Stellar Evolution

Protostar stage

- nebular material still contracting - core temperature increasing until nuclear fusion begins - gravity and gas pressure not yet balanced - rotating dense clumps flatten into disk

Stellar Lifetimes

1Ms with 1 LY, 10 billion years 2Ms with 20 LY, 1 billion years 30 Ms with 10^5 LY, 3 million years

The Sun will leave the main sequence in how long?

5 billion years

Why do big stars run out of fuel?

A bigger star has a hotter core, which means a much faster fusion rate and shorter lifetime.

Planetary nebula

A huge cloud of gas that is created when the outer layers of a red giant star drift out into space.

Star Birth

A star is born in a huge cloud of gas and dust known as a nebula (plural: nebulae). This nebula is about 21 light-years (125 trillion miles) across. Part of the nebula begins to shrink under the pull of its own gravity. This forms a protostar which is about 60 million miles across. The star begins to take shape. The temperature continues to rise and nuclear fusion begins to take place. The pressure from inside the star finally equalizes the gravity pushing in, and the star stops contracting.

Planetary Nebula Aftermath

A star like our Sun will lose almost half its mass, larger stars may lose 80-90% of their material. The remaining core settles into a white dwarf

Variable Stars

A star that regularly or repeatedly changes in brightness. Once helium fusion begins in the core of a red giant, it causes the star to turn yellow and begin pulsating. Many of these types are called RR Lyrae (lower mass) stars or Cepheid variable stars (require higher masses)

Stellar Mass

A stars mass determines its core temperature: a more massive star has a higher gravitational attraction than a less massive star. Hydrostatic equilibrium then requires a higher gas pressure for the larger gravity of a massive star. The higher pressure can be achieved from the perfect gas law, by a higher temperature.

Protostar Stage

At first, the energy to make the star glow comes from the gravitational energy gained by falling gases, later, the core gets hot enough for nuclear fusion to commence, adding energy to the protostar in addition to that from gravity. Astronomers consider it to be a protostar so as long as some gases are still falling inwards onto it. Protostars have a greater luminosity than normal stars. The protostar stage lasts only a few million years, then it becomes a main sequence star.

CNO Cycle

Carbon, Nitrogen, and Oxygen. These elements act as catalysts, still fusing hydrogen into helium, it can only occur within the very hottest of star cores, so it speeds up fusion in large stars but not smaller stars like our Sun.

The Life of a High Mass Star

Collapses from an interstellar cloud and resides on the main sequence. Proceeds through these stages much faster than the Sun, spending less than 100 million years on the main sequence. Then, it passes through the pulsating yellow giant phase and turns into a red giant, where the core begins to fuse one element into another creating elements as massive as iron. Once iron is reached, the core is out of fuel and it collapses, exploding into a supernova and leaving a neutron star or black hole.

Why they Pulsate

Core produces energy, outer layers are opaque, they trap radiation, outer layers heat and expand, layer becomes transparent, light and heat escapes, gases cool and fall back inwards, cycle repeats.

Structure of High and Low Mass Stars

Fusion in the Core of : Low Mass Stars: proton-proton chain High Mass Stars: CNO cycle, CNO act as catalysts for H fusion at higher core temperatures

Interstellar Gas Clouds

Gas: hydrogen 71% and helium 27% Dust: Microscopic particles of silicates, carbon, and iron Temperature: Around 10K

Further Collapse

Gravity continues to draw material inward, protostar heats to 7 million K in core and hydrogen fusion commences. Collapse of core ceases, but protostar continues to acquire material from the disk for about 10^6 years. In falling material causes violent changes in brightness and ultimately a strong outflow of gas.

Initial Collapse

Low temperature in interstellar gas clouds leas to too low pressure to support cloud against gravitational collapse. Collapse may be triggered by collision with another cloud, a star explosion, or some other process. Non- uniformity, awkward nature of gas leads to formation of smaller, warmer, and denser clumps.

Life of out Sun

Our Sun was born out of an interstellar cloud that gravitationally collapsed over a time span of a few million years. Fusing hydrogen into helium in its core, the Sun will reside on the main sequence for 10 billion years and in the process convert 90% of its core hydrogen into helium. Starved of fuel, the core will shrink and grow hotter as the outer surface expands and cools transforming the Sun into a red giant. After one billion years, the red giants core will be hot enough to begin fusing helium. The Sun will then transform into a pulsating yellow giant. As the core's helium begins to expire, the Sun will once again transform into a red giant, but only bigger than before. The high luminosity of the red giant will drive the Sun's atmosphere into space (planetary nebula) leaving behind its bare core, which will then dwindle and cool into a white dwarf.

Death of Stars like the Sun

Our sun will be a main sequence star- red giant- yellow giant- and then the core fills with carbon: stars like our Sun can do nothing with carbon so fusion stops, the final frantic fusion that occurs along with gravitational collapse of the core produces a final burst of energy. This last pulse causes the outer layers of the star to be blown out into space, above the escape velocity creating what is called the planetary nebula.

Stellar Mass Limits

Stars smaller than .1 are rarely seen since their mass is too small for their cores to initiate fusion reactions. Objects with masses between planets are called brown dwarfs, "failed stars" as they are extremely dim and difficult to observe. Upper mass limits of stars is about 30 due to extreme temperatures and luminosity preventing additional material from falling on them, intense radiation may even strip off outer layers of stars. Larger stars cannot form because solar winds generated by the first 30 will push away other gasses rather than allowing them to join the star.

Bipolar Outflows

Streams of material moving away from the protostars

Brown Dwarfs

Substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike main sequence stars. Less than .1 Ms

Mass

The collapse of larger clouds takes less time than that for smaller clouds, bigger clouds= more gravity. Where a star is on the main sequence depends on mass

period-luminosity relationship

The relation that describes how the luminosity of a Cepheid variable star is related to the period between peaks in its brightness; the longer the period, the more luminous the star.

Supermassive Stars grater than 10M

These stars begin their lives with the usual protostar and main sequence stages, then they may evolve into a blue supergiant or a red supergiant. These stars do not stop with carbon fusion, instead they advance through nucleosynthesis to, form heavier elements, when they reach iron, the heaviest element, larger elements will not release energy upon being fused. These materials fall by the force of gravity and the material collapses into the core, most of it rebounds back into space into a Type 2 Supernova explosion. w

What characteristic do all stars on the main sequence share?

They are all fusing hydrogen into helium in their cores

Which stars do what?

Very low mass stars: never have helium fusion and may never become red giant or yellow giant stars Low Mass Stars: Will become Red Giants and then RR Lyrae yellow giants or pulsating giants Higher Mass Low Mass Stars: Red giants and then Cepheid Variable type of yellow giant stars.

hydrostatic equilibrium

the balance of the inward gravitational force and the outward force of fusion within a star. This balance of forces is what keeps a main sequence star stable.