ASTR 209 - Ch.20: Stellar Evolution

A star like the sun will end up as a

White dwarf

Roughly how big (in AU) will the Sun become when it enters the red-giant phase?

A star like the Sun will evolve into a red giant with a size about 100 times its current size. This is equivalent to about 70 million km, or almost half an AU.

Do all stars eventually fuse helium in their cores?

A star must have sufficient mass to collapse its core enough to reach the critical temperature of about 100 million K to fuse helium into carbon. Only stars with a mass more than about 25% of the Sun's mass can manage. In less massive stars, the core stabilizes before the temperature can be reached.

Why is it important to understand the evolution of binary stars?

Because many, if not most, stars in binaries can follow evolutionary paths quite different from those they would follow if single.

Helium shell flashes

Caused by the enormous pressure in the helium-burning shell and extreme sensitivity of the triple-alpha-burning rate to small changes in temperature. Flashes produce large fluctuations in the intensity of the radiation reaching the stars outermost layers, causing those layers to pulsate violently as the envelope repeatedly is heated, expands, cools, and contracts.

Can you think of a way in which a helium dwarf might exist today?

If a solar mass star is a member of a binary system, it is possible for it's envelope to be stripped away during the red-giant stage by the gravitational pull of it's companion, exposing the helium core and terminating the stars evolution before helium fusion can begin.

Do many black dwarfs exist in our galaxy?

No, the universe is not yet old enough to form black dwarfs.

Core Hydrogen burning

On the main sequence, a star slowly fuses hydrogen into helium in it's core.

How can astronomers measure the age of a star cluster?

By determining which of their stars have already left the main sequence.

After the core of a sun-like star starts to fuse helium on the horizontal branch, the core becomes

Hotter

White dwarf

A dwarf star with sufficiently high surface temperature that it glows white. Carbon core is no longer concealed by the atmosphere of the red-giant star, and core becomes visible as the envelope recedes. Star shining only by stored heat. Once an isolated star becomes a white dwarf, it's evolution is over.

What is a planetary nebula? Why do many planetary nebulae appear as rings?

A planetary nebula is an extended region of glowing gas surrounding an intensely hot central star. They indicate the impending death of a star. The ring of planetary nebula is in reality a 3D shell of glowing gas. The nebula looks brighter on the edges because there is more emitting gas along the line of sight there, creating the illusion of a bright ring.

Pauli exclusion pressure

A rule of quantum mechanics that prohibits electrons in dense gas from being squeezed too close together.

What are white dwarfs? What is their ultimate fate?

A white dwarf is a star that is formed after a red-giant star's envelope has receeded. The core is about the size of earth and it shines only by stored heat, not by nuclear reactions. This small "star" has a white-hot surface when it first becomes visible, although it appears dim because of it's small size. Once an isolated star becomes a white dwarf, it's evolution is over.

Black dwarf

After a star is a white dwarf, it's evolution is over. An isolated white dwarf continues to cool and dim with time, following the white-yellow-red track near the bottom of the H-R diagram and eventually becomes a black dwarf: a cold, dense, burned out ember in space (graveyard of stars).

How long can a star like the sun keep burning hydrogen in it's core?

After approximately 10 billion years of steady hydrogen burning, a sun-like star begins to run out of fuel.

Helium flash

An explosive even in the post main sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves a stable equilibrium again.

What is a helium flash?

An explosive event in the post-main-sequence evolution of a low mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves equilibrium again.

What are the Roche lobes of a binary system?

An imaginary surface around a star. Each star in a binary system can be pictured as being surrounded by a tear-drop shaped zone of gravitational influence. Any material within the Roche lobe of a star can be considered part of that star. During evolution, one member of a binary system can expand so that it overflows it's own Roche lobe and begins to transfer matter onto the other star.

How do the late evolutionary stages of high-mass stars differ from those of low-mass stars?

