Nebulae and Star Formation

Orion Nebula

Nebulae: a cloud of dust and gas that we see in light

  1. Emission Nebulae or Bright Nebulae: a glowing gas (hydrogen); e.g. Great Nebula in Orion, heated by the Trapezium
  2. Absorption Nebulae or Dark Nebulae: dark dust clouds; e.g. Horsehead Nebula
  3. Reflecting Nebulae: reflecting dust cloud; e.g. Pleiades in Taurus
  4. Planetary Nebulae: excited by central star; e.g. Dumbbell Nebula
  5. Cirrus



Stars form in Giant Molecular Clouds about 100,000 to 1 million solar masses. A few thousand in the Milky Way Galaxy, Giant Molecular Clouds break into denser bits, contract, and eventually form stars. The Orion Molecular Cloud has about 500 stars. The Trapezium and the Orion Nebula have solar masses of matter with young stars.

  1. Non-stellar galactic objects reside in HII regions with molecular clouds of pre-main sequence stars and dense clumps of dust.
  2. Protostars and newborn stars about 1/2 to 1 solar mass reside in Molecular Clouds.

Interstellar Medium – The Material Between Stars


– Interstellar Medium

Interstellar Medium

Interstellar Medium is gas and dust between stars, nebulae, and giant molecular clouds (basic building blocks of galaxies in star formation). The four types of matter in interstellar medium are: interstellar dust, interstellar atoms, interstellar molecules, and interstellar snowballs.

Interstellar Dust

  • Interstellar Reddening: dust that scatters blue light and causes stars to look redder
  • Extinction of Obscuration: high dust content that diminishes the brightness of stars, by as much as 25 magnitudes
  • Can be smaller than smoke particles
  • Consists of graphite, silicates, or ices
  • In core of heavy elements (e.g. iron, magnesium), mantle of organic compounds (oxygen, carbon, nitrogen), and outer mantle of ice


  • Radio waves = longest wavelength of electromagnetic waves
  • Brightest optical objects not necessarily the brightest radio objects
  • e.g. Taurus A (Crab Nebula) and Sagittarius A (center of the Milky Way Galaxy)
  • Radio Spectral Line: the frequency or wavelength at which radio noise is slightly more or less intense
    • Hydrogen: 21 centimeter line
    • Radio spectra lines of molecules
      • OH (hydroxide): 1963
      • H20 (water): 1968
      • NH3 (ammonia): 1968
    • Over 50 molecules in interstellar space
    • Gives information on temperature, density, and motion
    • Molecular absorption line in UV

Interstellar Molecules

  • Molecules: two or more atoms bound together (e.g. H2O, CO, CH4, OH, H2, NH3)
  • Give absorption or emission bands
  • Observable in very cold, low density interstellar environments

Interstellar Snowballs

  • Between the sizes of  grains and comets
  • Composed of water, carbon, silicates, and other molecules

Interstellar Regions

  1. HI region: 200 K
  2. HII region: 10,000 K
  3. Molecular clouds: 50% gas in our galaxy
  4. Hot interstellar medium: 1 million K, super-heated gas from expanding supernova blasts (up to 90% of total volume)
  • HI Region
    • High density of neutral hydrogen atoms about a million atoms per cubic centimeter (e.g. Orion Nebula)
    • ~ 200 K
  • HII Region
    • Hydrogen with electron removed; e.g. ionized hydrogen gas (in emission nebulae)
      • Average density of hydrogen elsewhere is 1 atom per cubic centimeter
    • ~ 10,000 K

