Download NGC 3370 Spiral Galaxy

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 lowermass stars.
Open cluster M67
An H-R diagram for M67
Main
Sequence
An H-R diagram for M67
Giants
Main
Sequence
Subgiants
Compare M67 to the Pleiades
Age = 100 million years
Age = 4 billion years
Globular Cluster M 13
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Giants
Subgiants
Main
Sequence
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
• 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 mainsequence.
• 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
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.
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
Given these facts, who is
winning?
1. The changing
radius
2. The changing
temperature
3. Nobody wins when
a star dies
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.
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
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 hydrogen
Shrinking 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)
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
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 Hecore burning stars are more luminous than when
they were main-sequence stars.
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
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
Helium shell
burning
Plenty of hydrogen
Shrinking core
Hydrogen
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