As a result of helium fusion, low mass stars eventually form a carbon core that collapses but cannot collapse enough to attain a high enough temperature to allow the fusion of carbon. The outer part of the star continues to expand and as the final shells of hydrogen and helium fusion die out, this outer part of the star is ejected into space. This cloud of gas is known as a planetary nebula. The core of the star remains, continues to cool, and is known as a white dwarf. In contrast, more massive stars are able to reach the core temperatures required for more complex nuclear fusion processes: the fusion of carbon, oxygen, and heavier elements. As the fusion processes get more complex, they take less and less time. The most massive stars can fuse elements to create iron cores, and shortly thereafter they explode.

How do astronomers test the theory of stellar evolution?

Astronomers compare their comprehensive theory which ties together atomic and nuclear physics, electromagnetism, thermodynamics and gravitation with their painstaking observations of stars and star clusters which offer a look at all types of stars.

Why does a star get brighter as it runs out of fuel in it's core?

Because a non burning inner core, unsupported by fusion, begins to shrink, releasing gravitational energy, heating the overlying layers and causing them to burn more vigorously, thus increasing the luminosity.

Why are observations of star clusters so important to the theory of stellar evolution?

Because a star cluster gives us a "snapshot" of stars of many different masses, but of the same age and initial composition, allowing us to directly test the predictions of theory.

Why is the depletion of hydrogen in the core of a star such an important event?

Because once hydrogen is depleted, helium increases and nuclear burning subsides. Without nuclear burning to maintain it, the outward pushing gas pressure weakens in the helium inner core. However, the inward pull of gravity does not. Once the outward push against gravity is relaxed- even a little- structural changes in the star become inevitable. As the hydrogen is consumed, the inner core begins to contract. When all the hydrogen at the centre is gone, the process accelerates.

Why does fusion cease in the core of a low mass star?

Because the core's contraction is halted by the pressure of degenerate (tightly packed) electrons before it reaches a temperature high enough for fusion to begin. In fact, this statement is true whether the "next round" is hydrogen fusion (brown dwarf), helium fusion (helium white dwarf), or carbon fusion (carbon oxygen white dwarf).

When the sun leaves the main sequence, it will become

Brighter

Compared to other stars on the HR diagram, red-giant stars are so named because they are

Cooler

Compared to the sun, stars plotted near the bottom left of the H-R diagram are much

Denser

What is the essential evolutionary difference between high-mass and low-mass stars?

Fusion in high mass stars is not halted by electron degeneracy pressure. Temperatures are always high enough that each new burning stage can start before degeneracy becomes important. Such stars can continue to fuse more and more massive nuclei, faster and faster, eventually exploding in a supernova.

Helium fusion

In a red giant star, after shrinking and expanding stop (few 100 mill years after solar mass star leaves the main sequence) helium begins to burn in the core.

A star will evolve off the main sequence when it uses up

Most of the hydrogen in the core.

Asymptotic-giant branch

Path on the H-R diagram corresponding to the changes that a star undergoes after helium burning ceases in the core. At this stage, the carbon core shrinks and drives the expansion of the envelope and the star becomes a swollen red-giant for a second time.

A white dwarf is supported by the pressure of tightly packed

Photons

Main sequence turn off

Special point on the H-R diagram for a cluster, indicative of the clusters age. If all the stars in the cluster are plotted, the lower mass stars will trace the main sequence up to the point where stars begin to evolve off the main sequence toward the red-giant branch- the point where stars are just beginning to evolve off is the main sequence turn off.

Horizontal branch

Stage 10 star is now stably burning helium in it's core and fusing hydrogen in a shell surrounding it. A large increase in the luminosity occurs as a star ascends the red-giant branch, ending in the helium flash. Then star settles down on the horizontal branch.

Red giant

Star leaves main sequence- helium core shrinks and it's outer envelope expands.

Why don't stars live forever? Which type of stars live the longest?

