Free Astronomy Magazine May-June 2023 ARABIC VERSION

MAY-JUNE 2023 ing the motion of two co-orbiting stars, straightforward Newtonian physics can be used to measure their masses. However, these measure- ments can be uncertain if the dwarf’s companion star is in a long- period orbit of hundreds or thou- sands of years. Orbital motion can be measured by telescopes only over a brief slice of the dwarf’s or- bital motion. For this companion- less white dwarf, researchers had to employ a trick of nature, called gravitational microlensing. The light from a background star was slightly deflected by the gravitational warp- ing of space by the foreground dwarf star. As the white dwarf passed in front of the background star, microlensing caused the star to appear temporarily offset from its actual position on the sky. The results are reported in the jour- nal Monthly Notices of the Royal As- tronomical Society. The lead author is Peter McGill, formerly of the Uni- versity of Cambridge in the United Kingdom and now based at the Uni- versity of California, Santa Cruz. McGill used Hubble to precisely measure how light from a distant star bent around the white dwarf, known as LAWD 37, causing the background star to temporarily change its apparent position in the sky. Kailash Sahu of the Space Tele- scope Science Institute in Baltimore, Maryland, USA, the principal Hub- ble investigator on this latest obser- vation, first used microlensing in 2017 to measure the mass of an- other white dwarf, Stein 2051 B. But that dwarf is in a widely sepa- rated binary system. “Our latest ob- servation provides a new benchmark because LAWD 37 is all by itself,” Sahu said. The collapsed remains of a star that burned out 1 billion years ago, LAWD 37 has been extensively studied because it is only 15 light-years away in the constellation Musca. “Because this white dwarf is relatively close to us, we’ve got lots of data on it — we’ve got information about its spectrum of light, but the missing piece of the puzzle has been a measurement of its mass,” said McGill. The team zeroed-in on the white dwarf thanks to ESA’s Gaia mission, which makes extraordinarily precise measurements of nearly two billion star positions. Multiple Gaia obser- vations can be used to track a star’s motion. Based on these data, as- tronomers were able to predict that LAWD 37 would briefly pass in front of a background star in November 2019. Once this was known, Hubble was used to precisely measure over several years how the background star’s apparent position in the sky was temporarily deflected during the white dwarf’s passage. “These