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Educational Product
Grades 9-12
National Aeronautics and
Space Administration
Educational Brief
Subject: ACE Mission
Topic: Cosmology and Stellar Evolution

Crab nebula

The ACE (Advanced Composition Explorer) spacecraft was launched in August of 1997 to make observations that are being used to test current theories on the creation and evolution of the galaxy. The purpose of the ACE spacecraft is to sample the matter that comes near Earth from the Sun, the space between the planets, and the Milky Way galaxy beyond the solar system.

ACE instruments are being used to verify modern theories concerning the origin of the universe. Observations of light coming from distant galaxies show a shift in the spectrum toward the lower frequencies (Doppler effect). This phenomenon is called the red shift and has generally been the evidence cited by astronomers when they suggest that the universe was once much closer together than it is now. This evidence indicates that the universe began ten to fifteen billion years ago with a tremendous explosion they call the Big Bang.

Another important piece of evidence supporting the Big Bang was the discovery of cosmic microwave background radiation (CMB) in the 1960s by Arno Penzias and Robert Wilson. They discovered microwave radiation coming from all directions in the universe with equal intensity. They concluded that the radiation came from beyond our galaxy, from the universe as a whole.

A third standard used to establish the validity of the Big Bang hypothesis is the abundance of light elements in the universe. This standard is based on pioneering efforts by George Gamow and his collaborators. Their theory makes predictions about the density of baryons (neutrons and protons) and light elements three minutes after the Big Bang. ACE data is being used to test theories on how these elements were created and how they have evolved, and to give scientists a clearer picture of their abundance in our galaxy.

The elements we see all around us in the universe were created by nucleosynthesis. In this process, nuclear fusion occurs in stars. As fusion proceeds, lighter elements combine to produce heavier elements. Fusion also generates large amounts of energy (such as visible light) as matter is converted into energy. Einstein’s famous E = mc2 equation can be used to calculate the amount of energy released when a given mass is converted. The min the equation represents the nuclear mass defect. This mass defect is the difference between the mass of the stable nucleus that was produced during the process and the sum of the masses of its parent particles.

According to the theory, the light elements (hydrogen, helium, and lithium) were produced during the first few minutes. During the first three minutes after the Big Bang, the universe cooled from 10 E 32 K to 10 E 9 K. After this cooling took place, protons and neutrons that formed right after the Big Bang collided to produce deuterium (one proton combined with one neutron). The deuterium then either combined with an additional proton to create helium-3 and energy (gamma rays, which are high frequency electromagnetic radiation) or it combined with another deuterium to create helium-3 and a free neutron. A tritium (hydrogen-3, the heaviest isotope of hydrogen) and a free neutron are formed from the combination of two deuterium nuclei. The helium-3 would have combined with deuterium to form helium-4 and a proton, while the tritium would have combined with a deuterium to form helium-4 and a free neutron. The lithium -7 isotope could also have been created from the combination of a helium-3 and a helium-4.

This theory is collectively called Big Bang nucleosynthesis and makes the prediction that approximately 25% of the mass of the primordial universe (shortly after Big Bang nucleosynthesis) should have been helium, mainly helium-4. The abundance of helium-4 is slowly increasing as the universe ages due to its production in stars. ACE instruments are collecting data to test scientists’ theories concerning the abundance of helium isotopes and other elements.

Current theory suggests that the production of the heavier elements (from oxygen up through iron) only occurs on stars where temperatures are very much hotter than our Sun. These stars have temperatures above one billion degrees because they contain masses at least ten times that of our Sun.
Galaxy NGC 4314

The currently accepted theory of stellar evolution involves the following sequence of events. After the Big Bang, gravitation pulled together clouds of gas and dust to create giant clusters of matter. Continued contraction of these clusters eventually increased their temperature due to the interaction of colliding particles and the pressures created by the large gravitational attraction. As the temperature approached 15 million degrees, the electrons in the atoms were ripped off to create a plasma. (A plasma is the state of matter present when some or all an element's electrons have been separated from their nuclei.) Continued contraction occurred until the particles in the plasma moved with such high velocities (and therefore high energies and temperatures) that they began to fuse. The energy that was released eventually reached the surface of the cluster of matter, was released into space, and stars were born! It is believed that this fusion process produces enough energy to generate an outward pressure in the stars that reaches equilibrium with the inward pull of gravity. Our Sun
is currently at this stage in its evolution and is fusing hydrogen to create helium .

