The Life Cycle of Stars

Done by:

Jamila Salim Al-Mashani.

Nadia Suhil almhry.

The Life Cycle of Stars

The Birth of a Star:

• Stars form out of large interstellar gas clouds that collapse in on themselves getting hotter and hotter until the cloud is hot enough to support nuclear fusion. When this happens, a star is born.

• When a star first starts fusing hydrogen in its core into helium, it is said to be on the “main sequence” of the HR-Diagram (at Right) because it has not evolved to the point where it is fusing other elements other than hydrogen.

Red Giant Stage:

• No matter how large the star is initially, the next stage after being on the main sequence is becoming a red giant star. This occurs when there is no hydrogen left in the core of the star to be fused into helium, so the star starts to fall in on itself due to gravity until there is enough pressure and heat to start fusing the hydrogen around the core and once the core heats up even more, it starts fusing the helium into carbon.

• At right is the red giant star Betelgeuse, with a scale showing how large it really is.

• This stage of the star’s life varies in length depending on the size of the star. The “death” of the star also depends on the star’s size and there are three different fates for low, high, and very high mass stars.

Death of Low Mass Stars:

• Low mass stars (ones that aren’t big enough to fuse carbon) have a quiet death in store for them in the form of a white dwarf. Once they stop fusing hydrogen/helium in their cores, they collapse in on themselves until the electrons of the atoms will no longer be crushed any more (this is generally when the star is the size of the Earth). During this collapse, the star sheds its outer layers of gas and leaves only a small portion of itself as the white dwarf. The white dwarf does not have any fusion occurring in its core, so it is left to cool off for the rest of eternity until it no longer emits any light at all.

• The layers of gas given off by the white dwarf slowly drift outwards and are lighted by the cooling white dwarf forming planetary nebulae (which ironically have nothing to do with planets)

At Left are two example: on top the Helix Nebula and on bottom the Spirograph Nebulae

• If the white dwarf happens to have a binary partner nearby enough to it; it may steal material from its neighbor and every so often accumulate enough new hydrogen to start fusion again for a short time (this is because the surface of the white dwarf is often hot enough to fuse hydrogen if there is enough of it). In this case, the white dwarf is described as “going nova” and gets many time brighter than normal for a short period of time

Death of High Mass Stars:

• Stars of higher mass have a more violent death in their futures. More massive stars have hot enough cores to continue fusion beyond carbon unlike low mass stars, and are able to fuse all of the elements up to iron. Fusing iron causes the star to use energy and to cool down. When this iron fusion occurs, the star rapidly implodes on itself: when the implosion hits the core, it bounces off of it and starts traveling outward creating an enormous explosion known as a supernova. This explosion is so powerful that it can produce the equivalent of 1 billion times the light that a star like the sun produces. This explosion is also believed to create all of the heavier elements found in the universe.

• At right is the Crab Nebula which is a supernova remnant from over a thousand years ago that is still visible to this day. On the bottom is a depiction of a high mass star fusing all the elements up to iron in its core.

What is left over from this massive explosion is known as a neutron star (because it is believed to be made completely up of neutrons). This star is much smaller than even a white dwarf and is normally described as being as large as a city. Neutron stars are known to rotate incredibly fast and due to this rotation, light is focused into two bi-directional beams of light that rotate with the star. If we happen to be in the path of these beams, it appears that the star pulses very rapidly: leading to the name “pulsar”.

Death of Very High Mass Stars:

• The deaths of the highest mass stars (ones that are too big for neutron stars) are one of the most awe-inspiring objects in the universe: black holes. Neutron stars stop collapsing much like white dwarfs, only they rely on the neutrons to no longer be able to compress any further (there are no electrons because they are squeezed with protons to create neutrons and high-energy particles called neutrinos). However, there is a limit to what the neutrons can hold back, and when that limit is breached, a black hole is the result. A black hole’s gravity is so powerful that not even light itself can escape from it. This begs the question of how we know black holes exist if no light comes from them;

• and the answer is that we observe their affects on nearby stars as depicted in the image at left. The image is an artists representation of a black hole ripping apart its companion star based on the two actual images at the bottom of the depiction.