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Stellar Birth and Middle Age

What's covered here:

How do stars form? What are the basic ingredients? How does it occur?

How did a star like the Sun form?What happens during the Main Sequence phase of a stars life?

How do stars create energy?

What is the internal structure of a star like?

What determines the life span of stars?

Star formation

Before we start with the ways that stars are formed and evolve, there needs to be some

explanation as to how astronomers can actually talk about this stuff. Typically stars take

millions or billions of years to live their lives, so it is not possible to sit down behind atelescope and watch a star live out its entire life. We just don't live that long. However, we

do have some things to help us understand what is going on. One of these tools is the H-R

diagram. We can't actually see an individual star evolve, so we need to look at a large

number of stars at once. When you observer many stars, you have a pretty good chance of

catching stars in different stages of their lives. The more stars you observe, the greater

your chances of finding a star in one of the shorter, harder to catch phases. If you observe

thousands or millions of stars and describe their characteristics on the H-R diagram

(temperature and luminosity), you will see that there are patterns to their distributions on

the diagram. The information in H-R diagrams, coupled with computer models and

physical theories, help us to understand processes that we can't even see - or at least they

give us a good starting place to work from. By observing as many stars as possible, wehave a better chance of piecing together the picture of stellar evolution. What

characteristics of a star change over time? We can see the changes in temperature and

luminosity, which results in their changing positions on the H-R diagram. These are both

coupled with radius, so that changes as well. The masses of some stars are observed to

change, and to do so requires certain processes to operate inside the stars. We also know

that the generation of energy inside of a star will change its internal composition,

especially in the core. When you change its composition, you change its internal structure,

which of course has an influence on how it gives off energy and how hot its surface is and

how large its radius is and so on. In this way everything is sort of tied together - mass,

temperature, luminosity, radius, and chemical composition. As you'll soon learn, one ofthese physical characteristics will really have the greatest influence and basically

determines most of the other characteristics of a star (you'll have to keep reading to find

out which one is the most important; you think I'd give away the answer this early?).

When we describe the changes in a star's temperature and luminosity, its position on the

H-R diagram changes - we often stay that it "moves" on the H-R diagram. THIS DOES NOT

MEAN THAT IT MOVES IN SPACE!!! The "motion" is just the changes in the temperature

and luminosity of the star that we can observe. Stars do move through space but this

doesn't usually effect their lives. Don't get confused by the references to "moving" on the

H-R diagrams - it's just showing how they change their surface temperature and

ar Birth and Main Sequence Life http://www.uni.edu/morgans/astro/course/Notes/sec

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luminosity over time.

Stellar Birth

Observing stellar birth is rather tricky, since it happens so quickly (relatively speaking),

and it is often not directly visible but is buried under layers of gas and dust. There are

actually two main types of star formation - the large and small scale, or simply the

formation of many stars at once and the formation of only a few stars. Obviously the large

scale star formation will be easier to observe and it is sometimes really apparent. We'll

look at that process first.

Large Scale star formation

When we look into the sky, we often observe stars in groups - these groups are often

dominated by massive, young, hot stars. By this I mean these hot, big stars outshine all of

the other stars, so that you might not even notice the small stars that are there. These

stars are gravitationally bound together, so it is reasonable to conclude that the stars were

formed together. To form so many stars (hundreds, thousands or more) requires a lot of

resources. What is needed for large scale star formation? You need to have a lot of thebasic material that goes into stars, hydrogen and helium (or H and He for short).

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:

Masses of these clouds are typically on the order of millions of solar masses, and on

some occasions up to billions of solar masses (that's the Giant part).

They are cool, around 10 K (this is logical since there are a lot of molecules in them

which would not normally exist in a hot environment).

Gases are generally found in molecular form, with such molecules as H2, CO, CO2,CH, H2O, SiO, etc. Actually, about 150 different molecules have been found in GMCs,

some with up to 70 atoms! Some of these molecules are linked with the formation of

life (as seen in this recent finding from the Herscheltelescope).

