Hertzsprung-Russell Diagram

Hertzsprung (Ejnar Hertzsprung, 1873-1967, Danish astronomer) and Russel (Henry Norris Russell, 1877-1957, American astronomer), independently, at the beginning of the 20th century, placed, on a diagram, for many stars, on one side, on the horizontal axis, the spectral type of them, on the other side, on the vertical axis, their luminosity relative to the Sun (or absolute magnitude). They verified a correlation between the data. This correlation appeared under the form that the stars sorted into four groups: main sequence stars, supergiants, giants, white dwarfs. This diagram is the Hertzsprung-Russell diagram (see it below). These groups, in the same time, depict a static view of the stars (they sort stars according to such or such type) and the different stages of life of stars.

Bases of Stars' Sorting

Building of the development of spectrography in the second part of the 19th century (Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887), University of Heidelberg; elements of a star appear like lines in their spectrum, their scattered light), to which was added the systematic study (1884-1920s) at the Harvard College University by Annie Jump Cannon (1863-1941), stars have been sorted into 7 spectral classes: O, B, A, F, G, K, M. Strictly speaking, this sorting is not made on the basis of the chemical compounds of stars but on their surface temperature (from the stars with higher surface temperatures, class O, to stars with lower surface temperatures, class M). Spectral discrepancies which had been observed was indeed more the result of differences in temperatures at the surface of the stars than of such or such compounds being in their photosphere. However, some elements of stellar spectra are result of differences in the chemical composition, mainly differences in the so-called heavy elements (elements heavier than hydrogen and helium). Each of these spectral classes is at its turn divided into 10 spectral types; these spectral types are a range of temperatures. Class A, e.g. is divided into spectral types A0, A1, A2, A3, A4, A5, A6, A7, A8, A9. The lower the type, the warmer the surface temperature of the stars. Class O is divided only into the types O4, O5, O6, O7, O8, O9. In 1940, astronomers at the Yerkes Observatory added to sorting into spectral classes and types, an additional sorting: stars of same surface temperature are sorted into luminosity classes: class I is very bright supergiants, class II, bright giants, class III, giants, class IV, sub-giants, class V, main sequence stars, class VI (VII), dwarf stars.

On this basis, stars may so be sorted this way:

- class O stars are stars of which surface temperature lies between 28,000 and 60,000 K; their spectral lines are those of ionized helium, metals and hydrogen (in feeble quantity). These are blue stars. An example of such stars is z of Orion

- class B stars are stars of which surface temperature lies between 10,000 and 28,000 K; their spectral lines are those of neutral helium, ionized metals and hydrogen (in more important quantity). These are blue stars. Examples of such stars are Rigel and Spica

- class A stars are stars of which surface temperature lies between 7,500 and 10,000 K; their spectral lines are those of Balmer hydrogen and of singly-ionized metals. These are blue-white stars. Examples of such stars are Sirius and Deneb

- class F stars are stars of which surface temperature lies between 6,000 and 7,500 K; their spectral lines are those of neutral hydrogene (in weaker quantity) and of singly-ionized metals. These are white stars. Examples of such stars are Procyon and Canopus

- class G stars are stars of which surface temperature lies between 5,000 and 6,000 K; their spectral lines are those of singly ionized calcium, of hydrogen (in weaker quantity) and of neutral metals. These are yellow-white stars. Examples of such stars are the Sun and Capella

- class K stars are stars of which surface temperature lies between 3,500 and 5,000 K; their spectral lines are those of neutral metals and molecular bands begin to appear. These are orange stars. Examples of such stars are Aldebaran and Arcturus

- class M stars are stars of which surface temperature is under 3,500 K; their spectral lines are those of titane oxyde (TiO) and of molecular lines. These are red stars. Examples of such stars are Antares and Betelgeuse

This range of sortings do that a star may be named with a code. Thus, Sirius is a A1 V star (class A star; so blue-white; spectral type 1 of the spectral class -so having a high temperature; luminosity class V -so main sequence stars); Rigel is B8 I (class B star, so blue, type 8 of class B, so low temperature; luminosity class I, so very bright supergiant). Sun is G2 V (class G star, so yellow-white; type 2 of the class, so high temperature; luminosity class V, so main sequence star)

