3.1.3: Looking at Stars

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At this point you may still be unclear as to how we know all this. How can we know about processes and events that took place billions of years ago? Part of the answer lies in the fact that all the processes involved in the formation of new elements are still occurring today in the centers of stars. Our own Sun is an example of a fairly typical star; it is composed of ~74% (by mass,~92% by volume) hydrogen, ~24% helium, and trace amounts of heavier elements. There are many other stars (billions) just like it. How do we know? Analysis of the emission spectra of the light emitted by the Sun or the light emitted from any other celestial object enables us to deduce which elements are present.⁵⁶ Similarly, we can deduce which elements and molecules are present in the clouds between stars by looking at which wavelengths of light are absorbed! Remember that emission/absorption spectra are a result of the interaction between the atoms of a particular element and electromagnetic radiation (light). They serve as a fingerprint of that element (or molecule). The spectrum of a star reveals which elements are present. No matter where an element is found in the universe it appears to have the same spectroscopic properties.

Astrophysicists have concluded that our Sun (Sol) is a third generation star, which means that the material in it has already been through two cycles of condensation and explosive redistribution. This conclusion is based on the fact that the Sun contains materials (heavy elements) that it could not have formed itself, and so must have been generated previously within larger and/or exploding stars. Various types of data indicate that the Sun and its planetary system were formed by the rapid collapse of a molecular (mostly hydrogen) cloud ~4.59 billion years ago. It is possible that this collapse was triggered by a shock wave from a nearby supernova. The gas condensed in response to gravitational attraction and the conservation of angular momentum; most of this gas (>98%) became the Sun, and the rest formed a flattened disc, known as a planetary nebulae. The planets were formed from this disc, with the small rocky/metallic planets closer to the Sun, gas giants further out, and remnants of the dust cloud distributed in the Oort cloud.⁵⁷ As we will see, living systems as we know them depend upon elements produced by second and third generation stars. This process of planet formation appears to be relatively common and more and more planetary systems are being discovered every year.⁵⁸

Stars have a life cycle from birth to death; our Sun is currently about half way through this life cycle. There is not enough matter in the Sun for it to become a supernova, so when most of its hydrogen has undergone fusion, ~5 billion years from now, the Sun’s core will collapse and helium fusion will begin. This will lead to the formation of heavier elements. At this point, scientists predict that the Sun’s outer layer will expand and the Sun will be transformed into a red giant. Its radius will grow to be larger than the Earth’s current orbit. That will be it for life on Earth, although humans are likely to become extinct much sooner than that. Eventually the Sun will lose its outer layers of gas and they may become a part of other stars elsewhere in the galaxy. The remaining core will shrink, grow hotter and hotter, and eventually form a white dwarf star. Over (a very long) time, the Sun will cool down, stop emitting light, and fade away.

Questions to Answer

1. How do the properties of isolated atoms or molecules give rise to the world we observe? Why are objects different colors, or have different melting points?

2. Do isolated atoms/molecules exist in a state such as solid, liquid, or gas?

3. Where do the atoms in your body come from? (Trace their origin back as far as you can.)

4. How does the size of the universe influence the density of particles?

5. How many protons, neutrons, and electrons does ⁴He have? How about ⁴He²⁺?

6. Generate a graph that estimates the number of atoms in the universe as a function of time, beginning with the Big Bang and continuing up to the present day.

7. Draw another graph to illustrate the number of elements in the universe as a function of time. Explain your reasoning behind both graphs.

Questions to Ponder 

1. Can an atom of one element change into an atom of another element?

2. Is the number of atoms in the universe constant?

3. How does the big bang theory constrain the time that life could have first arisen in the universe?

References

⁵⁶ It has been estimated that it takes between 10,000 to 170,000 years for a photon released during a fusion reaction at the Sun’s core to reach its surface. http://sunearthday.nasa.gov/2007/loc...t_sunlight.php

⁵⁷ More physics that we will conveniently pass over, but it is worth noting that this is why the planets all move around the Sun in the same direction.

⁵⁸ You may want to search the web for “extrasolar planets.”