Stars cannot live forever because, as the main sequence star ages, it's core temperature rises and both it's luminosity and radius increase. These changes happen very slowly but eventually, as the hydrogen in the core is consumed, the stars internal balance starts to shift and both it's internal structure and it's outward appearance begin to change more rapidly; the star leaves the main sequence. M-type stars (red dwarfs) live the longest because they consume their fuel so slowly. O and B-type stats exhaust their fuel and leave the main sequence in only a few million years.

Describe an important way in which winds from red-giant stars are linked to the interstellar medium.

The central star fades and cools, and the expanding gas cloud becomes more and more diffuse, eventually dispersing into interstellar space. After awhile, the glowing planetary nebula (extended region of glowing gas) disappears from view. As the cloud rejoins the interstellar medium, it plays a vital role in the evolution of our galaxy. During the final stages of the red giants life, nuclear reactions between carbon and unburned helium in the core create oxygen and in some cases even heavier elements.

Planetary nebula

The ejected envelope of a red-giant star, spread over a volume roughly the size of our solar system.

Turn off mass

The mass of a star that is just evolving off the main sequence at any moment is known as the turn off mass.

Electron degeneracy pressure

The pressure produced by the resistance of electrons to further compression once they are squeezed to the point of contact (burning becomes unstable).

What is the internal structure of a star on the asymptotic-giant branch?

The star has a carbon core in which a small amount fuses with helium to form oxygen. Around the core, helium continues to fuse into carbon, and outside this region, hydrogen fuses into helium. The temperature of the core is about 300 million K, too cool to fuse carbon on a large scale. The heat of the interior is enough to expand the star to incredible size - hundreds of times its previous radius.

How long does it take for a star like the sun to evolve from the main sequence to the top of the red-giant branch?

The transformation from normal main-sequence star to elderly red-giant takes about 100 million years.

When the sun is on the red-giant branch, it will be found at

The upper right of the HR diagram

Why are white dwarfs hard to observe?

They are very small and therefore faint and hard to observe.

Why is it odd that the binary system Algol consists if a low-mass red giant orbiting a high-mass main sequence star? How did Algol come to be in this configuration?

This is odd because the more massive main sequence star should have evolved faster than the less massive component. If the two stars are formed at the same time (as is assumed to be the case) there should be no way that the 0.8 solar mass star could be approaching the giant stage first. Something has modified the evolution of the Algol system. Astronomers think that Algol started off as a detached binary, with both components lying well within their respective Roche lobes. Star 1 ascended the giant branch and it overflowed it's Roche lobe and gas began to flow into star 2. This transfer of matter had the effect of reducing the mass of star 2, which in then caused the Roche lobe of star 1 to shrink as it's gravity decreased.

The evolutionary track of a massive star it's starting point (stage 7) would be

Up and to the left

How can astronomers "see" stars evolve over time?

We can't observe a single star evolve, but we can observe large numbers of stars at different stages of their lives, and hence build up an accurate statistical picture of stellar evolutionary tracks.

Hydrostatic Equilibrium

When a main sequence star is in this state, pressure's outward push exactly counteracts gravity's inward pull.

What makes an ordinary star become a red giant?

When a star is far from the main sequence, it no longer has a stable equilibrium. The helium core is unbalanced and shrinking. The rest of the core is also unbalanced, fusing hydrogen into helium at an ever-increasing rate. The gas pressure produced by this enhanced hydrogen burning caused the stars non burning outer layers to increase in radius... While the core is shrinking and heating up, the overlying layers are expanding and cooling. The star is on it's way to becoming a red giant. Stars surface temperature has fallen to a point at which much of the interior is opaque to the radiation from within. Beyond this point, convection carries the cores enormous energy output to the surface. B

Helium shell burning

Within a few million years, after the onset if helium burning (stage 9), carbon ash accumulates in the Star's inner core. Above this core, hydrogen and helium are still behind in concentric shells.

On the main sequence, massive stars burn their hydrogen fuel

more rapidly than the sun