This system of classifying stars is based on luminosity, spectral type, absolute magnitude (star’s radius), and finally surface temperature in kelvin or celsius. The diagram is named after Danish and American astronomers Ejnar Hertzsprung and Henry Russell. The Hertzsprung-Russell (H—R) diagram gives us a view of our sun’s position in it, a standard of one in a main sequence. Once the temperatures of stars were plotted against their luminosities, it has been observed that stars tend to be in groups.
The real cause of the H-R diagram is nuclear fusion, the process that produces energy inside of a star. The place of each star among the diagram is dictated by nuclear reaction intensity and by what type of fuel it is fusing. The rate of nuclear fusion is exponential to the mass of the star. A higher mass means a higher temperature and higher pressure inside the core. The core tends to be in an equilibrium state, between the energy produced and the gravitational force.
So, the rate of nuclear fusion inside the core is correlated with the mass of the star. A more massive star will glow much brighter. In addition, the type of fuel the star is fusing also influences its brightness. Young stars usually don't fuse hydrogen yet, in fact they fuse deuterium, just like Brown Dwarfs. Main sequence stars fuse hydrogen. Red giants and other similar stars fuse helium or heavier elements, while stellar remnants like White dwarfs or Neutron stars no longer sustain fusion reactions.
Basically, any star can be placed somewhere on the H-R diagram. In each place, we will only find stars with similar properties and similar behavior.
Stars are classified based on their spectral class and luminosity class. Also, there are a few additional classes (see below).
Spectral Class Edit
The spectral class is indicated by a letter, followed by a number between 0 and 9.9. In the beginning, each spectral class was identified by the signature of a dominant molecule or atom in the light spectra. Currently, the classification is more related to the surface temperature of a star.
For the vast majority of stars, there are a few spectral classes which are strictly connected to the surface temperature of the star. Based on this, we have:
- O - type stars, they are the hottest stars, their light is blue.
- B - type stars, also very hot, Their light is white - blue.
- A - type stars, still hot and bright, with a white light.
- F - type stars, a bit hotter then the Sun, with yellow - white light.
- G - type stars, surface temperature like the Sun, with yellow light.
- K - type stars, cooler then the Sun, orange in color.
- M - type stars, far cooler then the Sun, their light is red.
In case of objects cooler then a star, like Brown Dwarfs, there are three additional spectral classes:
- L - type dwarfs, their light should be dark red.
- T - type dwarfs, their light is dimmer.
- Y - type dwarfs, they are too cool to produce visible light.
In addition, there are a few rare spectral classes:
Carbon stars are stars with much carbon in their photosphere. They also can be linked, based on their surface temperature, to a corresponding spectral class, in the h-r diagram.
White dwarfs have their own class, D. Depending on surface temperature, white dwarfs vary between D0 and D9.9. It is possible to make a correlation between classic stars and white dwarfs. A D2 white dwarf will have the same surface temperature with a B2 star. A D7 dwarf will have surface temperature like a F8 star.
White dwarfs end-up as Black Dwarfs. As this process happens, an additional spectral class will be needed, to list dwarfs that would have a surface temperature similar to M - type stars or brown dwarfs.
Neutron stars are a distinct category. Their surface is hotter then O - type stars (and this places them as beyond blue in the spectra), but because they are very small, their light is very little, placing them below white dwarfs. These stars can be placed somewhere, in the bottom-left corner of the H-R diagram.
Invisible objects like black holes, planets or Frozen stars can fit somewhere in the Y class. Also, planets that are hot enough to produce their own light can fit in a certain class. One way of creating an Artificial sun (a fake one, unable to sustain nuclear reactions) is to heat a giant planet up to a high enough temperature to become luminous.
Wolf - Rayet Stars are very hot stellar objects who are losing matter fast. Scientists group them as a single category, but, based on their surface temperature, they could fit into the O - type, sometimes into the B - type and most often in a hotter category then O - type stars.
Primordial Stars were far larger and brighter then stars that exist today. The smallest of them would fit around O0 - O2 spectral class, while the largest, would be in a hotter category.
Young stars are placed a bit right and up from the place where they will be when they will start fusing hydrogen.
