Star Facts :

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Star Size end states :

Stars up to 0.08 x Sun : do not have enough mass to start the fusion process and are called Brown Dwarfs

Stars from 0.08 to 0.8 x Sun : none will have yet exhausted their supply of Hydrogen and so are all still on the main phase - what happens next ?

Stars from 0.8 to 8 - 10 x Sun : will become Red Giant then planetary nebula and White Dwarf.

Stars from 8 - 10 to 20 - 30 x Sun : will become Red Giant then go supernova leaving Neutron Star.

Stars from 20 - 30 to ? x Sun : will become Red Giant then go supernova leaving Black Hole.

Stars over ? x Sun will ?

Core Sizes for end states :

? to 1.4 Solar Masses = White Dwarf

1.4 to 1.7-1.8 Solar Masses = Neutron Star

1.7-1.8 to ? Solar Masses = Black Hole

___

Sun Facts :

http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter

Size radius : 695,500 km

Size dia : 1,391,000 km

compared to Earth : 109 earths

volume : in fact 1,300,000 Earths would fit inside the sun.

Mass : 1,989,000,000,000,000,000,000,000,000,000 kg or 1.989 x 10 30 kg

compared to Earth : 5,973,700,000,000,000,000,000,000 kg : Sun is 333,000 x earth

Compared to Jupiter : 1,898,700,000,000,000,000,000,000,000 kg

Jupiter is very similar compostion to a star and if it had been 80 times more massive it would have become one.

Gravity at the surface : 274 m/s2

compared to Earth : 28 x earth (9.8 m/s2)

Escape velocity : 2,223,720 km/h - 55 x Earth

rotation :

velocity realtive to near stars : 19.7 km/s ( 70,920 km/hr )

Sun

Luminosity (1024 J/s) 384.6

Mass conversion rate (106 kg/s) 4300.

Mean energy production (10-3 J/kg) 0.1937

Surface emission (106 J/m2s) 63.29

Spectral type G2 V

Model values at center of Sun:

Central pressure: 2.477 x 1011 bar

Central temperature: 1.571 x 107 K

Central density: 1.622 x 105 kg/m3

Rotational and Orbital parameters

Sidereal rotation period (hrs)* 609.12 23.9345 25.449

Obliquity to ecliptic (deg.) 7.25 23.45 0.309

Speed relative to nearby stars (km/s) 19.4

*This is the adopted period at 16 deg. latitude - the actual rotation rate varies with latitude L as:

( 14.37 - 2.33 sin2 L - 1.56 sin4 L ) deg/day

North Pole of Rotation

Right Ascension: 286.13

Declination : 63.87

Reference Date : 1.5 Jan 2000 (JD 2451545.0)

Sun Observational Parameters

Apparent diameter from Earth

At 1 A.U.(seconds of arc) 1919.

Maximum (seconds of arc) 1952.

Minimum (seconds of arc) 1887.

Distance from Earth

Mean (106 km) 149.6

Minimum (106 km) 147.1

Maximum (106 km) 152.1

Solar Magnetic Field

Typical magnetic field strengths for various parts of the Sun

Polar Field: 1 - 2 Gauss

Sunspots: 3000 Gauss

Prominences: 10 - 100 Gauss

Chromospheric plages: 200 Gauss

Bright chromospheric network: 25 Gauss

Ephemeral (unipolar) active regions: 20 Gauss

Solar Atmosphere

Surface Gas Pressure (top of photosphere): 0.868 mb

Effective temperature: 5778 K

Temperature at bottom of photosphere: 6600 K

Temperature at top of photosphere: 4400 K

Temperature at top of chromosphere: ~30,000 K

Photosphere thickness: ~400 km

Chromosphere thickness: ~2500 km

Sun Spot Cycle: 11.4 yr.

Photosphere Composition:

Major elements: H - 90.965%, He - 8.889%

Minor elements (ppm): O - 774, C - 330, Ne - 112, N - 102 Fe - 43, Mg - 35, Si - 32, S - 15

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Good Link with diagrams

http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap16.html

Luminosity :

Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun.

However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass.

The more massive a main sequence star, the brighter and bluer it is.

Supernova :

Type 1a : White Dwarf Supernova

If accretion of matter from a companion star or the merger with another white dwarf, pushes a white dwarf star over the Chandrasekhar limit, it will collapse, heat up and explode like a thermonuclear bomb, leaving nothing behind. The expanding cloud of ejecta glows brightly for many weeks as radioactive nickel produced in the explosion decays into cobalt and then iron.

Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the same amount of light. This property makes them extremely useful as a distance indicator - if one Type Ia supernova is dimmer than another one, it must be further away by an amount that can be calculated. In recent years Type Ia supernova have been used in this way to determine the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy.

Neutron Star :

Pulsar

http://www.phys.vt.edu/~jhs/faq/pulsars.html

Black Hole :

Super Massive Black Holes

Quasar

http://www.phys.vt.edu/~jhs/faq/quasars.html#q2

Life Cycle

Birth of Stars :

Astronomers believe that molecular clouds, dense clouds of gas located primarily in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form "protostars". Initially, the gravitational energy of the collapsing star is the source of its energy. Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.

