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Discovering the Sun’s center, how big the Sun, silver among the gold

Q: My wife just asked how the guys that watch the skies know what is at the center of the sun. (George)

A composite image of the Sun, showing the solar wind fanning out, the Sun’s surface (the corona), the plasma rivers (blue, aqua) flowing under its surface, and its hot core (yellow). [SOHO/NASA]

A: Those watching guys theorize what the Sun’s center is made of, based on things about the Sun that they measure: for example, its mass, surface temperature, and the total amount of energy that shines out from its surface (its luminosity).

Astronomers build a mathematical model of the Sun from the theories and make predictions about the Sun’s behavior. Then they test the predictions to see if things really happen that way. If they don’t, then the astronomers tweak the model to make it fit the data better. Gradually, in this fashion, they determine the Sun’s structure.

The Sun’s core is an inner "ball" that extends from the center to about a quarter of the way to the surface and contains 60% of the Sun’s mass. It’s made of mostly hydrogen, some helium, and a tiny bit of heavier atoms like carbon, nitrogen, and oxygen. The core (and, indeed, the entire Sun) is neither a gas nor a liquid. It’s an electrically conducting medium called a plasma.

The plasma core is hot — 27 million degrees F (15 million C). A grain of sand this hot would cook a person 90 miles (145 km) away. The core is also dense, even denser than the next outermost shell where the Sun radiates its energy. Even this outer shell is so dense that it takes 170 thousand years for each energy particle to zigzag its way from the core out to space.

Further Surfing:

The Royal Observatory Greenwich: The Sun

How big the Sun

Q: What is the volume of the sun? (Aaron, Auckland, New Zealand)

A: The Sun’s volume is 338.8 thousand trillion cubic miles (1412 thousand trillion cubic kilometers) — big enough to contain over one million Earths.

This gigantic shining orb — an average yellow star, containing 99.9% of the solar system’s matter — gives us all our light and energy. If Earth could capture all of the Sun’s horrendous output for a year, it would be enough to meet the energy demands of 31,000 billion planet earths for that year.

Every second, the Sun generates this energy in a highly dense, extremely hot fusion-furnace core. In the furnace, the Sun changes 508 million tons of hydrogen into 504 million tons of helium and converts the "left over" mass (4 million tons) into energy, according to Einstein’s equation, E = mc². It has done this every second for the past 4.5 billion years of its life.

The Sun will run out of hydrogen fuel in another 5 billion years. It will then generate less energy and therefore push outward less. Gravity will dominate, pulling the Sun inwards. The contracting core will heat up, find more fusion fuel, and generate more energy that will greatly expand the outer part. The outer shell will then brighten (because its surface is bigger) and cool (an expanding gas or plasma cools). The Sun will become a red giant, still hot enough to burn planets to cinders as it engulfs Mercury and Venus and scorches Earth.

Finally, a couple of billion years later, it will run out of all fuel and collapse into a tiny (Earth-size) hot white dwarf that will shine as a moon-bright diamond in our skies.

Further Surfing:

SOHO: Our star the Sun

Shipman, U of Delaware: Our Sun's demise

Silver among the gold

Q: Why do people have gray hair? Is that because of the alteration of gene expression or the loss of certain pigments? (Terry, Atlanta, Georgia)

A: Our genes determine everything including when we gray and how white our hair will become. Caucasians gray earlier than Asians. We gray because our hair loses pigments.

As cars and people age, things start going wrong. Graying is one of those things. Often beginning in their 30s, gray hairs start to appear because hair shafts contain less melanin, the pigment that colors hair.

Graying woodcarver, Salzburg, Austria [Corel]

The color-producing cells (melanocytes) gradually disappear for some unknown reason. For a hair to have color — its root hair bulb must contain melanocytes actively making pigment and transferring the color to the hair shaft.

As people age, their hair bulbs become an "increasingly hostile place" for melanocytes to work properly and eventually these cells lose the battle and die, says Desmond J. Tobin, reader in cell biology at the University of Bradford, England.

Other parts of the hair follicle try to replace the cells with new melanocytes but usually the efforts fail. Consequently, the bulbs gradually run out of melanocytes. The hair gets grayer and grayer until, finally, the hair bulb has no melanocytes to make pigment and the hair becomes melanin free. Then the intrinsic color of the protein (keratin) that makes up the hair strand is revealed. It’s the same color as the keratin in our fingernails and toenails — an off white. Our hair looks white.

Further Surfing

British Journal of Dermatology 2004: 150:435-443 Human hair graying is linked to a specific depletion of hair follicle melanocytes affecting both the bulb and the outer root sheath by S. Commo, O. Gaillard and B.A. Bernard.

European Hair Research Society: The aging hair follicle pigmentary unit by DJ Tobin

American Academy of Dermatologists: How melanocytes produce hair color

(Answered April 30, 2004)