IN THE BEGINNING...
Stars are known to have existed eons before the first planets ever formed.
After the Big Bang, a lot of matter was created, quarks floated freely in the newborn space and time.
As
the volume of space increased over some million years, temperatures
decayed (volume, temperature and density relationships as Boyle
describes) enough so UP and DOWN quarks got married in groups of 3, the
Proton and the Neutron, some quarks still floated freely, creating
electrons and other sub-atomic particles.
Still,
a lot of stuff existed in the baby Universe, and yet, nothing existed
at all, everything was nothingness, it was too hot for those protons and
neutrons to come together and form any atoms, so, every matter wasn't
matter at all.
The simplest atom is Hydrogen,
it consists of one Proton in the nucleus, being orbited by one
electron, of course, chemistry nerds well remember that there are 3
types of Hydrogen, ie, Isotopes: Protium, which was already presented;
Deuterium, having 1 Proton, 1 Neutron and 1 electron; Tritium. having 1
Proton, 2 Neutrons and 1 electron, this isotope in particular is
radioactive.
Of
all Hydrogen in the Universe, at this point in history everything is
Hydrogen, the amount of each isotope is respectively, 99,985%, 0,015%
and ~0,00000000000000010%, of all mass.
The first stars, born
from the condensation of those pure Hydrogen clouds would have been
massive in unimaginable scales even for stars, imagine the largest star
ever found, UY Scuti, with gargantuan 1.708 Solar radii,
which is about 1.1 billion KM, in the place of our Sun, its surface
would scratch at 7,6 AU or slightly smaller than Saturn's orbit (9 AU).
Primitive stars of the first Eon of Light would have been 10x to 100x larger than UY Scuti, living only for a few thousand to 5 million years in average.
The
reason why they got so big was because of Hydrogen is light-weight,
once start fusing it into a new element, Helium, the star will generate
enormous amount of heat and light (radiation in general), it will
increase the temperature exponentially until it reaches some sort of
hydro-static equilibrium and will stay round and burning its hydrogen
until it haves to fuse Helium into Beryllium, and then into Oxygen, and
into Carbon and into Iron, which kills the fusion process as it is
required more power than a living star can generate to fuse.
I said
LIVING STAR, remember, those primitive stars where drowned in Hydrogen
gas clouds, they simply grow big and explode (die like in a Michael
Bay's movie :p) in a few thousand years, releasing those new elements
in the space to be re-absorbed by new stars.
If we could fast-forward
history in this point, the universe would see like someone had lighten
up a bunch of firecrackers in space, stars simply appear grow and
explode in almost no time...
As time pass, it will slow down the time stars live before explode, why?
Imagine,
or, get a bunch of gunpowder, light a spoon of it... It will burn very
fast, now, gunpowder is the hydrogen consumed by early stars, consumed
in no time at all. Now add up to the next spoon some sand and/or corn
starch, mix it well, and light up again, it will burn more slowly, the
impurities we added are just the other elements created by stars over
time.
Stars that are rich in carbon, oxygen and other metals live for
some longer amount of time, even tho those elements are no more than 1%
in the universe (99.9% of the universe are stars as well) or even less.
Longest living stars are more dense, thus creating more pressure and
dying in supernovas that are powerful enough to fuse Iron into other
heavier elements, like Uranium, Osmium, Cobalt and Gold...
Ok, so for now we learned a bit of history, let's go to astrophysics...
STAR TYPES
Stars are often classified by temperature, which leads to average composition as well.
You probably have heard of our Sun being a G2V star, or just G star.
The letter G is
the temperature class of our Sun, ie, other G stars are similar in
temperature, and varying slightly in composition as well. The 2
is the sub-class, each letter has 10 sub-classes by temperature from the
coldest to hottest, our Sun is a cool yellow star. The roman numeral 5 "V" means that our Sun is a main-sequence star, ie, our Sun has already reached "stellar adulthood".
There
are 7 main spectral types of stars, in the list bellow they are
organized from the hottest to the coldest, also hottest stars live less
due radioactive decay in stellar wind and photosphere, and cooler stars
live longer because they burn their fuel more slowly.
