5. The Sun

Question 1

The Sun’s characteristics (mass, radius, luminosity, rotation period)

Answer 1

Mass 1.98910^30kg
Radius 696,340km
Luminosity 1 L☉ (3.84610^26 watts)
Rotational period 24.47 days at equator, 38 days at poles

Question 2

The Sun is made of…

Answer 2

The same elements we find on the Earth but in different proportions: the Sun is mostly hydrogen and helium.

Question 3

How do we determine the Sun’s composition?

Answer 3

we can use a star’s absorption line spectrum to determine what elements are present

Question 4

What is the Sun’s structure?

Answer 4

Core

Radiative Zone

Convection Zone

Chromosphere

Corona

Question 5

The Sun’s core

Answer 5

Innermost 20–25% of the Sun’s radius

Temperature and pressure allow nuclear fusion

Hydrogen fuses into helium

Fusion process releases energy, and the core gradually becomes enriched in helium.

Question 6

The Radiative Zone

Answer 6

Between about 20–25% to 70% of the radius

Energy transfer occurs by means of radiation (photons) rather than by convection.

Question 7

The Convection Zone

Answer 7

Between about 70% of the Sun’s radius and a point close to the visible surface.

Sun is cool and diffuse enough for convection to occur

Primary means of outward heat transfer

Question 8

The Chromosphere

Answer 8

Thin layer of the Sun

Sculpted by magnetic field lines that restrain electrically charged solar plasma.

Occasionally prominences form and extend far into corona

Sometimes ejecting material away from the Sun.

Question 9

The Corona

Answer 9

Outermost part of suns atmosphere

The ionized elements within corona glow in x-ray and ultraviolet wavelengths

NASA instruments can image the Sun’s corona at these higher energies since the photosphere is quite dim in these wavelength.

Coronal Streamers - The outward flowing plasma of the corona is shaped by magnetic field lines into tapered forms called coronal streamers, which extend millions of miles into space.

Question 10

What are the different layers in the Sun’s atmosphere?

Answer 10

Photosphere - the deepest part of the Sun which we can directly observe with visible light.

Chromosphere

Transition Region - Very narrow (100 km) layer between the chromosphere and the corona where the temperature rises abruptly from about 8000 to about 500,000 K

Corona

Question 11

What are sunspots and where do they form?

Answer 11

Sunspots are areas that appear dark on the surface of
the Sun. They appear dark because they are cooler than
other parts of the Sun’s surface.

Question 12

What is the solar wind?

Answer 12

The solar wind is a stream of charged particles released from the upper
atmosphere of the Sun, called the corona. This plasma mostly consists of
electrons, protons and alpha particles

Question 13

What causes solar activity?

Answer 13

Electrically charged gases on surface that generate areas of powerful magnetic forces (magnetic fields).

The gases are constantly moving, which tangles, stretches and twists the magnetic fields. This motion creates a lot of activity on the Sun’s
surface, called solar activity

Question 14

What are coronal mass ejections and how are they connected to solar
flares?

Answer 14

CMEs are giant clouds of particles from the Sun hurled out into space, while flares are flashes of light—occurring in various wavelengths—on the Sun

Question 15

How does solar activity affect humans?

Answer 15

Solar flares and CMEs send enormous amounts of energy and charged particles hurtling into collision with the Earth’s upper atmosphere, where they can cause geomagnetic storms. Charged particles during geomagnetic storms cause disturbances in the Earth’s magnetic field, generating effects on electrical systems

Question 16

How does solar activity vary with time?

Answer 16

Every 11 years or so, the Sun’s magnetic field completely flips. This means that the Sun’s north and south poles switch places. Then it takes about another 11 years for the Sun’s north and south poles to flip back again.

Question 17

How does the Sun work?

Answer 17

The surface of the Sun is a dense layer of plasma that is continually in motion through the action of convective motions driven by heating from below. This constant motion of high-temperature causes a nuclear reaction. In the core of the Sun, hydrogen turns into helium and causes a fusion – which moves to the surface of the Sun, escaping into space as electromagnetic radiation, a blinding light, and incredible levels of solar heat

Question 18

What keeps the Sun shining?

Answer 18

The Sun shines by turning hydrogen into helium in its core. This process is called nuclear fusion. Fusion happens when lighter elements are forced together to become heavier elements. When this happens, a tremendous amount of energy is created.

Question 19

What possible sources of energy are there?

Answer 19

Gravitational contraction and fusion

Question 20

How long could gravitational contraction keep the Sun shining at its
current luminosity?

Answer 20

Since the solar luminosity is 4 × 10^26 watts (joules/second) or about 10^34 joules per year, contraction could keep the Sun shining at its present rate for roughly 100 million years.

Question 21

How does nuclear fusion occur in the Sun?

Answer 21

The Sun gets its energy when hydrogen nuclei are fused together to form helium nuclei within the solar core. This hydrogen burning is described by a sequence of nuclear fusion reactions called the proton-proton chain

Question 22

Why are neutrinos important for understanding what is happening inside
the Sun?

Answer 22

The Gamma rays produced by fusion takes thousands of years to reach the surface of Sun. But neutrinos travel at the speed of light and reach us. So the light is telling us about the fusion which took place millions of years back. But the neutrinos tells us the current status.

Question 23

What causes gas pressure?

Answer 23

The pressure a gas exerts comes from the motion of its molecules. When the molecules of a gas bounce off the walls of their container, they exert a force. Gas pressure is defined as the force per unit area produced by the gas.

Question 24

What is hydrostatic equilibrium?

Answer 24

Balance between thermal pressure and gravity

Question 25

Heat transfer in stars

Answer 25

radiation and convection zone.

Question 26

How we know what is happening inside the Sun?

Answer 26

Theoretical models

Question 27

How do we know that the theoretical models are accurate?

Answer 27

solar oscillations, detecting neutrinos

Question 28

How do we know the temperature of the Sun’s photosphere?

Answer 28

The Sun’s color provides a rough measure of where the Sun’s thermal radiation spectrum peaks, and this peak depends on temperature. Studying spectral lines gives us an even better indication of the Sun’s temperature, because different lines form more easily at different temperatures.

Question 29

What two forces are balanced in what we call gravitational equilibrium in the Sun?

Answer 29

Outward pressure from thermonuclear reactions and gravity from mass of the stellar core.