29:50 Modern Astronomy
Fall 1999
Lecture 13 ...September 22, 1999
The Formation of Stars

First exam one week from today. In the skies this week:
(0) Tonight, we'll try it again at 8PM in the lobby (1) On Thursday, (September 23) at 6:31 CDT we have the Autumnal Equinox. Notice that the Sun (and Full Moon) rise and set due east and west.

Stellar Birth and Death Let's start with the formation of stars. How does such an object get organized? Where do they come from? Outer space is obviously empty. The density of matter is extremely low. The mean density of matter in the Sun is about atoms per cubic centimeter. Where is all this stuff collected from?
Role of gravity is crucial in pulling matter together, scooping up large amount of matter thinly spread out.
We can see this process in the act of occurring, although it took a long time and lots of observations to figure out what was happening.
A crucial concept is the Main Sequence lifetime of stars.

SC1 Chart from winter sky.
Last time mentioned the Pleiades. a picture of the Pleiades is given on p511 of your textbook. Astronomers who have studied these stars conclude that they, and the Pleiades cluster, must only be about 70 million years old.
This star cluster must be close to the place it formed, and thus should give us some hints about star formation. You can see what appears to be wisps of gas that might be ``leftovers''.

Picture of Pleiades
Not far from the Pleiades are other examples of Open Star Clusters which are young swarms of stars.
M37 in Auriga.

Where does the matter come from to form these stars?
Look at Atlas of the Sky in the vicinity of the Pleiades, M37.... you can see examples of Dark Clouds.
Picture of Dark Clouds on p414 of textbook. For a long time the nature of these dark clouds was uncertain. In late 1960's came a big breakthrough. Astronomers developed the ability to detect radiation emitted by molecular gas as well as atomic gas. They found that these dark clouds contained huge amounts of gas in the form of molecular gas. It is primarily molecular hydrogen , although many other molecules have been found as well.
Pictures of molecular clouds.

These molecular clouds are fascinating objects in themselves. Astronomers have identified over 100 species of molecules, including some with as many as 13 atoms.
Table with list of molecules.
It has been suggested that the beginnings of life occur in these dark clouds, where organic molecules form.

The centers of these clouds are the coldest places in the universe. The opaqueness of the cloud keeps warming sunlight from coming in, and the cloud continues to cool itself off by emitting radiation from molecules. A warm cloud interior will be at 70K, the temperature of liquid nitrogen. A more typical temperature would be 50 K, and it is very common for the temperatures to reach 10 K...10 degrees centigrade above absolute zero.

In these refrigerators, 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 cause the gas around them to glow and they blow off the material from which they formed.

It is at this point that we see the star as a member of a very young star cluster. It is for this reason that we still see a few wisps around the Pleiades; this is part of the cloud from which the Pleiades formed.

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.

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