The Solar System: Basics

The Solar System


  • Eccentricity of Orbit: measures the ellipticity of orbit (ranges 0-1, with 0 as spherical and 1 as very elliptical)
  • Density: mass per unit volume; mass in grams and volume in cubic centimeters
  • Oblateness: measures how much the middle section of the planet bulges
  • Surface Gravity: the larger the surface gravity, the thicker the atmosphere as gravity pulls in more gases
  • Albedo: measures the fraction of light reflected compared to the amount of light received from the Sun; the higher the albedo, the more reflective the surface
  • Escape Velocity: minimum speed or velocity needed to escape the planet’s gravitational pull
  • Rotation: most planets rotate in counter-clockwise direction (prograde); others rotate in the clockwise direction (retrograde)
    • Rotational period is shortest for gaseous planets and longest for Venus
  • Roche Limit: about two and a half times the radius of the planet; within the Roche Limit, matter cannot accretes to form moons because the tidal force of the planet tears matter apart to form rings

Giant Planets: Giant planets have lighter elements such as hydrogen and helium in their atmospheres. They have stronger gravity and are at larger distances from the Sun. Jupiter, Saturn, and Neptune are stormy with great spots of lasting storms and belts and zones. However, Uranus is comparatively bland and uniform. All giant planets are home to convection, or hot gases rising and cold gases falling.

Terrestrial Planets: Terrestrial planets have heavier elements such as carbon, oxygen, and nitrogen. Mercury is most heavily cratered while Earth is least cratered. Larger terrestrial planets have plate tectonics. Earth has a sizable magnetic fields that can protect it from solar wind particles and Van Allen Belts. Earth has the “Goldilocks phenomenon,” or the right conditions for the development of life.


Stellar Properties

Stars: Stellar Properties

Stars are balls of gas held together by force of gravity and generate energy and light by nuclear fusion.

A star’s “color,” or wavelength gives information on:

  1. Temperature
  2. Composition
  3. Conditions
  4. Motion (Doppler Shift)
  5. Classification Scheme

What to Measure and How to Classify Stars:

  1. Spectroscopy
    • To determine composition
      • Absorption line produced when an electron absorbs a photon; emission line produced when an electron emits a photon
      • Dual nature of light: light behaves as waves (electromagnetic waves) or as particles (photons)
      • High energy electromagnetic waves are high energy photons, low energy electromagnetic waves are low energy photons
      • Energy of a photon defined by: E = hf, where h is Planck’s constant (h = 6.63 x 10 ^ – 34 joules sec) and f is the frequency of the electromagnetic wave
      • Three Types of Spectra:
        • Continuous Spectrum: appears as a rainbow spectrum
        • Emission Spectrum: appears as distinct color lines, characteristic of chemical elements
        • Absorption Spectrum: appears as black lines on a rainbow background, reverse of emission spectrum
    • To determine temperature
      • All objects give off thermal radiation
      • Peak wavelength corresponds to maximum intensity of radiation
      • Peak wavelength of electromagnetic radiation is related to temperature
      • Wien’s Law: W = 0.00290/T
      • As temperature increases, wavelength decreases
      • The hotter an object, the bluer the radiation
    • To determine density
      • The thicker the spectral line, the greater the abundance of the chemical element present
    • To determine motion
      • Doppler shift of spectral lines
      • Red Shift = moving away
      • Blue Shift = moving closer
    • To determine distance
      • Measured in light years (ly) – distance light travels in one year and parsecs (pc) – one parsec is 3.26 light years
      • Parallax: the only direct measure of stellar distance, the angle across the sky that a star seems to move with respect to a background of distant stars) between two observation points at the ends of a baseline of one astronomical unit (A.U.); a star one parsec from Earth has a parallax of one arc second
  2. Brightness
    • Apparent Brightness
      • Affected By: absolute (true) brightness, distance, intervening space, Earth’s atmosphere, and eyes’ visual response
      • Measured by apparent magnitude “m,” relative brightness as seen on Earth; brightest star (m=1) to faintest (m=6); a 1st magnitude star is 100 times brighter than a 6th magnitude star
    • Absolute Brightness
      • Measured by absolute magnitude “M”
      • The magnitude of a star observed from a distance of 10 parsecs (1 parsec = 3.26 light years)
      • Stars further than 10 parsecs would “appear” brighter; M increases
      • Stars closer than 10 parsecs would “appear” dimmer; M decreases
    • m-M = 5 log (r/10)
  3. Distances
    • Distance as the primary factor in the decrease of stellar brightness as perceived on Earth, used to determine absolute brightness
    • Inverse Square Law: the intensity of light varies inversely with the square of the star’s distance from the Earth
  4. Mass and Size
    • MASS: For binary stars, both the period of revolution of one star orbiting the other and the distance between the two stars can be measured
    • SIZE: Diameters of stars can be determined from temperature and luminosity (calculated from absolute brightness) =>  L  =   σ  T^4  A, where L = luminosity, σ = distance, T = temperature, and A = absolute brightness
  5. Classification Scheme
    • Spectral Types: O, B, A, F, G, K, M; subtypes 0 to 9 (e.g. B1, A4, G2, and M0)
    • O stars are more than 10 times hotter than M stars
    • Developed by Annie Jump Cannon in the late 1800’s
  6. H-R Diagram: to study evolutionary tracks

