Star Clusters & Stars

Open star clusters are some of the youngest pieces of the universe.  Astronomers have found open clusters situated along the spiral arms of the galaxy, and sometimes refer to them as galactic clusters.  Many star clusters contain hundreds of stars.  The study of galactic clusters is extremely important in the study of the evolution of the stars.

Most open clusters are situated within dust clouds called reflection nebulae.  These reflection nebulae appear as a blue haze. This is because dust particles in space reflect blue light easier than red light.  Stars form within gas clouds called dark nebulae.  Dark nebulae cannot be seen without special equipment.  The clouds are so dense they absorb any nearby starlight, so stars actually seem to hide behind them.  Dark nebulae are barely dense and cold enough to begin the chemical process of star birth. (Kaufmann, 498)

Interstellar dust clouds actually come as an enigma to scientists, because of the near vacuum medium the particles travel through to combine and form clouds.  Strong forces must play an essential role in their formation.  The stellar winds from dying redgiant stars (as they are burning off their last remnants of heavy elements they blast into interstellar space) are one example of the type of force needed. As these heavier elements cool off, they recombine as molecular dust particles. Over time they become increasingly dense and ionize from the starlight of nearby stars hitting m.  Gravitational forces continually pull the dust and gas toward the central part of the nebulae.  As more particles combine, the more friction is created and thermal heat can begin. (Kaufmann, 498)

This thermal heat is the beginning of the star formation called a protostar.  Protostars begin very large and condense through time.  As they condense, they give off energy.  They have not yet begun to create thermonuclear energy, which stars like our sun generate. These protostars are called T Tauri stars.  Nebulae are rather large and can churn out many of these protostars.  This is how an open cluster forms.  As the protostars increase in temperature, until they reach main sequence temperatures, they illuminate their surface through the process of fusion. (Kaufmann, 504)

The emission of photons from the older stars within the cluster can effect the formation of the protostars.  The photons blast away dark nebulae, which is the seed of stellar formation (Kaufmann, 504). This can cause a smaller star to form.

The stars within a cluster have barely enough gravitational pull to keep them together (Frommert, Online). This can create a rather large spread of open cluster members. The average age of a cluster is approximately 100 million years (Frommert, Online). This makes sense, because as stars are created they drift further and further from each other. These stars continue drifting until they are no longer within the grasp of the gravitational forces of the clusters.  The star constellation Ursa Major is actually part of a vastly spread out (23 degrees) open star cluster. (Fortier, 79)

These images are from the Nebula M16. Note that each poked out area is a stellar nursery, or a pocket housing a protostar.

Both images are courtesy of NASA at http://observe.ivv.nasa.gov

Chemical Reaction
Two types of interstellar clouds form from the red giant stellar blast.  Cirrus nebulae consist of dense particles, which absorb light.  Diffuse nebulae contain both gas and particles, but are inherently much more dense. They both contain a high abundance of CO, carbon monoxide (Verschuur, 381). The hydrogen element is the most abundant element found in the universe. Astronomers use CO to detect the presence of the H2 molecule (two hydrogen atoms).

As these clouds condense into colder matter, they combine and form into a type of dark nebulae called Bok globules. This density causes pockets within the nebulae, which become the cocoons for protostars (Verschuur, 381). This process can be initiated by the ionization from starlight.  Ionization means that the elements or molecules have become charged either positively or negatively.  This can lead to other chemical reactions as positive and negative charges attract.

The density of the molecules within the Bok nebulae establishes the reactivity of them and the energy radiating from the clouds. As the energy increases, the electrons within the atoms vibrate more rapidly. This is what causes the initial energy forming protostars (see figure 1).

MADE BY: Sara Petty-Powell and Christina Pince
LAST MODIFIED: 23*May*1999 @ 6:00 p.m. (PST)