Astrophysics: Stellar Physics and the Sun

Astrophysics Overview

  • Astrophysics explores physics beyond Earth, focusing on celestial objects like stars, galaxies, and black holes.
  • Space offers extreme conditions for physics, including high temperatures, large scales, and intense energies.
  • Discussion of Tim Curry and space.

Stellar Physics: The Sun

  • Stellar physics focuses on stars, including our sun.
  • Common misconception: Many people don't realize the sun is a star.
  • The sun's proximity makes it appear brighter and hotter than other stars.

Solar Observations

  • Solar telescopes are sometimes set up in front of Thornton Hall for direct sun observation.
  • Through a telescope, the sun appears as a featureless orb of plasma.

Plasma State

  • Plasma is superheated matter where electrons separate from atoms.
  • Free electrons make plasma highly reactive to electric and magnetic fields.
  • Electromagnetic activity on the sun causes ejections of material into space.

Sunspots

  • Sunspots are cooler regions on the sun's surface, often associated with material ejection.
  • Hot objects produce light; hotter objects emit more intense, bluer light, while cooler objects emit redder light.
  • Sunspots appear dark because they are slightly cooler than the surrounding area, though still around 4,000 degrees.

Electromagnetism in Space

  • Electromagnetism exists in space where charged particles are present.
  • The sun and planets like Jupiter have strong electromagnetic fields extending far into space.
  • Electromagnetic fields are visualized through mappings.

Stellar Classification

  • Stars vary in size, brightness, and color.
  • Classification systems categorize stars based on observed characteristics.
  • Early classification focused on hydrogen absorption spectra.

Hydrogen Absorption

  • Initial classification ordered stars by the intensity of hydrogen absorption lines.
  • Stars were labeled A, B, C, etc., based on absorption intensity.

Temperature and Color

  • Astronomer (possibly Harrietta Leavitt Swan) suggested organizing stars by surface temperature based on color.
  • Bluer stars are hotter; redder stars are cooler.
  • The letter-based classification was retained, resulting in the non-sequential order: O, B, A, F, G, K, M.

Modern Spectral Classification

  • The OBAFGKM sequence orders stars by temperature, from hottest (O) to coolest (M).
  • Surface temperature indicates the temperature and dynamics of the star's core.
  • Stars primarily consist of hydrogen with interesting core dynamics.

Stellar Appearances

  • Surface color reveals temperature, but brightness alone doesn't indicate size.
  • Brightness depends on both the star's luminosity and its distance from Earth.
  • Hotter, bluer stars are generally, but not always, larger.

Stellar Sizes

  • Stars vary significantly in size; O stars are much larger than M stars.
  • Some stars evolve into giant or supergiant stars.
  • The sun is an average-sized, G-type star.
  • Comparisons of planets to star sizes.

Blackbody Spectrum

  • A star's blackbody spectrum defines its color based on temperature.
  • The sun's temperature of about 5,700 degrees Kelvin places it in the G-type class.

Spectral Absorption Lines

  • Analyzing spectral absorption lines reveals the elements present in a star's atmosphere.
  • Each element has a unique atomic structure and absorbs light at specific wavelengths.
  • Elements like hydrogen, iron, oxygen, sodium, calcium, and magnesium can be identified in the sun's spectrum.
  • XKCD Comic reference.

Hertzsprung-Russell Diagram

  • The Hertzsprung-Russell (H-R) diagram plots stars by temperature (or spectral class) and luminosity (brightness).
  • Most stars lie on the main sequence, following the rule that hotter stars are brighter.
  • Giants and supergiants are outliers, being much brighter than main sequence stars of the same temperature.
  • The HR Diagram maps out the color and brightness of a star.

Notable Stars

  • The stars Rigel and Betelgeuse are part of the Orion constellation.
  • Betelgeuse is a red giant, relatively cool.
  • Rigel is a blue supergiant, very hot.
  • Constellations are largely arbitrary patterns seen in the sky and used in cultural stories.

Stellar Evolution

  • Stars evolve and change over time, including the sun.
  • The sun is about halfway through its life cycle and will eventually reach an end state.
  • Stars are not truly alive, but they do change and evolve.

White Dwarfs

  • The sun's final form will be a white dwarf, a small, dim remnant of the core.
  • White dwarfs are much smaller and less luminous than the sun is today.

Future of the Sun and Earth

  • The sun will eventually die, ending life in the solar system.
  • In approximately one billion years, the sun's increasing heat output will boil away Earth's oceans.
  • There may be an upper limit to life on earth.

Real HR Diagram Data

  • The HR diagram can be plotted with real data from thousands of stars.
  • Most stars fall on the main sequence, with giants and supergiants off to the side.
  • White dwarfs are also visible on the diagram, representing the end states of stars.

Solar System Formation: From Stardust

  • The solar system formed from a nebula of gas and dust.

Nebula

  • Giant clouds of gas and dust provide the raw materials for stars and planets.
  • Gravity causes the dust to coalesce which creates overdensities.

Energy Transformation

  • Gravitational potential energy converts to kinetic energy and then to thermal energy (heat) as dust particles collide.
  • The nebula begins to rotate.

