The Real Celebrities – The Stars

Vocabulary/new concepts:

Star: a celestial body that is a massive ball of glowing gas that produces energy by fusion, in which hydrogen is converted to helium. This energy propagates as electromagnetic radiation. Fusion processes in the cores of stars produce almost all the chemical elements in the periodic table.

Plasma: A high-energy state of matter – the most abundant in the universe. It is one of the four fundamental aggregate states of matter, along with the solid, liquid, and gaseous states. It is composed of neutral atoms, ions and free electrons. Stars, including our Sun, are huge spheres of plasma.

Red giant: When a star reaches a late stage in its life, it runs out of hydrogen in its core and starts burning helium. As a result, the star’s core shrinks and its outer layers expand, lowering its surface temperature. It becomes colder and starts to emit red light. The forces become unbalanced, and the star swells in size and gets bigger and cooler until the forces balance again. It is now a giant star with a characteristic red colour.

Black Hole: This is a region in space where the gravitational attraction is so strong that nothing can escape from it, not even a body moving at the speed of light. When the most massive stars run out of fuel at the end of their life cycles, their outer layers explode into space, and their cores shrink under their gravity until they become an infinitesimal point. A very strong gravitational field forms around it, far exceeding the pressure in the core, and the star collapses into a black hole.

Dwarf Planet: A small celestial body similar to a planet. It moves in an orbit around a star. It has enough mass to take on an orb shape from its gravity, but this mass is insufficient to clear its orbit of other astronomical objects. It is not a satellite of another planet or a dwarf planet.

Which is our Galaxy?

Galaxies are huge families of stars scattered across the infinite expanse of the Universe. Our galaxy is the Milky Way. The solar system is part of this galaxy. On a clear night, we can observe thousands of stars, which are a small fraction of the 200 billion stars in our galaxy. From Earth, the densest area of the Milky Way is visible as a faint, milky-white streak that crosses the sky. The shape of this huge galaxy is spiral. At its core are yellow and red stars, and its two arms are made up of nebulae of dust and bright blue and white stars. Earth is located at the end of one of the arms, 30,000 light years from the centre of the Milky Way. Travelling at the speed of light, it would take us about 100,000 years to travel around our entire galaxy.

The Milky Way. Source: https://depositphotos.com/photo/colorful-space-shot-of-milky-way-galaxy-with-stars-and-space-dust-32263367.html

How are stars born?

Stars are huge globules of hot gas. When we look at them, they look like tiny glowing dots in the sky because they are so far away from us. They have different sizes, colours, temperatures, and brightness. Stars glow because they have nuclear reactions going on in their cores. Cold stars are red and warm stars are blue or white.

Like human life, that of the stars is divided into different stages: birth, maturity and death.

Source: https://depositphotos.com/photo/life-cycle-of-a-star-63058181.html

Stars are born in giant dark and cold clouds of dust and gas, in which the main building block is hydrogen. The clouds collapse under their gravity, or as a result of an explosion of a nearby star, and break into smaller clouds. As the cloud contracts, the matter at its centre begins to heat up (as happens when you blow up a balloon – the air inside it is heated). Gravitational collapse results.

An increasingly hot and dense core is forming in the centre of the cloud. This is how the protostar appears. It is the initial stage of star formation. It consists of a stellar core and a low-density gas envelope. Gradually, the gas envelope begins to contract towards the core and then rotate around it. In the next stage, the gas gradually heats up and the protostar begins to glow faintly, emitting infrared rays generated by the heat. It emits no light from the visible spectrum.

The contraction continues until the temperature reaches about 10 million degrees Kelvin (10 000 000 K) and fusion reactions start in the core of the protostar, hydrogen is converted to helium and a huge amount of energy is released. The pressure induced by this process counteracts gravity, the protostar’s contraction ceases, the core “emerges from the dust cocoon” and the young STAR is born!  It scatters the debris from the formation that had obscured it, and the newborn star is now visible – radiating its light and heat. This is how our Sun came into being.

The birth process of a star can vary from several hundred thousand to several million years. This is a very long time in terms of human life, but a relatively short period on cosmic scales.

One of the most studied celestial bodies, an incubator of stars, is the Orion Nebula. It is a huge cloud of gas and dust in which new stars are actively forming. Around many of the young stars are observed disks of gas and dust from which planets may form in the future. Because it is one of the brightest nebulae that can be seen even with the naked eye and is located at a relatively close distance from us – about 1350 light years away – it is of interest to astronomers. Using powerful telescopes, including the Hubble Space Telescope, they study in detail the stages of star formation and evolution, the dynamics of gas-dust clouds, the influence of stars on the surrounding interstellar medium, the chemical composition of the nebula, and more.

Scientists say that what they observed in Orion probably happened in our solar system billions of years ago.

The Orion Nebula. Source: https://depositphotos.com/photo/great-orion-nebula-5087469.html

Maturity and death of a star

Stars enter the main phase, the maturity of their life, once nuclear fusion has begun and the star is in hydrostatic equilibrium: the pressure generated by fusion reactions in the core is balanced by gravitational forces pushing inwards and the star’s contraction ceases. At this time in their life, stars are stable and in the stage where they convert hydrogen to helium, and this period is called the main sequence. The Sun is now in this stage of its life.

