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This young cluster of about 3,000 stars in our Milky Way is called Westerlund 2 and contains some of the galaxy's hottest, brightest, and most massive stars. Hubble's infrared vision pierced dust around this stellar nursery to reveal the dense concentration of stars in the central cluster.
NASA, ESA, Antonella Nota (ESA, STScI), Hubble Heritage Project (STScI, AURA), Westerlund 2 Science Team
Today we know that stars are the essential sources of raw material in the universe, recycling and distributing the elemental building blocks of everything we observe: new stars, nebulas of gas and dust, planets, and even humans. All life on Earth contains the element carbon, and all carbon was originally formed in the core of a star.
Stars populate the universe with elements through their “lifecycle”—an ongoing process of formation, burning fuel, and dispersal of material when all the fuel is used up. Different stars take different paths, however, depending on how much matter they contain—their mass. A star’s mass depends on how much hydrogen gas is brought together by gravity during its formation. We measure the mass of stars by how they compare to the “parent star” of our system, the Sun. Stars are considered high-mass when they are five times or more massive than the Sun.
When high-mass stars have no more fuel to generate outward energy, their iron cores begin to collapse until the pressure overcomes the inward push of gravity and they explode in a spectacular supernova, dispersing elements into space to recombine as future stars, planets, asteroids, or even eventually life like us.
After supernova, massive stars can go one of two ways. If the remnant of the explosion is about 1.4 to 3 times the mass of our sun, it will collapse into a very small, very dense core of neutrons called a neutron star. If the remnant is more than three times as massive as the Sun, gravity overwhelms the neutrons and the star collapses completely into a black hole—so-called because the matter within is so compressed and the pull of gravity is so intense that even light is drawn in and not reflected, so that area is “black” or unobservable.
Surely it is a great thing to increase the numerous host of fixed stars previously visible to the unaided vision, adding countless more which have never before been seen.
—Galileo Galilei
Taken from: https://webbtelescope.org/webb-science/the-star-lifecycle
The Internal Structure of Stars
The Internal Structure of Stars
Gravitational Pull
Gravitational Pull
Origin: Gravitational pull in a star originates from its own mass. The more massive a star, the stronger its gravitational pull. This force tends to compress the star, pulling all its matter inward towards the center.
Effect: Without a counteracting force, gravitational pull would cause the star to collapse into a denser and denser object, leading to a catastrophic implosion.
Radiative Pressure
Radiative Pressure
Origin: Radiative pressure, or radiation pressure, is the outward force exerted by the energy released from nuclear fusion reactions occurring in the star's core. In these reactions, lighter nuclei, such as hydrogen, fuse to form heavier nuclei, like helium, releasing a tremendous amount of energy in the process.
Mechanism: The energy produced by nuclear fusion is initially in the form of gamma rays. As these gamma rays travel outward from the core, they interact with the stellar material, transferring momentum and exerting an outward pressure. This process not only transports energy to the star's surface but also provides the necessary outward force to counteract the inward pull of gravity.
Hydrostatic Equilibrium
Hydrostatic Equilibrium
Definition: The balance between the outward radiative pressure and the inward gravitational pull is known as hydrostatic equilibrium. It is this equilibrium that maintains the star's stability over most of its lifetime.
Dynamics: If the radiative pressure increases (for example, if the rate of nuclear fusion rises), it would push against gravity more strongly, causing the star to expand. Conversely, if the radiative pressure decreases (for example, if the fuel for nuclear fusion becomes scarce), gravitational forces would dominate, causing the star to contract. These adjustments help the star maintain hydrostatic equilibrium.
Role in Stellar Evolution
Role in Stellar Evolution
Main Sequence: During the main sequence phase of a star's life, which is the longest phase, the balance between gravitational pull and radiative pressure is relatively stable. The star efficiently fuses hydrogen into helium in its core, maintaining equilibrium.
Post-Main Sequence: As the star exhausts its hydrogen fuel, this balance is disrupted. The core contracts under gravity, leading to higher temperatures and pressures that can initiate the fusion of heavier elements (in stars massive enough to do so). The outer layers may expand, and the star evolves into a red giant or supergiant.
End Stages: The final stages of a star's life depend on its mass and how the balance between gravitational pull and radiative pressure is ultimately resolved. Low to medium mass stars may shed their outer layers and end as white dwarfs, while massive stars may undergo supernova explosions, leaving behind neutron stars or black holes.
