A galaxy, as seen from Earth, with the same structure as the Milky Way. The central region is the galactic bulge, with a thin disc surrounding it.
Credit: NASA
SYDNEY, 15 September 2006: The centre of our home galaxy, the Milky Way, formed independently of the region where Earth is located, astronomer Manuela Zoccali from the Universidad Catolica de Chile has revealed for the first time in the Journal of Astronomy and Astrophysics.
Zoccali used the chemical composition of stars in different regions of the Milky Way to investigate their formation. She has compared her study of our galaxy to heredity studies using genetics. "We have clearly established a ‘genetic difference' between stars in the disc and the bulge of our galaxy," she said.
Although there is clearly no genetic material in stars, the chemicals in the stars can identify similarities between groups of stars and is therefore likened to a genetic link.
The Earth is located in the outer reaches of our galaxy, on a thin disc, amidst pinwheel-shaped arms of gas, dust and stars. At the centre of this disc is a huge, tightly packed group of stars called the galactic bulge, named for the way it bulges out from the disc.
The centre of our galaxy has never been directly observed, due to the large number of stars obscuring the view, however most astronomers believe that the galactic bulge harbours a super-massive black hole.
Until now astronomers have been unable to establish the cause of the bulging centre. It was unclear whether the bulge was formed in a short period of time through a series of starbursts, or whether the stars formed in the disc and then moved toward the centre to form the bulge.
The team, led by Zoccali, tested the chemical composition of fifty giant stars in four different areas of the galactic bulge. The ratio of oxygen to iron can be used to infer whether there is a ‘genetic link' between the stars in the bulge and the stars in the disc. That is, whether or not one group of stars originated from the other.
Oxygen is predominantly produced in the explosions of massive, short-lived stars, while iron is formed when the pressure increases in small, compact stars before the entire star explodes. Small, compact stars often take much longer to reach the point of explosion than massive stars, so this data also provides insight into how quickly each region of the galaxy formed.
The astronomers clearly established that, for a given iron content, stars in the bulge possess more oxygen than their disc counterparts. This indicates a hereditary difference between bulge and disk stars.
"Bulge stars did not originate in the disc and then migrate inward to build up the bulge but rather formed independently of the disc," said Zoccali. "Moreover, the chemical enrichment of the bulge, and hence its formation timescale, has been faster than the disc."
