Chapter 10: Nebulas and the Birth of Stars and Planets
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
Low density ~ 100 - 1000 atoms/cm3
Temp ~ 10,000 K
Interstellar Absorption Lines:
Radio and Microwave Observations:
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).
More blue light scattered than red.
Dust similar in size to wavelength of photons.
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
Rotation - fragmentation, stops contraction
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
Stars form in groups in giant molecular clouds (Orion Molecular Cloud)
Infrared sources in Orion Molecular
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
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