Omicron Ceti - Mira

大写希腊字母Ω的天琴座 - 红巨星

The cool red giant star historically known as Mira but officially named Omicron Ceti has been known for about 400 years. It was discovered on August 13, 1596, by Dutch astronomer David Fabricus, who mistook it for a nova because it later faded from view. The name Mira means "The Wonderful". It is considered to be the first case of a variable star.

历史上被称为Mira的红巨星，正式名称为Omicron Ceti，已知约400年。它于1596年8月13日由荷兰天文学家David Fabricus发现，他误将其视为新星，因为该星随后亮度下降。Mira的意思是‘奇妙的’。它被认为是第一颗变星的案例。

Mira is a binary system with separation about 70 times more than that between Earth and the Sun, (equal to an angular size of only 0.6 arcseconds -- the apparent diameter of a dime at four miles away). It is the prototype for an entire class of stars known as "Mira-type variables." Although once like our Sun, Mira is now at the end of its life, and has evolved into a cool red giant star that is highly variable in brightness. Contracting and expanding every 332 days, Mira sheds vast amounts of material through its powerful "wind" of gas and dust. The orbital period of the companion star is about 400 years. (NASA)

Mira 是一个双星系统，其分离距离大约是地球和太阳之间的距离的70倍（相当于从四英里外看去，一枚硬币的直径为0.6角秒）。它是一个被称为‘Mira型变星’的整个星类的原型。尽管曾经像我们的太阳一样，Mira 现在已走到生命的末期，已经演化成一颗温度较低的红巨星，其亮度高度变化。Mira 每332天收缩和膨胀一次，通过强大的‘气体和尘埃风’抛射出大量物质。伴星的轨道周期约为400年。（NASA）

The top image above is a NASA Hubble Space Telescope image of the cool red giant star Mira A (right) and its binary companion taken on December 11, 1995 in visible light using the European Space Agency's Faint Object Camera (FOC). At bottom left isHubble's visible-light image of the disk of Mira, revealing an odd, asymmetrical shape resembling a football. This may be tied to dramatic changes occurring during its expansion-contraction cycles, or to the presence of unresolved spots on its surface. Hubble measurements show the red giant star is 700 times larger than our Sun. The lower right image is made in UV light. Hubble resolves a small hook-like appendage extending from Mira, in the direction of the companion that could be material from Mira being gravitationally drawn toward the smaller star. Alternately, it could be that material in Mira's upper atmosphere is being heated due to the companion's presence. PHOTO CREDIT: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics), and NASA

上图是1995年12月11日，使用欧洲空间局的浅场物体相机（FOC）在可见光下拍摄的NASA哈勃空间望远镜图像，显示了冷红色巨星Mira A（右）及其伴星。图底左方是哈勃在可见光下拍摄的Mira的盘状结构，显示出一个奇特的不对称形状，类似于足球。这可能与它在膨胀-收缩周期中的剧烈变化有关，或者与它表面存在未解析的斑点有关。哈勃的测量结果表明，这颗红巨星的大小是太阳的700倍。右下角的图像是在紫外光下拍摄的。哈勃分辨出一个从Mira延伸出的小钩状突起，朝向伴星方向，可能是由于Mira的引力被较小的恒星吸引而形成的物质。或者，也可能是由于伴星的存在，使Mira的上层大气被加热。PHOTO CREDIT: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics), and NASA

If Mira were at the center of our solar system, it would extend out more than 300 million miles, well beyond Mars' orbit and nearly two-thirds of the way to Jupiter. Mira's companion is a white dwarf that is surrounded by material captured from Mira's wind. At a distance of about 400 light-years, Mira is the closest wind-accreting binary system to Earth.

如果弥拉位于太阳系的中心，它将延伸出超过30亿英里，远超火星轨道，几乎到达木星的位置。弥拉的伴星是一个白矮星，周围环绕着从弥拉风中捕获的物质。距离地球约400光年，弥拉是最近的风吸积双星系统。

"Mira is the brightest and most famous long-period pulsating variable in the sky, and gave the name to this whole class of stars. It changes its brightness normally between maxima of about 3rd magnitude and minima of about mag 10, but occasionally brighter maxima up to mag 2.0 are observed (e.g. by William Herschel), or fainter when Mira stays at about magnitude 5. At a distance of about 400 light years, this corresponds to absolute magnitudes of about -2.5 near the maxima and +4.7 near its minima, so giant cool Mira is only about as, or even less luminous than our sun near its minima, but brightens up to about 700 and occasionally even over 1500 solar luminosities near the maximum of its cycle."(seds.org).

