Star Baby Book
Objectives
Participants will:
 Learn the life cycle of a star.
 Understand and define the different stages of a star life cycle.
 Create a model, PowerPoint, or other media that will explain the life cycle of a star.
Suggested Grade Level
3rd-12th
Subject Areas
Science
Timeline
2-4 class periods
Standards
Science
NS.K-12.4 Earth and Space Science
 Origin and evolution of the earth system
 Origin and evolution of the universe
Background
The evolution of a star through its various stages of life and death is a fascinating topic for students. It is vital that
students understand the life cycle of a star. That cycle has created our solar system, as we know it, as well as all of
the star systems in the universe. While there is a lot of information and several steps in the life cycle of a star, it can
be relatively easy to understand if a few basic physics and chemistry principles are kept in mind. This cycle can also
be a fun topic to cover and a way to bring in other subject areas such as art, language arts, technology, and music.
According to the latest observations of the Chandra X-ray Observatory,
http://chandra.harvard.edu/edu/formal/stellar_ev/, the following is the story of stellar evolution in the universe, as we
currently understand it.
Interstellar Medium and Nebulae
Interstellar medium (ISM) are the dust grains, and atomic and molecular gas interspersed between the stars of a
galaxy. The average density of ISM throughout the universe is roughly one atom per cubic centimeter, making it a
near perfect vacuum. (In comparison, our atmosphere contains 1019 particles per cubic centimeter)
Nebulae are the denser conglomerations of interstellar gas and dust. The three main types of nebulae are diffuse,
reflection, and absorption. A diffuse nebula produces an emission spectrum because of energy that has been
absorbed from one or more hot luminous stars that excite the hydrogen gas. A reflection nebula is a nebula that is
mainly composed of cool interstellar dust that reflects and scatters light from nearby stars. They are usually blue
because the scattering is more efficient for blue light by the dust particles. The Witch Head Nebula is an example of a
reflection nebula. Absorption nebulae are physically similar to reflection nebulae. They look differently due to the
geometry of the cloud of dust, the light source, and Earth. Absorption nebulae, or dark nebulae, are simply blocking
the light from the source behind them. The Horsehead Nebula is an example of an absorption nebula.
Giant Molecular Clouds and Protostars
Molecular clouds are interstellar gas and dust left over from the formation of the galaxy. They are composed mostly
of molecular hydrogen. They are the coolest (10 to 20 K) and densest (106 to 1010 particles/cm3) portions of the ISM.
The molecular clouds have diameters from 1-300 Light Years (ly) and contain enough gas to form about 10 million
Sun-like stars. These molecular clouds will eventually create clumps of gas and dust. As a gas clump collapses, it
heats up due to friction as the gas particles bump into each other starting the creation of a protostar.
As the clumps grow, more and more dust and gas is accreted into the swirling disc of matter. Protostars reach
temperatures of 2,000 to 3,000 K. This becomes a very violent area as more and more matter is drawn into the
emerging protostar while other matter is forming into protostars and eventual planets. It is so violent that only about
10% of protostars survive this time of formation.
Brown Dwarfs and Low Mass Stars
If a protostar forms with a mass less than 0.08 solar masses (our Sun=1 solar mass), its internal temperature never
becomes high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf. Imagine a stellar
body in between a Jupiter and a star like our Sun. Astronomers are unsure about the life cycle of a brown dwarf. The
time it takes to use up all of its hydrogen fuel is longer than the universe is old-13.7 billion years.
Mid-sized Stars
Mid-sized stars range in size from 0.8 solar masses to eight solar masses, or 8 times the mass of our sun. Gas and
dust will coalesce to form a protostar. Eventually, enough mass is accumulated to start fusion at the young star’s
core. Thermonuclear fusion in these sized stars produces the outward radiation pressure to counterbalance
gravitational forces for approximately 10 billion years. When the fuel is used up, fusion stops, the balance is tipped
towards gravity, and the core begins to collapse. As the layers collapse, the gas compresses and heats up. The
temperature becomes high enough for helium nuclei to fuse into carbon and oxygen nuclei, with hydrogen fusing in a
thin shell surrounding the core. The outer layers expand to an enormous size and the star is now called a red giant.
