Universe of Spectroscopy

13Extremely Metal-Poor Stars - to what extent can we measure small amounts of elements?

What kinds of substances constitute stars far away from Earth? There was a time when such knowledge was thought to be unattainable. However, spectroscopy has enabled detailed studies on chemical composition on stellar surfaces and measuring the abundances of a star with considerable accuracy.

Energy produced at the center of a star radiates as light. Gaseous Atoms and molecules in the stellar atmosphere, absorb characteristic light frequencies. To determine the chemical composition of a star, its starlight is dispersed into different wavelengths and the absorption lines are carefully analyzed.

Through investigation of the compositions of stars relatively close to our solar system, we have realized that many stars have a composition quite similar to that of the Sun. Most stars, including the Sun, are primarily composed of hydrogen and helium. Even in stars with relatively large amounts of heavy elements, these materials amount to only around 2% of the star's weight and less than 0.1% of its number of atoms.

However, among these nearby stars, some have much smaller amount of heavy elements (elements heavier than carbon) than does the Sun. It can be said that within these very small amounts of heavy elements, the history of the universe is engraved. After the Big Bang, only hydrogen, helium, and miniscule amounts of lithium existed in the universe. All of the other elements were formed nuclear reactions inside stars or during supernova explosion and have gradually accumulated, which means that stars born in the early ages of the universe would have very small amounts of heavy elements other than hydrogen and helium.

One of the representative heavy elements is iron, with an atomic number of 26. Because it has the most stable nuclei among all atomic nuclei, it is formed in larger amounts than other elements. Even when considering the composition of the solar system, it is evident that despite being a heavy element, iron exists in large quantities. Of all stars discovered until now, the star with the least amount of iron is HE1327-2326; its spectrum is shown in Figure 1. From its spectrum, we have determined that the abundance ratio of iron in this star is only 1/300,000 that in the Sun. Undoubtedly, this star was born in the early days of the universe (a few hundred million years after its birth) and has survived until now. This star is only a few thousand light years away from us. By studying these kinds of stars in detail, we can better understand the events that occurred in the early ages of the universe.

Column “To what extent can we measure small amounts of elements?”

How small an amount of element composition can be measured from the spectroscopy of stars? In terms of the ratio of the number of atoms, the composition of iron in the Sun amounts to nearly 1/30,000 of the amount of hydrogen. (Hydrogen is used as the reference because it is the Sun's main component.)

The ratio of iron abundances in HE1327-2326 is 1/300,000 of the Sun's ratio. In other words, compared with the amount of hydrogen in this star, the ratio of iron is nearly 1/10,000,000,000. Elements other than iron have been measured as well, although their numbers are far lower.

In a different low-metal star, a red giant, which contains the heavy element barium with an atomic number of 56, a composition ratio to hydrogen of 1/1,000,000,000,000,000 was measured. This measurement was possible because the spectral lines of barium are easily measured in the optical light region. In addition, barium tends to take an ionization/energy state that is likely to radiate these spectral lines when observed in red giants.

The current limit of measurement is around this range. However, when dealing with such miniscule amounts, many other factors which are usually ignored must be taken into account. For example, the accretion of interstellar matter on the stellar surface during the star passes the Milky Way's Galactic plane may need to be considered.

Image 1: Image of HE1327-2326 obtained from the MAGNUM telescope. From observation data, we determined the temperature of the star. (The background image is an image from DSS: AAO/ROE.)
Image 2: Comparison of the spectra of the Sun and HE1327-2326, which is a star with a very small amount of heavy elements. Because this star has nearly the same level of temperature as the Sun and is close to the main sequence, the difference in the strength of absorption lines can essentially be attributed to compositional differences. The top image is a low-resolution spectrum in the visible light region. The unevenness in the Sun's spectrum is due to the countless number of absorption spectrum lines from heavy elements in its atmosphere. In comparison, the spectrum of HE1327-2326 is smoother, and its evident absorption lines are all due to the Balmer series of hydrogen. No difference is observed with the Sun because hydrogen is the main component in the atmosphere. In the lower image, a high-resolution spectrum in the ultraviolet region is shown. In the Sun's spectrum, a large number of absorption lines can be seen overlapping. In contrast, for HE1327-2326, only a faint line of iron can be seen in this wavelength region.


July 19, 2013
High Dispersion Spectrograph on Subaru Telescope