The Sun is a star that formed 4.6 billion years ago in our Milky Way Galaxy. It is the largest and most massive object in our Solar System, whose energy enables life on our planet. What happened at the time of its birth? Was its formation similar to most stars in our Galaxy, or did it form in special circumstances? ERC grantee Maria Lugaro at the ELKH Konkoly Observatory in Budapest seeks to answer these questions by investigating the Solar System’s chemical origin. Her discoveries could help untangle the secrets of stars’ potential to harbour Earth-like planets, and ultimately life.

Since time immemorial, we have wondered if our planet is unique in the Universe. The answer to this age-old question depends on many factors, including the origin of our Sun. Were the circumstances of our Sun’s birth unique, or was it a typical occurrence? Knowing this is crucial for understanding how our Solar System and life within it compare to other planetary systems.

Our Sun is just one of hundreds of billions of stars in the Galaxy. Stars are born in cold and dense interstellar clouds of dust and gas called stellar nurseries. These star-forming regions of accumulated dust and gas collapse due to gravity and form stars.

Most stars are born in families and several generations may even coexist together in a stellar nursery. As it turns out, it is the size of the Sun’s stellar family that could give scientists clues about the uniqueness of our Solar System.

‘The Sun being born in a small or a large family may have affected its potential to harbour habitable terrestrial planets like Earth,’ says Maria Lugaro, an astrophysicist at the Konkoly Observatory and principal investigator of the ERC-funded RADIOSTAR project, who is exploring the circumstances of the Sun’s birth.

Lugaro is studying how nuclear reactions inside stars produced the chemical matter that builds up our Sun, planets and bodies. To look back 4.6 billion years, Lugaro and her team use radioactive nuclei as clocks that can reveal the time of astrophysical events before and around the Sun’s birth.

Powerful clues to cosmic history

The method they apply is similar to the one in which scientists use carbon-14 to determine the age of fossils and archaeological specimens. Scientists use radioactive elements because they decay in a predictable way. For example, every 5730 years, carbon-14 decays by half.

‘However, if we want to measure cosmic intervals related to the birth of the Sun, we need radioactive clocks that decay in much longer times than carbon-14’, says Lugaro. ‘Approximately twenty such nuclei exist, ranging from aluminium-26, with a decay time of nearly a million years, to nuclei such as curium-247 with a 16-million-year decay time.’

To measure a given cosmic interval using radioactive nuclei, scientists need to know the decay time, which can be measured in nuclear laboratories. They also need to know their value at the point in time in the Universe’s history that they are interested in measuring. Luckily, this historic data is stored in meteorites.

‘We know the numbers of many radioactive nuclei at the time of the Sun’s birth because they can be measured by analysing the composition of meteorites,’ explains Lugaro. ‘These nuclei provide powerful clues to investigate the circumstances of the Sun’s birth, but only if we can understand where they come from.’

Therefore, the project team aims to go even further back into cosmic history. They want to look at the time of the formation of the interstellar cloud that gave birth to the Sun.

The origin of the Sun’s interstellar cloud

In order to reproduce the evolution of the galaxy and stars, scientists are applying sophisticated mathematical programs. This allows them to derive the time that elapsed between the interstellar cloud formation and the Sun’s birth.

‘The longer this time, the more likely the Sun was born in a large family of different generations of stars. While the shorter time would mean it was born with just a few siblings, no parents, no grandparents coexisting, or even alone’, says Lugaro.

So far, Lugaro’s results confirm previous discoveries in this field, indicating that this pre-birth period lasted for at least 10 million years. This suggests that the Sun’s stellar cloud was large enough to allow the birth of several generations of stars.

Just the right amount of water 

Stellar members of our Sun’s large family may have created just the right conditions for the existence of a habitable terrestrial planet like Earth, explains Lugaro. This is because stellar members can produce and eject aluminium-26, whose radioactive decay produces a lot of heat, making ice melt and water evaporate.

Although we think of water as essential for life, too much of it can actually inhibit life. In fact, planets completely covered in water are less likely to sustain life than water-poor planets. That is why the heat created by aluminium-26 may have been essential for our planet, allowing just the right amount of water coverage.

Through the meteoritic analysis, scientists know that there was a lot of aluminium-26 at the Sun’s birth. However, they still do not know if this is a special or ‘normal’ case. ‘If we can understand where the aluminium-26 came from in the young Solar System, then we can have a reason for its origin, and determine how likely this could apply to other stars in the Galaxy’, says Lugaro.

By the end of the project, Lugaro and her team aim to understand the type of stellar nursery in which the Sun was born. Their goal is to produce a picture of the Sun’s birth that explains all radioactive nuclei known to be present in the early Solar System.

Expanding expertise and developing new tools

Lugaro says that the ERC grant enabled her to acquire a permanent position at the Konkoly Observatory, as well as strengthen her research team and develop new tools. She was able to expand her group’s expertise from focusing mostly on the modelling of nuclear reactions in stars to modelling the evolution of the galaxy as well. Her team currently consists of fifteen members with expertise ranging from astrophysics and astronomy to mathematics, computer science and meteoritic science.

‘With the ERC grant, I have been able to develop new concepts and tools and seek answers for topical questions around the Sun’s formation whose understanding is relevant not only for my specific scientific work but for the community as a whole,’ says Lugaro.