--Earth and Space Science
Students learn about the life cycle of stars and consider what will happen several billion years from now, when Earth’s sun dies.
Sun, star, life cycle, space, science, planets, solar system
First, assess students’ prior knowledge of the sun.
Second, explain to students that every star has a finite lifespan. Stars are born, exist and then die. The same laws govern the star nearest to Earth, our sun. This means that at some point in the future, the sun will cease to be. While that thought may be bleak, it’s helpful to think about what the death of our sun means for Earth, our solar system, the Milky Way and even the Universe.
Third, educators will want to cover basic facts about the sun including its size, temperature (varies in various zones), appearance and distance from Earth.
Good sources for size and temperature include NASA and Contemporary Physics Education Project (CPEP). Information on distance can be found here. For images of the sun, see this source or this alternate source.
Fourth, define (or ask students to define) the following terms. Good sources for student research include:
NOTE: Each of the following vocabulary terms is bolded when it appears in the student discussion section below.
Star: A massive, glowing sphere of plasma held together by gravity.
Plasma: A collection of free-moving electrons and ions (atoms that have lost electrons); intense thermal, electrical, or light energy is needed to strip electrons from atoms to make plasma.
Gravity: A natural phenomenon by which physical bodies attract each other with a force proportional to their masses. Gravity is most familiar as the agent that gives weight to objects and causes them to fall to the ground when dropped.
Nuclear Fusion: The process by which two or more atomic nuclei join together, or fuse, to form a single heavier nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to energy, which is released.
Red Giant: Toward the end of a star’s life, the temperature near the core rises. The hydrogen is depleted, the star collapses, and helium begins burning. The star radiates the resulting energy by expanding into a red giant.
White Dwarf: The remains of a dead star that is composed mostly of electron-degenerate matter.
Supernova: A very bright stellar explosion that causes a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months.
Neutron Star: The remains that can result from the gravitational collapse of a massive star during a supernova.
Black Hole: The remains of a star that becomes so dense that its gravity prevents anything, including light, from escaping.
Hydrogen: The most abundant chemical substance in the Universe. Stars are mainly composed of hydrogen in its plasma state.
Helium: A colorless, odorless, tasteless, non-toxic, inert gas. It is the second lightest element on Earth and is the second most abundant element in the Universe.
Discussion with students:
Familiarize yourself with the life cycle of stars, paying special attention to the end of the cycle. Once you have a firm grasp of the death process of a star, you can present the information to students. You’ll want to include the following points:
Stars are the most widely recognized astronomical objects and represent the most fundamental building blocks of galaxies. Stars are responsible for manufacturing and distributing heavy elements such as carbon, nitrogen and oxygen, and their characteristics are intimately tied to the characteristics of their orbiting planetary systems.
A star the size of our sun requires about 50 million years to mature from the point it is “born” to the time it reaches adulthood. Because our sun entered this “mature” phase roughly 4.6 billion years ago, it will remain active for approximately another 6 to 7 billion years. During that time, it will continue to convert hydrogen into helium in its core through a process called nuclear fusion.
Because there is a limited amount of hydrogen contained in the sun, it will eventually run out. At that point (roughly 6 or 7 billion years from now), the sun will begin to die. As with all stars, there are several ways in which this death can occur, and all of them involve the complete destruction of our solar system.
When a star has used all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter.
Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, and transforming the star into a red giant.
The process of becoming a red giant is long and painful for the surrounding celestial bodies. For example, when our sun begins the death process in about 1.1 billion years, it will start to grow hotter as it burns more hydrogen. The result is that the sun’s output will increase, and in 1.1 billion years, that output will be 10 percent greater than it is right now.
As the sun’s output increases, the polar ice caps on Earth will melt, resulting in a catastrophic, global flood. The flood will be short-lived, as the increasingly large and hot sun will cause all of the water on Earth to boil and then evaporate completely.
For the next 2.5 billion years, Earth will be a desolate, hot, dry planet devoid of life. The sun will continue its march to red giant status, growing larger and hotter. Roughly 3.5 billion years from now, the Earth will begin to glow red from the immense heat, and the planet’s mountain ranges will start melting. The view of the sky from Earth will show a huge sun whose borders cannot be completely seen.
Eventually the sun will grow so large that it will engulf the planets Mercury, Venus and Earth. At this point, the sun will officially be a red giant and enter into the final portion of the death process. What happens next depends on the size of the core.
