Stellar Evolution 1.1
Evolution of a sun like star
- As hydrogen is used up, the Helium core contracts and gets hotter - Hydrogen begins to fuse outside the core - The star expands and turns into a Red Giant
Red Giant Phase
- As the helium in the core fuses to carbon, the core becomes hotter and helium is consumed even faster; equilibrium is not stable - The outside of the star expands and contracts violently, causing the star to lose gas
Evolution of a High Mass Star
- High mass stars also fuse Hydrogen into Helium, and Helium into Carbon - but without a flash - The star leaves the main sequence and becomes a giant/supergiant star - However, high mass stars forge heavier elements via fusion to stay in equilibrium
White dwarf
- core composed of all carbon, or for more massive stars, oxygen - it has a mass similar to the sun, but a volume similar to earth - equilibrium with gravity is maintained through electric repulsion of atoms--electron degeneracy
Jocelyn Bell
- found a celestial source of periodic radio pulses in October, 1967 - emitted a periodic pulse every 1.337 seconds - LGMs= little green men - remnant neutron core after a type 2 supernova explosion
characteristics of high mass stars
- late in life fuse carbon into heavier elements - end life as a supernova - have higher fusion rate during main sequence life
characteristics of low mass stars
- the Sun is an example - end life as a planetary nebula - have longer lifetimes - final corpse is a white dwarf
What are the observational differences between Type 1 and Type 2 supernovae?
-Type I Supernova have a light curve similar to that of a nova. They are hydrogen poor. -Type II Supernova have a light curve with a plateau (hydrogen rich)
Stages during the life of a high mass star
1. contracting cloud of gas and dust 2. protostar 3. main sequence O star 4. red supergiant 5. supernova 6. neutron star
Elements produced during fusion in the core of a high mass main sequence star, based on order produced
1. helium 2. carbon 3. oxygen 4. iron
What mass will cause a white dwarf's core to collapse
1.4 solar masses (Chandrasekhar limit)
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?
100 million years
Roughly how big (in AU) will the Sun become when it enters the red giant phase?
100 times its current size, or almost half an AU
Under what circumstances will a binary star produce a nova?
A binary star will produce a nova when it is in the white dwarf stage. The white dwarf will pull materials from its binary star and these materials will then create an accretion disk around it. The accretion disk will cause the dwarf to heat up, and eventually at about 10(7) K, hydrogen will begin fusing helium. This is when the nova is most luminous.
What is a light curve? How can it be used to identify a nova or a supernova?
A light curve is the measure of Luminosity and Absolute Magnitude along a time continuum. It is used to identify a nova as there is shown to be a rapid rise and slow decline in luminosity as well as max brightness attained
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.
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.
Which of the following stars will become hot enough to form elements heavier than oxygen?
A star that is eight times more massive than the Sun
Type 1- A White Dwarf wakes up
A white dwarf can flare up very suddenly and then returns slowly to its former luminosity - also known as a nova
What are white dwarfs? Why are they hard to observe?
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.
How long can a star like the Sun keep burning hydrogen in its core?
After approximately 10 billion years of steady hydrogen burning, a sun-like star begins to run out of fuel.
Which of the following is the best description of the interior structure of a highly evolved high-mass star late in its lifetime but before the collapse of its iron core?
An onion-like set of layers forms, with the heaviest elements in the innermost shells surrounded by progressively lighter ones.
Supernova remnants
As the shock from a supernova collides with gas and dust in interstellar space, electrons and ions are accelerated All charges that accelerate, radiate Radio and X-rays trace the expanding shock
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.
Supernova explosion
Fusion up to iron yields energy - enough to ward off gravity Fusion with iron robs core of energy; it collapses in less than a second Iron atoms are crushed, electrons combine with protons, forming neutrons and neutrinos The star implodes and the outer layers bounce off the neutron core, causing a Type II supernova explosion
What is the helium flash?
Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium; occurs when core temp is 100 million K, 3 helium fuse into 1 carbon
What mechanism produces a nova?
If a white-dwarf star is close enough to a binary-companion star to steal material from it, the material eventually heats up to a temperature high enough to cause fusion on its surface.
What is the region of the HR diagram known as the horizontal branch?
It is a region of the HR diagram where stars remain for a time as they undergo helium core fusion
Will the Sun ever explode as a Type I supernova?
No. It lacks a binary companion.
What happens to a white dwarf when it explodes in a nova event?
