While there is mainly hydrogen floating around between the stars, in what we call the interstellar medium, there is also a little bit of dust also there - that's part of the 2% that makes up stars. This dust is very important for several reasons. Since each dust particle is more massive than a gas particle, it has a greater gravitational pull and would help in the process of forming stars. Also it can block energy/light that could break apart molecules. Another very important aspect of dust is its influence in our view of the sky. Dust is so good at blocking light that star light can not easily travel through clouds of dust and because of this many stars are not visible to our eyes. And if the dust does not entirely block the light it can just make stars look dimmer - this can really mess up our calculations of their distances since we think that the stars are fainter than they really are.
When you look up at the fuzzy band of the sky that we call the Milky Way you are looking at the thickest concentration of the gas and dust in our galaxy, and also where a lot of star formation is occurring - where the hydrogen and helium gets concentrated into stars. But enough about the raw ingredients, let's get to making some stars!
Looking out into the galaxy, we can find many Giant Molecular Clouds (GMCs). As the name implies, these are large clouds of gas and dust. They have very distinctive characteristics:
Let's say we get the thing compressed by some strong force. What happens to a GMC when you do compress them?
As with anything, when you start to compress it, material gets closer together. According to Newton's law of gravity, a decrease in distance leads to an increase in gravity (remember, gravity goes up as distance goes down). The material is feeling a higher gravitational pull (the gravity that each bit of the cloud feels from all of the other bits). If gravity is strong enough, it will bring the material together even more, which will increase the gravity and bring the stuff even closer, which increases the gravity... you sort of get the idea. Once you overcome the barrier of the cloud's resistance to the compression, it will pretty much give up and start collapsing down. This will continue until the individual stars form - remember, stars are just big gas balls, so as various parts of the cloud clump together, they will form clumps of gas which become stars. Figure 1 shows how this happens. You might want to make note of the fact that only part of the GMC will get compressed and have star formation occurring - not the entire cloud. With each batch of new stars forming, there could be hundreds or thousands of stars created at one time.
Figure
1. The compression of a GMC occurs only in a small part of the
cloud. The rest of the cloud is not effected. It should be noted that
the colors shown here are not accurate - in general you can't even see
the clouds since they are so dark. They would appear like a black
region to the eye.
What kinds of stars will form? Will they all be big stars? Will they all be little stars? The variety of stars that form is sort of like the variety of pieces of glass you get when you break a window or a drinking glass. Usually there are many small pieces - these would be the very low mass stars, which (when they reach the the Main Sequence) are the K, M, L and T types (less massive than the Sun). There will be few of the more massive stars, the A, F and G types (which have masses similar to the Sun) and very few of the really big stars, the O and B types. This kind of makes sense, because we see very few O and B stars out there - they are pretty much outnumbered by the little stars. As you'll see there is another reason why O and B stars are so rare, while the little ones are so common, but we'll get to that later.
Figure 2. The part of the cloud that was compressed
breaks up into stars in a distribution with few large, hot stars (OB
stars); more middle sized stars (AFG types); and many, many cool, small
mass stars (KLMT types). The coolest stars would not be visible to the
eye since they are mainly infrared sources.
Even though they are greatly outnumbered, the big stars, the O and B types, are the most important ones in the group. Why are they so important? What characteristic do these stars have that set them apart from the other stars? They are very hot! These beasts are so hot that they give off a lot of UV radiation. UV radiation is very important since it can ionize the gas around the stars (it ionizes the hydrogen mainly). Remember, when light ionizes something it means that it knocks an electron off of an atom. This happens quite a bit, as does the reverse process (the electron getting back into an orbit around the atom). The end result is that the gas glows. You get a lot of hot glowing hydrogen gas and perhaps other types of gas that are glowing, though there is much more hydrogen around the star formation region than anything else.
Now we have a region around the hot O and B stars that is just a huge cloud of hot glowing ionized gas (hydrogen). Such a region is called an H II region - the Roman numeral II means that one electron has been lost. How would you pronounce the name of this region? You say "H two." That makes it a bit confusing, since it sounds the same as saying H2, which is how we refer to molecular hydrogen. Yes, it is confusing, but we like it like that. Just remember, H II indicates that you are talking about hot, ionized gas, while H2 indicates that you are talking about cool, molecular gas.
