Exploring the Interstellar Medium, Stellar Evolution, and Galaxies

Posted on Jul 24, 2024 in Physics

Chapter 15: Interstellar Medium

The interstellar medium, composed of gas and dust between stars, plays a crucial role in stellar evolution. Let’s delve into its key characteristics:

Composition and Density

  • Primarily gas with a small fraction (1%) of dust.
  • Extremely low density.

Interstellar Extinction and Reddening

  • Solid dust grains (up to 300 nm) scatter and absorb light.
  • Blue light is scattered more effectively, leading to the reddening of starlight.

Dust Grain Emission

  • Dust grains absorb starlight and heat up to temperatures of 30-100 K.
  • They emit infrared radiation.

Interstellar Clouds

  • Interstellar clouds exhibit a range of temperatures and densities.
  • Some clouds reach temperatures of 106 K and emit X-rays. The Sun resides in such a bubble.
  • Intercloud gas has a temperature of around 8000 K.

HII Regions

  • HII regions are characterized by ionized hydrogen and temperatures of 104 K.

Neutral Hydrogen

  • Cooler neutral hydrogen emits radio waves at a wavelength of 21 cm.

Molecular Clouds

  • Molecular clouds, primarily composed of H2, are dense, cold (10 K), and appear dark.
  • Star formation occurs in the denser regions of these clouds.

Protostar Formation

  • Gravitational collapse within molecular clouds leads to protostar formation.
  • Spinning protostars form disks, which can evolve into planetary systems.
  • Protostars are bright in infrared due to their heat from gravitational contraction.

Protostar Energy Source

  • Gravity provides the energy source for protostars.

Main Sequence Transition

  • Protostars become main sequence stars when nuclear fusion ignites in their cores.

Brown Dwarfs

  • Brown dwarfs are objects not massive enough to initiate sustained nuclear fusion.

Hayashi Track

  • The Hayashi track describes the evolutionary path of protostars on the Hertzsprung-Russell diagram.

Chapter 16: Stellar Evolution and Stellar Types

Stars, the building blocks of galaxies, undergo a fascinating life cycle. Their mass dictates their fate and the rate of nuclear fusion within their cores.

Gas Distribution and Star Formation

  • Most gas in a galaxy resides in the galactic disk, where it collapses to form stars.

Low-Mass Stars

  • Stars with masses less than a certain threshold (M < …)

Chapter 17: Intermediate and High-Mass Stars

Intermediate-Mass Stars (3 Msun – 8 Msun)

  • Significantly more luminous than low-mass stars.
  • Shorter lifespans due to higher fusion rates.
  • Experience different fusion processes and exotic final stages.
  • Derive energy from the CNO cycle during their main sequence phase.

Luminous Blue Variables (LBVs)

  • Extremely massive and unstable stars.
  • Eject significant amounts of mass (up to 20%) through powerful outbursts.

High-Mass Stars

  • Possess convective cores that facilitate the transport of fuel for fusion.
  • Burn heavier elements, extending their fuel supply.
  • Experience nondegenerate helium core burning after leaving the main sequence.

Instability Strip

  • A region on the Hertzsprung-Russell diagram where stars exhibit pulsations.
  • Cepheid variable stars and RR Lyrae variable stars are found in this strip.
  • The period-luminosity relationship of these stars is crucial for distance measurements.

Element Fusion and Binding Energy

  • High-mass stars can fuse elements up to iron (Fe).
  • Iron fusion consumes energy instead of releasing it due to its high binding energy.

Neutrino Cooling

  • Neutrinos, weakly interacting particles, carry away energy during the fusion of heavier elements.
  • This reduces the amount of “usable” energy released.

Core Collapse and Supernovae

  • The iron core of a massive star collapses under its own gravity when it exceeds a critical mass.
  • This collapse triggers a Type II supernova explosion, enriching the interstellar medium with heavy elements (nucleosynthesis).

Neutron Stars

  • If the remnant core of a Type II supernova has a mass between 1.4 and 3 Msun, it forms a neutron star.
  • Neutron stars are incredibly dense, with radii of about 10 km.
  • They can be observed as pulsars or in X-ray binaries.

