The Great Chain of Origins

429 - 431

 

Good scientific theories are robust and adaptable. In previous chapters you looked at the basic ideas of star formation and accretion. These same theories can be "re-purposed" to provide a theoretical understanding of planetary formation. You will also see that other ideas - such as nucleosythesis play a critical role in our understanding that we, the solar system, the Sun and beyond are part of a marvelous chain of ideas and concepts that remind us how interlinked the universe is.

You are Star-Stuff!

You now know enough cosmology to understand that elements heavier than helium were not formed in any appreciable amounts in the Big Bang. The heavier elements of Carbon, Oxygen, Calcium etc were all formed in the intense heat of stellar cores and supernovae explosions. You are part of this universe - you are born of star stuff! The theory of Nucleosynthesis provides a detailed understanding of how heavier elements are produced. This is a wonderfully elegant idea - not only are massive stars able to literally "cook" heavy elements, their supernova explosions provide an efficient means to seed the galaxy with heavier elements.

Example 16.1 How does the discovery of stellar populations (Chapter 12.2) support the theory of Nucleosynthesis?

Solution: Population II stars have spectra that suggest that they are deficient in metals. Spectra for Extreme Population I stars on the other hand show much higher metal abundances. Since we also see Population I stars in regions in which we suspect new stars are forming it is logical to conclude that they have formed from material that has been enriched by earlier generations of stars.

The Origin of the Solar System - The Solar Nebula Theory

Historically, speculation concerning the formation of the solar system could be divided into two schools of thought. The solar system may have originated in a violent and rapid catastrophe when our Sun collided with another star; or the solar system may have formed gradually through a process of evolution from one form to another. The first idea is an example of a Catastrophic Process while the latter is an Evolutionary Process. Both types of processes do occur in nature but overwhelmingly we now understand that the solar system formed gradually, through an evolutionary process.

Example 16.2 Argue from your knowledge of the average distance between stars in the solar neighbourhood that stellar collisions ("catastrophes") are exceedingly un-likely.

Solution: Start with the facts: Compare the size of the Sun with the distance to the nearest star and then compare the volume of our Sun with the volume of space around our Sun that contains our nearest stellar neighbour.

Sun: radius = 7 x 108 m, Volume = 1.4 x 1027 m3

Nearest star distance = 3.8 x 1016 m, Volume = 2.3 x 1050 m3

The Sun occupies only a tiny fraction of the volume of space containing itself and its nearest neighbour as given by the fraction:

(1.4 x 1027 m3/2.3 x 1050 m3) = 6 x 10-24 This such a small fraction that it is telling us there is virtually no chance in the lifetime of the universe of our Sun and a nearby star colliding!

Of course example 16.2 does not "disprove" the catastrophic hypothesis. Perhaps our solar system is unique (can you think of evidence you could give to disprove this?). Also - even if stars did collide that still does not explain how a solar system would form. This also underscores a fundamental difference between a theory and a hypothesis:

A hypothesis is a plausible idea or suggestion used to explain a phenomenon.
A theory is a more elaborate set of ideas than a "mere" hypothesis. A theory may combine a number of ideas and principles to not only provide an explanation for a phenomenon but also provide testable claims. Usually a theory should make claims that in principle, if proven false, would negate the theory. As the philosopher of science Sir Karl Popper remarked - you cannot prove theories; you can only disprove them.

The Solar Nebula Theory (or SNT) is an evolutionary theory of solar system formation. It is a very old idea going back to Immanuel Kant's Nebular Hypothesis. In 1755 the German philosopher Immanuel Kant published a very small book with an enormous title! Kant's Universal Natural History and Theory of the Heavens: An essay on the Constitution and Mechanical Origin of the Whole Universe According to Newton’s Principles” proposed that the solar system of the then known 6 planets, Sun and moons formed out of a swirling disk of gas and dust.

The modern Solar Nebula Theory extends Kant's hypothesis into a theory by making very specific and detailed predictions. According to the SNT, our solar system formed from an accretion disk of material surrounding the newly formed Sun. Figure 16.1 captures the essential idea behind the SNT

Figure 16.1 A rotating, collapsing cloud of gas and dust heats and flattens into a disk. A proto-Sun forms in the center while planetary systems form in surrounding rings of condensing matter.

In the next few sections we will "unpack" this theory and judge how well its predictions have been tested.

 

Practice

  1. Explain in your own words the importance distinction between hypothesis and theory. Is the Big Bang a theory or a hypothesis?
  2. Can you suggest and observational evidence that would support the SNT?
  3. In the opening lines of his masterpiece Leave of Grass, the American poet Walt Whitman writes:
    I believe a leaf of grass is no less than the journey-work of the stars,
    This was written in 1855 - long before modern theories of stellar evolution. What modern idea does Whitman's poem "anticipate" - explain.

 

 


To understand that there is a logical "thread" connecting the cosmos with the formation of the solar system

Chp19.2

 

 

Celestia is a great, free planetarium program that allows you to explore the solar system! Download here.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Emanuel Swedenborg was the first to publish the nebular hypothesis in 1734. Kant was familiar with Swedenborg's work but extended it by applying Newton's laws of motion and gravitation.