Star Formation:
A Point-Form Summary.
This section of the course notes, and the associated PowerPoint presentation, makes the following critical points:
many stars are very old; the Sun, for example, formed about 4.6 billion years ago. But there is still active star formation going on in the galaxy, and we can study the formation process if we can identify the appropriate regions
there are many signposts of recent and ongoing star formation: the presence of short-lived massive stars; the residual gas and dust that provides the raw materials; objects which glow vigorously in the infrared; molcular clouds; and various other things
the Orion nebula is one particularly good region for study, largely because it is near enough to be studied in considerable detail
at present, there is a continuous cycle of star birth, life, and death, with gas coalescing into stars here and there, some of which is later released into the ISM as the stars reach the end of their lives
one particularly important aspect is the continuing enrichment of the ISM by massive stars: they release gas into which newly-made heavy elements (in small quantities) have been introduced
since the formation of stars leads to the capture of some material into remnants like white dwarfs and neutron stars, there will come a time when no further star formation will be possible: there will be too little gas available. But for a long time into the future, the cycle will continue much as we see it now
Associated Readings from the Text.Please look at: Chapter 17, pages 545-552. Chapter 19, pages 609-612.Stellar Nurseries.As we have seen, there are young stars as well as old stars in the heavens. In other words, some stars formed billions of years ago, while others formed very recently. Is it possible to regions in which stars are forming right now? How would we recognize such stellar nurseries? The ideal answer to this question would rely on a fairly well-developed idea of how the process of star formation operates. We explored some aspects of this in Physics 015 in discussing the formation of the Solar System, but the topic as a whole is discouragingly complex. Partly this is because stars form in inconvenient places, deep inside great cool clouds, surrounded by gas and dust which obscure our view. The onset of star formation may depend on external factors, like the recent explosion of a massive star, and may be complicated by all sorts of other things like the strength of the interstellar magnetic field. Moreover, the stars do not form as single objects: they are typically formed in clusters and associations. We noted earlier that the structure of a star is quite well determined by very simple physical laws; but the breakup of a big cloud into many stars is much more complex and less well understood. For this reason, therefore, I propose to give this topic a fairly quick overview, and then pass on to topics which are a little more clear-cut.Signposts of Star Formation.In seeking to identify stellar nurseries, there are a number of tell-tale things we can look for: O and B stars. These are massive stars which are so hot that they consume their fuel supply very rapidly. Such stars cannot last very long, so if you find a region which contains them, you can be sure that star formation occurred recently there (and may still be going on). HII regions (i.e. ionized gas clouds). O stars, being extremely hot, produce enormous amounts of ultraviolet radiation. Such photons ionize any surrounding gas, converting neutral hydrogen (denoted HI and read as ``H-one'') into ionized gas (HII, ``H-two''). The gas fluoresces, and gives off visible light with the characteristic emission-line pattern of hydrogen gas (and whatever other species are present). The existence of a large HII region is usually a tell-tale sign of the presence of bright, young O stars, recently formed. Dusty, gassy regions. Sometimes, star formation produces no O stars. (Remember that these stars are uncommon. We find many low-mass stars for every massive one.) In that case, we will see neither O stars nor conspicuous clouds of glowing ionized gas. Even so, there may be lots of neutral gas there, and clumps of interstellar dust within which somewhat smaller stars may be forming. In short, look for the raw materials of star formation. Reflection nebulae. Luminous stars surrounded by gas and dust may look like the Pleiades. One sees lots of scattered light, which looks quite blue because of the nature of the dust grains. Stars which are so conspicuously associated with a clump of gas and dust may have only recently formed. T Tauri stars. These are stars which are known to be of moderate mass (like the sun) but in a very early stage of their lives. They are recognizable because we can deduce from their spectra (in ways I need not go into) that they have strong `stellar winds' - that is, lots of material is flowing away from the stars at high speed. When we described the `Magic Broom' in the newly-formed Solar System, we were in fact asserting that the sun went through a comparable phase, which may be common to the vast majority of stars of moderate mass. Infrared sources. As you know, the lumps of gas which are slowly contracting to become stars - lumps which are commonly called `protostars' - gradually heat up as they do so. Although nuclear reactions do not begin until the blobs have contracted a great deal and gotten considerably hotter in the cores, the protostars are bright and reddish. (They have large surface areas, and can give off a lot of light even they are moderately cool.) The energy given off by protostars will be mainly in the infrared part of the spectrum and detected using the appropriate instruments. Molecular clouds. For a long time, astronomers believed that complex molecules could not exist in interstellar space, between the stars. In such a location, a molecule is at the mercy of any photon which happens to pass by. Many photons are energetic enough to completely rip apart any molecules they happen to run into, and it was thought that only the very simplest molecules might survive for a short time here and there. This was wrong. The development of millimeter-wave astronomy revealed that there are great clouds of molecular gas sitting out in space. Some of these clouds contain as much as one hundred thousand to one million times as much material as the sun contains. The molecules survive by virtue of being deep within the dense cores of great gas clouds, shielded from the harmful effects of energetic photons. Dense cool regions are exactly the locations in which stars are likely to form, so using a millimeter-wave telescope to pick up strong signals from interstellar molecules is a very useful way to identify likely sites of star formation. We will explore this further when we talk about the Interstellar Medium. Stars not yet on the main sequence. When a huge cloud contracts and fragments into pieces, not all the stars produced within it form at the same rate. As we have seen, the more massive stars do everything more quickly, right through their whole lives. An O star may in fact have run through its whole life cycle before the numerous low-mass stars in the region have even reached the main sequence, and a study of the properties of the stars in such a region may reveal that they are in this pre-natal state. Unbound systems. Some systems (small groups of stars) are not gravitationally stable - that is, the individual stars orbit around in complicated paths, affected by the gravity of the other stars, in such a way that fairly soon one or more of the stars will escape the system and head off into interstellar space. (We considered the evaporation of star clusters earlier. This is an analogous situation.) So if you find a small group of this sort, it is a pretty good bet that it has not been around very long, or else it would have already broken up.The Orion Nebula.It turns out that an ideal place to look for evidence of recent and continuing star formation is in the Orion Nebula, shown on page 610 of your text (and visible to the unaided eye!). Much of the reason for its usefulness, by the way, is its proximity to the Sun. Although other interesting sites of star formation are known, Orion is particularly easy to study because of the detail with which we can explore it. The Orion nebula is about 3-4 parsecs across (about 10 light years). This is absolutely tiny compared to the size of the galaxy, but much bigger than the solar system. Light takes only a few hours to cross the entire solar system, so the Orion Nebula is hundreds of thousands of times larger, and has room for many stars. Let us list the things we see in the Orion Nebula, in parallel with the list we made above. What signs of star formation can we recognize? O and B stars. Through a small telescope, it is quite easy to see four very conspicuous O stars in `the Trapezium,' a small grouping of stars right at the heart of the Nebula. The Trapezium stars are fantastically bright and hot, and produce lots of ultraviolet light. They must be very young. HII regions . The whole nebula is a great cloud of ionized hydrogen, a consequence of the aforementioned ultraviolet light from the Trapezium stars. Dusty gassy regions. In addition to the glowing hot gases, it is easy to pick out dark tendrils and wisps which amply demonstrate the presence of dense clumps of interstellar dust. (See page 612.) Reflection nebulae. As you can see from the image on page 612, there are regions (especially near the so-called Horsehead Nebula) in which we do indeed see this phenomenon. T Tauri stars. In the Orion complex, there is an `open star cluster' called NGC 2264. It contains lots of T Tauri stars, which are understood to be sun-like stars which have not yet completely formed and begun nuclear reactions. Infrared sources. The whole Orion complex is filled with objects which are glowing away brightly at infrared wavelengths. They do not show up very well in the visible partly because they are hidden behind obscuring dust clouds but also because they emit most of their light these longer wavelengths, a sure indication that they are cool 'protostars'. Molecular clouds. Millimeter-wave telescopes indeed show that there are enormous quantities of interstellar molecules in the Orion complex - obviously not in the HII region itself, where the temperature is high, but in some denser cool cloud. Stars not yet on the main sequence. The HR diagram for the open cluster NGC 2264, in Orion, shows that there are a few hot O and B stars already on the main sequence; but there are also many stars, not all of them T Tauri stars, which lie above the main sequence down at the lower right. They are still contracting and heating up. Unbound systems. The Trapezium represents such a system. These four stars are in a mutual gravitational dance, but will not remain bound together for long.Putting It All Together: The Full Cycle.You can see that Orion is a great example of a rich star formation region, and it has the extra advantage of being rather close to the Sun so that we can study it in some detail. It is, however, just one example of the thousands known in our own galaxy. In the Milky Way, star formation happen in a continuous cycle of birth-death-and-enrichment, encapsulated in the figure on page 600. Stars of various masses form, go through their lives, and die in ways which redistribute some or most of their constituent material to the interstellar medium. (Low-mass stars `puff off' a planetary nebula; massive stars explode catastrophically, as we have seen.) The importance of this cannot be overestimated. It is within the massive stars that all the heavy elements were created - all the calcium in your bones, all the carbon which is necessary for life as we know it, the oxygen we breathe, the elements which make up the solid body of the Earth, and so on. This process of birth and death, followed by the formation of new generations of stars, explains all the planetological and biological diversity we see around us. We are stardust! Previous chapter:Next chapter0: Physics 016: The Course Notes, spring 2005. 1: The Properties of the Sun: 2: What Is The Sun Doing? 3: An Introduction to Thermonuclear Fusion. 4: Probing the Deep Interior of the Sun. 5: The Sun in More Detail. 6: An Introduction to the Stars. 7: Stars and Their Distances: 8: The HR Diagram: 9: Questions Arising from the HR Diagram: 10: The Importance of Binary Stars: 11: Implications from Stellar Masses: 12: Late in the Life of the Sun: 13: The Importance of Star Clusters in Understanding Stellar Evolution: 14: The Chandrasekhar Limit: 15: Supernovae: The Deaths of Massive Stars, 16: Pulsars: 17: Novae: 18: An Introduction to Black Holes: 19: Gravity as Geometry: 20: Finishing Off Black Holes: 21: Star Formation: 22: Dust in the Interstellar Medium: 23: Gas in the ISM: 24: The Size and Shape of Our Galaxy: 25: The Discovery of External Galaxies: 26: Galaxies of All Kinds: 27: The Expanding Universe: 28: Quasars and Active Galaxies: 29: The Hot Big Bang: 30: The Geometry of the Universe: 31: Closing Thoughts: Part 1:Part 2:Part 3: |
(Wednesday, 22 April, 2026.)
