1. Stars from Adolescence to Old Age
Phys./Geog. 182 – Week 7 Mon.
Stars from Adolescence to Old Age
Start with the evolution of stars
on the main sequence
FIGURE A Summary of the Star Formation Process
This set of photographs and drawings takes you through the star formation process for stars with less than about 1.5 solar masses.

2. Stars with Masses between 0.08 and 0.4 times the mass of the Sun
have low core
temperatures,
live a long time,
convect helium
from the core,
so it mixes uniformly,
and will end up
composed entirely of helium.
FIGURE Fully Convective Star
This drawing shows
how the helium created in the cores of red dwarfs rises into
the outer layers of the star by convection, while the hydrogen
from the outer layers descend into the core. This process continues
until the entire star is helium.

3. A G-Type Star is similar to our Sun
A G-Type Star is similar to our Sun. The evolution is shown during an imaginary trek through space. At the end of the red giant stage, the core is small, the envelope huge, and the outcome depends on the total mass of the star.

4. Evolution of stars with more than 0.4 solar masses
FIGURE Evolution of Stars Off the Main Sequence
(a) Hydrogen fusion occurs in the cores of main-sequence
stars. (b) When the core is converted into helium, fusion there
ceases and then begins in a shell that surrounds the core. The
star expands into the giant phase. This newly formed helium
sinks into the core, which heats up. (c) Eventually, the core
reaches 108 K, whereupon core helium fusion begins. This
causes the core to expand, slowing the hydrogen shell fusion
and thereby forcing the outer layers of the star to contract.

5. Hydrostatic Equilibrium maintains a star’s size during Stage 7

6. Solar Composition Change During stage 7 hydrogen burning causes a build-up of helium in the star’s core. We will follow the evolution of a star like the Sun, with one solar mass.

7. Hydrogen Shell Burning occurs around an “ash” core, which is mostly helium, and the temperature is T = 10 million K

8. FIGURE 12-23 The Sun Today and as a Giant
(a) In about 5 billion years, when the Sun expands
to become a giant, its diameter will increase a
hundredfold from what it is now, while its core becomes more
compact. Today, the Sun’s energy is produced in a hydrogen fusing
core whose diameter is about 200,000 km. When the
Sun becomes a giant, it will draw its energy from a hydrogen fusing
shell that surrounds a compact helium-rich core. The
helium core will have a diameter of only 30,000 km. The
Sun’s diameter will be about 100 times larger, and it will be
about 2000 times more luminous as a giant than it is today.
(b) This composite of visible and infrared images shows red
giant stars in the open cluster M50 in the constellation of
Monoceros (the Unicorn). See Margin Chart Monoceros,
above. (T. Credner and S. Kohle, Astronomical Institutes of the
University of Bonn)

9. FIGURE 12-23 The Sun Today and as a Giant
(a) In about 5 billion years, when the Sun expands
to become a giant, its diameter will increase a
hundredfold from what it is now, while its core becomes more
compact. Today, the Sun’s energy is produced in a hydrogen fusing
core whose diameter is about 200,000 km. When the
Sun becomes a giant, it will draw its energy from a hydrogen fusing
shell that surrounds a compact helium-rich core. The
helium core will have a diameter of only 30,000 km. The
Sun’s diameter will be about 100 times larger, and it will be
about 2000 times more luminous as a giant than it is today.
(b) This composite of visible and infrared images shows red
giant stars in the open cluster M50 in the constellation of
Monoceros (the Unicorn). See Margin Chart Monoceros,
above. (T. Credner and S. Kohle, Astronomical Institutes of the
University of Bonn)

10. FIGURE 12-24 Post–Main-Sequence Evolution
(a) The luminosity of the Sun changes as our star evolves. It began as a protostar
with decreasing luminosity. On the main sequence today, it
gradually brightens. Giant phase evolution occurs more rapidly,
with faster and larger changes of luminosity. Note the
change in scale of the horizontal axis scale at 12 billion years.
(b) Model-based evolutionary tracks of five stars are shown on
this H-R diagram. In the high-mass stars, core helium fusion
ignites smoothly where the evolutionary tracks make a sharp
turn upward into the giant region of the diagram.

11. The hydrogen shell burning causes higher pressure on the envelope, which causes the star to expand into a Red Giant. The star follows the yellow curve on the H–R diagram. Stage 8 is the “subgiant branch” and the radius is about 3 times the solar radius. An example is the star Arcturus, M = 1.5 Msolar and R = 23 Rsolar, the luminosity is about 100 times solar.

12. With sufficient Helium, the star begins Helium Shell Burning and moves to the Horizontal Branch

13. Stage 10 follows the Helium Flash, which is like a huge nuclear explosion of helium “flashing” or burning quickly into carbon at 108 K Fusion of 3 He-4 nuclei produces a C-12 nucleus plus other products The star moves to the Horizontal Branch

14. Reascending the Giant Branch occurs in a way similar to the original move up to a giant. Burning in the H and He shells is even faster than before, so the star expands even more on this “asymptotic branch”

15. starting masses follow somewhat different paths on these branches.
Stars with different
starting masses follow
somewhat different
paths on these branches.
FIGURE Post–Main-Sequence Evolution
(a) The luminosity of the Sun changes as our star evolves. It began as a protostar
with decreasing luminosity. On the main sequence today, it
gradually brightens. Giant phase evolution occurs more rapidly,
with faster and larger changes of luminosity. Note the
change in scale of the horizontal axis scale at 12 billion years.
(b) Model-based evolutionary tracks of five stars are shown on
this H-R diagram. In the high-mass stars, core helium fusion
ignites smoothly where the evolutionary tracks make a sharp
turn upward into the giant region of the diagram.