Mo

50.000,000/

YEARS

^ YEARS

30,000 10,000 7,500

hotter

6,000 5,000 3,500 HI TEMPERATURE (K)

2,000

Figure 5.3. Theoretical tracks of evolution showing luminosity and temperature changes of contracting protostars of different masses. (Contraction times are marked on the tracks.)

Approximately how long does it take each of the following protostars to reach zero age main sequence (to be born)? (a) stars like our Sun_

_; (b) stars with mass much greater than the Sun's_;

(c) stars with mass much less than the Sun's_

Answer: (a) About 50 million years; (b) about 2000 years; (c) about 200 million years.

You can think of a main sequence star as an adult star. In comparison to changes in protostars, evolution of main sequence stars is very slow. A star spends most of its lifetime shining steadily, with luminosity and temperature values found along the main sequence of H-R diagrams.

A main sequence star gets its energy from nuclear fusion reactions in which hydrogen at the center of the star is converted into helium (Figure 5.4). Four hydrogen nuclei are fused into one lighter, helium nucleus. The disappearing mass is changed into energy and released. (The same process releases energy in hydrogen bombs.)

5.5 WHY STARS SHINE

Heavier mass: four hydrogen nuclei (before fusion)

Mass difference —>■ Energy —>- Starlight

Mass difference —>■ Energy —>- Starlight

Figure 5.4. An imaginary experiment showing why the stars shine. If you could weigh the hydrogen nuclei before and the helium nucleus after fusion, you would discover that the helium nucleus was lighter.

The energy from the nuclear fusion reactions eventually reaches the star's surface. Then the star shines energy into space.

The amount of energy released in a nuclear fusion reaction can be calculated from the famous special relativity theory equation of (German-born) U.S. physicist Albert Einstein:

where E = energy, m = mass difference, and c = speed of light.

According to Einstein's equation, when many nuclear fusion reactions occur together, enormous amounts of energy are released. The Sun is a huge, hot gaseous sphere that shines steadily without appreciable change of size or temperature. Although practically 5 million tons of hydrogen must be converted into helium each second to produce the Sun's luminosity, less than 0.01 percent of the Sun's total mass changes to sunshine in a billion years.

What is the source of energy that lets main sequence stars shine?

Answer: Nuclear fusion reactions in which hydrogen is converted into helium.

5.6 OLD AGE

A star will shine steadily as a main sequence star until all the available hydrogen in its core has been converted into helium. Then the star will begin to die.

Our Sun is an average medium-sized star. It has been shining as a stable main sequence star for about 5 billion years, and it should continue to shine steadily for another 5 billion years.

Very massive, hot, bright stars die fastest because they use up their hydrogen most rapidly. The very massive blue giant stars, such as Rigel in Orion, spend only a few million years shining as main sequence stars.

The least massive, cool, dim stars live the longest because they consume their hydrogen fuel least rapidly. The small-mass red dwarfs are the oldest and most numerous main sequence stars. They have lifetimes billions of years long.

What types of stars are expected to live (a) longest? _

much longer is the Sun expected to shine as it does now?

Answer: (a) Those with small mass, such as red dwarfs; (b) very massive stars, such as blue giants; (c) about 5 billion years.

5.7 RED GIANTS

After the hydrogen fuel in the star's core is used up, the star no longer has an energy source there. The core, which then consists primarily of helium, begins to contract gravitationally. Hydrogen fusion continues in a shell around the helium core, under the outside envelope of hydrogen.

Gravitational contraction causes the temperature of the helium core to rise. The high temperature makes the shell hydrogen fuse faster, and the star's luminosity increases.

The tremendous energy released by this hydrogen fusion and gravitational contraction heats up surrounding layers. The star expands to gigantic proportions. The star's density is then very low everywhere except in the core (Figure 5.5).

As the star expands, its surface temperature drops and its surface color turns to red. The star has changed into a huge, bright, red, aging star—a red giant or supergiant. It is cool but bright because of its gigantic surface area. It has the luminosity and temperature values of the upper-right region of the H-R diagram.

