The Hertzsprung-Russell diagram

In 1911 the Danish astronomer Ejnar Hertzsprung (1873-1967), who was mainly interested in studying star clusters, decided to plot the luminosities of the stars in a cluster against their colours. In 1913 the American astronomer Henry Norris Russell did something very similar when studying “local” stars in the neighbourhood of the Sun, although he used spectral classes rather than colours. Both colour and spectral class relate directly to surface temperature, so the two astronomers were really doing the same thing, although quite independently of each other. The type of diagram in question is therefore known by the names of two astronomers who never actually worked together but are forever linked in the “Hertzsprung-Russell diagram” (or “H-R diagram” as it is often referred to as).

The x axis of an H-R diagram is of the spectral classes from O to M (left to right), which is also of temperatures from high to low. The y axis is of absolute visual magnitude, brightest at the top, and dimmest at the bottom. A hot, bright star will therefore be plotted towards the top left of the diagram and a cool, dim star towards the bottom right.

An H-R diagram can be plotted for any population of stars that is desired. Hertzsprung was interested in plotting all the members of a star cluster, but it can also be used to plot stars seen in a particular area of the night sky, for example, or the named stars that are visible with the naked eye.

What becomes apparent when plotting virtually any group of stars is that some parts of the diagram are far better populated than others. It is noticeable that a band of stars appears in a progression from top left to bottom right, with other parts having far fewer members. This band has become known as the “main sequence”, and the Sun features at just about the mid-point of the sequence, being averagely bright and hot when compared with most of the stars that are easily visible from Earth. The H-R diagram can therefore be read as a means of comparing the Sun with any other star on the diagram.

Two particular classes of star lie outside the main sequence and are of interest in their own right. Towards the top right of the diagram are a number of highly luminous but relatively cool stars. They gain their high absolute magnitude from their large surface area, as they constitute the red giants and supergiants. Red giants are anything between 10 and 100 times larger than the Sun, but much cooler. Supergiants are much rarer than giants, but include colossal stars such as Betelgeuse in the constellation Orion which, were it in the same place as the Sun, would occupy all the space out to the orbit of Mars and beyond.

To the left of the main sequence is a group of stars known as white dwarves. These are hotter than most main sequence stars but not as bright. They are therefore not visible to the naked eye.

One important lesson to be learned from an H-R diagram is that stars are not distributed uniformly across it but there are whole areas where stars are absent (or extremely rare). This is because of the second very useful feature of these diagrams, which is that they demonstrate the evolution of stars; the position of a star on an H-R diagram can tell the user a lot about how old or young it is and the likely fate that will eventually befall it.

For example, a supergiant like Betelgeuse is quite likely to explode as a supernova, and a white dwarf is the remnant of a star that has burned nearly all its fuel but never had sufficient mass to become a supernova.

A main sequence star is “safe” for the immediate future, but, when its hydrogen fuel is exhausted it is likely to move out of the main sequence into the red giant area as it becomes cooler but not necessarily less luminous. The next stage is for it to throw off its outer layers to leave a dense and extremely hot central core, so it will move across the H-R diagram into the hotter and less bright zone, thus becoming a white dwarf.

The H-R diagram is a very useful tool when particular areas of the sky are being surveyed, because comparisons can then be made with the “standard” H-R diagram that includes the Sun. For example, H-R diagrams of globular clusters reveal them to be composed of older stars, many of them in the red giant stage, and with the upper end of the main sequence missing due to the lack of very hot stars.

© John Welford

The Orion Nebula

The constellation of Orion is one of the most familiar in the night sky, and it is unusual in being visible from both the northern and southern hemispheres, depending on the time of year. Its most notable feature is the chain of three stars of similar brightness in a short straight line (Orion’s belt), and two of the most familiar single stars in the sky, namely the red supergiant Betelgeuse at the top left-hand corner, and Rigel, a blue supergiant which is the sixth brightest star, at the bottom right.

However, probably the most intriguing feature of Orion is the nebula that is just visible with the naked eye (depending on how good your eyesight is), in the region below the belt that the ancients designated as Orion’s sword. The word “nebula” means mist or vapour, and that is how it appears, namely as an indistinct patch of fuzziness. With a reasonable telescope it is possible to see bright patches within the mist, and when seen through the orbiting Hubble telescope (see photo) it is truly spectacular, with huge regions of dust and gas illuminated by the stars within it. To make the images even more remarkable, the different light wavelengths are normally translated into “false colours” that give the nebula an extra level of beauty. 

