The curious case of anaemic stars

While observing the spectra of stars almost a century ago the astronomer Adams hit upon an unusual star. The faint star in the constellation of Libra at a distance of 190 light years, just had a catalogue number. But it attracted the attention because of many reasons - it had no metals at all. Metals in astronomy imply all elements other than hydrogen and helium. Metallicity is expressed as a ratio of abundances of iron to hydrogen, Fe / H. Therefore, physicians may permit metal-poor stars to be called “anaemic”.

The second reason was much more intriguing - it was running away with quite a high velocity. The third quality, that it was quite faint compared to stars similar to it spectroscopically, got its classification as “subdwarf”. However, apart from noting down these specialities, no hypothesis was put forward to explain these peculiarities, till 2013, after 100 years of Adam’s work.

100 years is quite a long time though not on an astronomical scale. New ideas developed, and new concepts were understood and applied to the stellar contexts. One of them is the structure of our Galaxy which is now established. All metal deficient stars are seen in the halo of the Galaxy while the young metal-rich stars are in the disc and spiral arms.

The sun and all the stars are going round the centre of the Galaxy in the plane of the disc. This star, numbered 140283 in the Henry Draper Catalog, was running in a direction which was distinctly different from that of the other stars, perpendicular to the disk of the Galaxy. Considering this motion as inherited property, we can fix its birth date. This turns out to be earlier than the formation of the disk itself.

The metallicity, or the abundance of elements other than hydrogen and helium, is understood now in terms of the different populations of stars. The massive stars of the first generation ended their life quickly within a billion years or so. A funny extension to the animal kingdom - an elephant would live only for a few years while a mouse would live for 100 years!

Their interiors produced heavier elements by nuclear processes which are now well understood. The ultimate cataclysmic event supernova, shreds these elements which mix with the interstellar matter as the essential ingredient for the next generation. Thus all the entries in our periodic table are contributed by supernovae. The sun, therefore, is a second-generation star (Population I). That lets us classify this anaemic star as the previous generation (Population II).

Now the question arises - how far back can we go to reach the older stars? This was precisely posed by this “old” star. Its estimated age took it back to the time of the Big Bang. The star earned the nickname Methuselah Star, named after a biblical patriarch who is reported to have lived 969 years.

Let us understand the methods used for estimating the distance and the age. One of the standard procedures is to measure its colour. This number, the colour index is compared with that of stars of the same spectral type. The reddening caused by the intervening material gives a measure of the distance, which is used for estimating the luminosity. Subsequently, the HIPPARCOS measurements provided a better estimate.

The inferred luminosity places it on the theoretical evolutionary tracks for fixing the age. The first estimate put it at uncomfortably high values of 15 to 14 Giga years. The possible sources of errors are being reinvestigated though this stage of evolution is known to be highly sensitive to differences of a hundred million years.

This also poses a new question to cosmologists on whether a revision of the age universe itself is needed.

Metallicity, therefore, is the critical factor in acquiring candidates for the new classification as Population III. These could be the stars hypothesised to have been formed just after 400 million years of the Big Bang. Searching for a star with only hydrogen and very little helium leads us to the Galactic Halo regions, as the natural choice. However, there is very little hope of finding any of them.

Over the humble time scale of 100 years, many modifications reviewed our understanding of the theory of stellar evolution. The age of this star still hovers around 13 Giga years.

Meanwhile, a star in a faraway galaxy has further complicated the issue. The dwarf galaxy in Sculptor is of poor metal content. A star in this galaxy numbered AS2009 appears to suffer from very “severe anaemia”. Its estimated distance is about 13 billion light years, fixing its date.

Yet another “old newspaper” clipping has surfaced recently. Hubble Space Telescope surveyed several clusters of galaxies. These massive galaxies bend the light and provide magnified images of fainter objects. This technique of gravitational lensing revealed the farthest star, at a distance of 12.9 billion light years. It was nicknamed Earendel after a mythological character.

Have we hit upon Population III? You went searching for your grandfather and suddenly confronted the great grandfather!!

(The author is a scientist at Jawaharlal Nehru Planetarium, Bengaluru)