Main Sequence Stars :

Main sequence stars are stars, like our Sun, that fuse hydrogen atoms together to make helium atoms in their cores. For a given chemical composition and stellar age, a stars' luminosity, the total energy radiated by the star per unit time, depends only on its mass. Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is. For example, Sirius, the dog star, located to the lower left of the constellation Orion, is more massive than the Sun, and is noticeably bluer. On the other hand, Proxima Centauri, our nearest neighbor, is less massive than the Sun, and is thus redder and less luminous.

Since stars have a limited supply of hydrogen in their cores, they have a limited lifetime as main sequence stars. This lifetime is proportional to f M / L, where f is the fraction of the total mass of the star, M, available for nuclear burning in the core and L is the average luminosity of the star during its main sequence lifetime. Because of the strong dependence of luminosity on mass, stellar lifetimes depend sensitively on mass. Thus, it is fortunate that our Sun is not more massive than it is since high mass stars rapidly exhaust their core hydrogen supply. Once a star exhausts its core hydrogen supply, the star becomes redder, larger, and more luminous: it becomes a red giant star. This relationship between mass and lifetime enables astronomers to put a lower limit on the age of the universe.

Death of an Ordinary Star :

After a low mass star like the Sun exhausts the supply of hydrogen in its core, there is no longer any source of heat to support the core against gravity. Hydrogen burning continues in a shell around the core and the star evolves into a red giant. When the Sun becomes a red giant, its atmosphere will envelope the Earth and our planet will be consumed in a fiery death.

Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and the star becomes a red supergiant. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star

White Dwarfs + Planetary Nebula

Death of a Massive Star :

Massive stars burn brighter and perish more dramatically than most. When a star ten times more massive than Sun exhaust the helium in the core, the nuclear burning cycle continues. The carbon core contracts further and reaches high enough temperature to burn carbon to oxygen, neon, silicon, sulfur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by burning it to any heavier element. Without any source of heat to balance the gravity, the iron core collapses until it reaches nuclear densities. This high density core resists further collapse causing the infalling matter to "bounce" off the core. This sudden core bounce (which includes the release of energetic neutrinos from the core) produces a supernova explosion. For one brilliant month, a single star burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. This heavy element enriched gas will be incorporated into future generations of stars and planets. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.

The fate of the hot neutron core depends upon the mass of the progenitor star. If the progenitor mass is around ten times the mass of the Sun, the neutron star core will cool to form a neutron star. Neutron stars are potentially detectable as "pulsars", powerful beacons of radio emission. If the progenitor mass is larger, then the resultant core is so heavy that not even nuclear forces can resist the pull of gravity and the core collapses to form a black hole.

Supernova

Neutron Stars

Black Holes

Life of Stars : Information

There seem to be an enormous number of stars that are visible to the naked-eye at a really dark site but, in fact, the eye can only see about two thousand stars in the sky at one time. We can see the unresolved light of many thousands more when we look at the Milky Way, and the light of the Andromeda galaxy, which can be seen by the eye, comes from thousands of millions of stars.

The Sun is our own special star yet, as stars go, it is a very average star. There are stars far brighter, fainter, hotter and cooler than the Sun. Basically, however, all the stars we can see in the sky are objects similar to the Sun.

The Sun (and any other star) is a great ball of gas held together by its own gravity. The force of gravity is continually trying to compress the Sun towards its centre. If there were not some other force counteracting gravity, the Sun would collapse. Outward pressure is produced by the radiation from nuclear energy generation in the Sun's interior.

Stars form from concentrations in huge interstellar gas clouds. These contract due to their own gravitational pull. As the cloud gets smaller it loses some of the energy stored in it as gravitational potential energy. This is turned into heat which, in the early days of the embryonic star, can easily escape and so the gas cloud stays cool.

As the cloud's density rises, it becomes more and more difficult for heat to escape and the temperature at the centre of the cloud rises. If the cloud is big enough, the temperature rise is sufficient for nuclear fusion reactions to begin. These generate more heat and the 'burning' of hydrogen into helium begins, as in the Sun. The object is then a main sequence star.

Early

In its early stages the embryonic star is still surrounded by the remains of the original gas cloud from which it formed. By this stage the cloud remnant takes the form of a disk around the star. The radiation from the star gradually dissipates this disk, possibly leaving behind a system of smaller objects, planets.

The main-sequence

The Hertzsprung-Russell diagram of the nearest stars and the brightest stars.The star now settles down to a long period of stability while the hydrogen at its centre is converted into helium with the release of an enormous amount of energy. This stage is called the main-sequence stage, a reference to the classical Hertsprung-Russell diagram (see Figure).

The horizontal axis shows spectral type and temperature from the hottest stars on the left to the coolest on the right. The vertical axis shows the luminosity of the stars with those 1,000,000 times brighter than the Sun at the top and those only 1/10,000th of its brightness at the bottom. The curved line marks the Main Sequence - stars, including the Sun, which are fusing hydrogen into helium. The group at the top right, including Betelgeuse and Aldebaran, are Red Giants. The group at the bottom left, including Sirius B, are White Dwarfs.