As the Orion Nebula is the closest nebula to Earth, at around 1,300 light years away, it is not surprising that it has been known about for hundreds of years. There are four bright stars at the heart of the nebula that were first found to be surrounded by nebulosity by Nicholas Pieresc in 1611. Galileo had observed the stars but not the nebula in 1617, and Christian Huygens produced a sketch of the object in 1656. William Herschel, who later discovered Uranus, made detailed observations of the nebula in 1774, describing it as “an unformed fiery mist, the chaotic material of future suns”. It was recorded by Charles Messier in his 1774 catalogue of deep sky objects at number 42 on his list, and it has been known as M42 ever since (it is also listed in the “New General Catalogue” as NGC 1976).

Herschel’s remark, made more than 200 years ago, was particularly prescient, given that this is precisely what the Orion Nebula is, as are many other similar regions of space. It is a stellar nursery where new stars are being born, coalescing from vast quantities of dust and gas and eventually “igniting”. The material in question may well have come from the disintegration of a much older star (or stars) that had reached the end of their life cycle, so the nebula is a half-way point in the process of the death and re-birth of stars.

Seen in the visible light spectrum, which is all that the early astronomers had available to them, the most prominent feature of the Orion Nebula is the group of stars mentioned above as being at the heart of the nebula. Originally thought to be a single star, and named Theta Orionis, Galileo and Huygens both noted that it comprised three stars. When a fourth star was detected later in the 17^th century it was given the name The Trapezium, but later observations have increased the number still further, with some of the stars proving to be binaries (two stars in orbit around each other). This is an open cluster of young stars that have started to burn their way out of the dust and gas surrounding them, which is why they can be seen and why the surrounding nebula is illuminated.

However, when other means of observation became available to astronomers in the 20^th century, it became clear that there was much more to the Orion Nebula than the Trapezium. When viewed in the infrared, the nebula is seen to be much larger, in that there are features that are invisible in the “ordinary” spectrum. For example, in 1968 the “Becklin-Neugebauer object” was detected, this being a compact source of infrared emissions located behind the Orion Nebula as we see it. This appears to contain a massive, newly-formed star that is still so deeply embedded in the cloud of dust and gas from which it has formed that its visible light cannot escape. Instead, the light waves that are absorbed by the dust are re-emitted at infrared wavelengths. The object is so massive (about the size of the solar system) and active that it is the brightest object in the whole sky at those wavelengths.

As a stellar nursery, the Orion Nebula is extremely active, with as many as 700 stars at various stages of creation having been detected. The Hubble telescope has also discovered some 150 protoplanetary disks within the nebula that are believed to be an early stage of star formation, and “brown dwarfs” that are proto-stars that have not acquired enough mass to trigger nuclear fusion.

Many complex and violent processes have been detected within the nebula, which contains areas of ionized hydrogen (known as an H II region), molecular gas, and dust. As well as emissions in the infrared and visible wavelengths, there is ultraviolet radiation that is responsible for much of the glow of the nebula.

The Orion Nebula, itself about 24 light years across, is now regarded as being part of a general area of nebulosity that extends for hundreds of light years. This is known as the “Orion Molecular Cloud Complex” that includes much of the area around Orion’s sword and belt, notable features of which are the Horsehead Nebula, Barnard’s Loop and the Flame Nebula.

The Orion Nebula continues to fascinate astronomers and astrophysicists who, by studying it in detail, are learning a great deal about how stars are formed. We are fortunate to have such an active region of space so accessible to view by both earthbound and space-based instruments. Despite the long history of its study, the Orion Nebula doubtless has many more secrets yet to be revealed.

© John Welford

Assessing the absolute magnitude and colour of a star

By using increasingly powerful telescopes, fitted with various instruments to analyse the quality of the light collected from objects such as stars and galaxies, and by making calculations based on their observations, astronomers down the centuries have been able to gather a considerable amount of information about a large number of stars and other objects.

One item of information that can be assessed is the “absolute magnitude” of a star. It is not surprising that stars that are relative close to Earth will appear brighter than those that are further away. However, it is only possible to compare the actual visual luminosity of stars if it is possible to calculate their distances and make the necessary adjustments. When this is done, astronomers can set a standard and declare the absolute, as opposed to the apparent, magnitude of every object in the night sky. The standard that is set is the magnitude that a star would have if it were at a distance of 10 parsecs from us (a parsec is equivalent to 3.26 light years).

Absolute magnitude is expressed as a factor of its apparent magnitude, either plus or minus. Thus the brightest objects, in absolute terms, record a figure of minus 10 and the dimmest are at plus 15. The Sun’s absolute magnitude is plus 4.8.

Star colour

Astronomers are also interested in how hot stars are, and this is directly related to their colour. Thus a relatively cool star such as Betelgeuse produces light at long wavelengths, which make the star look red, and a hotter star, such as Rigel, has its intensity curve skewed towards short wavelengths, so it appears blue. By using a light-sensitive device such as a photomultiplier, and a standard set of colour filters, it is possible to arrive at an accurate assessment of a star’s colour.

