The Milky Way and other Normal Galaxies

Review for Astronomy 123 Midterm Exam 1

posted April 20, 2001, jeb

Outline

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

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Astrophysical Times and Distances

Astrophysical Times (all approximate)

age of Milky Way Galaxy about 16,000,000,000=1.6 x 1010 years age of the Sun 5,000,000,000=5 x 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)

nearest spiral galaxy (Andromeda) 2,000,000 lt. years diameter of luminous disk of the Milky Way 120,000 lt. years distance from the Sun to center of Milky Way 30,000 lt. years distance from the Sun to nearest star (Alpha Centauri complex) 4 lt. years

Structure of the Milky Way Galaxy

Principal features
• Nuclear (central) bulge
  • Radius of 6,000 light years
• Galactic disk
  • Radius of 60,000 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)

Corona (or Dark halo) appears to contain about 90% 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)
• 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

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 I, Cepheid II
• 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

Great Rift
• dark region across Milky Way as seen from earth
• caused by overlapping molecular clouds

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

Interstellar reddening
• sky is blue, sunset is red

Globular clusters
• revealed position of the Sun in Milky Way
• scattered in galactic halo
• old stars

galactic (open) clusters
• found in disk
  • in spiral arms
• young stars

Center of Milky Way
• Sagittarius A - radio sources
• infrared radiation intense
• supermassive black hole

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 are they?
  • young, bright (O & B) stars
  • density 5-10% greater than between arms
• "Wind up Problem"
  • arms only persist for 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
= 7.2 x 10¹¹M_SUN
(or about 10¹²M_SUN)
• about 10¹¹ stars

Normal 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

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
tuning fork, again

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

The Distribution of Galaxies in Space
• Tully-Fisher
• Galaxy Clusters
• 20-30 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
    • 4 irregulars: Large Magellanic Cloud and Small Magellanic Cloud
    • 4 ellipticals
    • 16 dwarf irregulars or dwarf ellipticals
  • The Local Cluster is a galaxy" poor" cluster
    • rich clusters (such as Virgo and Coma) - thousands of galaxies
• Clusters of Clusters
  • superclusters

Galaxy Masses
• Rotation curves of galaxies
• Upward of 90 percent of the universe appears to be composed of dark matter
• What is the dark matter?
  • stellar remnants?
  • exotic subatomic particles?
  • we do not yet know
• Interstellar medium (between stars within a galaxy)
  • about 1 atom per cubic cm
• Intergalactic medium (between galaxies within the cluster)
  • about 10^-6 atoms/cm³, or about 1 atom per cubic meter
• Intercluster medium (between galactic clusters)
  • very little matter - no evidence for gas beyond the clusters, in the "extracluster" spaces

Galaxy Formation and Evolution
• close encounters or collisions
  • Starburst Galaxies - Rapid star formation
  • Ejection of Stars - Cartwheel galaxy and The antennae
  • Galactic Cannibalism
  • Shape distortion

Theories of Galaxy Formation
• Bottom-up Theory
• Top-down Theory (pancake model)

Hubble's Law
• The rate at which a galaxy recedes is directly proportional to its distance from us
• This redshift resulting from Hubble flow is called the cosmological redshift
• The whole universe is expanding
• Most distant observed galaxies about 15 billion light years away.
  • Look-back time - 15 billion years
  • Consequence -> The universe is at least this old
• best guess for Hubble constant is 65 km/sec/Mpc or 20 km/sec/million light years

Large Scale Structure
• On very large scales the galaxies are not distributed randomly
• Great Wall

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

examples
• M87
  
• Centaurus A
  
  

Solar Flares and Auroras

The Sun is currently passing through the maximum in sunspot activity on its surface

At the end of March, a gigantic sunspot group passed across the solar surface

A plasma of charged particles from this sunspot group struck the Earth during March 30-April 1, resulting in Auroras seen as far south as Mexico.
• This plasma came from a coronal hole ejection


The regions around the sunspots occasionally erupt into violent solar flares

On April 2, this sunspot group unleashed the biggest solar flare on record