Nov. 27th - Our Sun & Intro to Atmosphere Flashcards
Why does the Sun shine? - Ancient ideas:
Ancient thinkers often imagined the Sun to be some type of fire
A fair guess, because people did not know the size or distance of the Sun and therefore did not realize how incredible its energy output really is. Nor did they know how long Earth had existed
Why does the Sun shine? - 19th century
After the Sun’s distance and size had been measured with reasonable accuracy.
Scientists could then calculate the true energy output of the Sun, and this quickly ruled out coal, wood, or any other type of chemical burning.
There is simply no way that chemical processes can account for the Sun’s huge energy output.
Gravitational contraction:
Late 19th century, astronomers came up with an idea suggesting that the sun generates energy by slowly contracting in size, a process called gravitational contraction
Recall that a shrinking gas cloud heats up because the gravitational potential energy of gas particles far from the center of the cloud is converted into thermal energy as the gas moves inward
A gradually shrinking Sun would always have some gas moving inward, converting gravitational potential energy into thermal energy. This thermal energy would keep the inside of the Sun hot
Because of its large mass, the Sun would need to contract only very slightly each year to maintain its temperature—so slightly that the contraction would have been unnoticeable
Why was gravitational contraction disapproved?
Because of its large mass, the Sun would need to contract only very slightly each year to maintain its temperature—so slightly that the contraction would have been unnoticeable
Calculations showed that gravitational contraction could have kept the Sun shining steadily for up to about 25 million years
However, geologists pointed out a fatal flaw. Studies of rocks and fossils had already suggested that Earth was far older than 25 million years, which meant that gravitational contraction could not account for the Sun’s long-term energy generation
Why does the Sun shine? - Einstein’s breakthrough:
No known way that a large object like the Sun could generate so much energy for billions of years
Einstein’s special theory of relativity in 1905: included his famous equation, E=mc2, which tells us that mass itself contains an enormous amount of potential energy
Calculations showed that the Sun’s mass contained more than enough energy to account for billions of years of sunshine, if the Sun could somehow convert some of its mass into thermal energy.
Nuclear fusion requires extremely high temperatures and densities. In the Sun, these conditions are found deep in the core. **But how did the Sun become hot enough for fusion to begin in the first place?
**
Embedded in the mechanism of gravitational contraction
Sun was born from a collapsing cloud of interstellar gas (solar nebula)
The contraction of the cloud released gravitational potential energy, raising the interior temperature and pressure. This process continued until the core finally became hot enough to sustain nuclear fusion, because only then did the Sun produce enough energy to give it the stability that it has today.
What “balances” does the sun achieve to keep its size and energy output stable?
- Gravitational equilibrium (or hydrostatic equilibrium)
- Energy balance (between the rate at which fusion releases energy in the Sun’s core and the rate at which the Sun’s surface radiates this energy into space)
Gravitational Equilibrium
Gravitational equilibrium (or hydrostatic equilibrium), is between the **outward push of internal gas pressure and the inward pull of gravity.
**
EX: A stack of acrobats.
* The bottom person supports the weight of everyone above, so he must push upward with enough pressure to support all this weight.
* At each higher level, the overlying weight is less, so it’s a little easier for each additional person to hold up the rest of the stack.
Because the weight of overlying layers is greater as we look deeper into the Sun, the pressure must increase with depth.
* Deep in the Sun’s core, its pressure makes the gas hot and dense enough to sustain nuclear fusion.
* The energy released by fusion => heats the internal gas and maintains the pressure that keeps the Sun in balance against the inward pull of gravity.
Energy Balance (between the rate at which fusion releases energy in the Sun’s core and the rate at which the Sun’s surface radiates this energy into space)
Energy balance is important because without it, the balance between pressure and gravity would not remain steady.
If fusion in the core did not replace the energy radiated from the surface, thereby keeping the total thermal energy content constant, then gravitational contraction would cause the Sun to shrink and force its core temperature to rise.
OVERALL ANSWER: “Why does the Sun shine?
About 412 billion years ago gravitational contraction made the Sun hot enough to sustain nuclear fusion in its core.
Ever since, energy liberated by fusion has maintained gravitational equilibrium and energy balance within the Sun, keeping it shining steadily
What is the sun’s structure?
The Sun is essentially a giant ball of hot gas or, more technically, plasma—a gas in which atoms are ionized because of the high temperature
The differing temperatures and densities of the plasma at different depths give the Sun the layered structure
Determining properties of the sun
Spectroscopy tells you that…
…the Sun is made almost entirely of hydrogen and helium.