Analysis Questions
Analysis Questions
1. Explain how the process of nuclear fusion contributes to the stability of a star. Include in your explanation the types of particles involved and the outcome of their fusion.
2. Discuss the role of hydrostatic equilibrium in the lifecycle of a star, particularly how it affects the transition from the main sequence phase to the red giant phase.
3. Analyze the effects of gravitational pull on a star's lifecycle, especially in the context of a star exhausting its nuclear fuel. How does this gravitational pull influence the star's evolution into its final stages?
4. Consider a star significantly more massive than the Sun. Discuss how the balance between gravitational pull and radiative pressure would differ in such a star compared to a star like the Sun, particularly in terms of the nuclear reactions taking place and the star's eventual fate.
1. Core
1. Core
Location and Function: The core is the innermost part of a star, where the temperature and pressure are incredibly high.
Nuclear Fusion: This is where nuclear fusion occurs, the process that powers the star. In stars like the Sun, hydrogen nuclei fuse to form helium, releasing enormous amounts of energy in the process. This energy is crucial for the star's luminosity and supports the star against gravitational collapse.
Conditions: The core's temperature can exceed 15 million degrees Celsius, with the pressure being billions of times that of Earth's atmosphere.
2. Radiative Zone
2. Radiative Zone
Location and Function: Surrounding the core, the radiative zone is where energy produced in the core is transported outwards by photons through radiation.
Energy Transfer: In this zone, energy moves slowly outward through the dense gas, being absorbed and re-emitted by ions in a process that can take thousands of years.
3. Convective Zone
3. Convective Zone
Location and Function: Above the radiative zone, the convective zone is where energy is transported by convection. Hot plasma rises, cools as it moves upwards, and then sinks back down to be heated and rise again.
Convection Cells: This process creates convection cells, similar to boiling water, which efficiently transfer energy from the interior to the surface.
4. Photosphere
4. Photosphere
Visible Surface: The photosphere is the lowest layer of the star's atmosphere and the part we see as the surface. It's where the light from the star is emitted into space.
Temperature: It's cooler than the inner layers, with temperatures around 5,500 degrees Celsius for the Sun.
Features: It contains features like sunspots, which are cooler, darker areas caused by magnetic activity.
5. Chromosphere
5. Chromosphere
Above the Photosphere: The chromosphere is a layer above the photosphere, visible during solar eclipses as a reddish glow.
Features and Phenomena: This layer is known for its spicules and flares, which are eruptions of plasma.
6. Corona
6. Corona
Outermost Layer: The corona is the outermost part of the star's atmosphere, extending millions of kilometers into space.
High Temperatures: Surprisingly, it's much hotter than the layers below, with temperatures in the millions of degrees. The reason for its high temperature is still a subject of research but is thought to be related to the star's magnetic field.
Solar Wind: The corona is the source of the solar wind, a stream of charged particles that flows out into the solar system.
Energy Production and Transport
Energy Production and Transport
Understanding how energy is produced in the core and then transported to the surface and beyond is crucial. The balance between gravitational forces pulling matter inward and the outward pressure from nuclear fusion and thermal motion of particles is what keeps a star stable over its lifetime.
Importance in Physics
Importance in Physics
Studying the internal structure of stars provides insight into nuclear reactions, matter under extreme conditions, and the fundamental forces of nature. It also helps us understand the lifecycle of stars, from their formation to their eventual demise as white dwarfs, neutron stars, or black holes, depending on their initial mass.
Multiple Choice Knowledge Questions:
Multiple Choice Knowledge Questions:
1. What is the primary force that opposes gravitational pull in a star to maintain its stability?
- A) Magnetic force
- B) Radiative pressure
- C) Electric force
- D) Centrifugal force
2. Which process is responsible for generating the energy that creates radiative pressure within a star?
- A) Chemical reactions
- B) Nuclear fission
- C) Nuclear fusion
- D) Thermal conduction
3. What is hydrostatic equilibrium in the context of stellar physics?
- A) The balance between the magnetic forces and electric forces within a star
- B) The equilibrium between the centrifugal force and gravitational pull in rotating stars
- C) The balance between radiative pressure pushing outward and gravitational pull pushing inward
- D) The steady state of nuclear reactions occurring in a star
4. During which phase of a star's life is the balance between gravitational pull and radiative pressure relatively stable?
