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  • 星系

    The solar neighborhood
    ::太阳邻居

    The solar neighborhood is a small  volume   of space centered on the Sun.  Leaving the solar system, our closest neighbor is the triple star system, alpha Centauri, at a distance of about 4 light years. Spurred by Silicon Valley entrepreneur Yuri Milner's   ,   scientists are seriously tackling the engineering challenges for sending a probe to alpha Centauri . 
    ::太阳区是以太阳为中心空间的一小部分。离开太阳系,我们最近的邻居是三星系,阿尔法半人马星,距离大约4光年。在硅谷企业家尤里·米尔纳的激励下,科学家们正在认真应对向阿尔法半人马星发送探测器的工程挑战。

    The   below   shows a 3-D diagram for stars that are closer than 15 light years - stars in our immediate solar neighborhood. As humanity travels out into the galaxy, these are the places that we will visit first.  It is possible that school children of the future will memorize the names of these destinations in the same way that school children of the past memorized the names of planets in our solar system. 
    ::下面显示接近15光年的恒星的三维图,即我们太阳近邻的恒星。当人类进入银河系时,这些是我们将首先访问的地方。今后,学生们可能会像过去的学生们将太阳系中的行星名称记住一样记住这些目的地的名称。

     

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    Stars that are closer than 15 light years from the Sun. Notice that neighboring stars are not distributed in a plane. However the concentric rings show distances of 5, 10, and 15 light years. How many stars are closer than 5 light years? Roughly how many stars (e.g., 300, 30, or 3?) are closer than 15 light years? How many parsecs are in 15 light years?
    ::离太阳近15光年的恒星。 请注意, 相邻恒星不是在平面上分布的。 但是同心环显示距离是5、 10和15光年。 有多少恒星比5光年更近? 大约有多少恒星( 如300、 30或3? ) 接近15光年? 15光年中有多少分析?

    One might guess that bright stars are closest and faint stars are farther away.  If all stars were the same intrinsic brightness, that would be true.  However, for "normal" stars, the brightness of stars is correlated with the mass of the star.  And when stars evolve to become "red giants" or "supernovae" they also brighten.  With a telescope, we measure the   apparent brightness   of a star. We can translate this to  an   absolute scale  to learn  the true   luminosity   of a star only if we know the distance to the star.  Think for a moment about how you would measure the distances to stars.  It's not easy - this has been a major effort in astronomy over the past few decades. 
    ::人们可能会猜测,明星最接近,微弱的恒星距离更远。 如果所有恒星都是同样内在的亮度,那也是真实的。 但是,对于“ 正常” 恒星来说,恒星的亮度与恒星的质量相关。 当恒星演变成“ 红巨星” 或“超新星” 时,恒星的亮度也会更亮。 我们用望远镜测量恒星的亮度。 只有当我们了解恒星与恒星之间的距离时, 我们才能将此转换为绝对尺度来了解恒星的真正光度。 想想一下你如何测量恒星与恒星的距离。 这不是一件容易的事 — 过去几十年中天文学上的一项重大工作。

    Beyond the solar neighborhood: our galaxy
    ::超越太阳光环:我们的银河系

    How big is the Milky Way galaxy compared to the solar neighborhood? The scale of the Milky Way galaxy is nothing short of staggering. The   below  sketches   the diameter of the flattened disk of our galaxy, which   stretches 100,000 light years across.   The Sun is located about two thirds of the way between the center and the outer edge of the Milky Way.

    ::银河系与太阳区相比有多大? 银河系的规模几乎是惊人的。 下面绘制了银河系平面盘的直径,它横跨了10万光年。 太阳位于银河系中心与外缘之间大约三分之二的距离。

     

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    A sketch of the disk of the Milky Way (MW) galaxy from an edge-on view. The MW has several distinct structures: a thin disk, a central bulge, and a roughly spherical halo. In which of these component structures is the Sun located?
    ::银河系( MW) 磁盘的草图。 MW 具有几种不同的结构: 薄盘、 中央膨胀和大致球状光环。 这些组件结构中哪些是太阳的位置 ?