We have to get back to the star formation region and see what's happening there. The O and B stars are quite effective at ionizing the hydrogen. They can ionize a large area around them, producing a very bright H II region. When you observe these regions you are basically seeing hot, thin, glowing gas (mainly H). If you were to look at the light from the H II region, you could obtain a spectrum of it - what type of spectrum? An emission spectrum - remember, that is what is produced by a hot gas. The emission spectrum of hydrogen has a very strong red line in it, so there is often a rather pink-ish glow to these regions. Another tell tale sign that you are looking at an H II region is that there are often traces of the cool molecular gas that usually look like dark blobs. This is because it has dust in it and the gas in these dark clouds isn't hot enough to emit light, like the H II region gas can. It is sort of neat that these very different gases can be right next to one another. Other things to look for in the H II region are the culprits that are causing all the trouble - the O and B stars. These will be the brightest of the stars in the area, so they often stand out quite well. You may also note that these stars tend to be very bluish. Figure 3 shows the characteristics in and around an H II region.
Figure 3. The Triffid Nebula (M 20) is a good
example of an H II region since it shows the different features around
it. First of all is the H II region itself - distinguished by the
pinkish glow. Next is the presence of dark dust that blocks
light. This is referred to as a dark nebula or just basically dust.
However, under certain conditions, it will not look black but instead
will appear blue as is seen on the right. A reflection nebula is just a
region of dust that is reflecting blue light towards you. These bluish
regions are not always seen around H II regions but are seen often enough to
be recognized. They can also be seen in other circumstances.
At the center of the H II region are
the hot stars that are maintaining the high temperature of the gas in
the region. The image is over exposed so that these stars are not
visible amongst the hot gas. To see another view of the Triffid
Nebula, just click here. This shows both the
visible light view as seen at the left and an infrared view, as seen by the
Spitzer telescope. The IR view shows the extent of the gas and dust,
often in areas that aren't seen in the visible light view.
There are quite a few examples of H II regions, though one of the most spectacular is the Orion Nebula (Figure 4), which is visible in the winter sky. Other star forming region are shown in Figure 6.
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| Figure 4. A large view of the Orion Nebula, visible in the winter sky. Image credit: ESO/Igor Chekalin | A Hubble Space Telescope view of the inner region of the Orion Nebula. Compared to the other image, this covers only a tiny fraction of the bright inner core. To see how the image was obtained by the HST, you can view a little movie of it here. Image credit: NASA and C.R. O'Dell and S.K. Wong (Rice Univ.) |
 | Figure 5. Two Hubble Space telescope views of the central part of the Orion Nebula. The image on the left shows the visible light view. In this view you can see the four hot stars in the center that form the "Trapezium" and provides most of the energy that keeps the nebula hot. Also, individual gas and dust clouds are visible. The image on the right was obtained with an infrared camera on the Hubble, and in this instance, the many small, newly formed stars are visible. Notice how many of these stars are not visible in the image on the left. Credits for near-infrared image: NASA; K.L. Luhman (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.); and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory, University of Arizona, Tucson, Ariz.) Credits for visible-light picture: NASA, C.R. O'Dell and S.K. Wong (Rice University). |
 Figure 6. The Eagle Nebula (M 16) - this is another region of star formation, but unlike the Orion nebula, the stars are not formed on the inside of the cloud but are being formed on the outer edges. The huge pillars of gas shown are quite large, with the one on the left being about 1 light year long from top to bottom. There are bright stars located above the pillars (which are visible in x-rays as can be seen in this image) which are blowing away gas from the pillars. The gas that remains behind forms into stars. Image credit (both images): Jeff Hester and Paul Scowen (Arizona State University), and NASA. |
 Close-up This shows the top of the pillar on the left. The long tube-like structures on the edges of the cloud are parts of the cloud that are breaking away and forming into stars. These areas appear to contain a concentration of gas and dust that doesn't get entirely blown away by the hot massive stars located above the cloud. Whatever remains behind will eventually become a star. |
A bunch of stars have been formed. What happens now? It is possible
that the formation of stars will lead to further star formation. How
does that happen? The new stars, again the O and B ones especially,
have some rather strong winds. These winds can compress other parts of
the GMC and you know what happens when you compress a gas
cloud, right? Star formation! Tt is possible for the whole process to
continue like a domino effect until the cloud is gradually used up. In a way, the star
formation process slowly eats away the GMC. This also tells us that
when you see a region of star formation, there should be a pretty big
gas cloud in the area, much larger in size than the H II region that is
visible to your eye. There can also be a rather negative side effect to
having such massive stars involved. As just mentioned, these stars have
pretty strong winds. In Figure 6 the effects of some of these strong
winds are obvious, they can blow apart gas clouds - so it is
possible that some stars-in-the-making could be destroyed in the
process. Take a peak at Figure 7 for a view of some of these wanna-be
stars (which we call proplyds), and you may also want to check
out this animation showing the inside of the
Orion nebula and the future evolution of the stars in it (we'll get to
that later). In the animation you may notice how many of these blobby
stars are elongated - stretched out - by the strong winds from the hot
stars near the center of the nebula. Here
is an animation showing the zoom into the Hubble image and revealing
the proplyds as they really are - some of these are shown below in
Figure 7. In general these objects are several times larger than
our solar system (hundreds of A.U.s in size), but as they evolve, they'll
lose their cocoon of gas and dust.