You can see some red supergiant stars shining in the sky. Good examples are Betelgeuse in Orion and Antares in Scorpius, both over 400 times the Sun's diameter (Tables 1.1 and 2.1).

Hydrogen-

fusing center

1 million km

Hydrogen-

fusing shell

Outside envelope of hydrogen

Helium core

(size exaggerated)

4 million km-

(b) Age 10.3 Billion years

Figure 5.5. Sunlike star (a) at start of life on main sequence and (b) as it ages to red giant.

Our Sun, like all stars, is expected to change into a huge red giant when it dies. That red giant Sun will shine so brightly that rocks will melt, oceans will evaporate, and life as we know it on Earth will end.

When does a star begin to change from a main sequence star into a red giant?

Answer: When it has converted all of the available hydrogen fuel in its core into helium.

5.8 SYNTHESIS OF HEAVIER ELEMENTS

Gravitational contraction causes the temperature inside the red giant's helium core to rise to 100 million K. At that temperature, helium is converted to carbon in nuclear fusion reactions (Figure 5.6).

Redgiant Structure
Figure 5.6. Structure of a red giant star.

The helium core does not expand much once the helium fusion starts. The temperature builds up rapidly without a cooling, stabilizing expansion. Helium nuclei fuse faster and faster, and the core gets even hotter. This nearly explosive ignition of helium fusion is called the helium flash.

After some time, the temperature rises sufficiently so that the core expands. Cooling occurs inside, and helium fusion goes on at a steady rate, surrounded by a hydrogen-fusing shell.

Inside the more massive red giants, further fusion reactions can build up familiar elements heavier than carbon, such as oxygen, aluminum, and calcium (Appendix 4).

Astronomers believe that elements like carbon and oxygen, which we need for life, are made where?_

Answer: Inside red giant stars.

5.9 VARIABLE STARS

A star moves back and forth in the area between the red giant region and the main sequence several times, in a way not yet fully understood, before it enters the final stages of its life.

Most stars probably change from red giants to pulsating variable stars before they finally die. That is, they expand and contract and grow bright and fade periodically.

Cepheid variables are very large luminous yellow stars whose light output varies in periods of from 1 to 70 days. You can observe Delta Cephei, the first discovered and the star for which this class of variables was named (Figure 5.7). Cepheids are important because they provide a way of measuring distances too great to be measured by trigonometric parallax.

More than 700 Cepheid variables are known in the Milky Way Galaxy. Polaris, the North Star, is the nearest. Its brightness varies between magnitudes 2.5 and 2.6 about every 4 days.

U.S. astronomer Henrietta Leavitt (1868-1921) discovered that the longer the period of light variation of Cepheids, the greater the luminosity. Astronomers use this period-luminosity relation to determine the absolute magnitude of Cepheids after measuring their periods.

A comparison of the calculated absolute magnitude and the observed apparent magnitude yields the distance to the Cepheids and the star groups they belong to (Section 3.16). Cepheids are useful distance markers out to about 3 Mpc (10 million light-years).

RR Lyrae variables, named after variable star RR in the constellation Lyra, are pulsating blue-white giants whose light output varies from brightest

Period

5.4 days

----

N

\

\

\

Minimum

6 8 Days

10 12

Maximum

Minimum

6 8 Days

10 12

Figure 5.7. Light curve showing how the light output varies for Delta Cephei, the prototype Cepheid variable star.

to dimmest in periods of less than a day. About 4500 RR Lyrae stars are known in the Milky Way Galaxy. RR Lyrae stars are used to measure the distance to the star clusters they belong to, out to about 200,000 pc (600,000 light-years).

Long-period Mira variables, named for famous Mira in the constellation Cetus, are red giants that take between 80 and 1000 days to vary between brightest and faintest. Mira, about 40 pc (130 light-years) away, varies from its maximum bright red to its minimum output, where it becomes invisible, in a period of 332 days. Mira was named the "Wonderful" by amazed seventeenth-century observers, who first recorded its brightness fluctuations.

