Chapter 12 Stellar Evolution

Chapter 12 Stellar Evolution

12.1 Leaving the Main Sequence
During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure Star expands

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure Star expands Expansion causes cooling

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure Star expands Expansion causes cooling Fusion rate falls

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure Star expands Expansion causes cooling Fusion rate falls Pressure decreases

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure Star expands Expansion causes cooling Fusion rate falls Pressure decreases Star returns to original size

During its stay on the Main Sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium: A temperature increase in core increases fusion rate This increases pressure Star expands Expansion causes cooling Fusion rate falls Pressure decreases Star returns to original size A temperature decrease causes the opposite

Eventually, as hydrogen in the core is consumed, the star begins to leave the Main Sequence. Its evolution from then on depends very much on the mass of the star: Low-mass stars go quietly High-mass stars go out with a bang!

12.2 Evolution of a Sun-like Star
Even while on the Main Sequence, the composition of a star’s core is changing:

Even while on the Main Sequence, the composition of a star’s core is changing: As the fuel in the core is used up, the core contracts

When the fuel in the core is used up, the core begins to collapse.

When the fuel in the core is used up, the core begins to collapse. The collapsing core heats up

When the fuel in the core is used up, the core begins to collapse. The collapsing core heats up Hydrogen begins to fuse outside the core

When the fuel in the core is used up, the core begins to collapse. The collapsing core heats up Hydrogen begins to fuse outside the core The fusion rate is even higher than on the main sequence

When the fuel in the core is used up, the core begins to collapse. The collapsing core heats up Hydrogen begins to fuse outside the core The fusion rate is even higher than on the main sequence The star is on its way to becoming a red giant

Stages of a star leaving the Main Sequence:

Stage 9: The Red-Giant Branch As the core continues to shrink, the outer layers of the star expand and cool.

Stage 9: The Red-Giant Branch As the core continues to shrink, the outer layers of the star expand and cool. It is now a red giant, extending out as far as the orbit of Mercury.

Stage 9: The Red-Giant Branch As the core continues to shrink, the outer layers of the star expand and cool. It is now a red giant, extending out as far as the orbit of Mercury. Despite its cooler temperature, its luminosity increases enormously due to its large size.

Stage 9: The Red-Giant Branch As the core continues to shrink, the outer layers of the star expand and cool. It is now a red giant, extending out as far as the orbit of Mercury. Despite its cooler temperature, its luminosity increases enormously due to its large size. The increased luminosity comes from the increased fusion rate.

The red giant stage on the H-R diagram:

Stage 10: Helium fusion Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse at the helium flash

Stage 10: Helium fusion Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse at the helium flash Helium begins to fuse extremely rapidly

Stage 10: Helium fusion Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse at the helium flash Helium begins to fuse extremely rapidly Within hours the enormous energy output is over, and the star once again reaches equilibrium

Stage 10 on the H-R diagram:

Stage 11: Back to the giant branch As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster. The star is now similar to its condition just as it left the Main Sequence, except now there are two shells:

The star has become a red giant for the second time:

12.3 The Death of a Low-Mass Star
This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough (600,000,000 K) for carbon fusion to take place. This is because electron degeneracy pressure stops the collapse of the carbon core.

The carbon core, with no outward fusion pressure, continues to contract until the degeneracy pressure point is reached Meanwhile, the outer layers of the star expand to form a planetary nebula.

The star now has two parts: A small, extremely dense carbon core An envelope about the size of our solar system. The envelope is called a planetary nebula, even though it has nothing to do with planets – early astronomers viewing the fuzzy envelope thought it resembled a planetary system.

Stages 13 and 14: White and black dwarfs Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small. It is luminous only due to its high temperature.

The small star Sirius B is a white-dwarf companion of the much larger and brighter Sirius A:

The Hubble Space Telescope has detected white dwarf stars (circled) in globular clusters:

As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to glow.

A nova is a star that flares up very suddenly and then returns slowly to its former luminosity:

A white dwarf that is part of a close binary system can undergo repeated novas:

Material falls onto the white dwarf from its main-sequence companion. When enough material has accreted, fusion can reignite very suddenly, burning off the new material. Material keeps being transferred to the white dwarf, and the process repeats.

End of 16 April 2007 Lecture

This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough (600,000,000 K) for carbon fusion to take place. This is because electron degeneracy pressure stops the collapse of the carbon core.

Stages 13 and 14: White and black dwarfs Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small. It is luminous only due to its high temperature.

12.4 Evolution of Stars More Massive than the Sun
It can be seen from this H-R diagram that stars more massive than the Sun follow very different paths when leaving the Main Sequence:

High-mass stars, like all stars, leave the Main Sequence when there is no more hydrogen fuel in their cores.