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Figure 7. Two stars in the process of forming in the Orion Nebula, as seen by the Hubble Space Telescope. These "proplyds" (as they are called) are more gas cloud than star at this point and are feeling the effects of the strong stellar winds near them. Each proplyd is several times larger in size than our entire solar system and they have a long way to go until they become stars - mainly they have to shed a lot of their outer layers. Image credit: NASA, J. Bally (University of Colorado), H. Throop (SWRI), C.R. O’Dell (Vanderbilt University). |
GMCs are needed for large scale star formation. The formation is so massive that it can be seen over a great distance - even in other galaxies, since OB stars and H II regions are very bright. However, not all stars are found in such groups, so there must be a way to do small scale star formation. After all, the Sun is not part of a large group of stars - so we need to make stars on a smaller scale. We'll look at how that happens next.
Figure 8. A Hubble Space Telescope image of some
dark, small mass clouds. While these clouds look pretty dusty, they
really don't have that much dust in them; maybe only 1% is dust. The
dust, though, is very efficient in blocking out the light.
Image credit: NASA and The Hubble Heritage Team (STScI/AURA).
Figure 9. Disks around young protostars are shown in
these two Hubble images. In both cases the disks are tilted so that
they are seen edge on. There is dust in the disks, so they tend
to look very dark and are hard to see. This is especially
true for the disk on the right. Some hot gas around the newly forming
star is seen above and below the disk, while material getting shot out
from the poles (which look green here) indicates that there is a young
star in the middle (this will be explained further down the notes).
Image credit: Chris Burrows (STScI), John Krist (STScI), Karl
Stapelfeldt (JPL) and colleagues, the WFPC2 Science Team and NASA.
Figure 10.Early stages of stellar evolution,
pre-main sequence. As gas clouds get converted into protostars and the
protostars gradually stabilize into stars, they evolve in a generally
left-ward motion on the H-R diagram (towards higher temperatures). The
lines indicate the paths on the H-R diagram that protostars of
different masses will take as they head towards the Main Sequence.
Graph based upon stellar evolution computer models of
Siess,
Dufour and Forestini.
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Figure 11. Some time lapse images of T Tauri stars. These are images obtained from the Hubble Space Telescope showing the mass ejections due to strong winds from young stars (really still protostars). On the left is XZ Tauri, actually two stars in orbit about one another. The bubble of material bursting from them is moving at a speed of about 300,000 mph. On the right is HH 30 - the T Tauri star that has a disk of material around it and is erupting matter in two directions - you're just seeing one gusher here. Image credits: (left) John Krist (STScI), Karl Stapelfeldt (NASA JPL), Jeff Hester (Arizona State University), Chris Burrows (ESA/STScI); (right) Alan Watson (Universidad Nacional Autonoma de Mexico, Mexico), Karl Stapelfeldt (NASA JPL), John Krist (STScI), and Chris Burrows (ESA/STScI). |
Figure
12. Several T Tauri stars producing some H-H objects. These Hubble
images show mainly the H-H objects, since the T Tauri star is not
visible behind a layer of gas and dust or inside the disk of material.
However, the stream of material being ejected by the star is visible,
and the pile up of the material in the form of the two H-H objects is
also easily seen. You can click on the image to see a larger view.
The scale in each image is the equivalent to 1000 A.U., or 1000 times
the distance between the Earth and the Sun. Credit: C. Burrows (STScI
& ESA), the WFPC 2 Investigation Definition Team, and NASA.
It is interesting to note that the material is being ejected from the young star only in two directions, known as a bipolar flow. Why doesn't it just blow material out in a uniform direction or into a random direction? It is possible that there is some influence from the magnetic field of the star and/or the presence of a disk of material that would obstruct the flow of material in any random direction. In some cases disks are visible, but not all. As you'll see, a lot of things have a bipolar outflow - this is a very popular way for objects to spew material out into space. The T Tauri stage is short lived, and the star eventually settles down and becomes a boring, normal Main Sequence star.
