.

Return to Home Page

Feb. 13

| Lecture | Assignment | Links | Q&A |

Still under construction!

Today was a special class--devoted entirely to questions. I'll try to summarize here what I remember from the class, combining both the 1:15 and 2:45 sections.

Q: What do astronomers think of science fiction?

A: Well, there's quite a variety of opinions among astronomers. I think many of us enjoy it. Kepler is even credited with writing the very first science fiction!

My personal favorites are rather obscure: Stanislaw Lem, Olaf Stapledon, and I've recently started reading Greg Egan who is very well-versed in mathematics and physics.

Astronomers Fred Hoyle and Carl Sagan also wrote science fiction.

Isaac Asimov had a Ph.D. in biochemistry, I believe. I met him when I was a kid:

Physicist Lawrence Krauss has written books about "The Physics of Star Trek"

Q: What do astronomers think about astrology?

A: Well, most of the time we resent the attention and money astrologers get without having to check their ideas against reality the way we astronomers do. We try not to be arrogant. But there doesn't seem to be any compelling statistical evidence for it, and any theoretical reasons why it might work are unknown. There's much more of a physical effect from the lightbulbs in the room on us than from the planet Jupiter. Also, when astrology was first created, only 6 planets were known, and now 9 are. Yet the basis of astrology was never really affected. To astronomers, astrology is not a science. However, in the history of science, astrology and astronomy did grow up together. Here is some relevant biographical information on Kepler.

Q: What causes the beautiful colors in some of the photographs of nebulas?

A: Good question! That ties into what I've been talking about recently. There are several causes of the colors in nebulas. Start with this one for an example:

(Reminder: a nebula is a cloud of gas and dust between stars.)

What causes the red color? Well, it's sort of like a neon light, but a neon light has neon gas inside the tube, and here the gas giving off the red light is hydrogen the simplest and most common element in the Universe. Just like a fluorescent light glows because of an electrical spark, the energy that causes the hydrogen glow in a nebula is caused from the outside: the nearby stars light up the hydrogen gas.

The red color is from the electron in hydrogen going from energy level 3 down to energy level 2, and has 656 nanometers as its wavelength. This is something you'll experiment with in Lab 2.

So what about the other things you see in this picture? Well, notice that there are fewer stars in the bottom part of the picture. That's because the gas and dust, when there's enough of it, can absorb the starlight.

What about the blueish colors seen here? Does it mean that the gas is hot? No, that's a good guess though. This nebula is not a blackbody, it is giving off light not because it is hot but because it is reflecting, it's an example of a reflection nebula. Just like our sky is blue because blue light from the Sun is more likely to bounce around our atmosphere and red light from the Sun is more likely to come through straight (as in a sunset), the blue light from a star in a reflection nebula gets bounced around and spread through the entire nebula.

The picture above is of the Ring Nebula in the constellation of Lyra. Our own Sun will end its own life by expelling its outer layers into a nebula similar to this one. What causes the green glow?

It's similar to what causes the red glow from hydrogen, but it's from Oxygen. There are two energy levels of a form of Oxygen, and when an electron goes down to the lower level, it gives off its energy as a photon, a particle of light. However, the upper level takes a long time to spontaneously go down to the lower level. In that time, unless the gas has very low density, it's pretty likely for a collision between atoms to absorb that energy, bringing the electron down to the lower level.

In fact, the first people to look at spectra of these kinds of nebulas thought they had discovered an entirely new element, called nebulium, because they couldn't make the same spectral line here on Earth (because our gas was always too dense.) Here is a link on this interesting aspect of astronomical history.

Q: Do you have to pay to use a telescope? How does "accreditation" of astronomers work?

No, actually it works the other way around! You can actually get paid as an astronomer to use a first-class telescope!

To use a telescope you have to apply with a standard form, and a committee made of peers (other scientists) reviews your proposal. If accepted, you'll also write out a budget. Maybe you'll need assistants to help you analyze the data, or you'll need to travel to the observatory, or to a conference to present the results. Some telescopes can also give stipends so that astronomers can concentrate on the research instead of teaching. This is in the observatory's best interest, because then they can point to all the scientific publications that used data from their telescope.

