The Indian Express (Delhi Edition)
HOW DID STAR RESIDING IN ‘DRAGON’S EGG’ NEBULA GET A MAGNETIC FIELD?
TWO LARGE stars, residing inside a spectacular cloud of gas and dust nicknamed the ‘Dragon’s Egg’ nebula, have long presented a puzzle to astronomers. One of them has a magnetic field, like our Sun. But its companion does not. Moreover, such massive stars are not usually associated with nebulae.
Researchers now appear to have solved this mystery.
What has the new research found?
According to a study published on Thursday in the journal Science, the bigger star apparently gobbled up a (third) smaller sibling star, and the mixing of their stellar material during this hostile takeover created a magnetic field.
“This merger was likely very violent. When two stars merge, material can be thrown out, and this likely created the nebula we see today,” said Abigail Frost, astronomer at the Chile-based European Southern Observatory, and the study’s lead author.
Computer simulations previously had predicted that the blending of stellar material during such a merger could create a magnetic field in the star born in the process.
The two remaining stars — gravitationally bound to each other in what is called a binary system — are located in our Milky Way galaxy, about 3,700 light years from Earth in the constellation Norma. A light year is the distance light travels in a year, roughly 9.5 trillion km.
The magnetic star is about 30 times more massive than the Sun, and its companion is about 26.5 times more massive than the Sun. They orbit each other at a distance varying from seven to 60 times the distance between the Earth and the Sun.
What do we know about the Dragon’s Egg nebula?
The Dragon’s Egg is so named because it is located relatively near a larger nebula complex called the ‘Fighting Dragons of Ara’. The stars inside the Dragon’s Egg appear to have started out 4-6 million years ago as a triple system — three stars born at the same time and gravitationally bound.
The triple system’s two innermost members included a larger star — perhaps 25 to 30 times the mass of the Sun — and a smaller one — maybe five to 10 times the Sun’s mass.
The more massive one evolved more quickly than the other, with its outer layer engulfing the smaller star and triggering a merger that ejected into space the gas and dust that make up the nebula, the researchers said.
This occurred very recently in a cosmic time scale — only about 7,500 years ago, based on the expansion velocity of the material in the nebula. It consists mostly of hydrogen and helium, but also an unusually large amount of nitrogen, thanks to the merger. Many Sun-sized stars generate magnetic fields.