Hercules Globular Cluster (Messier 13)
Picture from: http://zebu.uoregon.edu/~js/ast122/lectures/lec12.html
The formation of a globular cluster:
In the hydrogen clouds where galaxies are made there are some giant molecular clouds. Caused by the heavy mass, central points appear and the hydrogen gathers. The temperature rice at the central points, and the hydrogen burning process starts, and stars will be born. The central points turns into many stars by one or a few stars, which starts a chain reaction. The heath from the first stars starts the process for a stars beginning for the other central points. It means that all stars in globular clusters are about the same age. In one cluster there are made about 100 000 stars. Because globular clusters are formatted at the same time as the galaxy, they are settled around the galaxy.
The remaining hydrogen in the cloud forms the actual galaxy. That is how our Milky Way is made. In such galaxies open star clusters arise. Because they are formed in the galaxy there are not enough molecular hydrogen mass available to form a globular cluster. Open star clusters therefore consist from a less number of stars and the stars are not placed in a ball but spread. Because of its young age, open star clusters may consist of remnants of supernovas. Contrary to closed clusters, there is a possibility for life in these clusters because the right elements exist (heavy elements).
H-R diagram for an open cluster. H-R diagram for a globular cluster.
The sketches above show a H-R diagram for respectively open galactic clusters and globular clusters. The variety of turn-off points in open star clusters is wide. The turn-off points in globular clusters are very similar.
Stars, from the very beginning to the end.
A star is made from a hydrogen cloud in space. The molecules in the cloud draw together towards a central point. This cause frictional resistance, which at some time grow until the hydrogen nuclei combine. They fuse together and become helium atoms. Our sun uses this process to create energy. When the process has been started in one star it causes a chain reaction, which starts the process all over the gas cloud. The actual star is this enormous production of energy.
The process goes off for millions or billions of years whereupon there is not hydrogen enough to preserve the energy production. For a short time the energy production totally stop. The star collapses more and more. The collapsing causes frictional resistance that makes it possible for the helium atoms to fuse together and create carbon. When this production has been started the star swell to giant size. At this stage it emits a lot of energy. Because of its size the surface temperature are not pretty high. This process does not continue for a long time. Afterwards, the star collapses and become a white dwarf. It remains this state and “cools down”.
Hertzsprung Rossel Diagram
A HR diagram shows the temperature and light intensity for different types of stars. In short time a large cluster becomes well arranged by putting different colour strainers over ccd-pictures of the stars. For example if you have visual, blue and ultraviolet light, you put the visual light on the y-axis, and the minus blue on the x-axis. That is how you create a HR-diagram.
In a HR-diagram for a group of stars with same age, there is a main line in which stars that burn hydrogen are placed. Red giants that burn helium are placed to the right in the diagram. Stars that do not even burn hydrogen or helium are in a position where they wait for becoming a white dwarf, a supernova or a black hole. They might swell up to start a new nuclear reaction instead. Lowest to the left in the diagram you find recently made white dwarfs which is cooling off. They have a strong central point.
You can tell the age of the cluster from a HR-diagram. You do it by reading the turn-off points for main sequence. You can tell the age of a star from its mass. This also indicates the age of the cluster.
Furthermore you can decide the distance to the stars from a HR-diagram. You just need a great number of data to do that. You need a great number of stars with same mass but with different distance, which you know. When you place them in the HR-diagram, you see the stars are placed almost vertically. This indicates that the stars have different distances. If you have enough points to measure, you can make a pretty exact calculation for the distance to the stars.
This is a HR–diagram of the globular star cluster M-13. The M-13 is a relative old star cluster, which you clearly can se on the diagram, because the main series are bending off quite fast, because there are many red giants, and many stars in the horizontal branch. The diagram are made after the Stromgren system, where you are using four colour filters, instead of the ”classic” HR-diagram, where you are using three filters. The data we have used, have we got from Frank Grundahl from University of Aarhus. The four filters are:
| Ultraviolet, u: | 250 nm |
| Visua, v: | 411 nm |
| Blue, b: | 467 nm |
| Yellow, y: | 547 nm |
You are using the visual filter on the 2. axis, and the visual – the yellow on the 1. axis. You can find which type of stars are the oldest, and still is in the main series, by reading the diagram. It is about 0.6. This number can, being put through a calculation, be a temperature in K. It is about 6200K. To find out which type of stars that have this temperature in the main series, we use a freeware program named “starclock”. It is programmed by Leos Ondra. With help from this program we find that it must be a star with a mass about one solar mass, and that these types of stars typically will be about 10,000,000,000 years old, before they leave the main series. From this we can conclude that the globular star cluster M-13 must be about 10,000,000,000 years old.
This HR-diagram is made by data from the M-45, or the Pleiades”, which is a very young open star cluster. The data we have used, have we got from Frank Grundahl from University of Aarhus. By the diagram you can very clearly se this, because that none of the stars really have left the main series yet. This is an example on one of the “classical” HR-diagrams, which is using three colour filters:
| Ultraviolet: | 365nm |
| Blue: | 440nm |
| Visual: | 550nm |
http://www.seds.org/billa/dssm/m45.html
We have a great difficulty by estimating the age of the cluster, because no stars have left the main series yet. Though, if we make the assumption, that the oldest star is just about to leave the main series, we can follow the same procedure as for M-13. The star is about 0.56, which is equal to a temperature on5550K. This means that it is a star on ca. 0.9 solar masses. This type of stars has a lifetime on ca. 8.000.000.000 years, before they start to leave the main series. From this we can deduce, that the cluster is maximum 8.000.000.000 years old. The cluster may be much younger because we see no stars that have left the main sequence to become red giants. This may indicate that the heavy blue giants for some reason are missing in this diagram.
Exercises:
- Look at the HR-Diagram for M-13, and find the main series, the Giants, the Supergiants, and the horizontal branch.
- The Betelgeuse is a red giant, and can be seen in the sky near Orion. Is the temperature of Betelgeuse higher or lower than the suns temperature?
- Explain the devolvement of a star.
- Obtain the Leos Ondra’s program StarClock20, by downloading sclock20.zip from ftp://ftp.seds.org/pub/software/pc/stars/. The program simulates the development of a HR-diagram in time. Run the program to see the HR-diagram of a 50 million year old cluster and a 15 billion years old galactic cluster.
Written by:
Johanne Solås Kjeldsen, 19 years old
Joachim Grandjean- Thomsen, 18 years old
Morten Gravgaard Poulsen, 18 years old.
Students at Morsø Gymnasium, Limfjordsvej 95, 7900 Nykøbing Mors, Denmark.
Teacher: Jens Peter Diget, teacher of astronomy at Morsø Gymnasium email: jens.peter.diget@skolekom.dk