Four Groups of the Hertzsprung-Russell Diagram

Four groups of the Hertzsprung-Russell diagram into which stars sort allow stars to be described in a static way, as they are in the same time a statistical sorting of stars and a sorting of their physical features

As far statistical sorting of stars is concerned, main sequence contains 90 per cent of all stars. White dwarfs mean 10 per cent of stars. Red giants contain less than 0,5 per cent of stars. Blue and red supergiants are marginal

As far as stars' physical features are concerned, the four groups of the H-R diagram are located in a reference frame which is defined by three groups of values. From the bottom to the top of the diagram (vertical axis), luminosity increases. For the left to the right of the diagram, temperatures decrease. From the left to the right of the diagram (horizontal axis), spectral classes are displayed in their order (O, B, A, F, G, K, M). These three groups of values, blended with other views on the stars' features (as above, other values), allow to describe the four groups of stars of the diagram:

- main sequence is formed by the curve of stars which starts top left of the diagram and goes down to bottom right. Main sequence stars, from this fact, are stars which span over all spectral classes, from O to M (the curve goes from left to right of the diagram) and which so span too over the whole range of temperatures; from this fact also (the curve goes from the top to the bottom of the diagram), main sequence stars span over the whole range of luminosity (from the very bright stars of the class O to very weak stars of class M -this range meaning from some ten-thousandths to 100,000 times Sun's luminosity). As a whole, main sequence stars form a luminosity class, the class V. Mass of main sequence stars vary on a weak range, from 0.1 solar mass (class M stars) to 100 solar mass (class O stars). At last it has to be known that an important correlation exist between mass and luminosity of main sequence stars. Luminosity of a main sequence star is equal, in average, to the 3.8 power of its mass. This correlation allows to think that all main sequence stars have something in common as far as their internal structure is concerned. Moreover, this correlation does that, as main sequence stars have an important range of luminosity, these 3.8 power values do that the range of the masses of main sequence stars is weak (from some hundredths to some hundreds solar mass)

- red giants are luminous stars of spectral classes F, G, K and M. Their group is located above the main sequence. Giants belong to a same luminosity class, the class III. They however vary in luminosity. Generally speaking, they have a surface temperature varying from 3,000 to 7,000 K, a mass of 1 solar mass and a radius of 100 solar radii; their mean density is so 10^-6 g per cm³

- supergiant stars sort into red supergiants if they are on the cold side of the H-R diagram (in spectral classes G, K and M), and blue supergiants if they are stars of the spectral classes O and B. Two types are stars of luminosity I and II classes

- white dwarfs are feeble stars, lying under the main sequence. Although few luminous, these stars have their external layers warmer than those of the Sun. They have a 1 solar mass mass, a radius of one-hundredth solar radius and a mean density of 106 g/cm³. They belong to spectral classes B, A and F

Correlation mass-luminosity does not apply to stars others than main sequence stars. These are above or under the curve. This lack of correlation allows to think that internal structures of luminosity classes stars which do not belong to main sequence is in a substantial way different of those of main sequence stars

Hertzsprung-Russell Diagram and Life of Stars

The Hertzsprung-Russell diagram allows too to spot main stages of the life of stars, as these four groups picture too the four stages of this life

Stars are being born in huge gas and dust molecular clouds, as various processes are at play to get the materials infall to the cloud's center, forming a "protostellar object", or protostar. At about 10,000 to 100,000 years into the core forming process (such protostellar objects are categorized into classes; class 0 is the youngest class), the cloud temperature is about 400 degrees below zero Fahrenheit (minus 240 Celsius). It's not until after a few million years that nuclear fusion ignites at the center of the cloud. Stars then appear on the main sequence, at the place matching the size, the temperature and the spectral class of them at their birth. The main part of the life of a star is going to take place on the main sequence, at this place. During 90 per cent of their life, stars, at their place on the main sequence, are going to shine by burning hydrogen into helium. Length, in years, of this main leg of a star's life depends upon the spectral class to which it belongs and upon its mass: this may vary from 30 million years for a class O7 star to theoretically 200 billion years for a M0 class star; the more a star has an important mass, the less the hydrogen stage lasts; the weaker the mass, the longer the hydrogen leg. Hydrogen burning, at last, tends to move the star away from the very axis of the main sequence, and slightly increases its radius.

