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Astronomy
Narrator
Listen to part of a lecture in an astronomy class.
Professor
I’ll tell you a story about how one astronomy problem was solved. It happened many years ago, but you’ll see that it’s interesting and still relevant. Two, three hundred years ago, astronomers already had telescopes, but they were not as powerful as those we have now. Let’s say ... they were at the level of telescopes amateur astronomers use today. Tell me, what do you see in the night sky when you use a telescope like that? Quick, tell me.
Female student
Planets ...
Professor
Right.. .
Male student
Even ... like ... the moons of Jupiter?
Professor
Right.. .
Female student
Stars.
Professor
OK . . . what else? . . . You think that’s all? . . . Ever heard of nebulae? ... I bet you have .. . Well, let’s just, um, put it up anyway ...
Nebulae are small fuzzy patches you see in the sky, they look like little clouds. Many of them have a spiral shape, and that’s why we called them spiral nebulae ... So astronomers in the eighteenth century . . . eighteenth century . . . when they looked through the telescope, they could see planets—and they knew those were planets ... the moons of Jupiter—and they knew they were the moons of Jupiter. . . and then they saw spiral nebulae and they didn’t have a clue.
What could those be? So, some of them thought—"these things are cloudy and fuzzy, so they’re probably small clouds of cosmic dust, and they don’t have to be very far away from us." But there were others who thought, "OK, the things look small and fuzzy, but maybe they’re actually distant galaxies of stars, but we can’t see the stars, because they’re so far away and they seem so tiny that they look like dust, and even the whole galaxy looks like a tiny little cloud."
Which of the two theories do you think was more . . . uh, surprising?
Male student
The galaxy one.
Professor
And why?
Male student
Well, I mean it assumed that the nebulae are not what they look like at first sight. The first theory assumed that, right?
Professor
OK. And now tell me this ... which one would have seemed more likely at the time?
Male student
Uh .. .They couldn’t tell.
Professor
Right. Two morals here: first, there can be different explanations for the same observation. And second, "obvious" doesn’t necessarily mean "right" ... What happened next was ... for a long time nothing. More than 150 years. No one could decide ... Both hypotheses seemed plausible .. . And a lot was at stake—because if the galaxy theory was right, it would be proof that the universe is enormous ... and if the dust theory was right.. . maybe nor so enormous. So the size of the universe was at stake .. . Finally in the 1920s we came up with a telescope that was strong enough to tell us something new here. When we used it to look at the spiral nebulae, we saw ... well, we were not absolutely sure . .. but it really looked like there were stars in those nebulae. So not dust after all, but stars .. .
But how far away were they, really? How would you measure that? Any ideas? Laura?
Female student
Well, how about measuring how strong those stars shine? Because, if the star is far away, then its light would be weak, right?
Professor
Yes .. . but there’s a problem here. You need to know how bright the star is in the first place, because some stars are naturally much brighter than others. So, if you see a star that’s weak ... it can mean one of two things ...
Female student
Oh ... it’s either far away or it’s just a weak star.
Professor
And you can’t really always tell which. But you’re on the right track. There is a kind of star where you can calculate its natural brightness . .. and—you guessed it—we found some in the nebulae. It’s called a variable star—or a "variable" for short— because its brightness varies in regular intervals. I won’t go into detail here, but. . . basically ... the longer the interval, the brighter the star, so from the length of those intervals we were able to calculate their natural brightness. This told us how distant they were—and many turned out to be very, very far away. So we can be sure that the spiral nebulae really are very distant galaxies—which is what some eighteenth-century astronomers guessed but didn’t have the instruments to prove ...
Now, one reason I told you this story is that today there are still plenty of situations when we see something out there, but we really aren’t sure what it is. An example of one such mysterious observation would be gamma-ray bursters.
We’ve known about these gamma-ray bursters for a long time now, but we can’t all agree on what they are.