    The 15 light year scale of our local neighborhood is puny in comparison and is not even resolvable in the    below.  It is impossible for us to take a  picture of the Milky Way because we have not traveled far enough through space to gain a full view of our galaxy. Instead, we have     looking toward the center of the galactic plane. The ensemble of images that we have made of the Milky Way  probe  the physical extent, including the thickness of the galactic disk. Considering the ratio of the diameter of the galaxy to the height of the galactic disk, the dimensions of the Milky Way are thinner than a dime.
    ::我们本地邻居的15光年规模比较起来是微弱的,在下面甚至无法破解。我们不可能拍摄银河的图片,因为我们没有在太空中走得足够远,无法全面观察银河系。相反,我们正向银河系的中心看。我们用银河系制作的图像集探索物理范围,包括银河磁盘的厚度。考虑到银河系直径与银河磁盘高度的比率,银河系的尺寸比硬币小。

    The Milky Way (MW) galaxy contains a stack of two disks: a thin disk with stars, gas, and dust, and a thicker disk that is comprised of older stars. We think that accumulated gravitational interactions cause the population of younger stars in the thin disk to wander out into what we call the thick disk. There is a central bulge in the MW galaxy that contains mostly young and massive stars, and a supermassive black hole (we now know that supermassive black holes reside at the centers of almost every galaxy and that the mass of the black holes scales with the mass of the host galaxy).
    ::银河系(MW)包含两张磁盘:一个由恒星、气体和尘埃组成的薄盘和一个由老恒星组成的厚盘。 我们认为,累积的引力相互作用导致薄盘中的年轻恒星群漫步到我们所称的厚盘中。 MW 星系有一个中央膨胀,其中多数是年轻和庞大的恒星,还有一个超大质量黑洞(我们现在知道,超大质量黑洞几乎存在于每个星系的中心,黑洞质量与宿主星系质量之比)。

     

     

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    A face-on diagram of our MW galaxy. The Sun orbits at a distance of about 27,000 light years (or about 8,000 parsecs) from the center. What do you think produces the spiral arms that are so obvious in the structure of the MW?
    ::我们的 MW 星系的表面图。 太阳的轨道距中心大约27 000光年( 约8 000 个分析器) 。 你认为什么能产生在 MW 结构中如此明显的螺旋臂?

     

    In addition to ~400 billion stars, our galaxy contains collections of stars called    that are  randomly distributed in the spherical volume that surrounds the flattened disk of the MW galaxy. A globular cluster looks like a spherical cloud of light from the great distances that we view them. However, telescopes have resolved these clouds into dense collections of millions of stars. The stars in globular clusters are uniformly old - they are the most ancient relics in our MW galaxy. The stars in globular clusters are gravitationally bound and orbit each other, while the globular cluster as a whole moves in an orbital path in the galaxy.    is an example of one of the ~150 globular clusters in our galaxy. The globular clusters in the Milky Way are a self-contained, gravitationally bound ensemble of stars that  orbit the  galaxy. 

    ::除了~4000亿颗恒星外,我们的银河系还收集了被称为随机分布在MW星系平面盘周围球体体体积中的恒星。一个球状星团看上去像来自我们所观察的远距离的光球云。然而,望远镜将这些云团溶解成成数以百万计恒星的稠密集合。球状星团的恒星群都是老旧的——它们是我们MW星系中最古老的遗迹。星团群群群中的恒星群是引力捆绑的,并相互运行,而球团群群群则作为一个整体在银河轨道上移动。这是我们银河系中大约150个球团群的一个实例。银河系中的球团群群群群群是一个自成一体、重力捆绑在一起的恒星群群,环绕着银河系。

     

     

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    Messier object 107 is a globular cluster with thousands of stars that are gravitationally bound. Globular clusters are distributed in a spherical volume centered on the galactic center.
    ::梅西尔天体107是一个星团群,由数千颗恒星组成,这些恒星具有引力约束。球状星团分布在一个球体体体积中,以银河中心为中心。

    In addition to the old, spherically distributed globular clusters, there are smaller clusters of stars that are called "open clusters."   Open clusters are found in the disk of the galaxy; like the globular clusters, all of the stars have about the same age; however, open clusters are associations of young stars.  Examples of open clusters include the   .
    ::除了旧的、球状分布式的星团外,还有较小的星团群群,称为“开放星团 ” 。 开放星团群在银河系的磁盘中找到; 与星团团群一样,所有恒星群的年代都差不多; 但是, 开放星团是年轻恒星的组合。 开放星团群的例子包括 。

     

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    The Pleiades is an open cluster of stars. Open clusters are located in the disk of the galaxy and are sites of recent star formation. Why do the stars in the Pleiades appear to be so blue?
    ::Pleiades是一颗开放的恒星群。 开放的星团位于星系盘中, 是最近恒星形成的地点。 为什么Pleiades 中的恒星看起来如此蓝色?