About How Long Open Clusters do Last
'Open clusters' seem a frequent consequence of star formation. When a gas and dust, star forming cloud collapse, it usually gives birth not to one star only, but to several ones at the same time. Those stars, then, are seen in the sky under the form of an 'open cluster', a relatively loose cluster of stars -the best examples of which being the famed Pleiades, or Hyades, for example. Such clusters of stars are now known to 'dissipate' along about 25 million years, as their stars, on the one hand, may be short-lived, like the massive, type B, blue stars which have a live duration of some tens of millions years only, or, on the other hand, as type O, more massive stars are shorter-lived still, with them exploding supernovae after some million years only. Those explosions further are pushing away the gas and dust possibly remaining in the cluster from the star births, leading to a further loss of mass in the cluster. The ultimate conclusion of such an evolution is that the cluster, eventually, do not exist anymore, in about 250 million years. That will be, for example, the case for the Pleiades

When the star's hydrogen is exhausted, it's made up only of helium. The central region of the star contracts until the remaining hydrogen around the helium core ignites hydrogen fusion processes. Such processes bring the star's outer region to expand. The star becomes a red giant (some stars become supergiants, as some even hypergiants -the latter with much more irregular an activity). It leaves the main sequence and belongs now to the group of giants. According to its mass, the star will continue the process of transforming elements, as it began to do by transforming hydrogen into helium and then burning helium: stars which mass is very much more important than one of the Sun, will be burning hydrogen, then helium, then carbon, then oxygen, neon and silicon; stars which mass is more important than one of the Sun will be burning elements down to carbon and will possibly continue down to silicon; stars of a mass equivalent to the Sun will stop at helium. Stars which mass is weaker than the mass of the Sun are burning hydrogen and possibly helium. Stars which mass is very much weaker than the one of the Sun do not become red giants (see below). A star burning helium as a red giant spends 10 to 25 per cent of its lifetime to do it. The process by which a star keeps on its cycle of compound burning is the following: in its red giant phase, the star cools, the core contracts again. It brings a heat of millions of degrees C, enabling helium fusion in turn. This is called "core helium burning". This may trigger an enormous amount of X-ray activity in the star's corona. The star ceases to be a read giant, it shrinks as the surface temperature increases

At last, the star arrives at the end of its life. This end depends upon the star's mass. Stars with an initial mass of less than 8 solar masses -and stars with a weak mass generally- end their lives as they stop burning helium and eject a hydrogen envelope, a planetary nebula -which is going

A white dwarf is a dense object, as a teaspoon of such a star would weigh about 10 tons on Earth! A white dwarf is about the size of the Earth. Such stars may endure then a burst of activity as hydrogen from the outer envelope is brought down into the helium shell (which surrounds the carbon, oxygen, and other heavy elements core) due to heat-spurred convection. This starts a flash of fusion activity. The latter would occur as swiftly as just a few years. Such a renewed activiy has the white dwarf reheat and ionize gases in its surroundings and ejecting a large amount of carbon from the core, as gas and as dust, providing some more material to star forming regions

