by Prof. Stefan Dreizler from Georg-August-Universität Göttingen – Institute for Astrophysics
A star is a sphere of very hot gas (called plasma) confined by its own gravity. Our Sun is the nearest star, astronomers therefore use it as a standard. Compared to our Earth, the Sun is more than 300,000 times more massive and its diameter is a factor of 100 larger. Compared to Earth and other planets like Jupiter, stars produce energy from nuclear fusion processes, which results in the emission of light over a broad range of wavelengths. The maximum of this emission, for example of our Sun, is in the region of the electromagnetic spectrum visible to our eyes, but the Sun also emits significant levels of ultraviolet and infrared radiation. These nuclear processes require very high temperatures (millions of degrees) and very high pressures in the central regions of the stars. Straight forward estimates show that a minimum mass of a bit less than a tenth of a solar mass or equivalent to 80 times the mass of Jupiter is required to reach the temperature and pressure to operate stable nuclear fusion processes over a very long time; in the case of our Sun comparable to the age of the Universe.
Objects below this critical mass of 80 Jupiter masses are called brown dwarfs. They can sustain some nuclear processes but only for a rather short time. They can be therefore seen as failed stars. Below a mass of about 13 Jupiter masses, not even short term nuclear processes are possible, objects below this mass limit are planets. It should be noted that this differentiation between planets and brown dwarfs is only one possibility—others are in use as well.
How do Stars Form?
Stars form out of very cold gas and dust clouds, called molecular clouds, which become gravitationally unstable and therefore collapse under their own gravity. The cause of the instability can vary from density waves in the Galaxy, close-by stellar explosions or even galaxy-galaxy collisions! These molecular clouds contain gas of typically many ten thousands of solar masses and have diameters in the range of tens of light years—on the order of a few ten thousand times larger than our solar system. During the collapse, the cloud may fragment, i.e. a single cloud forms a large number of stars in the range of hundred thousand stars. The resulting stars have a good chance to stay mutually gravitationally bound for a very long time. We find such clusters of stars, called globular clusters, in our own Galaxy as well as in other galaxies. Less populated clusters are visible as open clusters, with the Pleiades as a prominent example, but dissolve rather quickly contributing their stars to the Galaxy. Nevertheless, multiple systems—containing two, three or even more stars—are very common. Proxima Centauri for example is a member in a triple system.
The fragmentation and contraction process proceeds through several stages until the fragment, at that stage called a protostar, is not much larger than our Sun. The contraction leads to a continuous transformation of potential energy into kinetic energy resulting in significant heating of the gas. The star is born when the interior is hot and dense enough to sustain steady nuclear fusion which then brings the star into a stable equilibrium. On astronomical time-scales, the formation process is short, i.e. on the order of a million years. Star formation is self-terminating, the first stars forming in such a cloud start to heat the molecular cloud which quickly leads to dissolution of the cloud.
A rather near-by star forming region is the Orion nebula 1,300 light years away. With a small telescope, four prominent stars become visible which started to heat up the surrounding gas visible though the very characteristic light emission. Parts of the original cold molecular cloud gas are also visible as dark clouds. A closer look with the Hubble Space Telescope reveals a large number of newly formed stars.
The stellar composition reflects the material composition of our Universe, by number, hydrogen makes up about 90% of gas atoms, followed by helium with just less than 10%. All other elements, like carbon, oxygen, or iron, contribute less than 1% by number. We will see below that the amount of other elements is an indicator of the population the star belongs to, i.e. whether it has formed earlier in the life time of the Galaxy or more recently.
Since more than 99% of the stellar material is hydrogen and helium in a nearly identical composition from star to star, the properties of the newly formed stars are determined by its mass, which covers a wide range. The smallest objects formed are brown dwarfs; the most massive can have masses of the order of one hundred solar masses. The distribution of masses is quite universal, making the low-mass stars between a tenth and a half of the solar mass the most abundant ones. Proxima Centauri belongs to this category of low-mass stars. In that respect, it would be a better astronomical standard than our Sun.
Stellar Evolution and the Hertzsprung–Russell Diagram
As mentioned above, stars produce energy from nuclear reactions. Since hydrogen is the most abundant element and the amount of energy per atom gained in a fusion reaction is the largest, the fusion of hydrogen to helium is by far the longest lasting energy resource for stars. For example, our Sun can sustain this process for about 10 Billion years—the Sun is half-way through this stage in its evolution. Low-mass stars like Proxima Centauri have less hydrogen to be transformed, however, they emit less energy (light) per unit mass and can therefore sustain the hydrogen fusion (in the astronomers slang called “burning”) for much longer. On the other hand more massive stars than our Sun have a shorter life time since they consume their hydrogen more efficiently.
This leads us to the basic properties observable for stars, their luminosity, i.e. the total amount of emitted light per second, and the surface temperature. These two quantities are often displayed in a diagram, called the Hertzsprung–Russell diagram, which is one of the most important representations of stars in astronomy. The stars in their phase of hydrogen burning form the so called main-sequence. This is a sequence with the least massive stars having low temperatures and low luminosities, and temperature and luminosity increasing with mass. The surface temperature determines the color of the star; cooler stars appear red, hotter stars look blue. The surface temperature also determines the appearance of the stellar spectra. The emitted light of a star is not continuously distributed over the wavelengths as for example in a classical light bulb. The solar spectrum (the spectrum produced by the Sun) is characterized by millions of absorption lines indicating the presence of the chemical elements. Cooler stars like Proxima Centauri are characterized by the absorption lines of many molecules. These spectra are like finger prints and astronomers can learn a lot about the physical properties of stars by analyzing their spectra. In the context of planets around other stars, the spectral lines, or more precisely their shift in wavelength towards the red or the blue, is a measurement of the stars’ velocity. This is the basis of the Doppler Velocity Method which Pale Red Dot will be using to detect a planet orbiting Proxima Centauri.
