Chapter 3


This is challenging material, since it covers so much ground (or space rather). First thing to realize:

Distances to objects are very large, scales in the universe are beyond anything we can imagine on earth.


Here is a scale comparison:

Imagine the Sun were the size of an orange (10 cm = 4 inches in diameter). Then we the following relative dimensions:


Let's put it another way. Since the distances are so big, astronomers use "light years" as a unit of distance. A light year is the distance light travels in 1 one year in a vacuum. The speed of light is 300,000 km per sec, or about 190,000 miles per second (7 times around Earth in one second!).
A light year then is:

365 days x 24 hours/day x 3600 seconds/hour x 300,000 km/second = 9.5x1012 km.


Quick check: A light year is a unit of

a. time

b. distance

c. neither

d. both


Using light travel time as our distance measure, we can now more easily express the actual distances in the solar system and universe:


An easy way to remember this is, that if you picture the Sun to have the size of an orange, the universe that we can observe today spans about the size of the solar system. That doesn't mean it couldn't be bigger, it just means that light from more distant objects could not have reached us yet in the time the universe has been in existence.


Here is another scale comparison: A nice animation demonstrating big and small in the universe


On the bottom of page 53 in your book is a glossary of some common astronomical definitions of the objects we distinguish.

Important concept: Look Back Time
Since the light we see from distant galaxies came to us traveling at the speed of light (duh), we actually see distant objects the way they were in the past; our view of the Moon is about 1 second old, our view of the Sun 8 minutes, our view of Pluto 5+ hours, and our view of the Milky Way center about 25,000 years! Likewise, the most distant galaxy we observe we see now the way it was 12.5 billion years ago, when the universe was only 1 billion years old.

Conclusion 1: we do history as well as science!

Conclusion 2: interstellar travel is an enormous challenge!

Conclusion 3: interstellar communication can be a slow process! (makes postal service look pretty good!)

On the plus side, the large size of the universe implies there are literally billions and billions of galaxies in the universe, each containing billions of stars that may have planets like our Solar System, so the chances that life developed elsewhere are quite good from that perspective.

IMPORTANT LIMITATION: While we can see history by looking at distant objects, hence back in time, it does get harder and harder to see them: because of their large distance, the objects will appear very dim and small in the sky. That is why we cannot, for example, detect planets in other galaxies, only in our Milky Way. This is also why we need bigger and more expensive telescopes to study the distant galaxies. They are faint.


Clicker question light travel time:

Given the large distances, we see objects as they were in the past, with the most distant objects the furthest in the past, and the closest objects more closely to the present time. Why can we still do astronomy, if these objects are seen as they were in the past?

A. We can't really, in the sense that most objects we observe could already be gone by now.

B. We can, because changes to objects are slow enough that we can assume they are still there and not that different from how we see them.

C. Some objects indeed may have disappeared already by the time we can see them, or changed drastically, but we still learn useful information.

D. This makes astronomers very happy, since no one can prove they are wrong.


Make sure you review the material in the book on the structure of matter. Important concepts you should understand are:




If any of these is not clear to you, of course ask about it in class!





A few quick checks: Do I get matter?



Light, our source of information on the Universe

Light is a traveling wave of energy. We characterize the color of light by its wavelength. We can also use its frequency to characterize its color; wavelength is inversely related to frequency through the speed of the wave. Visible light, which we are most familiar with, is but one small part of the entire Electromagnetic ("EM") Spectrum. All EM waves in vacuum travel at the same speed, c, the "speed of light".


Much of what we know about the solar system comes from studying the visible and other forms of EM radiation that objects emit or reflect. Most of what we know about the universe outside our solar system comes from observing the EM radiation that we receive from stars and galaxies and other sources.

Why do objects emit light? The simplest way to describe is that every charged particle that is accelerated will emit EM radiation. This radiation can occur over a wide range in color, leading to a continuous spectrum. In addition, every charged particle that is bound to another charged particle (think of electrons bound to an atomic nucleus in an atom or in molecules) can change its state of energy and also emit light. In the latter case, think of electrons "changing orbits" inside an atom or molecule or ion. This change can only be "discrete" or "quantized" as we have learned from quantum physics in the 1920s. The discrete change produces light of one particular color, depending on the energy of the transition, leading to "emission lines" in a spectrum. Electrons changing orbits can also absorb light of that particular color, leading to "absorption lines" in a spectrum.

The hotter an object, the more energetic the radiation it emits. In the visible, this means the objects emit more blue and less red light if they are hot. Cool planets reflect visible sun light, but also emit their own infrared light. Your body also emits infrared light. Objects can be so hot or cool that our eyes are not sensitive to detecting the light they emit. We can build special detectors and telescope that can see this light.

Example spectra and more explanation: NASA website explaining different types of spectra.


The spectrum of the Sun


All the black lines you see in the solar spectrum come from certain chemical elements. They are like fingerprints that we can use to measure the chemical composition of the Sun. If you get a spectrum of a very old star, you will not find this multitude of lines, but the spectrum is dominated by hydrogen and helium lines only.