Only some of the accumulated material on the surface of the white dwarf explodes, so it slowly gains mass over time.
pulsating radio stars-pulsars
Rotation periods: from 1.4 ms to > 10 s Mass: 1.4 - 3 solar masses inferred from binary orbits Size: ≤ 10 km from nuclear equations of state Most pulsars emit radio emission A handful emit gamma-rays, X-rays, optical, and IR
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.
Why don't stars live forever? Which 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.
What proof do astronomers have that heavy elements are formed in stars?
The are formed by stellar nucleosynthesis. Through spectroscopic studies, the abundance of heavy elements in the stars has been observed.
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.
Roughly how often would we expect a supernova to occur in our own Galaxy? How often would we expect to see a galactic supernova?
The chance is very rare to see a supernova in our own galaxy, only happening approximately every 100,000 years. The chances of us observing a galactic supernova are much greater, once every 100 years.
What is the age range among the globular clusters associated with our Galaxy?
The globular clusters range from 10 to 12 billion years old.
As a low-mass main-sequence star runs out of hydrogen fuel in its core, it actually becomes brighter. How is this possible?
The outer layers expand due to the higher rate of fusion in a shell around the dead core
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.
Why is the depletion of hydrogen in the core of a star such an important event?
The star will begin to change its structure drastically
Why don't stars live forever?
They run out of nuclear fuel in their cores
Why does the spectrum of a carbon-detonation supernova (Type I) show little or no hydrogen?
This supernova is the detonation of a carbon white dwarf that contains virtually no hydrogen.
What evidence is there that many supernovae have occurred in our Galaxy?
We have plenty of evidence. Occasionally, explosions are visible from earth. In other cases, we detect the glowing remains, or supernova remnants.
What causes a nova
a continuous transfer of gas from the companion of a white dwarf in a semi-detached binary system
What is another name for an alpha particle?
a helium nucleus
What is the size of the core of a typical white dwarf star?
about the size of Earth
What is a planetary nebula?
an expanding shell of gas around a white dwarf star
When the sun leaves the main sequence, it will become
brighter
on the main sequence, massive stars
burn their hydrogen fuel more rapidly than the Sun
What is the primary composition of a white dwarf?
carbon
An observable supernova should occur in our Galaxy about once every
century
Figure 21.8 in the textbook ("Supernova Light Curves") indicates that a supernova whose luminosity declines steadily in time is most likely associated with a star that is
comparable in mass to the Sun
Eventually, the hydrogen in a main sequence star's core will be ____
converted by fusion into Helium; its evolution from then on depends on the mass of the star (low mass stars go quietly, high mass stars go out with a bang)
Compared to other stars on the HR diagram, red giant stars are so named because they are
cooler
Nuclear fusion in the Sun will
create elements up to and including oxygen
Compared to the Sun, stars plotted near the bottom left of the H-R diagram are much
denser
A white dwarf is supported by the pressure of tightly packed
electrons
Where can many white dwarf stars be found?
evolved globular clusters
A massive star becomes a supernova when it
forms iron in its core
When a star depletes its core supply of hydrogen, ____ dominates in the core and _____ dominates in the envelope
gravity; pressure
A white dwarf can dramatically increase in brightness only if it
has another star nearby
Hydrogen-shell burning proceeds increasingly faster due to which of the following?
heat released from the core's contraction
After the core of a Sun-like star starts to fuse helium on the horizontal branch, the core becomes
hotter with time
Interior of a high mass star
iron core, then SI, O, Ne, C, He, H fusion shells
A nova differs from a supernova in that the nova
is much less luminous
What property of a star cluster helps determine its age?
main sequence turnoff
A star will evolve off the main sequence when it uses up
most of the hydrogen in the core
Death of a low mass star
no fusion in core --> core contracts and increases in temp--> outer layers of star driven away--> planetary nebula formed
Supernova
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 I, which is a carbon-detonation supernova Type II, which is the death of a high-mass star
How can astronomers estimate the age of an isolated star?
spectroscopic studies
millisecond pulsars
spun up by matter falling in from a companion
The silver atoms found in jewelry originated in
supernova
Which stars eventually undergo supernova explosions?
the most massive stars
How do elements that have nuclei heavier than that of iron form?
through the sudden capture of neutrons and protons by other nuclei, followed by radioactive decay
When the Sun is on the red-giant branch, it will be found at the
upper right of the H-R diagram
Why does the core of a massive star collapse?
when it runs out of elements to burn through
When is a Type II supernova produced?
when the collapse of a supermassive star's core causes a shock wave that blows off the surrounding envelope of the star
A star like the Sun will end up as a
white dwarf