The Solar System

CHAPTER 7 | ATOMS AND STARLIGHT 137

Payne worked for many years as a staff astronomer at the

Harvard College Observatory with no formal position on the

faculty. She married Russian astronomer Sergei Gaposchkin in

1934 and was afterward known as Cecilia Payne-Gaposchkin. In

1956, Harvard accepted women to its faculty, and Payne-

Gaposchkin was appointed a full professor and made chair of the

Harvard astronomy department. By that time, the importance of

her research 30 years earlier had come to be widely recognized.

Cecilia Payne-Gaposchkin’s work on the chemical composi-

tion of the stars illustrates the importance of fully understanding

the interaction between light and matter. It was her detailed

understanding of the physics that led her to the correct composi-

tion. As you turn your attention to other information that can be

derived from stellar spectra, you will again discover the impor-

tance of understanding light.

The Doppler Effect

Surprisingly, one of the pieces of information hidden in a spec-

trum is the velocity of the light source. Astronomers can mea-

sure the wavelengths of the lines in a star’s spectrum and fi nd the

velocity of the star. Th e Doppler eff ect is the apparent change

in the wavelength of radiation caused by the motion of the

source.

silicon, iron, and aluminum. Even the most eminent astrono-

mers dismissed Payne’s result as illusory. Faced with this pressure

and realizing that as a female scientist in the 1920s she faced an

uphill battle, Payne could not press her discovery.

By 1929, astronomers generally understood the importance

of temperature on measurements of composition derived from

stellar spectra. At that point, they recognized that stars are mostly

hydrogen and helium, but Payne received no credit.

Density

4

MASS | ENERGY | TEMPERATURE AND HEAT | DENSITY | PRESSURE

A brick would be dense even in space where it had

no weight.

Y

ou are about as dense as an average

star. What does that mean? As you

study astronomy, you will use the term

density often, so you should be sure to

understand this fundamental concept. Density

is a measure of the amount of matter in a

given volume. Density is expressed as mass

per volume, such as grams per cubic centime-

ter. The density of water, for example, is about

1 g/cm^3 , and you are almost as dense as

water.

To get a feel for density, imagine holding a

brick in one hand and a similar-sized block

of Styrofoam in the other hand. You can

easily tell that the brick contains more matter

than the Styrofoam block, even though both

are the same size. The brick weighs more than

the Styrofoam, but it isn’t really the weight

that you should consider. Rather, you should

think about the mass of the two objects. In

space, where they have no weight, the brick

and the Styrofoam would still have mass, and

you could tell just by moving them around

that the brick contains more mass than the

Styrofoam. For example, imagine tapping each

object gently with your hand. The massive

brick would be easy to distinguish from the

low-mass Styrofoam block, even in

weightlessness.

Density is a fundamental concept in

science because it is a general property of

materials. Metals tend to be dense; lead, for

example, has a density of about 7 g/cm^3.

Rock, in contrast, has a density of 3 to

4 g/cm^3. Water and ice have densities of

about 1 g/cm^3. If you knew that a small moon

orbiting Saturn had a density of 1.5 g/cm^3 ,

you could immediately draw some conclusions

about what kinds of materials the little moon

might be made of—ice and a little rock, but

not much metal. The density of an object is a

basic clue to its composition.

Astronomical bodies can have dramatically

different densities. The gas clouds between

the stars can have very low densities, but the

same kind of gas in a star can have a much

higher density. The sun, for example, has an

average density of about 1 g/cm^3 , about the

same as your body. As you study astronomical

objects, pay special attention to their

densities.

Har

facu

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silicon, iron, and aluminum. Even the most eminent astrono-

■ Table 7-2 ❙ The Most Abundant

Elements in the Sun

Percentage by Percentage

Element Number of Atoms by Mass

Hydrogen 91.0 70.9

Helium 8.9 27.4

Carbon 0.03 0.3

Nitrogen 0.008 0.1

Oxygen 0.07 0.8

Neon 0.01 0.2

Magnesium 0.003 0.06

Silicon 0.003 0.07

Sulfur 0.002 0.04

Iron 0.003 0.1