How do the first stars die?

The first stars formed are the most massive that have ever existed. They have several hundred times the mass of the Sun! They have a life expectancy of just a few million years, because they use up their hydrogen at an incredible rate.

Once this fuel has been used up, they can no longer carry out fusion reactions. They lose their equilibrium, and their core collapses in a fraction of a second! The shockwave then creates a supernova, generating a fantastic ejection of matter into space.

The remaining dense core then collapses in on itself, as it is no longer able to counteract its own gravity. Its final fate may take the form of a neutron star or, for the stars that were originally the most massive, a black hole.

Why a spiral shape?

The areas of the Universe that will form galaxies are subject to several forces. Their own gravity causes them to condense. And they also rotate on themselves! So, the centrifugal force - the force that makes the passengers of a car feel like they are being thrown out of the car as it rounds a bend - pulls matter outwards.

When the matter of future galaxies condenses, it gradually flattens out and forms a disc. Inhomogeneities of matter remain within the disc. When the disc rotates, the areas that are denser than the others spread out: this is what gives rise to the spiral arms of galaxies like the Milky Way.

How are stars born?

Stars are born in the interstellar medium: a very cold, diffuse medium made up of 99% gas and 1% dust. It is almost empty, because it is billions of times less dense than air!

Under the effect of gravity, certain zones condense and become hotter. When the conditions of density and temperature are sufficient to initiate the fusion reactions of hydrogen nuclei, stars are born! When a star forms at the heart of a cloud of matter, a disc of gas and dust forms around it, giving rise to planets.

Generally, stars are born in clusters, as in the Pleiades cluster, visible to the naked eye.

How to identify planets that could sustain life?

First of all, the planet must be neither too close nor too far from its star, so that liquid water can exist on its surface. It also needs a solid surface, an atmosphere compatible with life, and, of course, liquid water!

Then, for life to emerge, it must contain several essential chemical elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. And finally, it needs a sufficient source of energy, such as the radiation from its star. The ideal candidate has yet to be unmasked...

Current detection techniques make it possible to obtain information about the atmospheric composition and physical conditions of the planet under study, such as temperature and density. This could help identify possible candidates for the presence of life...

How does solar weather influence Earth?

From time to time, solar flares eject matter that travels to Earth. The particles are guided by the magnetic field that surrounds the planet, and plunge into the atmosphere, mainly at the poles: this gives rise to polar auroras.

More rarely, particularly violent solar flares can also disrupt electrical currents on Earth. In 1859, during a peak in solar activity, a major geomagnetic storm occurred. It caused a number of fires in telegraph stations, disrupted telecommunications and caused very intense auroras to appear, even in tropical regions!

Today, satellites monitor the Sun's activity to anticipate its possible consequences on Earth. It is possible to observe the Sun in real time on SDO.

Where does water hide in the Solar System?

As soon as it is formed, the disc of matter surrounding the Sun contains water in the form of ice around grains of dust. Close to the star, in the zone where telluric planets are formed, heat sublimates the ice and dissipates the water found there. From the asteroid belt onwards, ice is protected from solar radiation. It is therefore abundant around grains of dust, but also in distant planets, their moons and comets! When ice is compressed under the effect of gravity, it liquefies and deep oceans form, as on Europa and Ganymede, two satellites of Jupiter.

The Moon, cause of tides on Earth?

Earth's gravity keeps the Moon in orbit around the Earth. The Moon also exerts an attractive force on the Earth! Once Earth has acquired its oceans, this force causes tides that periodically raise the level of the oceans. When the Moon is full and new, its alignment with the Sun reinforces the phenomenon: the tides are much more pronounced than during the first and last quarters of the Moon.

The Moon moves away from Earth by 3.8 centimetres per year. As a result, tides on Earth become less pronounced over time...

How does Earth become habitable?

Thanks to volcanism, the atmosphere and oceans form little by little. The planet is protected from solar wind by the atmosphere and the magnetic field generated by the liquid core. Also, plate tectonics stir up Earth's young crust: this recycling of matter has a major impact on carbon dioxide levels.  This process gradually stabilises the greenhouse effect: the climate is beginning to regulate itself!

Why are the first traces of life so controversial?

For traces of life to be proven, the rocks must be correctly dated, the reconstructed environment must be conducive to life and, finally, the traces observed must be verified as fossils of living organisms!

Microorganisms are so small that it is easy to confuse the traces and structures resulting from their activity with the products of purely chemical reactions. Small, rounded mineral precipitates can be indistinguishable from real microfossils! The problem is huge...

How do we go from non-living to living?

It took a long time for this question to be accepted as a scientific one. Today, however, it is certain that life has appeared somewhere. Is it the result of an infinitesimal and non-reproducible chance event, beyond the reach of the scientific approach? In order to make a decision, scientists lack a description of what defines the living state and the laws that drive it in physico-chemical terms, beyond the molecules that make it up, or even highly simplified models.