- A) Red giant phase
- B) Main sequence phase
- C) White dwarf phase
- D) Supernova phase
5. What can cause a star to expand?
- A) A decrease in nuclear fusion rate
- B) An increase in radiative pressure
- C) A decrease in gravitational pull
- D) A decrease in radiative pressure
Intrastella Processes
Intrastella Processes
4 Hydrogen to 1 Helium Fusion Reaction
4 Hydrogen to 1 Helium Fusion Reaction
3 Helium to 1 Carbon Fusion Reaction
3 Helium to 1 Carbon Fusion Reaction
Stars are not static objects. The lifetime of a star is determined by how massive it is: high mass stars are relatively short lived, whereas low mass stars last for longer. As a star consumes fuel in its nuclear reactions, its structure and composition changes, affecting its color and luminosity. Thus, the H-R diagram not only shows us the colors and luminosities of many stars, it shows these stars at different stages in their evolutionary histories.
Main Sequence: All stars on the main sequence have interiors hot enough fuse four hydrogen atoms into one helium atom, and this one helium atom is 0.7% lighter (Binding Energy - Fusion) than 4 hydrogen atoms were. The lost mass is converted into energy, and this energy is released, providing the star's luminosity.
Post-Main Sequence: Over billions of years, however, the residual helium in the star's core accumulates. When enough helium has accumulated, the helium can also undergo nuclear reaction. In this reaction, three helium atoms are converted into one carbon atom. The helium-burning nuclear reaction can occur only when the star's interior reaches a higher temperature (incr , and this higher temperature causes the star's outer surface to expand to a much larger size than it was while it remained on the main sequence.
Even though the core of the star is much hotter, the surface is now cooler, making the star redder. Thus, over time, a star becomes a red giant, moving from the main sequence area in the center of the H-R diagram to the red giant area in the upper right. The diagram below shows how a Sun-like star evolves on the H-R diagram.
End Stages: Eventually, all the helium in the core of the star is used up. At this point, what happens next depends on the mass of the star.
The heaviest stars, over six to eight times as massive as our sun, have enough pressure in their cores to start fusing carbon. Once carbon is gone, they explode as supernovae, leaving behind neutron stars or a black holes.
Less massive stars simply burn out, shedding their outer layers into beautiful planetary nebulae, and leaving the core as a hot white dwarf. White dwarfs lie in the lower left corner of the H-R diagram, a cosmic burial ground for dead stars.
Low mass stars evolve into red giants, and ultimately become white dwarfs and nebulae - our Sun will suffer a similar fate.
High mass stars, on the other hand, die spectacularly when their cores collapse under their own gravity, creating a massive supernova explosion. What's left behind is either a neutron star or a black hole. In fact, most of the elements in the periodic table are produced by supernova explosions, and they are only around today because of supernova explosions in the early universe.
Celestial Bodies
Celestial Bodies
Red Giants
Red Giants
Red giants are stars of similar mass as the Sun but at a later stage in their lifetime - the Sun will eventually become a red giant.
Red giants are much bigger than the Sun, and they have a relatively low surface temperature which is responsible for their red colour.
Red Dwarfs
Red Dwarfs
A red dwarf is a kind of star whose mass is between 0.08 and 0.4 times that of the Sun.
The energy released by nuclear fusion in a red dwarf is carried to the star's surface by the circular motion of the hot gasses inside it.
White Dwarf
White Dwarf
A white dwarf is what's left of the core of a red giant when it has lost its outer layers as nebulae. White dwarfs are extremely dense, with electrons packed very closely together in their core.
Chandrasekhar Limit
Chandrasekhar Limit
The Chandrasekhar limit is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about 1.4 M☉. The limit was named after Subrahmanyan Chandrasekhar.
Main sequence star:
A normal star that is undergoing nuclear fusion of hydrogen into helium.
Neutron Star:
Neutron Star:
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes, neutron stars are the smallest and densest known class of stellar objects.
Oppenheimer-Volkoff limit
Oppenheimer-Volkoff limit
Oppenheimer-Volkoff limits the largest mass a neutron star can have to approximately 2-3 solar masses. The uncertainty in this limit comes from the fact that the equation of state of the matter inside a neutron star is not precisely known.