Most stars lie in a well defined band in the diagram and the only parameter that determines where in the band they lie is the star's mass.

The more massive a star is the quicker it `burns' up its hydrogen and hence the brighter, bigger and hotter it is. The rapid conversion of hydrogen into helium also means that the hydrogen gets used up at a greater rate in the more massive stars than the smaller ones. For a star like the Sun the main-sequence stage lasts about 10,000,000,000 years whereas a star 10 times as massive will be 10,000 times as bright but will only last 100,000,000 years. A star one tenth of the Sun's mass will only be 1/10,000th of its brightness but will last 1,000,000,000,000 years, longer than the current age of the Universe.

Post main-sequence evolution

Stars do not all evolve in the same way. Once again it is the star's mass that determines how they change.

Small mass stars

Our knowledge of the evolution of these stars is purely theoretical because their main sequence stage lasts longer than the present age of the Universe, so none of the stars in this mass range has evolved this far!

We believe that the evolution will proceed as for the medium mass stars except that the temperature in the interior will never rise high enough for helium 'burning' to start. The hydrogen will continue to 'burn' in a shell but will eventually be all used up. The star will then just get cooler and cooler ending up after about 1,000,000,000,000 years as a 'black dwarf'.

Medium mass stars

The Ring Nebula (M57), a planetary nebula in the constellation Lyra.(Image: Hubble Heritage Project (AURA/STScI/NASA))During the red giant phase, a star often loses a lot of its outer layers which are blown away by the radiation coming from below. The star becomes a planetary nebula (like M57, right).

They 'burn' hydrogen into helium in their centres during the main-sequence phase but eventually there is no hydrogen left in the centre to provide the necessary pressure to balance the inward pull of gravity. The core of the star contracts until it is hot enough for helium to be converted into carbon. Hydrogen continues to fuse into helium in a shell around the core, but the outer layers of the star have to expand. This makes the star appear brighter cooler and it becomes a red giant.

Eventually the energy generation will fizzle out and the star will collapse to what is called a 'white dwarf'.

High mass stars

Hubble Space Telescope picture of Eta Carina - a massive, highly evolved star which is ejecting its outer atmosphere in a series of violent outbursts.(Image: Jon Morse (Uni. of Colorado) and NASA)There are very few masses greater than five times the mass of the Sun but their evolution ends in a very spectacular fashion. As was said above, these stars go through their evolutionary stages very quickly compared to the Sun. Like medium mass stars, they 'burn' all the hydrogen at their centres and continue with a hydrogen `burning' shell and central helium 'burning'. They become brighter and cooler on the outside and are called red supergiants. Carbon 'burning' can develop at the star's centre and a complex set of element `burning' shells can develop towards the end of the star's life. During this stage many different chemical elements will be produced in the star and the central temperature will approach 100,000,000°K.

For all the elements up to iron, nuclear fusion into heavier elements produces energy and so yields a small contribution to the balance inside the star between gravity and radiation. Fusion of iron into heavier elements, however, uses energy rather than releasing it. Once the centre of the star consists of iron, no more energy can be generated, and there is no longer a source of pressure to counteract the crushing pull of gravity. The star's core then starts to contract rapidly, collapsing on a timescale of less than one second. The protons and electrons in the core are crushed together to form neutrons, releasing a flood of neutrinos, which carry away most of the energy from the explosion.

The core collapse in the dying star releases a vast amount of gravitational potential energy, sufficient to blow away all the outer parts of the star in a violent explosion, and the star becomes a supernova. The light of this one star is then as bright as that from all the other 100,000,000,000 stars in the galaxy. During this explosive phase all the elements with atomic weights greater than iron are formed and, together with the rest of the outer regions of the star are blown out into interstellar space. The central core of neutrons is left as a neutron star which could be a pulsar.

What is remarkable about this is that the first stars were composed almost entirely of hydrogen and helium and there was no oxygen, nitrogen, iron, or any of the other elements that are necessary for life. These were all produced inside massive stars and were all spread throughout space by such supernovae events. We are made up of material that has been processed at least once, and probably several times, inside stars.

___

Supermassive Black holes:

http://chandra.as.utexas.edu/~kormendy/stardate.html

Swinburne Centre for Astrophysics and Supercomputing

Swinburne Astronomy Online:

http://astronomy.swin.edu.au/cms/astro/cosmos/S/Star

http://astronomy.swin.edu.au/cosmos/S/Stellar+Evolution

Star

The term star was originally associated with the visible stars we recognise from the night sky. Stellar means "star-like".

As our knowledge of the Universe increased, it was soon realised that our Sun was a fairly normal star, just close enough to be very bright due to the effect of the inverse square law.

The scientific use of the telescope brought many stars into view for the first time, and astronomers now believe there are some ~1022 stars in the observable Universe.

Stars have a wide range of masses, and their luminosity varies by many orders of magnitude. As stars increase in mass their lifetimes become dramatically shorter, with stars 10 times that of the Sun living for only about 0.1% of the time, albeit at much greater luminosity (about 10,000 times brighter). Astronomers refer to how stars live and die as stellar evolution even though it has nothing to do with Darwin's theories.

