exam 3

The Sun is

a star

We know more about the Sun than any other star, since it is by far the \__________ the Earth.

closest star to

How far is the Sun from Earth?

roughly one hundred and fifty million kilometers

The nearest stars besides the Sun are more than \______________ further from the Earth as compared with the Sun.

two hundred thousand times

T/F The Sun is indeed extremely close to the Earth by astronomical standards.

true

What symbol do astronomers use when talking about the radius of the sun?

the symbol R☉

What is another name for the symbol Sun R☉

solar radius

The Sun is enormous. Therefore, one solar radius R☉ is roughly equal to

seven hundred thousand kilometers

The solar radius is roughly how many times the Earth's radius?

one hundred

One solar radius R☉ is roughly equal to

100R⊕

What is the volume of the sun as compared to the Earth?

the Sun's volume is roughly one million times the Earth's volume

Why is the sun's volume one million times the earth's volume?

The volume of a sphere is directly proportional to the cube of its radius and one hundred cubed is one million

What symbol do astronomers use to denote the mass of the Sun?

the symbol M☉

What is another name for M☉(the mass of the sun)?

a solar mass

One solar mass M☉ is roughly one thousand times the mass of \_________

Jupiter

One solar mass M☉ is also roughly one thousand times the mass of \________________

the rest of the Solar System combined

One solar mass M☉ is roughly \____________ kilograms.

two nonillion

Which law was used to determine the mass of the Sun?

Kepler's third law

What is luminosity?

the total amount of energy the object radiates every second, commonly known as the power output

How do we calculate luminosity?

combine the distance to the Sun with the intensity of sunlight we receive from the Sun to calculate the luminosity of the Sun

The luminosity or the power output of any object is measured in what units?

watts

What is teh symbol for luminosity?

ℒ(cursive script)

For any object with luminosity ℒ, the intensity of the light I at a distance r from the object is given by the equation

I \= ℒ / 4πr2 This equation is true because the object radiates energy isotropically

The total energy radiated by the object cuts through a sphere centered on the object, and the surface area of a sphere of radius r is

4πr2. This equation also reveals why a lightbulb for example looks brighter when closer and dimmer when further.

Doesn't the lightbulb radiate a constant luminosity (constant power output) regardless of distance?

Indeed it does, but that same luminosity has spread out over a large sphere if we are far from the lightbulb.

When that constant luminosity is diluted over the large sphere, and a \______ fraction of that luminosity enters our eye. Smaller/Larger

smaller

Conversely, if that same luminosity is concentrated over a small sphere if we are close to the lightbulb, and thus a \_______ fraction of that luminosity enters our eye.

larger

We know our distance from the Sun, and we know the intensity of sunlight at our distance from the Sun. Thus, the only unknown remaining in the equation I \= ℒ / 4πr2 is

the luminosity of the Sun

What symbol do Astrophysicists use to denote the luminosity of the Sun?

the symbol ℒ☉

In fact, the luminosity of the Sun ℒ☉ is such a fundamental unit in stellar astrophysics that it is called

a solar luminosity

T/F The luminosity of the Sun is enormous

TRUE

one solar luminosity ℒ☉ is roughly how many watts?

four hundred septillion watts

The Sun has been radiating roughly four hundred septillion watts every second for roughly five billion years, and it will continue to do so every second for the next roughly

five billion years!

The surface temperature of the Sun is roughly how many kelvins?

six thousand kelvins

Astrophysicists have determined the Sun's surface temperature using how many different methods?

two

Firstly, we can graph the amount of light from the Sun as a function of the wavelength of the light. The resulting graph is

a continuous blackbody spectrum.

From the peak of this continuous blackbody spectrum, what can we calculate?

the surface temperature of the Sun

Essentially, we are calculating the surface temperature of the Sun from its

color

The amount of energy radiated from a hot, dense object often follows\______________ , which is a continuous spectrum with its peak radiation within a band of the Electromagnetic Spectrum determined by the temperature of the object.

the blackbody spectrum

In particular, hotter temperatures correspond to \____________ (which are also at higher frequencies and shorter wavelengths)

higher photon energies

while cooler temperatures correspond to \_____________ (which are also at lower frequencies and longer wavelengths).

lower photon energies

a hot, dense object's primary radiation is displaced as its \________________ changes.

temperature

The Wien displacement law

states that the wavelength of a hot, dense object's primary radiation is inversely proportional to its temperature, assuming we measure temperature with correct units such as kelvins or rankines.

At one or two thousand kelvins, objects radiate primarily \______ visible light.

red

At three or four thousand kelvins, objects radiate primarily \________ visible light.

orange

At five or six thousand kelvins, objects radiate primarily \______ visible light.

yellow

At roughly ten thousand kelvins, objects radiate primarily \_______ visible light.

blue

Hotter temperatures displace the primary radiation to higher and higher photon energies (which are also higher and higher frequencies and shorter and shorter wavelengths), while cooler temperatures displace the primary radiation to \________________ (which are also lower and lower frequencies and longer and longer wavelengths).

lower and lower photon energies

It is commonly known that the Sun is a \______ star.

yellow

The Wien displacement law calculates that the surface temperature of the Sun is roughly

six thousand kelvins

We can also calculate the surface temperature of any hot, dense object using the

Stefan-Boltzmann law

Stefan-Boltzmann law

states that the luminosity of any hot, dense object is directly proportional to both its surface area and the fourth power of its surface temperature.

Since the shape of the Sun is very nearly a sphere and the surface area of a sphere of radius R is 4πR2, the Stefan-Boltzmann law for the Sun states

ℒ \= σ(4πR2)T 4, where T is the surface temperature

σ (the lowercase Greek letter sigma)

is a fixed number called the Stefan-Boltzmann constant

We use lowercase r for

the distance from the hot object

We use uppercase (capital) R for

the actual radius of the hot object

In particular for the Sun, r is roughly

one hundred and fifty million kilometers (our distance from the Sun)

In particular for the Sun, R is roughly

seven hundred thousand kilometers (the actual radius of the Sun)

We already determined the luminosity of the Sun, and we certainly know the radius of the Sun. Therefore, the only unknown remaining in the Stefan-Boltzmann law ℒ \= σ(4πR2)T 4 is

the surface temperature of the Sun, which we again calculate to be roughly six thousand kelvins, consistent with the Wien-displacement method.

From the absorption spectral lines superimposed upon the Sun's continuous blackbody spectrum, we can determine

the composition of the Sun

We discover that the Sun is composed of

all the atoms on the Periodic Table of Elements, but not in equal amounts

Only two atoms account for close to \_________ percent of the Sun's mass; all the other atoms on the Periodic Table of Elements account for only a tiny fraction (tiny percentage) of the Sun's mass.

one hundred

Roughly seventy

five percent (three-quarters) of the Sun's mass is - hydrogen

Roughly twenty

five percent (one-quarter) of the Sun's mass is - helium.

T/F all the other atoms on the Periodic Table of Elements make up a tiny fraction (tiny percentage) of the Sun's mass.

true

The Sun radiates roughly how many watts every second?

four hundred septillion watts

Chemical reactions provide nowhere nearly enough energy to account for the Sun's luminosity over

its long lifetime

It is not difficult to calculate that the Sun would consume all of its \_____ in only several thousand years if it derived its luminosity from chemical reactions, but the Sun has been shining for roughly five billion years.

mass

Is Gravitational contraction the Sun's source of energy?

no

Gravitational contraction does convert gravitational energy into heat and light, it is not difficult to calculate that the Sun would need to \________ in several million years to account for its incredible luminosity.

collapse

Gravitational contraction

is also called Kelvin-Helmholtz contraction, named for the British physicist William Thomson Lord Kelvin and the German physicist Hermann von Helmholtz, the two physicists who developed the mathematical details of gravitational contraction

T/F the Sun was born as a collapsing cloud of gas from within a diffuse nebula.

true

The sun did derive its energy from

Kelvin-Helmholtz's (gravitational) contraction.

However, the Sun eventually attained gravitational equilibrium, meaning

outward pressure balances inward self-gravity

The Sun has been in gravitational equilibrium for roughly five billion years, and so Kelvin-Helmholtz (gravitational) contraction does not explain what?

why the Sun has been shining for most of its lifetime

Why does the Sun shine?

At the beginning of the 1900s (the twentieth century), the atomic theory of matter became firmly established.

Physicists discovered that atoms are composed of even smaller particles:

the nucleus at the center of the atom and electrons around the nucleus

Physicists discovered that chemical reactions involve

the electrons around the nucleus

The physicists also discovered nuclear reactions, which involve

the nuclei themselves

These nuclear reactions can generate thousands, even millions, of times more energy than

chemical reactions

Perhaps the Sun derives its energy from

nuclear reactions

There are four fundamental forces in the universe

the strong nuclear force, the electromagnetic force, the weak nuclear force, and finally the gravitational force is the weakest force

Actually, the gravitational force is by far the \_______ force in the entire universe.

weakest

The gravitational force is much weaker than

the other three forces

Gravity

causes everything in the universe to attract everything else in the universe

T/F there are both positive and negative electrical charges in our universe.

true

Do Positive and positive repel or attract?

repel

Do negative and negative repel or attract?

repel

Do positive and negative repel or attract?

attract

T/F charges repel, and unlike charges attract.

true

Protons are \_________ charged

positively

Electrons are \_________charged

negatively

Since unlike charges attract, the positive protons within the atomic nucleus attract

the negative electrons around the atomic nucleus

What holds the atom together?

the attraction between the positive protons within the nucleus and the negative electrons around the nucleus

What holds the nucleus of an atom together?

The atomic nucleus is composed of protons and neutrons. Since the neutrons are neutral, they are not attracted to or repelled from anything electromagnetically. More importantly, the protons are positive. Hence, they repel each other electromagnetically.

What holds the atomic nucleus together if the neutrons feel no electromagnetic attraction and all the protons feel electromagnetic repulsion from each other?

there must be another force in the nucleus that is stronger than the electromagnetic force so that it can overpower the electromagnetic repulsion of the protons, thus holding the nucleus together.

This force is the strongest force in the entire universe, and it is called

the strong nuclear force

The strong nuclear force must be stronger than the electromagnetic force, since the strong nuclear force must overpower the electromagnetic repulsion among the protons to

hold the atomic nucleus together

The strong nuclear force attracts protons and

protons together

The strong nuclear force attracts neutrons and

neutrons together