Determining properties of the sun
From the Sun’s angular size and distance, you can determine that…
…its radius is just under 700,000 kilometers, or more than 100 times the radius of Earth
Determining properties of the sun
Sunspots
visible splotches that appear darker than the surrounding surface, can be larger in size than Earth
Determining properties of the sun
You can measure the Sun’s mass using…
…Newton’s version of Kepler’s third law
Determining properties of the sun
You can observe the Sun’s rotation rate by…
…tracking the motion of sunspots or by measuring Doppler shifts on opposite sides of the Sun
Determining properties of the sun
Unlike a spinning ball, the entire Sun does not rotate at the same rate:
The solar equator completes one rotation in about 25 days, and the rotation period increases with latitude to about 30 days near the solar poles.
We define power as…
the rate at which energy is used or released
The standard unit of power is…
the watt, defined as 1 joule of energy per second; that is, 1 watt=1 joule/s
The Sun’s total power output…
AKA luminosity - an incredible 3.8×1026 watts
In what form does sunlight radiate towards earth?
Only a tiny fraction of the Sun’s total energy output reaches Earth, since it goes in all directions into space.
Most of this energy is radiated in the form of visible and infrared light, but after you’ve left the protection of Earth’s atmosphere, you encounter significant amounts of more dangerous types of solar radiation, including ultraviolet and x-rays.
The sun’s atmosphere:
Solar Wind
The stream of charged particles continually blown outward in all directions from the Sun.
The solar wind helps shape the magnetospheres of planets and blows back the material that forms the plasma tails of comets
Can be felt even from great distances at the Sun
The sun’s atmosphere:
As you approach the Sun more closely, you begin to encounter the low-density gas that represents what we usually think of as…
The sun’s atmosphere
The sun’s atmosphere:
The Corona
Outermost layer of the sun’s atmosphere
Extends several million kilometers above the visible surface of the Sun. The temperature of the corona is astonishingly high
The corona’s density is so low that your spaceship absorbs relatively little heat despite the million-degree temperature
The sun’s atmosphere:
The Chromosphere
The middle layer of the solar atmosphere and the region that radiates most of the Sun’s ultraviolet light
The temperature suddenly drops to about 10,000 K in the chromosphere
The sun’s atmosphere:
The Photosphere
Lowest layer of the solar atmosphere
Which is the visible surface of the Sun. Although the photosphere looks like a well-defined surface from Earth, it consists of gas far less dense than Earth’s atmosphere.
The photosphere is also where you’ll find sunspots, regions of intense magnetic fields that would cause your compass needle to swing about wildly
The sun’s interior:
The Convection Zone
Right under the photosphere
The photosphere above you is the top of the convection zone, and convection is the cause of the Sun’s seething, churning appearance.
Where energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection.
The sun’s interior:
The Radiation Zone
About a third of the way down to the center, the turbulence of the convection zone gives way to the calmer plasma
Where energy moves outward primarily in the form of photons of light
The sun’s interior:
Sun’s Centre - The source of energy
Nuclear fusion transforming hydrogen to make helium.
At the Sun’s center, the temperature is about 15 million K, the density is more than 100 times that of water, and the pressure is 200 billion times that on Earth’s surface. The energy produced in the core today will take a few hundred thousand years to reach the surface.
The Solar Thermostat - WHY?
Nuclear fusion is the source of all the energy the Sun releases into space.
If the fusion rate varied, so would the Sun’s energy output, and large variations in the Sun’s luminosity (total power output) would almost surely be lethal to life on Earth.
Fortunately, the Sun fuses hydrogen at a steady rate, thanks to a natural feedback process that acts as a thermostat for the Sun’s interior.
The Solar Thermostat - HOW: rise in temperature?
Suppose the Sun’s core temperature rose very slightly.
- The rate of nuclear fusion is extremely sensitive to temperature, so a slight temperature increase would cause the fusion rate to soar as protons in the core collided more frequently and with more energy.
- Because energy moves slowly through the Sun’s interior, this extra energy would be bottled up in the core, temporarily forcing the Sun out of energy balance and raising the core pressure. The push of this pressure would temporarily exceed the pull of gravity, causing the core to expand and cool.
- This cooling, in turn, would cause the fusion rate to drop back down until the core returned to its original size and temperature, restoring both gravitational equilibrium and energy balance.
The Solar Thermostat - HOW: decrease in temperature?
A slight drop in the Sun’s core temperature would trigger an opposite chain of events.
- The reduced core temperature would lead to a decrease in the rate of nuclear fusion
- Causing a drop in pressure and contraction of the core
- As the core shrank, its temperature would rise until the fusion rate returned to normal and restored the core to its original size and temperature.
General cycle of the Solar Thermostat
- Rise/fall in Sun’s core temperature
- Increase/decrease in fusion rate
- Raise/Lowering of core pressure
- Expansion/Shrinking of core pressure
The solar thermostat balances the Sun’s fusion rate so that…
…the amount of nuclear energy generated in the core equals the amount of energy radiated from the surface as sunlight.
