Star Life Cycle, Fusion Processes, and End States: Astronomy Notes

What is the initial stage of a star's life?

A star starts as a protostar, which is a collapsing cloud of dust and gas.

What fusion process occurs when a star is born?

Hydrogen (H+) fusion into helium (He2+) begins.

What is the significance of dynamic equilibrium in a star's life?

More mass leads to more gravity, which increases heat and pressure, requiring more fusion to generate heat.

How does a star's mass affect its lifespan?

Higher mass stars consume hydrogen faster and have shorter lifespans, while lower mass stars live longer.

What happens to a star's brightness as it ages on the main sequence?

As helium 'ashes' build up, pressure increases, leading to higher temperatures and gradually increasing brightness.

What characterizes the death of a low mass star (<0.25 M*)?

They are not observed to have died yet, likely living longer than the age of the universe.

What is the best guess for the death of low mass stars?

They are fully convective, mixing hydrogen and helium, allowing efficient use of hydrogen fuel.

What occurs when a low mass star exhausts its hydrogen fuel?

Gravity causes the star to collapse and heat up without a source of energy.

What stops the collapse of a low mass star?

The collapse is stopped by electron degenerate matter, a quantum mechanics effect.

What happens to a star after it stops collapsing?

It cools and fades away.

What is the Hertzsprung-Russell (HR) Diagram used for?

It plots the luminosity of stars against their color/temperature.

What is the relationship between a star's mass and its fusion rate?

More mass leads to a higher fusion rate, resulting in faster consumption of hydrogen.

What happens to the fusion process as helium builds up in a star?

Helium buildup interferes with fusion, requiring increased pressure to force hydrogen nuclei together.

What is the fate of a star after it has exhausted all hydrogen fuel?

It collapses under gravity and heats up until electron degeneracy halts further collapse.

What is the role of convection cells in low mass stars?

Convection cells mix hydrogen and helium, refreshing the fuel available at the core.

How does the temperature of a star change as it ages on the main sequence?

The temperature slowly increases as the star ages.

What is the effect of electron degeneracy on a collapsing star?

Electrons resist being crushed together, preventing further collapse.

What happens to a high mass star at the end of its life?

It explodes in a supernova.

What is SN 1987A?

A notable supernova observed on February 22, 1987.

What elements are primarily created during the nucleosynthesis of a supernova?

Elements heavier than iron (Fe), such as nickel (Ni), silicon (Si), and oxygen (O).

What is the significance of the remnant cloud from a supernova?

It mixes with the interstellar medium and contributes to the formation of new stars.

What is a white dwarf?

The remnant of a low to medium mass star, supported by electron degeneracy pressure.

What is the size limit for a white dwarf?

Approximately 1.4 solar masses (M*).

What is a neutron star?

The remnant of a high mass star (8 M to ~25 M), supported by baryon degeneracy pressure.

What is the size limit for a neutron star?

Approximately 3 solar masses (M*).

What is a Type II Supernova?

An explosion of a massive star that leaves behind a neutron star.

What is a pulsar?

A rapidly spinning neutron star that emits beams of light due to its strong magnetic field.

What is the Crab Nebula?

The remnant of a supernova first observed on July 4, 1054, featuring a central pulsar.

What is a nova?

An explosive fusion of hydrogen on the surface of a white dwarf, causing periodic brightening.

What is an X-ray burster?

Similar to a nova, but the light burst is in the X-ray spectrum instead of visible light.

What occurs during a Type Ia Supernova?

A white dwarf accumulates mass from a companion star until it exceeds the limit and explodes.

What is a black hole?

A remnant of a star where gravity is so strong that nothing, not even light, can escape.

What is the event horizon?

The boundary around a black hole beyond which nothing can escape.

What is the Schwarzschild radius?

The radius of the event horizon of a black hole.

How can black holes be detected?

By observing their effects on surrounding matter, such as the accretion disk of gas.

What is the first black hole to be imaged?

M87*, which has a mass of 6.5 billion solar masses.

What is the role of gravity in the formation of a black hole?

Gravity wins against all other forces, collapsing the star's mass into a singularity.

What happens to the core of a star after a supernova?

It can become a neutron star or a black hole, depending on the mass of the original star.

What is the alternative method of hydrogen fusion in stars that uses carbon as a catalyst?

CNO (Carbon-Nitrogen-Oxygen) cycle

What is the primary outcome of the CNO cycle?

1 C6+ + 4 H+ => 1 C6+ + 1 He2+ + energy (+neutrinos)

What happens to a high mass star when its helium core runs out of fuel?

The core shrinks, gets hotter, and hydrogen shell burning occurs, causing the star to expand.

What is the relationship between core fusion and star size?

Core fusion causes the star to shrink and heat, while shell fusion causes the star to expand and cool.

What occurs when a high mass star's iron core forms?

The iron core produces no energy, leading to a collapse under gravity.

What phenomenon occurs when the iron/nickel core collapses under gravity?

Electrons combine with protons to form neutrons, leading to core shrinkage.

What is the result of baryon degeneracy in a collapsing star?

It stops gravity's collapse but allows the outer layers to fall inward, creating a shockwave.

What happens during the death of a high mass star?

The star explodes, resulting in a supernova.

What is a 'standard candle' in relation to variable stars?

A pulse rate that is directly related to luminosity.

What are the two types of variable stars mentioned?

RR Lyrae (lower mass, shorter period) and Cepheid (higher mass, longer period).

What is the final fate of elements during the fusion process in high mass stars?

Fusion progresses from hydrogen to helium, then to carbon, neon, magnesium, oxygen, silicon, and ends at iron.

What happens to the outer layers of a star during a supernova?