Luminosity Class Edit
Luminosity types are listed with a number, written in Latin alphabet. There are stars There are stars that fit into a subcategory or are somewhere between categories. What is interesting, is that each type corresponds to a different type of fusion inside the star.
Based on luminosity classes, stars are known as:
- Class 0 - super-supergiants
- Class I - bright supergiants
- Class II - supergiants
- Class III - giants
- Class IV - subgiants
- Class V - dwarfs - also known as main sequence
- Class VI - subdwarfs.
There are many stellar remnants (white dwarfs and neutron stars) as well as stars expected to form in the future, which fell below VI luminosity class.
Class 0 - super-supergiants forms a horizontal line, starting from the place hosting main sequence O - type stars. These stars are rare and short lived. they fast evolve into Wolf - Rayet Stars, as radiation pressure overwhelms gravity and they start expelling their outer layers.
Class I - bright supergiants. They form a horizontal line, between classes 0 and II and are made of aged, very large, massive stars, that are fusing helium or heavier elements inside their cores.
Class II - supergiants. All stars in this category are aged stars, which had exhausted hydrogen and are fusing helium or heavier elements inside their cores.
Class III - giants. These stars form a special category. They are aged stars, with similar masses like the Sun, that are fusing helium.
Class IV - subgiants. Such stars had exhausted hydrogen in their cores and are fusing in a shell surrounding the core. Their cores are not massive enough to start helium fusion. It is a transitory state between classes V and III.
Class V - dwarfs.
The main luminosity class, inhabited by most of stars, is called the Main Sequence. Here stars spend most of their lifetimes and here one will find the highest number of stars in the Universe.
Class V stars fuse hydrogen in their cores and hydrogen fusion powers them for 90% of their lifetime. On the H-R diagram, they form a diagonal, stretching from top-left to bottom-right.
These stars are often called dwarfs. For stars in bottom right, the fade and small M - type stars, there is another term: red dwarfs. K - type stars are also called orange dwarfs and G - type stars, like the Sun, are known as yellow dwarfs. However, the bright and massive O and B stars are not often known as dwarfs.
Class VI - subdwarfs. These stars have a higher metallicity, which works as an insulation, decreasing the loss of energy and surface temperature. Also, part of their light is released in a spectra not visible to the human eye, making them appear less bright.
In the future, when the Universe will contain higher amounts of heavier elements (known in Astronomy as metals), there will be the new class VII and class VIII. The dimmest objects in this final class, known as frozen stars, will have surface temperature around water freezing point. However, massive stars, if formed, would be somewhere below O - type stars in class V - main sequence. See Stars In An Aged Universe for details.
Stellar remnants, like white dwarfs and neutron stars, are not listed among luminosity classes. Still, they can be plotted along the H-R diagram. White dwarfs and future black dwarfs could be linked into class IX, while neutron stars could fit into class X.
Before everything, we must acknowledge that human civilization is extremely young compared to the life of a star. All what we know is based on observation of various stars and theoretical models.
Our current understanding is that the whole evolutionary path of a star can be tracked on the H-R diagram. A star forms somewhere up-right from its place on the main sequence. Then, it spends most of its lifetime on the main sequence, then, depending on mass, it will move away, left or right from the main sequence, it will die and left behind a stellar remnant that will also evolve.
Small stars Edit
Objects too small to sustain fusion (like giant planets) are hot when they form and might belong to M or Y classes. This is why they are sometimes called sub brown dwarfs. However, they will not sustain deuterium fusion and will cool down and become like planets.
Brown Dwarfs are formed brighter, like M - type stars, but unable to sustain hydrogen fusion, they gradually cool down. Deuterium fusion keeps them hot for a while, then they keep on cooling, until they are cold and look like planets. In the end, they start to contract. Large brown dwarfs are expected to have enough mass to become hydrogen White dwarfs, which further cool down into Black Dwarfs. Smaller brown dwarfs will remain cold and looking like planets.