to last 10,000 to 50,000 years. Meanwhile, the star is becoming a white dwarf. Remains of the stars contract and the surface temperature reaches between 50,000° and 100,000° K, radiating ultraviolet photons which are absorbed by the planetary nebula. Some parts of the planetary nebula may be compressed by solar winds reaching speeds of up to 2 million mph (3.6 million km/h). The infrared-dedicated Spitzer Space Telescope found in 2007 that the swelling of the original star not only engulfes and burns some of its planets, as the remaining ones and the objects at its outskirts, like comets or Kuiper Belt Objects, have their orbits perturbated, which leads them to collide and to yield dust into the planetary nebula. 99 per cent of stars have the red giant period and then end as white dwarfs. A mystery might linger over white dwarfs as the younger ones (those who are hotter, thus bluer and brighter, compared to old ones), have been found, in clusters of stars, to lie at the outskirts of them, as they should have been near when their large progenitor stars had died, near the center of the clusters that is. A typical example of a planetary nebula is the nebula of the Lyre. This name of "planetary" nebula was given due to such objects having the aspect of a pale planet. The hottest white star known has a temperature of 400,000° F (200,000° C), as it seems to have had an episodic activity, leading to an irregularly shaped planetary nebula. A teaspoon of a white dwarf weighs 15 tons. Stars with an important mass (between 8 and 50 solar masses), after having burned the elements one after another -and in more and more swift sequences (the star burns helium during 500,000 years, carbon during 600 years, neon during 1 year)- have eventually their heart collapsing (in two-tenths of second) and they explode into a supernova. Among the remainders of the explosion, the star will end as a neutron star (2 to 3 solar masses, radius under 10 km, with a teaspoon of such a star weighing 4 billion tons) or as a pulsar -a neutron star rotating swiftly and emitting high-intensity radio radiation bursts, or sometimes like a 'magnetar' -a superdense neutron star, with an extremely strong magnetic field and emitting in the X-rays. A typical example of a remainder of supernova is the famous Crab Nebula (M1). The supernova events are well known to provide most of the dust injected into the Universe, that dust forming, then, the basis for the further formation of other stars, or the development of protoplanetary disks, and, eventually of planets and life. The dust elements are produced when the gas of the ejecta cools off, between some to several hundreds days after the explosion. Stars which mass is very important (mass greater than 50 solar masses) have the same story (they burn elements, the core collapses, they explode into supernovae) but, instead that the remainder of the star holds as a neutron star or as a pulsar, the collapse keeps on and the star becomes a black hole, a star which density is so high that spacetime is there warped closed and that, if matter may still come in, nothing more is able to go out, not even light! Supernovae events occur due to the fact that stars, core of which have turned into iron are no able anymore to yield energy. They would need, at the contrary, an input of energy instead to nuclear-burn the iron! It's likely that an intermediate range of stellar mass is yielding the smallest black holes possible in the Universe, with a 3-solar masses mass

A "Bang" or "Hiccups"?
Most recent data using observations by the gamma-ray bursts observing satellite Swift are showing that the last stages of a star going supernova are maybe not as simple as thought when the birth of a black hole is involved. The star, in this case, might not go "bang" once for all but it might endure a "bang" and a series of "hiccups". After the stars falls on itself as it runs out of energy and is no more able to sustain its mass, an initial blast is obliterating the outer shells of the star, as a chaotic black hole activity re-energizes the explosion again and again, creating multiple bursts all within a few minutes. First comes a blast of gamma rays followed by intense pulses of X-rays. The newly formed black hole gets immediately to work. Some matter falls as some other is expelled. Another explanation might involve no black hole at all and simply rest on the jets shooting away from the dead star falling back unto itself, creating shockwaves in the jet core

On a more longer timescale, it may be useful to know another categorization of stars, as 'Population I stars' are young stars (some billion years old, like the Pleiades), 'Population II stars' are stars which appeared 10 billion years ago during the bloom of most galaxies. 'Population III stars' are the oldest stars of the Universe and thought to be a hundred times more massive than our Sun, as short-lived -some million years only. Those Population III stars however did not yield any of the dust which the stars provided into the Universe

Most stars are powered by nuclear fusion. Other types of star activity are: rotation (neutrons stars), accretion (type Ia supernovae; a white dwarf pulls matter from a red giant companion and eventually goes supernova), maybe magnetic field (magnetars; magnetars are neutron stars with a magnetic field about thousand times stronger than ordinary neutron stars; also called Soft Gamma-ray Repeaters (SRGs) or Anomalous X-ray Pulsars (AXPs); they might be much numerous in our Milky Way than thought)