Name three characteristics of a pulsating variable star that change periodically. (1)_; (2)_; (3)_

5.10 DEATH

All stars evolve in about the same way, although over different periods of time, until their cores become mostly accumulated carbon (Figure 5.8). The last stage in a star's evolution, or the way it finally dies, depends greatly on its mass.

Hydrpgen enyelope^xtending out/ 1 several miliidn.kilometers

Hydrpgen enyelope^xtending out/ 1 several miliidn.kilometers

Figure 5.8. The structure of a star with an increasing inner core of mostly carbon.

Small stars, up to about 1.4 times the Sun's mass, finally die without a fuss, quietly fading away into the blackness of space. Very massive stars end with a violent explosion, flaring up brilliantly before giving up life.

What characteristic of a star determines the way it finally dies?_

Answer: Its mass.

5.11 MASS LOSS

When a star of mass like our Sun has depleted all of its available helium fuel, it becomes a bloated red giant star for the last time. (At this stage of its life our Sun will become so big that it will swallow up Mercury, Venus, Earth, and Mars.)

Figure 5.9. Famous Ring Nebula in Lyra, a planetary nebula and star core.

The star then throws off some of its mass. The star's outermost hydrogen envelope, enriched by heavier elements, flies off into space. Electrically charged particles stream away in a flow called a stellar wind. (The solar wind is described in Section 4.14.) Deeper layers are thrown off in a wispy, expanding shell of gas typically about 0.5 to 1 light-year across, called a planetary nebula, which continues to spread out at speeds of about 20 to 30 km/sec (45,000 to 67,500 miles/hour). The star's core is left behind.

About 1600 planetary nebulas have been recorded. They are probably less than 50,000 years old, because the gas atoms in the nebula separate rapidly. After about 100,000 years, the shell is too spread out to be visible.

Examine Figure 5.9. Identify the core of the star and the planetary nebula in the photograph. (a)_;

Answer: (a) Planetary nebula; (b) core of the star.

5.12 WHITE DWARFS

After it has thrown off its gas envelope, the star remains as a core of carbon surrounded by a shell of burning helium.

A star that has exhausted all of its nuclear fuel can no longer withstand the pull of gravity. It contracts again as gravity pulls matter in toward the center. Gravitational contraction makes the temperature and pressure go up very high, and electrons are stripped off atoms. The star becomes a small, hot, white dwarf. It is made mostly of electrons and nuclei. These subatomic particles can be squeezed much closer together than whole atoms can.

Eventually, when the white dwarf star reaches about Earth's size, it cannot contract any further. White dwarf stars of mass like the Sun are very dense because gravity packs all that mass into a star the size of Earth. The force of gravity on such a white dwarf star would be about 350,000 times greater than that on Earth. If you could stand on a white dwarf star, you would weigh 350,000 times more than you do on Earth.

If a white dwarf in a binary system accretes matter from its companion star, it may briefly blaze, called a nova. Or, it may explode brilliantly when a wave of nuclear fusion rips through a bigger buildup, called a Type Ia supernova.

Usually a white dwarf star cools, turns to dull red, and shines its last energy into space. Then the white dwarf becomes a dead black dwarf in the graveyard of space.

What is a white dwarf star?_

Answer: A small dense (dying) star of low luminosity and high surface temperature, typically about the size of Earth but with mass equal to the Sun's.

5.13 LIFE CYCLE OF SUNLIKE STARS

Identify each stage of the life of a star like our Sun, as labeled sequentially in Figure 5.10. (a)_;

hotter

1,000,000

I SPECTRAL CLASS G K M

3 1/100

Q CONTRACTING

SUM,

1/1,000,000

30,000 10,000 7,500 6,000 5,000 3,500 2,000 hotter I TEMPERATURE (K)

Figure 5.10. The life stages of a star like our Sun.

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