The Milky Way and the Distant Galaxies

Review for Astronomy 123 Midterm Exam 1

April 19, 2017, jeb

Outline

• Astrophysical Times and Distances
• Structure of the Milky Way Galaxy
• Basic Physics
• Spectral Types of Stars
• The Milky Way
• Galaxies
• Active Galaxies

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Astrophysical Times and Distances

Astrophysical Times (all approximate)

age of the universe 13,800,000,000 = 13.8 × 109 years age of Milky Way Galaxy more than 10,000,000,000 = 1 × 1010 years age of the Sun 5,000,000,000 = 5 × 109 years period of the Sun's orbit around Milky Way 225,000,000 years age of arms of Milky Way (also approx. lifetime of O and B stars) 10,000,000 years





Astrophysical Distances (approximate)

most distant quasars more than 10,000,000,000 lt. years more than 3,000 Mpc distance to Virgo Cluster 55,000,000 lt. years 17 Mpc nearest spiral galaxy (Andromeda, M31) 2,500,000 lt. years 800 kpc diameter of luminous disk of the Milky Way 100,000 lt. years 30 kpc distance from the Sun to center of Milky Way 26,000 lt. years 8 kpc distance from the Sun to nearest star (Alpha Centauri complex) 4.3 lt. years 1.3 pc

1 parsec (pc) = 3.26 light-years (lt. years)

Structure of the Milky Way Galaxy

Principal features (approximate dimensions)
• Galactic (central) bulge
  • Radius of 6,000 light years
• Galactic disk
  • Radius of 50,000 light years
• Galactic halo
  • Radius of 65,000 light years
• Dark halo
  • Radius of 200,000 or 300,000 light years

Massive object possible large black hole at the center of the nucleus

Appears to have spiral arms (like other spiral galaxies) where the brightest and youngest stars are

Contains many nebulae (clouds of gas and dust)
• emission (Orion)
• absorption (Horsehead)
• reflection (Pleiades)
• planetary (Ring)
• supernovae remnant(Crab)

Galaxy rotates
• Sun lies in the disk and circles Milky Way every 225,000,000 years (about 20 times since birth)

Dark halo appears to contain dominant fraction of the Milky Way's matter - BUT, it's invisible (Dark Matter)

Star formation occurs primarily in the disk today

Basic Physics

Doppler Effect
• red shift, moving away
• blue shift, moving towards

Electromagnetic Spectrum
• gamma-ray, x-ray, ultraviolet, visible, infrared, radio
• short wavelength ----> long wavelength
• visible at ~0.5 millionths of a meter (0.5 microns or 500 nanometers)
• all electromagnetic waves travel through space at the speed of light
  • speed of light = 300,000 km/sec

Blackbody radiation
• Perfect radiator of light that absorbs and re-emits radiation
• Light output depends only on temperature
• Stars emit radiation like a blackbody following the Blackbody spectrum
• As a blackbody is heated, the radiation it emits follows the blackbody spectrum with frequency increasing (wavelength decreasing) with temperature

Light-year
• distance light travels in a year
• speed of light = 300,000 km/sec
• 1 year = 3.16 x 10⁷ seconds
• So, light -year = 3 x 10⁵ km/sec x 3.16 x 10⁷ sec = 9.5 x 10¹² kilometers
  • 1 parsec = 3.3 light-years

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Spectral Types of Stars

Heavy	->	->	->	->	->	light
O	B	A	F	G	K	M
hot surface				6,000K(sun)		cool surface
bright						faint
short life						long life
heavy						lighter

The Milky Way

Population I
• like the Sun, young
• metal rich
  • few % of elements are heavy
• found in the disk of Milky Way

Population II
• like stars in globular clusters, old
• metal poor
  • few hundredths of % heavy elements, or less
• found in halo and in globular clusters

Generally the higher the heavy metal abundance, the closer stars are to the disk of the Milky Way

Sun's position in the Milky Way
• first determined by Shapley using the globular clusters
• center of the Milky Way in the direction of the constellation Sagitarius

Distances - determined from variable stars
• Cepheid
• R R Lyrae
  • in globular clusters
  • used to determine distances to globular clusters and size of the Milky Way
  • and that Andromeda is outside the Milky Way

Early formation of Milky Way
• globular clusters -->
  • Milky Way originated from large sphere of matter
  • diameter about 100,000 ly

Milky Way seen from Earth
• disk stretches across the sky, visible to naked eye in very dark locations on Earth
• dark regions obscure vision, caused by overlapping molecular clouds

Interstellar medium
• 90% hydrogen, 1% dust, 9-10% helium

Globular clusters
• tightly bound collections of hundreds of thousands to millions of stars
  • roughly spherical in shape
• revealed position of the Sun in Milky Way
• scattered in galactic halo
• old stars

Galactic (open) clusters
• loosely bound collection of tens to hundreds of stars
• found in disk
  • in spiral arms
• young stars

