Astronomy Matters for Chemistry Teachers

Chemistry Everyday for Everyone

Astronomy Matters for Chemistry Teachers Jay S. Huebner,* Robert A. Vergenz,* and Terry L. Smith* Department of Natural Sciences, University of North Florida, Jacksonville, FL 32224-2645 The purpose of this paper is to encourage more chemistry teachers to become familiar with some of the basic ideas described in typical introductory astronomy courses (1–9), including those about the origin of elements and forms of matter. These ideas would enrich chemistry courses and help resolve some basic misconceptions that are expressed in many introductory texts (10–16) and journal articles for chemistry teachers (17, 18). These misconceptions are typified by statements such as “we can classify all substances as either elements or compounds,” and “nature has provided 92 elements out of which all matter is composed.” If students accept these misconceptions, they could be deprived of (i) an appreciation of the history of elements and knowing that the elemental composition of the universe continues to evolve, (ii) knowing that of the first 92 elements in the periodic table, technetium and promethium do not occur naturally on Earth, and (iii) understanding that there are forms of matter other than elements and compounds. This paper briefly explores these ideas. Simplicity and Complexity Einstein said “Everything should be made as simple as possible, but not simpler” (19). This is especially true of science teaching: complex and challenging ideas should be made as simple as possible, but without being inaccurate. The standard definition of matter in chemistry is that it has mass and occupies space.1 This definition, coupled with the statement that all matter is composed of elements and compounds, is a powerful and simplifying idea, but it needs modification in light of modern observational data and interpretations given in astronomy. Astronomy is justifiably credited with awakening human understanding that Earth is not the center of the universe. An implication of this is that the types of matter found here may not be universal. Spectroscopic studies of X-ray, ultraviolet, visible, infrared, and radio emanations from extraterrestrial sources have demonstrated that atoms and molecules of the type found on Earth are widely distributed in the universe. But other forms of matter have also been identified, which confound the usual definitions and ways in which chemists classify matter. Synthesis of Elements There is good evidence that hydrogen, helium, and traces of lithium were created in the Big Bang, the process from which the natural history of the universe is traced, some 10 billion years ago (18, 24–27). The higher2 elements were synthesized mostly in stellar processes and dispersed within galaxies by solar winds from red *Corresponding authors. Email: [email protected], [email protected], or [email protected].

giant stars and by supernovae, as outlined below. Additional details can be found elsewhere (18, 24, 28, 29). The authors encourage inclusion of material about this subject in introductory chemistry texts, as in (16).

From Hydrogen to Helium The energy that stars radiate from their exteriors is generated in their hot cores by thermonuclear fusion, which converts lower elements into higher elements and also mass into energy. The energy for ignition of these processes comes from gravity compressing the clouds of gas and dust that form the stars. Protostars having sufficient mass for their cores to reach about 107 K will fuse hydrogen into helium and become stars. The most common nuclear reaction sequence in these stars is 1H

+ 1H → 2H + e + + ν

1H 3He

+ 1H → 3He + γ

+ 3He → 4He + 2 1H

where e+ , ν, and γ are positrons, neutrinos, and gamma rays, respectively. The energy released maintains the interior temperature and gas pressure and enables the star to be stable and resist further gravitational collapse, as long as sufficient hydrogen remains in the core. The luminosity produced by hydrogen-fusing stars increases approximately with the fourth power of the star’s mass (30). Stars remain stable in this state for periods that vary roughly as the inverse third power of their mass. Our sun is thought to be about half way through its 10 billion year period of hydrogen fusing. Even at 107 K, electrostatic repulsion between charged nuclei prevents them from approaching close enough for direct nuclear contact, but quantum mechanical tunneling makes fusion possible. The sun’s long-term stability is caused by the extremely low probability of fusion during each proton–proton collision.

Carbon and Oxygen When hydrogen is exhausted in a star’s core, hydrogen fusion will cease and gravity will again cause the core to contract. The core’s temperature will rise to the point where helium nuclei fuse to form carbon and oxygen, as 3 4He → 12 C +γ 12C

+ 4He → 16 O +γ

again converting mass to energy. When helium in the core is exhausted, the core will contract again; but with the mass available in our sun, it will not reach a high enough temperature to fuse higher elements. Therefore, this latter contraction will result in a carbon–oxygen white dwarf star. More massive stars evolve faster and achieve higher core temperatures in their later stages, and so fusion creates a succession of elements higher than oxygen. Stars fusing elements higher than hydrogen generate more power, which expands their outer layers, form-

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ing red giant stars. The visible surface, or photosphere, of our sun is expected to reach beyond the orbit of Earth in this phase. Convection transports portions of the higher elements synthesized to the surface of these stars, where along with other material they blow out into space in solar winds and form nebulae (29).