     

    The Sun is just one star among     (this is the range of accepted estimates and reflects the precision of our knowledge) that are gravitationally bound  to the Milky Way galaxy. Like all other stars, the Sun orbits the center of the galaxy.  It takes about 230 million years (230 Myr) for the Sun to complete one orbit around the galaxy.  This is a number that is so  large that it is meaningless to humans. Try putting 230 million years onto     that seems meaningful to you. For example, the dinosaurs became extinct about 75 million years ago (75 Mya), so, the Sun takes about three ``dino-extinction'' units of time to travel around the galaxy.  Or, the Cambrian explosion occurred about 500 Mya; so the Sun has traveled twice around the galaxy since the Cambrian explosion.
    ::太阳只是其中的一颗恒星( 这是公认的估计范围, 反映我们知识的精确度) , 与银河系有引力的星系。 和所有其他恒星一样, 太阳环绕银河系的中心。 太阳需要大约2.3亿年( 230万迈尔) 才能完成星系周围的一个轨道。 这是一个巨大的数字, 它对人类来说毫无意义。 尝试把2.3亿年的时间放在这个看来有意义的星系上。 例如, 恐龙在大约7500万年前( 75 Mya) 灭绝了, 因此太阳需要大约三个“ 深海灭绝” 的单位的时间来绕星系旅行。 或者, 坎布里安爆炸大约发生在500 Mya ; 因此太阳在坎布里安爆炸后两次环绕星系飞行。

      Review ...

    ::复议...

    1. What do you know about the Milky Way galaxy? How many of the structural components can you name?
    ::1. 你对银河系了解多少?你能说出多少结构组件?

    2. How would you estimate the number of stars in the Milky Way galaxy, given that it is not possible to count them all?
    ::2. 鉴于无法全部计算,如何估计银河系中的恒星数量?

     

     Other Galaxies
    ::其他星系

    In detail, galaxies are as unique as snowflakes. However, Hubble noticed that galaxies could be broadly classified by a few -- and only a few -- different large-scale morphologies. His first hypothesis was that he might be looking at an evolutionary sequence. There are beautiful spiral galaxies like the nearby   , with bright blue arms where massive young stars are forming. There are also distinct giant elliptical galaxies like     that astronomers call "red and dead" because they no longer harbor regions of active star formation. However, the evolution of galaxies  is not a simple linear transition from spiral to elliptical. We now know that galaxies grow by mergers, like the dwarf irregular galaxy,   . Galaxy mergers trigger new cycles of star formation.
    ::具体地说,星系和雪花一样独特。然而,哈勃注意到星系可以广泛分类为少数 -- -- 只有少数 -- -- 不同的大规模形态。他的第一个假设是,他可能正在研究进化序列。附近有一些美丽的螺旋星系,像附近的星系,有亮蓝臂,有大量年轻恒星正在形成。还有一些截然不同的巨型椭圆星系,如天文学家称之为“红与死”的天文学家,因为它们不再窝藏活跃恒星形成的区域。然而,星系的演化并不是简单的线性向从螺旋到椭圆的线性转变。我们现在知道星系通过合并而成长,如矮星系不规则的星系。银河合并引发了恒形成的新周期。

    Mergers can be subtle, with large galaxies sweeping up smaller ones, and the Milky Way galaxy is not innocent of this galactic canabalism. We have evidence for more than a dozen streams of stars that our galaxy has swallowed and is gravitationally digesting.     suggests that more than half the mass of our galaxy may have come from the accretion of other galaxies, as shown in the galaxy merger simulation below. The Large Magellanic and Small Magellanic clouds that can be clearly seen in the southern hemispheres are satellite galaxies that are being gravitationally lured into the Milky Way.  Mergers are part of the circle of life for galaxies.
    ::合并可能微妙,大星系的合并规模较小,而银河系并非与银河系的银河系无关。我们有证据表明,我们的银河系吞噬了十多个恒星流,这些恒星流正在引力消化。 这表明我们银河系的一半以上可能来自其他星系的积聚,正如下文银河系合并模拟所显示的那样。在南半球可以明显看到的大麦哲伦和小麦哲伦云是卫星星系,它们正被引入银河。合并是星系生命圈的一部分。

       The YouTube video below shows a simulation of a galaxy merger
    ::下面的YouTube视频展示了银河系合并的模拟

    The simulation follows the interaction of mass as realistically as possible.
    ::模拟尽可能现实地遵循质量的相互作用。

    What is the timescale for the merger (hundreds of years? thousands? more?)?
    ::合并的时标是多少(百年? 千年? 更多? ) ?