Black Holes

  • If the remnant core is more massive than about 3 Msun, it collapses further into a black hole.
  • Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape.

Chapter 19: Galaxies and the Milky Way

Early Attempts at Mapping the Galaxy

  • William Herschel (1785) attempted to map the Milky Way by counting stars.
  • His assumptions of uniform stellar luminosity and lack of extinction limited the accuracy of his map.

Harlow Shapley and Globular Clusters

  • Harlow Shapley studied the distribution of globular clusters, dense groups of old stars.
  • He discovered that the Sun is not located at the center of the Milky Way.

Classifying Galaxies

  • Edwin Hubble developed a classification system for galaxies, often depicted as a “tuning fork” diagram.
  • Spiral Galaxies: Flattened disks with spiral arms, a central bulge, and ongoing star formation.
  • Elliptical Galaxies: Smooth, ellipsoidal shapes with little gas and dust, and older stellar populations.
  • Irregular Galaxies: Lack a defined shape and often result from galactic interactions.

Spiral Galaxies

  • Classified as either barred or unbarred, based on the presence of a central bar-shaped structure.
  • Further categorized by the tightness of their spiral arms and the size of their central bulge.

Elliptical Galaxies

  • Classified by their ellipticity, ranging from E0 (spherical) to E9 (highly elongated).
  • Can be dwarf galaxies or giant elliptical galaxies.

Irregular Galaxies

  • Lack a regular shape and often exhibit ongoing star formation.

Stellar Orbits

  • Different galaxy types exhibit distinct stellar orbits.
  • Spiral galaxies have stars orbiting in a disk, while elliptical galaxies have stars in more random orbits.

Gas in Galaxies

  • Elliptical galaxies contain mostly hot gas, while spiral galaxies have both hot and cold gas.
  • Star formation occurs in the cold gas of spiral galaxies.

Spiral Arms

  • Spiral arms are thought to be density waves that propagate through the galactic disk.
  • These waves trigger star formation as they compress gas and dust.

Galactic Disk Warping

  • The disks of spiral galaxies can be warped due to interactions with neighboring galaxies or internal processes.

Stellar Halos and Tidal Debris Streams

  • Stellar halos surround galaxies and contain stars, globular clusters, and tidal debris streams.
  • Tidal debris streams are remnants of smaller galaxies that have been disrupted by the gravitational pull of a larger galaxy.

Rotation Curves and Dark Matter

  • Rotation curves plot the orbital velocities of stars and gas as a function of distance from the galactic center.
  • Observations of flat rotation curves in spiral galaxies indicate the presence of dark matter.
  • Dark matter is a non-luminous form of matter that interacts gravitationally but does not emit light.

Fritz Zwicky and Vera Rubin

  • Fritz Zwicky (1930s) first proposed the existence of dark matter based on observations of galaxy clusters.
  • Vera Rubin (1970s) provided strong evidence for dark matter through her studies of galaxy rotation curves.

Gravitational Lensing

  • Gravitational lensing, the bending of light by massive objects, provides further evidence for dark matter.

Dark Matter Halo

  • Galaxies are embedded in massive dark matter halos that extend far beyond their visible components.

Cosmic Microwave Background

  • The cosmic microwave background radiation, a faint afterglow of the Big Bang, provides estimates of the mass fraction of dark matter in the universe.

Nature of Dark Matter

  • The exact nature of dark matter remains a mystery.
  • Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates.
  • Other possibilities include Massive Compact Halo Objects (MACHOs) and modifications to gravity (MOND), although these are less favored.

Active Galactic Nuclei (AGN)

  • Active galactic nuclei (AGN) are extremely energetic regions at the centers of some galaxies.
  • They emit radiation across the electromagnetic spectrum, from radio waves to gamma rays.
  • AGN are powered by supermassive black holes, which accrete matter and release vast amounts of energy.

Types of AGN

  • Quasars: Extremely luminous and distant AGN.
  • Seyfert Galaxies: Spiral galaxies with bright, compact nuclei.
  • Radio Galaxies: Emit powerful radio jets.

