# stars

## particle accelerators

Scientists have built machines called particle accelerators. These amazing tools smash particles that are smaller than atoms into each other head-on. This creates new particles. Scientists use particle accelerators to learn about nuclear fusion in stars. They can also learn about how atoms came together in the early universe. Two well-known accelerators are SLAC, in California, and CERN, in Switzerland.

## how stars are classified

Stars shine in many different colors. The color relates to a stars temperature and often its size.

## color and temperature

Think about the coil of an electric stove as it heats up. The coil changes in color as its temperature rises. When you first turn on the heat, the coil looks black. The air a few inches above the coil begins to feel warm. As the coil gets hotter, it starts to glow a dull red. As it gets even hotter, it becomes a brighter red. Next it turns orange. If it gets extremely hot, it might look yellow-white, or even blue-white. Like a coil on a stove, a stars color is determined by the temperature of the stars surface. Relatively cool stars are red. Warmer stars are orange or yellow. Extremely hot stars are blue or blue-white.

## classifying stars by color

The most common way of classifying stars is by color as shown, in Table 26.1. Each class of star is given a letter, a color, and a range of temperatures. The letters dont match the color names because stars were first grouped as A through O. It wasnt until later that their order was corrected to go by increasing temperature. When you try to remember the order, you can use this phrase: Oh Be A Fine Good Kid, Man. Class O Color Blue Temperature range 30,000 K or more Sample Star An artists depiction of the O class star Zeta Pup- pis. B Blue-white 10,00030,000 K An artists depiction of Rigel, a Class B star. Class A Color White Temperature range 7,50010,000 K Sample Star Sirius A is the brightest star that we see in the night sky. The dot on the right, Sirius B, is a white dwarf. F Yellowish-white 6,0007,500 K There are two F class stars in this image, the super- giant Polaris A and Po- laris B. What we see in the night sky as the single star Polaris, we also known as the North Star. G Yellow 5,5006,000 K Our Sun: the most im- portant G class star in the Universe, at least for hu- mans. Class K M Color Orange Red Temperature range 3,5005,000 K 2,0003,500 K Sample Star Arcturus is a Class K star that looks like the Sun but is much larger. There are two types of Class M stars: red dwarfs and red giants. An artists concept of a red dwarf star. Most stars are red dwarfs. The red supergiant Betel- geuse is seen near Orions belt. The blue star in the lower right is the Class B star Rigel. The surface temperature of most stars is due to its size. Bigger stars produce more energy, so their surfaces are hotter. But some very small stars are very hot. Some very big stars are cool.

## constellations

The stars that make up a constellation appear close to each other from Earth. In reality, they may be very distant from one another. Constellations were important to people, like the Ancient Greeks. People who spent a lot of time outdoors at night, like shepherds, named them and told stories about them. Figure 26.1 shows one of the most easily recognized constellations. The ancient Greeks thought this group of stars looked like a hunter. They named it Orion, after a great hunter in Greek mythology. The constellations stay the same night after night. The patterns of the stars never change. However, each night the constellations move across the sky. They move because Earth is spinning on its axis. The constellations also move with the seasons. This is because Earth revolves around the Sun. Different constellations are up in the winter than in the summer. For example, Orion is high up in the winter sky. In the summer, its only up in the early morning.

## energy of stars

Only a tiny bit of the Suns light reaches Earth. But that light supplies most of the energy at the surface. The Sun is just an ordinary star, but it appears much bigger and brighter than any of the other stars. Of course, this is just because it is very close. Some other stars produce much more energy than the Sun. How do stars generate so much energy?

## nuclear fusion

Stars shine because of nuclear fusion. Fusion reactions in the Suns core keep our nearest star burning. Stars are made mostly of hydrogen and helium. Both are very lightweight gases. A star contains so much hydrogen and helium that the weight of these gases is enormous. The pressure at the center of a star is great enough to heat the gases. This causes nuclear fusion reactions. A nuclear fusion reaction is named that because the nuclei (center) of two atoms fuse (join) together. In stars like our Sun, two hydrogen atoms join together to create a helium atom. Nuclear fusion reactions need a lot of energy to get started. Once they begin, they produce even more energy.

