Goal: To understand the post main sequence evolutions of the

most massive stars

Objectives:1) To learn about Eta Carinae and LBVs

2) To learn about what Wolf-Rayet stars are3) To understand the effects these stars have on

their surroundings and why they are important for the evolution of the galaxy

4) To explore a little bit more in depth as to what happens when these stars die

Massive Stars

• Are rare, but special

• A 100 solar mass star only lives for about a million years

• Luckily that makes these massive stars very easy to find

• Where do you think we will have to find them?

Luminous Blue Variable stars (LBV)

• The first stage of post main sequence evolution for all stars > 25 solar masses.

• While on main sequence these stars are 10,000 to 1 million times brighter than our sun

• So, the Hydrostatic Equilibrium for the outer part of the star barely holds

Mass loss

• Needless to say they loose mass very fast to stellar winds

• About an EARTH mass to 100 Earth masses per YEAR.

• A solar mass in about a thousand to 100,000 years.

• The largest of these stars can loose a solar mass in a single “burp”

Eta Car

• “Eta Carinae (Eta) is one of the most remarkable of all well-studied stars and perhaps the most poorly understood. Observations with the Hubble Space Telescope and other modern instruments have solved a few of the mysteries concerning this object while opening a comparable number of new ones.”

• Kris Davidson and Roberta Humphries, U. Minnesota

Eta Car

• LBV

• 100-150 solar masses

• 2.5k parsecs from earth

(100X further than Sirius)

10k to 20k years to live

http://etacar.umn.edu/etainfo/history/

Eddington Limit

• There is a limit to the brightness of a star

• If a star is too bright the radiation pressure at the surface pushes plasma out MORE than gravity pulls in

• Gravity looses…

• For 20 years Eta Car seems to have exceeded this limit by a factor of 4.

So what are we seeing here?

• The loss of light is due to a massive ejection of material which then forms dust

• As much as 1 solar mass may have been burped out in this one even dubbed the “Great Eruption”

• Supernova impostor

• Keep in mind…• Not looking at the star necessarily but often

times the gas AROUND the star sometimes the gas thrown off of the star

Next stage:

• Wolf-Rayet star

• Eventually the LBV will throw all of its outer layers off.

• No more H

• This leaves a Wolf-Rayet star.• However, it also is throwing off lots of material.

Oddities

• Emission lines

• Lines which don’t quite match up with Hydrogen

Solution

• You are looking at the gas ejected from the star

• Not seeing the star

• The lines are all shifted

Doppler shift

• The gas is moving at us at hundreds to thousands of km per second

• This causes the lines to be “blue shifted” as the emitted wavelengths are compressed.

• A bit of explanation coming here…

Even Wolf-Rayet stars evolve

• These stars are so massive and do things so fast they evolve.

• They go from having poor Hydrogen lines and Nitrogen to no H and Carbon.

• This occurs as the layers get tossed off and the core continues to fuse heavier elements.

So why study stars so rare?

• What influence could they possibly have?

• Well, it turns out the rarest stars have the greatest impacts

Super bubbles

• The material they throw out is usually pretty hot (especially when they supernova at the end of their lives)

• This creates a giant bubble of hot gas

• Often you will get a few in the same small region also (they exist in clusters after all) whose bubble all combine to form a super bubble

Super bubble bursts

• If the bubble gets 1k parsecs in size it can break through the plane of the galaxy

• When this happens the metal rich gas gets thrown very far and spreads to a very large region of the galaxy

• So, these massive stars not only throw out large amounts of metals but spread them throughout the galaxy

Dust

• In addition these massive stars seem to create a majority of the dust in our galaxy

• This helps in the formation of planets.

Deaths

• Their deaths are thought to be spectacular.

• None witnessed though, they are that rare.

• Hypernova – a super strengthed super nova

• Gamma Ray burst – this one takes a bit of explaining

Conclusion

• Massive stars are powerful

• Destructive

• Almost unpredictable

• But alter the future of the galaxy