Chapter 10: Nebulas and the Birth of Stars and Planets


The Interstellar Medium (ISM):

Dust and gas between stars.



Close to central plane of galaxy.



Distributed in clouds, not uniformly



Densest clouds are birthplace of stars.

Nebulae (nebulas)- glowing clouds of gas and dust (e.g. Orion Nebula)



Emission nebula - clouds of gas excited by

hot O, B0 stars.



Ultraviolet light from stars excites and ionizes atoms in gas, produce emission line spectrum



Most of gas is ionized hydrogen ( HII region), some nitrogen, oxygen



Reddish color



Low density ~ 100 - 1000 atoms/cm3



Temp ~ 10,000 K



Interstellar Absorption Lines:



Produced by gas.


Very narrow


May be "wrong" ionization for star.


Multiple components - different clouds, different radial velocities.


HI clouds - neutral hydrogen.


Cold, T ~ 100 oK


10 - 100 atoms/cm3


Intercloud medium - ionized.


Warm T ~ 5000 oK


0.1 atoms/cm3


Gas - mostly H, He


Some elements low because they are in dust.




Radio and Microwave Observations:



21-cm radiation


Cool HI (neutral hydrogen)


Ground level of H has two levels.


Proton, electron spin


Spins aligned - higher energy state than unaligned spins.


11x106 years for HI to go from aligned to unaligned (highly forbidden transition).


Produces 21 cm photon.


Traces clouds of cold HI in space.




Molecules in ISM:


Vibrate, rotate


Changes in energy small.


Emit radio or infrared photons


H2 doesn't emit in radio, microwave.


Many other molecules observed.


CO used as a tracer for H2


Survive in clouds, shielded by dust from photons.


Giant molecular clouds.


Contain molecules but mostly H2


T ~ 10 oK


Very large


Birthplace of stars.




Interstellar Dust

Dark clouds - dense clouds of gas, dust block light beyond (Horsehead Neb.)

Bok globules - may contain forming stars.



Interstellar dust - not only in clouds.

Dims, reddens starlight (scattering, absorption).

Carbon, silicates, ice

Reflection nebula - light from stars reflected off dust in ISM (e.g. Pleiades).

Bluish light

More blue light scattered than red.

Dust similar in size to wavelength of photons.



Starbirth



Takes place in collapsing gas clouds



Central temperature increases during collapse

Central temperature reaches ~ 10 x 106 K, fusion starts, pressure halts collapse

Cloud must overcome:



Thermal energy of gas



Magnetic fields

Rotation - fragmentation, stops contraction





Collapse triggers:



Supernova explosions



Collision of molecular clouds

Spiral arms of galaxy

Once core gets dense enough, contraction begins (gravity wins!!!!)



Collisions increase during contraction,

atoms and molecules are excited, radiate at infrared wavelengths



Depending on amount of initial angular momentum, may get single stars surrounded by discs or multiple stars



Stellar Nurseries



Stars form in groups in giant molecular clouds (Orion Molecular Cloud)



Observational evidence



Infrared sources in Orion Molecular

Cloud

Young stars in Trapezium (Orion Nebula) with flattened discs of gas, dust



Streams of gas ejected from

protostars, perpendicular to planes of discs - bipolar outflows.



Excite and ionize ISM producing Herbig-Haro objects

Central condensing protostar must shed angular momentum



Transferred to disc



Carried away by bipolar flows, stellar winds



Star shrouded in gas and dust while forming



Eventually stellar wind and radiation pressure disperse gas, dust, and star is visible.



Formation of Planets



Planetary systems common, by-product

of star formation



Our Solar System



Nebular hypothesis - collapsing cloud of gas and dust formed Solar System



Inner nebula hotter than outer part



Inner nebula - only refractory materials survived or condensed



Outer parts - volatile materials also survived or condensed



Terrestrial planets and cores of jovian planets formed by accretion of available particles, collisions as larger bodies formed



Explains size, chemical composition of

planets, oddities of Venus and Uranus

Moons formed from disks of matter,

captures, or collisions



Other Planetary Systems



Extra-solar planets hard to detect -

angular separation, very faint compared to star



Detect by changes in star's radial velocity due to orbit around center of mass of planetary system



About 80 extra-solar planets known, some multi-planet systems



Orbits closer to stars than expected



Pulsar planets - first detected. Debris

from supernova



Disks detected around several young stars - sites of planet formation?

Update - TMR-1C, described in text,

is not a planet, but a star more distant than thought






Prof. Donna Weistrop

University of Nevada, Las Vegas