Potential to Emit Workbook: Part 2: How to Calculate Potential to Emit
The Sun. Discussion Why does the Sun emit light?
-
Upload
judith-goodman -
Category
Documents
-
view
233 -
download
1
Transcript of The Sun. Discussion Why does the Sun emit light?
The setting Sun is red because
a)The Earth is rotating away from the Sun, so it is redshifted. b)The setting Sun is cooler at sunset, so Wien’s law says the frequency of maximum emission shifts to lower frequencies thus appears redder.c)At sunset the light has to travel through more of the Earth’s atmosphere, which has lots of absorption lines in the blue portion of the spectrum. d)None of the above
Solar Data
Radius: 109 Earth radiiMass: 333,000 Earth massesMean density: 1.41 g/cm3
Composition: 74% hydrogen 25% helium
Luminosity: 3.86 1026 Watts
The Sun as a big cosmic light bulb
Suppose every human being on Earth turned on 1000 100-watt light bulbs. With about 6 billion people this would only be 6 1014 watts. We would need 670 billion more Earth’s doing the same thing to equal the energy output of the Sun.
Discussion
Why is there less solar intensity at sea level than there is at the top of Earth’s atmosphere?
Line blanketing
More heavy elements in a star’s atmosphere means more absorption lines, the redder a star will appear as higher frequency light is absorbed and re-emitted at lower frequencies.
Discussion
The Sun releases lots of energy each second, what if it were cooling down over time. How could we tell?
Discussion
Given the composition of the Sun, why is it unlikely that it could be heated by the burning of wood or coal?
Discussion
If the Sun is getting its energy from Kelvin-Helmoltz contraction, how could you prove this? Do you think this is an easy thing to do? Explain.
E = m c2
Matter is a form of frozen energy.
From Einstein’s Special theory of relativity, energy equals the mass times the speed of light squared.
H and He
H – one proton
He – two protons and two neutrons
Neutrons are electrically neutral protons with slightly more mass
Fusion converts the Sun’s Mass into Energy
4 hydrogen atoms have a mass of 6.693 10-27 kg
1 helium atom has a mass of 6.645 10-27 kg
Thus, 0.048 10-27 kg are converted into energy.
Free neutrons are unstable
A neutron, left by itself will decay into a proton, an electron and a neutrino.
Likewise a proton can change into a neutron by emitting a positron and a neutrino.
You can think of a neutron as a proton/electron pair.
Discussion
How can atoms with more than one proton in the nucleus stay together? Why don’t they just fly apart?
Discussion
Fusion keeps the Sun hot, but fusion requires the Sun to be hot. How did the Sun ever get hot enough to start fusion?
Pressure increases toward the center of the Sun
To maintain equilibrium, the pressure below each layer of the Sun must be greater than the pressure above that layer.
Discussion
What would happen if the Sun started to contract? What would happen if the Sun started to expand?
Heat Transport in the Sun
• Conduction – particles transfer energy via collisions
• Convection – energy transferred by movement of material from hotter to cooler regions
• Radiative Diffusion – energy transferred via photons
Discussion
Which would you rather do, put your hand in an oven at 450 degrees F or put you hand on a 450 degree F stove top?
Radiative Diffusion
Radiative zone – inner 71 percent of the Sun’s Interior were all atoms are ionized.
Takes a photon 170,000 years to reach the convective zone. Each time a photon is absorbed it loses energy.
Convection
Convective Zone – outer 29 percent of Sun’s interior. Bottom of convective zone is cool enough for heavy atoms to regain electrons and absorb light.
Discussion
What happens to the bottom layer of the convection zone as it absorbs light from the radiative zone.
Discussion
Will observations of the properties of the photons emitted by the Sun reveal much information about the interior of the Sun? Why or why not?
Surface temperatures
We can determine a star’s surface temperature from its color using Wien’s law, the temperature of a blackbody is proportional to the peak frequency of the blackbody radiation.
Stellar spectra
Spectrograph’s spread out the star light
Getting a star’s spectrum is time consuming
How do we measure a star’s color?
UBVIR photometry
We measure the stars apparent brightness through a number of colored filters.
U – ultraviolet I – infrared B – blue R – red V – visual
Color index
The ratio of the brightness through various filters can be compared to blackbody curves of different temperatures.
Discussion
We’ve discussed two potential problems with measuring a stars temperature using its color. What are they?
Distances to the Stars
The most accurate method of determining stellar distances is geometric parallax.
The parsec
With a baseline of 1 AU, a star that has a parallax of 1 arcsec has a distance of 1 parsec.
Thus distance in parsecs is given by
d = 1/p
where p is the parallax in arcsec.
Discussion
From Earth we can only measure a star’s parallax to about 100 pc. The distance to the center of the galaxy is 8 kpc or 80 times this distance.
Why are parallax measurements so limited?
Discussion
From Earth we can only measure a star’s parallax to about 100 pc. The distance to the center of the galaxy is 8 kpc or 80 times this distance.
Why are parallax measurements so limited? What could you do to get parallax measurements for more distant stars?
Hipparcos
Measured the parallax of the brighter stars (118,000 of them) from Earth orbit out to about 1000 pc or 1 kpc.
Discussion
How do you think astronomers determine the masses of stars?
Hint: Most stars are binaries.
Kepler’s 3rd law
The square of the sidereal period is proportional to the cube of the semimajor axis of the orbit: p2 a3
Newton’s laws require:
3
21
22
)(
4a
MMGp
Discussion
After observing this binary system for an entire orbit, what else do we need to know to determine the star’s mass?
Discussion
The strength of the hydrogen absorption lines does not correlate well with the amount of hydrogen present. Almost all stars have about 75% hydrogen, 24% helium and 1% metals.
Why do stars have absorption lines in their spectra?
Discussion
The hydrogen Balmer lines are produced by electrons absorbing a photon and jumping from the 2nd energy level to a higher energy level. How can this not take place with hydrogen still present the stellar atmosphere?
OBAFGKM
Oh Be A Fine Girl (Guy) Kiss Me. Oh Brother, Astronomers Frequently Give Killer Midterms.
O stars are the hottest M stars are the coolest
The Sun is a G2 star.
Discussion
I have two stars
HD 1378326 and HD 1177892
Both are B8 stars. What do they have in common?
Another inverse square law
The apparent brightness of a star falls as the square of its distance from you.
Luminosity
Luminosity is an object’s intrinsic brightness, the total amount of energy given off in a second.
This is different from an object’s apparent brightness
Discussion
What are some things that change the apparent brightness of a star, how bright that star appears in the sky?
Absolute magnitude
Luminosity is the same as apparent brightness, without considering the distance.
Absolute magnitude is magnitude a star would have if it were 10 parsecs away.
Apparent and absolute magnitudes
Apparent magnitude measures brightness in the sky.
Absolute magnitude measures luminosity.
Discussion
I have two light bulbs, one is 25 watts and the other is 100 watts. If the 25-watt light bulb is 1 mile away, how far would I need to place the 100-watt light bulb to have the same apparent brightness?
Luminosity and Distance
If we know the luminosity of a star or galaxy, we can figure out how far away it is by comparing it to its apparent brightness.