Stars like our Sun live for about 10 Billion years before they exhaust their primary source of fuel, the simplest element, hydrogen. After this occurs they swell up dramatically becoming red giants before losing their outer layers and resembling a planetary nebula. Once the outer layers peel off, the star becomes known as a white dwarf. White dwarfs are still referred to as stars. Whilst burning hydrogen in their cores, stars are said to be "on the main sequence" of the Hertzsprung Russell diagram.

Stars originally more massive than about 6-8 times the mass of the Sun can burn elements more massive than Hydrogen, and ultimately create cores that collapse catastrophically, creating neutron stars or black holes in a supernova explosion. Neutron stars and black holes are frequently referred to as stars, even though they are frequently invisible at optical wavelengths. The exact mass at which a star ceases to form a neutron star and starts creating a black hole is not known, but thought to be around 20 solar masses. Neutron stars manifest themselves in various ways, among them pulsars and magnetars.

Stars less massive than about 0.8 solar masses have not had sufficient time to exhaust their hydrogen since the Big Bang, and are still on the main sequence.

Very low-mass stars with masses less than about 0.08 solar masses, cannot burn Hydrogen at all in their cores and are often called "brown dwarfs".

Stars are not formed individually, but in massive groups and are usually associated with galaxies (collections of billions of stars) or globular clusters.

Shooting stars have nothing to do with stars whatsoever, and are small particles striking the Earth's atmosphere.

Stellar Evolution

Stellar evolution is a description of the way that stars change with time. On human timescales, most stars do not appear to change at all, but if we were to look for billions of years, we would see how stars are born, how they age, and finally how they die.

The primary factor determining how a star evolves is its mass as it reaches the main sequence. The following is a brief outline tracing the evolution of a low-mass and a high-mass star.

The life of a star

Stars are born out of the gravitational collapse of cool, dense molecular clouds. As the cloud collapses, it fragments into smaller regions, which themselves contract to form stellar cores. These protostars rotate faster and increase in temperature as they condense, and are surrounded by a protoplanetary disk out of which planets may later form.

The central temperature of the contracting protostar increases to the point where nuclear reactions begin. At this point, hydrogen is converted into helium in the core and the star is born onto the main sequence. For about 90% of its life, the star will continue to burn hydrogen into helium and will remain a main sequence star.

Once the hydrogen in the core has all been burned to helium, energy generation stops and the core begins to contract. This raises the internal temperature of the star and ignites a shell of hydrogen burning around the inert core. Meanwhile, the helium core continues to contract and increase in temperature, which leads to an increased energy generation rate in the hydrogen shell. This causes the star to expand enormously and increase in luminosity - the star becomes a red giant.

Eventually, the core reaches temperatures high enough to burn helium into carbon. If the mass of the star is less than about 2.2 solar masses, the entire core ignites suddenly in a helium core flash. If the star is more massive than this, the ignition of the core is more gentle. At the same time, the star continues to burn hydrogen in a shell around the core.

The star burns helium into carbon in its core for a much shorter time than it burned hydrogen. Once the helium has all been converted, the inert carbon core begins to contract and increase in temperature. This ignites a helium burning shell just above the core, which in turn is surrounded by a hydrogen burning shell.

What happens next depends on the mass of the star :

Stars less than 8 solar masses

The inert carbon core continues to contract but never reaches temperatures sufficient to initiate carbon burning. However, the existence of two burning shells leads to a thermally unstable situation in which hydrogen and helium burning occur out of phase with each other. This thermal pulsing is characteristic of asymptotic giant branch stars.

The carbon core continues to contract until it is supported by electron degeneracy pressure. No further contraction is possible (the core is now supported by the pressure of electrons, not gas pressure), and the core has formed a white dwarf. Meanwhile, each thermal pulse causes the outer layers of the star to expand, resulting in a period of mass loss. Eventually, the outer layers of the star are ejected completely and ionised by the white dwarf to form a planetary nebula.

Stars greater than 8 solar masses

The contracting core will reach the temperature for carbon ignition, and begin to burn to neon. This process of core burning followed by core contraction and shell burning, is repeated in a series of nuclear reactions producing successively heavier elements until iron is formed in the core.

Iron cannot be burned to heavier elements as this reaction does not generate energy - it requires an input of energy to proceed. The star has therefore finally run out of fuel and collapses under its own gravity.

The mass of the core of the star dictates what happens next. If the core has a mass less than about 3 times that of our Sun, the collapse of the core may be halted by the pressure of neutrons (this is an even more extreme state than the electron pressure that supports white dwarfs!). In this case, the core becomes a neutron star. The sudden halt in the contraction of the core produces a shock wave which propagates back out through the outer layers of the star, blowing it apart in a core-collapse supernova explosion. If the core has a mass greater than about 3 solar masses, even neutron pressure is not sufficient to withstand gravity, and it will collapse further into a stellar black hole.

The ejected gas expands into the interstellar medium, enriching it with all the elements synthesised during the star's lifetime and in the explosion itself. These supernova remnants are the chemical distribution centres of the Universe.

