The role of stellar clusters in empirically determining stellar evolution.
Key Concepts
(1) Stars sometimes form in clusters.
Very young clusters are still wrapped in their formation cocoon of dust and gas.
Laboratories are generally thought of as places where scientists can run controlled experiments to test their hypotheses and theories. The galaxy has helped us out to some extent by creating stars in clusters, instead of creating them one by one in random places, at random times, under wildly varying conditions of temperature, density, and chemical composition.
Stars in a cluster formed at the same time, in the same
molecular cloud.
Therefore, stars in a cluster
Thus, when stars form within a cluster, they differ only in their mass. The more massive stars evolve more rapidly, so to find the AGE of a cluster of stars, we need merely determine the mass of the stars which have just now exhausted the hydrogen in their cores and are turning into red giants.
The first diagram is of a cluster which is only 1 million
years old. The cool K & M stars have not yet settled down
onto the main sequence; they are still contracting
protostars, and have not yet ignited hydrogen fusion
in their cores. On the other hand, the hottest O star
has already been converted to a red supergiant.
. This would be about a 50 solar
mass star.
The next diagram shows the cluster at an age of
10 million years. Here the main sequence turnoff corresponds
to about a 10 solar mass star. The
The next frame is of the cluster
that is 100 million years
old. The main sequence lifetime of a 6 solar mass star
is 100 million years, so stars with M = 6 Msun
(L = 530 Lsun, spectral type A) are just
turning off the main sequence.
The next frame shows the cluster at an age of 1 billion years, which corresponds to the lifetime of a 2 solar mass star (spectral type F). By this age, a few of the more massive stars have no evolved to populate the white dwarf sequence.
The final diagram is of a cluster which is 10 billion years old. The main sequence lifetime of a 1 solar mass star is 10 billion years, so stars with M = 1 Msun (L = 1 Lsun, spectral type G) are just turning off the main sequence. All the stars on the red giant branch are more massive than 1 solar mass. More white dwarf stars are also present.
Fortunately, the mathematical models provide a good fit to the Hertzsprung-Russell (H-R) diagrams which are actually observed for clusters of stars.
For instance, the H-R diagram of the Pleiades (an open cluster in the constellation Taurus) indicates that the cluster is 76 million years old. When this technique is applied to other open clusters, they are found to have a fairly wide range of ages.
In class exercise.
Blue Image of Stellar Cluster -- represents an image of a star cluster taken with a blue filter (only blue light hits the detector).
Red Image of Stellar Cluster -- represents an image of a star cluster taken with a red filter (only red light hits the detector).
Click on the above two links to open two separate windows.
This is the CCD simulation applet that we used back in the early part of the term and that you used for your first homework assignment.
For each image, take a 100 second exposure.
Compare the two images. The numbers under each star are the same. However, a star that is very red, would appear much brighter (deposit more energy on the detector) on the red image than on the blue image. Typically such stars would have surface temperatures of 4000K or less.
Similarly, a very blue star will appear to be much brighter on the blue image than on the red image. Typically such stars would have surface temperatures of 10,000K or more.
Stars that are between 4,000 and 10,000 degrees have neutral colors and their brightnesses will appear to be about the same on both the red and blue images.
Using this table identify, by star number, whether the star is red, blue or neutral in color, by comparing the image size/brightness on the respective red and blue filter images. Enter all the stars on one line. For instance, under blue, your entry could look like 1,2,3,7,12,17,21,30
When done, publish to global view.
Note, the positions of each star are the same on the blue detector and the red detector. So your comparing image brightnesses on each of the detectors for the same star. Stars whose images appear approximately the same in each filter would not be either very red or very blue.
In rough terms, the ratio of blue stars to red stars would be an indicator of cluster age. The less blue stars there are, the older the cluster.
Now on to Supernova and Neutron Capture.
The Relative Abundances of the elements reflect these fusion reactions. Elements divisible by 4 are the most abundant.
To get beyond iron requires Supernova and neutron capture:
Neutron Capture: Two processes:
Rapid = capture of a neutron before a neutron to
proton decay can occur
Slow = Neutron capture decays into proton
another neutron is captured
Slow Process Sequence for Formation of Colbalt 59
Rapid Process Sequence for Formation of Colbalt 61
As the neutrons flood through the now exploded envelope of the star, the filaments get enriched in heavy elements.
In the solar system (meteorites) most heavy elements are proton-rich indicating S-processed elements, but some R-processing occurred which is of very important consequence to the earth.
But R-processing produced a small bit of Uranium which is extremely important.
Overall Composition of the Earth:
34.6%
29.5%
15.2%
12.7%
2.4%
1.9%
Composition of the Earth's Crust:
