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Chapter 14: Our Star

A Closer Look at the Sun

• Learning goals include understanding why the Sun shines and what its structure is.

Why Does the Sun Shine?

• The Sun shines because chemical and gravitational energy cannot sustain its luminosity for more than 25 million years.
• Gravitational equilibrium sustains the Sun's core, keeping it hot and dense enough for nuclear fusion and energy release.
• Nuclear energy can power the Sun, as described by E=mc².
• The chemical energy content divided by luminosity is approximately 10,000 years indicating this is not the reason for the Sun's shine.
• The gravitational potential divided by luminosity is about 25 million years, which is also not the reason the Sun shines.
• The nuclear potential energy of its core divided by luminosity is approximately 10 billion years, which is the reason that the Sun shines.

The Stable Sun

• Energy supplied by fusion maintains the pressure that balances the inward crush of gravity.
• Pressure is greatest deep in the Sun, where the overlying weight has the greatest impact.
• The rate at which energy radiates from the surface is the same as the rate at which it is released by fusion in the core.
• Gravitational contraction provided the energy that heated the core as the Sun was forming.
• Contraction stopped when fusion began creating enough outward pressure, achieving gravitational equilibrium.

What is the Sun's Structure?

• The Sun consists of the solar wind, corona, chromosphere, photosphere, convection zone, radiation zone, and core.

The Sun's Atmosphere

• The solar wind is a flow of charged particles from the Sun's surface in all directions.
• Charged particles escape along magnetic field lines until becoming part of the solar wind.
• The corona is the outermost layer of the solar atmosphere, with a temperature of ~1 million K (10^6 K).
• The corona is observed via X-rays and is of very low density.
• The density of matter in the corona increases as weight descends through the layers.
• The chromosphere is the middle layer of the solar atmosphere, with a temperature between 10,000-100,000 K (10^4-10^5 K), observed in ultraviolet (UV) light.
• The photosphere is the visible surface of the Sun, emitting visible light, and has a temperature of ~5800K.
• Density in the photosphere is less than Earth's atmosphere, and sunspots form here.
• The convection zone transports energy upward by rising hot gas (plasma).
• Each layer of the Sun and other stars are spheres of plasma, consisting of ions and electrons.
• The radiation zone transports energy upward by way of photons.
• The core generates energy through nuclear fusion at approximately 15 million K.
• The core's density is 100 times that of water, and its pressure is 200 billion times greater than Earth's surface.

Basics of Light and Observations of It

• As temperature increases, so does the energy of released photons which will be described in chapter 15.
• The energy of photons determines the wavelength and frequency of light.
• The corona emits X-rays due to its temperature of ~10^6 K (1 million K).

What Have We Learned?

• Why does the Sun shine?
• The Sun shines because gravitational equilibrium keeps the core hot and dense enough to release energy through nuclear fusion.
• What is the Sun's structure?
• The layers from inside out are: Core, radiation zone, convection zone, photosphere, chromosphere, and corona.
• The core is the hottest part of the Sun's structure.

14.2 Nuclear Fusion in the Sun

Nuclear Fusion in the Sun

• The learning goals cover how nuclear fusion occurs in the Sun, how energy from fusion exits the Sun, and how we know what's happening inside the Sun.
• At high speeds, the nuclei of atoms come close enough for the strong force to bind them together.
• Fission involves a big nucleus splitting into smaller pieces (e.g., nuclear power plants).
• Fusion involves small nuclei sticking together to make a bigger one (e.g., stars.)
• High temperatures enable nuclear fusion to happen in the core with strong force at small distances and is 100 times stronger than the EM force.
• Electromagnetic repulsion prevents collision of nuclei at low speeds.
• The Inverse Square Law: 1/r^2 for EM and gravitational forces
• The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.
• The proton-proton chain is how hydrogen fuses into helium in the Sun.
• In hydrogen fusion:
• Four protons go in.
• One helium-4 nucleus, two gamma rays, two positrons, and two neutrinos come out.
• The total mass is 0.7% lower.
• What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?
• The core would expand and cool which is how the solar thermostat keeps burning rate steady.
• A decline in core temperature causes fusion rate to drop, so the core contracts and heats up.
• A rise in core temperature causes fusion rate to rise, so the core expands and cools down.
• Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons. This is called radiative diffusion.
• Convection, involving rising hot gas, transports energy to the surface.
• Bright blobs on the photosphere show surface areas where hot gas is reaching the surface.
• Internal conditions are defined using mathematical laws of physics.