High-mass stars, like all stars, leave the Main Sequence when there is no more hydrogen fuel in their cores. The first few events are similar to those in lower-mass stars – first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells.

A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate. Eventually the star dies in a violent explosion called a supernova.

A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate. Eventually the star dies in a violent explosion called a supernova. This happens when the fusion product is iron

The “ashes” of earlier fusion reactions start fusing

The “ashes” of earlier fusion reactions start fusing Core looks like onion

The “ashes” of earlier fusion reactions start fusing Core looks like onion Finally, iron is produced

The “ashes” of earlier fusion reactions start fusing Core looks like onion Finally, iron is produced Iron can’t fuse

The “ashes” of earlier fusion reactions start fusing Core looks like onion Finally, iron is produced Iron can’t fuse Iron core collapses

The “ashes” of earlier fusion reactions start fusing Core looks like onion Finally, iron is produced Iron can’t fuse Iron core collapses SUPERNOVA

But just said >8!

12.5 Supernova Explosions A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star.

12.5 Supernova Explosions A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star. There are two different types of supernovae, both equally common:

12.5 Supernova Explosions A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star. There are two different types of supernovae, both equally common: Type II, which is the death of a high-mass star we already discussed

12.5 Supernova Explosions A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star. There are two different types of supernovae, both equally common: Type II, which is the death of a high-mass star we already discussed Type I, which is a carbon-detonation supernova

12.5 Supernova Explosions Carbon-detonation supernova occurs when a white dwarf has accumulated too much mass from a binary companion

12.5 Supernova Explosions Carbon-detonation supernova occurs when a white dwarf has accumulated too much mass from a binary companion…novas can occur beforehand

12.5 Supernova Explosions Carbon-detonation supernova occurs when a white dwarf has accumulated too much mass from a binary companion…novas can occur beforehand If the white dwarf’s mass exceeds 1.4 solar masses, electron degeneracy can no longer keep the core from collapsing.

12.5 Supernova Explosions Carbon-detonation supernova occurs when a white dwarf has accumulated too much mass from a binary companion…novas can occur beforehand If the white dwarf’s mass exceeds 1.4 solar masses, electron degeneracy can no longer keep the core from collapsing. Carbon fusion begins throughout the star almost simultaneously, resulting in a carbon explosion.

12.5 Supernova Explosions This graphic illustrates the two different types of supernovae:

12.5 Supernova Explosions The two types – each of which is more than a million times as bright as a nova – can be distinguished by their light curves and their spectra

12.5 Supernova Explosions Supernovae leave remnants – the expanding clouds of material from the explosion. The Crab nebula is a remnant from a supernova explosion that occurred in the year 1054.

12.6 Observing Stellar Evolution in Star Clusters
The following series of H-R diagrams shows how stars of the same age, but different masses, appear as the cluster as a whole ages. After 10 million years, the most massive stars have already left the Main Sequence, while many of the least massive have not even reached it yet.

After 100 million years, a distinct main-sequence turnoff begins to develop. This shows the highest-mass stars that are still on the Main Sequence. After 1 billion years, the main-sequence turnoff is much clearer.

After 10 billion years, a number of features are evident: The red-giant, subgiant, asymptotic giant, and horizontal branches are all clearly populated. White dwarfs, indicating that solar-mass stars are in their last phases, also appear.

This double cluster, h and chi Persei, must be quite young – its H-R diagram is that of a newborn cluster. Its age cannot be more than about 10 million years.

The Hyades cluster, shown here, is also rather young; its main-sequence turnoff indicates an age of about 600 million years.

This globular cluster, M80, is about billion years old, much older than the previous examples.

12.7 The Cycle of Stellar Evolution
Star formation is cyclical: stars form, evolve, and die. In dying, they send heavy elements into the interstellar medium. These elements then become parts of new stars. And so it goes.

Summary of Chapter 12 Once hydrogen is gone in the core, a star burns hydrogen in the surrounding shell. The core contracts and heats; the outer atmosphere expands and cools. Helium begins to fuse in the core, as a helium flash. The star expands into a red giant as the core continues to collapse. The envelope blows off, leaving a white dwarf to gradually cool. Nova results from material accreting onto a white dwarf from a companion star

Summary of Chapter 12 Massive stars become hot enough to fuse carbon, then heavier elements all the way to iron. At the end, the core collapses and rebounds as a Type II supernova. Type I supernova is a carbon explosion, occurring when too much mass falls onto a white dwarf. All heavy elements are formed in stellar cores or in supernovae. Stellar evolution can be understood by observing star clusters.

Chapter 12 Stellar Evolution

Similar presentations

Presentation on theme: "Chapter 12 Stellar Evolution"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Chapter 12 Stellar Evolution

Similar presentations

Presentation on theme: "Chapter 12 Stellar Evolution"— Presentation transcript:

Similar presentations

About project

Feedback