What are Quasars?


QUASARS (Quasi-Stellar Radio Source) [QSR]

  • Appear like stars
  • Emits strong radio signal
  • Distant active nuclei of galaxies
  • A compact region in the center of a massive galaxy surrounding its central supermassive black hole
  • Traveling away from Earth at tremendous speeds (largest Doppler red shifts known)
  • First discovered 20 years ago (3C273) by interaction of optical and radio astronomy
  • In 1963, Maarteen Schmid discovered a quasar with a 16% Doppler red shift (~3 billion light years away)
  • Since then, more than 1,500 quasars discovered with red shifts up to 473%
  • Provide information of the early phases of the Universe
  • Have been found in a cluster of galaxies (“host galaxies”)

Hubble’s Law: large redshift means large velocities of recession = great distances

Light and Telescopes


Longer Wavelengths vs. Shorter Wavelengths

Wavelength: the distance from the crest of one wave to the crest of the successive wave

Frequency: longer wavelengths corresponds to lower frequency and lower energy, shorter wavelengths correspond to high frequency and higher energy

Radiation: transmission of energy through space

Transmission: light rays or electromagnetic waves bending through a different medium

  • All waves have a source (e.g. electromagnetic waves originate from vibrating charged particles)
  • All waves, except electromagnetic waves, transmit through a medium

Wavelengths in Visible Light

Electromagnetic Spectrum: electromagnetic waves ranging from low frequency,  low energy, and long wavelength to high frequency, high energy, and short wavelength that originate from vibrating charges from the Sun; all electromagnetic waves travel at the same speed, or the speed of light (c = 300,000 km/sec or 186,000 miles/sec)


Reflection: light rays or electromagnetic waves bouncing off reflective surfaces (e.g. mirror)

Refraction: light rays or electromagnetic waves bending through a different medium (e.g. air to water)


Spherical Aberration

Spherical Aberration: when light rays incident on the edges of the spherical mirror are focused at a different point from light rays incident closer to the center of the mirror –> blurry images; corrected by using parabolic mirrors

Chromatic Aberration

Chromatic Aberration: as light rays travel through a lens, different wavelength rays are bent by different amounts, resulting in different focal points


Three Types: 1. Reflective (mirrors), 2. Refractive (lens); 3. Combined or Catadioptic (both mirrors and lens): combines advantages of refractive and reflective telescopes, while avoiding disadvantages

  • Objective: main lens or mirror
  • Eyepiece: lens that magnifies images
  • Focal Length: distance between the center of the lens and the its focus
  • Aperture: diameter of objective

Functions of Telescopes: to collect light, to resolve details, to magnify, to measure, to record

Problems of Optical Telescopes: “seeing” (Earth’s atmosphere refracts light), air transparency, light pollution

Hubble Space Telescope

Unusual Telescopes

  • Radio: Arecibo, VLA, COBE
  • Microwave, or RadarPIONEER, COBE
  • UltravioletCOPERNICUS, IUE
  • OrbitalHUBBLE
  • Multiple MirrorsKECK
  • Interferometry: VLA, VLT