Could We Colonize Giant Stars?
Giant stars are featured in several books, video games and science fiction films.
What are the salient features of this type of stars? These are much larger stars than
our Sun, with rays ranging from a few tens to about 200 times the solar radius, located
at the top of the Hertsprung - Russell diagram. This diagram takes its name from the two astronomers
Ejnar Hertzsprung and Henry Norris Russell who developed it independently in 1910. It
is a diagram that relates some stellar parameters: temperature and spectral class on the abscissa,
absolute magnitude and brightness on the ordinate. Temperature is measured in Kelvin. The absolute
magnitude is the brightness that the star would have if it were at a distance of 10
parsecs from the Earth. Finally, the brightness of a star is measured with respect to the
solar one, set by convention equal to 1. In the HR diagram there are different zones,
where the different types of stars are positioned according to the physical characteristics
mentioned above; the predominant evolutionary structure is the diagonal that starts from
the upper left corner (where the most massive, hot and bright stars are) to the lower right
corner (where the less massive, colder and less stars are positioned lights), called
the main sequence. At the bottom left is the sequence of white dwarfs, as they are celestial
bodies with effective high temperatures but not very bright; while above the main sequence,
to the right, the red giants and supergiants are arranged, as they are very bright celestial
bodies but at low temperatures. The giant stars mentioned at the beginning
therefore fall into this last category: they are stars characterized by large dimensions
and great luminosity. Specifically, the blue giants have a high surface temperature (even
up to 50,000 K) and a relatively short life (on average from a few million to a few tens
of million years), because, due to their large mass, they quickly burn hydrogen contained
in their core. The red giants, on the other hand, represent an advanced evolutionary stage
that is passed through by the majority of stars once they exit the main sequence phase,
that is, when they run out of hydrogen in the nucleus.
But would it be possible to colonize them? Or colonize the planets orbiting them? Let's
try to understand the feasibility of this project in this video.
The first big problem is the distance that separates us from any star other than the
Sun: to reach Proxima Centauri, the closest star to the Sun, traveling at the speed of
light, it will take us at least more than four years! So how do you get to the stars?
Let's analyze the possible means, starting from those currently available, namely the
chemical rocket and the ion rocket; later we will analyze some theoretical possibilities,
which do not yet exist.
Chemical rocket: the one used for example by the Space Shuttle or by Elon Musk's Space
Xs. In chemical rockets the propellant consists of the fuel and the oxidizer. Usually the
two components are joined only in the combustion chamber of the engine where, reacting chemically,
they produce the high temperature gas (about 3500-4500 ° C) which is expelled from the
rear nozzle of the rocket. The expulsion velocity V of the exhaust gases from the rocket, expressed
in km/s, is given by: in which
T is the exhaust gas temperature μ is the mass of the molecules.
From this relation, we note that V increases as T increases and μ decreases. From here
we understand because liquid hydrogen (H2) and oxygen (O2) are used: hydrogen is light
and the molecules expelled are of low mass (μ small) while the reaction with oxygen
leads to high values of T. The reaction that occurs in the combustion chamber it is of
the type: 2H2 + O2 → 2H2O which is the water production reaction.
The physical law underlying rocket motion is the conservation of momentum. The rocket
engine ejects hot gas at high speed from the rear nozzle and this, by reaction, gives the
rocket a speed in the opposite direction. The engine works best in empty space (the
atmosphere hinders the expulsion of gases). The speed at time t of a rocket starting from
a standstill is given by: in which
V is the ejection velocity of the gases with respect to the rocket calculated with the
previous relation M is the initial mass of the rocket (when
stationary on the launch pad) m (t) is the mass of the rocket at time t
(while in flight) M/m(t) is the mass ratio.
This relationship is known as the rocket equation, and was derived from Konstantin Tsiolkovsky,
one of the fathers of astronautics, in 1896: it is still the basis of space propulsion
today!
It is important to note that v(t) is directly proportional to the velocity of the exhaust
gases: the higher V, the higher v(t). Unfortunately, the final velocity growth is only logarithmic
as a function of mass ratio, therefore very slow! A typical chemical rocket with V = 3.5
km / s and M/m = 10 can reach a final speed of 8 km/s, just the one needed to enter orbit
at 400 km altitude, the same altitude at which the International Space Station orbits!
The travel times with a chemical rocket, however, would be prohibitive, at least in the span
of a human life: to reach Proxima Centauri, the nearest star to the Sun, would take about
75,000 years, traveling at the speed of 17.46 km/s, the same speed of Voyager 1 spacecraft,
launched in 1977 and now flying far beyond the Kuiper Belt. Despite this small inconvenience,
would you like to travel among the stars on a chemical rocket? Write it in the comments!
Ion rocket: an ion thruster is a type of electric propulsion, capable of creating a thrust
of the vehicle starting from the acceleration of ionized atoms without the need to resort
to high temperatures. The two previous relations also hold for it, in which typically we have
V = 30 km/s, and with M/m(t) = 10 the final speed reaches 70 km/s. Again, an ion rocket
would not be a viable solution to travel in interstellar space, because, given its very
small thrust, it would still take about 20,000 years to reach the star closest to the Sun.
In an attempt to find a viable alternative to chemical and ion rockets for interstellar
travel, between 1973 and 1978 the British Interplanetary Society developed the Daedalus
Project: a two-stage, unmanned interstellar probe with a nuclear fusion engine, able to
travel at 12.2% of the speed of light and arrive in 50 years at Barnard's star, a red
dwarf located 6 light years from the Sun, or in 300 years to Arcturus, an orange giant
in the constellation of Bootes and its star brighter. At the basis of the operation of
the Daedalus there is the interesting idea of a rocket based on the nuclear fusion of
deuterium and lithium to make the spaceship move:
D + Li → 2 He + charged particles
Deuterium and lithium react with each other without loss of energy through neutrons, and
the charged particles emitted can be contained and guided by magnetic fields up to the ejection
nozzle. Unfortunately, it has not yet been possible
to build this rocket, which would also be the basis of generational spaceships: hypothetical
interstellar spaceships capable of traveling at speeds lower (even slightly) than that
of light, self-sufficient and destined to host generations of human beings, in view
of a journey that could last centuries or even thousands of years. Again there would
be pros and cons. The pros certainly include low cruising speeds, given that you travel
at a fraction of the speed of light, and therefore the time of arrival can be measured in hundreds
of years rather than tens of thousands of years. But there are also several questions
still open: for example, how to maintain social peace for long periods? What if part of the
crew mutiny for any reason? How to keep an artificial biosphere inside the spaceship
for an equally long time? How to protect astronauts from ionizing radiation and cosmic rays? Do
you come up with other open questions? Write it in the comments!
Another possibility is represented by antimatter rockets. In physics, antimatter is the set
of antiparticles, having the same mass as the particles that make up ordinary matter,
but with opposite charge. For example, an "anti-hydrogen" atom is composed of a negatively
charged antiproton around which a positron (antielectron) positively charged positron
orbits. The existence of antimatter was theoretically predicted by P. A. M. Dirac in 1928 and experimentally
demonstrated by C. Anderson in 1932 with the discovery of the positron. According to some
theoretical studies, these rockets would allow to reach relativistic speeds, that is consistent
fractions of the speed of light, and the travel times to reach the stars would be noticed
considerably: to reach a star like Canopus, in the southern constellation of Carina (which
has a radius equal to 71.5 times the radius of the Sun), 312 light years away, it would
take "only" 354 years traveling at 0.88 times the speed of light! Would you like to travel
in a spaceship propelled by rockets to antimatter? Write it in the comments! Even in this case,
however, there are negative aspects: first of all, where to find a sufficiently large
amount of antimatter, given that our universe is made up almost exclusively of matter? One
could think of making it through particles accelerators, but how to isolate it from ordinary
matter and make it stable? How to keep it? In any case, even if a way were found to manufacture
antimatter, it would be a long and expensive process.
In case they succeed in building these rockets, the first colonists of exoplanets should face
another big problem: how to get the energy needed to survive? One possibility could be
to build a Dyson sphere around a giant star to absorb all of its energy. Originally proposed
in 1959 by British astronomer Freeman Dyson in the article in Science "Search for Artificial
Stellar Sources of Infrared Radiation", the Dyson sphere is a sphere with a radius equal
to that of a planetary orbit; it would consist of a shell of solar collectors or habitats
placed around the star so that a good part of the energy emitted by the star would hit
a surface on which the energy itself would eventually be reused.
But what and how many are the problems that humanity should face and overcome in order
to create a structure of this type? First of all, find the materials: the entire
mass of our planet would not even be enough to create an infinitesimal part of the structure!
So it would be necessary to use other materials present in the Solar System, such as minerals
present in asteroids, for example. And these materials should be of considerable thickness
and strength to adequately protect the Dyson sphere from impacts with any meteorites.
If the construction of the Dyson sphere were completed, other troubling issues would immediately
emerge. For example, what would happen in the sphere as the heat inside it increases?
Would a "ventilation" system be enough to dissipate excess thermal energy? In the event
of an intense stellar flare, how would the supporting structures of the sphere react?
Another aspect that must be considered is the "social" one; it goes without saying that
the project will have to be shared by all of humanity, but not only that, every single
human being will have to be actively involved in the achievement of the supreme goal, probably
for a period of about ten millennia if not much, much longer. Overlooking what may be
the social, cultural, ideological, religious and political structures that will be necessary
and functional to the needs of the project, this already allows us to glimpse dark and
gloomy scenarios, not only in relation to what is the today's concept of society, but
considering the probable and inevitable "degenerations", these could represent the stumbling block
on which the whole project could be wrecked in a catastrophic way.