The nuclear fuel (of light elements) which feeds the fusion process eventually runs out, and the stars end the first chapter in their lives. When the fuel runs out, the outward force which balances the gravitational attraction decreases. At that time gravitational forces again pull the outer layers of the star together, and the same processes that started the fusion process early in the stars’ history begin again. This time the star expands even more than before, and it becomes a red giant. The star may expand to one hundred times its original equilibrium size. The amount of time between a star’s birth and its red giant stage is dependent upon the original mass of the star.

The next step also depends upon the original mass of the star.
The Sun-like stars (stars having a mass approximately one-half to about three and one-half times the mass of the Sun) eventually deplete their nuclear mass to about 20% of what they had at birth. At that time they shrink to a white dwarf. This occurs as the inward pull of gravity again wins out over the decreasing outward pressure from the fusion process. These stars eventually cool and become cold and dark. They are often called black dwarfs.

Stars that began their history with masses ten or more times that of our Sun have quite a different fate. Similar to the Sun-like stars, these stars go through the red giant phase (they are called red supergiants) and then shrink to create forces which restart their nuclear furnace. But in this case the larger mass creates a larger gravitational pull and a larger number of internal collisions. The combined effect results in tremendously high temperatures capable of creating heavier atomic nuclei through fusion. The core eventually becomes mostly iron. Since the nuclear structure of iron does not allow its fusion to heavier elements (that would require the input of energy), fusion will cease. With the outward fusion pressure gone, the star goes through a rapid gravitational collapse (this is thought to occur in less than a second) and the temperature rises to 100 billion degrees. Since the nucleons (protons and neutrons) present in the star are being forced very close together, they create a tremendous repulsion of the positively charged nuclei from one another. This repulsion causes the star’s core to recoil into an unbelievable explosion. Scientists call this explosion a supernova. The fragments released from this supernova spew out in space to eventually form new stars, planets, and other celestial bodies. The nuclei of isotopes with masses heavier than nickel, created by this and previous supernovae, are thought to be distributed through the universe
during these supernovae.

Some of the remaining cores of these super giants (only the very largest) can form neutron stars. This can happen if the intense pressure from the gravitational attraction forces electrons to combine with nearby protons, thus forming neutrons. The other possibility for the heaviest cores is that the largest of the large may collapse with so much gravitational attraction that even light cannot escape. These massive collapsed stars are called black holes.

The ACE mission is important because of the ability of its instruments to study the origin and evolution of the elements. ACE detects many of the heavier isotopes which originated during the formation, evolution, and subsequent explosion of stars. The comparative number of different isotopes found in the galaxy is thought to be related to the life cycle of the massive stars.

It is important here to keep in mind that the material that the Earth and the rest of the solar system are made of has been changed and rearranged throughout the billions of years since their creation, so measuring its complete composition is difficult. ACE is measuring cosmic ray particles to draw comparisons between matter from the solar system (Sun, meteorites, planets, atmospheres of planets, the moon), comets, the local interstellar medium, and the galaxy. It is hoped that these comparisons will help answer some of the questions which astrophysicists have about the creation and evolution of the galaxy.

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Daniel Hortert
GESSEP Program
Pat Keeney
GESSEP Program
Dr. Eric Christian
ACE Deputy Project
Dr. John Krizmanic
Astroparticle Physicist
Beth Barbier
ACE Outreach Specialist
Author: Daniel Hortert

Goddard Scientists:
Dr. Eric Christian, <>
Dr. John Krizmanic, <>

ACE Outreach Specialist:
Beth Barbier<>