On the other hand, an astronomer often has to pay to get a research paper published. The Astrophysical Journal gets no money from ads, and libraries are just about its only subscribers. So instead it charges for papers to be published. Usually it costs about $1000 for a 10-page paper. Most of the time an astronomer can get this paid for by a grant.

There is no formal "accredidation" program in astronomy. Most active researchers have Ph.D. degrees. So the views of people from "outside" the field making contributions are not set in stone.

Q: What is NASA doing for education?

NASA has an education site that can tell you more.

I used to work at the NASA Goddard Space Flight Center, and they have a web site there all about high energy astrophysics. It's called Imagine the Universe. They also have a site called ask a High Energy Astronomer and I used to answer questions for them. They get e-mails from all over the world, and one week a month, a couple of scientists (the job rotates) will choose questions that come in through e-mail to answer.

Q: Why study astronomy when so many people are suffering around the world?

This is a tough question. Here are some justifications:

1. Solving human suffering is difficult, while solving something like Kepler's Laws is comparitively easier. Sometimes when you try to make the world a better place, it can backfire. There are some simple things one can do (say, visiting the Hunger Site), but poverty has been with us for millenia, while scientific understanding has only increased.

2. "Man does not live by bread alone."--there's more to life than survival. There needs to be something worth living for, and it's a basic human need to explore and put our lives in larger context.

3. People put much effort into other fields, such as sports or entertainment, that are also not directly linked to helping the most unfortunate survive. If we all of a sudden stopped doing these things, it wouldn't by itself solve the world's problems.

Q: What about the Big Bang?

There are two main lines of evidence for the Big Bang.

One is that all the galaxies appear to be flying away from ours. Or at least the further away we look, the faster a galaxy is likely to be moving away. (Galaxies have some random motion too, besides their motion away from us.)

If you extrapolate this outwards motion back in time, you find all the galaxies were once on top of each other at the same point.

The evidence that galaxies are flying away from us is that when we look at their spectra and see emission lines from elements like hydrogen, we see that they appear red-shifted. For example, a hydrogen emission line at 656 nanometers would appear to have a longer wavelength, 700 nanometers maybe. Actually, thinking of this redshift as being the result of the Doppler effect is not completely accurate; you can also think of the red-shift as resulting from the expansion of space itself. It's a little confusing.

The other main piece of evidence is the Cosmic Microwave Background. This was discovered in the 1960s, by scientists working for Bell Labs, the research institute of AT&T. These scientists, Penzias and Wilson, were trying to reduce the "noise" that was coming through on telephone lines. But they found radio waves coming in at all times from all directions, and after much work they convinced themselves it was coming from the sky (they had to scoop pigeon droppings out of their antenna!) Here is a link about the history of this discovery.

Our explanation for this radiation is that when we see it, we are seeing the Big Bang!

How can that be? Well, the Big Bang was very hot. So the radiation it gave off had short wavelength. (Think of Wien's Law, lambda_max=2.9x10⁶ nm/T (K).) There are a couple of ways--not entirely exact--you can think of the reason why we now see this radiation at microwave radiation (long wavelengths), typical for very cold material (2.7 K):

1. As the Universe has expanded, it has cooled off.
2. As the Universe has been expanding, the wavelength of this radiation has expanded also

So the radiation from the Big Bang is the most perfect natural blackbody ever discovered, deviating only from the theoretical spectrum by ten parts in a million or so. But it indicates a temperature of 2.7 K (2.7 degrees Celsius above absolute zero). From Wien's Law this tells us: lambda_max=2.9x10⁶ nm/T (K) approx=2.9x10⁶ nm /2.9

(because 2.9 is pretty close to 2.7):

lambda_max=10⁶ nm=10^-3 m, which is in the microwave part of the spectrum (see Feb. 11 notes).

Q: Just what is gravity anyway?

Here and here are web pages for lectures I've made about Einstein's General Theory of Relativity. This theory is our best current theory of gravity. Here is Einstein's semi-popular level book describing his theory.

Assignment

Chapter	Problem
6	5
6	7
6	8