Formation and Evolution of Stars - Guided Notes update
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Guided Notes – Formation and Evolution of Stars
Name:
Leah Walker
These notes cover Parts I through III of
Video Lecture – Stellar Evolution
.
Please type or handwrite notes as you watch the video lecture and answer the
included questions.
Modelling Stellar Evolution:
Stellar evolution is the process of how a star changes over time. Astrophysicists study it by observing numerous
stars at different points in their lifetime and using computer models to simulate stellar structure. A mathematical
model is used to compute the evolutionary phases of a star from its formation until it becomes a remnant, with
inputs being the star's mass and chemical composition, and the luminosity and surface temperature being the
only constraints.
Forces and Energy:
Key forces that drive stellar evolution:
Gravity & internal pressure
Important sources for generating heat in a star:
Nuclear Fusion & Gravitational Contraction
Test Yourself: Evolving Star I
The size of a star comes from the balance of two forces: gravity pulling all matter towards the core, and internal
pressure pushing matter out from the core.
1. If the interior of a star heats up, what happens to the internal pressure?
A. Increases
B. Decreases
C. Stays the same
Your answer:
A. Increases
2. If the internal pressure changes in the way you described in Question 1, what will happen to the overall size of
the star?
A. Gets bigger
B. Gets smaller
C. Stays the same
Your answer:
A. Gets bigger
Star Formation:
Star formation is the process of creating new stars from dense areas within molecular clouds in space. These areas are
called "stellar nurseries" or "star-forming regions". To understand how stars form, scientists study the interstellar
medium (ISM) and giant molecular clouds (GMC), as well as protostars and young stellar objects. Star formation theory
explains how single and binary stars form and how they vary in mass. Most stars form in groups called star clusters or
stellar associations.
Gravitational Collapse:
Gravitational collapse is a process whereby an astronomical object contracts due to its own gravity, which
attracts matter towards its center of gravity. This process is a vital mechanism for structure formation in the
universe, resulting in the creation of stars, white dwarfs, neutron stars, and black holes. When a massive star
undergoes gravitational collapse, it can result in a Type II supernova. Over time, a uniform distribution of matter
will collapse, forming pockets of higher density, and creating a hierarchy of condensed structures such as clusters
of galaxies, stellar groups, stars, and planets.
Protostar:.
A protostar is a young star that is still growing by collecting mass from its parent molecular cloud. This is the first
stage in the process of a star's life. It lasts around 500,000 years for a low-mass star like the Sun. The protostar
phase begins when a fragment of the molecular cloud collapses under its own gravity, forming a core inside the
fragment. The phase ends when the infalling gas is used up, leaving a pre-main-sequence star. This star contracts
and eventually becomes a main-sequence star, which produces helium through hydrogen fusion.
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Steps to becoming a protostar:
1.
Dense cores form within a molecular cloud.
2.
A protostar with a surrounding disk of material forms at the center of a dense core, accumulating additional material
from the molecular cloud through gravitational attraction.
3.
A stellar wind breaks out but is confined by the disk to flow out along the two poles of the star.
4.
Eventually, this wind sweeps away the cloud material and halts the accumulation of additional material, and a newly
formed star, surrounded by a disk, becomes observable.
Test Yourself – Evolving on the HR Diagram
What is happening to the radius of a star evolving in the direction shown with arrow. Use the temperature and
luminosity to answer.
A. The radius is shrinking (it’s getting cooler)
B. The radius is growing (it’s getting cooler)
C. The radius isn’t changing (the luminosity isn’t changing)
Your answer:
B. The radius is growing (it’s getting cooler)
Disks, Jets, and Winds:
A
disk
is a flat, rotating structure of gas and dust that surrounds a central object, such as a star or a black
hole.
Disks are often observed around young stars and are believed to be the birthplace of planets
.
A
jet
is an energetic outflow of ionized matter that is emitted as a beam along the axis of rotation of a central
object.
Jets are often observed in association with accretion disks, which are disks of gas and dust that surround
a central object and are actively accreting matter
A
wind
is a flow of gas that is driven by radiation pressure or thermal pressure.
Winds are often observed in
association with disks and jets and are believed to play a crucial role in regulating the growth of stars and black
holes
Formation Times:
Formation times refer to the duration it takes for celestial objects, such as stars, to form. This is a complex
process involving the collapse of interstellar gas and dust clouds due to their own gravity.
Test Yourself – Formation Times
In star clusters, by the time the largest stars have formed and died, stars of 0.5 solar mass have:
A. Also formed and died
B. Formed but are still on the Main Sequence
C. Are mostly formed
D. Have barely started the formation process
Your answer:
D. Have barely started the formation process
Mass Limits:
The mass limit of a star refers to the smallest and largest mass it can have. A star's mass determines its
lifespan, energy production, and fate. The smallest mass is about one-tenth of the Sun's mass, while the
largest is about one hundred times the Sun's mass.