     What do you think happens to individual stars when galaxies merge?
    ::你觉得星系合并时 单个恒星会怎样?

     

     The scale of the universe
    ::宇宙的规模

    Recall that our galaxy contains about 150     - spherical aggregates of light from many thousands of stars. In 1918, Harlow Shapley estimated the distances to globular clusters. He  made the simplifying  assumption that all clusters had nearly the same brightness, and reasoned that some clusters were fainter because they were farther away. Under this assumption (which turned out to be reasonable) the globular clusters  appeared to be  distributed in a spherical volume that was centered on a point in the Sagittarius constellation.  Shapley argued that this point was also the center of the galaxy and estimated that the Sun was 15,000 pc from the center of the galaxy. Copernicus unseated the Earth as the center of the solar system, and Shapley  showed that the Sun  was not at  the center of the galaxy. Subsequent studies have shown that the Sun is actually about 8 kpc from the center of the galaxy, so Shapley overestimated the physical extent of the Milky Way, but only by a factor of two.
    ::回顾我们的银河系包含着来自数千颗恒星的约150个球状集光。 1918年, Harlow Shapley 估算了离星团星团的距离。 他做了简化的假设, 所有星团的亮度几乎相同, 并推理说有些星团由于离星系更远而更加暗淡。 根据这个假设( 事实证明是合理的 ) , 星团似乎分布在球体体体积中, 球体积集中在Sagittarius星座的一个点上。 Shapley 争论说, 这个点也是星系的中心, 并且估计太阳是来自星系中心的15000 个方位。 Copernicus 将地球作为太阳系的中心, Shapley 显示太阳不是星系的中心。 随后的研究显示, 太阳实际上离星系中心约8 kpc, 所以Shapley 高估了银河系的物理范围, 只有两个系数。

    In the 18 th   and 19 th   centuries, natural scientists had observed fuzzy, extended objects in the night sky that they called "nebulae.''  In 1755, Immanuel Kant interpreted these nebulae as "Island Universes'' - large collections of gravitationally bound stars. Shapley believed that Kant's island universes were part of our galaxy. Indeed, he had no reason to believe that there was a physical edge to our galaxy. In 1920, when  physicists were still struggling to determine the speed of light, a     about the nature of island universes took place between two prominent astronomers, Harlow Shapley and Heber Curtis, at a meeting of the National Academy of Science. Shapley, who had overestimated the size of the Milky Way, maintained that the nebulae were part of our galaxy, and Curtis  argued that they were outside of our galaxy. This debate was a good example of how  humans struggle to piece together scientific theory with imprecise or inaccurate data.  
    ::在18世纪和19世纪,自然科学家在夜空中观察到了模糊的延伸物体,他们称之为“Nebulae ” 。 1755年,Immanuel Kant将这些星云解释为“Island宇宙”——大量重力结合恒星的集合。Shapley认为Kant岛宇宙是我们星系的一部分。事实上,他没有理由相信我们的星系有物理边缘。1920年,物理学家仍在努力确定光速,关于岛屿宇宙的性质,在国家科学院的一次会议上,Harlow Shapley和Heber Curtis这两个著名的天文学家之间发生了。Shapley高估了银河的大小,认为星系是我们星系的一部分,Curtis认为它们不属于我们的星系。这一辩论是人类如何用不精确或不精确的数据来拼凑科学理论的一个很好的例子。