The evolutionary development of stars is shown in the Hertzsprung-Russell diagram – this is the curve around which most stars are arranged. This diagram shows the mathematical relationship between the luminosity (radiant power) and surface temperature of stars (spectral class) in the main phase of their life. How long a star’s life will last in this main phase depends on how much nuclear fuel (hydrogen) it has and how quickly it uses it up (power). The more massive the stars, the faster they use up their fuel. Consequently, they have shorter lifetimes on account of the more intense processes going on in their cores.

Source: Own elaboration by We Teach Data project based on https://physicstime.com/sites/default/files/Diagrame%20Hertz-Russel.jpg

The death of a star occurs when it runs out of hydrogen fuel in its core and nuclear reactions stop. This takes away the source that maintains the star’s equilibrium and it leaves the main sequence. It then begins to evolve differently depending on its mass.

Stars with a small mass similar to the Sun

Source: https://depositphotos.com/photo/extremely-hot-star-flaring-of-sun-elements-of-this-image-furnished-by-nasa-274027146.html

• The Red Giant

After billions of years, the hydrogen in the core of the star is depleted and the pressure from nuclear reactions decreases and stops. This takes away the source that keeps the star in equilibrium, the balance between pressure and gravitational forces is disturbed. Its core begins to shrink and get hotter. But the outer layers begin to cool and expand – up to 100 times the star’s previous size. So it becomes a red giant.

Source: https://depositphotos.com/photo/astronomy-red-giant-star-522894196.html

• The White Dwarf

Source: https://depositphotos.com/photo/white-dwarf-core-extinct-star-core-illustration-587998246.html

After becoming a red giant, the outer layers of the star continue to expand and cool as the core contracts and heats up. As the helium burns through the cores of the stars, peculiar thermal pulses are formed that spread outwards and “blow” the outer layers of the stars into the surrounding space. These outer layers can contain more than half the stellar mass. As a result, an expanding gas envelope forms around the star, which we call a planetary nebula.

You could look up further information and images of some nebulae that can be observed with a small telescope – e.g. Cat’s Eye, Ring Nebula, Eskimo Nebula.

After the planetary nebula dissipates, the hot core of the star – the white dwarf – remains. This is an extremely dense and hot star about the size of a planet. It radiates energy left over from the star’s nuclear fusion processes. Over billions of years, it emits light and slowly cools.  When the white dwarf radiates all its energy outwards, it stops glowing and dies as a brown dwarf – this is the end of the star’s life.

Stars with mass greater than the Sun

These are stars 9 to 40 times the mass of the Sun – true giants in the Universe. They are extremely bright and radiate a huge amount of energy. They contain most of the heavy elements needed for galaxies to form and evolve.  However, their lifetimes are considerably shorter than those of stars with less mass.

When the residual core in an exploded star is less massive than 1.4 solar masses, it cannot become a white dwarf. Instead, it continues to contract as the pressure of electrons cannot balance gravity. The sudden and rapid contraction produces a huge explosion, generating a shock wave that ejects the outer layers of the star and heats them. The brightness of the star becomes extremely high and is comparable to the brightness of an entire galaxy. This is a supernova. It then becomes a tiny dot. It is not a new star, but a dying old star. Supernova explosions in the universe eject all elements heavier than oxygen and are the only source of elements heavier than iron. They enrich interstellar space with heavy elements, thickening the gas clouds that subsequently form new stars.

Check out the following video:

After exploding as supernovae, massive stars leave behind neutron stars or black holes.

Neutron star – produced by the final gravitational contraction of the supernova core. In this violent collapse, the nuclei of the atoms are destroyed as protons bond with electrons to form a mass containing mostly neutrons. As a result, the pressure of the neutron gas stops the collapse and the remainder of the core becomes a neutron star.

The density inside a neutron star is enormous – on the order of 1018 kg/m3. It’s as if a small dice cube had a mass of a trillion kilograms. On the other hand, its size is relatively small – it shrinks to a body between 10 and 30 kilometres across. That’s as big as a city. So imagine a few stars the size of our Sun fitting within the confines of, say, London! Their magnetic field is billions of times stronger than the Earth’s magnetic field.

Source: https://upload.wikimedia.org/wikipedia/commons/9/97/A_Young_Pulsar_Shows_its_Hand.jpg

Rapidly rotating neutron stars that emit beams of electromagnetic radiation (radio waves, X-rays, etc.) directed towards Earth and resembling a periodically glowing lighthouse are called pulsars. They are a kind of “cosmic clock” since their rotation period is very stable and precise.

A black hole – one of the most mysterious and powerful objects in the Universe. It forms when many massive stars collapse at the end of their life cycle.

If the star has a mass more than 20 times that of the Sun, the collapse after the supernova is even faster. Even the pressure of neutron gas can’t stop it. The core continues to shrink until it becomes a point of almost infinitely high density, shrunk to an infinitesimally small size. A black hole forms around this point. Such objects possess a tremendous gravitational force that distorts space considerably and prevents even light from propagating in a straight line. The light is also absorbed and the object becomes invisible. Therefore, an astronomical object whose gravity is so strong that nothing, not even light, which has the highest speed in the universe, can escape from it, is called a black hole. The boundary around a black hole beyond which there is no return is called the event horizon. Any body that crosses this boundary is swallowed by the black hole.

Black holes cannot be observed directly.  The only way to detect them is to observe the effects such objects have on space and other objects around them – if matter from another nearby object spirals down into them, it heats up significantly and emits X-rays and gamma rays that we can detect.

Source: https://depositphotos.com/photo/stars-and-material-falls-into-a-black-hole-77862928.html