Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the universe, part one

I've long wanted to write something about the Big Bang, and about how astronomers describe the universe. And I've wanted to explain to you how astronomers have gone about gleaning information about the universe that has made them believe in the Big Bang.

You need to keep a few things in mind when you read this. First, that it is an overwhelmingly huge and complicated subject. Second, that there is so much of what astronomers know that I don't know or understand myself. And third, that there are just so many things that astronomers themselves don't know.

Astronomy is a work in progress. I have been interested in space since 1969, for forty-one years. And the astronomically accepted description of the universe has been utterly revolutionized since then. Watching all this new evidence about the universe unfold before my eyes, and seeing the universe literally undergo a "mental landslide" in my mind's eye, has been like riding a scientific roller coaster. It has been fantastic! And the ride isn't over by any means!

The thing to remember is that the universe itself is what it is. What has changed is what we know about it. I'm sure you have heard the story about three blind persons who touched different parts of an elephant. One felt the trunk, one an ear and the third one the tail. Not only did they have "different mental ideas" about what the animal looked like, but also their combined knowledge of the elephant was not in any way enough to describe or understand this fantastic herbivore in its entirety. I can tell you that astronomers have been doing something similar - they have "touched the trunk of the universe" and tried to describe the entire universe from what they know about "its trunk". Obviously they are going to fail. On the other hand, a person who has touched the trunk of an elephant does have some real knowledge about that trunk, and similarly, astronomers do have a real knowledge about some of the aspects of the universe. The question is how complete the information will be that they are eventually going to glean about it.

All right. One person who really touched the trunk of the larger universe in a pioneering way was the Italian astronomer Guiseppe Piazzi:



In this painting, Piazzi is pointing at an orange star, 61 Cygni (which, by the way, is an extremely insignificant-looking star, nowhere near as bright as it looks here).

61 Cygni and its vicinity. 61 Cygni is a point of light at the lower left. Click on "Labeled image of Northern Cygnus" to find it.

For some reason Piazzi had started measuring the exact position in the sky of 61 Cygni, and he noticed that over the years, 61 Cygni seemed to move slowly across the sky, compared with all the other stars in its vicinity. The movement of 61 Cygni was extremely insignificant and only detectable with scientific instruments, but the movement was there nevertheless. No other stars that Piazzi measured seemed to move. Why was 61 Cygni moving, if all other stars were stationary?

A German astronomer, Friedrich Wilhelm Bessel, suspected that 61 Cygni seemed to move because it was much closer to the Earth than the stars that seemed to lie close to it.



Friedrich Wilhelm Bessel, honored on a German stamp.

If Bessel was right, all the other stars were immeasurably far away, but 61 Cygni just might be close enough to show a parallax. So what is a parallax? Well, imagine that you are inside a moving car, looking out at the landscape. As your car moves, the nearest objects seem to whiz by very fast. The "intermediately distant" objects seem to pass by more slowly. The very distant objects, such as a mountain far in the background, hardly seem to move at all. The fact that things appear to move more and faster the closer they are as you pass by them is the key to understanding parallax.

It should be the same thing with stars. They should seem to move as the Earth orbits the Sun, provided they are sufficiently nearby. The distance from the Earth to the Sun is 150 million kilometers, and it takes the Earth exactly one years to make one turn around the Sun. This means that in six months, the Earth should make "a half orbit" around the Sun, and move around 300 million kilometers away from the position it had in space six months earlier. Take a look at this picture:



Here you can see the Sun in the middle, surrounded by a circle. The circle is the orbit of the Earth. The figure 1 represents the position of the Earth at a certain time of the year, for example on January 1, and the figure 2 represents the position of the Earth six months later, which in this case would be on July 1. On July 1, the Earth will have moved 300 million kilometers away from the position it had on January 1. These 300 million kilometers form the baseline of a triangle, whose third point is at the star whose position we want to measure. You can see the star whose distance we want to measure in the picture.

In the picture you can see that when the Earth is at point 1, the star appears to be in position B. But when the Earth is at position 2, the star appears to be in position A. How much the star appears to have moved in the sky is determined by the angle theta, θ, that you can see in the picture. The angle θ and the known size of the baseline (300 million kilometers) tell us how far away the star really is. In reality, the angle θ, even for the most nearby stars, is extremely tiny. The "baseline" of 300 million kilometers worth of Earth orbit around the Sun is incredibly insignificant compared with the lightyears that separate us from the stars, and the angle θ becomes correspondingly hard to measure. You get a triangle with an unbelievably tiny baseline and two incredibly long "legs".

Nevertheless, the distance to the most nearby stars is measurable, you do get an angle, and the angle θ of 61 Cygni, its so-called parallax, was measured by Friedrich Wilhelm Bessel in 1838. The angle was 313.6 milliarcseconds, which is so tiny that it is hard to imagine. Think of it like this. A circle has 360 degrees. A degree can be divided into sixty arcminutes. An arcminute can be divided into sixty arcseconds. One arcsecond can be divided into a thousand milliarcseconds. 313.6 milliarcseconds is less than one part of 3,600 of a degree. But Bessel succeeded in measuring it, and he came close to pinning down the true distance of 61 Cygni, which is eleven lightyears.

By proving that 61 Cygni was about 11 lightyears away, Friedrich Wilhelm Bessel demonstrated that the stars were far away, but not immeasurably far way, at least not the most nearby ones. Bessel gave the universe a third dimension, a dimension of depth. The sky is not a black canopy sequined by equidistant little points of light. The stars are at different distances. Also, as Guiseppe Piazzi had demonstrated, 61 Cygni had a movement all of its own, a movement that was unrelated to the Earth's orbit around the Sun. The stars were at different distances and they were not stationary at all, since they had their own movements in the sky.



The sky is not a canopy, and the stars are not decorations painted on the inside of a canopy.

Piazzi and Bessel showed humanity that there is a real incredibly large universe outside our own solar system. By showing us that, Piazzi and Bessel touched the tip of elephant's trunk of the universe.



Ann

Thank you, Ann, for the first of what promises to be a fascinating series of posts! I remembered the science from the astronomy course I took in undergrad, but I hadn't known the names behind the science.

I loved the pictures you included, BTW.

May I say as an aside that you do a remarkable job of presenting complex material clearly and concisely? You mentioned that much of what astronomers know you don't understand yourself; otherwise, I would have suspected you to be an astronomy professor. Do you teach in a related field? Or are you a professional communicator in some other type of occupation?

Thanks again for the educational post.

An eager student,
Lynn

Thank you, Lynn! I'll be back, but possibly not for another two weeks, as RL promises to be a beast.

Ann

P.S. As for what I teach, it is English and Swedish. I am a bit embarrassed to admit that I teach English, because you will undoubtedly have noticed that I keep making small grammatical mistakes. I'm so much more comfortable teaching my Swedish students Swedish.

I don't teach astronomy, and I wouldn't be able to, since I don't understand the math of astronomy (or at least, I only understand the most basic mathematical principles of it). Besides, I'm not equally interested in all aspects of astronomy by any means. I wouldn't want to teach students about the Moon, since I would be too bored myself. The best thing about the Moon is the funny outlines you can draw on its surface!



The rabbit, the man and the lady in the Moon.

The Kings of Bores, however, may be the asteroids. You can't even draw funny pictures on them.



Kings of bores! Oink!

Ann