An average star like the sun continues the process of ejecting its outer layers until its core is exposed. This core is dead, but still incredibly hot, and it appears white due to this heat. At this point it’s called a white dwarf because it is roughly the size of Earth, despite containing the entire mass of the original star.
The star doesn’t shrink or collapse any further, because pressure from fast-moving electrons keeps them stable at this point. The more massive the core, the denser the resulting white dwarf. This means the smaller a white dwarf is in diameter, the larger it is in mass. White dwarfs are the most common way in which a star dies, and this will be the end result when our sun dies.
Particularly large stars will die in a titanic explosion known as a supernova. In a supernova, the star’s core collapses and then explodes. Because the star no longer has any way to support its own mass, the iron core collapses. In just a matter of seconds, the core shrinks from roughly 5,000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more.
The outer layers of the star also begin collapsing; however, because of the enormity of the release of energy, they rebound and are shot out into space. This explosion is so immense—and the amount of energy released so huge—that it is possible for a supernova to outshine entire galaxies for weeks.
If the collapsing stellar core at the center of a supernova contains the right amount of mass, the collapse continues until electrons and protons combine to form neutrons. The result is a neutron star. These stars are incredibly dense, and because they contain so much mass packed into such a small volume, the gravitation at the neutron star’s surface is immense.
A neutron star also has a powerful magnetic field that can accelerate atomic particles around its magnetic poles and produce powerful beams of radiation.
When the largest stars die, their cores collapse completely to form a black hole. A black hole is an infinitely dense object whose gravity is so strong that nothing—not even light—can escape its immediate proximity. A black hole cannot be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists also can study stars to find out if they are flying around, or orbiting, a black hole.
While all stars eventually die, those that result in supernovae eventually are recycled and used to create new stars and planets. The dust and debris left behind from a supernova eventually blend with the surrounding interstellar gas and dust, enriching it with the heavy elements and chemical compounds produced during the death process. Eventually, those materials are recycled, providing the building blocks for a new generation of stars and accompanying planetary systems.
After class instruction and discussion, encourage students to demonstrate learning by letting them select one of the two multiple-intelligences activities below:
Activity 1: Bodily-Kinesthetic Intelligence
In this activity, students will act out the stellar death process.
Choose six students to play the part of the inner six planets: Mercury, Venus, Earth, Mars, Jupiter and Saturn. The rest of the class will form a group that is the sun. Place the sun group in the center of the class with the remaining students “orbiting” the sun in their respective places.
The teacher will act as the “timekeeper,” announcing to the class the passage of years as the planet students march around the sun. When the teacher announces 1.1 billion years, the sun students begin to gradually separate to represent the increasing size of the sun. This will continue until the students playing the roles of Mercury, Venus and Earth have become part of the sun group. When the teacher announces 3.5 billion years, the sun group will quickly huddle in the middle of the room to represent the collapsed sun.
Encourage students to add personality to their “planet” roles by having them complain about how hot it’s getting as the sun expands, or by having them wipe their brows due to the heat. Teachers can use props as they see fit, including white smocks or T-shirts for the sun students to wear once they become a “white dwarf.”
Activity 2: Spatial Intelligence
Making models of the solar system and its elements gives students an accurate perspective of their place in the cosmos. Models can vary in complexity from single planets and space objects to multi-element, orbiting models. Teachers may wish to assign groups of students to create a model of (1) the solar system as it now exists, (2) the red giant sun, and/or (3) the white dwarf sun.
Step-by-step instructions for constructing model solar systems can be found at:
An alternative to the three-dimensional model would be a series of drawings (much like a “graphic novel”) depicting the death of the sun. The key focus should be accuracy of the planets’ positions relative to the sun and the sun’s size, shape, etc. as it goes through the death process.
Make sure to show students a sample model or illustration beforehand and communicate expectations regarding level of effort. Providing a rubric can help to communicate expectations.
Activity 3: Linguistic Intelligence
Have students create “eyewitness” narrative accounts of the sun’s death. They can choose from a variety of perspectives such as an astronaut observing from space, a mountain or body of water on the earth, or the sun itself. What is the sequence of events? What does the process look/feel/sound like?
An alternate form of this activity would involve asking pairs or small groups of students to put in order a shuffled set of teacher-created 3-inch by 5-inch index cards (or slips of paper), on each of which is written a step or stage in the star death process. For an added challenge, leave out some key terms in the written steps, and have students fill in the blanks.
Assess students in terms of the following: