What is the eventual fate of the Sun?
- We expect that the Sun will eventually change, because at some point,
all of the hydrogen in the core of Sun will get converted into helium, so
there won't be any left for nuclear reactions. The hydrogen in the very
center runs out first, leaving a helium core with a shell of nuclear reactions
around it. During this stage in the Sun's evolution, the pressure forces
change, and the outer regions of the Sun will expand to be very large.
- When the hydrogen is depleted, there will be nothing keeping the
center of the Sun hotter than the outside, so there will be no pressure
which balances gravity. Consequently, the central regions of the
Sun will begin to contract.
- As the core of the Sun contracts, the central regions will heat
up. Eventually it will get hot enough for another nuclear reaction
to start. This reaction changes three Helium atoms into one Carbon
atom. This nuclear reaction keeps it hotter in the center, so the Sun
stabilizes for a while.
- Eventually, the helium in the core of the Sun is all converted into
carbon. Again, the core of the Sun will contract, because nothing balances
the inward gravitational force.
- As the core of the Sun gets denser and denser, the core material
eventually changes state into a kind of matter called degenerate matter.
Degenerate matter has the property that it provides outward pressure
even when energy is not being generated. This electron pressure
will stop the collapse of our Sun. When this sets in, the core of the
the Sun will be essentially pure carbon. The outer regions of the Sun
will most likely be blown off of the Sun during the later stages of
its evolution and return to interstellar space. We observe this for
other stars that have reached the end of their life (
Planetary nebulae gallery)
- Consequently, at the end of its lifetime, our model predicts that
the Sun will be a dense, hot, carbon core which will gradually cool
off. We actually observe many stars that have the properties we expect
the Sun will eventually have: these are called white dwarfs.
Do all stars go through the same phases of evolution as the Sun?
- Next we want to consider other stars. Are they similar to or
different from our Sun? How do we learn about stars which are so far
away?
- We already learned that we can measure masses of stars which are
in binary systems using our understanding of gravity. From observations
of many different systems, we find that stars come in a wide range of
masses; there are stars both less massive and more massive than our Sun.
- The fact that stars have different masses imply that they may
have a different evolutionary history from that of the Sun. The reason
for this is that different amounts of mass imply different strengths
of gravity in the different stars, and this implies that different stars
will reach different temperatures in their cores when they initially form.
- Stars form when a parcel of gas in the interstellar matter starts
to come together under the force of gravity. The gravitational attraction
acts to accelerate the gas, thus heating it up. More massive protostars
heat up more than less massive ones because they experience more
gravitational acceleration. Consequently, the life history of a star
depends on its mass.
- Since we know that we must have a minimum temperature in order
that nuclear reactions can occur, this implies that there is a minimum
mass for stars. Models of how the physics of gas balls work suggest
that if a protostar has less than about 1/10th the mass of the Sun, that
it will never get hot enough for nuclear reactions to start. Observations
seem to confirm this, in that we do not see stars with masses much less
than this.
- For stars which are more massive than this, up to stars with
masses somewhat larger than that of the Sun, the evolution is
similar to that of the Sun.
During most of their lifetimes, these stars convert hydrogen to helium
and the pressure generated by the heat in their cores balances the
gravitational pull which tries to compress the star. As with the Sun,
the hydrogen eventually runs out, gravity acts to compress the star
until a new nuclear reaction which converts helium to carbon starts
up. Eventually, the helium runs out, the star stars to collapse again,
but the collapse is halted by electron pressure when the carbon core
takes on a degenerate form.
- Different stars take different amount of times to go through
the stages of evolution.
- The most massive stars get the hottest in the cores, so they
are the ones that use up their fuel quickest; even though they
have more fuel (since they are more massive), the rate of consumption
is much faster, so they live shorter.
- The more massive the star, the brighter it is during most
stages of evolution.
- For stars which are much more massive than the Sun, a somewhat
different evolutionary path is followed. These have the same stages
of hydrogen and helium nuclear reactions as the less massive stars.
But after the helium is converted to carbon, these stars collapse
and even the conversion of the carbon into degenerate matter does
not provide enough pressure to stop the gravitational collapse, which
is stronger in these more massive stars. The core continues to
collapse until a new series of nuclear reactions set in which convert
the carbon into successively heavier elements until the entire core
is converted into iron. Iron is a special element from the point of
view of nuclear reactions, because there are no nuclear reactions
involving iron which can produce energy. Consequently, once an iron
core is reached, the star collapses under its gravitational force and no
nuclear reactions can stop it. Several things can happen to these
very massive stars:
- If the star is not extremely massive, the collapsing core
can turn into a different state of matter called neutron degenerate
matter, in which protons and electrons fuse together to create an
incredibly dense star known as a neutron star. Neutron degenerate
matter produces a sort of pressure called neutron pressure which can
balance the gravitational force. We observe actual neutron stars when
we see a sort of object called a pulsar.
- For the most massive stars, even neutron pressure cannot hold
the star up after nuclear reactions have finished running their course.
We know of no other force which can hold up these stars, so we suspect
that these stars may collapse into an infinitely dense point which
has such strong gravity at its surface that even light cannot escape
from it. Such is object is called a black hole, and there are indications
that such objects actually exist in the Universe.
- Many, if not all, massive stars experience a massive explosion
as they near the end of their life which expels a large fraction of
their mass back into the interstellar medium. These explosions are
called supernova explosions. Different stars probably expel different
fractions of their mass back into space.
- The end stages of stellar evolution, and supernovae explosions in
particular, are very important for the existence of
life in the Universe because they are the means by which heavy elements
are distributed into the interstellar matter.
- We think that when the Universe original formed, it contained only
hydrogen and a tiny bit of other light elements. There were no heavier
atoms such as carbon, nitrogen, and oxygen which are so important for
our existence today (we are made of these!).
- The first stars converted some of the light elements into heavier
ones by the process of nuclear reactions. Some of these heavier elements
were distributed back into the interstellar matter by ejection of matter
at the end of the lives of stars (planetary nebulae for lower mass stars
and supernovae explosions for higher mass stars).
These elements mixed with the original light elements, and
a new generation of stars were born which had some heavier elements.
This process has continued up to the current time; stars are successively
formed with more and more heavier elements, which are deposited into
the interstellar matter by supernovae explosions.
- Planetary nebulae gallery
- Stingray
- NGC 7027
- M2-9
- MyCn18
- NGC 6543
- Final stages of massive star evolution:
Eta Car
- Supernovae remnant: Cygnus
- Supernovae remnant: Crab
- Some of the heavier elements form into planets around new stars,
and it is from this material that life evolved. We believe that the
heavy elements which we are made of were originally created inside
of massive stars! In a literal sense, we are all made of stardust.
Where might we find life in the Solar System?
- Presence of liquid water is thought to be critical, at
least for life similar to life on Earth.
- Where can liquid water be found? Requires correct
range of temperature: between freezing (0 C/32 F) and boiling
(100 C/212 F).
What determines the surface temperature of a planet?
- Planets are heated primarily from the energy coming from the Sun
- To a large extent, temperature
is determined by the distance of the planet from the Sun. How does this
work? Sunlight is absorbed by the planet causing it to heat up. As the
planet gets warmer it starts to glow with its own blackbody radiation.
The warmer it gets, the more it glows. The process stops when the amount
of glowing from the planet balances the amount of heating from the Sun.
When the details are worked out, one finds that planets farther from the
Sun should be cooler than those closer to the Sun.
- The simple progression of cooler temperature with larger
distance from the Sun assumes that all planets absorb light
equally, i.e. that all planets reflect the same amount of
light. Since the surfaces/atmospheres of the planets differ,
this actually isn't the case
- Since the Sun shines only on the side of a planet facing
the Sun, it will be hotter on this side than on the back
side. If the planet rotates then the heat can be fairly
well distributed (although we know that it is hotter during
the day than during the night on Earth!). The presence of
an atmosphere can also distribute the heat more evenly
around a planet.
- temperature table
- Even accounting for these effects, the actual observed
temperatures of planets don't match
these simple expectations. The proximity of Venus to the Sun
is nowhere near enough to cause its very high temperature: simple
balance calculations show that Venus should have a temperature only
a bit warmer than Earth, and if one takes account of the fact that
it is covered by clouds, which reflect light well, we might even
expect that it would be a bit cooler than Earth!
- Mercury: expect 163 C, get -175 to 425 on opposite sides
- Venus: expect -40 C, get 470
- Earth: expect -16 C, get 15
- Mars: expect -56 C, get -50
- Why are some of the planets (especially Venus!) much hotter
than expected? Note that the kind of light that the planet emits is not the
same kind of light that is incident on it from the Sun. Since the planet
is cooler than the Sun, it glows predominantly at longer wavelengths,
in the infrared part of the spectrum. So incoming sunlight is predominantly
visible light, but outgoing radiation is predominantly infrared light. The
can give rise to the greenhouse effect
- The greenhouse effect can occur when light (energy) is incident on
a planet with an atmosphere. The atmosphere of some planets transmits
some wavelengths of light, but is opaque to other wavelengths because
of the chemical composition of the atmosphere, clouds, etc. On Venus,
sunlight, which is composed largely of visible light, passes through
the atmosphere and warms the planet. The planet, as a solid, dense
object, shines light back into space, but since it is much cooler than
the Sun, this radiation is predominantly infrared light. However, the
infrared light cannot penetrate the dense clouds of carbon dioxide in
Venus' atmosphere, so the energy cannot escape. As a result, Venus is
heated. It keeps getting hotter until the re-radiation of the Sun's
energy is able to escape - but this doesn't occur until Venus is several
hundred degrees hotter than it would have been without an atmosphere!