An important tool in the study of stellar evolution is the Hertzsprung-Russell diagram (HR diagram), which plots the absolute magnitudes of stars against their spectral type (or alternatively, stellar luminosity versus effective temperature). As a star evolves, it moves to specific regions in the HR diagram, following a characteristic path that depends on the star's mass and chemical composition.

Astronomy Cast Episode 30:

The Sun, Spots and All

http://www.astronomycast.com/solar-system/episode-30-the-sun-spots-and-all/

Fraser Cain: Hi Pamela

Dr. Pamela Gay: Hey Fraser, how's it going?

Fraser: Good, well I've got a very strange story for you. You know I live out on the west coast

of Canada, on Vancouver Island, and we get a lot of rain here over the winter. This

winter was particularly hard – very, very rainy.

So yesterday, it wasn't raining and I decided to venture outside and there was

something really strange in the sky: the clouds were on fire! Well, not exactly on fire,

but there was some kind of burning orb peeking out from behind them.

Pamela: I think that might be this thing called the Sun...

Fraser: The Sun? I'm intrigued, tell me more!

Pamela: Well, it's this nearby star and its been keeping us warm for about 4.5 billion years.

Although in the past it used to be significantly cooler and its currently warming up, in

about 5 billion years it's going to stop the nuclear processes in its centre that keep it

going. Between now and then, it's going to about double in brightness.

Fraser: Wow.

Pamela: And we'll die.

Fraser: (laughing) "And we'll die." Well as long as you put that in... So this orb will kill me??

Pamela: This orb will kill you.

Fraser: Seriously though, the weather is finally turning here in rainy Vancouver island and we

just had the first day of spring in the northern hemisphere, I think that's how it works.

Also (and I think this is the coolest part) the Hinode spacecraft just released a series of

animations of the Sun's surface, and they will blow your mind. I have never seen

anything like this. They're these really close up animations of plasma moving around

on the Sun's surface. So we're going to put a link to that in the show notes.

This week, we want to talk about the Sun. So let's talk about the Sun: what do you

know?

Pamela: Where do you want to start?

1

Fraser: There's got to be layers to it,

Pamela: Well, a good place to start then is probably the centre of the Sun.

In the very centre of the Sun, we have this very, very dense, very, very hot region sort

of like the centre of a nuclear explosion. In fact, it is a nuclear explosion.

In the core of the Sun, we have protons slamming together with so much energy – well,

energy is not just what they're slamming together with, it's what they're releasing.

These protons, when they collide, will actually first form deuterium and then eventually

you'll get deuterium colliding with things and other things going on and you end up

with helium and then helium collides and you end up with a new form of helium.

Along the way, in these collisions, energy is released in the form of light and neutrinos.

Fraser: What's making this reaction happen?

Pamela: You simply crush stuff together closely enough and heat it up, and the heat causes the

atoms to move; heat anything up and it gets excited and runs around. Everything is

packed closely together, and when a whole bunch of stuff is packed closely together

and trying to move at high velocities, it can't help but collide with one another.

So these atoms are heated up, packed closely together, and colliding. Along the way,

when they collide, they're producing heavier elements and releasing light in the form of

gamma rays and x-rays.

Fraser: So the fusion process is actually the atoms vibrating into other atoms so hard that they

merge together?

Pamela: They're colliding, they're racing around in a swarm of different velocities and different

directions, colliding together, merging to form heavier atoms and releasing light.

Fraser: Is that where the light from the Sun comes from, then?

Pamela: Originally, but that light goes through a lot of processes before it finally reaches the

planet Earth.

Like I said, initially, they start out as gamma rays and x-rays. Thankfully, the Sun isn't

hitting the Earth with gamma rays and x-rays because that might (over time) do really

bad things to our atmosphere. Instead, that light races away from where it was formed.

The inner 25% of the Sun is where these nuclear reactions are taking place. That light

shoots off in all directions, but as it tries to shoot off, it ends up hitting other atoms in

the Sun and it'll get absorbed into the atom, carried along for a little while, and then re-

released sometimes with a different colour, a different temperature, and then it will

continue on.

These absorptions, scatterings, re-emissions... this will happen 10^30 times, that's 1

followed by 30 zeros different times as the photons of light try to move from the core to

the surface of the Sun. This process can take a photon 10 million years.

Fraser: It takes 10 million years from when the photons were created in the nuclear process to

when they can actually get out of the Sun!

Pamela: It's kind of shocking. Every photon has to be around for 10 million years before it gets

to the surface and then it takes it a meagre 8 minutes to reach the planet Earth.

Fraser: We talked about, in one of the earlier shows, how it's the light pressure from those

photons that's counter-balancing the gravity pressure of the star itself. Is this what this

light pressure is, these photons bonking into their next absorption target?

Pamela: That's exactly what's happening. The centre of the Sun is releasing all these photons

and they're trying to escape, trying to escape and as they try to leave, they're pushing on

the outer layers.

Think of it this way: imagine you have a steady stream of famous people being

produced in the centre of a crowd and these famous people are trying really hard to

push their way out the edge of the crowd but as they go everyone's grabbing a hold of

them, demanding an autograph and sometimes they end up going off in the wrong

direction initially before they finally get to the edge of the crowd.

