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Nebula
Fuzzy Patch
• Any source of light in the night sky that was not a point
was called a nebula.
–
–
–
–
–
comets
gas clouds
dust clouds
galaxies
star clusters
• In order to help comet hunters, Charles Messier began to
catalog these objects in the 1700’s.
Interstellar Medium
• Interstellar space is 99%
gas.
– Hydrogen 74%
– Helium 25%
– All other elements 1%
• Dust (1%) includes
chemical compounds.
– methane, ammonia,
water, monoxides
• Atoms are widely spaced,
about 1 atom per cm3, a
nearly perfect vacuum.
– 1019 atoms/cm3 in air
• The temperature is cold,
less than 100 K.
• Voyager I reached the
ISM in 2012.
Molecular Clouds
• The small mass of atoms creates very weak gravity.
– Pull atoms and molecules together
• Concentrations equal to 1 million solar masses can form
giant molecular clouds over 100 ly across.
Diffuse Nebulae
• Clouds of gas do not
naturally emit light.
– Light blocked beyond
• The Coal Sack is a dark
nebula.
• Some nebulae reflect the
light of nearby stars.
– H I region
• The Pleiades are in a
reflection nebula.
Emission Nebulae
• Gas clouds can be heated
by nearby stars.
– Emits own light
– H II region
• Like a star the spectrum
can be analyzed.
– chemical composition
– Temperature and density
– velocity (by Doppler
shift)
• The Orion Nebula is an
emission nebula.
Star Formation
• A cool (10 K) nebula can be compressed by shock waves.
– Shock waves from new stars and exploding stars.
exploding star
shock waves
nebula with areas
of higher density
Protostars
• Local concentrations in a nebula can be compressed by
gravity.
– Keeps together at low temperature
– Contracts to form centers and radiates energy
• These are protostars, called T Tauri stars (G, K, M).
Birth of the Sun
• Gravity continues to pull
the gas together.
-20
-15
Abs. Magnitude
-10
-5
0
1 M
• Hydrogen fusion begins
at 5 million K.
5
10
15
20
O B A F G K M
Spectral Type
• At this point the star has
reached the main
sequence.
Birth of Other Stars
• Large masses become
brighter, hotter stars.
-20
-15
Abs. Magnitude
-10
-5
10 M
0
3 M
0.5 M
5
10
0.02 M
15
20
O B A F G K M
Spectral Type
– Fusion starts sooner,
about 100,000 years
• Small masses become
dimmer, cooler stars.
– Fusion starts later, up to
100 million years