Astronomers have always been fascinated by the different sizes and colors of stars that they observed. In 1817 an German instrument maker named Joseph Fraunhofer attached a spectroscope to a telescope and pointed it at the stars. He found that different stars have different absorption lines in their spectra. At first astronomers did not understand why different stars would have different absoprtion lines. Nevertheless in the early 1900s, a team of astronomers at Harvard College Observatory started a project to examine the spectra of hundreds of thousands of stars. They wanted to develop a detailed spectral classification system based on the absorption lines they were seeing. They adapted an existing spectral class system which had assigned stars a letter from A to O based on the strength of Balmer series absorption lines.

The new system reordered the classes into the order OBAFGKM where O stars are the hottest and each successive class is cooler with M being the coolest stars. Each letter was also divided into tenths of the range by adding a number 0-9 to the end. O stars are the least common and M are the most common found in the main sequence of stars. Stars near the beginning or end of their lives are not part of this classification. The new system of classification was published in the 1920s and included 225,300 stars. It was called the Henry Draper Catalogue because the funding for the project had been provided by Henry Draper.

Much of the work on the project was done by Annie Jump Cannon, Williamina Fleming and Antonia Maury and Edward Pickering.

The spectral sequence they developed is summarized in the table below:

Having the information about spectral types was useful, but astronomers wanted to look for trends in the data. In 1911 Ejnar Hertzsprung plotted the absolute magnitude of stars against their colors. Two years later Henry Norris Russell independently did a similar graph using spectral types. Graphs of this type are known as Hertzsprung-Russell diagrams or H-R diagrams.

The most surprising thing about the H-R diagram was that the stars were not randomly scattered on it, but clustered in certain regions and along certain lines. The band that stretches across the diagram includes 90% of the stars in the night sky. This band is called the main sequence stars. The stars clustered at the upper right of the diagram include about 1% of the stars on the diagram, and are called giants and supergiants. Because of their cooler temperatures, they must be large to be as luminous as they are. The stars in the lower left of the diagram are called white dwarfs. They are very hot, but their luminosities are low, so they must be small. They make up about 9% of the stars on the diagram.

The interactive H-R diagram below and our new Star In A Box app both show the life cycle of a star. The interactive H-R diagram is for a star with the same mass as the Sun, and shows how it forms and where it is on the H-R diagram at various stages of its life. The Star In A Box allows you to choose the initial mass of a star and follow it through its life.

Explore the lifecycle of a star similar to the Sun.

Stars begin to form from clouds of gas in space.  The cold temperatures and high densities (compared to elsewhere in space, but would be considered a vacuum on Earth) of these clouds allow gravity to overcome thermal pressure and start the gravitational collapse that will form a star.

A protostar looks like a star but its core is not yet hot enough for fusion to take place. The luminosity comes exclusively from the heating of the protostar as it contracts. Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum.

Sometimes the formation of stars can be encouraged or sped up by disturbances in the gas clouds that compress the gas such as other nearby stars or supernovae. 

As the cloud collapses, is begins to spin and by the time a protostar is formed, the cloud flattens and there is a protostellar disk spinning around the protostar. These disks probably slow the rotation of the protostar, and sometimes coalesce into planetary systems.

As the protostar rotates, it generates a strong magnetic field. The magnetic field also generates a strong protostellar wind, which is an outward flow of particles into space. Many protostars also send out high-speed streams or jets of gas into space. Usually there are two jets flowing out along the rotation axis of the protostar. Eventually the wind and the jets clear away the extra gas around the protostar and allow the protostar to come into view.

A protostar becomes a main sequence star when its core temperature exceeds 10 million K. This is the temperature needed for hydrogen fusion to operate efficiently.

The length of time all of this takes depends on the mass of the star. The more massive the star, the faster everything happens. Collapse into a star like our Sun takes about 50 million years. The collapse of a very high mass protostar might take only a million years. Smaller stars can take more than a hundred million years to form.

