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How life on Earth will end: This Week in Astronomy with Dave Eicher
When we contemplate how long life has left on Earth, it’s natural to think about the ultimate end of the celestial body that sustains us — the Sun. It will be another 5 billion years before it begins to run out of fuel and expands into a red giant that will likely engulf and destroy our planet. That seems like plenty of time, right?
But in fact, time is going to run out for us on Earth a good deal sooner than that. While our star still has plenty of nuclear fuel to sustain itself, it is nevertheless steadily evolving, as all stars do during their life. That’s because at the Sun’s core, its nuclear reactions fuse hydrogen atoms into helium — and there’s nowhere for the helium to go. These ashes build up, the core becomes denser, its pressure becomes higher, and our star’s nuclear furnace starts to runs a little bit hotter.
As a result, over time, the Sun becomes brighter, currently by around 10 percent every billion years. And in about another billion years, that increase in energy will inevitably warm the planet and cause the oceans to evaporate and disappear. Our solar system’s habitable zone — what scientists call the region where liquid water can exist on a planet’s surface — will have migrated outward and left Earth behind.
So, it turns out that the story of life on Earth is already 80 percent finished, and moving into its final act.
注释：
contemplate: v
表示"沉思；打算"，means " think deeply and thoughtfully"，如：He stood contemplating the painting. 他站着审视那幅图画。
中文简要说明：
当我们思考地球上的生命还有多少时间时，自然而然会想到我们赖以维生的天体—太阳的最终结局，它最终将会膨胀成一颗红巨星，并吞噬掉我们的星球，不过天文学家表示，即使太阳还有50亿年的寿命，地球上的生物也不一定能够活到那个时候，实际上大概我们星球上的生命只剩下10亿年的时间。
据《天文学》（Astronomy）报导，天文学家艾奇尔（Dave Eicher）表示，地球上的生命远不及太阳寿命的极限，虽然太阳仍还有足够的核燃料来维持核融合反应，但它仍在稳步演变，就像所有的恒星一样。
这是因为在太阳的核心，核反应将氢原子融合成氦，而氦却无处可去，所以当这些氦元素持续增加，太阳核心密度会变大、压力增加，这将会使得太阳的核反应变得越来越热。因此，随着时间的推移，太阳变得更加明亮，目前每十亿年增加约10％，因次大约10亿年后，太阳能量的增加将不可避免地使地球变暖，导致海洋蒸发和消失。
届时地球已经不在太阳系的适居带（habitable zone）里面，也就是科学家所称的液态水可能存在于行星表面的星系内区域。因此，地球上的生命故事已经完成了80％，准备步入最后的篇章。
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