The main reaction operating in the Sun and other low mass MS stars is the Proton-Proton chain (or the p-p chain). To get this reaction to work you need a temperature of at least 13 million K and a density of about 100 gm/cc. If you are not familiar with density, then you might find it interesting that the density of metal is around 7 gm/cc. This is much denser than most metals that you come into contact with. Something you might also want to remember is that a proton is really just a hydrogen atom without an electron (an ionized hydrogen atom). The core of a star is very hot, and the atoms are colliding around quite a bit, so it is really easy for the atoms in the core to get ionized. There are a whole bunch of protons just bouncing around in the core - but you can also think of them as hydrogen atoms bouncing around in the core - it doesn't matter how you visualize it.
Here is the step by step process:
First have two protons come together and form into deuterium
(H2), with by-products of a positron and a neutrino.
Deuterium is an unusual form of hydrogen, sort of like over-weight
hydrogen. The positron (e+) is just like an electron,
except that it is positively charged. A neutrino ( ) is a weird little
particle that is very difficult to detect. Here's the reaction
written out:
 H2 + e+ + |
 |
Now have the deuterium fuse together with another proton to
form a type of helium that is a bit light weight (He3) and
also a gamma-ray photon  . The gamma-ray is the energy that the star has produced.
Here it is written out:
He3 + |
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| Take the light weight helium and have it combine with
another light weight helium to produce normal helium (He4)
with two protons left over .
He4 + H1 + H1 |
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Basically, you start with four protons (hydrogen nuclei) and end up with a helium nucleus (which contains two protons and two neutrons) and some other stuff (the positron and neutrino, oh yes, and some energy). If you were to put this stuff on a scale and measure the mass of what went into the cycle and what came out of the cycle, you would see that they don't weigh the same. The difference in weight between these items is the mass used to create the energy in the reaction. Even though only a minuscule amount of energy is produced in each reaction, stars do this reaction so many times that there is a great deal of energy given off. A star like the sun does this reaction about 100,000,000,000,000,000,000,000,000,000,000,000,000 times each second. Even though a single reaction doesn't produce enough energy to keep a gnat alive, stars produce a huge amount of energy, since the reaction is performed so often.
In stars more massive than the Sun, another fusion reaction is at work, the CNO cycle. This does basically the same thing as the p-p chain but uses other elements, namely carbon, nitrogen and oxygen, to bring about the production of energy and the other byproducts (Helium, positrons, and neutrinos).
Fusion is the source of energy (photons) in a star. The density of the interior of stars is so high that the radiation can't go very far without being absorbed, deflected and bounced around by all the stuff on the inside. Actually, it is rather difficult for the photons to go very far at all without hitting something. The path that the light takes from the core to the surface is rather chaotic.
The light produced in the core will take a Random Walk towards the surface. This little walk can take around 200,000 years or even more. At the onset of the walk, the photon is a high energy gamma-ray, but it loses most of its energy in all those collisions it has as it works its way through the Radiative Zone. In this region, radiation (energy, light, photons, etc.) is transported according to the rules that govern how radiation interacts with materials - all of those collisions, absorptions, and emissions that go on in the dense material. That's why this layer in a star is called the Radiative Zone.
Figure 13. Energy
produced in the center of the Sun (or any other star) bounces around
for years as it works its way towards the surface. This Random Walk
is due to the way that matter and energy interact. The process will
eventually stop once the density of the material is too low for there
to be significant interaction - once it gets out of the radiative zone
in the Sun.
Further out from the center, the material is less dense, so the radiative processes are not as important. Here the energy is transported via convection in what is called the Convective Zone (isn't that clever?).
Remember, convection is also the process you see when you boil a pot of water on the stove. Energy is transported by the motion of the material - the churning and bubbling of the gas. At the top of the convective zone, the gas is thin enough for radiation to easily escape - these are the layers of the star's surface that we see - the atmosphere. If you remember the stuff about the Sun's surface, the tops of the convective bubbles are what we call the granules. The situation for larger stars is actually quite different - they have the convective zone near the core with the radiative layer near the surface. This just has to do with the way that the material acts under different circumstances - whether there is convection or not going on. Either way, energy produced in the core of a star, where it was originally a high energy photon, will eventually get to the surface of the star where it can then fly off into space in the form of a relatively low energy photon, usually in the visible part of the spectrum.
Figure 14. The interior of a star like the Sun is
shown. The three main layers of the core, radiative zone and convective
zone are depicted. For larger stars, the order of the various regions
in the interior does not appear to be the same as is shown here. Those
stars tend to have convection near their cores and their radiative
layer just below the surface.