They fall inward and may create a bright light in the sky.

What is the significance of SN 1987A?

It was a notable supernova observed in 1987, marking the death of a giant star.

What is the role of neutrino energy in the supernova explosion?

It drives the shockwave that propagates outward during the explosion.

What is the result of no heat production in a dying star?

The core shrinks under gravity, leading to further changes in the star's structure.

What is the pattern observed in the fusion process of high mass stars?

Shell fusion leads to expansion, while core fusion leads to shrinkage and heating.

What happens to the core of a high mass star as it reaches the point of electron degeneracy?

Electrons resist gravity, but gravity ultimately causes the core to collapse.

What is the consequence of an iron core in a high mass star?

It signifies the end of fusion processes, leading to gravitational collapse.

What is the mass range for medium mass stars?

~0.25 M to ~8 M

What happens to the core of a medium mass star as it approaches death?

The core shrinks under gravity and gets significantly hotter.

What fusion process ignites in the shell of a dying medium mass star?

Hydrogen fusion (H+ → He2+).

What is the result of hydrogen fusion igniting in the shell of a medium mass star?

Fusion heats outer layers, causing them to expand and cool.

What happens to the star's surface and core during the expansion phase?

The surface becomes big and cool, while the core remains very hot.

What prevents the helium core from collapsing in a medium mass star?

Electron degeneracy pressure.

What is the Triple Alpha Process in the context of medium mass stars?

The fusion of 3 He2+ to form 1 C6+ and energy.

What is the net gain in lifespan from helium fusion in a medium mass star?

~109 years.

What happens to the core as carbon 'ashes' form a new core?

Fusion in the core ends, leading to core shrinkage and heating.

What is the outcome for medium mass stars after carbon fusion?

They evolve into a white dwarf, with a planetary nebula forming from ejected outer layers.

What is a planetary nebula?

An expanding bubble of cooling gas ejected from a star, eventually merging into the interstellar medium.

What happens to the outer layers of a medium mass star during its final stages?

Stellar wind blows the outer layers off, forming a planetary nebula.

What is the structure of a planetary nebula?

It often features a bipolar outflow.

How does a medium mass star's evolution appear on the HR diagram?

It moves off the Main Sequence into the Giants region.

What occurs during the carbon fusion phase in higher medium mass stars?

Carbon fusion creates neon and magnesium.

What is the final remnant of a medium mass star after its death?

A white dwarf.

What is the significance of the helium fusion process for the star's lifespan?

It provides a temporary new source of energy but is less energetic than hydrogen fusion.

What is luminosity in relation to stars?

Luminosity refers to the actual brightness or light output of a star.

How does distance affect the perceived brightness of a star?

As distance increases, light spreads out, causing the flux to decrease by the square of the distance.

What is parallax and how is it used in astronomy?

Parallax is the apparent shift of a star's position due to Earth's movement, used to measure distances to stars.

What is the relationship between apparent brightness and absolute brightness?

Apparent brightness plus distance can be used to determine absolute brightness.

What classification system is used for star temperatures?

Stars are classified from hot to cool as O, B, A, F, G, K, M, with mnemonics to remember the order.

How can temperature be used to determine a star's size?

Knowing a star's temperature and luminosity allows us to calculate its size.

What is the Doppler shift in relation to stars?

Doppler shift refers to the change in the spectrum of a star as it moves toward or away from us, indicating its speed.

What are binary stars?

Binary stars are pairs of stars that orbit each other, with over 50% of bright stars having one or more companions.

How can we determine the mass of a star in a binary system?

By observing the orbital motion of the stars, we can calculate the mass of the star based on Newton's Law of Universal Gravitation.

What is the Hertzsprung-Russell (HR) Diagram?

The HR Diagram is a plot of stars showing the relationship between their luminosity and color/temperature.

What fusion process occurs in the main sequence of stars?

In the main sequence, stars fuse hydrogen (H) into helium (He2+).

What determines a star's characteristics?

A star's mass determines its luminosity, temperature, and evolutionary path.

What is the significance of knowing a star's distance?

Knowing a star's distance is crucial for determining its luminosity and understanding its true brightness.

What is the limit of parallax measurements?

Parallax measurements are limited by the precision of angle measurement, currently allowing distances up to about 10,000 parsecs.

Who reclassified stars based on temperature in the early 20th century?

Astronomer Annie Jump Cannon reclassified stars by temperature, examining 250,000 stars.

What mnemonic helps remember the star temperature classes?

The mnemonic 'Oh, Be A Fine Girl/Guy, Kiss Me' helps remember the order of star temperature classes.

How does a star's size relate to its luminosity and temperature?

A star's size can be determined using its absolute brightness and temperature.

What is the role of light in understanding stars?

Light is the primary means by which we understand stars, as we cannot send probes to them due to their distance.

What are sunspots?

Dark spots visible on the Sun's surface that are cooler than their surroundings but more energetic.

Who made early sketches of sunspots?

Galileo in the mid 17th century.

What do spectra taken around sunspot locations indicate?

They show light consistent with charged particles in a magnetic field.

How do sunspots affect convection?

Sunspots suppress convection, preventing heat from rising in those areas.

What is the rotation pattern of the Sun?

The Sun rotates faster around the equator than at the poles.

What happens to the Sun's magnetic field due to its rotation?

The magnetic field gets dragged by the surface and twisted up, then slowly breaks free.

What is the cycle period for sunspot activity?

Sunspot activity repeats on an 11-year cycle.

What is a solar prominence?

A loop of material stuck on the solar magnetic field that emerges from the surface and is fairly stable.

What occurs during a solar flare?

When the magnetic field snaps, charges on the Sun's surface can get ejected, resulting in highly energetic emissions.