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We propose to study the last HST-accessible white dwarf (WD) cooling sequence (CS) for a nearby globular cluster (GC) comma the chemically complex comma extreme blue horizontal branch cluster NGC 6752. Over 97% of stars end their lives as WDs comma and the WD CS provides constraints not only on the age comma but also potentially the star formation history of a GC. The CS of WDs also lies in the least-explored region of the color-magnitude diagram of old stellar populations. Recent deep imaging with HST has successfully reached the end of the WD CS in only three d_commaclassicald_comma old GCs comma M4 comma NGC 6397 and 47 Tuc comma and reveals an unexpectedly complex comma and double-peaked comma WD CS in the metal rich old open cluster NGC 6791. One more investigation is in progress on the massive globular Omega Centauri comma where over 14 sub-populations are known to exist. While almost every cluster is known to host multiple populations comma every single cluster is unique. NGC 6752 is a bridge between the relatively simple globular clusters comma and Omega Cen comma the most complex globular cluster known. NGC 6752 has an extended blue horizontal branch comma a collapsed core and 3 chemically distinct populations. It is our last chance to add diversity to our very limited sample of WD CS comma so far containing only 3 globular clusters comma one old open cluster comma and the complex Omega Cen system. We need to undertake this investigation while HST is still operational comma as there is no foreseeable opportunity in the post-HST era to have one extra WD CS in the homogeneus optical photometric system of HST.我们提出针对近距球状星团（globular cluster, GC）——化学组成复杂的极端蓝水平分支星团NGC 6752，开展其最后一批可通过哈勃空间望远镜（Hubble Space Telescope, HST）观测的白矮星（white dwarf, WD）冷却序列（cooling sequence, CS）研究。超过97%的恒星最终会演化为白矮星，而白矮星冷却序列不仅可用于约束球状星团的年龄，还有望为其恒星形成历史提供限制条件。白矮星冷却序列还处于老年恒星种群色-光度图中探索程度最低的区域。近期借助哈勃空间望远镜开展的深场成像已成功在仅3个经典老年球状星团（M4、NGC 6397与47 Tuc）中抵达白矮星冷却序列的终点，并在富金属老年疏散星团NGC 6791中发现了结构出乎意料复杂且呈双峰分布的白矮星冷却序列。针对大质量球状星团半人马座ω星团（Omega Centauri, Omega Cen）的另一项研究正在进行中，该星团已知存在超过14个子星族。尽管几乎每一个星团都被证实拥有多星族，但每个星团都独具特色。NGC 6752是介于相对简单的球状星团与目前已知结构最复杂的半人马座ω星团之间的桥梁。NGC 6752拥有延展的蓝水平分支、坍缩核心以及3个化学组成迥异的星族。目前已观测到白矮星冷却序列的样本非常有限，仅包含3个球状星团、1个老年疏散星团与结构复杂的半人马座ω星团系统，而本研究是我们为该样本增添多样性的最后契机。我们必须在哈勃空间望远镜仍处于服役期内开展此项研究，因为在后哈勃时代，我们暂无机会借助哈勃的同质光学测光系统获取额外的白矮星冷却序列观测数据。