Protostar:
Protostar:
A protostar looks like a star but its core is not yet hot enough for fusion to take place. The luminosity comes exclusively from the heating of the protostar as it contracts. Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum.
Brown Dwarfs:
Brown Dwarfs:
Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars.
Dim brown dwarfs can also be difficult to find; eventually the visible light left over from their birth fades completely beyond the red end of the visible spectrum, and they emit only infrared light
Blue Giants:
Blue Giants:
In astronomy, a blue giant is a hot star with a luminosity class of III or II. In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.
Nebula:
Nebula:
A nebula is a cloud of dust and gas, usually tens to hundreds of light years across. A galaxy is much larger — usually thousands to hundreds of thousands of light years across. Nebulae are one of the many things that galaxies are made of, along with stars, black holes, cosmic dust, dark matter and much more.
Stellar evolution on HR diagrams
Stellar evolution on HR diagrams
Taken from: Option D - Astrophysics : Stellar Evolution on HR Diagrams
From the description below, annotate your HR diagram with the appropriate labels and arrows.
The nebulae in space from which stars are created are actually the remains of a previous star that has reached the end of its lifecycle and died. Generally speaking, they consist of hydrogen and helium and small amount of the other heavier elements. The nebula, under the influence of gravity, begins to condense, and eventually, a protostar is formed.
The slow inwards collapse of clouds of interstellar matter (because of gravitational forces) is opposed by the random motions of the particles, creating an outwards gas pressure. In order for star formation to begin, the total mass of the gas cloud has to be great enough to create sufficient inwards gravitational forces to overcome the gas pressure. Then part of the cloud will collapse inwards until nuclear fusion begins and opposes the collapse with greater thermal gas pressure and radiation pressure outwards.
Stars form when a portion of interstellar cloud collapses gravitationally. The Jeans mass criterion is determined by asking when the magnitude of the gravitational potential energy exceeds the magnitude of the gas’s kinetic energy. For a given temperature, the minimum mass required of a cloud of interstellar matter for star formation is called the Jeans mass, MJ. The collapse of an interstellar cloud may begin if M>Mj where Mj is the Jeans criterion. (10.4). It is important to realize that the Jeans mass is very temperature dependent: a greater mass is required for star formation at higher temperatures.
Such protostars can be observed in nebulas such as the horsehead nebula and the crab nebula. It is in this stage that the process of nucleosynthesis begins. Nucleosynthesis, in contrast to the nuclear processes that we are used to on Earth, is fusion, not fission. That is, instead of splitting a heavy nucleus, light nuclei are smashed together and fuse to produce a heavier nucleus, and gamma rays. It is called the proton-proton cycle. The star will continue to react its core of hydrogen into helium for all of its main-sequence lifetime (see previous section: the nature of stars).
Once the star runs out of hydrogen, the core collapses, and, under the additional gravitational pressure, the hydrogen in the core will start to undergo fusion. This causes the outer layers of the star to expand, however, the outer layers also cool, and the star becomes a red giant. The core continues to react and elements such as carbon, neon, oxygen, silicon and iron are produced. It is here that the elements that compose our world are created. Without the stars then universe would be composed of hydrogen and little else.
When the star finally runs out of fuel completely; usually when the core becomes iron, the red giant star collapses. The next stage of the star is determined by the mass of that star and the Chandrasekhar limit.
If a star is below 1.4 solar masses (Type G), it is less that the Chandrasekhar limit and when it collapses, its forms a white dwarf of 1.4 solar masses or less, along with a planetary nebula. The white dwarf star continues to cool and eventually becomes invisible.
If a star is above 1.4 solar masses (Type A, B, O), it is above the Chandrasekhar limit and instead of becoming a regular red giant, it becomes a super red giant. In this case, when the star dies, it takes a rather more spectacular path than the star below the Chandrasekhar limit, becoming a supernova. Depending on the mass of the star, it will either go on to become a black hole or a neutron star.
For stellar masses less than about 1.4 solar masses, the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star so the collapse is halted by electron degeneracy to form white dwarfs. Electron degeneracy is a stellar application of the Pauli Exclusion Principle, as is neutron degeneracy. No two electrons can occupy identical states, even under the pressure of a collapsing star of several solar masses.
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