Stellar Evolution Diagram Answer Key:



Stellar Evolution Diagram Answer Key:

Nebula

This is a cloud of dust and gas that can last for millions of years. A nebula consists commonly of about 70% Hydrogen, 28% Helium, and about 2% of other heavier elements. Nebulas rarely start to clump together on their own- often they require an outside force to nudge them into coalescing. Such a force could be provided by a nearby supernova. When the matter in a nebula begins to collapse- it begins to spin rapidly and flattens into a disk shape.

Protostar:

A protostars lifetime is only a few years. As the nebula shrinks a central mass will collect towards the center. It heats up because of the collision of particles and the increase in pressure. A protostar becomes a main sequence star when it reaches temperatures above 10,000,000 C- the temp at which fusion begins.

Main Sequence:

Main sequence stars last for about 5- 10 billion years. The larger the star- the shorter the life of this stage. The energy for fusion comes from the massive amounts of gravity and pressure. This causes Hydrogen atoms to collide and fuse into helium atoms. The star is considered stable because the force of fusion causing the star to expand is equal to the force of gravity. This stage ends when it runs out of fuel (most of the hydrogen is fused into helium)

Giant/Supergiant:

A supergiant star forms when the main sequence star runs out of Hydrogen to fuse in the core. This causes the star to collapse. When the M.S. star collapses- the increase in pressure causes and increase in temperatures that are high enough to fuse Helium into larger atoms up to Carbon atoms. Only Giant stars are much larger than M.S stars because they are expanding. The Giant stars expand because they not only fuse helium in their core- but they are also fusing left over hydrogen into helium in the outer regions of the star- causing the force of fusion to be greater than the force of gravity. Giant stars come to an end when most of the Helium in the core is fused to Carbon.

Planetary Nebula:

This stage is very short (years). A planetary nebula is a small giant star that has run out of helium to fuse in the core. This causes the star to collapse. As it collapses- the outer regions of the star can be left in space- giving the star a ring of gases that are shed off into space. This stage has no fusion because it is not big enough and the collapse does not allow it to get hot enough to cause Carbon to fuse into larger elements.

White Dwarf-

A giant star becomes a white dwarf when the star looses its outer gases and reveals its core. A white dwarf consists primarily of Carbon and can last for 100’s of billions of years. This is the final stage of small stars. These stars still glow because they are still very hot. Black dwarfs are believed to be white dwarfs that no longer glow because they have cooled- however the universe is not old enough for this to have occurred.

Novas:

Novas are white dwarfs that have an explosion that relseases energy and causes the star to increase in brightness for a short period of time (weeks). Novas are not large enough to fuse their primary element Carbon. However, they do have enough gravity/pressure to fuse smaller atoms (H & He). If there is a companion star that they can pull these gases off of and they collect a significant amount of hydrogen and helium- they can cause it to fuse and become a nova. When the nova runs out of fuel it becomes a white dwarf again until enough gases are collected to go nova again. Repeating cycle.

Supernovas:

Supernovas occur when a very large star (Supergiants) collapses. The collapse creates enough pressure to raise the temperatures high enough to fuse carbon into larger elements up to Iron. This stage of a star is very short (weeks). When the core is almost completely iron there is an additional collapse as fusion stops. This causes a final explosion in which the fusion of elements past iron can occur. After this final explosion – stars will typically reveal an extremely dense core of neutrons (called a neutron star). Only the very largest of supernovas will cause the formation of a black hole.

Neutron Star

Once a neutron star forms- it should have a very long life (100’s of billions of years). It is a core of a supernova after an explosion which causes it to contract into a very small but incredibly dense ball of neutrons. A spoonful of matter from a neutron star would weight 100 million tons on earth.

Black Hole:

The lifetime of black holes is not known. A black hole is when the core of an extremely large supernova contracts with such great force that it collapses leaving a hole in space. Black holes have tremendous amounts of gravity- so much that not even light can escape it. Black holes give off x-rays just before it pulls matter in from its companion stars. Astronomers think that black holes are found in the center of galaxies.

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