DISTANCES AND SIZES OF THE STARS.

James had a long talk with the captain one day about the telescope, and got a capital lesson about the magnifying power being according to the character and size of the glasses used. The sailors called the instrument the “Bring ’em near;” and it does make distant objects look as if near. It was explained to the boy that the stars were of different[Pg 36] magnitudes or sizes, first, second, third, &c., according to their relative distances. The following conversation followed between the father and son:—

“Now, my boy, I must try and give you some idea of the penetrating¹ power of light, that you may get an idea of the enormous distances of the stars. You are aware that the more distant they are the less their light. A star of the first or nearest magnitude will have one hundred times the light of one of the sixth magnitude. A telescope, therefore, gathering² one hundred times the ordinary light, will make the sixth look as near as the first.

And will it be one hundred times further off?

No; light increases or diminishes according to the square of the distance.

I know. If the light be one hundred times less, the star will be ten times further off, for the square of ten is one hundred. I can understand now that a thing is only seen by the help of light. I do not see many stars, because their light is too little for my eyes to take in. The telescope has bigger eyes to take in the light of the distant stars and nebul?.

The pupil of the eye is but one-eighth of an inch in diameter. An object glass of twelve inches diameter is, therefore, ninety-six, or say one-hundred times as long. As the light seen is according to the square of the diameter, the telescope of twelve inches will receive one[Pg 37] hundred times one hundred, or ten thousand times more light.

But is there a way of measuring the quantity of light?

There is. We find that the sun has twenty-two thousand millions of times more light than the nearest of the fixed³ stars.

Then, the square root of this ought to tell how much further off it is. Let me see. It will be about 150,000.

Yes. If the sun were put back 150,000 times further than it is, it ought to look as brightly as that star. If it does not, it is because it is really smaller than the star.

What! 150,000 times 95 millions!

But that is nothing; for it is only to the first rank. What of the twentieth magnitude?

Yes. But you say the nebul? are further off than that.

I may tell you that if the sun moved three times as fast as the world does, in its six hundred millions of miles a year, it would take two hundred and fifty millions of years to get to as far as Lord Rosse’s telescope could see.

That takes my breath away.

Hear a little more. Light comes from the sun to us in eight minutes. It will take sixty thousand years to come from one of those stars Lord de Rosse saw. In fact his telescope has enlarged our universe one hundred and twenty-five million times.

Then I think religious people ought to thank astronomers⁴ for showing them more of the greatness of God. Those who only thought[Pg 38] of him as the Creator of the three thousand stars, to be seen by the naked eye, could not have such a notion of his vast power as those who know of millions upon millions of suns.”

After this, James was left to digest such wonderful lessons. When his first astonishment⁵ had passed away, his curiosity was excited to know more about the distances of the stars, so that he might form a simpler idea of the thing. He took, therefore, another occasion of bringing up the subject in these words:—

“Father, do you really believe the stars are so far off?

I am obliged to believe many things I do not understand, upon the testimony⁶ of trustworthy witnesses; but in this case I can form a good guess of the truth. Do you remember what I once told you of the parallax, or angle of observation of the sun or moon?

Yes. That of the moon was 57 minutes, and the sun 83?4 seconds. A degree is 60 minutes, and a minute 60 seconds.

Very well. Then 57 minutes, or 3420 seconds, will be four hundred times as much as the other. If the moon be 240,000 miles off, the sun will be four hundred times further or 96,000,000.

But how do you get this parallax?

Distances are calculated by the angle made in looking at an object from two places. The two lines of sight cross one another. A great[Pg 39] base is needed to view a distant object, or else no angle can be observed. Astronomers take the diameter of the earth’s orbit.

That is twice ninety-five millions of miles.

With that base—that is, looking at a star from both sides of our orbit, or at six months’ interval—we could get the two lines crossing one another, and so making an angle. The further the object, the more minute the angle. Only a few of the fixed stars could be observed in this way, as they generally are too far off to give an angle.