There are a few thousand GMCs in our galaxy, and they tend to be found in the areas of

large scale star formation and near very massive, hot stars. GMCs are so cool that the only

way that they can be detected is with telescopes that detect their very low energy light - in

this case radio telescopes would be best. They are very good for detecting complex

molecules in space. You might know these regions as the Spiral Arms of a galaxy (we'll

get to those later on in the course). GMCs are basically the raw materials that make up

stars. There is only one problem - they don't want to be stars (obviously they should not goto Hollywood). Cool gas clouds tend to want to stay as they are - as cool gas clouds. In this

way, GMCs are sort of like Nerf balls - they resist squeezing. If you were to squeeze them

together just a little bit, they would tend to heat up, and this increases the motions of the

material in the clouds (atoms and molecules), which increases the gas pressure, and this

will counter-act the squeezing. If you want to make stars, you have to squeeze the GMCs

so hard that they can't resist or overcome the force of gravity! Somehow or another a very

strong force of compression must be exerted upon a gas cloud, so much so that it can't

counteract it with an increase in the gas pressure. After all, without the compression,

there would be no stars formed and if that were the case, then we won't have anything to


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talk about - and that would be really boring, wouldn't it? Where does the compression

come from? It doesn't really matter where it actually comes from, it just has to be strong

enough to counteract the ability of the cloud to resist compression. You'll later learn about

some of these compression mechanisms that are out there that can do the trick.

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. Accordingto 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 ofnew 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 smallpieces - 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); andmany, 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


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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 ofthis 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 alwaysseen 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 clickhere. 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.


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Figure 4. A large view of the Orion

Nebula, visible in the winter sky. This

image is about 1 degree wide.

AAO/ROE, photo by David Malin

A Hubble Space Telescope view of the innerregion 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 ithere. Image

credit: NASA and C.R. O'Dell and S.K.

Wong (Rice Univ.)

Figure 5.

Two Hubble

Spacetelescope

views of the

central part

of the Orion

Nebula. The

image on the

left shows

the visible

AAO, photo by David Malin

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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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 theright 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-infraredimage:

NASA; K.L.

Luhman

(Harvard-

Smithsonian

Center for

Astrophysics,

Cambridge,

Mass.); and


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G. Schneider,

E. Young, G.

Rieke, A.

Cotera, H.

Chen, M.

Rieke, R.

Thompson

(StewardObservatory,

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

Close-upThis 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


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year long from top to bottom. There are

bright stars located above the pillars (not

included in the pictures, but they would be

above them) 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), andNASA.

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. Asjust mentioned, these stars have pretty strong winds. In Figure 6 the effects of some of

these strong winds are obvious, they can blow apartgas 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 Hubbleimage

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.

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 toshed a lot of their outer layers. Image credit:

NASA, J. Bally (University of Colorado), H.

Throop (SWRI), C.R. ODell (Vanderbilt

University).

Of course, the formation of so many stars at once is easily noticed, particularly because

many of these stars stay in groups or clusters. Sometimes these clusters are sitting right

next to H II regions - which also helps us see where star formation is going on in the

galaxy. Where the stars form in a relatively loose group that eventually breaks down


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(what's known as an association), it is harder to track the point of origin for the stars.

The Orion nebula has been mentioned as a good example of an H II region, and it is also

one of the most studied ones out there. This is due in part to its relatively close distance to

us (about 1500 light-years away) and the fact that the part of the nebula facing us is the

part where all the action is. You are sort of looking straight in at the star formation, so it is

rather obvious what is going on. The hot stars really give it away. Often, we don't have

such a good view. If we were on the wrong side (the cool dark side) of the GMC, we would

not be able to see any hot stars or H II regions since the cool gas and dust would block our

view. To see anything that is made up of cool gas and dust, we cannot use visible light

telescopes, but rather IR telescopes. The IR images that are of star forming regions show

not only the stars and gas associated with the star formation but also the gas that isn't

currently involved in any star formation, since cool gas and dust show up easily at these

wavelengths. IR telescopes reveal the presence of a lot of "invisible" material, which can

eventually go into the formation of stars. Figure 5 shows many of these strong, newly

formed infrared stars in the Orion Nebula. Here is an image from the VLT (visible light)

and the Spitzerspace telescope showing a region of star formation. The cloud's dark dusty

appearance hides all of the star formation that is going on, which can be seen with the

Spitzer'sinfrared eye. It also helps that the stars in this cloud are also "misbehaving" byblasting material out - a good way to spot young stars as you'll see in the next section. The

basic lesson here is don't just trust what you can see with your eyes - use other "eyes" like

radio and infrared to get the entire picture!

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.