In a newly formed star cluster, there are many more stars with low masses than stars with high masses. For every star with a mass between 10 and 100 solar masses, there are typically 10 stars with masses between 2 and 10 solar masses, 50 stars with masses between 0.5 and 2 solar masses, and a few hundred stars with less than 0.5 solar masses. As time passes the balance shifts even more toward smaller stars because the higher mass ones die first.

Stars above about 200 solar masses generate power so furiously that gravity cannot contain their internal pressure. These stars blow themselves apart and do not exist for long if at all.

A protostar with less than 0.08 solar masses never reaches the 10 million K temperature needed for efficient hydrogen fusion. These result in “failed stars” called brown dwarfs which radiate mainly in the infrared and look deep red in color. They are very dim and difficult to detect, but there might be many of them, and in fact they might outnumber other stars in the universe.

Main Sequence

Low mass stars spend billions of years fusing hydrogen to helium in their cores via the proton-proton chain. They usually have a convection zone, and the activity of the convection zone determines if the star has activity similar to the sunspot cycle on our Sun. Some small stars have very deep convection zones. Some of these stars also rotate very quickly which twists their magnetics fields. When these field lines line up, the result can be a flare of radiation including X rays.

Over its lifetime, a low mass star consumes its core hydrogen and converts it into helium. The core shrinks and heats up gradually and the star gradually becomes more luminous. Eventually nuclear fusion exhausts all the hydrogen in the star's core.

A star's lifetime is proportional to its mass divided by its luminosity t ∝ M/L. A star's luminosity is roughly proportional to the 3.5 power of its mass so L ∝ M^3.5. Substituting t ∝ 1/M^2.5 where t is the Sun's main sequence lifetime, a star with a mass 4 times the Sun's would have a lifetime of 1/4^2.5 or 1/32 solar lifetimes.

Red Giant

When hydrogen fusion can no longer happen in the core, gravity begins to collapse the core again. The star's outer layers expand while the core is shrinking and as the expansion continues, the luminosity begins to increase. For a star with the mass of the sun, this expansion takes about a billion years and the star's radius increases 100 times, and its luminosity increases even more. The star is called a red giant. A hydrogen burning shell forms around the helium core, and she shell contributes more and more helium to the core over time.

Eventually the core becomes hotter and denser and reaches a temperature of 100 million K, and helium nuclei begin to fuse into carbon. The helium fusion then heats the core rapidly even more and  a helium flash takes place. This causes the core to expand, which lowers the temperature of the core and reduces the total energy output from what it was during the red giant phase. The outer layers then contract and the star's temperature increases a bit.

After about 100 million years, the star fuses all its core helium into carbon. Then a helium fusion shell forms around this core, and the hydrogen fusion shell remains around that. It then becomes a red giant again and remains like this for a few million years with its outer layers continuing to expand.

Planetary Nebula

Eventually gravity can no longer contain the outer layers of the red giant and the star ejects these layers into space. The remaining carbon core is still very hot and emits ultraviolet radiation that ionizes the gas in the expanding shell and makes it glow brightly. This glowing gas is called a planetary nebula, but has nothing to do with planets. Planetary nebulae are relatively common and astronomers estimate that there are between 20,000 and 50,000 in our galaxy. Planetary nebulae often have elongated shapes. One theory is that the star first ejects a doughnut shaped cloud of gas and dust from its equator, then ejects gas from the entire surface. The doughnut blocks some of this ejection and it is channeled in two opposite directions. As the core cools, the glowing gas fades and disperses and the nebula disappears within a million years or so.

White Dwarf

The cooling carbon core is all that is left. At first its surface temperature is around 100,000 K and emits ultraviolet radiation which ionizes the gas in the nebula and makes it glow. The cooling core is called a white dwarf, and eventually can no longer be seen and is then called a black dwarf. The matter in a white dwarf is very dense about 10⁹ kg/m³, which is a million times denser than water. A teaspoonful of white dwarf matter if brought to Earth would weigh about 5 tons!