Higher Elements In stars with masses above ten times the mass of the sun, at core temperatures of around 109 K, iron is fused. But iron is at the peak in a graph of nuclear binding energy per nucleon versus mass number (31, 32). Because of this, iron stellar cores cannot produce additional energy, but rather absorb energy when iron is fused to form higher elements and fissioned to form lower elements. Consequently, evolving stellar cores containing iron absorb energy, cool suddenly, and are collapsed by gravity in a time of probably less than a second (29). The infalling outer layers progressively collide with the compressed core and each other. Neutrons are produced, which, being electrically neutral, avoid the coulomb barrier and readily combine with all elements. Successive and multiple neutron capture and beta decay reactions create intensely radioactive isotopes, probably of all elements (18, 24, 28, 33). There is a spectacular explosion, as some of the in-falling materials rebound and are thrown out into space. This is a supernova. A single star in this stage may outshine a galaxy of a billion other stars. Many details of these processes were confirmed in a recent supernova, designated SN1987A (29, 33, 34). The radioactive materials from supernovae are observed to heat supernova remnants (35). These expanding nebulae collide with other nebulae in interstellar space and compress them to start condensations that result in the formation of new stars and planets. These ideas explain (i) the observed distribution of higher elements in galaxies, (ii) the formation of new stellar systems in old galaxies, (iii) the existence of radioactive materials in Earth, Moon and meteoroids, and (iv) the meaning of statements such as “…each one of us and all of us, are truly and literally a little bit of stardust” (18, 36). Natural Elements A related topic on which there is disagreement in the teaching literature is the number of naturally occurring elements. Elements that have only radioactive and short-lived isotopes, such as technetium, promethium, and the transuranium elements,3 would not have survived long enough after being created in supernovae to be incorporated into Earth (37, 38). Other elements between bismuth and uranium, with the exception of thorium, also would not survive, but these nuclides are continuously created from the radioactive decay series of uranium and thorium4 (39). So, 90 elements occur naturally on Earth, but more are synthesized in stellar processes. Astronomers consider stellar processes to be natural. The spontaneous fissioning of naturally occurring uranium on Earth also creates another short-lived nuclide, the neutron. Confounding Examples Several types of objects considered in this section contravene the usual definition of matter. These include the final state of some evolved stellar cores. The form of 1074

such a stellar core depends on its mass. It may be a white dwarf star, neutron star, or a black hole.

White Dwarfs and Neutron Stars White dwarfs have masses from about 0.5 to 1.4 times the solar mass and consist internally of atoms such as helium, carbon, and oxygen. These atoms are completely ionized and packed by gravity to a density of about 109 kg/m3 (41). For comparison, the density of lead is about 104 kg/m3, while the uranium nucleus is about 3 × 10 17 kg/m 3 (42). One such white dwarf, called Sirius B, accompanies Sirius A, which is the brightest star in the night sky. It is estimated that there are as many as ten billion white dwarfs in the Milky Way galaxy (43). When a stellar core has a mass between about 1.4 and 3 times the sun’s mass, a neutron star results, with a typical density of about 1018 kg/m3. There are uncertainties about the interior of neutron stars, which may have cores of elementary particles called hyperons (44). It is estimated that there are 100 million neutron stars in the Milky Way (45). In terrestrial matter, coulomb repulsive forces between electrons in adjacent atoms keep one atom from occupying the space of another, providing the physical basis for the definition that matter occupies space. In massive stellar objects, the crushing force of gravity overwhelms the atomic structure of matter inside. Resistance to gravitational collapse in a white dwarf results from its electrons behaving as quantum particles in a spherical box. For a star of one solar mass, 1057 electrons fill energy levels in a box 107 m in diameter. The star’s resistance to gravitational collapse can be understood by considering the analogy of quantum mechanical particles of mass m in an infinitely deep potential energy square well (46) of width L. The nth energy level is En = n2h2/8mL2, where h is Planck’s constant. In this model, if gravity is to cause the star to contract, L will be reduced, requiring that the energy of all enclosed electrons increase with 1/L2. This provides a mechanism to resist gravitational collapse that is much stronger than coulomb repulsion. Astronomers call this state a degenerate electron gas (41, 47, 48), and it produces bulk properties that are beyond our terrestrial experience. For example, the diameter of a white dwarf decreases with increasing mass (42, 49). In neutron stars, a degenerate neutron gas resists collapse and creates an excluded volume. White dwarfs and neutron stars are certainly matter by the traditional definition in chemistry, since they have mass and occupy space. They are common in our galaxy and result from natural processes. But the bulk of a neutron star is not composed of either elements or compounds. Black Holes Stellar cores more massive than neutron stars become black holes. No known mechanism for generating repulsive forces can withstand the overwhelming attraction of gravity in black holes. A black hole absorbs everything, including light, that passes within a distance called its event horizon. Current understanding indicates that all information on the properties of materials inside (other than mass, net electrical charge, and angular momentum) has disappeared from our universe. Many of the objects believed to be black holes are identified by radiation resulting from streams of matter being compressed and heated prior to falling through the event horizon (50).

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Chemistry Everyday for Everyone

Black holes defy attempts to apply the usual definition of matter. Black holes have mass and their event horizons enclose a volume, but this volume does not occupy space to the exclusion of other objects. The particles inside a black hole are theoretically falling towards a point singularity, which does not occupy a three dimensional volume. Black holes absorb matter, but are they matter?