Center of Milky Way
• Sagittarius A - radio sources
• intense radiation at all wavelengths (radio, IR, X-ray, gamma rays)
• supermassive black hole
  • about 4,000,000 solar masses

Infrared survey shows nuclear bulge

Radio measurements
• VLA in New Mexico
• can "see" into center of the Milky Way galaxy

Spiral Arms
• tracers in survey for spiral arms:
  • 21 cm spin-flip (HI)
  • O & B type stars
  • galactic clusters
  • HII regions
  • molecular clouds
• what makes the arms so bright?
  • young, bright (O & B) stars
  • glowing, emission nebulae
  • density 5-10% greater than between arms
• "Wind up Problem"
  • arms only persist for about 10,000,000 years
• Formation
  • density wave theory
  • self-propagating star formation (or Supernovae chain reaction)

Rotation curve (stars & clouds)
• dark matter (80-95% of mass in Milky Way)

Mass of Milky Way
about 6 × 10¹¹M_SUN within 40 kpc
(or about 600 billion M_SUN,
nearly 10¹²M_SUN)
• about 10¹¹ stars and 90% dark matter

Dark Matter
what is it?
• exotic subatomic particles (WIMPs) likely make up large fraction, if not all
• stellar dark matter might also contribute
  • search using gravitational lens effect
    • effect explained by Einstein's theory of General Relativity
  • stellar dark matter is a small fraction of total

Galaxies

Spiral Galaxies
• young stars



SO	spiral w/o arms
Sa	large bulge, tight arms
Sb
Sc	small bulge, loose arms

Barred Spiral Galaxies
• young stars
• nucleus is like a bar (elongated)



SBO	spiral w/o arms
SBa	large bulge, tight arms
SBb
SBc	small bulge, loose arms

Elliptical Galaxies
• mostly old stars
• very little dirt left



EO	spherical
E1
E2
..
..
E6
E7	elongated ellipsoid

The Milky Way is SBb or SBc

S0 and SB0 Galaxies
• Intermediate between E7 and Sa
• thin disk and a flattend bulge, but no gas and no spiral arms

Irregulars
• like Large Magellanic Cloud
• the majority of galaxies are of this type

Hubble's "tuning fork" diagram

Frequency of Galaxy Types
• The majority of galaxies in the universe are low luminosity, irregular galaxies

The Distribution of Galaxies in Space
• Tully-Fisher (standard candle)
• Galaxy Clusters
• about 50 galaxies populate the local neighborhood of the Milky Way
  • Local Group
  • Composition of the Local Group
    • 3 spirals: the Milky Way, Andromeda (M31), and M33
      • distance to Andromeda (M31) is about 2,000,000 light-years
    • irregulars: Large Magellanic Cloud and Small Magellanic Cloud
    • dwarf irregulars and dwarf ellipticals
  • The Local Cluster is a galaxy" poor" cluster
    • rich clusters (such as Virgo and Coma) - thousands of galaxies

Hubble's Law
• The rate at which a galaxy recedes is directly proportional to its distance from us
• The redshift resulting from Hubble flow is called the cosmological redshift
• The whole universe is expanding
• Most distant observed galaxies have look-back times of about 13 billion years.
  • Consequence -> The universe is at least this old
• Hubble constant (H₀) is 70 km/sec/Mpc or 20 km/sec/million light years
  • recessional velocity = H₀ × distance

Active Galaxies

a small fraction of all galaxies

emit hundreds to thousands of times the energy of normal galaxies

much of the emission is in the radio portion of the EM spectrum (non-thermal spectrum)
Seyfert Galaxy - spiral
• bright nucleus, sub-yearly variation -------> small source

Radio Galaxies
• synchrotron radiation (charged particles spiral in magnetic field)
• non-thermal spectrum (radio and infrared)
• radio emissions - thousand x normal
• types:
  • core-halo -----> bright core
    • region of radio emission smaller than visible
  • radio lobe ----> very large
    • region of radio emission much larger than visible
    • largest objects in the universe
• Example - Cygnus A lobe radio galaxy
  • 1 billion light years away
  • 1 million times the radio output of the Milky Way
• Other examples
  • Centaurus A - unusual dust
    • lobe radio galaxy
  • M87 - jet of gas
    • core - halo galaxy
    • 3,000,000,000 solar mass black hole at core

Quasars
• Quasi-Stellar Radio Sources
• First discovered as very intense radio sources from star-like objects
• Found to be moving away from us at tremendous velocities
  • large red shifts of the emission lines
    • redshift = ( observed wavelength - true wavelength ) / true wavelength
  • nearly the speed of light (from Doppler red-shift)
• Also, they are small (light-weeks)
  • rapid variability
• "Look-back time" tells us we're looking at the universe in the distant past
  • 1-10 billion years ago

Quasars --------> radio galaxies (galaxies in evolution?)

Central Engine
• Super massive black hole - million/billion solar masses
• Accretion disk
• Jets of high-speed particles
  • synchrotron radiation
• Appearance depends on viewing angle.
  • Appears as either lobe or core-halo radio galaxy, depending viewing angle.

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