29:50 Modern Astronomy
Fall 2002
Lecture 16 ...October 11,2002
Dark Clouds and the Formation of Stars

Preliminary Announcements

• Details on test next week
• Astronomy picture of the day

(1) Why are dark clouds dark? URL of Dark Cloud. URL of M8 Dark CloudThey are opaque because small pieces of matter (interstellar dust) attenuates the light by the processes of absorption and scattering. These two processes together are referred to as extinction.
Demonstration of extinction of light.

(2) In molecular clouds, these small pieces of matter (interstellar dust) are about meters in size, comparable or smaller than the wavelength of visible light. A diagram showing their structure is given in Figure 22.21 of your textbook.

(3) Stardust If we see large concentrations of these small particles where stars form, and the Sun and solar system formed out of such a cloud, there should be some of these particles in solar system objects. Tiny examples of these particles have been found in pieces of comets and are referred to as stardust.
Picture of stardust. URL with picture of stardust.

(4) Big discovery about dark clouds made in the late 1960's. At that time, observations began at millimeter radio wavelengths, i.e. electromagnetic waves with wavelengths in the range 1 - 3 millimeters.

(5) Why millimeter wavelengths? Molecules have spectral lines just like atoms. We can pick out the spectrum of carbon monoxide, water, ammonia, etc as distinctive of that molecule. Most molecules have spectral lines in the wavelength range 1 - 3 millimeters. These spectral lines are associated with rotation of the molecules, and are therefore called rotational transitions.
Blackboard drawing of rotational spectrum of molecule.

(6) Another reason radio observations are useful is that radio waves penetrate the opaque dust of the dark clouds, so we can ``see into'' the center of the dark clouds. This is just like the ability of satellite TV signals to be received even on overcast nights when you can't see the stars.

An essay on molecules and photodissociation

Light carries energy. The small packets of electromagnetic waves that are emitted and absorbed by atoms and molecules act in many ways like small particles. We use the term photons to describe these BB-like wave packets. The shorter the wavelength of the photon (the higher the frequency) the more energy it carries. It is for this reason that good ole yellow sunlight is good for you, while ultraviolet light wrecks your skin.

Molecules are held together by an attractive force between the atoms. However, if you pump enough energy into a molecule, it breaks apart or disassociates. When this dissociation energy is provided by the absorption of light, we speak of photodissociation. Most molecules would be photodissociated by the ultraviolet radiation that permeates space. The molecules can survive and thrive in the interiors of dark clouds because the clouds are opaque and absorb UV light at the periphery.

After photodissociation, the molecule has been converted to free-floating, electrically neutral atoms. Atoms are held together by the attractive electrical force between the positively charged atomic nucleus and the negatively charged electrons. Usually it takes more energy to break apart an atom into negatively charged electron and a positively charged ion than it does to dissociate a molecule. Breaking up an atom is called ionization.

If photons carry enough energy (the light is ultraviolet enough), it is possible to photoionize the atoms. For hydrogen (a rough-tough atom) the light must have a wavelength of 91 nanometers. By comparison, visible light is between 400 and 700 nanometers, and UVB radiation, which will make a mess of your complexion, has wavelengths between 290 and 330 nanometers.

In giant molecular clouds, the opacity of the interstellar dust blocks the UV light from the interior, so molecules can form. These molecules aid in the formation of stars. However the formation of stars are the undoing of the molecular clouds. In the refrigerators of the molecular clouds, clumps form, contract under their own gravity, and form stars. We can see this in the process of formation. When the protostars are sufficiently compact and hot to ``turn on'', they begin radiating ultraviolet light. This light photodissociates the molecules and photoionizes the dissociated atoms. This causes the gas to glow and make pretty nebulae. This absorbed ultraviolet light also heats up the previously cold gas, causing it to expand and ``blowout'' part of the molecular cloud.

(7) These radio observations showed that the gas in the dark clouds was molecular in form, like the Earth's atmosphere. For this reason, we can call dark clouds molecular clouds.

(8) Molecules can exist in this part of space because the dust blocks the UV light.

(9) There are two important (and interesting) aspects of molecular clouds.

• Their role in star formation
• Their nature as ``chemical reactors''.

(10) Molecules and star formation.
A Carbon Monoxide view of the constellation of Orion

The existence of molecules in the dark clouds almost certainly plays a crucial role in their ability to form stars. One of the ``crucial ingredients'' is the fact that the molecules allow the temperatures in these clouds to go as low as about 70K, and even less than 10K.

According to our current understanding, this is the sequence of events in which stars form.

1. Something compresses the very dilute gas in the interstellar medium into bigger blobs or concentrations.
2. These concentrations begin contracting under their own gravity, forming clouds with about 100,000 solar masses of material.
3. As the clouds contract, they drag interstellar dust with them that blocks starlight from entering. The gas also changes from atomic to molecular.
4. In the densest, coldest parts of the cloud, clumps and condensation with masses from less than the mass of the Sun to 50 or 100 times the mass of the Sun begin contracting under the influence of their own gravity.
5. These clumps contract to the size of the solar system, and form protostars. At about the time, it is almost certain that planetary systems form around most stars. Icky chemicals from the molecular clouds rain down on the planets and generate grotesque life forms.
6. The protostars ``turn on'' as Main Sequence stars, and blast away the cloud they formed from. Molecular clouds last about 30 million years before they are disrupted by the stars they produce.
7. The star clusters hand together for 100 million to a few hundred million years until they drift apart, and the stars continue their existence as single stellar systems.

The Sun must have formed in a molecular cloud, and been part of a open star cluster. However, this was 4.5 billion years ago, and we have no idea which stars were the Sun's early playmates.

As we look out in the galaxy, we see that star formation is an active, ongoing process.
Figure of CO map of the galaxy.
URL of galaxy in CO
This image shows mostly the High Mass Star Formation Regions like the Orion Nebula. There are estimated to be such regions in the Milky Way galaxy, along with many more smaller regions like the Taurus-Auriga association.

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