Grades 9-12 or Adult Sophisticated Learner
The young Sun, just like the young Earth, was very different than it is now. To understand why, we need to know a little bit about how the Sun and other stars give off so much energy.
Stars creates all of their light and heat through the process of nuclear fusion. This is where atoms of one kind of element merge together to make atoms of another element! You may recall that the Law of the Conservation of Energy tells us that energy cannot be created nor destroyed, but it can change forms. We also know from the Special Theory of Relativity that energy and mass have a very special relationship. We call it the “mass-energy equivalence”, but you’ve probably heard it more often stated as the famous equation E=mc2\. This tells us that any amount of mass also has an equivalent amount of energy, and vice versa. Well, it turns out that the process of nuclear fusion is also one where mass is converted into energy. When stars are fusing together elements, they producing lots and lots of energy. But they’re also making bigger and bigger chemical elements in the process.
The most common nuclear fusion reactions in stars are ones that convert hydrogen into helium. This is actually how most of the energy coming from the Sun is produced. However, stars will also then fuse together helium atoms to make elements like carbon, oxygen, and nitrogen. Many of the chemical elements are made this way. It’s such an important process that is has a special name, “stellar nucleosynthesis”, and it’s responsible for our Sun giving off so much light and heat.
Over time, as stellar nucleosynthesis has gone on in the Sun, making more and more helium and carbon and such, the Sun has slowly gotten hotter and brighter. The main reason for this is that as these heavier elements form and then start undergoing stellar nucleosynthesis reactions, the core of the Sun starts to contract (it shrinks down and becomes more dense) and that increases the temperatures and pressures inside, which then in turn causes even more reactions to release even more energy.
The Sun was about 30% less luminous in its youth than it is now, but scientists also know that there was liquid water on Earth’s surface at that time (from studying the rock record). How could water on Earth’s surface remain liquid with that much less light and heat coming from the Sun? How could life – which requires liquid water – have gotten started on the young Earth in freezing conditions? Scientists call this the “faint young Sun paradox.” Resolving this question is an active area of research, and there are likely many contributing factors. One way to resolve the conflict is with the greenhouse hypothesis, which states that Earth’s early atmosphere likely contained higher levels of greenhouse gases – especially methane, carbon dioxide, and water vapor. Those and other gases were coming from degassing and volcanic processes following the accretion of rocky particles during Earth’s formation. On Earth today, the carbon dioxide in the air is converted into oxygen by photosynthetic organisms like plants and algae. On the early Earth, there were no such organisms to do that conversion, so carbon dioxide remained in the atmosphere and contributed to global warming, which kept the water liquid on the surface despite the fainter, less luminous Sun.
Disciplinary Core Ideas
ESS1.A: The Universe and Its Stars: The star called the Sun is changing and will burn out over a lifespan of approximately 10 billion years. (HS-ESS1-1) *The study of stars’ light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth. (HS-ESS1-2, HS-ESS1-3) *Nearly all observable matter in the universe is hydrogen or helium, which formed in the first minutes after the big bang. Elements other than these remnants of the big bang continue to form within the cores of stars. Nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases the energy seen as starlight. Heavier elements are produced when certain massive stars achieve a supernova stage and explode. (HS-ESS1-2, HS-ESS1-3) *Stars go through a sequence of developmental stages — they are formed; evolve in size, mass, and brightness; and eventually burn out. Material from earlier stars that exploded as supernovas is recycled to form younger stars and their planetary systems.
ESS1.B: Earth and the Solar System: Cyclical changes in the shape of Earth’s orbit around the Sun, together with changes in the tilt of the planet’s axis of rotation, both occurring over hundreds of thousands of years, have altered the intensity and distribution of sunlight falling on the earth. These phenomena cause a cycle of ice ages and other gradual climate changes.