Actually, there was a major problem with neutrino studies of the Sun. There have been neutrino experiments going on for some time, some working for over 30 years, and all of the detectors got basically the same result. We detected fewer neutrinos than we were supposed to detect based upon the laws of physics. This used to be a major problem in astronomy since it has some rather nasty aspects. Why were too few neutrinos detected? Was there something wrong with the inside of the Sun? If that were the case, it may be bad news for us, since our existence depends on the Sun. Was there something wrong with all these theories that we have to explain what happens inside of stars? If that were the case, we would have some serious conflicts between the rules that we are using and the experiments where the results do come out okay. Is there something wrong with the neutrino detectors? Well, every neutrino detector is different, and every detector seemed to get the same result - that there were too few neutrinos coming from the Sun.
What's the answer? It seems that neutrinos aren't all alike. Actually,
we knew this previously, but we didn't know that the neutrinos coming
from the Sun do change in subtle ways, becoming a type of neutrino that
is incredibly hard to detect. Before, neutrinos were just really
difficult to observe, but this other type of neutrino was so close to
impossible to detect that no one ever saw it. That was until a really
amazing neutrino detector was built (follow
this link), and it started to
pick up all these very difficult to detect neutrinos. It seems that the
neutrino problem is no longer a problem. Sometimes it takes better
theories to figure out what's going on; sometimes it takes better
equipment.
Looking at neutrinos helps us to figure out what the Sun is like, but
other stars are so far away that it is nearly impossible for us to look
for their neutrinos. How can astronomers figure out what is going on
inside all of the other stars out there? How do we know if there is
convection or radiative energy transport occuring inside of them? In order to
understand how stars work, astronomers must apply several laws of
physics to them and these laws, combined with observations, provide
astronomers with a better pictures of stellar physics. These rules are
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| Figure 15. Results from computer models of the Sun. These graphs show the calculated temperature and density for a star like the Sun. The graphs are set up with radius increasing to the right, and extend from the center (where the radius =0) to the surface (radius=1). The temperature and density values vary over a wide range of values. Click on each image to see a larger version. Data is from the solar model of J. Christian-Dalsgaard et al (1996). | |
I think that is enough talk about physics and computers. Let's get back to the Main Sequence discussion. When a star finally turns on (starts fusion reactions), it is at the starting point of its life on the Main Sequence. While on the MS, it is burning hydrogen in its core - and nothing else. Initially stars are said to be on the Zero-age MS, the point when they have just started to burn their fuel - brand new stars. Most stars we see in the sky are not on the ZAMS, since they have generally been burning fuel for some years. The ZAMS is really only a theoretical concept that is most useful when we make computer models to study stellar evolution.
Figure 16. The Zero-Age Main Sequence (or ZAMS) is
shown - it is the green line. This is only a theoretical location on
the H-R diagram and is used to show the likely location where a newly
formed star would be found before it has burned any hydrogen. As stars
undergo fusion they gradually follow the paths indicated by the light
blue lines until they reach the end of their Main Sequence phase -
indicated by the dark blue line. The masses of the individual stars are
indicated. This graph is based on computer model data and is not based
upon actual stars.
where M is the mass of the star in units of solar masses. If you put the Sun's mass in there - you'd get the time the Sun will spend on the Main Sequence, about 10 billion years. If you put in greater masses, the fact that the mass is being divided into10 billion will always give you a shorter time. For example, a 5 solar mass star would give you an age of 1/52.5 x 10 billion = 1/55.9 x 10 billion = 10 billion/55.9 = 179 million. This formula is only an approximation, but you can see that a greater mass means a shorter Main Sequence life. Even though high mass stars have more mass, they burn it much more quickly and end up having very short lives (remember the mass-luminosity relation of MS stars? Massive stars have very high luminosities and use their fuel up very quickly).
Stars on the MS are arranged in very orderly way -
The entire history of a star, how quickly it forms, where it resides on the MS, its life span on the MS, the mechanism it uses to produce energy, its internal structure and what happens to it after it leaves the MS is dependent primarily on MASS! Pretty much everything about a star's life depends on its MASS. Don't forget that! The table below gives some values of time that a star will spend on the Main Sequence based upon computer models. You would not get these exact ages using the formula given above, since that is sort of an approximation for all stars, so it isn't too good in some cases.
| Mass | Spectral Type | Years on Main Sequence |
| 25 | O7 | 6.4 Million |
| 20 | O9 | 8.1 Million |
| 15 | B0 | 11.6 Million |
| 12 | B1 | 16.0 Million |
| 9 | B2 | 26.4 Million |
| 7 | B4 | 43.1 Million |
| 5 | B6 | 94.3 Million |
| 3 | A0 | 351.7 Million |
| 2 | A5 | 1.1 Billion |
| 1.5 | F2 | 2.7 Billion |
| 1 | G2 | 9.4 Billion |
| 0.8 | K0 | 22.8 Billion |