By using a spectroscope it is possible to obtain a full spectrum of the light coming from a star, and the nature of the spectrum is also directly related to the surface temperature of a star. Astronomers use a spectral scale of star types which are given the letters O, B, A, F, G, K and M, with O stars being the hottest (surface temperatures above 35,000 degrees K) and M stars the coolest (at around 3,000 degrees K).

© John Welford

Algol, the Demon Star

Algol (Beta Persei) has another name, namely the Demon Star. The main reason for this is that it forms the eye of Medusa the Gorgon in the constellation of Perseus. The ancients thought that they could see an outline in the night sky of Perseus, the Greek hero, slaying the monster who could turn a man to stone with her stare. It is a somewhat fanciful notion, but then so are most of the assumed constellations! Perseus is close to the W-shaped constellation of Cassiopeia, and is pointed to by the left-hand V.

However, Algol had another claim to fame, in the eyes of medieval astrologers, as being a star that implied danger and misfortune. The reason for this is its odd behaviour. Far from shining steadily in the night sky, it dims to 44 per cent of its usual brightness, every two days, twenty hours, forty-eight minutes and fifty-six seconds. This is not what most stars do, so there must surely be some devilry at work here!

  

The explanation was given in 1782 by John Goodricke, a young amateur astronomer from York, who was profoundly deaf and who died at the age of only 21. He not only measured the periodicity of the star but also surmised that it was caused by Algol being not one star but two, orbiting around each other and with one star eclipsing the other.

However, the second star is invisible to observers because it is outshone by its brighter companion. It has only been “seen” by virtue of spectroscopic observations of the spectral lines from the two stars as they orbit each other. As one star comes closer towards us and the other recedes, the spectral lines of each are shifted due to the Doppler effect (the same effect that is noted when the pitch of an approaching emergency vehicle siren rises as it approaches and sinks as it leaves), and can therefore be distinguished from each other. This calculation was only made as recently as 1978, at the McDonald Observatory in Texas.

So Algol is what is usually termed a double star (or binary system), meaning that two stars have been caught by each other’s gravitational pull and have never been able to escape since they were formed in a stellar nursery (the Orion Nebula is an example of such a nursery). One of the stars, which is regarded as the primary star, emits virtually all the light that we see, whereas the secondary star, which is dark by comparison, obstructs the light from the primary star as it passes between it and our line of sight. Actually, the two stars orbit around a common centre of gravity, making their orbits elliptical.

(To be completely accurate, Algol is actually a triple star system, but the third component orbits at a much greater distance than the other two stars do from each other – see illustration).

We now know that the system is about 100 light years away, and that the primary star is a white, hydrogen-burning star that is about 2.6 million miles in diameter and with a mass approaching four times that of the Sun, but shining a hundred times as brightly. For comparison, the Sun is approximately 0.86 million miles in diameter. The companion star is larger than the primary star, at 3 million miles, but with a mass and luminosity similar to that of the Sun.

The two stars are 6.5 million miles apart (measured from surface to surface). If we could imagine the primary star of Algol as being where our Sun is, both stars would fit easily within the orbit of Mercury, which at an average orbit of 36 million miles is usually thought of as being virtually on top of the Sun (Earth’s orbit is 93 million miles). The two stars of Algol are, in astronomical terms, practically touching each other.

Given that the “dark” star is larger than the “bright” one, the question arises as to why it does not completely eclipse its companion as opposed to merely reducing its light. The reason for this is that they are not in exactly the same plane as seen from Earth, and part of the surface of the primary star is always going to be visible.

Algol presents something of a puzzle, in that the two stars, which must be assumed to have been born at roughly the same time, have features suggesting that they are of very different ages. The primary star is a massive supergiant that is still burning hydrogen (as opposed to helium when all the hydrogen has been used up). Its maximum age can therefore only be about 100 million years. However, the secondary star appears to be on its way to becoming a red giant, having reached the stage that our own Sun will reach when it is about twice its present age. This suggests that the secondary star of Algol must be about 10,000 million years old. So how can the two stars have been born as twins?

The explanation, as suggested in 1955 by John Crawford, is that the secondary star is not what it seems. Indeed, it was once far more massive than it is now, probably even more massive than its companion, and it soon reached the stage at which it had burned all its hydrogen and was ready to become a red supergiant. However, as it grew it was distorted by the gravity of the other, very close, star and started to lose material to it, having become pear-shaped rather than globular. This process has continued, so that it has now lost so much mass that it only has as much as our own, much smaller, Sun. The two stars have swapped roles, with the original primary star becoming the secondary, and vice versa.

Algol, a “close primary”, has some very interesting features that are shared by some, but by no means all, similar systems that have been investigated. For example, some binary systems have been discovered where one partner has become a black hole that is rapidly consuming its other half. Perhaps there are therefore other systems that are far more deserving of the “demon star” tag!

© John Welford