How We Learn About the Inside of the Sun

• Solar vibrations are observed.
• Solar neutrinos are observed.

Learning About the Inside of the Sun

• Making mathematical models which use the laws of physics.
• Basic models of a star's composition and mass can solve equilibrium and energy balance equations.
• Computer models can determine pressure P, temperature T, and density at all measured depths of the Star.
• Good models accurately predict star properties like size, luminosity, age, and surface temperature.

Solar Vibrations

• Vibration patterns on the Sun’s surface reveal qualities of what the Sun is like on the inside.
• The Sun vibrates, causing ripples on the surface that are revealed by way of Doppler shifts.The data that is collected from these vibrations coincides with mathematical models of the Sun's interior.
• This study is known as helio-seismology.

Solar Neutrinos

• Neutrinos created during fusion directly fly through the Sun.
• These neutrinos only interact with weak and gravitational forces.
• Observations of these solar neutrino can tell us more of what happens in the Sun.
• Early neutrino searches failed to find the predicted number.
• More recent observations find the right number of neutrinos, some have been known to have changed their form.
• Neutrinos can change passing through matter; early detectors weren't capable to detect the change.

What Have We Learned?

• How does nuclear fusion occur in the Sun?
  • The core has extreme temperature and density which is optimized for nuclear fusion of hydrogen to helium through the proton-proton chain.
  • Gravitational equilibrium acts as a thermostat to regulate the core temperature, since fusion rate is very sensitive to temperature.
• How does the energy from fusion get out of the Sun?
• Randomly bouncing photons carry energy through radiation zones.
• Rising hot plasma carries energy through the convection zone.
• How know what is happening inside the Sun?
  • Mathematical models agree with observations of solar vibrations and solar neutrinos.

14.3 The Sun-Earth Connection

• the learning goals address what causes solar activity and how it varies over time.

What Causes Solar Activity?

Solar flares. Sunspots. Solar prominences. All are connected to magnetic fields.

Solar Activity

Solar Activity (Sunspots)

• Sunspots are cooler than other parts of the Sun (4000 K).
• Sunspots exist in regions with strong magnetic fields.
• High magnetic fields may split absorption bands observed in spectra of sunspots.
• Lines closer together indicate a strong magnetic field.
• Loops of bright gas often connect sunspot pairs.
• Pairs of sunspots connect tightly with magnetic field lines.
• Charged particles spiral along magnetic field lines.
• Magnetic activity causes solar prominences, that erupt high above the Sun's surface.
• The corona appears radiant in X-ray photos where the magnetic fields trap hot gas.
• Coronal mass ejections are driven by the releases of electromagnetic energy. They are caused by the energetic releases of plasma from stressed regions in the Sun's magnetic field corresponding to sunspot groups or prominences.
• A coronal hole forms where the magnetic field extends far out into space. This allows particles there to escape to become part of the solar wind.
• Magnetic activity causes solar flares that send X-rays and charged particles into space, which are the most explosive form of coronal mass ejections.

Coronal Holes

• Dark regions exist in X-ray photos near the poles where magnetic field lines extend into the space.
• This results in charged particles escaping and becoming part of solar winds.

Effects of Solar Activity on Earth

• Charged particles from the Sun disrupt electrical grids and disable communication satellites.