We could say that stars are born, change over time, and eventually die. Most stars change in size, color, and class at least once during their lifetime.

## formation of stars

Stars are born in clouds of gas and dust called nebulas. Our Sun and solar system formed out of a nebula. A nebula is shown in Figure 26.2. In Figure 26.1, the fuzzy area beneath the central three stars contains the Orion nebula. For a star to form, gravity pulls gas and dust into the center of the nebula. As the material becomes denser, the pressure and the temperature increase. When the temperature of the center becomes hot enough, nuclear fusion begins. The ball of gas has become a star!

## red giants and white dwarfs

A star like our Sun will become a red giant in its next stage. When a star uses up its hydrogen, it begins to fuse helium atoms. Helium fuses into heavier atoms like carbon. At this time the stars core starts to collapse inward. The stars outer layers spread out and cool. The result is a larger star that is cooler on the surface, and red in color. Eventually a red giant burns up all of the helium in its core. What happens next depends on the stars mass. A star like the Sun stops fusion and shrinks into a white dwarf star. A white dwarf is a hot, white, glowing object about the size of Earth. Eventually, a white dwarf cools down and its light fades out.

## main sequence stars

For most of a stars life, hydrogen atoms fuse to form helium atoms. A star like this is a main sequence star. The hotter a main sequence star is, the brighter it is. A star remains on the main sequence as long as it is fusing hydrogen to form helium. Our Sun has been a main sequence star for about 5 billion years. As a medium-sized star, it will continue to shine for about 5 billion more years. Large stars burn through their supply of hydrogen very quickly. These stars live fast and die young! A very large star may only be on the main sequence for 10 million years. A very small star may be on the main sequence for tens to hundreds of billions of years.

## neutron stars and black holes

After a supernova explosion, the stars core is left over. This material is extremely dense. If the core is less than about four times the mass of the Sun, the star will become a neutron star. A neutron star is shown in Figure 26.4. This type of star is made almost entirely of neutrons. A neutron star has more mass than the Sun, yet it is only a few kilometers in diameter. If the core remaining after a supernova is more than about 5 times the mass of the Sun, the core collapses to become a black hole. Black holes are so dense that not even light can escape their gravity. For that reason, we cant see black holes. How can we know something exists if radiation cant escape it? We know a black hole is there by the effect that it has on objects around it. Also, some radiation leaks out around its edges. A black hole isnt a hole at all. It is the tremendously dense core of a supermassive star.

## supergiants and supernovas

A more massive star ends its life in a more dramatic way. Very massive stars become red supergiants, like Betelgeuse. In a red supergiant, fusion does not stop. Lighter atoms fuse into heavier atoms. Eventually iron atoms form. When there is nothing left to fuse, the stars iron core explodes violently. This is called a supernova explosion. The incredible energy released fuses heavy atoms together. Gold, silver, uranium and the other heavy elements can only form in a supernova explosion. A supernova can shine as brightly as an entire galaxy, but only for a short time, as shown in Figure 26.3.

## star systems

Our solar system has only one star. But many stars are in systems of two or more stars. Two stars that orbit each other are called a binary star system. If more than two stars orbit each other, it is called a multiple star system. Figure 26.5 shows two binary star systems orbiting each other. This creates an unusual quadruple star system.