The star brightens to near a factor between 1,000 and 10,000, but the surface temperature drops giving the star its
reddish appearance. A strong wind begins to blow from the star’s surface, which blows away most of the hydrogen
shell surrounding the star’s core.
Eventually, the material ejected by the star forms an envelope of gas called a planetary nebula, which expands into
the surrounding ISM at speeds of between 17-35 km/hr (10-22 mph). The core of the star left is called a white dwarf.
Within 50,000 years the layers of the planetary nebula become so thin they are no longer visible. Therefore, all
planetary nebulae that we see are less than 50,000 years old. Eventually, the white dwarf will use all of its energy
and become a black dwarf-a cold, dark mass. The universe is not old enough for any white dwarf to have become a
black dwarf, so they are not considered to be part of the evolutionary stage of a star.
Massive Stars
Massive stars, larger than eight solar masses, form in the dense nebula star-forming regions as well. Massive stars
are fundamental to the production of the heavy elements in the universe. The beginning life cycle of a massive star is
similar to that of a mid-sized star. Gas and dust will condense to form protostars. The difference is that a massive
star is able to accrete more mass depending on the material available and, therefore, grows larger.
Just as with mid-size stars, the gas and dust continues to compress until the temperature and pressure is sufficient
for hydrogen fusion. Hydrogen continues to be the star’s main fuel source for several million years. One of the main
products of hydrogen fusion is helium. When the star begins to run out of molecular hydrogen, it begins to cool and
condense to a slightly smaller size. As it collapses, the friction of the gases begins to heat the core up eventually
give the star a temperature high enough to begin fusing helium. Once again, heat causes the star to expand to a
fairly static size for several million years until the helium begins to run out. When helium is depleted, the star begins
to collapse again. Eventually, the core reaches a temperature high enough for carbon fusion. This growing and
shrinking process continues throughout the star’s main sequence life. It is a dance of equilibrium between the
stresses of gravity compressing the star with the heat of fusion expanding the star. This dance becomes more
chaotic and dramatic later in its life.
Many of the lighter elements found on the periodic table are created during this main sequence life cycle. Only
massive stars have enough material and heat of fusion to create these molecules. There is a limit to the elemental
synthesis. That boundary is iron. Once the star reaches iron fusion, the continued fusion of iron requires more
energy than is available. When this point is reached the collapse of the star’s iron core is imminent. The pressure of
gravity overwhelms thermal expansion. Instantaneously, the core collapses from a diameter of approximately 8,000
km to approximately 19 km. The collapse happens so fast that the outer layers have no time to collapse along with
the core. The energy released during the collapse of one massive star is more than that produced by the lifetime
fusion of 100 Suns. Most of the energy is expelled into space as neutrinos. The remaining energy triggers a Type II
supernova explosion.
The core collapses so fast that it momentarily goes past its equilibrium point and instantaneously rebounds. The infalling layers meet the rebounding core, creating a strong shock wave that runs outward through the layers toward
the star’s surface. The shock wave heats the outer layers, inducing explosive nuclear reactions and ejects the outer
layers in speeds in excess of 16 million km (9 million mph). The energy of the shock wave creates elements heavier
than iron, which is impossible in the core of a star. The exploding star becomes brighter than a billion Suns. The
material being released from the explosion, or Type II supernova, seeds the surrounding ISM and may trigger the
next generation of stars. The end of a massive star’s life cycle depends upon the initial mass of the star. It could
become a neutron star, pulsar, magnetar, or black hole.
Neutron stars have a collapsing core remnant between 1.4 and 2.5 solar masses. However, it is compressed to an
area of about 12km. It is so tightly packed that the electrons are forced into the atomic nuclei where they combine
with protons and become neutrons. A mere teaspoonful of a neutron star would weigh more than 6 billion tons. The
gravitational pull on the surface of a neutron star is 2x1011 times stronger than that found on the surface of the earth.