Time Variance of Solar Activity

• A graph shows sunspot numbers and how they change with time.
• The vertical axis reflects sunspots that cover the Sun's surface.
• There is an approximate 11-year sunspot cycle.
• There is an interesting outcome in the solar cycle, where magnetic poles flip every 11 years and it takes 22 years for the process to return to its starting point.
• There are graphs that indicate latitudes where shifted groups of sunspots exist during the cycle.
• There are long-term variances that date back hundreds of years.
• There is indirect information, such as the information found between tree rings via Carbon-14 patterns, that has given scientist data on weather patterns and other atmospheric cycles.
• The sunspot has to do with the winding and twisting of the Sun's magnetic field along the Earth and other planets in the universe. This causes magnetic poles to flip which resets the cycle.
• The amount of sunlight that reaches Earth remains constant over these 11-year cycles, even as Earth continues to warm.

What Have We Learned?

• What causes solar activity?
• Stretching and twisting of magnetic field lines near the Sun cause solar activity.
• How does solar activity vary with time?
  • The activity waxes and wanes on a roughly 11-year timescale.

Chapter 15: Surveying the Stars

15.1 Properties of Stars

• The learning goals encompass understanding the meaning of and how to measure stars’ luminosities, temperatures, and masses.

Stellar luminosities

How Do We Measure Stellar Luminosities?

• Luminosity is the total power (energy per second) emitted by a star into space.
• Apparent brightness is the amount of energy (starlight) reaching Earth per second per square meter.
• It depends on both luminosity and distance.
• Alpha Centauri and the Sun have about the same luminosity, but the Sun appears brighter to us.
• The amount of luminosity passing through each sphere is the same, with the area of a sphere being 4π x (radius)^2. You can measure this by dividing luminosity by area to get brightness.
• ApparentBrightness=L(total-luminocity)/4π×distance2. if the star's luminosity could be accurately determined, it is possible to measure its distance and apparent brightness.
• Detectors don't measure light across the EM Spectrum. Total lumen can't equal lumen.
• Instead, luminosity (bolometric luminosity) and brightness describes lumen measurement or brightness by EM Spectrum.

Stellar Distance

• Stellar distances are measured through something called stellar parallaxing.
• Parallax is the apparent shift, where objects appear to move against a backdrop.
• That parallax angle is dependent on distance.
• Parallaxing is the process of comparing snapshots against different times in history to find out if any angled changes have occurred as one looks toward the stars.
• The equations of stellar paralax depend on if the values are metric, parsecs, AU or light years.
• The smaller the parallax angle, the greater the distance is from the star.
• We can represent arcseconds as arcsecs or "
• π (radians) = 180° and 1° = 60 arcmin = 3600 arcsec
• Luminosity = 4π (distance)² × (brightness)
• The brightness of a star depends on both distance and luminosity
• The range of most radiant stars is 10^6 Lsun while the range that accounts for the least lumen range is 10^-4Lsun.
• This value is derived by apparent magnitude.
• A fainter star can be 6.
• Hipparchus originally designated this system (190-120 BC).

Stellar Temperatures

• The colour of a star can help one accurately figure out its temperature .
• When one looks at EM Emission Spectra of a star, it can tell us the surface temperature.
• Each object emits thermal radiation by thermal radiation.
• As an object of fixed size becomes a Fixed-Size Object, luminosity will also continue to rise during the same time period.
• The properties of thermal radiation:
• Hotter objects emit more light per unit area, across a wider range of frequencies.
• They also emit photons, which have a higher-than-average energy (at their peak).

Spectral Types

• Absorption can occur through gaps in the spectrum, with the light passing by the gas.
• Types of Spectra:
  • Continuous Spectrum: Encompasses wavelengths with hot and dense light sources.
  • Absorption Spectrum: Some light is absorbed while the rest is transmitted which wavelengths that are absorbed depend on elements.
  • Emission Spectrum: Gas emits a spectrum that corresponds with emitted gas which relies on composition and density.
• There are also corresponding lines that relate to temperature.
• We can express temperature with a easy to remember phase where OBAFGKM is Cooler to Hotter.
• To further classify spectral types, astronomers further organize, or sub classify the numbers or letters used in the method.

The Pioneers With Help From Others

• Annie "Jump" Cannon developed the foundation of today's current classification style while supported by other scientists who called computers at Harvard.