    The distances to stars 
    ::与恒星的距离

    A breakthrough in understanding the distances to stars came about when Henrietta Swan Leavitt measured the brightness variations for more than 2400 stars in the Magellanic Clouds, satellite galaxies of the Milky Way that are visible from the southern hemisphere of Earth. A certain class of these stars, Cepheid variables, showed a regular periodicity in their cycle of brightness and dimming.  Leavitt pointed out that the repetition timescales, or periodicity in the brightness variation cycles, were longer for the brighter variable stars. By correlating the periodicity of the variation brightness with the intrinsic brightness (the period-luminosity relation), Leavitt discovered an important "standard candle."   Measure the period of variability in a Cepheid star and you know the   . Then, compare the apparent brightness that you observe with the intrinsic brightness; because more distant stars appear fainter, the distance to the star can be calculated this way.
    ::当亨丽埃塔·斯旺·莱维特测量了麦哲伦云中24个以上恒星的亮度变化时,在了解恒星距离方面出现了一个突破。从地球南半球可见的银河卫星星系“麦哲伦云”中,有24个以上的恒星的亮度变化。这些恒星的某类,即塞斐德变量,显示了其亮度和亮度周期周期的周期性。莱维特指出,对于更亮的变异恒星来说,重复的时间尺度或周期性是更长的。通过将变化亮度周期与内在亮度(周期光度关系)相联系,莱维特发现了重要的“标准蜡烛 ” 。 在切斐德恒星中测量变异期,您知道这一点。然后,将您观察到的亮度与内在亮度进行比较;因为更遥远的恒星出现,与恒星的距离可以以这种方式计算。

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    (left) Henrietta Swan Leavitt at her desk at Harvard College Observatory. (right) The period of variability in days is plotted on the x-axis against the min (lower curve) and max (upper curve) brightness on the y-axis for Cepheid stars in the Magellanic Cloud. By measuring the period of brightness variation, one can read off the intrinsic brightness of the star. Why is the minimum brightness of a star a larger number than the maximum brightness?
    ::Henrietta Swan Leavitt在哈佛大学天文台的办公桌上。 (右) 天的变异期是在X轴上绘制的,以相对于麦哲伦云中切斐德星的微小(低曲线)和最大(最高曲线)Y轴(最高曲线)亮度。通过测量亮度变化的时间,人们可以从恒星的内在亮度中读出。 为什么恒星的最小亮度大于最大亮度?

     Making scientific plots.
    ::制作科学图案。

    It is very bad form not to label the axes with parameters and values! The plot of Cepheid period vs brightness above is included as an example of   how to lose points  on your homework, in case you were wondering.
    ::不将轴标为参数和值是非常糟糕的。 Cepheid 时期的图案与上面的亮度相比, 被作为如何在作业中丢失分数的例子, 以防您怀疑 。

    The debate about whether nebulae were part of our galaxy was resolved a few years later in 1923 when Edwin Hubble used the 100-inch Mount Wilson telescope to resolve individual stars in M31 and M33, two examples of "spiral nebulae." Hubble measured the brightness variations for a Cepheid variable, and from the period-luminosity relation (above) he knew the true luminosity of the stars. Knowing the luminosity of the star allowed Hubble to estimate a distance to M31 and M33 that was even larger than Shapley's over-estimated size of the Milky Way galaxy. The conclusion was that M31 and M33 were not part of our galaxy.  
    ::1923年,爱德温·哈勃用100英寸的威尔逊山望远镜解决了M31和M33中的单个恒星,两个“螺旋星云”的例子。哈勃测量了Cepheid变量的亮度变化,以及从恒星的周期光度关系(以上)中,他知道恒星的真正光度。知道恒星的光度使得哈勃能够估计到M31和M33的距离,这个距离甚至比Shapley高估的银河系面积还要大。其结论是M31和M33不是我们银河系的一部分。

    The realization that we were looking at other galaxies well outside of our own Milky Way meant that the universe was much bigger that anyone imagined. The true nature and the distances to the "island universes" was a mystery for almost two centuries. Once this mystery was solved, humans began to realize that the Milky Way galaxy is only a tiny speck in a vast universe. How big is the universe?  That's a great question - current estimates put the "size" of the universe at more than 90 billion light years.
    ::我们所看到的银河系远在我们的银河系之外的其他星系的认知意味着宇宙比任何人都想象的要大得多。 真正的自然和与“ 岛屿宇宙”的距离是近两个世纪以来的一个谜。 一旦这一谜团被解开, 人类开始意识到银河系只是一个巨大的宇宙中的微小斑点。 宇宙有多大? 这是一个伟大的问题 - 目前的估计认为宇宙的“规模”在900亿光年以上。

    As Douglas Adams says in The Hitchhiker's Guide to the Galaxy:
    ::正如Douglas Adams在《银河系希克指南》中所说:

    Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
    ::太空是巨大的。你不会相信它是多么巨大, 巨大的, 令人难以置信的大。我的意思是,你可能会认为它 远在通往化学家的道路上, 但这只是空间的花生。

