40.80]Earlier experiments involving supernova activity
[04:45.88]had suggested that in some cases,
[04:49.44]white dwarf stars were able to survive these kinds of events.
[04:55.20]"It's exciting to think that was telling us something
[05:00.12]about the actual physics of these supernovas," McCully added.
[05:07.36]Scientists have discovered about 50 supernovas of this kind.
[05:13.84]But until now, they had not been able to identify
[05:19.00]the surviving white dwarf star.
[05:22.32]Scientists believe that our sun
[05:25.64]is also likely to become a white dwarf one day.
[05:30.36]This happens to about 97 percent of stars.
[05:35.96]But the event involving our sun is not expected to happen
[05:41.96]until at least 5 billion years from now.
[05:46.44]I'm Bryan Lynn. 更多听力请访问21VOA.COM
END OF TRACK. "END OF TRACK." The two men bowed. "Whoever was that person you were talking to?" she enquired, as soon as they stood together. The took of triumph faded from her eyes, she had grown worn and weary. The roses were wilting on the walls, the lights were mostly down now. Hetty, looking in to see if anything was wanted, found herself driven away almost fiercely. I only saw Master Jervie once when he called at tea time, The year 1747 was opened by measures of restriction. The House of Lords, offended at the publication of the proceedings of the trial of Lord Lovat, summoned the parties to their bar, committed them to prison, and refused to liberate them till they had pledged themselves not to repeat the offence, and had paid very heavy fees. The consequence of this was that the transactions of the Peers were almost entirely suppressed for nearly thirty years from this time, and we draw our knowledge of them chiefly from notes taken by Horace Walpole and Lord Chancellor Hardwicke. What is still more remarkable, the reports of the House of Commons, being taken by stealth, and on the merest sufferance, are of the most meagre kind, sometimes altogether wanting, and the speeches are given uniformly under fictitious names; for to have attributed to Pitt or Pelham their[112] speeches by name would have brought down on the printers the summary vengeance of the House. Many of the members complained bitterly of this breach of the privileges of Parliament, and of "being put into print by low fellows"; but Pelham had the sense to tolerate them, saying, "Let them alone; they make better speeches for us than we can make for ourselves." Altogether, the House of Commons exhibited the most deplorable aspect that can be conceived. The Ministry had pursued Walpole's system of buying up opponents by place, or pension, or secret service money, till there was no life left in the House. Ministers passed their measures without troubling themselves to say much in their behalf; and the opposition dwindled to Sir John Hinde Cotton, now dismissed from office, and a feeble remnant of Jacobites raised but miserable resistance. In vain the Prince of Wales and the secret instigations of Bolingbroke and Doddington stimulated the spirit of discontent; both Houses had degenerated into most silent and insignificant arenas of very commonplace business. "It certainly will be. Miss Widgeon," answered Maria, with strictly "company manners." "One who has never had a brother exposed to the constant dangers of army life can hardly understand how glad we all feel to have Si snatched from the very jaws of death and brung back to us." "Just plug at 'em as you would at a crow, and then go on your way whistlin'?" persisted Harry. "Hurroo!" echoed Hennessey; "that's the ticket." "Come forward, keeper," continued the baron, "and state how these arrows came into your hands!" "Yes." HoMEJULIA京香2018下载
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