"The stars are made of iron, copper, zinc, and other such metals, but
the heat is so intense that these metals are turned into vapor. You have
seen the steam coming from the spout of a kettle when water is boiling,
and you know then that the water is scalding hot. But imagine heat so
great that masses of iron and copper are not only melted but turned
into vapor. Then you have some idea of the intense heat that prevails on
the stars. The rains that fall on earth are made up of drops of water,
but the rainfalls on the stars must be drops of melting iron, while the
clouds that form are sheets of molten metal."
"How wonderful!" said Harry; "and how do we know this, as the stars are
so far away?"
"By means of a little instrument known as the spectroscope, or
light-sifter. But you must wait till you are a little older before I can
explain that to you, as it is something very difficult to understand.
At any rate, I can tell you this, that when we want to find out what
a star is made of we catch a ray of its light and examine it with the
light-sifter. As Professor Ball quoted in one of his lectures:
"'Twinkle, twinkle, little star,
Now we find out what you are,
When unto the midnight sky
We the spectroscope apply.'"
"And can you tell how old the stars are?" asked Harry; "because when you
were talking about the planets you said some are old and some are young."
"This same little spectroscope tells us that as well, and we can recognize
the stars that are in their infancy, and others that are middle-aged or
nearly worn-out."
"How strange to think of worn-out stars," said Harry; "yet I suppose
they must grow old sometime, just as we do; only I suppose they take
ever so much longer growing up."
"Hundreds of years," said Mary, laughing at the idea of grown-up stars.
"There are young stars and old stars, and even the star that gives us
light and heat will grow cold and dead some day, and not warm its planets
any longer. But that will be millions of years hence, long after we are
dead and gone."