A set of stellar spectra for stars of different temperatures and luminosities, and also one for a star with low abundance of heavy elements (a "metal-poor" star). In this figure, we see stars from hot at the top to cool at the bottom. The letter/number code on the left describes the type of star (a code adopted by astronomers related to the mass and temperature of the star, the Sun is a G2 star), the number on the right is the star's name. Notice how the general color shifts from blue to red as the stars get cooler. Also, the different lines due to chemical elements change for the different stars. This is related to the star's temperature and its chemical composition.


Big Bang cosmology, the key evidence.

Two-dimensional analog of an expanding universe with no center: the surface of a sphere that is blown up. The distances between all points gets bigger over time, and this is true for any point. There is no center on the surface, all observers on the surface would see the same features.


Einstein showed that matter and energy are equivalent (the famous equation E = M c2). Thus, in the early universe matter was created from energy (light), and even today in high energy physics experiments we can observe the change of light into matter and vice versa. As matter was created from energy in the early universe, gravity began to accumulate the matter to form new objects, since gravity is caused by matter (mass). The first stars formed, and galaxies began to form. We can still observe stars forming in the Milky Way and in other galaxies today; when they form, invariably a disk of material is formed around them from which it is likely that planets are forming. The universe is about 13.8 billion years old, whereas our Sun, Earth and Moon have all been shown to be about 4.5 billion years old. If the Sun had formed immediately after the Big Bang, we would not have had the heavy elements (carbon, oxygen, silicon, iron, etc etc) required to make the planets as we know them.

How do we measure what the composition of the early universe was: we can observe the chemical composition of the oldest stars we can find, by analyzing the light they emit, using our big telescope. A spectrum is the detailed breakup of a stars light into all its colors or wavelengths.

We can also observe the helium abundance in other stars. We always find, no matter how many stars we look at, that all stars have a lot of helium in them, never less than 23-24%. That implies the helium existed before the star was formed, and Big Bang models predict that helium abundance. It also implies that stars make the other elements through nuclear fusion and when they explode as supernova. We can also study this process by studying many stars and see how their chemical composition is as a function of age, or what materials are brought up to the surface of a star that were made inside the nucleus. Finally, in exploding stars we the formation of heavy elements through the radiation they emit.




Some concluding comments of this first section:

The book goes into more detail on nuclear fusion. The key is that stars are very hot in the center, so hot that matter is compressed into other matter (hydrogen gas in particular) and in that process hydrogen is converted into helium and a lot of energy is released since some mass is converted into energy (the helium that forms has less mass than the 4 hydrogen nuclei (protons) that went into it). Einstein predicted that mass and energy are equivalent, and can be converted into one another. The mass difference between the 4 hydrogen nuclei and the helium nucleus is only 0.7%, yet this is enough to produce enormous amounts of energy and allow the sun to shine for 10 billion years. The energy is equal to (mass difference) * (speed of light squared), the famous equation E= mc2. Billions of hydrogen nuclei are converted into helium and energy all the time in the sun.

The authors discuss how galaxies, stars and planets can form from contracting gas clouds. Make sure you understand the difference between a "galaxy", a "star" and a "planet".

In general, planets orbit around stars, and stars are so massive that they generate their own visible and other light through a process called "nuclear fusion" in the core. A planet has not enough mass to have nuclear fusion in its center (it is not hot enough there). We will see later-on that there are intermediate objects that astronomers call "brown dwarfs" which are stars that are too massive to be called a planet yet did not have enough mass to sustain nuclear fusion of hydrogen gas in their cores.

We observe contracting gas clouds and new stars forming in many places in our Milky Way. The most famous place perhaps is in the Orion Nebula, which you can see on a dark night with no city lights and no moon light with the naked eye (and spectacularly well with binoculars) in the sword of the constellation Orion. We see dust disks around new stars that are forming, and it is likely that the dust disks are the first stage towards formation of planets.

The process of planet formation is discussed in some detail. We distinguish rocky planets (we call them "terrestrial planets" in the solar system) from "jovian planets" which are like Jupiter, hence large gas spheres. The terrestrial planets are found closer to the Sun than the Jovian planets, which has likely to do with how they formed. We will get back to differences between the two.



Questions

1. What is the difference between nuclear fusion and nuclear fission? Give an example of each.


2. If our Sun would have been as old as the Universe, what would we expect about its planets?
a. No changes from current situation necessarily.

b. If the Sun still had planets, they were most likely all giant planets

c. It probably would have no planets at all.

d. We will never be able to find out.


3. Regarding nuclear fusion in the Sun, which of the following is correct?

a. The hydrogen in the center of the Sun is converted to He, some mass is lost in the process.

b. All the hydrogen in the Sun will be converted to Helium and then the Sun runs out of energy.

c. All the hydrogen in the Sun is converted to energy, following, E= mc2, and is essentially gone as matter.

d. The Sun produces energy through nuclear fusion but no mass is lost.