银河系有多少星星？

When you gaze up into the night sky, there are thousands of stars you can see without any help from a telescope. That might sound like a lot. But those stars are actually just the ones that are brightest and closest to Earth. The Milky Way—the galaxy Earth calls home—is bigger. A whole lot bigger. But just how many stars are there in the galaxy?

当你凝视夜空时，你可以在没有望远镜的情况下看到成千上万的星星。[HW：我们就别想了]这可能听起来很多，但那些实际上只是最亮和最接近地球的恒星，银河系里可是有一大堆星星呢!但是银河系中究竟有多少颗恒星呢?

Unfortunately, we can’t simply look at all the stars with telescopes and count them up. That’s because our galaxy isn’t only full of stars. There are also clouds of dust and gas that block our view of dim or faraway stars. So astronomers have to answer this question by working backwards.

很抱歉，我们不能用数的。那是因为我们的星系不仅充满了星星，还有尘埃和气体阻挡了我们对昏暗或遥远星星的视线，所以天文学家必须反着来回答这个问题。

The first step is to figure out how much stuff, or mass, there is in the entire galaxy. Astronomers at the University of Arizona recently estimated that the Milky Way weighs about 960 billion times as much as our own sun. It’s made up of things like stars, dust, gas… and dark matter. Dark matter is mysterious and scientists aren’t really sure what it is. However, they can see how gravity from dark matter affects stars as they orbit the center of the galaxy. “Basically the faster your stars are moving, the more dark matter there is,” explains Malanka Riabokin, an instructional specialist in the astronomy department at the University of Arizona in Tucson. Dark matter is believed to make up around 90 percent of the material in the galaxy. Once you subtract the dark matter and dust and gas, you’re left with all the star stuff—the equivalent of about 30 to 50 billion suns.

第一步是弄清楚整个星系中有多少物质或质量。亚利桑那大学的天文学家最近估计，银河系的质量约为我们太阳的9600亿倍。它由星星、尘埃、气体……和暗物质组成。暗物质是神秘的，科学家们并不确定它是什么。然而，他们可以看到暗物质的重力在绕星系中心运行时如何影响恒星。“基本上恒星移动得越快，暗物质就越多，”图森市亚利桑那大学天文系教学专家马兰卡·里博金解释说。据信暗物质占星系中约90%的物质。一旦减去暗物质和尘埃和气体，就会得到所有的星星 - 相当于大约30到500亿个太阳。

But the actual number of stars is much higher than that. That’s because stars can be anywhere from 8 percent the mass of our own sun to 20 times as large. When astronomers look at the places where stars are born, they see tiny stars are much more common than massive ones.

但实际的恒星数量远远高于此数。因为恒星可以是我们自己太阳质量的8%到20倍之多。当天文学家观察恒星诞生的地方时，他们看到的小恒星远多于大恒星。

So we know how much star stuff is floating around the galaxy. And we know roughly how much of that mass should be big stars, and how much should belong to small stars. That means we should know how many stars there are in total, right?

所以我们知道在银河系周围漂浮了多少恒星。而且我们大致知道那些质量应该多少大恒星和多少小恒星，这就意味着我们应该知道了总共有多少颗星星了吧?

Well, kind of. There’s a lot of guesswork that goes into each step of counting the stars in the galaxy. This means that there isn’t just one estimate for how many stars are scattered across the Milky Way. Depending on whom you ask, there may be anywhere between 200 billion and 1 trillion stars out there.

差不多吧。计算银河系恒星的每一步都有很多猜测。这意味着，对于银河系中有多少颗恒星，只是有一个估计值。根据你问的对象，我们银河系可能有2千亿到1万亿颗恒星。

Any way you slice it, though, that’s a lot of stars.

但是不管如何区分，这也是很可观的数字了。

“When you look up at the night sky and realize that there are literally billions upon billions of stars that you cannot see, and just how deep that space must be to accommodate all those stars (and other galaxies, so much farther away!) it starts to give you an appreciation of the scale of the Universe,” Riabokin says.

里博金说：“当你抬头仰望夜空，意识到那里有数千万亿颗恒星，以及这个空间必须有多广袤才能容纳所有这些恒星(以及其他星系)，这就让你开始了解宇宙的规模了。”