- The greenhouse effect also occurs on Earth - Earth is warmer than
it should be based on its distance from the Sun, although it is not
nearly so dramatic as for Venus, because our atmosphere has a composition
that is much more transparent to infrared light.
- There is a serious
concern, however, that the amplitude of the greenhouse effect on Earth
is increasing because human activity (in particular the burning of
fossil fuels, as well as deforestation) is increasing the
amount of carbon dioxide in the Earth's atmosphere by a significant amount.
The increase in carbon dioxide has been
measured);
note there is a general increase from human production as well as
a seasonal variation which shows the importance of biological processes.
Although it known that there are long term climatic variations of
carbon dioxide in the Earth's atmosphere, there is very little doubt
that the recent increases in abundance of greenhouse gasses.
arise from human activity.
- This increase in greenhouse gases has
the potential of raising the Earth's temperature by a few degrees,
which could have some very serious consequences including partial
melting of the polar caps and global climate change. There is already
strong scientific evidence that
global temperatures are increasing.
There is reasonable evidence that human production of
greenhouse gasses is the
cause of this effect.
- There are certainly astronomical factors, such as variation
of the Sun's energy output, and variations in the Earth's orbit, that
can affect the temperature on Earth. Certainly, the Earth's climate
has been significantly different
in the past (probably both warmer
and colder than it currently is). However, these changes all have
occurred much more gradually than the recent changes, so the recent
changes very likely have a different cause, and also, have a different
impact (since change is occurring much faster).
- There are also other natural factors that can contribute
to global temperatures, e.g. volcanic activity can increase the dust
content in the atmosphere leading to some extra reflection of incoming
sunlight. But again, it seems unlikely that these are the cause of
recent observed warming.
- Increased warming can have potential serious consequences (e.g.,
sea level rise, disruption of biological systems, etc.), although understanding
exactly what will happen is challenging, because the weather system on
Earth is very complex.
- Although there is not complete agreement on this, the potential consquences
of global warming are sufficiently serious that it seems prudent to
avoid behavior which is likely to increase the greenhouse effect. We
definitely know that the greenhouse effect is real - all one needs
to do is learn about Venus!
- Many people and governments believe it is smart to work under
the precautionary principle; we should avoid actions that
have a good chance of causing significant harm even if there is not
complete scientific consensus on the issue
- Given that fossil fuel resources are
finite in any case, it seems hard to justify not decreasing our
consumption of them.
- Technology already exists to improve efficiency
of burning of fossil fuels (in particular, in automobiles), and there
are several promising technologies for alternative energy production;
it seems likely to be cost-effective in the long run to invest in these.
- Things you can do about global warming issues
- Talk about the issue with people!
- Communicate to political representatives that you think the
issue is important. Unfortunately, society is not changing voluntarily
enough to make an impact, so some non-voluntary regulation is
required. Vote for political candidates who have the intention of doing
something about the issue, and, ideally, a record of having done
something
- It seems difficult to increase energy consumption and reduce
greenhouse gas emission at the same time, even with regulation
on energy generation. So we need to reduce energy consumption
- When you buy a car, make fuel economy a critical part of
your decision
- Drive less: carpool, walk, bike, consolidate trips
- Request workplaces and stores to settle for a bit warmer
indoor temperatures in summer, and a bit cooler in winter
(reduce heating and cooling energy consumption)
- Reduce energy consumption where possible
- Be aware that if we as a society don't make investments
that deal with the issue now, the cost may be significantly
longer in the long run, i.e. it's not just a question of
costs now, but of costs later
- This discussion has been about average temperatures of a planet. However,
on any given planet, there will be locations that are warmer, and locations that are
colder, depending on the tilt of the axis and the time of year.
- The temperature of a planet can also be affected by internal
heat sources. These come from several sources: heat left over from planetary formation,
radioactive decay, and tectonic motion (which probably exists because of the previous
two heat sources). On the terrestrial planets, internal heat sources are
relatively small compared to heating from the Sun.
Up: AY110 class notes
Previous: PART 4 - THE
Jon Holtzman
2013-12-06
