The Life of a Star

Stars come in all kinds of sizes, colors, ages, brightness, temperatures, and masses. But all stars are "born" in pretty much the same way. Parts of our Milky Way galaxy is filled with giant clouds of gas called nebulae. A nebulae is a stellar nursery.

Trifid Nebula
Keyhole Nebula
Great Nebula in Orion

The internal pull of gravity eventually caused a collapse of a nebulae. Just as water "beads up" due to the internal attraction of water molecules, gas in a nebula also "beads up" into individual lumps which eventually become stars.

(animation) A perfect example is the nebula known as "The Seven Sisters" or the Pleiades.

Around many of these new stars astronomers are finding disks of debris called proplyds which could very well be planetary systems forming around the central star.

Most of the gas making up these nebulae is hydrogen (about ¾) and helium (about ¼) ... leaving only a few percent for all the other elements on the periodic table.

It is also worth mentioning here that each of these nebula is capable of producing a "litter" of hundreds to thousands of individual stars. Most having a mass less than our sun.

What makes the stars shine?

When Einstein wrote the famous equation E = M c² early last century, he discovered the key to sunshine. Einstein saw that energy and matter were different forms of the same thing (a wild concept even today). In discovering the nature of the atom, scientists realized that nuclear reactions were possible that were able to prove the validity of this famous equation. Nuclear fission generates electricity by splitting up heavier atoms into lighter ones and releasing energy (another example being the atomic bomb). Nuclear fusion combines lighter atoms into heavier ones and also releasing energy. The hydrogen bomb is direct evidence that it can be done.

It is now known that if you can get hydrogen hot enough ... and close enough ... it will fuse into helium. In fact, it takes 4 hydrogen atoms to create 1 helium atom (in a series of complicated reactions). This is the key to understanding the stars. Deep in the interior of young stars, the hydrogen (which makes up most of a star) is hot enough and under enough pressure to fuse into helium. Once this process starts, it can keep going for a very long time (depending on the mass of the star). A "normal" star is a fusion machine in perfect balance. Gravity is trying to pull the star closer together and the pressure from the nuclear reaction trying to push the star apart.

For example, our sun has been fusing hydrogen into helium for about 5 billion years and the gas tank is only half empty. That is, there is enough hydrogen in the core of the sun to keep it going for another 5 billion years (although, in time, the sun will be producing slightly more energy output than it is currently producing).

Approximately 90% of all stars seen are producing energy in the fashion described above. These stars are known as main sequence stars. That is, they are producing energy by fusing hydrogen to helium in their cores. Astronomers say that our sun will have a main sequence lifetime of 10 billion years. However, not all stars are burning their fuel at the same rates. This depends on the mass of the star. High mass stars (up to 50 times the mass of the sun) can go through their supply of fuel in a very short time (by comparison) ... remaining on the main sequence for only 10 million years. Low mass stars have such a long main sequence lifetime that the universe is not old enough for any of them to have run out.

The H-R Diagram

To help with the details, it becomes necessary to introduce you to a famous diagram used in all astronomy classes. It is known as the Hertzsprung-Russell Diagram or H-R diagram for short.

The H-R Diagram

As stated before, stars vary in brightness and temperature. By brightness, we do not refer to the way it looks in the sky but rather the amount of light it emits (rather like comparing the light output of a 50 watt light bulb vs. a 100 watt light bulb). It is useful to compare the light output of a star by comparing it to the sun (with the sun =1). It is interesting to know that some stars give off only 1/10,000 the light of our sun and others put out over 1,000,000 times the light of the sun.

Stars also vary in temperature. All stars are gaseous with several layers in the outer atmosphere that we know of. When we refer to the "surface temperature" of a star, we refer to the layer known as the photosphere (where the sun looks like it ends to our eyes). Our sun's photosphere is about 6,000 K (11,000 F). The range of stellar temperatures is from 3,000 to 40,000 K. Cool stars appear red in color and hot stars appear blue in color. Astronomers give a designation called spectral class to indicate the temperature of stars. The sequence (from hottest to coolest) is : O B A F G K M. this is further subdivided from 0 - 9 ... so a B3 star is slightly hotter than a B4 star. Using this standard, our sun is classified as a G2 star.

The Main Sequence

About 90% of stars lie on the main sequence. Recall that this means they are fusing hydrogen to helium in their cores. When stars are plotted on the H-R diagram, main sequence stars stretch all the way from the upper left corner to the lower right corner in a continuous pattern. Our sun appears around the middle of the main sequence. Not too hot, not too cold, but that does NOT mean our sun is an "average" star. Stars are not distributed evenly along the main sequence. Most stars are main sequence but most main sequence stars appear in the lower right corner of the HR diagram. That is to say, our sun is slightly hotter and slightly more luminous than most stars in the sky.

What causes this distribution of stars along the main sequence? The reason for the dispersion of stars along the main sequence results from the different masses of the individual stars. Stars with a high mass have huge fusion cores and burn their fuel at incredible rates ... making them hot, bright and short lived. Low mass star (the most common by far) burn their fuel at a snail's pace and are therefore cool, dim and live a long, long life on the main sequence.

Mass	Location on H-R	frequency	properties	life span on MS
High	Upper left corner	very rare	hot and luminous (blue)	very short
Medium (sun)	middle	somewhat rare		sun = 10 billion yrs
Low	Lower right corner	very plentiful	cool and dim (red)	very long

What happen when a star runs out of hydrogen to fuse?

The answer depends on the star's mass. Let's go through the life cycle of a star with the mass of the sun.

