This week’s material is the heart of astronomy, because it includes the standard stellar model of star structure. The scope goes from star formation (from the stuff of the interstellar medium) up to the end of the first phase of nuclear reactions powering a star’s energy output. In the next chapter, you will see how stars make a transition from the end of the first phase of nuclear reactions to following phases, and what happens after the nuclear burning phases involves some weird (aka "modern") physics.

Last unit you studied what astronomers observe about our Sun’s surface properties and about stellar luminosity and color (or surface temperature) - recall the H-R diagram that shows a vivid observed correlation that underlies the standard stellar model. The H-R diagram convinced astronomers many decades ago that stars live a very long time - far longer than various non-scientific beliefs would allow. With the advent of radioactive dating methods, scientists began to realize that Earth has been around for billions of years. Following that discovery, over the past few decades we have seen that every object in the solar system from which we have samples also dates back billions of years. The problem as of the 1930’s was that, if the solar system has existed (with its Sun) for billions of years, what could the Sun’s source of power be? We knew the mass and size of the Sun, its surface composition, and its total power output, but not what the power source is. With the discovery of nuclear reactions and intensive laboratory exploration of nuclear physics in the 1930’s and early 1940’s and beyond, astronomers finally were able to understand stellar energy production (not the last time astronomers would benefit from research and development of interest to the military). Of all the possible explanations for energy production in the Sun, only nuclear fusion could provide the level of power required for billions of years. Gravitational energy release (from contraction) and chemical burning simply could not provide the total amount of power produced for long enough. Rates of the nuclear reactions shown in your book have been measured in Earth laboratories, and those data are in the standard solar computer models describing how our Sun (and other stars) works. Last week you studied the nuclear reactions taking place in the core of our Sun.  Although the details vary depending on stellar mass, nuclear reactions in all main sequence stars result in energy production while converting hydrogen (in the form of protons) into helium (in the form of doubly-charged helium nuclei).

Some of the most beautiful pictures in astronomy come from the study of star formation. Once astronomers realized how mature stars work, they could also begin to understand quantitatively how a star might form. The answer is simple in hindsight - gravity causes ultra-cold material to fall toward a center of mass, the material heats up by collisions, and eventually high enough temperature and material density occur to support nuclear fusion conditions. These are severe conditions indeed, and difficult to imagine. The density of gas in the core of the Sun far exceeds the density of even uranium (U density about 20 grams per cubic centimeter - water has a density of 1 gram per cc). The temperature in the core of the Sun is over 10 million Kelvins. How is the Sun able to sustain this reaction in a controlled mode? This is one of the key questions for you to contemplate as you read the chapter. We have been trying to develop controlled fusion energy production (for electricity) on Earth for several decades, and we remain many decades away from success, which by no means is a sure thing. Achieving and maintaining the severe conditions remain the problem, but somehow the Sun and other main-sequence stars control those reactions and maintain the required conditions for millions to tens of billions of years.

Observing star formation is one of astronomy’s greatest challenges. Only a tiny fraction of stars are in the formation phase, because it happens so quickly - so there are limited observing possibilities.  The action is at the center of a collapsing cloud that contains dust, so we can’t see the center in visible light. Theory says that collapsing and heating centers should radiate strongly in the radio and far infrared at first, then gradually radiate in the nearer infrared, and finally begin to radiate strongly in the visible range when the star is about ready for nuclear reactions to occur; yet during all those phases a dusty cocoon surrounds the forming star and hides it from viewing in visible light. You may need to review pages 69-70 concerning radiation from space that we can detect at Earth’s surface. Visible light gets through our atmosphere, and some near-infrared. So complete study of star formation had to await development of satellite/rocket/balloon astronomy and development of infrared detectors that could work in space for extended periods of time. The Hubble space telescope has very good infrared imaging capability, and other satellites are dedicated completely to infrared observing. One of the main missions of HST is to investigate star forming regions, and it has been very active and successful on that front.

Finally, a few words on the meaning of evolution in a stellar context. Remember that we have a model of evolution based on statistical data for hundreds of thousands of stars with known parallax (distance). As an analogy, consider a bug that lives one day only. That bug might observe lots of humans, and it would see that during its lifetime most humans did not change at all - a very small number are born and an almost equal number die each day (compared with the total population). Now recall that we are the bugs, and during our lifetime most stars undergo no measurable change; however, we do see a very small number of variable stars and the occasional explosion and a few other odd short-time phenomena that you will learn about later. Working out a stellar evolution scenario took a lot of work and gobs of physics background material. A common misperception about a star and the H-R diagram is that when a star moves on the H-R diagram it moves in space. Movement on the main sequence has nothing to do with physical sky motion. When a star changes luminosity, its point on the H-R diagram changes, or moves, but the evolution does not affect spatial location and motion. ALSO, stars on the main sequence are in hydrostatic equilibrium (stable) and do NOT move diagonally along the main sequence. They move only slightly as they burn their fuel, from the bottom of the band toward the top of the band, while moving very little horizontally on the diagram. The location of a mature star on the main sequence depends almost entirely on its mass - so all stars with the same mass as our Sun will lie about the same place on the H-R diagram as our Sun.

Finally finally, keep your eye on (i.e., put in long-term memory) stellar elemental composition as it changes with evolution. Not so much the details, but the net changes. In this chapter, you learn about changes that happen in the core of the star - the net change is that hydrogen gradually becomes helium while a star is on the main sequence. Hydrogen outside the core does not undergo change, and other elements anywhere in the star (helium, carbon, etc.) change very little while a star is on the main sequence. Next chapter you will learn what happens when the core no longer has enough hydrogen to maintain equilibrium:  major stellar changes, some recycle of the star's outer layers to the interstellar medium, occasionally some violent recycle of material to interstellar space (from which everything started), and some of the most bizarre objects known to astro-physicists - stellar remnants.

The following table provides a listing of approximate values of luminosity (relative to our Sun) and main-sequence lifetime (years), assuming the total main-sequence life of our Sun will be 10 billion years.  The mass is relative to our Sun's mass (1 unit).  The luminosity and lifetime values are from a spreadsheet calculation using the approximate equations in your textbook for luminosity (page 151) and life expectancy (pages 175-176), with a modification for very low mass stars (0.4 and less) to match the data points in Figure 8-21on page 150, at the low end of the mass range.