For an idea, expect blue giant stars O to not live more than 10 Million years in average, G yellow stars like our sun are more likely to give birth to life in their Billion year life-spans, M red-dwarf stars could also be likely to give rise to life and sustain it in a Trillion year life-span.
O - Giant Blue stars
B - also Giant Blue stars
A - White-Blue Giant stars
F - White large stars
G - Yellow sun-like stars
K - Orange small stars
M - Red dwarf stars
If
you are going 3D with those, I strongly recommend you to download the
image and pick the RGB code from each star as base for you lighting
apparatus, for more realistic touch.
HABITABILITY
As
we have seem, hot stars not live enough for any planet to form around,
if it does so, the star will die before any life form even rise.
LETS GET RID OF O, B, A
to be home stars, forget them, unless you wanna establish a outpost or
mining colony around those radioactive time-bombs in your story, but is
like 99% under any possibility that life could naturally occur or thrive
in such environment.
This left us with F, G , K and M stars, those stars are not so radiative, and live enough so any planets form and naturally stay safe enough to occur life.
Looking back at our planet's history, its been know that primitive life existed way back in the end of the Hadean Eon,
4 to 3,8 billion years ago, that is an important aspect, it tells us
that, by the average formation and cooling time of an Earth-sized planet
is at least 500 million years.
This is very helpful is establishing the evolutionary state of overall life in such a world around a known star.
If
we take Epsilon Eridani as an example, 10 LY away in the Eridanus
constellation, it is around 650 million years old, as well, planets
around it are more or less that age, if any Earth-sized planet exist in
its habitable zone and stayed safe enough for life to occur, it would be
no more than bacteria and a few thriving stromatolites, or mostly
archea and primitive algae, like the Earth in the Archean Period 3,85
billion years ago.
Unless your story involves planetary seeding
(panspermia type event), is rather impossible for jurassic-tier
creatures to live in a planet as young as the Cambrian Earth, even
atmospheric conditions like lack of oxygen, ozone, and excess methane
make those young planets uninhabitable to advanced life, as well the
young bedrock can't yet sustain plant and vegetable growth yet without
enough time to erosion to mix substances and break volcanic rock into
sand and dirty.
GETTING EVERYTHING FROM MASS
Stars
have different masses, usually the lower the temperature the smaller it
is as well its mass. Giant stars have similar spectra of some small
stars and are a very confusing deal to astrophysics in establishing
fixed values for proportions, so ugh, stay away from giant stars.
Our ranges for a habitable star, capable of hosting a home planet to alien species, usually go from 0,6 to 1,4 M⊙.
Consider some larger stars and brown dwarfs as places suitable for settlement, and we have something around 70 MJupiter to 1,6 M⊙.
If
we are going to make our star entirely fictional then there are some
formulas to figure out what it will look and behave like, all calculated
from its given mass:
These
formulas are proportional to our Sun, input 1 solar mass for Mass and Sun's Age, and
it will give our solar system's parameters. The further away we move from
sun-like stars to small red dwarfs or bright F stars, it will be rather
inaccurate - though adjustments have been made to accommodate stars up to 10 Solar Masses.
BROWN DWARFS
What about L, T and Y classes? Well they're not properly stars, actually they are Brown Dwarfs, Jupiter-sized objects, with somewhat dozens to hundreds of times more mass, enough to fuse Tritium and Deuterium, is very unlikely that those worlds ever had "moons" (gas giant moons as usually planet-sized, take Callisto for example). If one of those objects ever had a system like the Galilean moons, their heat and light wouldn't be enough to sustain life.
If a brown dwarf is 0,01% as luminous as our Sun, 0,01%L⊙ is slightly more luminous than Teide 1, the first Brown Dwarf discovered and still rather very bright, a planet/moon would have to orbit around 15 million km (0,10001 AU) from its brown dwarf to receive the same light as Earth does from the Sun, theoretically having a zone suitable for planet existence as far as 2 AU.
Still, with as much mass as 57~65 Jupiters in such a small place, roughly 3 RJupiter planets in there would suffer from some serious tidal pull, turning any world in some unstable mess of earthquakes. Going further away to 0,35 AU, where only 8% of total light reaches, average temperatures for a terran atmosphere would fall around 1ºC. Safer from tidal force, still very cold, likely having life in some sub-surface ice or cold pool, but very rare.
THE HR DIAGRAM & STELLAR EVOLUTION
In astronomy, the Hertzsprung-Russell diagram is a distribution graph that shows the relationship between the absolute magnitude or luminosity versus the spectral type or stellar classification and the effective temperature. The Hertzsprung-Russell diagrams are not mapping the location of stars. Instead, they place each star on a graph indicating its absolute magnitude or brightness against its temperature and color.
Hertzsprung-Russell diagrams are also referred to by the abbreviations diagram H-R or HRD. They were created around 1910 by Ejnar Hertzsprung and Henry Norris Russell and represent an important step towards understanding stellar evolution.
A simplified HR diagram looks like this:
Depending the stellar class and age, a star can move through various different regions of the HR Diagram, like so...
Different stars make slightly different paths along the HR diagram and at different speeds, most of the star's useful lifetime it's on the Main-Sequence stage, which for Sun-like star's is about 10 billion years, for the exception of the Main-Sequence and White/Black Dwarf stages, the other parts of its life-cycle are radically shorter - only a couple thousand to million years...
This totally breaks most of people's conceptions about stars and planetary systems as being rather stable over time - they're not at all for the vast majority of cases we know of, the rule of thumb is being highly dynamic.
While on Main-Sequence, a star increases slowly in brightness - Sun-like stars start their lives with ~70% of their nominal luminosity (ie, the luminosity we calculate for it's MS phase), and steadily increase to 200% by the start of the Red Giant phase.
To calculate your star's luminosity according to it's age in billion years, use:
One could assume this is to tryhard science too much - but for our purposes, building up the entire history of an alien world, this is very much needed, this is also useful in case we want to have a very young or old star in our setting.
The above graph shows us estimates for the Initial Mass of the Sun according to geological records of Mars and Earth - these are still somewhat debated due the (in precise astronomical terms) huge difference. Because stars burn their fuel away to keep shining, they lose a tiny amount mass over time, which over the eons can amount to several percent.
Depending on the star's initial mass, it's early phase as a T-Tauri star may be dimmer or brighter than it's nominal luminosity, thus, the star have a weaker or stronger light-pressure and stellar-wind strength which blows away some of it's mass.
For Sun-like stars this can be described based on it's age:
The first parentheses is the adjustment for mass-loss rate and fraction of the Sun's mass, while the second one weights the age on the process - this particular format will estimate the initial mass by about +0,07 M⊙.
This mass-loss and age relationship feeds us with quite some material to build up and simulate our star system's evolution over time.
Assume for a moment that we have a lone planet orbiting our star in a perfect circle by the time it forms, as the star loses mass, our planet drifts outwards with each orbit, becoming more eccentric over time, because it's mass decreases along with gravitational pull, whereas the lone planet retains it's momentum.
As T-Tauri stars decrease quickly in mass as the circunstellar disk develops, we have two factors at play that alter a planet's orbit around it.
A - Planetary mass increase quickly over time, and so does the gravitational pull, thus causing the planet to de-orbit over time.
B - The decrease in stellar mass also decreases the gravitational pull, thus causing the planet to drift outwards.
The balance between these factors will cause the planet to migrate inwards, outwards, or stay in place while it develops - I say "while it develops" because when in MS phase, this mass loss is very minute.
If there are other planets in the system, this shift in orbital positions can easily mess up their orbits as well - keep that in mind.
The Red Giant phase lasts only about a billion years for stars of moderate mass, increasing their nominal luminosity by a factor of 3 (1000x) very quickly, before dropping back to a brief period of stability that lasts about ~100 million years, after that time, it shreds into a planetary nebular or goes supernova if it's sufficiently massive, becoming a degenerate stellar object.
For stages beyond Main Sequence, the following graph can be used as reference:
If your star exists, like Epsilon Eridani, or Gamma Serpentis, fill the given values above with the star's actual values, is very useful because it get your work down to figuring out just habitable zone and system boundaries in some cases.
Go ahead and pursue your ideal star, or you can use the Python applet below:
- M. O. Valent, 18/02/2019
- M. O. Valent, last updated in 02/06/2021