events are rare, and the ef- fects are tiny,” said McGill. “For in- stance, the size of our measured offset is like measuring the length of a car on the Moon as seen from Earth.” Since the light from the background star was so faint, the main challenge for astronomers was extracting its image from the glare of the white dwarf, which is 400 times brighter than the background star. Only Hub- ble can make these kinds of high- ﻣﺠﻠﺔ FreeAstronomy ﺎﻟﻌﺮ ﺔ ٢٧ ﺗُﻈﻬــــﺮﻫـــــﺬﻩ اﻟﺮﺳـــــﻮم اﻟﻤﺘﺤﺮﻛـــــﺔ ــﻢ ﺣﺮﻛــﺔ ﻧ ﻗــﺰم أﺑــ ﺾ ﻤــﺮأﻣــﺎم ـﺪة ـﺔ ﻌ ﻧ ﻤﺔ ﺧﻠﻔ . أﺛﻨـﺎء اﻟﻤـﺮور ــــــــــــ ــــــــــــﺪو أن اﻟــــــــــــﻨ ﻢ اﻟ ﻌ ﻳ ﺪ ﻐـــــــــــــ ﻣﻮﻗﻌـﮫ ﻗﻠــ ﻼً ﻷن اﻟﻀـﻮء اﻟﻘــﺎدم ﻣﻨــــــﮫ ــــــﺔ ﻗــــــﺪ اﻧﺤــــــﺮف ﻔﻌــــــﻞ ﺟﺎذﺑ اﻟﻘــﺰم اﻷﺑــ ﺾ . ﺎﺳــﺘﺨﺪام ﻫـــﺬﻩ ـ اﻟــ ﺤ ﻠﺔ ﻣـــﻦ اﻟﻄﺒ ﻌـــﺔ ﻗـــﺎم ﻋﻠﻤـــﺎء اﻟﻔﻠـﻚ ﺎﺳـﺘﺨﺪام ﺗﻠﺴـ ﻮبﻫﺎﺑــﻞ اﻟﻔﻀــــــﺎﺋﻲ اﻟﺘــــــﺎﺑﻊ ﻟﻨﺎﺳــــــﺎ / و ﺎﻟــــــﺔ اﻟﻔﻀــﺎء اﻷورو ــﺔ ﺑ ﻟﻠﻤــﺮة اﻷوﻟــﻰ ـــــــﻢ ﻣﻨﻔـــــــﺮد ﻏــــــــ ـــــــﺎسﻛﺘﻠـــــــﺔ ﻧ ﻘ ﺷﻤﺴـﻨﺎ . ﺸــ ﻞ ﻣ ﺎﺷـﺮ ﺗــﻢ اﻟﺘﻨﺒــﺆ ﺎ ﻟﺘـــــــــــــــــﺄﺛ اﻟﻤﺴـــــــــــــــــﻤﻰ ﻌﺪﺳـــــــــــــــــﺔ ـ اﻟـ ﺠﺎذﺑ ﺔ ﻛﻨﺘﻴﺠــﺔ ﻟﻨﻈﺮ ـﺔ اﻟﻨﺴــﺒ ﺔ اﻟﻌﺎﻣـــﺔ ﻷ ﻨﺸـــﺘﺎﻳﻦ ﻣﻨـــﺬ ﻣﻨـــﺬ ﻗـــﺮن ﻣﻀﻰ . ﻣﻀﻰ ﻗﺪﻣﺖ ارﺻﺎد ﻛﺴﻮف اﻟﺸﻤﺲﻓﻲﻋﺎم ١٩١٩ أول دﻟﻴﻞ ﻣ ﺎﺷﺮﻋﻠﻰ اﻟﻨﺴـﺒ ﺔ اﻟﻌﺎﻣـﺔ . ﻟﻜـﻦ آ ﻨﺸـﺘﺎﻳﻦ ﻟـﻢ ﻌﺘﻘﺪ أ ـﺔ ﻧﮫ ﻤﻜﻦ إﺟـﺮاء اﻟﺘﺠﺮ ـﺔ ﻧﻔﺴـﻬﺎ ﻟﻠﻨﺠـﻮم ﺧـﺎرجﺷﻤﺴـﻨﺎ ﺴـ ﺐ اﻟﺪﻗـﺔ اﻟﻤﻄﻠﻮ . ] NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI) ﻳﻤﻜﻦ ﺧﻼل ﻣﺸﺎﻫﺪة ﺣﺮﻛﺔ ﻧ ﺠﻤ ﻳـﺪوران ﰲ ﻣـﺪار ﻣﺸـﱰك اﺳـﺘﺨﺪام ﻓﻴﺰﻳـﺎء ﻧﻴـﻮﺗﻦ ﻣ ﺒﺎﴍة ﻟﻘﻴﺎس ﻛﺘﻠﺘﻴﻬﻤﺎ . وﻣﻊ ذﻟﻚ ﻳﻤﻜﻦ أن ﺗﻜﻮن ﻫﺬه اﻟﻘﻴﺎﺳﺎت ﻏـ ﻣﺆﻛـﺪة إذا ﻛـﺎن اﻟﻨﺠﻢ ا ﺼﺎﺣﺐ ﻟﻠﻘﺰم ﰲ ﻣﺪار ﻃﻮﻳﻞ اﻟﻔـﱰة ﻳﺼﻞ إﱃ ﻣﺌﺎت أو آﻻف اﻟﺴﻨ . ﻳﻤﻜﻦ ﻗﻴﺎس اﻟﺤﺮﻛﺔ ا ﺪارﻳﺔ ﺑﻮاﺳﻄﺔ اﻟﺘﻠﺴﻜﻮﺑﺎت ﻓﻘـﻂ ﻋﱪ ﴍﻳﺤﺔ ﻗﺼـ ة ﻣـﻦ اﻟﺤﺮﻛـﺔ ا ﺪارﻳـﺔ ﻟﻠﻘﺰم . ﺑﺎﻟﻨﺴـﺒﺔ ﻟﻬـﺬا اﻟﻘـﺰم اﻷﺑـﻴﺾﻏـ اﻟﺮﻓﻴﻖ ، ﻛﺎن ﻋﲆ اﻟﺒﺎﺣﺜ اﺳﺘﺨﺪام ﺧﺪﻋـﺔ ﻣ ـﻦ اﻟﻄﺒﻴﻌ ـﺔ ﺗﺴـﻤﻰ اﻟﻌﺪﺳـﺔ اﻟﺘﺜﺎﻗﻠﻴ ـﺔ اﻟﺪﻗﻴﻘﺔ . اﻧﺤﺮف اﻟﻀﻮء ا ﻨﺒﻌـﺚ ﻣـﻦ ﻧﺠـﻢ اﻟﺨﻠﻔﻴﺔ ﻗﻠﻴﻼً ﺑﺴﺒﺐ اﻻﻧﺤﻨﺎء اﻟﺜﻘﺎﱄ ﻟﻠﻔﻀﺎء ﺑﻮاﺳﻄﺔ اﻟﻨﺠﻢ اﻟﻘﺰم اﻷﻣـﺎﻣﻲ . ﻋﻨـﺪﻣﺎ ﻣـﺮ اﻟﻘﺰم اﻷﺑﻴﺾ أﻣـﺎم ﻧﺠـﻢ اﻟﺨﻠﻔﻴـﺔ ﺗﺴـﺒﺐ اﻟ ﻌﺪﺳﺔ اﻟﺪﻗﻴﻘﺔ ﰲ ﻇﻬﻮر اﻟﻨﺠﻢ ﺑﻌﻴـﺪًا ﻋـﻦ ﻣﻮﻗﻌﻪ اﻟﻔﻌﲇ ﰲ اﻟﺴﻤﺎء . ﻟﻘﺪ ﻫﺬه ﴩ ـ ﺗﻢ ﻧ اﻟﻨﺘﺎﺋﺞ اﻟﺠﺪﻳﺪة اﰲ ﺠﻠﺔ اﻟﺸﻬﺮﻳﺔ ﻟﻠﺠﻤﻌﻴـﺔ اﻟﻔﻠﻜﻴﺔ ا ﻠﻜﻴﺔ . ا ﺆﻟﻒ اﻟﺮﺋﻴـ ـ ﴘ ﻫـﻮ ﺑﻴـﱰ ﻣﺎﻛﺠﻴﻞ اﻟﺬي ﻛﺎن ﻳ ﻌﻤﻞ ﺳـﺎﺑﻘ ﺎً ﰲ ﺟﺎﻣ ﻌـﺔ ﻛﺎﻣﱪﻳﺪج ﰲ ا ﻤﻠﻜﺔ ا ﺘﺤـﺪة وﻣﻘـﺮه اﻵن ﰲ ﺟﺎﻣﻌﺔ ﻛﺎﻟﻴﻔﻮرﻧﻴﺎ . ﺳﺎﻧﺘﺎ ﻛﺮوز اﺳﺘﺨﺪم ﻣﺎﻛﺠﻴﻞ ﺗﻠﺴـﻜﻮب ﻫﺎﺑـﻞ ﻟﻴﻘـﻴﺲ ﺑﺪﻗﺔ ﻛﻴﻒ ﻳﻨﺤﻨﻲ اﻟﻀﻮء اﻟﻘـﺎدم ﻣـﻦ ﻧﺠـﻢ ﺑﻌﻴﺪ ﺣﻮل اﻟﻘﺰم اﻷﺑﻴﺾ ، ا ﻌـﺮوف ﺑﺎﺳـﻢ LAWD 37 ﻣﻤــﺎ ﺗﺴــﺒﺐ ﰲ ﺗﻐﻴــ ﻧﺠ ـﻢ اﻟﺨﻠﻔﻴﺔ ﻮﻗﻌﻪ اﻟﻈﺎﻫﺮي ﻣﺆﻗﺘًﺎ ﰲ اﻟﺴـﻤﺎء . ﻛﺎﻳﻼشﺳﺎﻫﻮ ﻣﻦ ﻣﻌﻬﺪ ﻋﻠـﻮم اﻟﺘﻠﺴـﻜﻮب ا ﻟﻔﻀﺎﺋﻲ ﰲ ﺑـﺎﻟﺘﻴﻤﻮر ﺑ ﻤﺎرﻳﻼﻧـﺪ ﺑ ﺎﻟﻮﻻﻳـﺎت ا ﺘﺤـﺪة اﻷﻣﺮﻳﻜﻴـﺔ ، اﻟﺒﺎﺣـﺚ اﻟﺮﺋﻴـ ﴘﰲ ـ ﺗﻠﺴﻜﻮب ﻫﺎﺑﻞ ﰲ ﻫﺬ ا اﻟﺮﺻﺪ اﻷﺧ اﺳﺘﺨﺪم اﻟﻌﺪﺳـﺔ اﻟﺪﻗﻴﻘـﺔ ﻷول ﻣــﺮة ﰲ ﻋـﺎم ٢٠١٧ ﻟﻘﻴﺎس ﻛﺘﻠﺔ ﻗﺰم أﺑﻴﺾ آﺧﺮ ، Stein 2051 B ﰲ ﻧﻈﺎم ﺛﻨﺎﺋﻲ ﻣﻨﻔﺼﻞ ﻋﲆ ﻧﻄﺎق واﺳﻊ . ﻗﺎل : " ﺳﺎﻫﻮ ﺗـﻮﻓﺮ ﻣﻼﺣﻈﺘﻨـﺎ اﻷﺧـ ة ﻣﻌﻴـﺎرًا ﺟﺪﻳﺪًا ﻷن LAWD 37 ﻫﻲ ﻛـﻞ ﳾء ﰲ ﺣـﺪ ". ذاﺗﻬﺎ اﻟﺒﻘﺎﻳﺎ ا ﻨﻬﺎرة ﻟﻨﺠﻢ اﺣﱰق ﻗﺒﻞ ﻣﻠﻴﺎر ﻋﺎم ، ﺗﻤﺖ دراﺳﺔ LAWD 37 ﻋﲆ ﻧﻄـﺎق واﺳـﻊ ﻷﻧﻪ ﻻ ﻳﺒﻌﺪ ﺳﻮى ١٥ ﺳﻨﺔ ﺿﻮﺋﻴﺔ ﰲ ﻛﻮﻛﺒﺔ اﻟﺬﺑﺎﺑﺔ . ﻗﺎل ﻣﺎﻛﺠﻴﻞ " : ﻧﻈﺮًا ﻷن ﻫﺬا اﻟﻘـﺰم اﻷﺑﻴﺾ ﻗﺮﻳﺐ ﻧﺴﺒﻴًﺎ ﻣﻨﺎ ﻓﻠﺪﻳﻨﺎ اﻟﻜﺜـ ﻣـﻦ اﻟﺒﻴﺎﻧﺎت ﻋﻨﻪ ، ﻟﺪﻳﻨﺎ ﻣﻌﻠﻮﻣـﺎت ﺣـﻮل ﻃﻴـﻒ اﻟﻀﻮء اﻟﺨﺎص ﺑﻪ ﻟﻜﻦ اﻟﺠﺰء ا ﻔﻘـﻮد ﻣـﻦ ــــ اﻟﻠﻐﺰ ﻛﺎن ﻗﻴﺎﺳًﺎ ﻟﻜﺘﻠﺘﻪ . رﻛـﺰ اﻟﻔﺮﻳـﻖ ﻋـﲆ اﻟﻘﺰم اﻷﺑﻴﺾ ﺑﻔﻀـﻞ ﻣﻬﻤـﺔ ا ﺮﻛﺒـﺔ ﻏﺎﻳـﺎ اﻟﺘﺎﺑﻌـﺔ ﻟﻮﻛﺎﻟـﺔ اﻟﻔﻀــﺎء اﻷوروﺑﻴـﺔ واﻟﺘــﻲ ﺗﺠﺮي ﻗﻴﺎﺳﺎت دﻗﻴﻘﺔ ﺑﺸﻜﻞ ﻏ ﻋـﺎدي ـﺎ ﻳﻘـﺮب ﻣـﻦ ﻣﻠﻴـﺎري ﻣﻮﻗـﻊ ﻧﺠـﻢ . ﻳﻤﻜـﻦ اﺳﺘﺨﺪام أرﺻﺎد ا ﺮﻛﺒﺔ ﻏﺎﻳﺎ ا ﺘﻌﺪدة ﻟﺘﺘﺒـﻊ ﺣﺮﻛﺔ اﻟﻨﺠﻢ . ﺑﻨﺎءً ﻋﲆ ﻫـﺬه اﻟﺒﻴﺎﻧـﺎت ﻛـﺎن ﻋﻠﻤﺎء اﻟﻔﻠﻚ ﻗﺎدرﻳﻦ ﻋﲆ اﻟﺘﻨﺒﺆ ﺑﺄن LAWD 37 ﺳﻴﻤﺮ ﻟﻔﱰة وﺟﻴﺰة أﻣﺎم ﻧﺠﻢ ﰲ اﻟﺨﻠﻔﻴﺔ ﻓﻤﱪ ﰲ ﻧﻮ ٢٠١٩ . ﺑﻤﺠﺮد أن ﻋُﺮف ﻫﺬا ﺗﻢ اﺳﺘﺨﺪام ﺗﻠﺴـﻜﻮب ﻫﺎﺑﻞ ﻟﻠﻘﻴﺎس اﻟﺪﻗﻴﻖ ﻋﲆ ﻣﺪى ﻋﺪة ﺳـﻨﻮات ﻛﻴﻒ ﻛﺎن ا ﻮﻗﻊ اﻟﻈﺎﻫﺮي ﻟﻠﻨﺠﻢ ﰲ اﻟﺴﻤﺎء . اﻧﺤﺮﻓﺖ ﻣﺆﻗﺘًﺎ أﺛﻨﺎء ﻣـﺮور اﻟﻘـﺰم اﻷﺑـﻴﺾ . ﻗﺎل ﻣﺎﻛﺠﻴﻞ " : ﻫﺬه اﻷﺣﺪاث ﻧـﺎدرة ر واﻵﺛـﺎ ﻓ ﺿﺌﻴﻠﺔ ﻌـﲆ ﺳـﺒﻴﻞ ا ﺜـﺎل ﺣﺠـﻢ اﻹزاﺣـﺔ ا ﻘﺎﺳﺔ ﻟﺪﻳﻨﺎ ﻣﺜﻞ ﻗﻴﺎس ﻃﻮل اﻟﺴﻴﺎرة ﻋـﲆ اﻟﻘﻤﺮ ﻛﻤﺎ ﺷﻮﻫﺪ ﻣﻦ اﻷرض . ﻧﻈﺮًا ﻷن ﺿﻮء اﻟﻨﺠﻢ ﻛﺎن ﺧﺎﻓﺘًﺎ ﺟـﺪًا ﻛـﺎن اﻟﺘﺤﺪي اﻟﺮﺋﻴﴘ ﻟﻌﻠﻤﺎء اﻟﻔﻠﻚ ﻫﻮ اﺳـﺘﺨ ﺮاج ﺻﻮرﺗﻪ ﻣﻦ وﻫﺞ اﻟﻘﺰم اﻷﺑﻴﺾ وﻫـﻮ أﻛﺜـﺮ ﺳﻄﻮﻋًﺎ ﺑـ ٤٠٠ ﻣـﺮة ﻣـﻦ ﻧﺠـﻢ اﻟﺨﻠﻔﻴـﺔ . ــــــ - ﻣﺎﻳﻮ ﻳﻮﻧﻴﻮ 2023