□ 曹 燕

雙語小知識

□ 曹 燕

Galactic Hub 銀心

It’s a good jaunt from the suburbs to downtown — a journey into bright lights, heavy traffic, and non-stop action. That’s especially true of the“downtown” region of the Milky Way— the galaxy’s core. It’s populated by some bright, massive stars that race around at high speeds. And those stars are packed together much more tightly than the stars out where we live, in the galactic suburbs.

從郊區(qū)到鬧市是次不錯的短途旅行——一場進入絢麗燈光、繁忙交通，以及永不停止活動的旅程。銀河系的“鬧市區(qū)”——“銀心”尤其如此。這里被諸多以高速運轉(zhuǎn)的、明亮而巨大的恒星所占據(jù)。這些恒星的擁擠程度遠比我們所居住的銀河系“郊區(qū)”大得多。

S S T空間望遠鏡（S pitzer S pace Telescope）紅外照相機下的銀河系中心，來源：NASA

The very heart of the Milky Way contains a black hole that’s about four million times the mass of the Sun. Gas or debris occasionally funnel into the black hole, causing a short flare-up before they disappear from view.

在銀河系中心包含著一個大約四百萬倍于太陽質(zhì)量的黑洞。氣體和碎片偶爾漏入其中，在消失前短暫地閃耀一下。

Thousands of stars orbit within a few light-years of the black hole. Most of them are fairly old and feeble. But there’s also a cluster of hot, young stars that are many times heavier than the Sun, and many thousands of times brighter. Some of these stars get so close to the black hole that they can whip around it at millions of miles per hour.

在距黑洞幾光年范圍內(nèi)，有幾千顆恒星圍繞其運動，其中大多數(shù)衰老而虛弱，但也有一群熾熱、年輕的星球，它們比太陽重許多倍、亮幾千倍。很多恒星距離黑洞如此之近，使得它們以數(shù)百萬英里每小時的高速度轉(zhuǎn)動著。

A ring of cold gas also encircles the black hole. It’s probably about 10 lightyears across, give or take. It’s possible that the ring could fragment into smaller clumps, which could then collapse to give birth to new stars.

一條寒冷氣體環(huán)同樣圍繞于黑洞，直徑差不多10光年左右，氣體環(huán)有可能會碎裂成更小的團塊，團塊塌縮后將誕生新恒星。

Many more stars lie not far outside the ring, including a couple of other big, bright clusters. So do ribbons of hot gas that are sculpted by strong magnetic fields, and much more— all part of our galaxy’s dynamic downtown.

許多恒星位于寒冷氣體環(huán)以外不遠處，包括幾個巨大而明亮的星團，還有一條條被強磁場塑造而成的炙熱氣體帶，以及其他一些物質(zhì)，共同組成銀河系的鬧市區(qū)。

Far, Far Away 很遠很遠

It’s a long way from Earth to the center of the galaxy. But the exact distance is uncertain — measurements vary by several thousand light-years.

地球距離銀河系中心很遠，但到底有多遠還不能精確地測定，因為用不同的測量方法得出的結(jié)果相差了幾千光年。

The most accurate method of measuring astronomical distances is called parallax. To understand how it works, hold out a finger and look at it with first one eye, then the other. The finger appears to shift against the background. The angle of the shift reveals how far the finger is from your eyes.

測量天文距離最精確的方法叫作視差法。用一根手指就能弄明白視差法是怎么回事：伸出一根手指，先用一只眼睛注視它，之后再換作另一只，這根手指先后在背景中呈現(xiàn)出不同位置，兩個位置間的夾角就能揭示這根手指距離眼睛有多遠。

Astronomers determine parallax by looking at an object when Earth is on opposite sides of the Sun. The change in position causes a target to shift a tiny bit compared to the background of more-distant objects.

天文學(xué)家們是這樣運用視差法的：他們在地球剛好運行到太陽兩端時觀測同一目標(biāo)，在由更遠的天體所構(gòu)成的背景中，觀測目標(biāo)發(fā)生了微小的位移。

As the distance increases, though, the angle gets smaller. Eventually, it gets so small that it’s almost impossible to measure. That’s especially true when you’re looking through Earth’s atmosphere, which blurs the view. New technology has sharpened the view, but there’s still a lot of wiggle room in the measurements.

當(dāng)然，觀測目標(biāo)距離越遠，位移夾角便越小。最終，夾角小得難以測量，當(dāng)你透過地球那令人視線模糊的大氣層觀測時就更是如此了。新技術(shù)已經(jīng)銳化了視野，但在提高測量精度方面仍有余地。

因為地球繞日公轉(zhuǎn)，在地球上觀測恒星會出現(xiàn)視差。如上圖，每年1月份和7月份，在地球上觀測同一顆恒星會產(chǎn)生微小的位置位移，被觀測的恒星離我們越近，視差就越大。來源：ESA

Another technique looks at how certain stars change in brightness. There’s a relationship between the length of each brightness cycle and the star’s true brightness. But clouds of dust between Earth and the center of the galaxy absorb a lot of light, complicating the measurements.

另外一種方法是觀測某些恒星亮度的變化。亮度變化周期的長度與恒星的真實亮度間具有聯(lián)系。但地球與銀河系中心之間的塵埃云吸收掉很多光線，使得測量更加復(fù)雜。

So when you take all the difficulties into account, different studies yield a distance of between about 24,000 and 28,000 light-years away — a long jaunt any way you look at it.

總之，把所有這些困難都考慮進去，不同研究得出的地球與銀心距離在24,000到28,000光年之間，不管怎么看這都是很遠的距離了。

（責(zé)任編輯 張長喜）