Explore 2.9 Transitions in the Life Cycles of Stars
Learning Objectives
By the time you have completed the 2.9 Introduction & Exploration Activities, you should be able to:
Understand and be able to identify each of the following: low-mass star (<8 times the Sun’s mass), intermediate-mass star (8-20 times the Sun’s mass), high-mass star (>20 times the Sun’s mass), white dwarf star, neutron star, black hole, radioactive isotope, stable isotope and half-life.
Describe the roles of gravity and gas pressure (from fusion or collapse) in the stability of normal stars and in the transitions between normal, giant and mini stars. Also, describe the response of a star (e.g., a temperature or size change) to the exhaustion of a fuel supply (first hydrogen fuel, then helium fuel, …).
Describe the states and transitions that comprise the lifecycle of low-, intermediate- and high-mass stars using terms such as gravity, fusion, size (diameter), temperature, gas pressure, balance, collapse and expansion. Also, understand where most heavy elements forms.
Scientific Terms/Concepts
Terms: Low-mass Star (<8 times the Sun’s mass), Intermediate-mass Star (8-20 times the Sun’s mass), High-mass Star (>20 times the Sun’s mass), White Dwarf star, Neutron Star, Black Hole.
Define and give an example of each term:
Term:
Low-mass Star
Definition:
Answer: Lightweight stars form slowly, live long lives and die in relatively quiescent transitions that produce nebulae and white dwarfs. They contain less than about 8 times the mass of our Sun.
Lightweight stars form slowly, live long lives and die in relatively quiescent transitions that produce nebulae and white dwarfs. They contain less than about 8 times the mass of our Sun.
Example:
Answer:Most stars (~99%) are low mass stars. The least massive are red, heavier stars are yellow (like the Sun), and the most massive are white. All low mass stars eventually produce a mini star (called a white dwarf star) and a nebula (called a planetary nebula). Our sun is an example of a low-mass star.
Most stars (~99%) are low mass stars. The least massive are red, heavier stars are yellow (like the Sun), and the most massive are white. All low mass stars eventually produce a mini star (called a white dwarf star) and a nebula (called a planetary nebula). Our sun is an example of a low-mass star.
Term:
Intermediate-mass Star
Definition:
Answer: Stars that contain 8-20 times the Sun’s mass. Like heavyweight stars, these stars explode in supernova explosions and form neutron stars.
Stars that contain 8-20 times the Sun’s mass. Like heavyweight stars, these stars explode in supernova explosions and form neutron stars.
Example:
Answer:Intermediate-mass stars are massive enough to undergo supernova explosions and become neutron stars at the end of their life. Spica, the bright star in Virgo, is a visible intermediate mass star.
Intermediate-mass stars are massive enough to undergo supernova explosions and become neutron stars at the end of their life. Spica, the bright star in Virgo, is a visible intermediate mass star.
Term:
High-mass Star
Definition:
Answer: Heavyweight stars that contain more than about 20 times the mass of our Sun and are relatively rare. They are extremely brilliant and explode as supernovae, forming black holes. Both intermediate and High-mass stars produce iron in their core during their red giant phase
Heavyweight stars that contain more than about 20 times the mass of our Sun and are relatively rare. They are extremely brilliant and explode as supernovae, forming black holes.
Both intermediate and High-mass stars produce iron in their core during their red giant phase
Example:
Answer:The lightest high mass stars are white, but most are brilliantly blue. Betelgeuse and Antares are examples of high mass stars.
The lightest high mass stars are white, but most are brilliantly blue. Betelgeuse and Antares are examples of high mass stars.
Term:
White Dwarf Star
Definition:
Answer: Incredibly dense stars that consist mostly of helium, carbon, and oxygen, but can also contain heavier elements like neon, magnesium and sulfur.
Incredibly dense stars that consist mostly of helium, carbon, and oxygen, but can also contain heavier elements like neon, magnesium and sulfur.
Example:
Answer: Sirius B has about the same mass as the Sun, but is about fifty times smaller than the Sun. So, imagine compressing the matter in the Sun into a sphere only twice the size of Earth!
Sirius B has about the same mass as the Sun, but is about fifty times smaller than the Sun. So, imagine compressing the matter in the Sun into a sphere only twice the size of Earth!
Term:
Neutron Star
Definition:
Answer: An extremely dense stellar remnant composed almost entirely of neutrons. It is the remnant of the supernova explosion of an intermediate-mass star.
An extremely dense stellar remnant composed almost entirely of neutrons. It is the remnant of the supernova explosion of an intermediate-mass star.
Example:
Answer: A single teaspoon of neutron star matter would weigh billions of tons. A pulsar is also a neutron star. They are about 10 miles across. A neutron star resides at the center of the Crab Nebula in Taurus.
