Uploaded January 2023 | Updated September 2026, 1 week ago
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Key notes for exam preparation
The story of a star starts with a nebula. A nebula is a giant cloud of gas, ice, and dust. Eventually, gravity brings the matter in the nebula together to form a protostar.
A star forms when the pressure at the protostar’s core reaches 15,000 degrees Celsius and nuclear fusion starts. During nuclear fusion, four hydrogen atoms fuse to form a helium atom. A helium atom has slightly less mass than four hydrogen atoms. The difference in the masses is released as energy. Albert Einstein’s famous equation, E = mc2, showed how matter could be converted into energy. The c in the equation represents the speed of light, which is 186,282 miles per second – a huge number! It doesn’t take much mass to make a lot of energy.
The amount of energy a star makes, and its resulting brightness, depends on how much matter was in the nebula that formed the star. Large stars are brighter than smaller stars. The life cycle of a star depends on its mass. Smaller stars are dimmer, but they also burn for longer than larger stars. A large, bright star can burn for just a million years, while a dimmer star can burn for billions of years. The Sun is an average star with a life expectancy of about 10 billion years. We are nearly halfway through the Sun’s life.
MAIN SEQUENCE STARS
When a star has nuclear fusion creating hydrogen at its core, we call it a main sequence star. In a main sequence star, the pressure of the energy being generated in its core is equal to the pull of gravity, so the star stays the same size. However, when the star runs out of hydrogen atoms, the pressure of gravity is unbalanced, and the core collapses in on itself, becoming even hotter. The outer layers of the star expand away from the hot core and then cool. At this point in the life cycle of a star, the low-mass stars become red giants, and high-mass stars become red super-giants. A high-mass star has about ten times the mass of our Sun.
GIANTS AND SUPERGIANTS
Inside the incredibly hot cores of giants and super-giants, helium atoms fuse to form carbon atoms and energy. When the star has used up all of its helium, the core will again condense and get hotter while the outer layers expand again.
In red giants, the outer layers are pushed so far away from the super-dense core that they escape into space. All that is left is a tiny, dense core about the size of Earth called a white dwarf. Gravity pulls the atoms of the white dwarf together until only the force of electrons repelling each other keeps the star from collapsing in on itself. After about four billion years, the white dwarf will have cooled and become a black dwarf.
Things get crazier for red super-giants. As a supergiant runs out of fuel, its core collapses similarly to a red giant. However, a red supergiant is so massive that it can create helium, carbon, oxygen, neon, silicon, and iron through nuclear fusion. Iron is so stable that it will not undergo nuclear fusion in the star’s core.
SUPERNOVAS
When the star has a core full of iron, the pull of gravity is so strong that it collapses the star in one second. Temperatures in the core reach 100 billion degrees Celsius. The repulsive forces of the atoms within the core are stronger than gravity, so they explode out. This explosion superheats the other layers of the star so much that even heavier atoms and radioactive isotopes are created from nuclear fusion. The shockwave of the exploding star sends these heavy atoms into space. We call an exploding red supergiant a supernova.
NEUTRON STARS
In smaller high mass stars, the core left over after a supernova is called a neutron star because the only thing left in it are neutrons. A neutron star is so dense that just a teaspoon of it would weigh 100 million tons on Earth.
BLACK HOLES
In larger high mass stars, the core left over after a supernova is so dense that nothing can stop the pull of gravity, and the star’s core collapses to the point that it has no volume. It has so much gravity that not even light can escape. We call this a black hole. While the gravity inside a black hole is extreme, it does not pull matter into itself. It acts like any other massive object in space. In fact, planets, stars, and even galaxies can orbit around black holes.
Please don't hesitate to send an email for comments, advices, recommendation, even for support and classes. My email address is here
thephysicspartner@gmail.com
Key notes for exam preparation
The story of a star starts with a nebula. A nebula is a giant cloud of gas, ice, and dust. Eventually, gravity brings the matter in the nebula together to form a protostar.
A star forms when the pressure at the protostar’s core reaches 15,000 degrees Celsius and nuclear fusion starts. During nuclear fusion, four hydrogen atoms fuse to form a helium atom. A helium atom has slightly less mass than four hydrogen atoms. The difference in the masses is released as energy. Albert Einstein’s famous equation, E = mc2, showed how matter could be converted into energy. The c in the equation represents the speed of light, which is 186,282 miles per second – a huge number! It doesn’t take much mass to make a lot of energy.
The amount of energy a star makes, and its resulting brightness, depends on how much matter was in the nebula that formed the star. Large stars are brighter than smaller stars. The life cycle of a star depends on its mass. Smaller stars are dimmer, but they also burn for longer than larger stars. A large, bright star can burn for just a million years, while a dimmer star can burn for billions of years. The Sun is an average star with a life expectancy of about 10 billion years. We are nearly halfway through the Sun’s life.
MAIN SEQUENCE STARS
When a star has nuclear fusion creating hydrogen at its core, we call it a main sequence star. In a main sequence star, the pressure of the energy being generated in its core is equal to the pull of gravity, so the star stays the same size. However, when the star runs out of hydrogen atoms, the pressure of gravity is unbalanced, and the core collapses in on itself, becoming even hotter. The outer layers of the star expand away from the hot core and then cool. At this point in the life cycle of a star, the low-mass stars become red giants, and high-mass stars become red super-giants. A high-mass star has about ten times the mass of our Sun.
GIANTS AND SUPERGIANTS
Inside the incredibly hot cores of giants and super-giants, helium atoms fuse to form carbon atoms and energy. When the star has used up all of its helium, the core will again condense and get hotter while the outer layers expand again.
In red giants, the outer layers are pushed so far away from the super-dense core that they escape into space. All that is left is a tiny, dense core about the size of Earth called a white dwarf. Gravity pulls the atoms of the white dwarf together until only the force of electrons repelling each other keeps the star from collapsing in on itself. After about four billion years, the white dwarf will have cooled and become a black dwarf.
Things get crazier for red super-giants. As a supergiant runs out of fuel, its core collapses similarly to a red giant. However, a red supergiant is so massive that it can create helium, carbon, oxygen, neon, silicon, and iron through nuclear fusion. Iron is so stable that it will not undergo nuclear fusion in the star’s core.
SUPERNOVAS
When the star has a core full of iron, the pull of gravity is so strong that it collapses the star in one second. Temperatures in the core reach 100 billion degrees Celsius. The repulsive forces of the atoms within the core are stronger than gravity, so they explode out. This explosion superheats the other layers of the star so much that even heavier atoms and radioactive isotopes are created from nuclear fusion. The shockwave of the exploding star sends these heavy atoms into space. We call an exploding red supergiant a supernova.
NEUTRON STARS
In smaller high mass stars, the core left over after a supernova is called a neutron star because the only thing left in it are neutrons. A neutron star is so dense that just a teaspoon of it would weigh 100 million tons on Earth.
BLACK HOLES
In larger high mass stars, the core left over after a supernova is so dense that nothing can stop the pull of gravity, and the star’s core collapses to the point that it has no volume. It has so much gravity that not even light can escape. We call this a black hole. While the gravity inside a black hole is extreme, it does not pull matter into itself. It acts like any other massive object in space. In fact, planets, stars, and even galaxies can orbit around black holes.