The Missing Mass Current research in astronomy provides good evidence for additional “unseen” mass in galaxies, which so far has only been detected by the effects of its gravity (i.e., mass) on visible matter and light. This is referred to as the “dark matter” (51) or “missing mass problem”5 (52). Some estimates suggest that a significant fraction of the Milky Way galaxy’s mass, and perhaps as much as 99% of the mass of galaxy clusters, is unaccounted for (53, 54). It may be that dark mass does not interact through electromagnetic fields, which, of course, would be unlike mass made of elements6 (55, 56). If these estimates turn out to be true, then the elements constitute only a minor part of the universe’s mass. Particle Beams The existence of various isolated particles that have mass, called rest mass or mass energy in relativity, also blurs the concept of matter. Examples are alpha particles, electrons, muons, neutrons, pions, and positrons. There are many industrial and scientific uses for beams of these particles, including in nuclear chemistry. An electron beam, for example, is as close as the nearest operating cathode ray tube. Most scientists would probably instinctively consider beams of molecules or electrons to be matter. But do they occupy space? Measurements on electrons suggest they are virtually point particles down to at least 10{18 m, the current experimental limit (57). Many of these particles do not exist in bulk form. Should the definition of matter depend on the existence of bulk forms of the particles? If the above particle beams are in fact matter, then they provide additional examples of matter not made of elements or compounds. Teaching about Elements and Matter We have observed that many students gain the impression in introductory chemistry that matter made of elements and compounds constitutes everything in the universe except energy. The existence of neutron stars, and perhaps other examples described above, makes it clear that this notion is false. The classifications (i) of natural things into matter and energy and (ii) of matter into elements and compounds are useful, but should not be taught as universal. The concomitant presentation of interesting exceptions, such as neutrons and neutron stars, would make clear the limits of this classification scheme. The ACS and IUPAC could define neutrons to be an element, say neutronium, with atomic number zero. Then neutron beams and the matter in neutron stars, ignoring the possibility of hyperons existing there, could be correctly stated to be made of elements. But there would be drawbacks to this change, as neutrons do not form atoms in combination with electrons as all other elements do. Given the choice, it seems better to leave the definition of elements alone. Black holes and particle beams are examples for which the commonly taught definition of matter is not

useful. They illustrate the care required in formulating fundamental definitions. Precisely what does it mean to “occupy space?” Is this essential to what is intended in trying to classify all things as either matter or energy? Perhaps it is better to classify all things as either having rest mass or not. If occupying space is unimportant, how is the concept of matter different from that of mass? The definition of matter is important because it is often used to define chemistry, the study of matter and its transformations. Such definitions are important because they affect the perspective of future chemists and influence who will be drawn to study the subject. Chemistry is also described as the study of the electronic interactions of atoms and molecules. This avoids using “matter”, but excludes the study of some of the materials mentioned above. We think this limitation is undesirable. One can muse over the wonders that may be in store for chemistry when or if samples of dark mass are acquired for investigation, and about industries which might evolve from understanding and exploiting its properties.7 It may be foolish to mention these possibilities in classes. Such speculations are surely not science. But it is wrong and a disservice to chemists and other scientists of the future to implant a mind-set that the elements of the periodic table provide the only forms of matter. The perspective of astronomy clearly indicates that more care is needed in describing these fundamental concepts. Acknowledgments We thank R. Bowman, M. Everly, J. Garner, D. Gay, E. Healy, and K. Venkatasubban, and also Alan Campbell Ling, recently deceased, for helpful discussion and critique. Notes 1. Chemistry text books seem almost universally to define matter in this way. Of eight chemical dictionaries consulted, four did not define matter, and, surprisingly, the others each defined it differently. Matter was defined as: 1) “Anything subject to gravitation; hence any substance that occupies space.” (20) , 2) “That which occupies space; substance; that which is composed of molecules and atoms.” (21) , 3) “anything that has mass or occupies space.” (22) , and 4) “anything that has mass and occupies space” (23). The definition used in chemistry texts will be used here. 2. The term “higher elements” is recommended rather than “heavier elements”, as heavy refers to weight, a terrestrial concept which is unnecessarily limiting. 3. The chemical symbol for Tc, Pm, and the transuranium elements are printed only in outline form on the Sargent-Welch periodic table, indicating their lack of stable (i.e., non-radioactive) isotopes. The half-lives of the longest lived isotopes of Tc and Pm are 4.2 x 106 and 18 years, respectively (37,38). 4. The decay of these elements in the rocks of Earth, Moon, and in meteorites provides some basis for determining that the Solar System is about 4.6 billion years old (3,39,40). 5. We recommend the term “dark mass” to avoid uncertainties about other properties required in the definition of matter. One of the definitions of matter given in chemical dictionaries, quoted above, would include dark mass as matter even if it does not occupy space. 6. Detection of neutrinos from the Sun’s core and SN1987A demonstrate that it is possible to study remote objects with other than electromagnetic radiations. 7. Speculations about the forms of dark mass should acknowledge the possibility that it may be as varied as mass made from elements.

Literature Cited 1. Chaisson, E.; McMillan, S. Astronomy Today; Prentice Hall: Englewood Cliffs, NJ, 1993.

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