     

    We now know that there are hundreds of billions of galaxies in the universe. The     has made a dramatic change in our understanding of what's out there. HST orbits the Earth once every ~90 minutes. In 1994, the HST photographed     near the Big Dipper. This patch was selected because it did not have very many stars - as far as our observations with ground-based telescope could tell, this was just a boring, dark, empty part of the sky. Astronomers stacked more than 340 images to construct a     of the universe. The     - an area that is about one tenth the size of the full moon - contains about 1500 galaxies. There is nothing special about this direction in the sky (now we have Hubble deep field images taken in several different directions); the celestial sphere is wall-papered with hundreds of billions of galaxies, each with hundreds  of billions of stars. Read that last sentence again; it is a staggering result that re-ordered our place in the universe.
    ::我们现在知道宇宙中有数千亿个星系。 宇宙中的星系已经对我们对宇宙的理解发生了巨大变化。 HST每~90分钟环绕地球一次。 1994年, HST在大飞地附近拍摄了照片。 这个补丁之所以被选中,是因为它没有很多恒星, 就我们用地基望远镜观测到的观察来看, 这只是一个无趣的、黑暗的、空的天空的一部分。 天文学家堆积了340多幅图像来构建宇宙。 这个面积大约是整个月的十分之一, 它包含大约1500个星系。 天空的这个方向没有什么特别之处( 现在我们有不同方向的哈勃深场图像); 天体是墙面,有数百亿个星系, 每个星系都有数千亿颗。 读上一句, 是一个惊人的结果, 重新排列了我们在宇宙中的位置。

     

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    The Hubble Deep Field shows about 1,500 galaxies beyond the Milky Way galaxy in a patch of the sky that is about one tenth the area of the full moon. Everything in this image that does not have bright diffraction spikes is a galaxy. Why do the galaxies have different colors?
    ::哈勃深场展示了银河系之外大约1500个星系, 星系在天空中大约是满月面积的十分之一。 这个图像中没有明亮的折射钉的一切都是一个星系。 为什么星系的颜色不同?

     

    What's beyond galaxies? We've recently learned that a     surrounds galaxies. This gas is visible only  at ultraviolet wavelengths, so it couldn't be observed from the surface of Earth (where the atmosphere blocks out UV light).
    ::星系之外是什么?我们最近了解到环绕星系。这种气体只有在紫外线波长时才可见,因此无法从地球表面观测到(大气层将紫外线隔开的紫外线)。

     

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    What's beyond galaxies? A lot! This image shows some galaxies in a visible band image on the right. On the left, an ultraviolet (UV) image shows an incredible amount of circumgalactic gas. The mass of the circumgalactic material exceeds the mass of all the stars in the galaxies.
    ::星系之外是什么? 很多! 此图像显示一些星系在右侧的可见波段图像中。 在左侧, 紫外线( UV) 图像显示有惊人数量的环银河气。 环星系材料的质量超过了星系中所有恒星的质量 。

    We are newcomers (our place in time)
    ::我们是新来者(我们在时间上的位置)

    The universe came into existence 13.7 billion years ago (Gya). This is a number that is beyond the comprehension of mere mortals, so Carl Sagan  cleverly re-scaled the cosmic timeline to one calendar year.  At midnight on January 1, the Big Bang occurs and it is now Dec 31 and the clock is about to tick over to the new year.  On this scale, 37.5 million years whiz by every day and 26048 years click by each minute. The     helps us to visualize the vast expanse of time that has passed since the Big Bang. When do we humans arrive on the scene on this cosmic calendar?
    ::宇宙诞生于137亿年前( Gya) 137亿年前( Gya ) 。 这是一个无法被普通人理解的数字, 因此Carl Sagan巧妙地将宇宙时间调整到一个日历年。 1月1日午夜, 大爆炸发生, 现在是12月31日, 时钟将到新年。 在这个规模上, 每天3,750万年, 每分钟点击26048年。 这有助于我们想象大爆炸以来的广大时间。 我们人类何时到达这个宇宙日历上的舞台?