Mira 是天空中最亮且最著名的长周期变星，它为这一类恒星命名。它在亮度变化正常范围内，最大亮度约为3等，最小亮度约为10等，但偶尔会达到高达2.0等的最大亮度（例如由威廉·赫歇尔观测到），或在Mira处于约5等时显得较暗。在约400光年距离处，这对应于其最大亮度时的绝对星等约为-2.5，最小亮度时约为+4.7，因此，Mira 这颗红巨星在亮度最低时仅与太阳亮度相当，甚至更小，但在其周期最大时亮度可达约700倍，偶尔甚至超过1500倍的太阳亮度。

A whole class of variable stars have been named after Mira. "Mira-type" stars are variables with a period of around 100 - 1000 days and with visual light variations of more than 2.5 magnitudes. Over 6000 "Mira variables" are now known.

一大类变星被命名为Mira。'Mira型'星是周期在100至1000天之间，视亮度变化超过2.5等的变星。目前已知的'Mira变星'有超过6000颗。

This light curve from AAVSO shows the variation in intensity from the Mira binary system.

这张来自AAVSO的光变曲线显示了Mira双星系统的光强变化。

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Beta Ceti

天琴座

"Beta Ceti is a bright, giant star with a hot corona that radiates about 2,000 times more X-ray power than the Sun. Scientists suspect that this X-ray activity is somehow related to its advanced stage of evolution called core helium burning. During this stage, the core of the star is very hot (more than a hundred million degrees Celsius) and converting helium to carbon via nuclear fusion reactions."(Chandra)

贝塔天琴座是一颗明亮的巨星，其表面存在一个炽热的电晕，辐射出的X射线功率是太阳的约2000倍。科学家怀疑这种X射线活动可能与它所处的进化的后期阶段——核心氦燃烧阶段有关。在这一阶段，恒星的核心温度非常高（超过一亿摄氏度），并通过核融合反应将氦转化为碳。

Credit NASA/CXC. This Xray image was taken by the Chandra Xray Observatory on June 29, 2001 .

NASA供图。这张X射线图像于2001年6月29日由钱德拉X射线天文台拍摄。

"Using the theory of how stars evolve, we can reconstruct the history of Beta Ceti, a star with a mass of about 3 Suns. Over the first billion years of its existence, Beta Ceti was powered by nuclear fusion reactions converting hydrogen to helium in the core."(Chandra)

利用恒星演化的理论，我们可以重建贝塔天琴座（Beta Ceti）的历史，这是一颗质量约为太阳约3倍的恒星。在其存在的前十亿年里，贝塔天琴座由核融合反应提供能量，这些反应将氢转化为氦。

"After the hydrogen in the core was exhausted, the central region of the star contracted until hydrogen gas around the helium core became hot and dense enough for hydrogen fusion reactions to ignite there. This powerful new energy source caused the outer regions of the star to expand greatly and cool. At this point Beta Ceti became a red giant. During the red giant phase, Beta Ceti would have been a very weak X-ray source."(Chandra)

在核心的氢耗尽之后，恒星的中心区域收缩，直到围绕氦核的氢气体变得足够热和密集，从而引发氢融合反应。这种强大的新能量来源导致恒星的外层区域大幅膨胀并冷却。此时，Beta Ceti变成了红巨星。在红巨星阶段，Beta Ceti会成为一个非常弱的X射线源。

"After about 10 million years, the core of the star contracted and heated to more than 100 million degrees, enabling helium fusion reactions to occur there. In this core helium burning stage, which will last 100 million years or more, the overall diameter of the star has shrunk to about 20 times that of the Sun and the surface temperature has increased, so it is no longer a red giant star. " (Chandra)

在大约1000万年后，恒星的核心收缩并加热到超过1亿度，使氦融合反应得以发生。在这一核心氦燃烧阶段，将持续1亿年或更久，此时恒星的总体直径缩小到太阳的约20倍，表面温度升高，因此它不再是红巨星。