Fraser: And they probably get grumpier and grumpier and grumpier as they go until they're able

to make it out.

Pamela: And with photons they get lower and lower energy as they go.

Fraser: But I know that gamma rays and x-rays are coming from the Sun, so is that just some of

the photons are able to make it out unchanged while others do get changed pretty

significantly?

Pamela: There are actually processes at the surface of the Sun that can create new hot photons.

We have magnetic field lines poking up through the surface of the Sun and these

magnetic field lines contain huge amounts of energy and they'll get themselves all

twisted up. Sometimes they'll spontaneously reconfigure into a simpler shape. When

they do this, they release the energy that it took for them to stay in the complex shape

and that energy comes off as really hot photons.

Fraser: So the x-rays and the gamma rays that we see coming from the stars are not the original

ones that were generated in the middle, they're produced at the surface.

Let's go back to our layers then, so we've talked about that inside layer where the

nuclear reactions are happening, what's outside of that?

Pamela: So the inner 25% is the core, and then next up to 70% out from the centre of the Sun (so

you go from 25% out to 75% out) there's this radiative zone. This is the area where the

light is getting absorbed and re-emitted and absorbed and re-emitted. Eventually it

reaches this layer where the Sun basically becomes a giant lava lamp. As the radiation

comes out of the radiation zone it goes through this layer and starts heating up gas. The

heated up gas starts to rise so you end up with cells of rising, hot material going

towards the surface of the Sun. When it reaches the surface of the Sun, it radiates away

its energy, cools off and sinks back down.

This is the same process that's going on with a lava lamp. You have a hot light (a light

bulb) underneath the lava part, and that light is heating up blobs of the oil inside the

lava lamp, which then rises, cools off at the surface and sinks back down. The Sun has

the exact same radiation level followed by lava lamp level.

Fraser: But when I think of the Sun, I think of it as all gas, right? The inside is hydrogen gas,

but I guess it's in this radiation thing. With this outer layer, it has a chance to radiate

away into space and so its not quite as hot, not quite as intense as the rest of the Sun?

Pamela: The temperature structure of the Sun changes radically as you go from the centre to the

outer layers. The very centre of the Sun is about 15 million degrees Celsius. As you go

out toward the surface of the Sun, the surface of the Sun is about 5 thousand degrees, or

5,700 degrees Kelvin. That's a huge change in temperature. This change in temperature

happens in a large amount across the radiative zone, where the temperature falls from

about 7 million degrees to 2 million degrees. At the bottom of the convective zone, it

has to go from that 2 million degrees all the way down to the 5,700 degrees at the

surface. So in each of these different temperature regimes, you have different physics

dominating how the gas behaves.

Fraser: In this convective zone, we've got these bubbles of gas boiling up to the surface,

releasing their energy, cooling down and then sinking back down. What does that look

like in our telescopes? What do we see?

Pamela: We can actually watch the surface of the Sun boil the same way oil in a pot boils, where

you get granules of material coming up and then streaming back down. We'll put links

to these animations on our website; it's just all these individual cells where you can

watch them flowing up in the centre and down on their edges. All the edges intermix

and they slowly change shape and move around and it's fascinating to see the Sun do

the same thing that happens when I'm cooking sopapillas.

Fraser: Yeah, the photos of the Sun doing this kind of stuff are just astonishing.

So, let's talk about some of the other features on the outside. You talked about the

magnetic field lines. How does that happen?

Pamela: At the transition layer, between the convective zone and the radiative zone, there's a lot

of weird physics going on. We talk about the Sun's magnetic field being generated by

magnetic dynamo that occurs in the interface between the radiative zone and

convective zone, and I have to say we're not entirely sure how it happens. But, however

it happens, it occurs at this interface and you end up with magnetic field lines streaming

away from the magnetic dynamo and in some places they come up through the surface

of the Sun and one of the really weird things is this magnetic field, the north pole of the

magnet and the south pole of the magnet will actually flip back and forth over 11 year

periods.

So we have a magnetic field getting generated at the interface between the radiative

zone and the convective zone, and it's an unstable magnet that flips back and forth.

Different magnetic field lines pop up through the surface of the Sun, create really weird

shaped structures, and leave footprints in the form of Sunspots where they're poking up

through the surface.

Fraser: Oh, so the sunspots are the places where the magnetic field lines are poking up through

the surface of the Sun. I didn't know that.

Pamela: One of the weird things about them is that, just like magnets have a north pole end and

a south pole end, when you look at these sunspots you can find them where one of them

will be a north sunspot and the other one will be a south sunspot so we can see exactly

how the magnetic field lines are flowing from spot to spot.

Fraser: When we talk about this 11 year cycle, this is the solar maximum and solar minimum,

right?

Pamela: Exactly. The number of sunspots we see on the surface of the Sun varies from year to

year over an 11 year cycle, more or less; it occasionally betrays us. There was one

marked period from about 1645 to 1715 where there were no sunspots, but in general

it's a nice, healthy, 11 year cycle. We went through solar minimum about 2005 and the

number of sunspots changes and where on the Sun the sunspots are located also

changes. During solar maximum, we have the most sunspots. They'll star to appear in

the northern mid-latitudes and the southern mid-latitudes of the Sun. As we move

toward minimum, the sunspots move toward the equator.

Fraser: So where do we stand now, then? We're a couple years past the solar minimum, heading

toward the solar maximum again.

Pamela: Exactly. So the sunspots are starting to crop up at the mid-latitudes, we're starting to get

higher numbers of them, and it's a good time to watch the Sun evolve because it's going

to start to do more and more things as we watch.

Fraser: I've heard that this upcoming solar maximum is supposed to be pretty significant.

Pamela: We try to make predictions about how many sunspots we'll see, what we expect to see

and right now they're expecting the next solar maximum is going to have more sunspots

than the previous solar maximum, so it should be fun to watch, and we have more

satellites in orbit than ever before, trying to watch what's going on.

Fraser: That's right, we've got the Hinode spacecraft, that I mentioned, and there's the new

STEREO spacecraft that was just launched as well. Those are put, I think, one space

craft trailing the Earth and one ahead of the Earth in our orbit, and they'll be able to

make a 3-D picture of the Sun's surface as well as any things that happen on the

surface.

Let's talk about some of those things they might see. I know there's a few other terms

we have to get through, one is flares. What are those?

Pamela: Flares occur when those magnetic field lines reconfigure and in the process they have

to break. When this happens, you end up with the material streaming away from the

broken field lines. This is a magnetic flare – at least, this is how we think we

understand it. We're still learning new things every day, and magnetic fields are one of

those things that are really hard to understand. The Sun, even though we've been able to

see it as long as humanity's been around to watch, we're still learning new mysteries

about it every day.

Fraser: Is that when we get one of those coronal mass ejections, those great big sprays of plasma

that come off the surface of the Sun?

Pamela: Coronal mass ejections are one of the most exciting mysteries there are in solar science.

They seem to be associated sometimes with sunspots and the flares that are associated

with the reconfiguring of the magnetic fields, but sometimes they just seem to happen

just because, and we're not always completely sure why. We're working to try to figure

out how to make predictions. Understanding coronal mass ejections is actually fairly

important because when these things happen, they can release huge amounts of energy

that can harm astronauts and satellites that are orbiting the Earth.

Fraser: But what's actually in a coronal mass ejection?

Pamela: Well, it's a lot of high energy particles. You get particles coming off the Sun at high

energies, streaming toward the Earth and they can cause all sorts of weird interferences

as they hit the atmosphere, and there's also radiation. So a lot of bad stuff can hit our

atmosphere all at once when one of these things happen.

Fraser: We also get to see the aurora borealis and aurora australis, so that's good.

Pamela: That's good, but the x-rays and stuff coming off of them that make such pretty pictures

can be rather harmful for astronauts, so we want to know how to predict when one is

going to happen. Thus knowing when to say "Um, guys? You need to hide in the safest

part of your space station"

Fraser: Our understanding of the Sun so far, has that affected our understanding of some of the

other stars we can see? Can we see some of the features on the Sun – like, can we see

some of the features that we see on our Sun, on other stars?

Pamela: We're actually able to see both flares and sunspots on other stars. There are stars that

have flares much greater than the flaring activity of the Sun, and these flares will cause

sudden flickering in the brightness, where you're taking a string of images with a digital

detector of some sort, and you're going along... star's behaving, star's behaving, star's

behaving and all of a sudden you get an anomalously high reading of the star. A couple

of different things could be happening. It could be that your detector's screwed up, and

a lot of times we blame our detectors. But if two different people simultaneously see

the star suddenly brighten for a moment or two, then we can say that was a real

brightening, and it was probably flare activity with the star.

We can also watch stars change in brightness in ways that we believe are due to star

spots. If you get enough star spots on a star, it will change the amount of light coming

off of the star and we can see that happening. There's all sorts of complex programs out

there to try and reconfigure using extremely high-res images what those sunspots might

look like.

Fraser: That might be a way then, you can detect the rotation speed of a star.

Pamela: In some ways it's easier to look at the line broadening because the left edge of the star

might be rotating toward you while the right edge rotates away from you, and we can

use that difference in the rotation rate of the two edges to get the rotation rate of stars,

but we can also use star spots.

Fraser: So, you mentioned at the beginning of this podcast, we're talking about what the future

holds for the Sun. We've got a more elaborate podcast about how stars die, but can we

talk a bit about the future for our Sun?

Pamela: Sure. The Sun right now is burning hydrogen in its core, producing helium. This is all

confined to a region that is at a high enough temperature and pressure to allow protons

to get close enough together that they can merge together to form different elements.

Most of the time, when two protons get together their electromagnetic forces between

them will cause them to try to repel one another, but if they're going fast enough, they

don't have time to react and they'll get close enough that a different force will take over

and they'll merge together.

Now, the whole Sun isn't at a high enough temperature/pressure to allow nuclear

reactions to go on. So eventually, all of the hydrogen in the part of the Sun capable of

nuclear burning is going to get used up, and when that happens the Sun is initially

going to start to collapse down. As it does that, it will create a new layer, a new shell,

around that core that's capable of hydrogen burning.

So that shell will burn hydrogen, burn hydrogen, burn hydrogen, and the hydrogen in

that shell will produce helium, and that helium is heavier so it sinks towards the core.

So the core is getting denser and denser and denser and hotter and hotter and hotter

until eventually it reaches a temperature around 100 million degrees Kelvin, at which

point helium is capable of starting its own nuclear burning. The helium is going to go

into what's called a CNO cycle, that ends up producing carbon, nitrogen and oxygen in

different stages.

So now, we have the core of the Sun burning and the core of the Sun burns, and the

core of the Sun burns, and we still have this shell of hydrogen going. This is going to,

over time, build a carbon core to the Sun.

Our Sun doesn't have enough mass that once it gets a nice carbon core it's going to be

capable of then burning that carbon core into anything. But, it will then (once it has the

carbon core) go through one more phase and in that last final phase it's going to be a

supergiant star. It's going to be something resembling a Mira variable star: big, bright,

large variations, easy to see. It's going to be undergoing helium shell burning and

hydrogen shell burning, so you have this onion layered Sun, where the core is this

remnants of the carbon-nitrogen-oxygen cycle, you have a helium burning shell around

that, and a hydrogen burning shell around that.

Now, once that fuel is burned up, the Sun is just going to sort of fall apart. The outer

layers of the star are going to drift away and form a beautiful planetary nebula,

something like the Helix nebula. The core of the Sun is going to get left behind as a

white dwarf star. That white dwarf star doesn't have the ability to produce anymore

energy, so it can't support itself except by the atoms pushing one another apart.

That leftover core of our Sun is going to collapse down to roughly the size of the moon.

The atoms are going to get so close together that they're essentially going to form a

crystalline structure similar to the most dense diamond you can imagine. That white

dwarf, over time, is going to slowly cool off and cool off and cool off until it fades

away, given the length of the age of the Universe.

Fraser: That's – it's kind of ironic that the Sun gets hotter as it runs out of fuel. It just seems

strange to me. Right now, the Sun is getting hotter and hotter and hotter, isn't it? Not

that you can notice over a couple of years, but over millions and millions of years the

Sun is becoming hotter.

Pamela: In about 50 million years it's going to be hot enough to do bad things to the water on

the surface of the planet.

The many ironies of the way stars evolve... when they first start out, that initial burning

is able to support a fairly small star, but the future hotter burning that's going to happen

is going to cause the star to bloat out until it's roughly one and a half times the Earth's

orbit in radius. So the Sun will grow to be bigger than our Earth's orbit by 50% before

it dies.

Fraser: And what does that mean for the Earth?

Pamela: Well, along the way, the Sun actually loses lots of mass, and when the Sun loses mass

its planets move away because the gravity isn't pulling on them quite as strongly. We're

not quite sure how these two things are going to play out: the mass loss allowing the

planets to escape to larger distances and the star expanding. But, some of the papers

that I've read most recently have indicated that the mass loss is going to allow the Earth

to get far enough away from the Sun that it's not quite going to get sucked in.

But, like I said, the Sun's going to get twice as luminous and because of this, the

surface of the Earth is going to get burnt off, basically. We're not going to be around.

So we need to figure out how to get even further away and find someplace that is safer.

Fraser: So, with all of the new instruments that are going up to the Sun, what would you say are

the big mysteries that scientists are trying to solve about the Sun right now?

Pamela: Trying to understand how changes in the apparent magnetic field lines, the places

where we can see hot x-ray emitting gas twisted up into bizarre shapes going between

sunspots... understanding what specific shapes indicate that a flare or coronal mass

ejection is about to happen is probably one of the most interesting thing.

Fraser: And useful.

Pamela: And useful. New pictures that you were talking about from the Hinode, those we think

are starting to give us proof that when you find a twisted up S-shaped, x-ray emitting

thing, that thing is going to break apart and we're going to get a flare or a coronal mass

ejection.

Fraser: Once again, I think it's going to be a good time for this kind of astronomy as well. We're

just totally in the golden age of astronomy. It's great.

That was great, Pamela. Thank you very much for explaining what that burning orb

was, now I feel a little safer but I'll keep an eye on it.

And we'll talk to you next week.

Pamela: Sounds great Fraser.

This transcript is not an exact match to the audio file. It has been edited for clarity.

How can a star be compressed to form a black hole?

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/990210b.html

The Answer

Gravity does the work. When you have enough material together, gravity can be very strong. And the more mass you have, the lower the density needs to be in order to make a black hole.

If all nuclear burning in a star greater than about 1.4 solar masses were to stop, and the star allowed to cool and solidify, the solidified material would not be strong enough to support its own weight, and it would collapse as the electrons were pushed into the protons to make neutrons. This neutron star material is stronger, but the star would be only about 20 km in diameter. If you piled on more material, you would eventually get to the point where there is so much gravity that not even the neutrons could hold it up, and the star would collapse into a black hole.

David Palmer and Samar Safi-Harb

for Ask an Astrophysicist