Life-Cycle of Stars

Task Using the information around the room, find out the life cycle of a star. Put all of this into order, from the beginning of a star, to the end.

Nebulae Clouds of dust and gas

Protostar Hot gas heated through pressure Lasts about 10,000,000 years Pre fusion

Main Sequence Star Hydrogen fusion begins and the star is born Usually lasts 5 billion years or so Stable due to balanced forces of gravity and pressure from fusion reactions

Red Giant Hydrogen fuel runs out The core contracts due to gravity and the outer layers are shed The star grows and cools into a red giant

White Dwarf When the red giant has run out of all fuel and can fuse nothing more, it will lose its outer layers. This leaves just the core, which is still extremely hot. It is so hot it glows white hot, hence the name “white dwarf” Temp: 107 K interior but surface temps vary greatly (as low as 4000 K (about the same temp as a sun spot) to about 150 000 K)

White dwarf stars eventually cool and become black dwarfs … theoratically! None of these exist yet as the process takes longer than age of the universe to get to this stage. We are almost certain this WILL happen but it just HASN’T HAPPENED YET.

Useful diagram but note that it misses protostar phase.

Nebulae Clouds of dust and gas

Protostar Hot gas heated through pressure Lasts about 10,000,000 years Pre fusion

Massive Star Stars much larger than our sun also have a main sequence phase but this tends to be shorter. Many burn so hot that they become blue giants

Red Super Giant Unlike a Red Giant, a Red Super Giant is large enough and hot enough to maintain fusion reactions with ever heavier elements up to iron.

Supernova At the end of a Red Supergiant’s life it leaves layers of elements in the structure of the star like layers in an onion

Supernova Fusion ends with Iron and with no outward radiation pressure the star begins to collapse under it’s own gravity

Supernova The collapsing inner core is compressed into neutrons

Supernova The outer layers then rebound off this neutron core in a shockwave (red ring)

Supernova The rebound is then temporarily slowed by infalling material

Supernova Before being re-activated by an as yet not fully understood process

Supernova All this happens in just a few seconds and the supernova can shine as bright as an entire galaxy for a number of weeks

Supernova Betelgeuse – highly unstable red supergiant about 600 ly away

Option A – Neutron Star This is the fate of the cores of stars around 3x the mass of the Sun (or less) Very dense (about the size of Earth but as heavy as the Sun)

Option B – Black Hole When a star is more than 3x bigger that the sun, this leads to a black hole The neutron core collapses further under extreme gravity Nothing can escape a black hole, including light

Life Cycle of a Star Nebula Clouds of gas and dust come together. Gravity pulls them together as a spinning ball of gas. This is getting so hot that it starts to glow. Protostar Gravity acting on the protostar makes it become smaller , hotter and shines brighter. This leads to fusion. This is when hydrogen atoms combine to form helium and releases energy. It is gravity that holds the star together, and the build up of pressure from fusion that makes the star the size it is. As long as there is hydrogen there is energy being released. Massive protostars burn faster than smaller protostars and don’t last as long Red Giant (small stars only) When hydrogen runs out, the core contracts due to gravity but the outer part expands and turns red. Which is a red giant. It will be so big, it will consume Mercury, Venus and Earth. Red Supergiant (massive stars) Similar to a red giant. The core contracts due to gravity and the pressure from fusion expands on a much bigger scale. The core collapses almost instantly causing it to explode (Supernova) White Dwarf Eventually, the core of the red giant remains, and will emit what ever energy is left Black Dwarf Eventually the white dwarf will no longer have any energy, and will become a black dwarf Supernova The remnants of the supernova gather with dust and gas in space. Gravity pulls this in and a new star is formed. Known as a second generation star. Neutron Star After the supernova, a very dense core is left. The neutron star. It could collapse into a black hole