Tag: star formation

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Although, as we look further and deeper into the sky, we are always looking into the past – there are other ways of gaining information about the universe’s ancient history. Low mass, low metal stars may be remnants of the early universe and carry valuable information about the environment of that early universe.

The logic of stellar archaeology involves tracking generations of stars back to the very first stars seen in our universe. Stars born in recent eras, say within the last five or six billion years, we call Population I stars – which includes our Sun. These stars were born from an interstellar medium (i.e. gas clouds etc) that had been seeded by the death throes of a previous generation of stars we call Population II stars.

Population II stars were born from an interstellar medium that existed maybe 12 or 13 billion years ago – and which had been seeded by the death throes of Population III stars, the first stars ever seen in our universe.

And when I say death throes seeding the interstellar medium this includes average sized stars blowing off a planetary nebula at the end of their red giant phase – or bigger stars exploding as supernovae.

So for example, the low metal spectral signature of HE 0107-5240 matches that predicted for a very early low mass Population II star built from the end-products of a Population III supernova.

This is about as close as we can get gathering any information about Population III stars. Telescopes that can look deeper into space (and hence look further back in time) may eventually spot one – but it’s unlikely that any still exist. Theory has it that Population III stars formed from a homogenous interstellar medium of hydrogen and helium. The homogeneity of this medium meant that any stars that formed were all massive – in the order of hundreds of solar masses.

Stars of this scale, not only have short life spans but explode with such a force that the star literally blows itself to bits as a ‘pair-instability’ supernova – leaving no remnant neutron star or black hole behind. Supernova SN2006gy was probably a pair-instability supernova – mimicking the last gasps of Population III stars that lived more than 13 billion years ago.

It was only after Population III stars had seeded the interstellar medium with heavier elements that fine structure cooling resulted in disruption of thermal equilibrium and fragmentation of gas clouds – enabling smaller, and hence longer lived, Population II stars to be born.

Around the Milky Way, we can find very old Population II stars in orbiting dwarf galaxies. These stars are also common in the galactic halo and in globular clusters. However, in ‘the guts’ of the galaxy we find lots of young Population I stars.

This all leads to the view that the Milky Way is a gravitational hub nearly as old as the universe itself – which has been steadily growing in size and keeping itself looking young by maintaining a steady diet of ancient dwarf galaxies – which, deprived of such a diet, have remained largely unchanged since their formation in the early universe.

Further reading:

A. Frebel. Stellar Archaeology – Exploring the Universe with Metal-Poor Stars http://arxiv4.library.cornell.edu/abs/1006.2419

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Scientists have found evidence of a catastrophic event they believe was responsible for halting the birth of stars in a galaxy in the early Universe. According to their findings, just 3 billion years after the Big Bang, a massive galaxy exploded in a series of blasts trillions of times more powerful than any caused by an atomic bomb. The blasts happened every second for millions of years. “We are looking into the past and seeing a catastrophic event that essentially switched off star formation and halted the growth of a typical massive galaxy in the local Universe,” said lead author Dr. Dave Alexander from Durham University.

Using the Gemini Observatory’s Near-Infrared Integral Field Spectrometer (NIFS), scientists looked at SMM J1237+6203 and noticed properties seen in other massive galaxies near to our own Milky Way, which suggest that a major event rapidly turned off star formation in early galaxies and halted their expansion.

This catastrophic event occurred when the Universe was a quarter of its present age. The explosions scattered the gas needed to form new stars by helping it escape the gravitational pull of the galaxy called, effectively regulating its growth, the scientists added.

They believe the huge surge of energy was caused by either the outflow of debris from the galaxy’s black hole or from powerful winds generated by dying stars called supernovae.

Theorists, including scientists at Durham University, have argued that this could be due to outflows of energy blowing galaxies apart and preventing further new stars from forming, but evidence of this has been lacking until now. The team hopes the new findings can increase our understanding about the formation and development of galaxies.

“Effectively the galaxy is regulating its growth by preventing new stars from being born,” said Alexander. “Theorists had predicted that huge outflows of energy were behind this activity, but it’s only now that we have seen it in action. We believe that similar huge outflows are likely to have stopped the growth of other galaxies in the early Universe by blowing away the materials needed for star formation.”

The Durham-led team now plans to study other massive star-forming galaxies in the early Universe to see if they display similar characteristics.

The research is published in the Monthly Notices of the Royal Astronomical Society.

Source: Royal Astronomical Society

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When a giant cloud of interstellar gas and dust collapses to form a new cluster of stars, only a small fraction of the cloud’s mass ends up in stars. Scientists have never been sure why.  But a new study provides insights into the role magnetic fields might play in star formation, and suggests more than the influence of gravity should be taken into account in computer models of stellar birth.

Gravity favors star formation by drawing material together, so if most material does not coalesce into stars, some additional force must hinder the process. Magnetic fields and turbulence are the two leading candidates. Magnetic fields channel flowing gas, making it hard to draw gas from all directions, while turbulence stirs the gas and induces an outward pressure that counteracts gravity.

“The relative importance of magnetic fields versus turbulence is a matter of much debate,” said astronomer Hua-bai Li of the Harvard-Smithsonian Center for Astrophysics. “Our findings serve as the first observational constraint on this issue.”

Li and his team studied 25 dense patches, or cloud cores, each one about a light-year in size. The cores, which act as seeds from which stars form, were located within molecular clouds as much as 6,500 light-years from Earth.

The degree of polarization of light from the clouds is influenced by the direction and strength of the local magnetic fields, so the researchers measured polarization to determine magnetic field strength. The fields within each cloud core were compared to the fields in the surrounding, tenuous nebula.

The magnetic fields tended to line up in the same direction, even though the relative size scales (1 light-year-sized cores versus 1000 light-year-sized nebulas) and densities were different by orders of magnitude. Since turbulence would tend to churn the nebula and mix up magnetic field directions, their findings show that magnetic fields dominate turbulence in influencing star birth.

“Our result shows that molecular cloud cores located near each other are connected not only by gravity but also by magnetic fields,” said Li. “This shows that computer simulations modeling star formation must take strong magnetic fields into account.”

In the broader picture, this discovery aids understanding of how stars and planets form and, therefore, how the universe has come to look the way it is today.

Source: Harvard-Smithsonian Center for Astrophysics