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Ouch! A spiral galaxy as viewed edge-on collides with a small blue galaxy. From the Hubble Ultra Deep Field. Photo by NASA, ESA, S. Beckwith (STScI) and the HUDF Team.


astro101 -- Stellar Evolution

The Horsehead Nebula

The interstellar medium

The interstellar medium (ISM) is the space between the stars, which is composed of low density gas and dust. One way to observe the ISM to to observe dark clouds or dark nebulae, where space dust blocks our view of the stars behind it. The illustration shows the horsehead nebula, an example of dark cloud in the constellation Orion.
Image Credit: NASA, NOAO, ESA and The Hubble Heritage Team (STScI/AURA)

The protostar

Stars form out of the gas and dust in the ISM. Inside a dark cloud, if enough material clumps together, gravity takes over and causes the object to undergo a rapid collapse. This object becomes a protostar, which is an object that is destined to become a star.

As the protostar collapses, its interior heats up. This causes the atoms in the interior to move faster, which creates gas pressure and and slows the collapse. When the gas pressure becomes substantial, the object becomes known as a pre-main sequence star. The pre-main sequence star is characterized by a slower contraction. These are now legitimate stars that can shine in the sky.

The Orion Nebula

The Great Orion Nebula

When a hot star forms inside a dark cloud, the cloud will undergo dramatic changes. The hot stars energize the surrounding hydrogen gas by forcing electrons to have higher energies. When the electrons fall back down, they produce the characteristic pink glow known as Balmer emission. This type of object is known as a biright nebula or emission nebula. Image Credit: NASA,ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team.

Onto the Main-Sequence

As the pre-main sequence star continues to contract,the core continues to heat. Protons in the core of the star slam into each other with such f orce that nuclear fusion can occur. As a result, hydrogen fuses into helium, releasing energy in the process. Now the star has a new energy source. The gas pressure becomes high enough that the contraction of the star stops. The star now enters an important state known as hydrostatic equilibium, in which gravity pushing inwards is balanced by gas pressure pushing outwards. When this occurs,the star is said to be on the main sequence, where it will spend most of its useful life.

Brown Dwarfs

For an object that has a mass less than about a tenth of a sun, it will never get hot enough in the core to fuse hydrogen. These objects are not considered stars, but are called brown dwarfs. These are simply objects that are considered too small to be a star.

The Pleiades

The Pleiades

Since a star forms out of gas and dust, younger stars may have some remaining dust surrounding them. Pictured is an open cluseter of stars, the Pleiades or seven sisters, which is visible with the naked eye. As photographed through a telescope, we see reflection nebulae around some of the stars. This is caused by the star's light reflecting off left over dust. Blue light is reflected more easily than red light (a phenomenon which explains why our sky is blue). The stars in the Pleiades our relatively young and have just reached the main sequence. As stars age, the surrounding dust will generally dissipate.

Stellar Evolution After the Main Sequence

How a star evolves is dictated primarily by the mass of the star. Let us first consider the case of a massive star. A main sequence star generates its energy by nuclear fusion of hydrogen in its core. Eventually, the hydrogen in the core will become exhausted. The star will no longer be in hydrostatic equilibrium due to a drop in gas pressure. Gravity takes over and the star contracts. As the star contracts, the interior heats. Eventually it will be hot enough for some hydrogen in a shell outside the core to start to fuse. The gas pressure in the shell increases,which caused the outer regions of the star to expand and cool. Meanwhile,the core of the star continues to contract and heat until it gets hot enough for nuclear reactions to take place with helium. Once the helium fusion begins,the gas pressure causes the star to expand and cool and become a red supergiant.

Eventually, the helium in the core runs out. Again the lack of a central energy source causes the gas pressure to drop and a contraction of the star. Eventually the contraction causes enough heating for the helium to fuse into carbon in a shell.

For what little time remains in the star's life, it will alternate between contracting and heating (after running out of energy in the core) and expanding and cooling (when finding a new nuclear source in the core).Through the various nuclear sources, the star continues to fuse lighter elements into heavier elements until it finally creates a core of pure iron. Nuclear energy cannot be generated by fusing iron into heavier elements. The core undergoes a rapid collapse, but the iron core is so dense, radiation cannot escape. Instead the entire star explodes, creating a supernova-- the catastrophic end of the star.

The Horsehead Nebula


When a supernova explodes,it generally appears as a "new"star. The illustration shows a supernova that went off in the year 1987 in a nearby galaxy known as the Large Magellanic Cloud. The image on the right (the "before"picture) shows the star as a blue supergiant. The image on the left shows the brightness of the explosion. We also have historical evidence for supernovae that went off in our own galaxy. In 1054, astronomers in China noted a "new" star that appeared in the constellation Taurus that was bright enough to be seen in the daytime. Other supernovae were seen and recorded in Europe in 1572 (Tycho's supernova) and 1604 (Kepler's supernova).

The Crab Nebula

Supernova Remnants

If we look at the point in space where a supernova went off in the past, we can view a supernova remnant. An example is the Crab nebula, which is the remnant from the supernova that was seen in 1054. After the explosion, the remant travels outwards where it sweeps up gas and dust in the interstellar medium. As it gets heavier, it slows down and eventually blends into and rejuvenates the interstellar medium. New stars form from this material.

Neutron Stars

During the explosion of a massive star, the core of the star will survive. One form that the core can take is to become a neutron star. During the collapse, protons and electrons and forced together where they transmute into neutrons. A neutron star has more than the mass of the sun, but has been collapsed into the size of a small city. Because of the conservation of angular momentum, a neutron star can rotate as fast as thousands of times per second.

Because neutron stars are so small, it might be thought that they would be impossible to observe. But the discovery was accidently made in 1967 by Jocelyn Bell, a graduate student of Cambridge Unviersity, who observed rapid radio pulses coming from a place in space. Originally they were thought to be signs of extraterrestrial life, and were dubbed LGM's (Little Green Men). Eventually it was discovered that a rapidly rotating object could emit such pulses, and the objects become known as pulsars, which are the observational examples of neutron stars. There is a pulsar located inside the Crab Nebula.

Black Holes

Neutron stars cannot exist above about 3 solar masses. If the mass of what remains in the core of a star after a supernova is greater than this, gravity will cause the object to collapse. This time there is no known force that can stop it, and the object will collapse until the radius becomes zero. This object is known as a black hole, which has several suns worth of mass, but no size. A black hole has an escape velocity that is higher than the speed of light. If you could stand on a black hole with a flashlight, the light would fall back down.

The fate of other stars

For objects with masses between about 0.1 and 0.4 suns, the graviational force is strong enough to cause the interiors to heat so that they will become main sequence stars. Once they exhaust their hydrogen fuel in their cores, they contract due to gravity, but never get hot enough to fuse helium. As a result, they shrink down and become white dwarfs. White dwarfs are small, faint objects that slowly radiate away all of their energy at which time they will become black dwarfs, the burned out cinders of stars.

For objects with masses between about 0.4 and 4 suns, the stars will fuse hydrogen into helium as main sequence stars. They will contract and heat enough to fuse helium,which causes the gas pressure to rise. The stars expand into red giants.

The Ring Nebula

Planetary Nebulae

During the red giant phase, the outer parts of the star expand away from the core. The outer shell becomes a planetary nebula, named because it resembled a planetary disk as seen through a small telescope. The example is the Ring Nebula. At the center of the plaentary nebula is an object that will shrink into a white dwarf. This will be the fate of the Sun. Image by Adam Block/NOAO/AURA/NSF.