Stars Clusters General Definitions

General definitions - Star Clusters -

What is a star cluster ?

A star cluster can be defined as an undoubted concentration of several stars, gravitationally bounded, emerging clearly over the surrounding stellar background.

Why star clusters are important ?

As already said star clusters are important in relation to various astrophysical problems, including disk structure of our galaxy, as soon as stars formation and their evolution.

Generally open clusters show a great range of different characteristics:

they span a wide range in ages and these ages can be obtained quite reliably from observations
they can be seen at large distance from us and their brightest stars can be found, practically, in all spectroscopic and luminosity classes, depending upon parent cluster age.

So, is evidently that the studies of colour-magnitude diagrams, allows us to test models and theories of stellar evolution.

During their evolution the more massive stars reach Zams, from pre-main sequence stage first and then, less and less massive stars, complete the same itinerary reaching Zams. However, during the time spent by a stars of two solar mass to reach the Zams, a ten solar mass star would be evolved until the end of the AGB phase.

Naturally all this demonstrate that the older is cluster, the lower will be the star mass on the Zams and still, the lower will be the turnoff point on the main sequence.

Thus, the main sequence turnoff is a good indicator for the cluster age.

The analysis of both galactic and globular cluster has helped us in formulating the following concepts:

Zams ( zero age main sequence ) that represents the initial position of a stars on the color-magnitude diagram or HR diagram while it actually convert hydrogen into helium through core nuclear reactions. From an observational and evolutionary point of view the zero age main sequence can be defined, neglecting alone white dwarf, as the low luminosity locus (Balona).
Determination or verification of the initial luminosity or mass function ( LF or IMF ).
Calibration of distance scale in our galaxy.
Accurate determination of the ratio ( Fe/H ) and his variation with the galactocentetric distance.
Determination of the nature of interstellar reddening law.
Statistical distribution of stars along the color-magnitude diagram with indentification of various gaps that are associated to different physical processes.
Identification of various stellar activity associated to individual peculiar objects.

The open galactic clusters can be divided, on the basis of their evolutionary stages, into three broad categories, these are:

Old galactic clusters – Age estimated at least 10⁹ yr. Ex. M67 or Ngc 752.
Intermediate age galactic clusters – Ages ranging from 5 × 10⁶ ÷ 10⁹ yr. Ex. Hyades, Coma.
Young clusters – Ages ≤ 5 × 10⁶ yr. Ex. Ngc 6530, Ngc 6611, Ngc 2264.

In all previous three groups, an accurate determination of member's intrinsic colours provide information about their position on colour-magnitude diagram, the nature of their reddening and also the eventual existence of circumstellar shells.

But over this matter, what can really do an advanced amateur ?

I think that a really advanced amateur, can do very high quality job in open clusters research field.

Often an advance amateur can be much skillful with photometric jobs and for this reason, can lead high quality job. This can be done, for instance, using UBVRI Johnson or Kron-Cousins systems or at least: BVRI Johnson or BVRcIc Johnson-Cousins.

For photometric systems and all sky photometry, we refer our readers to the following books:

Arne A. Henden and Ronald H. Kaitchuk - ” Astronomical Photometry” Willman Bell.

Edwin Budding - " Introduction to Astronomical Photometry " Cambridge U.P.

Two more complete and advanced books on this matter that, unfortunately, require extensive mathematical and astronomy backgroud, are:

M. Golay - " Introduction to Astronomical Photometry " Dordrecht Reidel.

V. Straizys - " Multicolor Stellar Photometry " Pachart Publishing House.