I wonder how many new astronomers consider stars only as tools to help locate the really spectacular celestial objects?  After all most of us grew up gazing at the heavens filled with stars using only our naked eyes, and once we can identify many constellations, perhaps the novelty wears off.  By the time we purchase our first scopes, stars take on a different attraction in the form of open clusters, or globular clusters who seem to congregate together in close knit stellar communities.  What about all those renegade stars who live a life of solitude far from others of their kind?  Herein lies the fascination of stars as each has a life cycle that carries them on many common and divergent paths from birth to death.

This life cycle begins in an interstellar cloud of gas and dust.  Our galaxy and the universe in general contain huge pockets of such clouds that serve as stellar nurseries.  One such stellar nursery is M16 (Eagle nebula).  These clouds contain mostly hydrogen and must have sufficient mass to be able to contract gravitationally.  It is unknown what triggers a gravitational collapse in these clouds (near by supernova shock waves?), but something prompts a collapse and the process of a new star begins.  As the gravitational force is greatest at the centre of the cloud, material at the centre collapses faster (becomes more dense) than at the edges of the cloud.  With this increase of density at the centre, the cloud collapses faster and grows in density, thus the cloud collapses even faster.  During this process hydrogen molecules gain kinetic energy and begin to bang into one another and into the surrounding bits of dust.  These collisions cause the dust to clump together and generates heat in the form of infrared radiation.  At a certain point the mass becomes dense enough to trap this radiation and the core of the cloud is called a protostar.

M16 by Brad Wallis and Robert Provin 

A protostar is like an infant in its mother's womb as the bits of dust envelope the core and block most energy from escaping.  With more mass being drawn into the cloud's core from the cloud edges, a shock wave forms and more heat is produced.   At about 2000° Kelvin the hydrogen molecules break up and, in the process, begin to absorb heat.  Gravitational energy takes over and heats the core until the core becomes hot enough to support nuclear fusion reactions and a star is born.  The enveloping dust bits that have served as a womb are now either drawn into the young star or are blown off allowing the star to emit light and energy.  This birthing process takes place over a period of 50 million years.

Mass now become the defining characteristic of this new star.  If the mass is less than 0.08 solar masses, it fails to reach the 10 million degrees Kelvin required to sustain thermonuclear fusion (Jupiter is possibly an example of this process).   If the mass is greater than 100 solar masses, the star becomes unstable (force of outward gas pressure exceeds inward pressure of gravity) and explodes apart.  If the mass is between these two extremes (our sun = 1 solar mass), a stable main-sequence star can form.  When the star is able to obtain most of its energy from thermonuclear reactions rather than gravitational collapse, it is called a zero-age main sequence star and settles down to the longest stage of its life cycle.

A star on the main sequence is really a ball of hot gas that is at its essence a hydrogen reactor.  Hydrogen is converted into helium when two hydrogen protons (H¹)collide with enough energy (at a temperature of at least 8 million degrees Kelvin) that they stick together forming  heavy hydrogen (H²) that is composed of a proton and a neutron. As part of this collision a positron (positively charged electron)is released which collides with an electron(negatively charged) which causes mutual annihilation and a gamma ray is released.  The heavy hydrogen molecule then collides with another proton and forms light helium (He³) and another gamma ray.  If the cycle continues another light helium will be produced which combines with the other light helium to produce normal helium (He4) plus two more protons and a gamma ray.  The net result of this reaction is the release of helium and energy.   This energy that is released is spelled out in Einstein's special theory of relativity

E = mc²

where E is the energy (in joules) released in the conversion of mass m (in kilograms), and c is the speed of light (in m/sec).  One joule of energy per second is one watt of power.  Translated this means that c² is a large number that can produce allot of energy from a small amount of mass.  For example 1000kg of matter can release enough energy to supply the needs of the entire country of the United States for one year.  This process is like a slowly exploding hydrogen bomb with gravitational forces keeping the star from flying apart. 

   Where the star ends up on the main sequence depends on its mass.  The more mass a star has the hotter it burns and the more luminous it becomes and vice versa.  The main sequence (position 1) as shown below in the Hertzsprung-Russell (H-R)diagram represents a series of stars of decreasing mass from the upper left-hand corner O-stars, to the lower right-hand corner M-stars.  Massive stars have higher core temperatures that burn faster than low mass stars that burn slower.  Thus massive stars spend less time on the main sequence even though they have more fuel to burn to begin with.  Our sun at 1 solar mass, spends 80% of its time on the main sequence as it slowly transforms its hydrogen core to helium (life span of ~500 million years).   A 15 solar mass star would convert its mass to energy ~25,000 times faster than our sun and would spend a shorter period on the main sequence, thus have a shorter life.

