Detection of stars outside the main sequence in open clusters


From the HR diagrams of the open clusters, you can identify candidate stars for giants, red supergiants or blue stragglers by their placement in the diagram. (graphic by ESA Science & Technology)

The red giants are stars of small or intermediate mass that have consumed the hydrogen of the nucleus during its stage in the main sequence turning it into helium by nuclear fusion and its current state is burning hydrogen from a layer around the nucleus, which increases its size and surface temperature be lower.

The supergiant stars constitute an evolution of the red giants in which a temperature in the nucleus of more than 100 million degrees has been reached and the triple alpha process of the nuclear fusion is produced that generates an expansion of the star and a new cooling Of the surface.

The blue stragglers would be in the diagram those that appear somewhat separated in the prolongation of the main sequence. They are stars that are beyond the point of takeoff of the sequence and that possibly are the result of mergers of stars.

Methodology for the detection of red  giants and blue stragglers in open clusters

Once the HR diagram is made (see the section "measuring distance to open cluster"), the stars belonging to the diagram that meet these conditions are searched:


  • Red giants: B-V > 0.8 and apparent magnitude < estimated value from the HR diagram so that they are outside the main sequence. It is not a criterion that 100% of giant stars and red supergiants necessarily meet. In some diagrams there are special cases such as stars belonging to the giant red asymptotic branch.

  • Blue stragglers: B-V < 0.5 and apparent magnitude < value estimated from the HR graph so that it results in a point outside the main sequence, such as prolonging it. In some cases there are special stars with metallic lines (Am type).


The result is represented graphically using GNUPLOT highlighting the stars that meet the condition with a red dot (red giants) or blue (blue stragglers) and a CSV file is generated with the crossed data of:

  • Measurements made of B-V, V, V-modulus of diastance and signal noise

  • Catalog values ​​of B-V and V performing the data crossing from the values ​​of RA and DEC. The WCS2000 scat tool is used on the Tycho2 catalog


The list of stars obtained are candidates for special stars. To verify it, two sources are used:


The results found for some open clusters are shown below.

Open Cluster M37

Candidate stars for red giants:

The graph shows red stars with B-V> 0.8 and brighter than magnitude 12.


The table shows the candidate red stars with the data obtained, either from the Tycho2 catalog or from the measurements, as well as additional information from the WEBDA and SIMBAD databases. Of the 14 stars, 9 are confirmed for WEBDA, and another 2 are confirmed by SIMBDA, since they have data of spectral type and belonging to the cluster.

Red giants
Blue stragglers stars


Candidate stars for blue stragglers

In the graph, stars with B-V> 0.5 and apparent magnitude <11 (to be outside the main sequence) have been indicated in blue.


The table shows the stars found as candidates for blue stragglers. 3 of the 4 have been confirmed with the WEBDA database, confirming also its pernecencia to the cluster.

Open Cluster M67

Candidate stars to Red Giants

In red, the stars that meet the condition of B-V> 0.8 and the brightest apparent magnitude of 12.5 are shown

By the ascending form of isochrone, the red stars may be in the phase of the giant asymptotic branch (Asymptotic Gian Granch) very interesting stage of some intermediate mass stars.

The table shows the stars found (crossing RA and DEC with the Tycho2 catalog). From the denomination in Tycho2, the candidate stars are searched in the database WEBDA and SIMBAD to obtain more information, especially of the spectral type and of the cluster membership.

In this case, no data were found in WEBDA with the Tycho2 names. But in SIMBAD it is verified that all but one are confirmed as belonging to the group, and all have spectral types compatible with the red giants.

The most interesting part is that many are confirmed as stars belonging to the giant red asymptotic branch, which on the other hand is in accordance with the arrangement of the stars in the HR diagram. These types of stars are in their last evolutionary phase. It has already burned all the helium in the nucleus (it has burned hydrogen before) and burns helium from the outer layer, reducing the surface temperature and increasing its size. In the end you can get reactivate burn hydrogen from external layers, leading to instability processes.

Red giants
Blue stragglers stars


Candidate to blue stragglers stars

In the graph, the stars with B-V> 0.5 and apparent magnitude <12.5 (to be out of the main sequence) have been indicated in blue.


The table shows the stars that meet the criteria. The membership of all of the cluster is confirmed. All but the num 261 are confirmed as blue lags either by the WEBDA database or by SIMBAD.

Especially interesting two stars (num 153 and 55) of type Am (with metallic lines). They are stars whose spectrum has strong lines of absorption of metals such as zinc, strontium, copper, zirconium and barium, and deficiencies of others, such as calcium and / or scandium. This particularity is due to the fact that some elements that absorb light better are pushed towards the surface, while others sink due to the force of gravity. This effect occurs if the rotation speed is low.