A single teaspoon of neutron star matter would weigh billions of tons. A pulsar is also a neutron star. They are about 10 miles across. A neutron star resides at the center of the Crab Nebula in Taurus.
Term:
Black Hole
Definition:
Answer: A single, infinitely small and dense stellar remnant formed when the neutrons (the strongest known particles) are crushed and the star continues collapsing.
A single, infinitely small and dense stellar remnant formed when the neutrons (the strongest known particles) are crushed and the star continues collapsing.
Example:
Answer: Black holes are relatively bright objects due to the gases that orbit them giving off light as they collide with and decelerate each other. The system Cygnus X-1 is a blue supergiant that is being orbited by a black hole.
Black holes are relatively bright objects due to the gases that orbit them giving off light as they collide with and decelerate each other. The system Cygnus X-1 is a blue supergiant that is being orbited by a black hole.
Term:
Radioactive Decay
Definition:
Answer: Is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive.
Is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive.
Example:
Answer:Uranium 238 will decay over time to form lead 206. This is a good example of radioactive decay.
Uranium 238 will decay over time to form lead 206. This is a good example of radioactive decay.
Term:
Radioactive Isotope
Definition:
Answer: An atom that undergoes radioactive decay and becomes other isotopes, which can themselves be radioactive or stable.
An atom that undergoes radioactive decay and becomes other isotopes, which can themselves be radioactive or stable.
Example:
Answer: Carbon 14 is an example of a radioactive isotope. It will decay into carbon 12, which is stable and will no longer decay. Uranium 238 is another example of a radioactive isotope.
Carbon 14 is an example of a radioactive isotope. It will decay into carbon 12, which is stable and will no longer decay. Uranium 238 is another example of a radioactive isotope.
Term:
Stable Isotope
Definition:
Answer: An atom that does not undergo radioactive decay.
An atom that does not undergo radioactive decay.
Example:
Answer: As carbon 14 decays to carbon 12, it becomes a stable isotope that will no longer decay. After uranium 238 decays, it becomes lead, which is stable.
As carbon 14 decays to carbon 12, it becomes a stable isotope that will no longer decay. After uranium 238 decays, it becomes lead, which is stable.
Term:
Half-life
Definition:
Answer: The time required for half of the radioactive atoms in a sample to decay.
The time required for half of the radioactive atoms in a sample to decay.
Example:
Answer: When zircon forms it contains uranium. As uranium decays, it turns to lead. If a zircon is found and it contains lead in its sample, we can know that at least one half-life has elapsed.
When zircon forms it contains uranium. As uranium decays, it turns to lead. If a zircon is found and it contains lead in its sample, we can know that at least one half-life has elapsed.
This image from the reading material is a great illustration of the life cycles of stars and shows the difference between the masses as well as where a White Dwarf, Neutron Star, and Black Hole come from.
This image gives you an idea of the different sizes in the mass of stars in the Milky Way Galaxy.
This website gives great information on how our sun compares with other stars:
Using your knowledge of the relationship between gravity and fusion-fueled gas pressure, predict what will happen at the beginning of the transition from normal to giant star. Record your prediction in the box below.
Answer
When fusion in the core exhausts its supply of hydrogen fuel, the fusion energy that has balanced gravity disappears, gravity overcomes gas pressure, and the core of the star begins collapsing. As collapse proceeds, temperature and gas pressure in the core increase.
Using your knowledge of giant star evolution, make a prediction about what causes the giant star stage to end. In other words, what event causes the transition to mini stars to begin. Record your answer in the box below.
Answer
Imbalances between gravity and gas pressure produce stellar transitions. When a giant star consumes the last of its fuel or its core can no longer resist the pull of gravity, the star collapses—for the last time. Thus, star collapse initiates the transition that produces nebulae & mini stars from giant stars.
To understand how quietly some stars pass through the transition that produces mini stars from giant stars, consider how fusion in the interior of a star will respond to losing significant fractions of a giant star’s mass. Record your description in the box below.
Answer
As giant stars shed matter from their surfaces, the temperatures and gas pressures inside these stars drop, lowering fusion rates. In some low-mass giant stars, mass loss can continually drop temperatures & pressures until fusion rates diminish to zero and fusion ceases. In these cases, the giant star passes quietly from giant star to nebula + mini star. The nebula, of course, results from material shed from the star during and shortly after the giant star stage. And, without fusion, the core of the star collapses to maximum density (for matter containing free electrons) and becomes a white dwarf. If fusion rates, instead, remain high and the end of fusion results from the exhaustion of nuclear fuel in the interior of the star or from other processes that cause star collapse, the transition from giant to mini star is much more energetic… and the energy of the transition rises with the mass of the star.
Watch this video (4 minutes) that shows the transition expected to come to Betelgeuse, a high-mass star.