Unified AGN Model

  • The unified AGN model suggests that different types of AGN are the same underlying phenomenon viewed from different angles.

Chapter 20: The Milky Way and the Local Group

Mapping the Milky Way

  • Mapping our own galaxy is challenging due to the presence of interstellar dust, which obscures our view.

Milky Way Classification

  • The Milky Way is classified as a barred spiral galaxy (SBbc).

Globular Clusters as Distance Indicators

  • Globular clusters, containing old stars like RR Lyrae variables, are useful for mapping distances within the Milky Way and to nearby galaxies.

Milky Way Structure

  • The Milky Way consists of a central bulge, a flat disk, and a spherical halo.
  • The Sun is located about 8.3 kiloparsecs (kpc) from the galactic center.

Milky Way Rotation Curve and Dark Matter

  • The Milky Way’s rotation curve reveals that a significant portion (about 90%) of its mass is in the form of dark matter.

Chemical Evolution of the Milky Way

  • The Milky Way’s stellar populations exhibit a range of metallicities (abundances of elements heavier than helium).
  • Older stars in the halo have lower metallicities, while younger stars in the disk are more metal-rich.

Milky Way Components

  • Thin Disk: Contains young stars, gas, dust, and is the site of ongoing star formation. The Solar System resides in the thin disk.
  • Thick Disk: Composed of older stars and has a lower density of gas and dust.
  • Bulge: A central spheroidal region containing a mix of stellar populations.
  • Halo: A diffuse, spherical region containing old stars, globular clusters, and tidal streams.
  • Galactic Center: The innermost region of the Milky Way, harboring a supermassive black hole.
  • Extended Gas Component: A low-density halo of gas surrounding the Milky Way.
  • Cosmic Rays: High-energy particles that are trapped by the Milky Way’s magnetic field.

Galaxy Formation Theories

  • Hierarchical Formation: Galaxies form through the gradual merging of smaller structures.
  • Monolithic Collapse: Galaxies form from the collapse of large gas clouds.

Galactic Archeology

  • Studying the properties of stars in the Milky Way’s halo provides insights into its formation and evolution.

Milky Way Halo Components

  • Stellar Halo: Contains stars, globular clusters, and tidal streams.
  • Dark Matter Halo: A massive halo of dark matter that dominates the Milky Way’s gravitational field.
  • Gas Halo: A halo of hot gas detected in X-rays.
  • Fermi Bubbles: Giant lobes of gamma-ray emission extending above and below the galactic plane.

Galaxy Groups

  • Galaxies are often found in groups, held together by gravity.
  • Galaxy groups typically span 1-2 megaparsecs (Mpc) and contain a few large galaxies and many smaller ones.

The Local Group

  • The Milky Way is a member of the Local Group, a small group of about 50 galaxies.
  • Andromeda Galaxy (M31): The largest galaxy in the Local Group, about 2.5 million light-years (Mly) away.
  • Triangulum Galaxy (M33): A smaller spiral galaxy, also a member of the Local Group.

Major Galaxies’ Contribution

  • Andromeda, the Milky Way, and Triangulum account for about 90% of the Local Group’s luminosity.

Lesser Members of the Local Group

  • Large Magellanic Cloud (LMC): A dwarf galaxy that is a satellite of the Milky Way.
  • Small Magellanic Cloud (SMC): Another dwarf galaxy and Milky Way satellite.
  • M32: A small elliptical galaxy that is a satellite of Andromeda.
  • NGC 205 (M110): A dwarf elliptical galaxy, also a satellite of Andromeda.

Dwarf Galaxies

  • The Local Group contains numerous dwarf galaxies, which are much smaller and less massive than spiral or elliptical galaxies.

Galactic Cannibalism

  • Larger galaxies can absorb smaller galaxies through a process called galactic cannibalism.
  • This process can contribute to the growth of large galaxies and trigger new star formation.

Wein’s Law Calculation

: 

 λmax = b / T

(λpeak = 2900mK/T)

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