Galaxies

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Galaxies

• Dr Bryce

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Our galaxy probably formed from a giant gas cloud
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Halo stars formed first as gravity caused cloud
to contract
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Remaining gas settled into spinning disk
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Stars continuously form in disk as galaxy grows
older
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Infrared light from center
Radio emission from center
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Swirling gas near center
Orbiting star near center
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Stars appear to be orbiting something massive but
invisible a black hole? Orbits of stars
indicate a mass of about 4 million MSun
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X-ray flares from galactic center suggest that
tidal forces of suspected black hole occasionally
tear apart chunks of matter about to fall in
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Hubble Deep Field

• Our deepest images of the universe show a great
  variety of galaxies, some of them billions of
  light-years away

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Galaxies and Cosmology

• A galaxys age, its distance, and the age of the
  universe are all closely related
• The study of galaxies is thus intimately
  connected with cosmology the study of the
  structure and evolution of the universe

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Hubble Ultra Deep Field
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy
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halo
disk
bulge
Spiral Galaxy
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Disk Component stars of all ages, many gas clouds
Blue-white color indicates ongoing star formation
Spheroidal Component bulge halo, old stars, few
gas clouds
Red-yellow color indicates older star population
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Thought Question

• Why does ongoing star formation lead to a
  blue-white appearance?
• A. There arent any red or yellow stars
• B. Short-lived blue stars outshine others
• C. Gas in the disk scatters blue light

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Barred Spiral Galaxy Has a bar of stars across
the bulge
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Lenticular Galaxy Has a disk like a spiral
galaxy but much less dusty gas (intermediate
between spiral and elliptical)
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Elliptical Galaxy All spheroidal component,
virtually no disk component
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Irregular Galaxy
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Hubbles galaxy classes
Spheroid Dominates
Disk Dominates
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Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
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Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
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The distance ladder
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Step 1 Determine size of solar system using radar
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Step 2 Determine distances of stars out to a few
hundred light-years using parallax
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Luminosity passing through each sphere is the
same Area of sphere 4p
(radius)2 Divide luminosity by area to get
brightness
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The relationship between apparent brightness
and luminosity depends on distance We can
determine a stars distance if we know its
luminosity and can measure its apparent
brightness A
standard candle is an object whose luminosity we
can determine without measuring its distance
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Step 3 Apparent brightness of star clusters
main sequence tells us its distance
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Knowing a star clusters distance, we can
determine the luminosity of each type of star
within it
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Cepheid variable stars are very luminous
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Step 4 Because the period of a Cepheid variable
star tells us its luminosity, we can use these
stars as standard candles
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Cepheid variable stars with longer periods have
greater luminosities