M8.5 and smaller M - type stars have very long lifetimes. Their solar winds can, over a long timescale, remove a significant part of their mass, until they are no longer able to fuse hydrogen. These stars will then behave like brown dwarfs, will pass the Y, L and T spectral classes and then will contract, to become white dwarfs and finally black dwarfs.
M4.5 to M8 M - type stars are formed as a bit brighter and redder stars. Then, they spend their long lifetimes on the main sequence. Finally, when they start to run out of hydrogen, they increase fusion rate, as their cores contract. These stars will heat-up and become Blue Dwarfs. In the end, they also finish all their hydrogen and gradually compress until they become helium White dwarfs, which further cool down and become black dwarfs.
M0 to M4 M - type stars are born a bit brighter and cooler (spectral class IV). They spend most of their lifetimes on the main sequence. As they start exhausting hydrogen, they will increase fusion rate and will become subgiants (so, back to spectral class IV). Still, they don't have enough mass to start helium fusion. So, they will then start to contract, until they will become helium white dwarfs and after a long timescale black dwarfs. However, it is possible that as some point helium fusion will start in a powerful flash. This will make the white dwarf expand into something similar to Blue Dwarfs, then contract back into a helium white dwarf.
Average size stars Edit
K - type stars have a different lifetime. As young stars, they are formed to the corresponding spectral class M0-M4 and luminosity class IV. They spend most of their lifetimes on the main sequence. Then, they move upwards on the H-R diagram, becoming again class IV - subgiants, fusing hydrogen in a shell surrounding their cores. Helium fusion starts as a powerful blast that consumes all helium in the core, without making the star explode. The star will expand in what is known as the blue loop. Such stars belong to luminosity class III and are large like red giants. In case of former K - type stars, the blue loop will last long, but the star will not go too far on the H-R diagram from its previous position. After the star returns from the blue loop, it remains red (spectral class M) and big (luminosity class III). Then, helium fusion starts in a shell, in violent bursts that occur once every 10000 years or so. These helium bursts will expel the outer layers of the star into a planetary nebula. However, after only a few bursts, the outer layers are expelled and the star contracts into a white dwarf, composed of carbon, with very little oxygen. The formed white dwarf will have nearly the same mass the star once had. Carbon white dwarfs are more susceptible to explode as a supernova then carbon - oxygen dwarfs. The explosion will leave no stellar remnant.
G - type stars, like our Sun, are formed a bit up-right on the H-Z diagram. For the Sun, which is a G2 star, the forming position as a young star is G8, luminosity class IV. They spend most of their lifetime on the main sequence, then, as they start fusing hydrogen in a core and become subgiants, they go back to the place they were formed and even further, into the K spectral class. Core helium fusion pulls them into the blue loop, left on the H-R diagram. Then, the star expands gets redder, reaching its place as a giant (spectral class M, luminosity class III). Helium fusion then occurs in powerful flashes, in a shell surrounding the core. It is the helium flashes that expel the outer layers of the star, reducing its mass from 1 to 0.5 Solar masses and forming a planetary nebula around. When there is not enough helium or hydrogen, no more fusion occurs. Without the surrounding envelope, only the compressed carbon - oxygen core remains and a white dwarf is formed. The white dwarf then cools down and becomes a black dwarf.
F - type stars have a similar fate like G - type stars. The major difference is that the blue loop last less time. Also, since these stars are more massive, they will have more shell helium flares. The resulting white dwarf will also have a mass around 0.5 Solar masses and will have a higher concentration of oxygen and smaller of carbon.
A - type stars are a bit different. First of all, they are formed faster. As Young stars, they migrate to their position in the H-R diagram quickly. When they clear their surroundings of gas and dust, they already are on the or very close to the main sequence. They spend less time on the main sequence, as they fuse all hydrogen faster. Then, when hydrogen is exhausted in their cores, they start fusing hydrogen in a shell and become subgiants. This phase is short. When core helium fusion starts, they pass the blue loop very fast, without getting too far left on the H-R diagram. They become red giants for longer then former G - type stars. Many more helium flashes are needed to expel their outer envelope. Helium flashes occur at smaller intervals of time. In the end, a white dwarf remains, composed mostly of oxygen, with traces of carbon.
