Nailing fingerprints in the stars

What are stars made of? After astronomers detected a bright-yellow, unknown spectral line in sunlight in 1868, they named the new element helium after the Greek sun god Helios. But it took about 30 years before physicists on Earth managed to detect – and therefore confirm – the discovery of helium in a laboratory.

It is a pattern that has been repeated many times: the indirect detection of elements and molecules through spectral signatures in space has preceded detailed studies on the ground. Lab spectroscopy has long lagged behind binocular observation, but it is striking how wide this gap has now widened.

A state-of-the-art infrared spectrograph, for example, installed in 2011 on the Sloan Digital Sky Survey (SDSS) Telescope in Sunspot, New Mexico, records the spectra of 1,800 stars per night, located in the Milky Way’s bulge, where dust forms a stream of light. blocks visible wavelengths from reaching Earth.

The result is the detection of thousands of unknown spectral lines – dips or peaks of electromagnetic waves at specific energies, caused by the absorption of light by gas on its way to Earth or emission by gas on stars.

Some physicists are now pointing to the irony that multimillion-dollar projects like the SDSS are producing data that cannot be analyzed because of a failure to support very cheap lab work on the ground.

They have a point, and support should be extended for laboratory-based research understanding such spectra. A good rule of thumb is that agencies funding telescope projects that do state-of-the-art spectroscopy should spend a small fraction, perhaps a few percent, of funding on associated laboratory spectroscopy.

Lab-based measurements are less glamorous, but big questions about the evolution of galaxies will be solved by understanding small but important details about the physics and chemistry of the millions of stars revealed by the spectra.

For example, the spectra can provide clues as to whether stars formed in the galactic bulge or moved into it later. The spectra can also shed light on the amount of dark matter near a star, by revealing information about the star’s motion, which alters its spectral lines.

A good example of the benefits of such work comes from a November paper in The Astrophysical Journal by nuclear physicists at Imperial College London, the National Institute of Standards and Technology in Gaithersburg, Maryland, and the Astrophysical Institute of the Canary Islands in Tenerife, Spain ( MP Ruffoni et al. Astrophys. J. 779, 17; 2013).

They report 28 possibilities of electron transitions between sets of energy levels for the element iron. These can now be used in conjunction with spectra to estimate the abundance of iron in stars in the galactic bulge – a step towards their age and their formation. None had previously been measured in the laboratory.

Such research is necessary because, in order to identify and quantify elements in space from spectra, astronomers must know the probability that electrons in the elements’ atoms will move between energy levels. For light elements with few electrons, such as hydrogen and helium, the transition probabilities can be calculated using the laws of quantum mechanics.

But heavy elements have many electrons that can take part in the transition – iron has 26, making the potential transitions between levels too complex to calculate accurately.

The only option is to measure emissions in the laboratory. Physicists can use tunable lasers to excite electrons into greater levels and measure further transitions. This information can then feed back to astronomical observations. Additional funds will greatly improve this capability, leading to better access to powerful lasers and detectors.

Even as experimentalists face the challenges of taking lab spectra, there is an astronomical spectroscopy boom. In addition to the infrared instrument taking data on the US$55-million SDSS, astronomers plan to build giant 30–50-metre telescopes such as the €1-billion (US$1.3-billion) European Extremely Large Telescope, which will be located close by. is Cerro Paranal, Chile, which will take hundreds of thousands of stellar spectra.

In addition, NASA’s planned $8.8 billion James Webb Space Telescope, which uses state-of-the-art mercury cadmium telluride infrared detectors like the Sloan instrument, will look at stars and, it is hoped, in the atmospheres of planets outside the Solar System.

Although spectra can be used to estimate the amounts of different elements in the atmospheres of stars or planets, a special area of ​​interest is in identifying molecules, which also emit distinctive spectral lines during transitions between different states. Huh.

Other laboratory-based experiments may also solve one of the longest-standing questions in astronomy: the origin of the diffuse interstellar band — the space between stars and Earth caused by diffuse matter.