Ch 20 - Stellar Evolution

Mass Loss from Giant Stars

All stars lose mass via some form of stellar wind. The most massive stars have the strongest winds: O-and -type stars can lose a tenth of their total mass this way in only a million years. These stellar winds hollow out cavities in the interstellar medium surrounding giant stars.

Evolution of a Sun-Like Star

As fuel in core is used up, the core contacts; when it is used up the core begins to collapse. Hydrogen begins to fuse outside the core. 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 10 - Helium Fusion Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse, through a 3 alpha process: 4He + 4He converts to 8 Be + energy 8Be + 4He converts to 12C + energy The 9Be nucleus is highly unstable and will decay in a bout 10x -12s unless an alpha particle fuses with its first. This is why high temperatures and densities are necessary. The helium flash: The pressure within the helium core is almost totally due to "electron degeneracy" - two electrons cannot be in the same quantum state, so the core cannot contract beyond a certain point. This pressure is almost independent of temperature - when the helium starts fusing, the pressure cannot adjust Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium. 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 2 shells.

Leaving the Main Sequence`

Because star life span (even the short-lived stars) are too long, can't observe a single star going through its whole life cycle. Observing stars in clusters give us a look at stars in all stages and construct a complete picture. During its stay on the Main Sequence, any fluctuations in a star's condition are quickly restored; the star is in equilibrium. Eventually, as hydrogen is consumed, the star begins to leave the Main sequence. In evolution from then on depends on the mass of the star - low-mass stars go quietly but high-mass stars go out with a bang. Even while on the Main Sequence, the composition of a star's core is changing

Star Evolution in Binary Systems

If the stars in a binary-star system are relatively widely separated, their evolution proceeds much as it would have if they were not companions. If they are closer, it is possible for material to transfer form one star to another, leading to unusual evolutionary paths. Each star is surrounded by its own Roche lobe; particles inside the lobe belong to the central star. There are different types of binary-star systems, depending on how close the stars are. Detached binary - each star has its own Roche lobe Semi-detached binary - one star can transfer mass to the other Contrast binary - much of the mass is shared between two stars. As the stars evolve, their binary system type can evolve as well (Algol system). It is thought to have begun as a detached binary. As the blue-giant star entered its red-giant phase, it expanded to the point where mass transfer occurred. Eventually enough mass accreted onto the smaller star that it became a blue giant, leaving the other star as a red sub-giant.

Death of a Low-Mass Star

Low-mass stars never become hot enough for fusion past carbon to take place. There is no more outward fusion pressure being generated in the core, which continues to contract. The outer layers become unstable and are eventually ejected. The ejected envelope expands into interstellar space, forming a planetary nebula. The star now has 2 parts: a) A small, extremely dense carbon core b) 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 viewed the fuzzy envelope thought it resembled a planetary system. Planetary nebulae can have many shapes. As the dead core of the star cools, the nebula continues to expand and dissipates into the surroundings. Stage 13 & 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 Hubble Space Telescope has detected white dwarf stars in globular clusters . As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to grow

Summary of Chapter 20

Stars spend most of their life on the main sequence. When fusion ceases in the core, it begins to collapse and heat. Hydrogen fusion starts in the shell surrounding the core. The helium core begins to heat up; as long as the star is at least 0.25 solar masses, the helium will give hot enough that fusion (to carbon) will start. As the core collapses, the outer layers of the star expand. In Sun-like stars, the helium burning starts with a helium flash before the star is once again in equilibrium. The star develops a non burning carbon core, surrounded by shells burning helium and hydrogen. The shell expands into a planetary nebula, and in the core is visible as a white dwarf. The nebular dissipates, and the white dwarf gradually cools off. High-mass stars become red supergiants, and explosively. The description of stars' birth and death can be tested by looking at star clusters, whose stars are all the same age but the different masses. Stars in binary systems can evolve quite differently due to interactions with each other.

black dwarf

The endpoint of the evolution of an isolated, low-mass star. After the white dwarf stage, the star cools to the point where it is a dark "clinker" in interstellar space.

subgiant branch

The section of the evolutionary track of a star corresponding to changes that occur just after hydrogen is depleted in the core, and core hydrogen burning ceases. Shell hydrogen burning heats the outer layers of the star, which causes a general expansion of the stellar envelope.

white dwarf

a dwarf star with sufficiently high surface temperature that it glows white

Sirius

brightest star in the northern sky All sightings record between 100 BCE and 200 CE and describe it as being red, but now it is blue-white. Why? a) Was there an intervening dust cloud? b) Could its companion have been a red giant (it became a white dwarf quickly)?

asymptomatic giant branch

path on the Hertzsprung-Russell 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

horizontal branch

region of the H-R diagram where post-main-sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core and fusing hydrogen in a shell surrounding the core

blue straggler

star found on main sequence of the Hertzsprung-Russell diagram, but which should already have evolved off the main sequence, given its location on the diagram; thought to have formed from mergers of lower-mass stars.

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 stable equilibrium again.

Evolution of Stars More Massive than the Sun

Stars more massive than the Sun follows 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 the cars. 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. Stars with masses more than 2.5 solar masses do not experience a helium flash - helium burning starts gradually. A 4-solar mass star makes no sharp moves on the H-R diagram - it moves smoothly back and forth. A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate. Its path across the H-R diagram is essentially a straight line - it stays at just about the same luminosity as it cools off. Eventually the star dies in a violent explosion called a supernova.

Observing Stellar Evolution in Star Clusters

Stars of the same age, but different masses appear as the whole cluster ages. After 10 million years, the most massive stars have already left the main sequence, while many of the least massive have not yet 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 sub-giant, asymptomatic giant and horizontal branches are all clearly populated. White dwarfs, indicate that the solar-mass stars are in the last phase also appear. chi Persei - is a newborn double cluster (can't be more than 10 million years) Hyades cluster - young star, its main-sequence turnoff indicates an age of about 600 million years. 47 Tucanae - globular cluster about 10 - 12 billion years old

roche lobe

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

hydrogen-shell burning

fusion of hydrogen in a shell, that is driven by contraction and heating of helium core. Once hydrogen is depleted in the core of a star, hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star and increasing the burning rate there

main-sequence turnoff

special point on the Herzsprung-Russell diagram for a cluster, indicative of the cluster's age. If all the stars in the cluster are plotted, the lower -mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence towards the red giant branch. The point where stars are just beginning to evolve is the main-sequence turnoff.

subgiant

star on the subgiant branch of the H-R diagram

Blue stragglers

stars that formed much more recently, probably as the result of the merger of smaller stars

triple-alpha process

the creation of carbon-12 by the fusion of three helium-4 nuclei (alpha particles). Helium-burning stars occupy a region of the Hertzsprung-Russell diagram known as the horizontal branch.

planetary nebula

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

core hydrogen burning

the energy burning stage for main-sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by ore hydrogen burning

lagrangian point

the point where gravitational forces are equal

electron degeneracy pressure

the pressure produced by the resistance of electrons to further compression once they are squeezed to the point of contact

red-giant branch

the section of the evolutionary track of a star corresponding to intense hydrogen shell burning, which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant