Reporter:
Ronny Green reporting Live –
WE INTERUPT YOUR NORMAL PROGRAMMING TO BRING YOU THIS SPECIAL NEWSBREAK. I’M HERE, AT UCSD’S CENTER FOR ASTRONOMY WHERE GRADUATE STUDENT ERIC WHITE, STUDYING UNDER THE DIRECTION OF DR. RED, HAS DISCOVERED S STAR. WE HAVE GATHERED A PANEL CONSISTING O F ASTRO-PHYSISIST DR. BLACK, STELLAR EVOLUTION EXPERT DR. BLUE, AND STELLAR STRUCTURE EXPERTS DR. NEAL YELLOW WHO WILL JOIN US FOR THIS SPECIAL DISCUSSION ABOUT THE DISCOVERED STAR.
Dr. Red – tell us about your discovery
Dr. Red:
This particular star, named fondly Etoile, is simply a M2.
Reporter:
What exactly is a M2?
Dr. Red:
The M2 comes from the Harvard Spectral Classification. It is a categorization based on lines that are mainly sensitive to the star’s temperature rather than its gravity or luminosity. It was found that this type of classification could be put into to a decreasing temperature order It goes: O - B - A - F - G - K - M. There is also subclasses between 0 - 9. An example of a Harvard classification is our own particular sun. The sun is a G . Its surface temperature is about 5500 degrees Kelvin with H and K lines very strong but the H I lines getting weaker. Etoile, being a M2, has a surface temperature about 3000 degrees Kelvin. The color is red due to the TiO bands getting stronger and the Ca I lines.
Reporter:
How big is Etoile?
Dr. Red:
Etoile has a solar radii of approximately 1.5 Au.
Reporter:
What is the Mass?
Dr. Red:
The mass is about 4 solar masses.
Reporter:
Can you describe the structure of Etoile?
Dr. Red:
I could but we have here two specialists here today on stellar structure: Professor Black and Professor Yellow.
Dr. Black:
Thank you Professor Red. The star has six regions. Core, radioactive zone, convention zone, photosphere, chromosphere, corona
Reporter:
I heard that energy is produced in the core. Am I correct?
Dr. Black:
Etoile and other stars obtain their energy through nuclear fusion. In the core, the existing pressure (about 10^11 atmosphere pressure) and the density is about 10^26 particles per cm) is sufficiently high enough for nuclear fusion to occur.In this pr ocess, 4 protons fuse to form an alpha particle. In this case, it’s the helium nucleus. The difference is mass is converted into energy. This can be explained by Einstein’s famous equation, E=mc^2. Most of this energy is in the form of gamma rays such as Xays radiation. The energy of these emitting photons is proportional to the frequency F.
Reporter:
Is energy produced the same way in the radioactive zone?
Dr. Black:
Energy is really produced in the radiactive zone. Instead, the energy, in the form densely packed, fast moving particles. The moving atoms create waves of radiant energy called infrared rays. Since the photons are so close together that the photons c ollide with each other, combine and then re-emit outwards. This type of motion can be described by the mean free path of the photons. The moving photons create waves of radiant energy called infrared rays. As distance increases from the core, the temper ature begins to drop. Because radiant heat normally flows from a hot place to a cooler one, the energy produced in the sun's core flows through the radiation zone, toward the surface of the sun
Reporter:
Does nuclear fusion exist in the radioactive zone?
Dr. Black:
Less pressure, no fusion, but energy radiates outward
Reporter:
Why is there a convection zone?
Dr. Black:
Beyond the core, the pressure is so low due to the decrease in pressure that nuclear fusion does not take place so much. Instead, the radioactive energy propagates outward and collides with each other in their attempt to move outward. During this proce ss, the energy of the individual photons thus the frequency.
Reporter:
How does the convection work?
Dr. Black:
As the distance further away from the core, the temperature decreases and creates a large temperature gradient. This lost of temperature does not allow the photons energy outward. Instead, energy transport is dominant by convection.
Dr. Black:
Convection can be seen in the photosphere as granulation.
Dr. Yellow:
The photosphere is a relatively thin region (Only in the hundreds of kilometers thick in our sun) that lies between the convection zone and the chromosphere. "Photosphere" literally means sphere of light; It is the region responsible for a majority of the visible light emitted from a star. It is the densest part of the atmosphere - much more dense than gases found in the chromosphere and corona. Still, the density here (for an average sized star like the sun) is much less than even the density of air o n earth. Because the photosphere is dense and bright enough to keep us from seeing through it, it is often referred to as the ‘surface’ of the star. Appearing on this surface are many features of great interest, including granulation, sunspots, and sola r flares.
Granulation arises from the effects of convection in the convection layers. As convection cells rise they become smaller and smaller, the smallest ones reaching the photosphere- this process may take as long as 10 million years before radiation from t he core reaches the surface- even longer in Beta Seven. Brightness is directly related to temperature- so as these convection cells reach the surface, bringing hotter plasma up to the photosphere, we see a small bright spot, And in areas where the plasma has cooled and is falling back down through the convection zone, we see a slightly darker spot. These spots are called granulation cells. Granulation cells are 1500 to 2000 km across. This pattern of lighter and darker regions is called granulation. And though granulation cannot be seen on any star besides the Sun, the existence of convection on other stars is inferred from spectroscopy by the mixing to the surface of elements produced deep within the interiors and by other, less direct observations.
Our esteemed collegue, Mr Grad Student White, will tell us about sunspots.
Reporter:
Please tell us about Sunspots.
Dr. Yellow:
The presence of sunspots has been identified on StarX. However, we have not yet had time to examine in depth sunspot development on this newly found star. GRAD STUDENT WHITE will explain what we have determined, from study of our own Sun.
GRAD STUDENT WHITE: (Show Sunspots overview Frame 1) Sunspots appear darker than the rest of the visible solar surface because they are cooler. Most of the visible surface of our Sun has a temperature of about 5400 C, but in a large sunspot th e temperature can drop to about 4000 C. They usually come in sizes between about 2500 km and about 50,000 km
DR. RED: The temperature of the visible surface of StarX has been calculated to be approximately 3000 C. The larger sunspots there may have a temperature of 2000 to 2500 C. This is not known yet.
GRAD STUDENT WHITE: Yes. (Show Sunspots overview Frame 2)
Shown here is a detail of a sunspot from the Sun. They emerge when magnetic field pops up at the solar surface from below in the form of many thin, bent strands, called magnetic flux loops tubes. These are less than about 150 km across and clump t ogether at the surface, forming greater concentrations of magnetic field. When theses concentrations are a few hundred kilometers across, they show up as pores. When the pores grow to be about 2400 km across, they develop a penumbra and change into sunspo ts. On our Sun they generally grow for a few days, and then start to slowly decay again. The biggest of sunspots may live for up to a few weeks.
DR. RED: We speculate, however, that sunspots will exist for much longer periods of time on Etoile.
GRAD STUDENT WHITE: Yes. The amount of magnetic flux that rises up varies with time in a cycle called the sunspot cycle or solar cycle. This solar cycle is commonly used as a gauge of solar activity. Our Sun’s cycle lasts 11 years on average.
DR. RED: We have not had time to examine a full period of StarX’s solar cycle. However, we speculate that it will be much longer than that of the Sun’s, easily exceeding our life span.
Reporter: Do sunspots appear in a pattern or randomly on a star?
GRAD STUDENT WHITE: We have observed that sunspots do not appear everywhere on the Sun.
(Show Distribution Frame 1)
They are usually concentrated in two bands, about 15 - 20 degrees wide in latitude, that go around the Sun on either side of the solar equator, and the average latitude of those bands varies with the solar cycle. Just after the minimum of the solar cycle, sunspots appear at an average latitude of about 25 - 30 degrees. As the solar cycle progresses, newer sunspots appear closer and closer to the equator, and the last sunspots of the solar cycle appear at an average latitude of about 5 - 10 degrees. This behavior is sometimes called Spoerer's Law. It is very rare to ever find sunspots at latitudes above 70 degrees.
Reporter: You mentioned that the solar cycle was used as a gauge of solar activity could you explain how this is done?
GRAD STUDENT WHITE: With the use of a term defined as the sunspot number, R (Show Solarcycle Frame 1) Shown here. R equals the number of sunspots, I, plus ten times the number of sunspot groups, G, seen on the Sun with a particular standard tele scope. K is a constant "smoothing factor"
DR. RED: The smoothing factor K is used to rationalize the number sunspots observed with the resolving power of that individual’s telescope. (ie Hubble vs. back yard RadioShack type)
GRAD STUDENT WHITE: (Show Solarcycle Frame 2)
The Solar cycle displays a record of the sunspot number vs. time. On the Sun this relationship approximates a sine-squared function with a period of about 11 years. Notice, however, two subsequent solar cycles are not well separated, almost regar dless of what measure you use. Even the time of lowest sunspot number is not well defined: There are many days with a sunspot number of 0. When the old cycle is winding down the new one is already gearing up, so one can have sunspot groups from the old and the new cycle on the Sun at the same time. If you take the appearance of the first sunspot of the new cycle as your definition of the start of the new cycle, then you run into the problem that the identification of the cycle of a new sun spot group is not always straightforward, especially at the beginning of the new cycle when sunspots are commonly very small. Due to such issues with cycle date identification, the start of a new cycle is stated with an uncertainty of one o r two months.
Reporter: What creates the disturbance in the magnetic field that causes the sunspots?
DR. RED: The differential rotation of the Sun is thought to be the cause of its solar cycle.
GRAD STUDENT WHITE: The most popular explanation is the Babcock model. (Show Babcock Model Frame)
To begin with the star’s magnetic field is poloidal. The differential rotation of the Sun causes the magnetic field near the equator to rotate faster than the magnetic field near the poles, giving the magnetic field a toroidal component and causin g the strength of the magnetic field to increase. Babcock estimates that after 3 years the magnetic field near the equator has rotated 5.5 times more around the Sun than the magnetic field near the poles. When the toroidal magnetic field gets s trong enough in places, then it erupts toward the surface and forms an active region with sunspots.
DR. RED: Eventually, part of the magnetic field of the decaying active regions moves toward the equator and cancels with field of opposite polarity coming from the other side.
GRAD STUDENT WHITE: Yes the remaining field components move to the poles. After all the cancellations, the Sun has again a poloidal magnetic field but of the opposite polarity as before.
DR. RED: It is hard to say if our Sun would show any sunspots if there were no differential rotation. Differential rotation explains why magnetic field appears at the surface, but not why it should have the form of many sunspots with the observed s izes, shapes, and other characteristics. It seems that other processes on much smaller scales than differential rotation must be occurring to cause such variation.
GRAD STUDENT WHITE: True, but as of yet such information has not been determined. Perhaps the future study of the sunspots of StarX, and comparisons to the sunspots of our Sun, will provide the insight necessary to answer such questions.
Reporter: Very Nice. Where do solar flares come in?
Dr. Yellow: The intense magnetic activity connecting sunspot pairs sometimes causes an eruption of plasma originating in the photosphere that follows the magnetic field lines. When these eruptions are large enough they can escape the magnetic field l ines and become large solar flares reaching out even beyond the corona. Solar flares on our sun can disrupt tv reception here on earth, but last only for an hour or so.
Reporter: If the photosphere is considered the surface of the star, then what’s in the atmosphere?
Dr. Yellow: Above the surface is the chromosphere. The word 'Chromosphere' means sphere of color, named so because during a solar eclipse it appears as a thin ring of red color around disk of the moon. Because the chromosphere is much less dense than the photosphere, light from the photosphere is able to pass through it. The interesting thing about the chromosphere is its temperature gradient. At the outer edge of the chromosphere, there is a sharp rise in temperature. Since the source of heat is t he fusion taking place in the core, one would expect that the star would be cooler the farther from the core you go. That holds true until you get to this point in the chromosphere. This rise in temperature has yet to be explained, but it is speculated that some kind of magnetic waves may be responsible for transporting heat from inside the star out to this layer.
Reporter: So after this rise in temperature do thing finally begin to cool down?
Dr. Yellow: Exactly. Outside the chromosphere is the corona. The corona is even less dense than the chromosphere. It expands outward into space, becoming less dense and cooler as it expands. In our solar system, this expansion is called the 'solar w ind', and has effects on the entire solar system, including the earth.
Reporter:
Could we go back a step to the central structre of the star. Why exactly is it important to known the content of the core?
Dr. Black:
Determine the age
Determines the cycle
Dr. Black:
Luckily today we have professor blue who can talk about the life cycles is a star.
Dr. Blue:
Before I start talking about the growth and death of the star. It is probably important to understand the origins of the star. Professor Red could you please explain the birth of a star?
Dr. Red:
A. star-birth is influenced by such fundamental effects as:
rotation
magnetic fields
gravitation
gas pressure
Grad Student White:
In very simple terms, then, we believe that stars form bythe condensation and collapse of huge interstellar clouds.
Reporter:
Can you describe in more detail how this Star originated?
Grad Student White:
(Step1)
Start with cool gas (a "chilly" 10 K - 50 K!) like a nebula (show nebula
pictures), which begins to collapse.
It can't be too hot or the gas pressure will be to high and nothing will
happen.
Thus it keeps away from hot stars, while dust to block the starlight
aides in this.
Reporter:
How long does this Collapsing stage take?
Dr. Red:
Somewhere around several million years
After that, the collapsing cloud fragments into numerous clouds that each
continue to collapse.
we call these "cloudlets". Show picture (initial collapse.jpg)
Grad Student White:
(Step2)
The cool gas is attracted to the center by gravity.
Recall that every atom attracts every other atom. The closer the atoms
are, the bigger the attraction.
(Step3)
The cloud gets smaller and denser.
(Show sequence of cloud.jpg pics up tp cloud3.jpg to illustrate this.)
Dr. Red:
to add to that
After the collapsation starts slowing, due to the heat in the core, the
cloud becomes very dense (10^6 particles/cc)
Then the core heats more and more as material from the envelope continues
to rain onto the core.
Reporter:
Can you expand on the part where density rises in the center?
Dr. Red:
Material piles up at the cloud center and density rises in the center
The core begins to collapse faster than the outer envelope.
(Show cloud4.jpg).
Grad Student White:
(Step4)
Then It starts to rotate because of conservation of angular momentum.
The easiest way to see this is to demonstrate it. It's like a figure
scater doing a spin.
(Step5)
The outside parts form into a disk.
A disk is the only possible shape for gas in orbit around a central mass.
The disk configuration keeps gas cloudlets from hitting each other.
The disk is called a protoplanetary disk.
(Step6)
As the central part of the disk continues to contract, it gets warmer.
Whenever you compress a gas, it gets warm.
This is energy conservation at work: gravitational potential energy is
turned into heat energy.
(Step7)
As the temperature goes up to several thousand K, the protostar starts to
radiate.
The inside continues to heat up as it contracts.
When the center gets dense enough and hot enough ( a few x 106K), nuclear
fusion begins.
Reporter:
At what temperture is the core at this point?
Grad Student White:
The core reaches 2000K when the H2 molecules break apart. Due to the
fact that this takes a lot of energy, the core collapses again
Reporter:
And at what temperature does the cloud begin to look like a star?
Grad Student White:
Approximately 10,000K, and at this point it is called a Protostar.
(Step8)
And after the Protostar stage, the central heat source stabilizes the
pressure, halting further contraction.
Now the energy radiated is supplied by nuclear fusion instead of further
contraction.
(Step9)
A star has been born.
Reporter:
To Add to that an outer envelope - like a womb - shields the entire event
from view at optical wavelengths. The dust in the envelope heats up and
becomes a very large region glowing brightly in the infrared part of the
spectrum.
Then the outer envelope is blown away and a pre-main sequence star emerges.
This is a violent process - a rapidly spinning and still collapsing outer
disk collides with a now expanding shell from the newly formed star.
Motion above and below the rotational plane of the nebula is easier and
hence material "squirts" away at right angles to the disk.
The images show clearly the jet like structures (bi-polar flows) ejected
from the collapsing stellar cloud and protostar.
[Image]
Associated with jets are bright knots of light called Herbig-Haro objects.
We think that these are the result of shock-wave heating produced by
supersonic matter ejected along the jets.
As this matter plows into dust and gas left over from the collapsing
nebula it heats the gas and produces these bright, fast moving "blobs".
Reporter:
How long does this whole process take?
Grad Student White:
The entire process - from large cloud to sun-like star takes about 30
million years (give or take a day or two).
Reporter:
After it is formed, how long does a star stay in stability.
Grad Student White:
Once on the main-sequence the star enters a period of relative
stability and will spend approximately 90% of its lifetime in this phase.
Dr. Red:
Stability is due to hydrostatic equilibrium.
Reporter:
What is hydrostatic equilibrium?
Dr. Blue:
Hydrostatic Equilibrium
Gravitational forces that tend to collapse the star are opposed by pressure derived from the heat of fusion in the core region of the star.
As fusion fuel is reduced, the pressure generated is reduced, allowing gravity to compress the core further.
Compression of the star generates heat that can increase the rate of fusion, thus reacquiring equilibrium.
Reporter:
So what’s the next stage?
Dr. Blue:
Electron Degenerate Stars
As the core is compressed, heat is generated.
The added heat excites the electrons, freeing them to form an electron cloud.
As this cloud grows, it generates its own pressure to counteract the gravitational forces independent of fusion and temperature.
Stars whose mass is not large enough for gravity to overcome this "electron degenerate" pressure (about 1.4 Solar Masses) will stop compressing.
Since these stars no longer compress, the fusion process will slow down and the star will begin to cool. This is known as a "White Dwarf".
Reporter:
Is that like a red Giant?
Dr. Blue:
Red Giants
If there is sufficient mass to overcome electron degenerate pressure, the star will continue to compress and heat up.
Eventually, there will be enough heat to cause the Helium in the core to ignite creating a red giant.
A hydrogen shell will still be present around the core, which will also undergo fusion.
Reporter:
What exactly is a red giant?
Dr. Blue:
The energy release here will push outward and cool.
The visible outer layers of these stars are much larger and cooler than a main sequence star.
Because of its large size, and the redder colors emitted from cooler gasses, they are called red giants.
Reporter:
What happens if it was larger?
Dr. Blue:
Larger stars will progress to burning Neon, Oxygen and Silicon.
Neutron Degenerate Stars
Compression continues until the free electrons in the core are forced to combine with the protons in the core to form a core of only neutrons.
Inverse b decay.
Photodisintegration of atomic nuclei.
These neutrons are then compressed until they generate a "neutron degenerate" pressure that act similarly to electron degenerate pressure.
Stars that cannot overcome this pressure end their lives as "Neutron Stars".
Neutron stars have a strong magnetic field that can pulse as observed from Earth due to their rotation. This is similar to a lighthouse.
Black Holes
If there is sufficient mass to overcome all degenerate pressure, hydrostatic equilibrium will continue to regulate the stars size and fusion rate.
Since there is no end to its compression it compressed into a single point called a "Singularity", which is the center of a black hole.
Black holes are formed when the remaining mass is around 4 or 5 times the size of the Sun.
The Big Boom
As very large stars advance to burn silicon, an iron core will begin to form where no fusion takes place.
Since there is no fusion in this core, there is no pressure to counteract gravity.
When the iron core exceeds 1.4 Solar masses, it will collapse.
This core collapse takes place very fast (» 1mSec), so the outer layers fall inward to the core and bounce off in an explosion called a "supernova".