Australasian Science Magazine

By David Reneke

Astronomers have discovered the oldest known star in the universe, and ice and water vapour have been detected on Ceres.

In an Australian first, a team led by astronomers at the Australian National University has discovered the oldest known star in the universe, which formed shortly after the Big Bang 13.7 billion years ago. The discovery has allowed astronomers for the first time to study the chemistry of the first stars, giving scientists a clearer idea of what the universe was like in its infancy.

“This is the first time that we’ve been able to unambiguously say that we’ve found the chemical fingerprint of a first star,” said Dr Stefan Keller of the ANU Research School of Astronomy and Astrophysics. “It’s one of the first steps in understanding what those first stars were like. What this star has enabled us to do is record the fingerprint of those first stars.”

The star was discovered using the ANU SkyMapper telescope at the Siding Spring Observatory in NSW. It’s a dedicated 5-year research program searching for the earliest stars in an effort to produce the first digital map of the southern sky.

The ancient star is around 6000 light years from Earth and is one of the 60 million stars photographed by SkyMapper in its first year of operation.

“The stars we are finding number one in a million,” says team member Professor Mike Bessell, who also worked on the research. “Finding such needles in a haystack is possible thanks to the ANU SkyMapper telescope that is unique in its ability to find stars with low iron from their colour.” The composition of the newly discovered star shows it formed in the wake of a primordial star that had a mass 60 times that of our Sun.

“Making a star like our Sun requires hydrogen and helium from the Big Bang, plus an enormous amount of iron, the equivalent of about 1000 times the Earth’s mass,” Dr Keller says. “To make this ancient star you need no more than an Australia-sized asteroid of iron and lots of carbon.”

It was previously thought that primordial stars died in extremely violent explosions that polluted huge volumes of space with iron. This ancient star, however, shows signs of pollution with lighter elements such as carbon and magnesium, and no sign of pollution with iron. The result may resolve a longtanding discrepancy between observations and predictions of the Big Bang.

Ceres Is a Waterworld

Ceres is the largest and roundest body within the main asteroid belt between the orbits of Mars and Jupiter. First discovered in 1801, Ceres was classified as a planet. It would later be reclassified as the first named asteroid by Sir William Herschel.

In 2006, the International Astronomical Union voted on specific definitions for different planetary bodies, resulting in yet another reclassification of Ceres, this time as a dwarf planet. This was the same meeting that reclassified Pluto to dwarf planet status as well.

Although Ceres has always been thought to have ice, it’s never actually been seen. Using the Herschel Space Telescope to study radiation deflecting off Ceres, astronomers found indications of water vapour and lots of ice on the surface. They believe that surrounding its rocky core is a mantle of ice so thick it could hold more water than Earth does!

The plumes of water vapour are a total mystery, raising lots of questions about how they appear. It could be that part of the dwarf planet’s orbit brings it slightly closer to the Sun, which warms up the ice and then vents off as steam, although there could be radioactivity within the core that causes the sublimated water to be expelled.

Water vapour does not appear to be venting all the time, and the amount coming out does not seem to be all that constant. The vents do not appear sporadically and appear to be restricted to two separate areas. At maximum, the vents were observed to release about 6 kg of water per second.

In 2007, NASA launched the Dawn space probe to study Ceres and Vesta, a large asteroid. The spacecraft is expected to rendezvous with the dwarf planet in February 2015. This is incredibly good timing as scientists won’t really have to wait long to take up-close measurements and follow up with these preliminary observations.

Information collected will help researchers understand how water was distributed throughout the solar system, and how it ended up on a planet capable of retaining it as a liquid – the prerequisite for life.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com