We've looked at different types of stars on the main sequence - but how do they form ? First we need some stuff to make stars with...
With telescopes we can see that our galaxy contains more than just stars. There are lots of clouds of gas and dust (mainly made from flakes of Carbon) around. We can see them as dark patches, completely blocking (absorbing) light from stars which are behind them. There is a huge amount of gas and dust around in our galaxy, yet space is an excellent vacuum. Both these statements are true as space is so big!
Any passing shockwave (like that from a supernovae - we'll do more on this in a few lectures time) can trigger gravitational collapse of these giant clouds of gas and dust. As the clouds collapse, the gravitational energy is converted into higher velocity of the gas and dust, i.e. the cloud is heated. The energy gets radiated away and the cloud contracts further... eventually the cloud becomes so dense that the heat (long wavelength light) cannot escape freely. It rattles around inside, interacting with the electrons many times before it finally struggles out to the surface (i.e. blackbody radiation). The temperature of the core goes up, so the collapse is slowed down and the proto-star is in hydrostatic equilibrium, with the pressure from the weight of the outer layers pushing in on the core is matched by the pressure from the hot gas inside the core, though unlike stars on the main sequence the energy to keep the gas hot comes from gravitational contraction, and it has to keep on contracting to remain hot. Their temperatures are generally lower than those of the mainsequence star they will eventually become.
If the cloud is massive enough then the core temperature eventually reaches the temperatures needed for nuclear fusion reactions to begin, forming a star. But for cloud masses below 0.08x that of the sun then the core temperature never goes high enough to get fusion reactions going.The cloud keeps on contracting, converting gravitational energy to heat - these are called brown dwarfs, until at long last electron degeneracy pressure (and we'll do more on this in a few lectures time as well) finally stops it collapsing any further.
Big clouds which will form high mass stars collapse faster than smaller clouds because they have more gravity. So the high mass stars are born first and they emit lots of blue and ultraviolet light. If there is still lots of gas and dust around then this light can't get out. Instead it interacts many times with the dust - absorbed, emitted, absorbed, emitted.... - i.e. blackbody radiation. Since the cloud is much bigger than the original star then the temperature has to be much lower - cloud glows in the infrared. But this also pushes away the gas and dust. Eventually the cloud gets less dense, so the photons and electrons don't come into equilibrium and intead we see emission lines from the gas. The blue and ultraviolet photons can excite the electrons in hydrogen atoms, causing the electrons to leave the atom altogether (ionisation). Any other electron in the gas can recombine with the proton, but the electron need not go straight into the very bottom energy state. It can recombine first to a higher energy state, and then drop down the rest of the energy levels in steps, giving the energy out as emission lines (as in the a few lectures ago) The most prominant line is generally one from Hydrogen, called the Halpha line. Its wavelength is such that the gas glows pink! These are called emission line nebulae. See Nick Strobel pages on gas.
So there we have a young star embedded in diffuse gas and dust. The size of the dust particles is generally a bit smaller than the wavelength of blue light. Particles scatter light most effectively if the light has wavelength comparable to the size of the particle. So dust scatters blue light more effectively than red light (e.g. cigarette smoke look blue). When we look at starlight that has to travel through a lot of gas and dust then the star looks redder than it actually is, since its blue light has been scattered out from our line of sight (think of how red the sun seems at sunset! - and how blue the sky is which is just the scattered light!). So if we have a cloud of gas and dust away to the side of a star, the dust can scatter the blue light from the star and reflect it. So we see a fuzzy blue patch, called a reflection nebulae. See Nick Strobel pages on dust.
The NASA Observatorium has a superb site for the birth of stars from giant clouds of gas and dust. Read all of it! Also Giant clouds to proto-stars are covered nicely by Nick Strobel, together with a section on their evolutionary tracks on the HR diagram.
The cloud rotates faster and faster as it contracts - as in the rotating lecturer - but for those of you who missed it, think of a spinning skater: they start spinning with their arms out, then as they bring their arms towards their body they spin faster. Its called conservation of angular momentum). This causes the cloud to flatten into a disk (think of how my hair went out horizontally!) rotating around the central core. There are compilations of pictures of disks around young stars, and a nice view where we are looking straight down on the disk. The disk can then condense into planets, while the radiation from the young star blows away the remaining cocoon of gas and dust.
If this is correct then planets round single stars should be fairly common. They are!! A nice compilation of the known extrasolar planets around nearby sun-like stars. These have mostly been found using the doppler shift to measure small wobbles in the orbit of the star caused by the planet's gravity pulling on the star (see especially the animation on how it works. This site also links to a a huge amount of information compiled under general information, including a list of articles about planet finding
So in order to have life we'd want there to be
a) a planet (can't do life on the surface of a star)! Probably a rocky
planet would be best as well.
b) some sort of solvent for biological activity - water is the one we
use for life on earth, and its a very very common molecule, so in all
probability it'd be the one used by alien lifeforms too
We've seen that its very easy to make planets - there are a lot of
them about. But these were jupiter mass and above so NOT rocky. Its
very hard to detect rocky planets as they have low mass, so don't
produce much of a doppler shift. But since its easy to produce
jupiters, then maybe its easy to produce rocky planets too. Then look
at their distance from the star - are they in the habitable zone,
where liquid water can exist ? In our solar system only earth is -
mercury and vensus are too close to the sun, and too hot. No liquid
water, only steam. And mars is too cold - no liquid water only ice...
But maybe there was water on mars in the past - there are features on
the surface that look VERY like water erosion features. See more on
surface features of mars. So maybe life
could have started evolving there in the past ? We don't know, though
there was all the fuss a few years ago about the martian meteorite,
which was claimed to show evidence for fossilised bacteria
A nice compilation of articles from the debate which followed is
here.
The jury is still out as to whether this is fossilised bacteria or
crystaline structures. And if it is fossilised bacteria then could
these have grown from the time the meteorite spent on Earth rather
than on Mars? Whatever the outcome, what most people would like is not
dead bacteria but live, intelligent, self conscious, alien lifeforms.
And for that you probably need lots of time for evolution to get to
such complexity. On earth we've had about 4.5 billion years. So don't
go looking for intelligent life around high mass stars as they don't
live long enough! Have more of a look at
conditions
for extraterrestrial life.