Stellar Lifecycles The process by which stars are formed and use up their fuel. What exactly happens to a star as it uses up its fuel is strongly dependent on the star’s mass. The Orion Nebula - Birthplace of stars

A Star is Born Stars form from huge, cold, clouds of gas and dust. At some point this cloud collapses on itself. Its own gravity causes clumps of material to form. These clumps heat up as material continues to fall upon them. Eventually temperatures are high enough in the center of these clumps to allow nuclear fusion reactions to occur. Often several large clumps can form within the cloud. Clusters of stars can all form at the same time from the same cloud. A cluster of massive, hot blue stars have formed still surrounded by clouds of gas that may form new stars.

Main Sequence Stars Once nuclear fusion has begun, pressures in the core grow high enough to stop the stars from collapsing any further. It is then in Hydrostatic Equilibrium. These are now Main Sequence stars Their position along the line of the Main Sequence depends on their mass. Almost the entire lifetime of a star is spent on the Main Sequence. The H-R diagram showing the Main Sequence line (in purple). More massive stars are to the upper left, less massive stars to the lower right.

Differences Between High Mass and Low Mass Stars Stars that are more massive than the Sun have stronger gravitational forces. These forces need to be balanced by higher internal pressures. These higher pressures result in higher temperatures which drive a higher rate of fusion reactions. The Hydrogen within the core of a high mass star therefore gets used up much faster than in the Sun and “ages” faster. Low mass stars “age” slower. A star like our Sun will remain on the Main Sequence for about 10 billion years. A very massive star may only be on the Main Sequence for a few million years.

When the Sun Leaves the Main Sequence When a star uses up the Hydrogen in its core it can no longer support itself against gravity. The core compresses and temperatures begin to rise. Temperatures may get high enough outside the core to begin Hydrogen fusion there instead. The pressure from this shell around the core pushes the outer layers of the star out. These outer layers cool and get redder. The life cycle of a star like the Sun

The Last Years of the Sun During this Red Giant stage the core of the Sun will continue to contract and heat up. Eventually temperatures will be high enough for the fusion of Helium in the core. The Sun then converts Helium into Carbon & Oxygen. The surface temperature of the Sun increases and it becomes a Yellow Giant. This stage lasts as long as there is Helium available in the core. The motion of the Sun through the H-R diagram as the Sun ages. Notice that the Sun spends most of its life on the Main Sequence.

The Sun’s Planetary Nebula As the core exhausts its Helium fuel it begins to contract and heat causing the Helium to get used even faster. The Sun increases its luminosity. The outer layers of the Sun expand, cool and redden again. The outer layers of the Sun start streaming away from the core. This material forms a nebula surrounding the Sun. Except for the core, the rest of the Sun will eventually be dispersed into space forming a planetary nebula like this one.

White Dwarf Stars The core of the Sun eventually stops all nuclear fusion but remains extremely hot. The core will form a White Dwarf star, a very dense, small object about the size of the Earth. Over time the White Dwarf will cool and dim. By measuring the temperature of white dwarfs you can estimate how long ago they formed. White Dwarf stars are very hot but also very small. They appear in the lower left corner of the H-R Diagram.

How do High Mass Stars Evolve? For stars greater than 8 times the Sun’s mass after Helium fuel is exhausted the core of the star contracts, heats up and starts to fuse Carbon & Oxygen into Neon and Silicon. Helium and Hydrogen fusion continue in shells around the core. As long as the star can raise its core temperature high enough it can continue to fuse new elements. Until iron is created.

High Mass Stars Evolve Quickly Compare the duration of the stages of a high mass star’s life to that of a solar-mass star (two slides back).

Supernova The formation of iron actually absorbs rather than releases energy. Nuclear fusion at the core stops and it begins to collapse. The pressures of the surrounding layers are so high that the atoms of the iron core are crushed, smashing the electrons into the protons forming neutrons. Once neutrons are formed the collapse stops, the surrounding gas is heated and explodes off the core. This is a supernova explosion. The explosion is so energetic that it can outshine the combined light of a galaxy! Heavy elements are formed in the material blown off the star. These elements are dispersed into space where they can be used to form planets and new stars. Depending on its mass the core may become a neutron star or collapse further to a black hole.

Testing Stellar Evolution Theories

Open Clusters: Young Stars

Pleiades HR Diagram: A Young Cluster Blue Dots: Pleiades Orange crosses: Coma Bernice Gray Circles: Hyades

Hyades: Older than the Pleiades

M67: Open Cluster Getting older

Globular Clusters: Old Stars

47 Tucanae: Globular Cluster Red Giant Stars

47 Tucanae: Globular Cluster Blue Straggler Binaries