Stellar Masses

• Mass is measured through the gravitational properties.
• Mass measurements can only occur in Binary stars can offer.
• These equations used newtons Version of Kepler's 3rd Law.
• With that, an explanation would relate to period and separated a star, then astronomers could accurately find each star through the data.

Types of Binary Star Systems

• Eclipsing Binary: Can find out and determine the masses better by following transitions.
  • Can measure periodic eclipses where binary stars allows on the true measurement, and radii.
• Kepler's 3rd Law, p2 = a3, for all 3 Binary stars

Visual Binary System

• Directly measure star-orbital motions

Spectroscopic Binary System

• A system where you track the shifts through Doppler.

Eclipsing Binary System

• Stars in alignment with the line of sight allows one to see shifts in light

Stellar Masses

• The weight of all stars. is 100 M Sun.
• Small stars are .08 M Sun

What Have We Learned? (How do we measure properties of stars)

• Luminosities are measured through, Inverse Square Law, parallax.
• Temperature can be determined best, by use spectral type, and color.
• Kepler's 3rd Law is how the masses are also tracked and that is by way binary star system. Studying transits can indicate mass of the star more correctly

15.2 Star Patterns (H-R Diagram)

• What it is, significance behind it as well, giant supergiants and dwarfs. Why some are.

• HR Diagrams, can depict all the properties of stars temperature, spectral type

• Also referred with the term bolometric

  • Stars in high luminosity would be higher temperature due to, radius.

• These stars are called and supergiants by brightness, volume

• Stars are high temperatures so the ratio, or diameter needs more light than white dwarf.

Stellar Luminosity

• Full classification of what luminosity is.
• This process is referred to as Sun at G and or any other.

The Significance Behind these diagrams

• All stars on this list are not really. They just give off heat which relates to radiation type
• These masses were noted to exist during the main sequence, which is a phase of the HR diagram, one with short lives and long lives.

The most fundamental part of light?

• Both core radius and temperature are the major forces that balance out to create gravitational effects

Solar Vibrations

Solar Neutrinos

• Neutrinos made through, Fly over through the sun
• They can only effect by weaks that are gravitational forces
• Observations about the tell core or how their
• Early in the search, they could not be determined in the amount.
• There has been some recent observations. There now that the total neutrinos can now change forms.

What Have We Learned?

• HR Diagrams can stellar and surface colors.
• Normal stars that are used up, they release heat.
• This can effect how these stars have their high mass is blue and vice versa

15.3 Star Clusters

• Two Types: Are their clusters and measure how old.
• Most clustered packed by 1 thousands of stars.
• The other can pack one Million starts, a large ball is gravity
• The order, is first by stars, then yellow then red is released.
• Stars, must form at the same time and distance.
• Open type stars, has to exist.
• Galaxy halo exist.

Star Time

• The radius shows few stars that live under the sun.

What Have We Learned From This:

• What are the types?
  • Open are loosely packed and thousand type stars, and more stars are in dense packages.
• How do we measure it?
  • It is measured by how much more mass is still left.

16.1 Stellar Nurseries

• The learning objectives are to learn where do stars form, and why?

Where Do Stars Form?

• Stars are born in dark clouds and the dust in interstellar

Forming Clouds

• All stars live in dark clouds of gas also help make up the medium.
• We used the composition of interstellar gas lines of stars.
• With up 70 percent and more of mass is always in their environment which is at temperature.
• A has to have one or more elements .
• The matter has to be cold and be in order 10-3 K 30. K must be with cold dense temperature.
• Most knowledge with a few lines but the H2 gas.

Interstellar Dust

• The particles are small they are known better as smoke/sand in the sky that block elements.

Interstellar Reddening

• Stars, go the edges will appear more red due to more short waves
• When happens then what happened.
• The visible is what is happening over barnard.

Glowing Dust Grains

• This helps produce some new stars.
• The infrared is still bright by dust.

Mass of a Star Forming Cloud

• The cloud has to be about a couple and be even for there be pressure.
• The CO can go to a point will lose a certain amount of heat. We must use some other factors
• Those parts all have some elements will resist.
• Other gravity forces are, and it is easier to collapse when there are smaller
• More of new stars than the rest