Small Scale Star Formation

It is possible to have only a few stars form, rather than thousands of stars forming. These

are formed from obviously much smaller clouds of gas and dust. From such clouds only a

few stars or maybe only one star will form. Such clouds are typically only around 10 solar

masses in size. These gas clouds are pretty small and difficult to find due to the fact that

not only are they small, but they tend to be cool - they don't give off any or hardly any

visible light. I guess we could call them PMC - puny molecular clouds.

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 inthem; 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).

They are already small to begin with, so it is easier

to get them to compress, though there is less mass

involved (remember, gravity depends strongly on

distance, with a small distance having a very large

gravitational pull). As in the case of large scale star

formation, these clouds will give off light primarily in


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IR wavelengths. However there wouldn't be any easy

to see H II regions, since it is unlikely that large

stars will form in this small scale process. The stars

that do form are buried inside of the dusty cloud and are rather cool protostars.

Protostars are what you'd call something that isn't quite a star yet, but is more star-like

than cloud-like. The reason it isn't a star yet, is because it hasn't started any fusion

processes - the mechanism by which stars make energy. I'll get to fusion in a little, just

hang on a bit. It is very difficult to spot these critters, since not only are they cool stars,

but they are also buried deep inside the dust and gas cloud.

As the small gas cloud compresses, it often flattens out into a disk. Why does it do that? It

is sort of like the way that good old fashioned pizza is made. When the dough is being

shaped, it is often spun around - this stretches it out. In the case of these small gas clouds,

they are already spinning around, and spinning things that are compressing have a

tendency to flatten out into disk shapes. It is possible to find these stars by looking for the

disks around them - since the disk can be much larger than the star. Figure 9 shows some

of these disks around young newly forming stars. In case you were wondering, yes, this is

a way that planets can get formed - the material in the disk can eventually form into

planets. Now you should be wondering about whether disks would form around the bigstars - you were wondering that, weren't you? It is possible that disks could form around

them, but since they are in groups, there is also a greater chance that the other stars in

the area could mess up and destroy any disks. Also, the very strong winds that are

produced by massive stars would disrupt a disk of material very quickly.

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 especiallytrue 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.

A star with the mass of the Sun in this stage of its life would have a surface temperature of

only about 4000 K and a radius about 20 times the Sun's current radius. The luminosity is

rather large, mainly due to the large radius, typically about 100 times the Sun's.

These stars are cool and luminous. Are these stars Red Giants? No, as you'll learn, Red

Giants are stars near the end of their lives, while these protostars haven't even ripened

(haven't "turned on" yet). They do have similar outward appearances, but what is going on

inside a Red Giant and a protostar is quite different (of course we can't see inside of them

directly; we just think we know what is happening inside the stars - I'll talk about this

more later). Also, protostars in this stage of their lives evolve very fast, so it is hard to

catch them during this stage. By "fast" I mean around a few million years for the really

small mass protostars to get their act together (become real stars) and only a few


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thousand years for the very large mass protostars to become real stars. Figure 10 shows

where these stars are on the H-R diagram and how they change their temperatures and

luminosities as they evolve.

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 generallyleft-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.

Also with small scale star formation, it

is very difficult to find protostars when they are still surrounded by gas and dust. Oncethey do get close to the Main Sequence, they often start making their presence known by

being rather active. I like to think of this as their "terrible twos stage," sort of like a young

child having a temper tantrum - these small stars do sort of the same thing. This stage,

known as the T Tauri Stage (TT - just like "terrible twos") only happens to small mass

stars like the Sun. What they do is develop very high velocity winds which blow off their

outer layers and the surrounding material (mainly hydrogen). On occasion, this material

will run into other material in the region and form shock fronts, what we call H-H

Objects.

We can see that the various H-H Objects are moving either towards or away from us, so

they have a pretty good velocity away from the protostar. Figures 11 and 12 show some

views of T Tauri stars and their associated activities, including H-H objects.

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 TTauri 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).


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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.

On the Main Sequence (MS)

The "adult" stage of a star's life is when it becomes a Main Sequence (MS) Star. When

does this happen? A star becomes a MS star when it starts fusion reactions in its core.This is the process that produces the energy in a star's core. Along with these fusion

reactions, there are other characteristics of a star that help astronomers understand the

nature of stars. Some of these properties make MS stars distinct from others and help us

understand the way in which stars evolve. You have to remember that we have never seen

inside a star, that we can only study what is on the surface, but there are many ways of

figuring out what is going on deep below the surface.

