June 23, 2014
Star Measurement Made Easier With A Novel Spectral Ruler
Brett Smith for redOrbit.com - Your Universe Online
Being hailed as a critical first step in a massive attempt to map the Milky Way, a team of European astronomers has devised a novel way to categorize stars based on their metallic composition, according to a new paper published in the journal Astronomy & Astrophysics.
The study team developed their ‘spectral ruler’ by selecting 34 'benchmark' stars to characterize the distinct kinds of stellar groups in our galaxy, such as hot stars or cold stars; as well as red giants and white dwarfs. The researchers also considered stars that include distinct chemical patterns, or 'metallicity,’ in their spectrum. The study team said these compositions act as a "cosmic clock" which allows astronomers to learn a star's age.
This range of data constitutes the first value set for calculating the millions of stars Gaia seeks to catalogue. Many of the benchmark stars are well-studied and can be observed with the human eye.
"This is the first attempt to cover a wide range of stellar classifications, and do everything from the beginning – methodically and homogenously,” said study author Paula Jofre, an astronomer from the University of Cambridge. "We took stars which had been measured a lot so the parameters are very well-known, but needed to be brought to the same scale for the new benchmark - essentially, using the stars we know most about to help measure the stars we know nothing about."
"In previous galactic studies, the Sun is used as the standard to show a method is working, along with a few other well-known stars,” she continued. “But I choose this one because it works for my method, you choose a different one for different reasons; data may not match.”
The new study is being published as the European Space Agency is embarking on its Gaia mission to catalog the entire Milk Way. Launched in December 2013, the satellite-based mission will allow astronomers to conduct a detailed study of the galaxy’s myriad stars and planetary systems.
The study team said their system was required to make certain that the investigation of the extraordinary amount of information generated by Gaia is performed in the most effective and efficient way.
As a basis for a new system, the researchers focused on metallicity – or the metallic elements found within a star. Using spectroscopic techniques, scientists can split the light from a star into its chemical elements. Typically, the higher a star's metal content the newer it is.
Jofre and her study team created a 'spectral' library including the best information on the atmospheric structure of the 34 benchmark stars to ascertain a standard scale for metallicity. The team also included definitive measures for the stars' temperatures and surface gravities in their spectral ruler.
"Now this set of data scales for the benchmark stars can be used as a way of making definitive measurements of others stars – invaluable to astronomers working on a wide range of projects," Jofre said.
"Advances in understanding the history and structure of our Galaxy with ambitious projects are possible only because, like Newton, we see farther by standing on the shoulders of giants,” added Gerry Gilmore, a Cambridge astronomer and Gaia scientist who was not involved in the study. “For reliably determining what chemical elements the stars are made of, those giants are the benchmark stars. All our vastly expanding knowledge depends on really understanding the few."