ESS2.A: Earth Materials and Systems: The geological record shows that changes to global and regional climate can be caused by interactions among changes in the Sun’s energy output or Earth’s orbit, tectonic events, ocean circulation, volcanic activity, glaciers, vegetation, and human activities. These changes can occur on a variety of time scales from sudden (e.g., volcanic ash clouds) to intermediate (ice ages) to very long-term tectonic cycles. (HS-ESS2-4)
PS3.D: Energy in Chemical Processes and Everyday Life: Nuclear Fusion processes in the center of the Sun release the energy that ultimately reaches Earth as radiation. (HS-ESS1-1) The main way that solar energy is captured and stored on Earth is through the complex chemical process known as photosynthesis. (HS-LS2-5)
PS1.C: Nuclear processes: Nuclear processes, including fusion, fission, and radioactive decays of unstable nuclei, involve release or absorption of energy. (HS-PS1-8)
PS3.A: Definitions of Energy: Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from one object to another and between its various possible forms. (HS-PS3-1, HS-PS3-2) At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. (HS-PS3-2, HS-PS3-3)
PS3.B: Conservation of Energy and Energy Transfer: Conservation of energy means that the total change of energy in any system is always equal to the total energy transferred into or out of the system. (HS-PS3-1) Energy cannot be created or destroyed, but it can be transported from one place to another and transferred between systems. (HS-PS3-1, HS-PS3-4)
PS4.B: Electromagnetic Radiation: Electromagnetic radiation (e.g., radio, microwaves, light) can be modeled as a wave of changing electric and magnetic fields or as particles called photons. The wave model is useful for explaining many features of electromagnetic radiation, and the particle model explains other features. (HS-PS4-3) When light or longer wavelength electromagnetic radiation is absorbed in matter, it is generally converted into thermal energy (heat). (HS-PS4-4)
Cause and Effect: Cause and effect relationships can be suggested and predicted for complex natural and human designed systems by examining what is known about smaller scale mechanisms within the system. (HS-PS3-5) Systems and System Models: When investigating or describing a system, the boundaries and initial conditions of the system need to be defined and their inputs and outputs analyzed and described using models. (HS-PS3-4) *Stability and Change: Change and rates of change can be quantified and modeled over very short or very long periods of time. Some system changes are irreversible. (HS-ESS3-5)
Big Ideas: The Earth and the Sun have changed over time since their formation. Our knowledge of stars and stellar processes tells us that the Sun gave off less energy when it was young than it does now. The “Faint young Sun paradox” was presented as a scientific problem to question how water on Earth remained liquid with less heat coming from the early Sun. Elevated levels of greenhouse gases in the atmosphere helped keep the early Earth warm even though the Sun gave off less heat and energy. With a liquid water supply and ideal atmospheric conditions, the birth of life on Earth was possible with a young Sun.
Boundaries: Emphasis is on the energy transfer mechanisms that allow energy from nuclear fusion in the Sun’s core to reach Earth and the way nucleosynthesis varies as a function of the mass of a star and the stage of its lifetime. Grade level appropriate observations include the masses and lifetimes of other stars, as well as the ways that the Sun’s radiation varies due to sudden solar flares (“space weather”), the 11-year Sunspot cycle, and non-cyclic variations over centuries. (HS-ESS1-1). Does not include details of the atomic and subatomic processes involved with the Sun’s nuclear fusion (HS-ESS1-1) or details of the many different nucleosynthesis pathways for stars of different masses (HS-ESS1-3).
6-9 SpaceMath Problem 395: Death Stars. Some stars create super-flares that are capable of eliminating life on planets that orbit close to the star. Students learn about these flares on common red-dwarf stars and compare them to flares on our own sun [Topics: scientific notation; percentages; rates of change] https://spacemath.gsfc.nasa.gov/astrob/7Page59.pdf
6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Another Look at Solar Energy (page 45) and The Greenhouse Effect and Planetary Temperatures (page 41). NASA. https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf
8-10 SpaceMath Problem 189: Stellar Temperature, Size and Power. Students work with a basic equation to explore the relationship between temperature, surface area and power for a selection of stars. [Topics: algebra] https://spacemath.gsfc.nasa.gov/astrob/5Page44.pdf
9-12 SpaceMath Problem 483: The Radioactive Dating of a Star in the Milky Way! Students explore Cayrel’s Star, whose age has been dated to 12 billion years using a radioisotope dating technique involving the decay of uranium-238. [Topics: half-life; exponential functions; scientific notation] https://spacemath.gsfc.nasa.gov/stars/9Page5.pdf
9-12 SpaceMath Problem 482: Exploring Density, Mass and Volume Across the Universe. Students calculate the density of various astronomical objects and convert them into hydrogen atoms per cubic meter in order to compare how astronomical objects differ enormously in their densities. [Topics: density=mass/volume; scientific notation; unit conversion; metric math] https://spacemath.gsfc.nasa.gov/stars/9Page4.pdf