If an object were dropped from 1 meter high, it would take 1 microsecond to hit the ground and it would be traveling
over 2000 kilometers per second!
Pulsars are spinning neutron stars that have jets of particles moving almost at the speed of light streaming out from
the magnetic poles. These jets produce very powerful beams of high-energy particles that emit X-rays. Because the
axes of true north and magnetic north are different the beam of particles and X-rays sweep around as the pulsar
rotates.
If the core remnant of a collapsed massive star exceeds 3 solar masses, the neutron degeneracy pressure which
keeps neutron stars and pulsars stable, is unable to stop the total collapse of the star. The neutrons get pushed into
each other until the star becomes a boundary in space around the black hole, called the event horizon, beyond which
we cannot see. The extreme gravitational field within the event horizon emits no radiation; however, it can be
indirectly detected by its effects on the space-time around it.
Gamma-ray bursts are among the most energetic and most luminous explosion in the Universe. They occur roughly
once a day and last from a few thousandths of a second to a few hundred seconds. The energy from the burst in just
one second is comparable to the amount of energy production of our Sun in its entire lifetime. It is thought that the
source of these bursts of energy come from rapidly spinning black holes, which form when the central core of a very
massive star becomes unstable, and collapses under its own gravity. The in-falling stellar material becomes part of
the newly formed black hole, which releases enormous amounts of energy in two jets. The jets expand at nearly the
speed of light along the rotational axis.
Stellar Evolution Chart (Moving from low stellar mass to high stellar mass)
NebulaProtostarBrown Dwarf
NebulaProtostarRed DwarfWhite Dwarf
NebulaProtostarMid-sized Star (Sun)Red GiantPlanetary NebulaWhite Dwarf
NebulaProtostarBlue SupergiantRed Giant/Blue GiantType II SupernovaNeutron Star
NebulaProtostarBlue SupergiantType II SupernovaBlack Hole
NebulaProtostarBlue SupergiantBlack Hole
Vocabulary
Interstellar medium, stellar evolution, protostar, brown dwarf, red dwarf, white dwarf, red giant, planetary nebula, blue
supergiant, red supergiant , type II supernova, neutron star, magnetar, black hole
Materials
computer(s) with PowerPoint
construction paper
crayons
markers
paint
8 ½” x 11” white paper
poster paper
other materials students will need to create their books/presentations
Lesson
1. Discuss with students stellar evolution at an appropriate level. More up-to-date, advanced-level information
can be found at the Chandra X-ray Observatory Web site at
http://chandra.harvard.edu/edu/formal/stellar_ev.
2. The purpose of this activity is to have students put together a book that highlights each stage in a star’s
stellar evolution in the same way a mother puts together a baby book for her child.
3. Depending on the age of your students, they can either use an existing star or create a fictional star. For
young learners, they can use a fictional star name and for older learners they may use an existing star.
When creating the baby book, they will insert the current image of their star into the sequence.
4. Once the star has been chosen, either the students must research the mass of their star to determine its
stellar life cycle, or if creating a fictional star they may choose any life cycle from the above chart.
5. Now that the star’s life cycle has been determined, each student or group of students must decide how they
will convey their star’s life cycle. They can go “old school” and put together a photo album, they can create a
PowerPoint, or do artist drawings. The point is to let each student or group display their information that
suits them best.
6. At this point, the students must organize their star’s life cycle for their presentation.
7. Have students work on their presentations and practice presenting.
8. Have students present to the class their star’s life cycle.
Extensions
Other items or activities can be added to enhance the presentation such as childhood memory items from the star’s
childhood, or even a favorite lullaby that was sung to the young star.
Evaluation/assessment
Evaluate students on creativity and accuracy of stellar evolution.
Resources
Chandra X-ray Observatory Education Page
http://chandra.harvard.edu/edu/formal/stellar_ev
Hertzsprung-Russell Diagram downloaded from
http://www.physast.uga.edu/~jss/1020/ch15/16-10.jpg
Hertzsprung-Russell Diagram