     

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    The 13.7 Gyr since the Big Bang to this moment in time is rescaled to a calendar year. This allows for an intuitive sense about the scale of time. (This image is the original work of Eric Fisk. How old was the Milky Way galaxy when the Sun came into existence? How long did the dinosaurs persist on the cosmic calendar?
    ::从大爆炸到这个时刻的13.7焦耳时间被重新缩放为日历年。 它允许对时间规模有直觉感知。 (这是埃里克· 菲斯克的原始作品。 太阳诞生时银河系的年代有多大? 恐龙在宇宙历程上持续了多久? )

      Working with BIG numbers: powers of ten
    ::与BIG数字合作:10人的权力

    The numbers involved in understanding science in general and astronomy in particular are staggering.  To simplify calculations, you will want to work with scientific notation.  A good review of   order of magnitude estimates   is given in this 4-minute     presentation.

     

    Olbers Paradox
    ::Olbers Paradox( 单极人 )

    One of the most important ways to make progress in science (and life) is to rule out things that cannot be true. Those humans who can extract information from what seems like a vacuum to everyone else have changed the course of science. As Sherlock Holmes more eloquently stated:
    ::科学(和生命)进步的最重要方法之一是排除不可能真实的事情。 那些能够从真空中提取信息的人改变了科学进程。 正如夏洛克·福尔摩斯更雄辩地指出的那样:

    Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.
    ::一旦你消除了不可能的,不管还剩下什么, 无论多么不可能, 都必须是真理。

        Think about it...

    ::考虑一下...

    Can we deduce anything about the physical extent of the universe just by noticing that the night sky is dark?

      The question of why the night sky was dark was discussed by the 15th century mathematician Thomas Digges and the 16th century astronomer Johannes Kepler. However, the paradox of why the night sky is dark and not as bright as the Sun is generally attributed to 19th century German amateur astronomer Heinreich Olbers, who stated that if the universe was: (1) infinite in both size and age, (2) uniform, and (3) not expanding, then the night sky would glow with a constant brightness, similar to the brightness of the Sun.
    ::十五世纪数学家托马斯·狄格斯和十六世纪天文学家约翰内斯·开普勒讨论了夜空为何黑暗的问题。然而,19世纪德国业余天文学家海因里希·奥尔伯斯(Heinreich Olbers)通常对夜空为何黑暗、为何夜空为何不如太阳那样明亮的悖论自相矛盾,他说,如果宇宙是sad1) 大小和年龄无限,(2) 制服和(3) 不扩张,那么夜空会以恒定的亮光照耀,类似于太阳的亮光。

    Here is Olbers' thought experiment: if you move the Sun twice as far away, it is fainter by a factor of four. However, its angular size has also decreased by a factor of four, so that the number of photons per unit area has stayed constant. In an infinite universe, every bit of the sky would have a star somewhere along our line of site. Given an infinite age and a static universe, the light from even the most distant stars would have reached us. If all other stars were as bright as the Sun, then the night sky should be as bright as the Sun. Many possible resolutions have been offered for Olbers' paradox. Perhaps:
    ::这是奥尔伯斯的思考实验:如果你将太阳移动两倍远,它会以四分之一的系数昏倒。然而,它的角体大小也减少了四倍,这样每个单位区域的光子数量就保持不变了。在一个无限的宇宙中,每个天空的每一个部分都会在我们站点线的某个地方有一颗恒星。鉴于一个无限的年代和一个静态的宇宙,即使最遥远的恒星也会有光照着我们。如果所有其他恒星都像太阳一样亮,那么夜晚的天空应该像太阳一样亮。对于奥尔伯斯的悖论,人们已经提出了许多可能的决议。也许:

    • our Sun is special (humanity's favorite go-to explanation)
      ::我们的太阳是特殊的(人类最喜爱的解释)
    • there is obscuring dust in the universe
      ::宇宙中弥漫着灰尘
    • the universe is finite in size or the distribution of stars is not uniform
      ::宇宙大小有限或恒星分布不统一
    • the universe is finite in age and the light from distant stars has not yet reached us
      ::宇宙在年龄上是有限的 远远恒星的光线尚未到达我们
    • the universe is expanding so that the light is red-shifted and no longer visible to us.
      ::宇宙正在膨胀 光线是红色的, 不再可见于我们。

    Each of these explanations has profound implications for the universe. In particular, if the universe is finite in age, this implies that it had a beginning!  Let's put the resolution to Olbers' paradox on hold for a moment, and look at the evidence that emerged in the 20 th   century.
    ::这些解释对宇宙都有深远的影响。特别是,如果宇宙在年龄上是有限的,这意味着它有一个开始!让我们暂时搁置奥尔伯斯的悖论,看看20世纪出现的证据。