Most of the energy released by fusion starts its journey out of the solar core in the form of…
Photons
Photon movement from the solar core through the radiation zone
Random Walk, Radiative Diffusion
Although photons travel at the speed of light, the path they take through the Sun’s interior zigzags so much that it takes them a very long time to make any outward progress.
The technical term for this slow outward migration of photons is radiative diffusion; to diffuse means to “spread out” and radiative refers to the photons of light, or radiation.
Deep in the solar interior, the plasma is so dense that a photon can travel only a fraction of a millimeter in any one direction before it interacts with an electron.
Each time a photon “collides” with an electron, the photon gets deflected into a new random direction (ping pong)
The photon therefore bounces around the dense interior in a haphazard way (sometimes called a random walk) and only very gradually works its way outward from the Sun’s center
At the top of the radiation zone, where the temperature has dropped to about 2 million K, **how does the solar plasma absorb photons? What does this lead to?
- More readily, rather than just bouncing them around.
- This absorption creates the conditions needed for convection, so above this level we find the Sun’s convection zone.
Photon movement from the convection zone:
Recall that convection occurs because hot gas is less dense than cool gas: like a hot-air balloon, a hot bubble of solar plasma rises upward through the cooler plasma above it. Meanwhile, cooler plasma from above slides around the rising bubble and sinks to lower layers, where it is heated.
The rising of hot plasma and sinking of cool plasma form a cycle that transports energy outward from the base of the convection zone to the photosphere
Atmospheric Basics
The Moon and Mercury
Have so little atmosphere that it’s reasonable to call them “airless”; they have no wind or weather of any kind.
Atmospheric Basics
Venus
is enshrouded by a thick atmosphere composed almost entirely of carbon dioxide, giving it surface conditions so hot and harsh that not even robotic space probes have survived there for long.
Atmospheric Basics
Mars
also has a carbon dioxide atmosphere, but its air is so thin that you’d die within minutes if you ventured outside without a pressurized space suit.
Atmospheric Basics
Earth
Has the only “just right” conditions that allow liquid water on the surface, making it hospitable to life.
What is an atmosphere?
Atmosphere
a layer of gas that surrounds a world. In most cases, it is a surprisingly thin layer (like a dollar bill)
What is an atmosphere?
Atmospheric air is a mixture of
Gases that may consist either of individual atoms or of molecules.
Temperatures in the terrestrial atmospheres are generally low enough (even on Venus) for atoms to combine into molecules.
* For example, the air we breathe consists of molecular nitrogen (N2) and oxygen (O2), as opposed to individual atoms (N or O)
Atmospheric pressure:
Collisions of individual atoms or molecules in an atmosphere create pressure that pushes in all directions
On average, each molecule in the air around you will suffer a million collisions in seconds. These collisions create pressure that pushes in all directions, and this pressure holds up the atmosphere so that it does not collapse under its own weight.
EX: The air molecules inside a balloon exert pressure, pushing outward as they continually collide with the balloon’s inside surface. At the same time, outside air molecules collide with the balloon’s outer surface, exerting pressure that by itself would make the balloon collapse. A balloon stays inflated when the inward and outward pressures are balanced
We can understand atmospheric pressure by applying similar principles. Gas in an atmosphere is held down by gravity. The atmosphere above any given altitude therefore has some weight that presses downward, tending to compress the atmosphere beneath it. At the same time, the fast-moving molecules exert pressure in all directions, including upward, which tends to make the atmosphere expand.
Atmospheric Pressure - OVERVIEW:
Planetary atmospheres exist in a perpetual balance between the downward weight of their gases and the upward push of their gas pressure.
The higher you go in an atmosphere, the less the weight of the gas above you, and less weight means less pressure. That is why the pressure decreases as you climb a mountain or ascend in an airplane.
Atmospheric pressure measured in
BARS (as in barometer)
- One bar is roughly equal to Earth’s atmospheric pressure at sea level
You don’t notice the atmospheric pressure for two reasons:
- First, the pressure pushes in all directions, so it pushes upward and inward on you as well as downward.
- Second, the fluids in your body push outward with an equivalent pressure, so there is no net pressure trying to compress or expand your body. You can tell that the pressure is there, however, because you’ll quickly notice any pressure changes. For example, even slight changes in pressure as you go up or down in altitude can cause your ears to “pop.”
The “Top” Of an Atmosphere
There is no clear boundary between the atmosphere and space above, because pressure and density decrease gradually with increasing altitude.
At some point, the density becomes so low that we can’t really think of the gas as “air” anymore. Collisions between atoms or molecules are rare at these altitudes, and the gas is so thin that it would look and feel as if you had already entered space.
This altitude is often described as “the edge of space.”
MATH INSIGHT