In the Hertzsprung–Russell diagram displayed here, the main stellar properties are indicated. For historical reasons, the stars are categorized in so called spectral classes, labeled with the capital letters indicated on the horizontal axis (OBAFGKM, O star are bluer, M stars are redder). Among others, Proxima Centauri‘s position in the Hertzsprung-Russell diagram is on the right bottom, indicating that it is a low-mass red star of spectral type M, with a surface temperature of the order of 3,000 degree Kelvin and a luminosity of about a thousandth of the solar luminosity.
In the Hertzsprung–Russell–Diagram, other branches of stars are also visible. These positions in the diagram are populated by stars which finished their phase of hydrogen burning. For the sake of a compact description, most of the details of the further stages of stellar evolution are left out here. Basically, stars expand after the nuclear burning of hydrogen has ceased. This transforms the star into a giant. Most of the stars start a second phase of nuclear processes, now transforming helium into carbon and oxygen. This is less efficient and lasts therefore shorter. For most stars, the exhaustion of helium marks the end of nuclear processes and the star ends up as a white dwarf. Stars which are about ten times more massive then our Sun go through more nuclear processes, which end when a significant fraction of the stellar material is transformed to Iron. The final stage is either a neutron star or a stellar black hole formed in a violent supernova explosion.
The Peaceful End of Stellar Life
The later stages of stellar evolution are interesting for planets and planetary systems in two aspects. First of all, the transition of our Sun into a giant will terminate all life on Earth. The increasing solar radius and the increasing luminosity will bring the mean Earth temperature far above 100 degrees Celsius. The inner planets Mercury and Venus are very likely swallowed by the expanding Sun. When the Sun finally turns into a white dwarf, the Earth on its current orbit would be far too cold to sustain life. Moreover, there are observations indicating that we can observe decaying former planetary systems around white dwarfs which were destabilized due to the evolution of its host star.
Secondly, stars lose a large fraction of their mass during the late phase of stellar evolution. Since this material is enriched in heavy elements like carbon, oxygen, silicon, iron, etc. and since the next generation of stars and their planets is formed out of this processed material, later generations of stars have more material to form planets like our Earth which mainly consist of iron and silicon and sustain life which relies on carbon and oxygen.
Stars and their Planets
Planets are a natural by-product of the star formation process. The contraction of the fragments of the star forming molecular cloud, based on angular momentum conservation—a fundamental principle of nature, leads to the formation of a disk around the protostar. These disks are the birth places of planets which form either in a bottom-up process from the growth of dust particles to larger and larger objects, finally of planetary size, or in the top-down, direct gravitational collapse of a fragmenting disk. Many details of these processes are still debated, but the basic principles seem to be clear. While this may indicate that all stars should have planets, planets might be lost, for example due to close stellar encounters, leaving stars without planets.
The properties of the star determines the distance at which a planet can sustain habitable conditions, i.e. having a temperature suitable for liquid water. This depends on the stellar luminosity which sets the planet’s mean temperature depending on the star–planet distance. A less luminous star such as Proxima Centauri has a habitable zone much closer to the star compared to our Sun, where obviously our Earth is in the habitable zone.
Stellar properties also determine how difficult it is to detect planets around it. Detecting planets around red dwarfs like Proxima Centauri is easier than around Sun-like stars. This is because red dwarfs are less massive (larger wobble due to a planet around them), smaller (larger dip in light when a planet crosses in front of them) and less luminous (higher planet–star contrast). An important aspect for life on other planets is also the stellar activity. Since we can study our Sun from nearby, we know that it is not a uniformly emitting sphere. The turbulent motion of the outer solar layers in connection with the solar magnetic field lead to a variety of phenomena summarized under the keyword activity. This activity determines the amount of high energy radiation and particles a planet is exposed to. This does not only impact possible life directly, it also influences the conditions in the planet’s atmosphere.
Overall we can state that the knowledge of planets around other stars is only as detailed as our knowledge of the host star making stellar astrophysics a key aspect in our exploration of other worlds.
About the author. Stefan Dreizler’s expertise is in Stellar physics and observational spectroscopy. He studied physics and obtained his PhD at University of Kiel (1992). After that he worked as a Postdoctoral fellow at University of Erlangen-Nurnberg between 1992 and 1995. He then moved to University of Kield as a Research Assitant (1995 and 1996), and to University of Tuebingen (1997-2000), where he was appointed as Lecturer between 2000-2003. His last move was to University of Goettingen where he is today a Full Professor in Astrophysics. He was Dean of the departmenbt of Physics between 2007 and 2009, and he has been deeply involved in the construction of the Multi Unit Spectroscopic Explorer (MUSE installed at ESO’s VLT), whose mechanical structure was assembled in Goettingen. He is member of the Kuratorium of the Max-Planck-Institut fur Sonnensystemforschung (Solar System Research). He runs astronomy outreach activities with the MONET telescopes (Texas and South Africa), and he is a very active member in the CARMENES project, which recently started operations and will also search for small planets around M-dwarfs in the northern hemisphere.