MADIWALA:City astro-physicists were recently surprised to find some stars with moderate levels of hydrogen.
Stars are usually rich in hydrogen, but some are deficient, and some show moderate levels. “We don't usually expect hydrogen-deficient stars, but we see them in nature,” said Dr Gajendra Pandey of the Indian Institute of Astrophysics, Bengaluru, whose student B P Hema reported this discovery in a paper published in The Astrophysical Journal Letters.
“This kind of star was not discovered earlier,” said Hema, who will soon be defending her doctoral thesis, of which this study is a part.
A typical star derives its energy from the fusion of hydrogen nuclei to form helium, the next higher element in the periodic table.
When the hydrogen at its core runs out, it begins to fuse helium to form elements higher still. It also swells in size to become a red giant. But its atmosphere, the outer layer, will still have hydrogen, the most abundant element in the universe.
There are some stars, however, that may not have much hydrogen in their atmospheres. Finding out how such a star forms and evolves would require, as for any other star, knowing its surface or effective temperature, and its intrinsic brightness or luminosity.
To find such specimens with almost no hydrogen, Hema used the Vainu Bappu Observatory in Kavalur, Tamil Nadu, to survey the globular cluster Omega Centauri, which is at a known distance. Although it is the largest and brightest such aggregation in our galaxy, it is 16,000 light years away. “These are very faint stars,” Hema said.
If the stars in a globular cluster are formed from the same material, scientists expect them similar composition: the same proportions of hydrogen, helium, and metals, the catch-all term astronomers use for all other elements. The presence and relative amount of any element or molecule in a star’s atmosphere can be determined from the characteristic dark lines it leaves in the spectrum.
Pandey suspects some stars appear more metal-rich than they really are because of their hydrogen deficiency, which leads to their atmospheres being more transparent to light at visible wavelengths. So to detect hydrogen-poor stars, it might be a good idea to search for metal-rich ones.
Unusually for a globular cluster, the stars in Omega Centauri don't have the same metallicity, the proportion of metals they contain. “The range in the metallicity is the main reason we selected this cluster,” she explained.
“If you pick two stars of the Omega Centauri cluster which are of similar temperature and gravity, their spectrum should be exactly the same if they have the same metallicity,” Pandey said.
If they’re not, it means they have different amounts of the element or molecule responsible for the dissimilar spectra. “That is the criteria we use. We look for similar effective temperature and surface gravity," he added.
The researchers went to look for stars that are hydrogen-deficient, with almost no hydrogen, but ended up discovering stars that are relatively hydrogen-poor.
"This is the first detection of such stars,” he said.