Stars behind the bubble

September 21, 2001

A cluster of stars that lived fast and died young may have let loose a fusillade of supernova explosions as the cluster drifted through the Sun's galactic neighbourhood 5 million to 6 million years ago.

Jesus Maiz-Apellaniz, an astronomer at the Space Telescope Science Institute in Baltimore, United States, believes the event may explain the mysterious origin of what is known as the "local bubble". This is the region of our Milky Way galaxy in which the Sun and other stars sit, where the local interstellar medium is particularly thin and hot.

Scientists have speculated that the local bubble was most likely created by several supernova explosions. However, their source had not been identified.

Maiz-Apellaniz used data from the European Space Agency's Hipparcos astrometry satellite to identify and track the movement of a loose group of stars called the Scorpius-Centaurus OB association, which is moving within the Milky Way.

It seems this cluster contains the type of young, hot star that was more likely to die in a cataclysmic supernova explosion than those currently found in our part of the galaxy. A computer model calculated that the association probably experienced about 20 such violent stellar deaths in the past 10 million to 12 million years.

This estimate is backed up by the detection of four or five runaway stars that appear to have been flung out of the association by the force of nearby explosions.

What is more, Maiz-Apellaniz's calculations show the tempestuous Scorpius-Centaurus OB association was in the Sun's neighbourhood between 5 million and 7 million years ago.

He has estimated that about six supernovae would have been produced by a subgroup of stars within the stormy cluster at just the right time to create the local bubble.

The results, published in The Astrophysical Journal , support the idea that the interstellar medium is pitted with such bubbles.

Maiz-Apellaniz and colleagues have analysed the possible consequences of the flurry of supernovae for life on Earth. The results of this follow-up study have been submitted to the Proceedings of the National Academy of Sciences .

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