Star Forming Clouds
● Stars form in dark clouds of dusty gas in interstellar space.
● The gas between the stars is called the interstellar medium.
Composition of Clouds
● We can determine the composition of interstellar gas from its absorption lines in the
spectra of stars.
● 70% H, 28% He, 2% heavier regions in our region of the Milky Way.
● In general, the gas and dust is too cold to produce visible light
● However, the tiny solid particles of interstellar dust block our view of stars in the other
side of a cloud.
● Particles are <1 micrometer in size and made of elements like C, O, Si, and Fe.
● Stars viewed through the edges of the cloud look redder because dust blocks (shorter-
wavelength) blue light more effectively than (longer-wavelength) red light.
● Long wavelength infrared light passes through a cloud more easily than visible light
● Observations of infrared light reveal stars on the other side of the cloud.
● Sometimes these clouds can be seen in silhouette when there is sufficient background
starlight as can be seen in these images.
● Although the gas is too cold to produce visible light, the molecules in these clouds can
produce observable spectral line emission at radio wavelengths.
● Most of the matter in star-forming clouds in the form of molecules (H2, CO, etc.)
● Most of what we know about molecular clouds comes from observing the radio emission
lines of carbon monoxide (CO).
● These molecular clouds have a temperature of 10-30 K and a density of about 300
molecules per cubic centimeter.
● An example of medium sized molecular cloud is located in the constellation Taurus about
140 parsecs away.
Observing Newborn Stars
● Visible light from a newborn star is often trapped within the dark, dusty gas clouds where
the star is formed.
● Observing the infrared light from a cloud can reveal the newborn star embedded inside it.
Glowing Dust Grains ● At visible wavelengths the dust in this clouds very effectively blocks the background
starlight at visible wavelengths. However at very long wavelengths (in the far infrared)
the cold dust can emit thermal radiation (remember Wien’s law).
● Dust grains that absorb visible light heat up and emit infrared light of even longer
● Long wavelength infrared is brightest from regions where many stars are currently
Why do stars form? Gravity versus Pressure
● Gravity can create stars only if it can overcome the force of thermal pressure in a cloud.
● Atypical molecular cloud (T=30 K, n=300 particles/cm^3) must contain at least a few
hundred solar masses for gravity to overcome pressure.
● Emission lines from molecules in a cloud can prevent a pressure buildup by converting
thermal energy into infrared and radio photons that escape the cloud.
Resistance to Gravity
● Acloud must have even more mass to begin contracting if there are additional forces
● Both magnetic fields and turbulent gas motions increase resistance to gravity.
Fragmentation of a Cloud
● Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.
● Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to
break apart into multiple fragments, each of which may go on to form a star.
● This simulation begins with a turbulent cloud containing 50 solar masses.
● The random motions of different sections of the cloud cause it to become lumpy.
● Each lump of the cloud in which gravity can overcome pressure can go on to become a
● Alarge cloud can make a whole cluster of stars.
Isolated Star For