Download - Quiz #6 Most stars form in the spiral arms of galaxies Stars form in clusters, with all types of stars forming. O,B,A,F,G,K,M Spiral arms barely move,

Quiz #6

• Most stars form in the spiral arms of galaxies• Stars form in clusters, with all types of stars

forming. O,B,A,F,G,K,M• Spiral arms barely move, but gas clouds and

stars orbit around the galaxy moving in and out of spiral arms

• From the HR diagram, by far the most luminous stars are the O-type stars. Their luminosity can be 100,000 times the Sun’s.

• Why is the spiral structure in galaxies so noticeable, even at great distances?

Here are the evolutionary tracks for various mass stars. Stars that never have convection do not

have the down turn. Also the very massive stars form fast, due to their large gravity.

Interesting, but does it really happen. Here is the cluster at the center of the Orion Nebula

This is the HR diagram for the Orion cluster

Main sequence line

Massive stars on MS, but lower mass stars not.

Close up of low mass stars

Same thing in Lagoon Nebula.

Pleiades, what about them?

Still some gas around

And dust shows up in the infrared image taken by Spitzer telescope

All stars are on the main-sequence except the O-stars which are already running out of

fuel and moving off the main-sequence.

Star clusters have been extremely important to understanding how stars die.

• Stars in a cluster are all virtually the same distance away from us. The apparent brightness is directly related to the star’s luminosity.

• Stars in a cluster all have the same chemical abundances. They form from the same cloud so the amount of elements heavier than helium are all the same.

• They move together in their orbit, so they all have about the same velocity. (Co-moving)

• MOST IMPORTANT: They all form at the same time. (Co-eval)

Co-eval is important because a star cluster is NOT a mixture of old and young stars like the general field of the Galaxy.

Being co-eval means that it is possible to see the effects of evolution and what type of stars are produced after the hydrogen in the core runs out.

Let’s first consider what happens to lower-mass stars.

Open cluster M67

An H-R diagram for M67

Main Sequence

An H-R diagram for M67

Main Sequence

Sub-giants

Giants

Compare M67 to the Pleiades

Age = 100 million yearsAge = 4 billion years

Globular Cluster M 13

Age of M13: 14 billion years. Mass of stars leaving the main-sequence ~0.8 solar masses


Main Sequence

Sub-giants

Giants


Main Sequence

Sub-giants

Giants

Helium core-burning stars

• The stars in M13 which are now in the giant phase are just slightly more massive than the stars on the main-sequence. (~0.85 times the Sun’s mass)

• The giants are not from high mass O,B or A type stars. Those stars died billions of years ago.

Time to become a red giant

• It takes about 1billion years for a lower mass star that is leaving the main-sequence to reach the tip of the red giant branch. This is a long time by most standards. But it is short compared to the 12 billion years on the main-sequence.

• It is only 8% of the main-sequence lifetime.• Although the expansion is slow for these stars,

they are not in a stable equilibrium. They are expanding.

Let’s consider what is happening.

• When the core of the star runs out of hydrogen, the core can no longer balance the inward force of gravity.

When the fuel runs out, what happens to the core of the star?

1. It shrinks

2. It expands

3. It explodes

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It shrinks.

• Gravity is forcing the core to contract.• This is almost like running star formation in

reverse. The core was also shrinking when the star formed. Now, however, the core starts out small and is getting smaller.

Where does new energy come from when the core begins shrinking

1. There is no source of energy in the core. The hydrogen has run out.

2. The helium is changed into carbon and produces energy

3. Radiant energy is produced from the decreasing potential energy.

Gravitational potential energy

• When the core begins to shrink, the particles in the core gain kinetic energy and then they radiate. This shrinking produces more energy then was being produced from nuclear fusion when it was on the main-sequence.

• The core shrinks, but the added energy being sent out into the overlying layers of the star causes the outside (outer envelope) to expand.

• The star’s radius grows. And the luminosity increases. But the envelope being more spread out allows heat to escape more quickly. The surface temperature actually goes down.


Main Sequence

Sub-giants

Giants


Given these facts, who is winning?

1. The changing radius

2. The changing temperature

3. Nobody wins when a star dies

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The growing radius wins.

• The luminosity is increasing, at least in these 0.8 solar mass stars, and the radius is increasing while the surface temperature is decreasing.

• Since L = σT4(4πR2) the changing radius is winning.

• This is the Sub-Giant phase.


Main Sequence

Sub-giants

Giants


Next, something new happens…

• Remember, the core is out of hydrogen, but the rest of the star has plenty of hydrogen.

• With no reactions in the core it is shrinking.

Here is a way to think about it.

Outside of star

Plenty of hydrogenShrinking core

Where core use to be. And where conditions were right for fusion.

Result…

• Because the core is shrinking there is hydrogen that is introduced into the area around the core where temperatures and pressures are high enough for hydrogen fusion to take place.

• Hydrogen begins to fuse into helium, in a shell around the shrinking helium core.

• Now there are two energy sources in the star.

Two energy sources.

• Gravitational potential energy is being used to make radiant energy in the core.

• The shell around the core is producing energy from the fusion of hydrogen.

• The result of all this energy is that the outer envelope of the star expands enormously. The star becomes a red giant. (luminosity class III)


Main Sequence

Sub-giants

Giants


Helium core burning

• The core contraction and hydrogen shell burning until at last the temperature is high enough in the core to begin helium fusion. This is around 100 million degrees.

• When this happens the star is fusing Helium into Carbon in its core, and still is fusing Hydrogen into Helium is a shell around the core.

The triple alpha process

Interestingly…

• After hydrogen and helium, carbon is the most abundant element in the universe.

• This is because helium makes carbon next in the chain of elements that are generated.

Core and shell burning produces more energy than the star produced on the main sequence so the He-

core burning stars are more luminous than when they were main-sequence stars.

Main Sequence

Sub-giants

Giants


Helium gone in the core

• Helium fusion rate is much faster than the Hydrogen fusion rate was. Within a few hundred million years the supply is gone in the core.

• The core once again shrinks, releasing gravitational potential energy.

• The material in a shell closest to the core begins to fuse helium into carbon, in bursts, as the temperature increases.

• Above this shell, hydrogen is being converted into helium.

Here is a way to think about it.

Outside of star

Plenty of hydrogenShrinking core

Hydrogen shell burning

Helium shell burning

Three energy sources.

• At this point there are three sources of energy in the star, the shrinking carbon core, and two shells.

• The star rapidly expands and heads back up to the giant stage. This is called the asymptotic giant branch, because it asymptotically approaches the red giant branch.

Asymptotic Giant branch phase

• During this phase the helium core burning is not stable. It rapidly turns on and off in bursts. Small explosions.

• The results of these explosions is to eject shocks into the outer envelope of the star. Material in the envelope is lifted off the star, over and over again.

• When the carbon core can no longer contract, everything stops.

• The lost envelope becomes an expanding planetary nebula.

• The exposed carbon core is a white dwarf star.

Planetary nebula & White Dwarf

White Dwarf star

Download - Quiz #6 Most stars form in the spiral arms of galaxies Stars form in clusters, with all types of stars forming. O,B,A,F,G,K,M Spiral arms barely move,

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