Death of the Sun

In 5 billion years, the core of our sun will run out of fuel (hydrogen). What happen next? Since there is no longer any radiation pushing outward, the force of gravity will force a contraction at all layers of the sun. This means that not only will the core (now all helium) shrink ... but so will all layers above the core. But this contraction just outside the (former) core is still mostly hydrogen and it will heat to the point where it gets hot enough to start a fusion reaction. This fusion of hydrogen to helium takes place in a shell surrounding the old core of the sun ... giving our sun a new lease on life. However, the sun modifies its equilibrium in such a way that the outer layers are expanded to great distances ... vastly increasing the diameter of the sun. It becomes about 100 time its present size ... becoming a red giant in the process. Red Giants are actually quite rare... making them fewer than 1% of all stars.

A Red Giant (animation)

The bad news for the earth is ... when the sun becomes a red giant ... the earth is vaporized (and will most likely be swallowed up by the sun). The sun will remain a red giant for a few hundred million years (maybe less) until ...

Temperatures within the shrinking helium core become high enough so that even helium fuses. Yes, at high enough temperatures you can actually fuse 3 helium atoms into a carbon atom. This new energy source causes the core to suddenly expand outward ... which pushes on the upper layers ... causing the sun to eject a substantial amount of mass into space. We see this as an object known as a planetary nebula.

Here are some beautiful examples of planetary nebula

Helix Nebula
The Ring Nebula
Cat's Eye Nebula
Butterfly Nebula
Eight Burst Nebula

Each of these images have something in common ... a small object is left behind after this "mini-explosion". It is the remnant exposed core of the red giant. The planetary nebula phase was triggered by a "helium flash" in this core when it underwent fusion. Only this fusion was a fusion of helium to carbon. So this exposed core is a very hot ... dense ball of carbon atoms about the size of the earth. Technically it isn't really a star ... because there is no fusion anywhere in this core, but .... it is still called a star with the name white dwarf. When you heard the nursery rhyme .. Twinkle, twinkle little star ... how I wonder what you are, this object really is a "diamond in the sky". Astronomers are currently finding that about 10% of all "stars" are white dwarfs. The number may be even higher. White dwarf appear on the H-R diagram in the lower left corner.

A white dwarf will slowly cool to a black dwarf.

Death of low mass stars

The universe is not old enough for most stars to evolve off the main sequence. Models predict that these stars will run out of hydrogen fuel in their cores and fade directly into white dwarfs. These stars are the most common ... so we'll just have to wait 35 billion years and see.

Death of high mass stars

Although very rare, high mass stars die in spectacular fashion. And their deaths are extremely important for you and me. After a high mass star uses up its hydrogen fuel in the core (changing it to helium), it simply starts fusing the helium to carbon. At the same time the star initiates a shell of hydrogen to helium fusion directly outside the core. The star reconfigures itself as a Super Giant star on the H-R diagram ... growing to a size over 1000 times that of the sun.

Deep inside a super massive star (animation)

The story of super giants gets even stranger. The carbon core contracts further and reaches high enough temperature to fuse carbon to neon, then neon to oxygen, then oxygen to silicon, then finally silicon to iron. When the iron core reaches a critical mass ... it suddenly collapses ... reaching nuclear densities and superheats. In addition, the shock wave of this event ripples through all the layers above the core ... superheating each layer as well. This produces fusion reactions of all kinds resulting in a supernova explosion. For one brilliant month, a single star burns brighter than a whole galaxy of 200 billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. These heavy elements are ejected into space and eventually become incorporated into future generations of stars and planets. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible. In a very real sense ... we are all made from the death of these massive stars!

Crab Nebula - In 1054 Chinese astronomers reported seeing a "guest star" ... visible even during the daytime. Today we see the result of this explosion as the crab nebula.

Cas A Supernova

Supernova 1987 A - the most recent supernova visible without the aid of a telescope.

Not all supernovae are good ....

Credit: J. Morse (U. Colorado), K. Davidson (U. Minnesota) et al., WFPC2, HST, NASA

If a nearby supernova were to occur, things could get ugly. The picture above is the star Eta Carinae and it about to go supernova anytime. It is close enough to the earth that it could produce massive damage to our planet if it were to explode. Predictions range from minor increases in skin cancer ... all the way to total extinction of life on earth.

Strange supernova by-products

Buried deep within the crab nebula is the remains of the collapsed iron core (which got the explosion going in the first place). Only now, it is compressed down to a ball of solid neutrons only a few miles across. Sometimes known as a neutron star, it is also called a pulsar because it spins at incredible rates ... emitting pulses of energy in the process. The mass of a neutron star is over 1.4 times the mass of the sun!!!! Astronomers believe even more violent stellar explosions (recently labeled hypernova), could leave behind an even more massive core. If it had a mass of about 3 times the mass of the sun, it would continue to collapse .... In fact, the collapse never stops ... so the object collapses down to zero volume (much smaller than an atom)!!!! This a very strange object having the property that it could pull in everything close to itself ... including light. Called a black hole, astronomers are quite certain that they have "found" several examples. Although impossible to actually "see" ... they are detected by observing objects they interact with.

Mass of Original Star	Main Sequence lifetime	How it explodes	What it leaves behind
less than our sun	longer than the universe is old	it doesn't	white dwarf (which cools to a black dwarf)
mass like our sun	about 10 billion years	planetary nebulae	white dwarf (which cools to a black dwarf)
slightly more than the sun (~15x)	a few 100 million years	supernovae	neutron star (if > 1.4 solar masses)
much more than our sun (~70x)	only ~10 million years	hypernovae	black hole (if > ~3 solar masses)