Large stars Edit
B - type stars, depending on their size, follow different paths. These stars are formed very fast. When their Stellar Wind finally clears the surroundings and we can see them, they are always on the main sequence. Also, their life in the main sequence is short.
Very dim, B8.5 to B9.9 stars follow a different path. After spending most of their lifetimes in the main sequence (luminosity class V), they run fast through the subgiant (IV) phase. Helium fusion does not occur in a powerful flash, but rather continuously. These stars become orange (spectral class K) or red (spectral class M) and climb to luminosity classes III and II. However, they are not massive enough and are unable to fuse something heavier then helium. When helium is exhausted in the core, they continue to fuse helium in a shell surrounding the core, releasing a high amount of energy and expelling the outer layers into space. Unlike smaller stars, helium bursts don't occur in flashes, but in a more continuous flame. After the outer layers are expelled, what remains from these stars are White dwarfs, which slowly cool down into Black Dwarfs or explode in supernovas.
B4 to B8 stars have a different evolution pattern. They fuse hydrogen when they are in the main sequence and continue fusing hydrogen as subgiants. As giants, they fuse helium in a continuous way, accumulating a high amount of carbon in their cores. Neutrino radiation cools the core, preventing carbon from starting fusion. So, these stars evolve to spectral class M and luminosity class III. Then, at some point, carbon fusion starts off-center, at the border of the carbon core. Because the core is highly pressurized, carbon fusion occurs very fast, in what is called carbon detonation. The powerful flash might be considered a small supernova and expels the outer layers into cosmos. What remain from these explosions are White dwarfs composed of sodium, neon and magnesium (fusion products of carbon) and some oxygen (produced from helium fusion). These white dwarfs are around 1 Solar masses weight and slowly cool into Black Dwarfs. Since temperature needed for neon to fuse is much higher, the chance for a supernova to occur later is very small. The main risk is that these massive white dwarfs can accrete matter and when their mass exceeds 1.4 Solar masses they can collapse into Neutron stars. Sirius B is considered to be a the remnant of a B5 star.
B0 to B3.5 stars are able to fuse carbon in their cores without exploding. They pass the subgiant phase, then become red supergiants (spectral class M or K, luminosity class II). These stars lose a significant amount of matter in their supergiant stages, but far less matter then the brighter and more massive O - type stars. In their cores, they fuse carbon, then neon, oxygen, magnesium and finally silicon, until they can no longer produce energy and collapse into Neutron stars after a supernova explosion. Betelgeuse is supposed to be an evolved B0 star. On the outside, red supergiants formed from B - type stars are variable stars, with multiple and complex cycles. On the inside, not related to what we see on the outside, during their last stages of fusion, the core expands and contracts gradually, as fusion of heavier elements occurs in bursts. However, there is no way from the outside to see what is happening in the inside.
O - type stars exhaust their fuel very fast. Sometimes, they don't have time to clear the surrounding space of gas and dust until they leave the main sequence. So, it is impossible to know how they look as Young stars. Depending on their mass, they can follow very different paths.
Small O - type stars (O8.5 to O9.5) leave the main sequence when hydrogen gets exhausted in their cores. While fusing helium, they become red supergiants (spectral class K or M, luminosity class II). They remain like this, very large, until they exhaust helium in their cores. While fusing carbon, which produces more heat, they can expel their outer layers of gas and reveal hotter layers. This makes them change color into blue a few hundred years before going supernova. They leave behind a neutron star.
Average O - type stars (O7.5 to O8) have a different life. While fusing helium, they can expel a part of their outer layer, revealing a hotter, blue layer, which then cools. The process repeats several times, making the star change color between red and blue. They usually go supernova while they are blue. Their supernovas leave behind Neutron stars. However, the neutron stars continue to accrete matter from the supernova nebula. If they pass a critical mass, they will collapse into a black hole. In case of larger O - type stars (O7), a black hole can be directly created during core collapse and supernova
Larger O - type stars (O4.5 to O6.5) produce too much energy during helium fusion. They evolve into blue supergiants (luminosity class I, spectral class O or B). Smaller stars continue to evolve into red supergiants (luminosity class I, spectral class K). As they increase in size, surface gravity decreases until gravity equals light radiation pressure. At that point (which is short lived), they become super - supergiants (luminosity class 0). Then, as radiation pressure overwhelms gravity, the outer layers of gas are pushed into space. Such celestial objects, known as Wolf - Rayet Stars, are very hot and have a very powerful Stellar Wind, losing mass fast. However, the loss of matter is not enough to prevent the incoming core collapse, supernova and formation of a black hole. Wolf - Rayet stars are found on the H-R diagram left to the main sequence, left to where the star spent most of its life.