I know an equilateral triangle has its three angles equal to two right angles; and with ninety degrees for one right angle, each angle of the triangle will have sixty degrees. But I suppose no star parallax could be one degree.

No; nor a minute, the sixtieth part of one degree. When the object makes an angle of a second, or sixtieth of a minute, from a base line of one hundred and ninety millions of miles, the distance of the star will be about twenty millions of millions of miles.

Is there any star making the second angle?

The alpha of the Centaur⁷ is about that, and is one of the nearest of fixed stars.

That the nearest to us, and yet so far! Do tell me the distance of some others.

There is one, 61 Cygni, of the Swan, with one-third of a second; and, therefore, three times the distance of the alpha Centaur. There is a star in the Lyre which is one-fifth of a second. Grand Arcturus is one-eight. The North Polar Star is one-tenth. Pretty[Pg 40] Capella, of the Kid, is one-twentieth; that is, twenty times farther back than a Centaur. As it looks one of the brightest stars, it must be very large.

What of old Sirius?

The angle he makes with our orbit diameter is one-fourth of a second; so that he is about 80,000,000,000,000 miles.

Thank you, dear father, for these terrible long figures.

Their great distance may give us a good guess of their great size.

I know the size of the sun to be half-a-degree in the heavens, at a distance of ninety-five millions of miles, and yet it is really eight hundred and eighty-six thousand miles diameter, I have been told. When, then, a star is millions of millions of miles away, I am sure it must be a big one to be seen at all.

It is so. If the sun were thrust back as far as the star a Centaur, it is calculated that it would not shine with more than one-third of the light that star now gives us; it must, therefore, be not more than one-third its size.

But you said Sirius was four times farther off than the Centaur, father.

Yes; and it gives four times as much light. It is, then, probably four times as large. It must be, therefore, many times larger than the sun.

I am sure it ought to have more planets turning round it than the sun has, or else have them much larger in size.”

1 penetrating   

adj.（声音）响亮的，尖锐的adj.（气味）刺激的adj.（思想）敏锐的，有洞察力的

参考例句：

• He had an extraordinarily penetrating gaze. 他的目光有股异乎寻常的洞察力。
• He examined the man with a penetrating gaze. 他以锐利的目光仔细观察了那个人。

2 gathering   

n.集会，聚会，聚集

参考例句：

• He called on Mr. White to speak at the gathering.他请怀特先生在集会上讲话。
• He is on the wing gathering material for his novels.他正忙于为他的小说收集资料。

3 fixed   

adj.固定的，不变的，准备好的；(计算机)固定的

参考例句：

• Have you two fixed on a date for the wedding yet?你们俩选定婚期了吗？
• Once the aim is fixed,we should not change it arbitrarily.目标一旦确定，我们就不应该随意改变。

4 astronomers   

n.天文学者，天文学家( astronomer的名词复数 )

参考例句：

• Astronomers can accurately foretell the date,time,and length of future eclipses. 天文学家能精确地预告未来日食月食的日期、时刻和时长。 来自《简明英汉词典》
• Astronomers used to ask why only Saturn has rings. 天文学家们过去一直感到奇怪，为什么只有土星有光环。 来自《简明英汉词典》

5 astonishment   

n.惊奇，惊异

参考例句：

• They heard him give a loud shout of astonishment.他们听见他惊奇地大叫一声。
• I was filled with astonishment at her strange action.我对她的奇怪举动不胜惊异。

6 testimony   

n.证词；见证，证明

参考例句：

• The testimony given by him is dubious.他所作的证据是可疑的。
• He was called in to bear testimony to what the police officer said.他被传入为警官所说的话作证。

7 centaur   

n.人首马身的怪物

参考例句：

• His face reminded me somehow of a centaur.他的脸使我想起半人半马的怪物。
• No wonder he had soon been hustled away to centaur school.也难怪父母匆匆忙忙就把他送到了半人马学校。