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Chapter 16: Problem 2

Place the following steps in the evolution of a low-mass star in order. a. main-sequence star b. planetary nebula ejection c. horizontal branch d. helium flash e. red giant branch f. asymptotic giant branch g. white dwarf

Short Answer

Expert verified
a, e, d, c, f, b, g

Step by step solution

01

- Main-sequence star

The evolution of a low-mass star begins when it is a main-sequence star. In this phase, the star is fusing hydrogen into helium in its core.
02

- Red giant branch

After exhausting the hydrogen in its core, the star expands and becomes a red giant. Hydrogen fusion continues in a shell around the core.
03

- Helium flash

Once the core temperature is high enough, helium fusion begins explosively in what is known as the helium flash.
04

- Horizontal branch

Following the helium flash, the star stabilizes and burns helium in its core, placing it on the horizontal branch of the Hertzsprung-Russell diagram.
05

- Asymptotic giant branch

After helium in the core is exhausted, the star ascends the asymptotic giant branch (AGB) where it burns helium and hydrogen in shells around the core.
06

- Planetary nebula ejection

The outer layers of the star are expelled, creating a planetary nebula.
07

- White dwarf

The remaining core becomes a white dwarf, which will cool down over time.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

main-sequence star
A low-mass star begins its life as a main-sequence star. During this phase, the star undergoes hydrogen fusion in its core, converting hydrogen into helium. This process releases energy that makes the star shine brightly. The main-sequence phase is also the longest phase in a star's life.
It is characterized by:
  • Stable fusion of hydrogen into helium
  • Balanced gravitational and radiation pressures
  • Consistency in luminosity and temperature
The Sun, for example, is currently in its main-sequence phase.
red giant branch
When a low-mass star exhausts the hydrogen in its core, it leaves the main-sequence and enters the red giant branch. In this phase, the core contracts while the outer layers expand, causing the star to swell significantly. This results in a cooler and redder appearance.
A red giant has:
  • Hydrogen shell fusion around an inert helium core
  • Increased luminosity
  • Expanded outer layers
This is a transitional phase leading up to more dramatic changes in the star's lifecycle.
helium flash
After the red giant phase, the core's temperature increases until it reaches roughly 100 million Kelvin, hot enough for helium fusion to commence. The helium flash is a brief but intense burst where the helium in the core begins to fuse into carbon and oxygen. This event happens rapidly and releases a tremendous amount of energy.
Key points about the helium flash:
  • It occurs suddenly and explosively
  • Only in low-mass stars
  • Marks the end of the red giant phase
The star then stabilizes and moves on to the next phase.
horizontal branch
Following the helium flash, the star enters the horizontal branch phase on the Hertzsprung-Russell diagram. Here, it burns helium steadily in its core, which now consists of carbon and oxygen.
Characteristics of the horizontal branch phase include:
  • Core helium fusion producing carbon and oxygen
  • Stable and defined position on the HR diagram
  • Balanced energy output and size
This phase sustains the star until the helium in the core is depleted.
asymptotic giant branch
Once the helium in the core is exhausted, the star ascends the asymptotic giant branch (AGB). In this phase, the star burns helium and hydrogen in shells around an inert carbon-oxygen core, causing intense pulsations and mass loss.
The asymptotic giant branch phase is marked by:
  • Helium and hydrogen shell burning
  • Large pulsations and variability
  • Massive loss of outer layers
This stage is a precursor to the ejection of the star's outer layers.
planetary nebula ejection
During the later stages of the AGB phase, the outer layers of the star are ejected into space, forming a planetary nebula. This nebula is illuminated by the remaining hot core, resulting in a beautiful glowing shell of ionized gas.
Key aspects of planetary nebula ejection:
  • Release of outer stellar material
  • Formation of ionized gas cloud
  • Short-lived phase compared to others
This marks the shedding of the star’s outer envelope, leaving behind the core.
white dwarf
After the planetary nebula disperses, the remnant core of the star becomes a white dwarf. This is a dense, Earth-sized object primarily composed of carbon and oxygen, no longer undergoing fusion.
White dwarfs are characterized by:
  • High density and small size
  • Gradual cooling over billions of years
  • A stable final stage barring external influences
The lifecycle of a low-mass star concludes here, leaving behind a cooling white dwarf.

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Most popular questions from this chapter

Go to the Hubble Space Telescope's planetary nebula gallery (http://hubblesite.org/gallery/album/nebula/planetary). For each of the three types of symmetry, find an example of a nebula that shows clearly the type of symmetry: spherical (being symmetric in every direction, like a circle), bipolar (having an axis about which they are symmetric, like a person's face, and point-symmetric (being symmetric about a point, like the letter \(S\) ). Print each of the three images you chose, and label the type of symmetry each one represents. For all three nebulae, identify the location of the central star. For bipolar symmetry, draw a line that shows the axis about which the nebula is symmetric. For point symmetry, identify several features that are symmetric across the location of the central star.

If a main-sequence star suddenly started burning hydrogen at a faster rate in its core, it would become a. larger, hotter, and more luminous. b. larger, cooler, and more luminous. c. smaller, hotter, and more luminous. d. smaller, cooler, and more luminous.

T Coronae Borealis is a well-known recurrent nova. a. Is it a single star or a binary system? Explain. b. What mechanism causes a nova to flare up? c. How can a nova flare-up happen more than once?

When the Sun runs out of hydrogen in its core, it will become larger and more luminous because a. it will start fusing hydrogen in a shell around a helium core. b. it will start fusing helium in a shell and hydrogen in the core c. infalling material will rebound off the core and puffs up the star. d. energy balance will no longer hold, and the star will drift apart.

Place the main-sequence lifetimes of the following stars in order from shortest to longest. a. the Sun: mass \(1 M_{\text {sun }}\), luminosity \(1 L_{\text {sun }}\) b. Capella Aa: mass 3 \(M_{\text {sun }}\), luminosity 76 \(L_{\text {Sun }}\) c. Rigel: mass \(24 M_{\text {sun }}\), luminosity \(85,000 L_{\text {sun }}\) d. Sirius A: mass 2 \(M_{\text {sun }}\), luminosity 25 \(L_{\text {sun }}\) e. Canopus: mass \(8.5 M_{\text {sun }}\), luminosity \(13,600 L_{\text {sun }}\) f. Achernar: mass \(7 M_{\text {sun }}\), luminosity \(3,150 L_{\text {sun }}\)
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