Fusion

How is energy in a star produced? Originally people thought that the Sun was actually

something that was on fire, like a big block of wood or some sort of "normal" fuel. Whenpeople determined how long a big block of wood, gasoline or some other "normal" fuel

would last, they determined that the age of the Sun would be really young, since these

types of fuels don't last very long. There must be some other mechanism that is creating

the energy. We know that gravitational compression (squeezing) produces heat, so could

gravity be doing it? No, there isn't enough mass in the Sun to produce all of the energy we

get from it. Eventually,Albert Einstein provided the answer in his Special Theory of

Relativity(1915). While there were several different formulas and concepts in this theory,

the most familiar is the famous equation

E=mc2


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What does this mean? All it really says is that if you convert mass into energy, you get a

certain amount of energy out of the reaction that only depends upon the mass involved -

the more mass, the more energy. The formula doesn't tell us howthe energy is produced

from the mass, but just gives us a way to measure the amountof energy produced. Other

physicists came along and determined the processes where the mass is converted into

energy, but Einstein is the one who showed that mass and energy are related. As you'll see

this formula can go either way - you can convert mass into energy or energy can be

converted into mass.

How do you convert mass into energy? I'm glad you asked. The process is known as

fusion, or the fusing together of atoms. This is not what goes on in nuclear power plants,

and actually it is rather difficult to do even the simplest fusion reactions, which sort of

explains why we have no nuclear fusion plants on the Earth. These reactions require very

high temperatures and pressures to work - the main reason it is so hard for people to

duplicate this process on Earth. Sometimes we use the word burning to describe the

energy production in a star, but this is not a very accurate term, since the material isn't

really burning - it just sometimes makes it easier for people to visualize the process.

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:

H1

+ H1 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:

H2 + H1 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 .

He3 + He3 He4 + H1 + H1

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 Walktowards 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 Walkis 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


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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 nearthe 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 haveconvection near their cores and their radiative layer just

below the surface.

Stellar interiors

How do we know what the inside of a star looks like? How

can we really talk about fusion and such with any

confidence when it is pretty much impossible to see

anything below the surface of a star? There are several ways to study the interior of a star.

One already mentioned is helioseismology, which can also be applied to other stars(where it is referred to as asteroseismology). The study of a star's light is not a good

indicator of the interior since it is altered so much on its journey from the center, and it is

also old light - it takes a long time to travel through all those layers. The only thing that

comes immediately from the center of a star is the neutrino, one of the by-products of the

fusion reaction. Neutrinos are really weird little critters. These are particles that we don't

know much about, in part because we have a very hard time detecting them. There are

thousands of neutrinos going right through your body right now. Does it hurt? No, because

neutrinos travel through pretty much everything without causing any problems - they

don't interact well with matter. That's why it is so difficult to study them - we can hardly

ever catch them! It isn't really impossible to study them, but it is just very difficult.

Neutrinos are so elusive, that we are not even sure if they have mass. Our best currentestimates put there mass at less than 5 x 10-37 kg, though we aren't certain about the

lower limit to the mass. For comparison, a proton has a mass of around 1.67 x 10-27 kg,

which means that a proton is at least 3 billion times more massive than a neutrino.

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?


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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.

Figure 15. The Super-Kamiokande Neutrino detectorin Japan. This large vat of ultra pure water will detect

different types of neutrinos. The spheres seen along

the side of the tank are light detectors that register the

rare interaction of a neutrino with a water molecule.

Image courtesy of ICRR (Institute for Cosmic Ray

Research), The University of Tokyo .

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 neutrinothat 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 (see Figure 15), 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

Hydrostatic Equilibrium - This is the balance between weight (gravity) and

pressure (air pressure or gas pressure). This is like how the old dome on theUNI-Dome worked (and how the Metrodome in Minneapolis works). The cloth dome

was held up by air pressure inside of the UNI-Dome, while the weight of the dome

was always exerting a downward force due to the pull of the Earth's gravity. The air

pressure and the gravity need to be evenly balanced so that the roof doesn't cave in -

which happened at those times when it wasn't balanced. A star is similar. The outer

layers of the star are being continually pulled in by the star's gravity. These layers

don't fall in because the high pressure of the gases inside of the star prevent that.

