The Hertzsprung Russell Diagram Stellar Evolution at a Glance

Jupsat Pro Astronomy Software

Secrets of the Deep Sky

Get Instant Access

The astronomers Einar Hertzsprung and Henry Norris Russell noticed very early on (towards the beginning of the 20th century) and independently, that stars tend to gather in specific parts of a diagram in which their luminosities are plotted as a function of their "effective" (surface) temperatures. Modern theory shows that stars evolve, and that they actually move as a function of time in such a diagram, widely referred to as the "Hertzsprung-Russell diagram", or "HR diagram" for short. This evolution generates an accumulation of stars where it is slow, and relative voids where it is rapid. A version of this diagram is shown on Fig. 1.15, based on 22 000 well-studied stars mostly drawn from a catalog based on data from the European satellite "Hipparcos", and also from the Gliese catalog of nearby stars.8

Figure 1.15 shows that the HR diagram is crossed by a thick, long, and wavy diagonal, running from hot, luminous stars, down to cool, low-luminosity stars. This diagonal is called the "main sequence", on which most stars lie. Note that, as is customary among astronomers, luminosities are indicated in solar units (on the vertical axis), and temperatures are increasing towards the left (on the horizontal axis). Figure 1.15 also shows the empirical division of stellar luminosities into "luminosity classes", from high (I) to low (V), as well as their "spectral types", indicated by letters (from O to M for historical reasons), which basically reflect their effective temperatures.

The theoretical works of Hans Bethe and collaborators showed, as early as 1938, that stars draw their luminosity from nuclear reactions in their cores. The "main sequence" is the locus where stars like the Sun are turning their hydrogen into helium in their centres, by means of a network of thermonuclear reactions (starting at a temperature of ~ 15 MK). The main factor placing the stars on the main sequence is their mass. The stars on the upper left-hand side are the most luminous (up to 10 000 Lq), i.e., the most massive (up to 100 Mq), while on the lower right-hand side lie the least luminous ones 0.00001 Lq), which have a small mass (down to less than 0.1 Mq). The duration of the main sequence depends very much on the stellar mass: for massive stars it is only a few million years, while for the Sun it is about 10 billion years. The smallest stars stay on the main sequence "eternally", i.e., burn their hydrogen so slowly that this phase may take as long as hundreds of billions of years, i.e., much longer than the age of the present-day universe, which is estimated to be 12 billion years old. The present-day Sun is 4.5 billion years old, and thus lies half-way through its main sequence stage.

Figure 1.15 also shows a dense "wing" of stars to the right of the main sequence. This is the post-main sequence, or "giant" branch, into which stars evolve when they convert hydrogen into helium by a more energetic network of

8 See http://www.anzwers.org/free/universe/hr.html

Fig. 1.15. The "Hertzsprung-Russell diagram". This diagram can be expressed in various units: absolute magnitude vs. colour or spectral type (directly from observations), luminosity vs. temperature (after the stellar type has been determined). The long diagonal is the "main sequence", on which stars turn hydrogen into helium via nuclear reactions. The dotted rectangle highlights the area of the "pre-main sequence" stages shown in Fig. 1.17 below

Fig. 1.15. The "Hertzsprung-Russell diagram". This diagram can be expressed in various units: absolute magnitude vs. colour or spectral type (directly from observations), luminosity vs. temperature (after the stellar type has been determined). The long diagonal is the "main sequence", on which stars turn hydrogen into helium via nuclear reactions. The dotted rectangle highlights the area of the "pre-main sequence" stages shown in Fig. 1.17 below thermonuclear reactions (called the "CNO cycle"). The basic factor that makes these reactions happen is the increase in central temperature. As a result of this new energy source, the stars (mostly solar-type stars in this case) increase enormously in size. Subsequently, new thermonuclear reactions occur, in which helium burns and converts the stellar interior into carbon and oxygen during a few million years, while the envelope continues to expand. For instance, when it reaches that stage, 5.5 billion years from now, the Sun will become larger than the Solar System. In other words, it will engulf all the planets in a vast, expanding tenuous envelope, including the Earth: this is Doomsday as astronomers see it

Fig. 1.16. Example of a "planetary nebula". This beautiful object (here in the constellation Lyra) is actually a dying star. The ring-like emission comes from an expanding envelope that may be much larger than the solar system, while the original, solar-type star shrinks to become a planet-sized "white dwarf" (on the picture, the small dot at the centre of the ring)

