Special Sub-Topic: The Life and Death of a Star Click Here To Play: The Life and Death of a Star

Most stars tend to form in large clouds of gases in space. What are these clouds called?

Nebulae. A nebula (plural - "nebulae") is a large cloud of gases and dust in space. Over time, these gas and dust particles accumulate and grow larger due to gravitational forces acting upon them. This is the basis of star formation.

Which element contained in nebular clouds acts as the fuel for newly-formed stars?

Hydrogen. Hydrogen is the most abundant element in the universe by a long way. It is the smallest and lightest of the elements and is burned inside of stars through nuclear fusion to form other elements.

When gases and dust in nebulae start gathering together due to gravity, they form into large clumps. What are these clumps of gas and dust known as?

Bok globules. Bok globules are vast dense clouds of dust and gas in which stars generally tend to form. They were discovered by Bart Bok in the 1940's.

When the gas/dust compression due to gravity is balanced by the pressure created inside the forming star, what state is said to have been achieved?

Hydrostatic equilibrium. Hydrostatic equilibrium is a state reached when a clump of gas and dust has compressed (due to gravity) to such a state that the internal pressure of the clump does not allow it to be compressed any further.

Once a star has gathered enough mass and achieved hydrostatic equilibrium, the pressure (and resultant increased temperature) ignites nuclear fusion in the star's core. It will spend approximately the next 90% of its life fusing hydrogen into helium. What is this stage of the star's life called?

Main sequence. Since hydrogen is by far the most abundant element in the universe, a star will spend the vast majority of its life burning hydrogen in its core. During this main sequence of its life, helium accumulates in the core while hydrogen is used up.

Once the hydrogen in the star's core is used up, the star begins to change dramatically. What change first takes place after the main sequence?

The star starts expanding. Once a star's supply of hydrogen is used up, the star loses its outer gas layers, which expand outwards in the form of a gigantic bubble away from the star's core. This is the formation of a red giant.

Once the hydrogen is all used up, a star will begin fusing the helium instead. Heavier stars will be able to fuse progressively heavier elements together, if their masses can support it. One element, however marks the end of regular fusion in a star's core, as it cannot be fused any further. Which element is this?

Iron. The nuclei of iron atoms are more tightly packed than those of other elements. Because of this, it will not release any energy from fusion, but will instead consume energy. As the iron builds up, it gets pushed into the outer layers of the star. The energy released by a supernova however, is enough to fuse iron atoms even further.

If the mass of the star's core exceeds a critical mass point (called the "Chandrasekhar Limit"), the star experiences a catastrophic explosion called a supernova. What happens in the core of a supernova?

Neutrons are formed. The gravitational forces of the supernova compress the core to such a state that the electrons are forced into the protons to form neutrons. The neutrons are tightly packed together in what is known as a neutron star. As it gets smaller and denser, its rotation speed starts to increase.

Once neutron stars exceed a certain critical mass, even the neutrons are compressed to such a state that their composition particles are forced together. The core is packed together so tightly, that a "singularity" is believed to have been formed. This creates what is known as a black hole. What are the particles that make up a neutron called?

Quarks. Neutrons are each composed of three particles called quarks. Once the gravitational pressure in the neutron star exceeds a critical mass point called the "Tolman-Oppenheimer-Volkoff Limit", the individual quarks are forced together to form what is known as "degenerate quark matter".

While a black hole signifies the ultimate end to a star's life cycle, it is not the only way for a star to end its life. Which of the following is not a final stage of a star?

Protostar. A protostar is a star that is still beginning its life cycle. White dwarf stars form from less massive stars (which include our Sun), brown dwarfs are stars that did not have sufficient mass to sustain nuclear fusion, while neutron stars may or may not degenerate further into a black hole, depending on its total mass. Note, white dwarfs are theorized to eventually cool down into "black dwarfs", but since no black dwarf star is known to exist in the universe as yet, the white dwarf can be considered a final stage of a star's life.