The star is balanced - it doesn't cave in on itself or blow itself apart.

Conservation of Energy- Energy is given off but it is continually produced. Another

way of saying this is that stars keep on shining; there are no gaps in the production of


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energy. This is important because it helps regulate the temperature and density

inside of a star. The energy from the core flows out from a star continuously - it

doesn't get trapped inside various regions but flows through them. This is a good

thing, because if it didn't happen, if energy got stuck in a certain layer, then that

layer would heat up to an abnormally high value, which would screw up the internal

structure of the star.

Conservation of Mass - A star's total mass is the sum of all of its layers. This one

actually sounds kind of silly, because it is saying if you take all the parts of a star and

add them up, you'll get the total mass of the star. What this rule really does is

account for the fact that not all layers of a star have the same mass - some layers are

thicker than others; some are thinner since the density inside of a star changes from

one layer to the next. You have to account for this change in density when you do add

up all the layers. It is actually rather important.

Energy Transport laws - These are the rules governing energy flow mechanisms

such as convection and radiative transport. These rules also tell us how much energy

is being transported by these various mechanisms; in some layers it is all radiative,

in some layers it is all convective, and in some layers it is a mix of both.

By combining these rules with other laws of physics and observations, astronomers can

make computer models of stars which can then be altered to show how stars will live their

lives. Of course, the results from the computer models can be compared to observations of

stars. This makes life pretty easy for astronomers since stars take a very long time to get

from one phase of their lives to another, so sitting behind a telescope and waiting for a

star to change is pretty much a waste of time. A computer can speed up the process quite

a bit.

What is the inside of a star like? We know that various physical characteristics like

temperature, density and pressure change as you go from the center of the star to the

surface. In the center the temperature and density values are the highest (temperatures of

several million K, for example), which is a pretty good thing since there would be no fusionwithout these very high temperatures and densities. As you get further and further from

the center of the star, the temperature and density decrease quite drastically. By the time

you get to the surface of the star, the temperature is in the range of only thousands or tens

of thousands degrees, quite a bit cooler then the core. The density is really low at the

surface. Actually, the density is so low that it is lower than the density of air molecules in

the room that you are sitting in now - the gas is really thinned out at the surface.

Remember, in the core, densities are more than 10 times the density of lead! Stars are

objects of very extreme conditions. See Figure 16 for some results from a computer model

for the Sun.


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Figure 16. 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 17. 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.

How long will a star be a MS star? A

looooooonnnnngggg time! Stars spend 90%

of their life on the MS. Boy, is it dull! Being

on the MS is sort of like middle age. You get

up, go to work, come home, watch tv, go to bed, repeat. This type of routine is boring!

Except, a star never stops working; it is constantly fusing hydrogen into helium (and

making energy) while it is on the Main Sequence. Main Sequence life spans for a variety of

stars are given in the table below.

It is easy to figure out how long stars will last on the Main Sequence, since it depends onhow much fuel they have (mass) and how quickly they use it (luminosity). The time a star

spends on the Main Sequence can be approximated using the following formula

time on MS = (1/M2.5)x 10 billion years

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


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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 -

At the top left end of the MS- High Temperature, High Luminosity, High Mass, Short

Life (millions of years)In the middle of the MS - Medium Temperature, Medium Luminosity, Medium Mass,

Longer Life (billions of years) - this is where you'd find the Sun

At the lower right end of the MS - Low Temperature, Low Luminosity, Low Mass,

Long Life (10s or 100s of billions of years).

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, sincethat is sort of an approximation for all stars, so it isn't too good in some cases.

MassSpectral

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 Million5 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

Now that you've read this section, you should be able to answer these questions....

What is needed for large scale star formation?

What types of stars (mass, spectral type) are formed in large scale star formation?

How are H II regions formed?

How does small scale star formation differ from large scale star formation (apart from

the size)?

What are TT Tauri stars and H-H Objects?

How does the mass of a star influence the pace of its formation?

What do Main Sequence stars do that makes them Main Sequence stars?


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What happens during fusion in the Sun?

How does the energy from center of the Sun get to the surface?

What are neutrinos?

How are astronomers able to understand the workings of the interiors of stars?

What does the Main Sequence life span depend upon?

How do characteristics of stars vary along the extent of the Main Sequence?


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