Main Sequence:
Test Yourself: Evolving Star II
When the core of a Main Sequence star runs out of fuel, it will start to contract due to gravity. What will this
contraction do to the temperature of the core of the star? (Hint: think about gravitational potential energy).
A. Hotter
B. Cooler
C. Stays the Same
Your answer:
A. Hotter
What should happen to the overall size (not just the core!) of a star that has just run out of fuel in its core?
A. Gets Bigger
B. Gets Smaller
C. Stays the Same
Your answer:
A. Gets Bigger
In the case described here, what should happen to the luminosity of the star? (Hint: Luminosity depends on how
much heat is travelling out of the core)
A. Gets more luminous
B. Gets less luminous
C. Stays the Same
Your answer:
A. Gets more luminous
Post Main Sequence:
The post-main sequence phase in astronomy refers to the stage of a star’s life after it has exhausted the
hydrogen fuel in its core and has moved off the main sequence. A star's post-main sequence stage depends on
its mass. During this phase, the star grows large and may pulsate due to instabilities in its outer layers. Cepheid
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variables are pulsating stars used to measure distances. Helium fuses into heavier elements in the star's core.
The exact events depend on the star's mass.
Shell Burning:
Shell burning in astronomy is the fusion of hydrogen, helium, and other elements in a star's interior outside
its core. As fuel is exhausted, the shell moves outward until it reaches regions too cool to sustain reactions.
Post-main sequence, shell burning occurs in stars that depleted hydrogen fuel in their core. For example, in
a star like our Sun, hydrogen shell burning is more intense than on the main sequence, causing the star to
brighten and expand into a red giant.
Helium Fusion:
Helium fusion is a nuclear reaction that occurs in low-mass star cores during the red giant phase. When the
hydrogen in the core is depleted, helium gets compressed into degenerate matter, which increases the density
and temperature of the core. This causes helium burning and leads to a runaway reaction where the
temperature keeps rising due to increased fusion rate.
Test Yourself: Helium Fusion
Once helium fusion starts in a stellar core, how does the star’s outer surface change?
a) It becomes larger and hotter
b) It becomes smaller and cooler
c) It becomes smaller and hotter
d) It becomes larger and cooler
Your answer:
c) It becomes smaller and hotter
Low-Mass Stars:
Low-mass stars are stars with less than a few tenths of the sun's mass, but enough to burn hydrogen in their
cores. They have large convection zones compared to intermediate- and high-mass stars.
White Dwarfs:
A white dwarf is a small, dense star that's the final stage of low-mass stars. It's made of electron-degenerate
matter and has a mass similar to the Sun's but a volume similar to the Earth's.
Test Yourself: White Dwarfs
A white dwarf star is generating energy from what source?
a) nuclear fusion of hydrogen into helium
b) It no longer generates energy but is cooling slowly.
c) gravitational potential energy as the star slowly contracts
d) nuclear fusion of heavy elements in the central core
Your answer:
b) It no longer generates energy but is cooling slowly.
Your Turn: Yellow Stars
You find two yellow stars at the same distance (but not in a binary system!). They have the same mass, but one is
more luminous than the other. Which one is older? Explain how you can tell.
Your answer: The less luminous star is older than more luminous star.
High-Mass Stars:
High-mass stars are rare and have a mass of at least 8 times that of the Sun. They are very bright and short-lived,
lasting only a few million years. These stars create heavy elements in their cores, explode as supernovas, and
distribute these elements into space. Most of the elements in the universe, including those on Earth, came from
high-mass stars.
Test Yourself: Iron Core
After the material in the core of a massive star has been converted to iron by thermonuclear reactions, further
energy can be released to heat the core ONLY by
a) thermonuclear fusion of iron into heavier elements.
b) gravitational contraction.
c) absorption of neutrinos.
d) nuclear fission or splitting of nuclei
Your answer:
Click or tap here to enter text.
Supernova:
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Neutron stars:
Test Yourself: Pulsar Spin
Why do pulsars slow down?
a) Any rotating object needs fuel to keep spinning, and pulsars have no more nuclear fuel.
b) They are expanding and hence their rotation rate must slow down to conserve angular momentum.
c) They are transferring energy to a black hole companion.
d) They convert spin energy to light, which radiates out into space.
Your answer:
Click or tap here to enter text.
Test Yourself: Clusters
You study a cluster of stars with main sequence stars ranging from 0.08 to 10 M. There may be other non-main
sequence stars in the cluster. What stellar corpses would you expect to find here (assume there were never any
stars over 25 solar masses in the cluster?
a) white dwarfs and neutron stars
b) just white dwarfs
c) just neutron stars
d) There should be no stellar corpses in this cluster
Your answer:
Click or tap here to enter text.