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.


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


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