When the last of the hydrogen in a 1 solar mass star's core is used up, thermonuclear reactions cease there and the core begins to heat up and contract (position 2).   Reactions of hydrogen in the star's outer layers speed up producing more energy and light and the star expands in size and is called a red giant (position 3).  Meanwhile the core continues to contract and get even hotter.  When the core gets hot enough it starts to burn helium until it is out of control and a helium flash occurs (lasts only minutes).   After this, the star decreases in size and dims a little and it moves downward and to the left (position 4) on the H-R diagram (thus it leaves the main sequence becoming dimmer and smaller with time). 

Eventually the burning of helium converts the star's core to carbon and the reaction stops in all but the star's outer layers and the star becomes a red giant again as it expands (position 5).  This time the red giant becomes unstable and gravity takes over and causes the star's core to contract releasing huge amounts of energy which again causes the star's outer layers to expand (position 6).  The star pulsates rapidly with growing vigor until a final violent pulsation rips off the cool outer layers and a hot core is left behind.  The outer layers form a nebula that expands away from the core until it dissipates into the void of space (M57 the ring nebula is an example of this process).  The star's core contracts and is unable to achieve a high enough temperature to burn carbon, thus over a period of 75,000 years  the star shrinks to become a white dwarf (position 7).  Without energy to burn the star cools to become an invisible black dwarf.  Larger mass stars (>5 solar masses) behave differently and do reach temperatures sufficient to burn their carbon cores which can result in a carbon flash that can blow the star apart and a supernova explosion signals the death of this massive star.

M57

Attributes of a star that amateur astronomers need to be aware of can be divided into magnitude, and color spectra.  The magnitude scale we now use has evolved from a system of measurement created by Hipparchus a second-century B.C. mathematician, philosopher, and astronomer.  Hipparchus cataloged stars by their apparent magnitudes by rating the brightest star he could see as magnitude 1 and the faintest as magnitude 6.  As this system evolved some stars were found to be brighter than magnitude 1 and were added to the scale.  For example, Vega is magnitude 0, and Sirius is magnitude -1.4.  The first thing to note is that the larger the negative magnitude of an object, the brighter it appears.  The larger the positive magnitude, the dimmer the object appears.  Also this scale is not proportional, which is to say that a 6th magnitude star is not 6 times dimmer than a 0 magnitude star.  This all relates to how the eye perceive brightness.  For example, a 200W light bulb does not appear to be twice as bright as a 100W light bulb.  But the difference in brightnes between a 200W light bulb and a 400W light bulb will appear to be the same as it was for  the 100W and 200W light bulbs.  The eye senses equal ratios of brightness as equal differences.  Thus, a difference of 5 magnitudes corresponds to a brightness ratio of 100 (1 magnitude equals a difference of 2.512).

Thus we can see from the table below a full moon at a magnitude of -10 is 10,000 times brighter in appearance than the star Vega at a magnitude of 0 rather than being 10 times brighter.

The spectral type of a star can tell us allot about a star, such as its color, size, stage of evolution, and how it compares to other stars.  In a way, the spectral type can give us a biography on the history and possible futures for a given star.  The H-R diagram can help us out here as the star's spectra is defined by its temperature and its luminosity.  The previous H-R diagram was used to demonstrate the life cycle of a 1 solar mass star.  The H-R diagram below is used to show the relationship between the star's temperature and its luminosity.  If you remember, the more mass a star has, the faster its nuclear reactions occur related  to both higher gravity and higher temperatures generated.  Thus, the more luminous the star becomes.   On the H-R diagram below you will notice that stars on the upper left are more massive, brighter and hotter (blue spectra) than stars on the bottom right (red spectra).   Our sun is located some where in the middle.  This diagram is intended to give you a feel for the difference between stars of different spectral classes.

The classification of stars spectral type runs O, B, A, F, G, K, M.   O stars are the brightest and hottest stars, and M stars being the dimmest and coolest stars.  The easiest way to remember this progression is via a mnemonic:   Oh Be A Fine Girl Kiss Me.  These letters can further be subdivided into letter /number combinations from 0 to 9.  A spectrum whose appearance placed it halfway between standard B0 and A0 stars would be called a B5 star.  Although this is the primary classification of spectra it is not the only one.  Below you will find a table of other spectral codes that you might see used to describe a star.

Our Sun's spectral code is G2V, that tells us that as a star it is near the middle of all stars for both brightness and color, it is also a dwarf.  Epsilon Ursae Majoris in the Big Dipper is type A0p IV:(CrEu), this tells us that the star is a subgiant A type star with strong emission lines indicating the elements chromium and europium.  The colon indicates the uncertainty in the IV luminosity class.

Although this page is a tad long, it is hoped that increased understanding of stars will allow all to perceive the general story of the life and times of a star.   Next time you are using the stars to hop to one object or another, stop to smell the roses, or rather, see the spectra.