The largest O - type stars that exist today (O2 to O4) have a different fate. They don't change position too much on the H-R diagram. As they start fusing helium, they become Wolf - Rayet Stars without becoming a giant. They fast expel their hydrogen layer, exposing a helium core. In some cases, they also expel the helium layer, revealing a carbon core, a sign that they exhausted helium. However, with all their mass loss, they cannot prevent the supernova that will destroy themselves. In most cases, the resulting celestial body is a black hole. However, stars with a mass between 140 and 250 Solar masses (former O2) will undergo pair instability supernova, which will leave no stellar remnant behind.
Primordial Stars were far more massive then current stars. As they were formed, they stayed in the main sequence for a short time. Some of them would fit along O - type stars, spectral class O0 to O2. Still, others were much brighter, fitting up-left on the H-R diagram. As these stars exhausted hydrogen in their cores, they evolved in different ways.
Smaller stars (with mass below 1000 Solar masses) exploded in very powerful supernovas, creating Black holes. What is interesting about these stars is that as they fused helium, carbon or heavier elements, they remained the same on the outside, because extra heat produced in the core had not enough time to reach surface.
Stars between 1000 and 10000 Solar masses were massive enough to absorb the explosion shock of the forming black hole. They became quasi-stars. These stars were powered not by nuclear fusion, but by matter falling into the black hole. They are expected to be orange (spectral class K) and far larger and more luminous then any other star (luminosity class should be two units above class 0).
Stars larger then 10000 Solar masses are supposed to evolve in a complete different way. Their life on the main sequence is very short. However, when the star tries to fuse helium into carbon or oxygen, the process produces a huge amount of energy in a short time, resulting in a very powerful explosion, known as pair - instability supernova, completely destroying the star without any stellar remnant.
Stars that will be formed in the future (see Stars In An Aged Universe) will be dimmer. On the H-R diagram, most of them will fit in spectral classes M, L, T or even Y and in luminosity classes VII and VIII. However, as Young stars, they will look more similar to current young stars. At the end of their lifetimes, they will also behave like main sequence stars currently behave at the end of their lifetimes.
White dwarfs gradually cool and become Black Dwarfs if nothing happens to them. However, if they accrete matter, they can create temporary explosions (known as nova) or they can explode in a supernova. If they slowly accrete matter and pass over 1.4 Solar masses, they collapse into Neutron stars.
Neutron stars gradually cool if left alone. Their fate is not clearly understood. Neutrons might, over time, disintegrate. In this case, the neutron star can become a quark star, producing a nova or a supernova. Other scientists speculate that neutron stars can explode at some point, producing a small black hole. If they accrete matter, they can increase mass over a certain limit and become Black holes. Still, some of them can remain around for a very long time. Because of their small radiation surface, they mostly cool by neutrino radiation. so, they could still be glowing when nothing will produce any light around.
Black holes can swallow anything, increasing their sizes. However, there is a theory that they slowly lose energy, through Hawking radiation. This theory is not approved by all scientists. If it is true, black holes slowly lose mass as they lose energy.
The H-R diagram is very useful, since it can be used to virtually any star.
For terraforming, it is very important to know the basic parameters of a star. The H-R diagram can be used to pinpoint any existing, past or future star and determine its properties. A terraformer can draw on the H-R diagram which areas host the best stars for a planet suitable for terraforming and which not.
The evolution of a star has some importance for terraforming. Stars pass very slow through some parts of their path on the H-R diagram but very fast through other parts. Pioneers and technicians will ask if their newly terraformed planets are habitable for at least a thousand years.