High mass stars go through a similar process to low mass stars in the beginning, except that it all happens much faster. They have a hydrogen fusion core, but much of the hydrogen fusion happens via the CNO cycle. After the hydrogen is exhausted, like low mass stars, a helium core with a hydrogen shell forms, then a carbon core, with helium and hydrogen shells. Then unlike low mass stars, they have enough mass that gravity contracts the core raising the temperature and carbon can fuse into neon, then neon into oxygen, then oxygen into silicon, then iron. Each stage of burning lasts a shorter time than the previous one. For example, in a 25 solar mass star, hydrogen burning would take about 7 × 10⁶ years, helium burning 7 × 10⁵ years, carbon burning, 600 years, neon burning 1 year, oxygen burning 6 months and silicon burning one day.

Supernova

Iron cannot release energy by  fusion because it reuqires a larger input of energy than it releases. So the iron core continues to be subjected to gravity, which pushes the electrons closer to the nuclei than the quantum limit allows, and they disappear by combining with protons to form neutrons, giving off neutrinos in the process. Once this process starts, in a fraction of a second, an iron core the size of the earth and with a mass like our Sun, collapses into a ball of neutrons a few kilometers across. This gravitational collapse releases an enormous amount of energy (more than 100 times what our Sun will radiate over its entire 10 billion year lifetime). This energy blows the outer layers of the star off into space in a giant explosion called a supernova. The ball of neutrons left behind is called a neutron star and is incredibly dense. In some cases the remaining mass is large enough that gravity continues to collapse the core until it becomes a black hole.

The explosion sends a shock wave of the star's former surface zooming out at a speed of 10,000 km/s, and heating it so it shines brilliantly for about a week. This shock wave compresses the material it passes through and is the only place where many elements such as zinc, silver, tin, gold, mercury, lead and uranium are produced. Over several months the gases cool and fade in brightness and join the debris of interstellar space. This debris has in it all of the elements that were created in the star's core. Millions or billions of years later, this debris may be incorporated into new stars. The fact that the Earth contains elements that are produced only in supernovae is evidence that our solar system, planet and bodies contain material that was produced long ago by a supernova.

The crab nebula is a remnant from a supernova that went off in 1054 A.D. When Betelgeuse explodes as a supernova it will be more than 10 times brighter than the full moon in our sky. It is only 640 light years away, and could have already become a supernova, but the light from it just hasn't reached us yet.

Supernovae occur in stars with at least 8 solar masses.

A star is called a variable star if its apparent brightness as seen from Earth changes over time. There are two basic types of variable stars: intrinsic variables, whose luminosity actually changes, and extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth. A star could be an intrinsic variable because it periodically swells and shrinks. A star could be an extrinsic variable because it has an orbiting companion that sometimes eclipses it.

Intrinsic Variables:

Pulsating variable stars:

There are many different types of pulsating variable stars. Some of them vary in brightness by as much as 100 times, and some have cycles that repeat as often as every few days, while others vary over months or years. In most cases these stars pulsate because they are at the end of their lives and have become unstable.

Eruptive variable stars:

These stars are more likely to have very irregular cycles. They include protostars, which in the process of becoming main sequence stars, often have variations in their brightness. Giants and supergiants lose their matter relatively easily and may also experience eruptions. White dwarfs that are part of a binary system may also experience eruptions as they take matter from their companion star.

Extrinsic Variables:

About half the visible stars are not isolated, they are part of multiple star systems. Pairs of stars that orbit each other are called binary stars or binaries. Observing how binaries orbit each other gives astronomers information about their masses.

Spectroscopy makes it possible to study binary star systems where the two stars are close together. Sometimes a binary system is so far from us, and the stars are so close together, that from Earth even with a telescope, we would only be able to see it as one star. Spectroscopy can tell astronomers the composition of the surface of a star, and if a star has absorption lines of elements that would normally not appear in a singe star, they know that there are two different star types orbiting each other.

Some binary systems are oriented so that the two stars periodically eclipse each other when seen from Earth. In that case the apparent brightness of what appears to be a single star decreases when one star goes behind the other. Astronomers use photometry to measure changes in a star's brightness to study these systems.