Fig. 1.16. Example of a "planetary nebula". This beautiful object (here in the constellation Lyra) is actually a dying star. The ring-like emission comes from an expanding envelope that may be much larger than the solar system, while the original, solar-type star shrinks to become a planet-sized "white dwarf" (on the picture, the small dot at the centre of the ring)

happening today in the so-called "planetary nebulae", such as the one located in the constellation Lyra shown on Fig. 1.16. In parallel, the core of the Sunlike stars shrinks to become a star in itself, called a "whitedwarf", a very hot (100 000K), very small (planet-sized) and faint star (the small white dot at the centre of Fig. 1.16). White dwarfs slowly cool until they become dark after hundreds of billions of years.

One notable exception to this nuclear origin for stellar energy and evolution is a class of young stars called the "pre-main sequence" (PMS) stars, or "T Tauri stars" in the case of solar-like stars. Indeed, when they are young, stars draw their energy only from their slow contraction under the pull of gravity: The PMS phase is a "non-nuclear" phase, because at this stage the stars are not hot enough in their centres to ignite nuclear reactions. The PMS phase is very important for the problem of the origin of life because it is the phase during which planets may form.

T Tauri stars are too faint to be have been observed by Hipparcos, so none is included in Fig. 1.15. In contrast, Fig. 1.17 shows a dedicated HR diagram obtained for a cluster of low-mass young stars 1 Mq ), located in the constellation Chamaeleon, in the southern hemisphere, and drawn from another source9 This HR diagram extends over a much smaller area than the "global"

9 Simplified version, adapted from Lawson, Feigelson, and Huenemoerder 1996, Monthly Not. R. Astron. Soc, 280, 1071.

Stellar Evolution Ttauri

Fig. 1.17. The "Hertzsprung-Russell diagram" for solar-like pre-main sequence stars, also called "T Tauri" stars. By means of sophisticated models, one can assign an age and a mass to a star, once its luminosity and temperature are known. Stars with filled dots are surrounded by circumstellar disks, while stars with open dots are not. It is thought that disks may disappear because of planet formation. This diagram occupies the area indicated by the dotted rectangle in Fig. 1.15

Fig. 1.17. The "Hertzsprung-Russell diagram" for solar-like pre-main sequence stars, also called "T Tauri" stars. By means of sophisticated models, one can assign an age and a mass to a star, once its luminosity and temperature are known. Stars with filled dots are surrounded by circumstellar disks, while stars with open dots are not. It is thought that disks may disappear because of planet formation. This diagram occupies the area indicated by the dotted rectangle in Fig. 1.15

HRD of Fig. 1.15. The dots represent the observational points, and the lines are so-called "evolutionary tracks" drawn from theoretical calculations of the evolution of PMS stars. Such tracks have been calculated by different authors10, with slightly different results, and their intercomparison leads to uncertainties on stellar masses and ages that may reach 50% in some parts of the diagram. The so-called "birthline" shown on Fig. 1.17 is the locus where newly formed stars become optically visible.

Whereas the observational points are labeled in luminosities and temperatures, the theoretical tracks are labeled in stellar masses (dotted lines) and ages (continuous lines). The last continuous line indicates the start of the nuclear reactions turning hydrogen into helium ("zero age main sequence"). Using these tracks, one is able to assign (within some uncertainty) a theoretical mass and age to a star, from its observed luminosity and temperature. For instance, reading off the left panel, a T Tauri star with L = 1.1 Lsun and T = 4500 K

10 e.g., D'Antona and Mazzitelli 1994, Astrophys. J. Suppl., 90, 467; Siess, Dufour, and Forestini 2000, Astron. Astrophys. 358, 593.

has a mass of 1 Mq and an age of 4.1 Myr. The present-day Sun is located at M = 1Mq, L =1 Lq and T = 5800K.

The observational points in Fig. 1.17 are filled or open, depending on whether the stars have disks ("classical" T Tauri stars, filled points) or not ("weak-line" T Tauri stars, open points). One notes a concentration of filled points peaking near 3Myr: in the present case, this is the estimated age of the Chamaeleon cluster (peak of star formation). At later ages, open points are more numerous, indicating that on average the disks disappear over timescales of order a few million years, although older disks are present in this particular cluster. A possible, but poorly understood, cause for this disappearance is the formation of planets.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment