The science of stars

Reader's Digest Editors

The stars have fascinated humanity since the dawn of time, and now science is finally starting to reveal their secrets. 

The life and death of stars

gazing up at the death of a star

Although the stars seem constant to us, there is, in fact, a relentless cycle of life and death going on in space. Astronomers can identify regions where new stars are being born as well as the remains of stars that have exploded.

The stars appear unchanged to us only because they take an almost inconceivably long time—millions and billions of years—to develop. Spectacular and rapid events like a supernova—a star exploding—are very rare. The course of a star’s life and its eventual death depend on the amount of mass it accrued during its formation.

 

How are the stars and planets formed?

the life cycle of a star

The life of a star begins with a gigantic cloud of cool hydrogen gas with an initial temperature no more than a few degrees above absolute zero (–273.15° C). The cloud gradually collapses under its own gravity and clumps of material begin to form inside it, causing it to collapse at ever-increasing speeds, while the temperature of the gas rises steadily.

The cloud eventually starts to glow, giving off thermal radiation which can be detected by infrared telescopes. The internal density and temperature ultimately increase to a point where nuclear fusion takes place, in which hydrogen is converted into helium, and so a new star is born.

Typically, many stars of different sizes emerge from one large gas cloud. Sometimes, star clusters made up of thousands or even millions of stars are created all at once.

A rapidly rotating cloud of gas and dust (the remnants of the original cloud) floats around many of the young stars, and inside this cloud, there are further clumps of matter which have the potential to develop into planets and other, smaller celestial bodies. It is thought that planets may be formed around most stars.

 

How old are the oldest stars?

starry night sky

The first stars in the cosmos were formed 400 million years after the Big Bang. Some stars are very frugal in using their supply of fuel, which means that even today we are still able to see stars belonging to the very first generations. This is especially the case with globular clusters, where we find stars that are more than 13 billion years old, although new stars are being formed all the time.

One of the best-known star birth regions is the Great Orion Nebula, which is 1600 light-years away and visible with the naked eye as part of the constellation Orion. The Hubble and Spitzer telescopes have revealed that many young stars in the Orion region have discs of matter rotating around them in which planets may be forming.

 

What determines how long stars live and how they die?

nasa animation of the death of a star
Death of a star captured by Nasa

The duration of a star’s life and the manner of its death depend on its mass. This is because large, high-mass stars use up their energy reserves more rapidly than smaller, low-mass stars. This is the reason why large stars shine brightly for no more than a few million years, while smaller stars can live for billions of years.

Stars produce their energy as a result of a process known as nuclear fusion—during which hydrogen is converted into helium. However, the time inevitably comes when all the hydrogen at the core of a star has been used up.

 

What are red giants and white dwarfs?

red giant star

Once a star’s hydrogen reserves are used up, the star enters the final phase of its life. It swells up to become what is known as a red giant.

Our own Sun will one day turn into a red giant, and in so doing may even devour the Earth, along with several more of the inner planets of the solar system.

Further fusion processes occur in the core of the enlarged star over millions of years, during which, for example, helium is converted into carbon and oxygen. When all of a star’s stock of potential nuclear fuel has been exhausted, it collapses and turns into a white dwarf.

At this stage, the star is only about the same size as Earth, and it takes billions of more years for it to cool down.

 

What is a supernova?

supernova in its galaxy
Supernova pictured in its galaxy. Image via Wiki Commons

When a star has a mass of more than eight times that of our Sun its end is considerably more dramatic than that of most regular stars.

In its final stages, the dying star hurls its outer layers into space with a mighty explosion which is known as a supernova.

At the same time, the star’s interior collapses to form a small, extremely dense body, which can take the form either of a neutron star or a black hole.

 

What is a neutron star?

the Central neutron star at the heart of the Crab Nebula
The central neutron star at the heart of the Crab Nebula. Image via Wiki Commons

The matter in a neutron star is as densely packed as the matter in an atomic nucleus. It is so tightly packed, that a tiny piece of a neutron star the size of a pinhead would weigh more than one million tonnes.

The immense pressure in the interior of this kind of object pushes the electrons into the atomic nuclei’s protons—leaving nothing but neutrons behind. Neutron stars have a diameter of only about 20 km but contain about the same mass as our Sun.

 

Can we see black holes?

black hole as visualised by Nasa
Nasa's visualisation of a black hole

If the mass of a star’s collapsing interior is more than about three times that of our Sun, its gravity becomes so strong that even the neutrons cannot halt the process.

Inexorably, the star continues to collapse until it turns into a black hole—the name given to a region of space where gravity is so powerful that nothing, not even light, is able to escape. This is why these gravity holes are "black". However, we are still able to detect black holes because their colossal gravity draws in the surrounding matter which, as it descends into the black hole, becomes very hot and begins to glow—thereby revealing the black hole’s position.

Black holes don’t only occur when a star dies. There are also monster black holes at the centres of galaxies which have a mass of millions if not billions of times that of our Sun. These are thought to have originated in the early stages of the universe, and there's even one of these supermassive black holes in the centre of our Milky Way.

 

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