Universe of Spectroscopy

14Stars in Globular Clusters - Compositions are actually not uniform -

A globular cluster is a group of stars. A large globular cluster consists of more than one million stars. In the Milky Way Galaxy, it is known that most globular clusters were born more than 10 billion years ago in the early stage of the Galaxy's formation. The color-magnitude diagram of a globular cluster that is plotted the relationship between stellar color and brightness clearly shows stellar evolutionary sequences such as a main sequence and a red giant branch that are known in the so-called HR diagram. The metallicity (the amount of chemical elements heavier than helium) and age of the cluster are estimated from this diagram.

It had been believed for a long time that the stars in a globular cluster were born at the same time from a gas cloud that had a homogeneous chemical composition. Therefore, globular clusters have been studied to investigate various characteristics of stars, and have served as "laboratories to study stellar evolution."

However, recent observations have overturned this accepted idea. The globular cluster Omega Centauri was known to contain stars with a variety of metallicity. It had been thought that Omega Centauri s just an exception. However, recent observations have revealed that, in regards to other globular clusters, the composition of light elements, such as carbon and oxygen, also significantly differ depending on each star. Such phenomena were known previously only for red giants in some clusters, and effects of stellar evolution were suggested for the reason for the variation of chemical composition, because, when a star evolves into a red giant, light elements that are newly made inside of the star emerge on the surface. It was not clear whether the variations of the abundances of light elements in red giants reflect the chemical composition at the birth of the star or whether the light elements have been affected by the star's evolution.

On the other hand, accurate photometric observations in recent years have found examples, one after another, in which main sequence and subgiant sequence (stars on the way to evolving into red giants) are divided into multiple branches on color-magnitude diagrams. If stars were formed at the same time with the same chemical composition, these sequences should have been single line. Thus, these examples indicate that, in a globular cluster, stars with different compositions were possibly born in different eras.

Although we can roughly estimate stellar metallicity based on their color, spectroscopic observations are essential to deriver accurate metallicity. For the cases written above, there are also some reports that claim differences of stellar compositions by spectroscopic observations. Among those reports, there is also a surprising finding that differences in metallicity obtained from spectroscopic observations are the opposite of the differences in metallicity that are expected from the stars' colors. Generally, stars that have higher metallicity are redder in color. On the other hand, stars that have lower metallicity appear bluer. However, the spectroscopic observations indicate that the sequence of redder stars have lower metallicity. In an attempt to explain this result, it has been suggested that as much as 50% of the composition of helium, an element that is not included in "metal," in these two sequences could be different. Helium is an element that was produced in large amount, along with hydrogen, at the Big Bang. This would be a huge issue if the amount of helium significantly differs depending on the star.

In this way, new mysteries are emerging through observational development regarding globular clusters that have been closely studied for a long time. When you turn a telescope toward globular clusters, many stars come in the field of view. Using multi object spectrograph (that enables us to obtain spectra of lots of objects simultaneously) is an efficient method of observations. To derive accurate metallicity, wavelength resolution should be high enough. The Very Large Telescope (VLT) of the European Southern Observatory is equipped such an instrument. Thus, this research field studying chemical composition of star clusters is actively performed with the VLT.

Column “Enigma also Seen in Heavy Elements”

There are also globular clusters in which the composition of elements that are heavier than iron significantly differ between stars. A study of M15, a metal-poor globular cluster, reported this for the first time in 1997. Detailed spectroscopic observations with the Subaru Telescope not only confirmed the result, but also revealed differences in the compositions of a variety of heavy elements between the stars (2006).

Figure 2 shows an area around a spectral line of a heavy element europium (atomic number 63). The spectra of two similar red giants in M15 are compared. The absorption lines of iron are so similar that it is difficult to tell the difference. However, the absorption line of europium in one red giant is much stronger. You may find small differences in other absorption lines. These also correspond to the strength of absorption lines of heavy elements, such as lanthanum and samarium.

It is known that these heavy elements were produced by explosive nucleosynthesis. Differences in the composition among stars are an important point when studying how the original explosive phenomenon was related to the birth process of stars in globular clusters.

Image 1: Globular Cluster M15 (an overall image taken by a 50cm public telescope of NAOJ and an infrared image obtained with the Subaru Telescope's adaptive optics system)
Image 2: Comparison of the spectra of two red giants in M15. There are noticeable differences in absorption lines of heavy elements, including europium.


July 19, 2013
Subaru Telescope (HDS)