Protoplanetary Disk

  • A rotating blob of material forms a protoplanetary disk.
  • Centrifugal force causes the material to flatten into a disk.
  • Real images of protoplanetary disks show dust lanes where planets are accreting.
  • The earth, as well as other planets come from these disks rotating around the star.

Planet Formation

  • Gravity coalesces dust in the disk, forming planets.
  • Earth, Mars, Jupiter, and Saturn all originated from the same disk.
  • Studying other forming star systems allows us to witness planet formation.

Sun Ignition

  • The sun "turns on" when nuclear fusion begins in its core.
  • The rotation of dust can be observed.

Solar Wind

  • The sun's light and solar wind disperse dust from the protoplanetary disk.
  • Planets sweep up remaining dust as they form.

The Sun's Interior: Fusion Engine

  • Cross-section views of the sun reveal its internal layers (photosphere, chromosphere, corona, convective zone, radiative zone, core).
  • It takes light 10,000 - 170,000 years to make it's way out from the center of the sun to the surface, according to the image. (Other estimates were at one million years.)
  • Neutrinos only take about 2.3 seconds.

Solar Layers

  • Photosphere: Where photons are made; about 5,700 degrees.
  • Chromosphere: Where the sun's atmosphere resides, this is where some of the light created from the photosphere is absorbed.
  • Corona: The outermost layer, millions of degrees hot, though diffuse.
  • Solar flares and prominences: Electromagnetic ejections of material powered by the electromagnetic force.

Solar Density

  • The density of the sun on the surface is very diffuse; around .0002 grams per cubic centimeter.
  • In comparison, water is 1 gram per cubic centimeter.

Convective Zone

  • Heat is transferred via convection; hot material rises, cools, and sinks.
  • Temperature drops as the material moves towards the surface.

Radiative Zone

  • Energy is transported via radiation (photons).
  • Light bounces around inside the matter.
  • The density of the core moves up to that of 20 grams per cubic centimeter.

Core: Nuclear Fusion

  • The core reaches 15-16 million degrees and a density of 150 grams per cubic centimeter.
  • The earth would be squashed by the force.
  • Nuclear fusion occurs in the core, converting hydrogen to helium.

Nuclear Forces

  • Gravity:
    • Is the most important fundamental force of the universe.
    • Allows dust and material to coelesce to form structures.
  • Electromagnetism:
    • Is what creates the balance inside planets and stars in order to ensure that the particles don't squish together.
  • Strong Nuclear Force:
    • Sticks atomic nuclei together to form things like hydrogen, helium, etc.
  • Weak Nuclear Force:
    • transmutes protons into neutrons.

Proton-Proton Chain

  • The proton-proton chain converts hydrogen to helium, releasing energy and producing neutrinos and gamma rays.
  • Gamma rays in the radiative zone is where many of the photons in the process were created.
  • Fusion converts mass into energy, powering the sun.

Stellar Equilibrium

  • A push and pull between gravity and fusion.

Hydrostatic Equilibrium

  • There is hydrostatic equilibrium.
  • Gravity would love to squish and condense everything into a smaller size, if it could.
  • Fusion, which is pushing outwards is working to prevent those particles from squishing down.
  • Describes that there is not an expansion or construction of the planets.
  • Maintains the stability of a star.

Gradual Changes

  • As helium accumulates, fusion becomes easier, and the sun gradually expands.
  • A slight release valve of energy and matter that are used in creating a new star.
  • This process will eventually evolve the sun into a red giant.

End States: Red Giants and Nebulae

  • The sun will eventually evolve into a red giant, potentially engulfing the earth.
  • After the red giant phase, the sun may become a planetary nebula, ejecting its outer layers.

Stellar Recycling

  • Material from dying stars enriches new stellar nurseries.
  • Our sun and solar system formed from the remnants of previous stars.

Solar Activity

  • Time-lapse videos show the sun's rotation and surface activity (solar flares, prominences, sunspots).
  • Follows a cycle.

Magnetic Fields

  • Magnetic fields, generated by moving charged particles, influence the plasma on the sun's surface.
  • There is an oddly enough a solar cycle that follows solar maximums, and solar minimums during this part of the cycle.

Coronal Mass Ejections (CMEs)

  • CMEs are large ejections of solar material and magnetic fields.
  • Intense emissions of energetic particles can disrupt cameras and satellites.

Geomagnetic Storms

  • On occasion the CMEs can result in geomagnetic storms of energetic particles, under certain conditions.

Earth's Magnetic Field

  • The earth's electromagnetic magnetic field protects the planet from most charged particles from the sun.
  • Energetic particles get deflected. The bulk of the energetic particles generally don't make it to Earth.

Auroras

  • During geomagnetic storms, some charged particles enter Earth's atmosphere near the poles which create Auroras.

Atmospheric Interactions

  • Charged particles transfer energy to atmospheric molecules (oxygen, nitrogen).
  • Electrons are boosted to higher energy levels and the atoms absorb certain kinds of energy.
  • Atoms release energy in the form of light.

Northern Lights (Aurora Borealis)

  • A direct correlation between gravity pulling inwards and fusion with light being dumped into our atmosphere.
  • Auroras are produced by energized oxygen and nitrogen atoms fluorescing.
  • They are more frequent near the poles because high energy particles pierce through the electro magnetic field.