## measuring star distances

Astronomers use light years as the unit to describe distances in space. Remember that a light year is the distance light travels in one year. How do astronomers measure the distance to stars? For stars that are close to us, they measure shifts in their position over time. This is called parallax. For distant stars, they use the stars brightness. For example, if a star is like the Sun, it should be about as bright as the Sun. They then figure out the stars distance from Earth by measuring how much less bright it is than expected.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

A group of stars that seem from Earth to form the outline of a familiar shape is called a

``````a. binary star system.

-->  b. constellation.

c. solar system.

d. galaxy.
``````

Particle collisions in accelerators simulate

``````a. nuclear fission in a black hole

b. fusion of hydrogen into helium

-->  c. the conditions of the birth of the universe

d. all of these
``````

Constellations appear to move across the sky each night because

``````a. all stars have orbits.

-->  b. Earth rotates on its axis.

c. Earth revolves around the sun.

d. constellations are affected by Earths gravity.
``````

Stars emit energy as

``````a. neutrino streams

c. solar wind

``````

Which color of star has the highest temperature?

``````a. red

-->  b. blue

c. yellow

d. orange
``````

When our Sun stops fusion it will first become a(n)

``````-->  a. red giant

b. red supergiant

c. white dwarf

d. blue neutron star
``````

What is the energy source for all stars?

``````-->  a. nuclear fusion

b. nuclear fission

c. solar

d. hydrothermal
``````

Which class of star is our sun?

``````a. B

b. F

-->  c. G

d. K
``````

A star forms from a nebula when the temperature is high enough for

``````a. a supernova to occur.

-->  b. nuclear fusion to start.

c. a black hole to collapse.

d. heavy elements to form.
``````

Energy production in a star takes place in the

``````a. convective zone

-->  b. core

d. corona
``````

Astronomers measure the distance to very distant stars by comparing the stars to our sun. Which factor do they compare?

``````-->  a. brightness

b. location

c. parallax

d. color
``````

A star spends most of its life as a

``````-->  a. main sequence star.

b. red supergiant.

c. white dwarf.

d. supernova.
``````

The hottest stars blue-white; the coolest stars are red.

``````-->  a. true

b. false
``````

Stars in a constellation are near each other in space.

``````a. true

-->  b. false
``````

Our Sun is about half way through its life span.

``````-->  a. true

b. false
``````

A black hole emits dark electromagnetic radiation that we cannot see.

``````a. true

-->  b. false
``````

The same constellations appear in a location all year-round.

``````a. true

-->  b. false
``````

Constellations appear from Earth to move with the seasons.

``````-->  a. true

b. false
``````

Our sun is the biggest and brightest star in the galaxy.

``````a. true

-->  b. false
``````

Stars are made mostly of hydrogen and helium.

``````-->  a. true

b. false
``````

The coolest stars are red in color.

``````-->  a. true

b. false
``````

Once a star forms, it never changes.

``````a. true

-->  b. false
``````

Gravity causes a nebula to become denser at the center.

``````-->  a. true

b. false
``````

A larger star remains on the main sequence longer than a smaller star.

``````a. true

-->  b. false
``````

The next stage our sun will go through is white dwarf.

``````a. true

-->  b. false
``````

Betelgeuse is an example of a red supergiant.

``````-->  a. true

b. false
``````

A black hole is an empty place in space.

``````a. true

-->  b. false
``````

giant ball of glowing gas that is very hot

``````a. binary star system

b. black hole

c. main sequence

d. nebula

e. red giant

f. supernova

-->  g. star
``````

stage of a stars life in which hydrogen atoms fuse to form helium

``````a. binary star system

b. black hole

-->  c. main sequence

d. nebula

e. red giant

f. supernova

g. star
``````

stage of a stars life in which helium atoms fuse to form heavier elements

``````a. binary star system

b. black hole

c. main sequence

d. nebula

-->  e. red giant

f. supernova

g. star
``````

explosion of a red supergiant star

``````a. binary star system

b. black hole

c. main sequence

d. nebula

e. red giant

-->  f. supernova

g. star
``````

core of a star that has too much gravity to let light escape

``````a. binary star system

-->  b. black hole

c. main sequence

d. nebula

e. red giant

f. supernova

g. star
``````

cloud of gas and dust from which a star forms

``````a. binary star system

b. black hole

c. main sequence

-->  d. nebula

e. red giant

f. supernova

g. star
``````

two stars that orbit each other

``````-->  a. binary star system

b. black hole

c. main sequence

d. nebula

e. red giant

f. supernova

g. star
``````

## diagram questions

No diagram questions associated with this lesson