However, research on the origins of life is progressing in different directions, based on different hypotheses. Depending on the case, research focuses on the chemical nature of biomolecules such as nucleic acids, amino acids or membrane components, or on the environment that could be responsible for the appearance of life, such as hydrothermal vents. Other avenues focus on the systemic and dynamic aspects of life, resulting from interactions between multiple components. This translates into studying the link between life and energy use, and its non-equilibrium nature, which is essential to consider the possibility of self-organising systems.

To assess the respective contributions of the different research avenues to this fundamental question, we will have to wait until a solution is found and accepted by the scientific community...

How do living things evolve?

Certain characteristics in organisms emerge by chance. How to know if they are advantages or disadvantages? The environment decides! If these characteristics can be passed on to other organisms, particularly those of the next generation, those that are advantageous will be maintained over time, while the others will disappear. This is natural selection. After LUCA, three major lineages of living organisms emerge: bacteria, archaea and eukaryotes. 

Bacteria are the most abundant and diverse living beings on the planet. They live in almost every type of environment: seawater, soil, sediment, on our skin, in our intestines... and even deep in the earth's crust!

Archaea are small cells that resemble bacteria, but they are much more resistant. They beat all records for adaptation to extreme environments: some live in boiling hot springs, others in salt or even acid!

How are continents formed?

Earth is cooling down: this is changing the way continents are made from now on. The very hot water rising from the deep basalts of the oceanic crust is now responsible for the formation of the continents. Basalts are volcanic rocks that line the bottom of the oceans and sink into Earth's mantle. The hot water that comes out melts the mantle into magma, which then becomes new continental crust.

Oxygen: an energy source for the evolution of species?

As soon as it accumulated in the atmosphere, oxygen was a poison for most living species. But they eventually adapted. Either by taking refuge in protected environments, or... by becoming able to breathe it! During respiration, oxygen reacts with organic carbon, releasing tremendous energy for the cellular machinery, provided cells know how to protect themselves from it. This is the beginning of a diversification of living organisms, allowing multi-cellular organisms to appear...

How does Earth defrost?

During the snowball Earth episode, volcanoes keep emitting carbon dioxide into the air... Gradually, the gas accumulates and the greenhouse effect increases. As the ice melts, oceans absorb even more energy, causing temperatures to skyrocket: up to 50 degrees Celsius!

The chloroplast, an ancient independent living being?

Like cyanobacteria, their distant predecessors, the chloroplasts of plants and algae are capable of oxygen photosynthesis. These compartments in their cells are essential to their functioning! The chloroplast is fed by light, carbon dioxide and water. And above all, it has its own genome! A genome that closely resembles that of a cyanobacterium, only smaller...

Initially, ancient eukaryotic cells feeding on organic matter absorbed a cyanobacterium and lived in symbiosis with it. Little by little, the cyanobacteria became chloroplasts. Photosynthesis in plants and algae therefore takes place in an ancient bacterium!

What story do fossils tell us?

Palaeontologists have found animal fossils dating from the Cambrian period in Canada, Australia, Greenland and China. They have reconstructed between 150 and 200 species from these fragments. There was undoubtedly already a diversity of animals without skeletons but they left very few fossil traces. The variety of animal forms found has led some specialists to argue that animal biodiversity was richer then than it is today!

Who populated the continents before vertebrates?

Micro-organisms were the first to colonise Earth's surface. Plants and fungi then took root on the continents 410 million years ago, followed by arthropods such as millipedes and spiders.

Any regular meteorite impacts?

The term ‘meteorite’ refers to any solid extraterrestrial object reaching Earth's surface. While meteorite impacts such as the one that hit Chicxulub are very rare, the arrival of much smaller meteorites is very frequent. Over the course of a year, the flow of micrometeorites - fragments less than a millimetre in size - amounts to more than 5,000 tonnes!

More massive meteorites, on the other hand, account for around ten tonnes a year. To prevent the risks, NASA is monitoring near-Earth objects that could potentially cross Earth's orbit with the NEO Surveyor mission. For objects between 30 and 139 metres in size, the probability of impact is around 1 every 100 years. For objects larger than a kilometre, the frequency of impact is counted in millions of years, so there's little risk of suffering the same fate as the dinosaurs!

Everyone living in France can take part in monitoring meteorites by joining the Vigie-Ciel participatory science programme: VIGIE-CIEL

Hybridisation between human species?

The genome of today's Eurasians is derived from 1.5% to 4% of that of Neanderthal man: this is evidence of proven hybridisation between Homo sapiens and Homo neanderthalensis.

Yet, according to one of the criteria used to define species, two different species should not be able to produce hybrid lines! This reflects the difficulty of applying this definition of species to very closely related groups such as Homo sapiens and Homo neanderthalensis. These two Homo should be conventionally united under the same name...

Similarly, genes shared with ancient Homo denisovensis have been found in Oceanian populations, while they are absent in other current populations.