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Pillars of Creation. The Main Sequence. Main Sequence – relates luminosity to temperature. Cool stars tend to be faint (less luminous) Hot stars tend to be bright (more luminous) The top left end of the sequence are the blue giants – large, hot and very luminous.

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the main sequence
The Main Sequence
  • Main Sequence – relates luminosity to temperature.
    • Cool stars tend to be faint (less luminous)
    • Hot stars tend to be bright (more luminous)
  • The top left end of the sequence are the blue giants – large, hot and very luminous.
  • The bottom right end are the red dwarfs – small, cool and faint.
star formation
Star Formation
  • Star formation begins with huge clouds of stellar dust and gas (hydrogen atoms) – molecular clouds.
    • Huge in terms of both distance and mass
  • Gravity begins to pull the hydrogen atoms together and the mass becomes denser, and temps increase
  • At core temps approaching 10million K, the hydrogen atoms begin the process of fusion into helium – ignition
    • * Surface temps are lower than our Sun’s about 4500 K, and radius would be slightly larger
  • This results in an enormous amount of energy which pushes outward and balances out the pull of gravity and keeps the mass from collapsing in on itself as long as this fusion process is continuing – as long as there is fuel (H) to be fused.
star formation1
Star Formation
  • How many hydrogen atoms

are needed? - generally

about 1057.

  • And generally something has to jumpstart the clumping process – for example, a neighboring star goes supernova and the shockwave hits….or part of the molecular cloud becomes to cold for its internal pressure to support it against its own gravity….whatever, once the collapse begins, if enough atoms are present, star formation begins.
star formation2
Star Formation
  • Over the next 30million years or so, the star will shrink, central temp will increase to about 15 million K and surface temp will reach approx 6000K to the current state of our Sun on the Main Sequence.
  • The current gravity in/pressure out equilibrium state of hydrostatic equilibrium burns steadily for around 10 billion years
star formation3
Star Formation
  • Research is divided on whether more massive stars form from denser/more massive clumps of matter, or whether all stars start out as relatively lightweight stars and then grow through accretion.
  • In either case, in cluster environments, the strong gravitation fields of the most massive protostars seem to give them a competitive advantage over the rivals when attracting gas from surrounding nebula….at least in simulations.
leaving the main sequence
Leaving the Main Sequence
  • A main-sequence star is in a state of hydrostatic-equilibrium called hydrogen core-burning.
    • Any change in temperature results in a change in pressure, which brings it back into equilibrium
    • Small increase in core temp causes pressure to increase = star expands and cools
  • As hydrogen (fuel) is consumed, the balance begins to shift and the star leaves the main sequence.
failed stars
Failed Stars
  • Some stars hit pressure/gravity equilibrium before the central pressure is high enough to begin fusion and never ignite – the continue to cool and are called brown dwarfs.
    • The theoretical minimum to reach nuclear fusion is 80x the mass of Jupiter or .08x the size of our Sun.
star evolution
Star Evolution
  • As a rule, low mass stars die gently, high-mass stars die catastrophically.
  • The dividing line between the two lies about 8x the mass of the Sun, with a great deal of variation in each category.
star evolution1
Star Evolution
  • Let’s start with one the size of the Sun……
  • There is less and less H at the core, and more and more He. It is not hot enough at the core for He to fuse, so the core begins to cool and the area of H burning moves out into higher layers.
  • Outward pushing pressure decreases, but inward pushing gravity does not, so the area where H fusion is still going on gets even hotter, and fusion speeds up – this stage is called hydrogen shell-burning.
The hydrogen shell generates energy faster and therefore the star gets even brighter than it was before.
  • Non-burning layers increase in radius, expanding and cooling – on its way to becoming a red giant.
star evolution2
Star Evolution
  • Eventually, the temps in the core will reach 108K – high enough for He to fuse into carbon and the nuclear fires will reignite, called helium flash.
  • The star settles into equilibrium again as a horizontal branch star, and its actual designation depends on its mass – a great deal was lost by strong stellar winds which may have ejected up to 20-30% of the original amount during the red giant phase.
  • It will remain here a relatively short period of time, fusing He to C in its core, still fusing H to He in its outer shell and it will become a red giant again n the asymptotic giant branch when the He is exhausted.
star evolution3
Star Evolution
  • Stars the size of the Sun will not reach another stage of core fusion – temps would need to reach 600 million K to make carbon fuse.
  • So, the carbon core is dead while the outer hydrogen- and helium-burning shells consume fuel at furious and ever increasing rates.
  • As the Sun expands it cools and begins to fall apart, sending its outer layers into space from the intense radiation.
star evolution4
Star Evolution
  • Eventually, the red giant is left with two distinct pieces.
    • The exposed core – very hot and very luminous
    • The escaping envelope – a cloud of dust and cool gas surrounding the core and expanding away at a typical speed of a few tens of kilometers per second
  • As the core exhausts the last of the fuel, it contracts and heats up, becoming so hot that the UV radiation ionizes the inner parts of the surrounding cloud, producing a planetary nebula.
star evolution5
Star Evolution
  • The central star fades and cools, the gas cloud becomes more diffuse and disperses into space, taking with it all of the elements that were formed during the death throws of the star.
    • Nuclear reactions between the carbon and the unburned helium create oxygen and sometimes other heavier elements, like neon and magnesium.
    • The elements get caught up in the envelope and enrich the interstellar medium.
star evolution6
Star Evolution
  • The carbon core of the star has been squeezed down until it can be squeezed no further – the electrons can’t be forced any closer together, and this is what supplies the internal pressure pushing back against gravity.
  • Internal temp is about 300million K – too cool to fuse carbon….so we are done.
  • White dwarf – may be a the center of a planetary nebulae or ‘naked’, their envelopes expelled to invisibility long ago.
star evolution7
Star Evolution
  • The white dwarf continues to cool and dim, and eventually becomes a black dwarf.
    • It doesn’t shrink – it just fades away….remember, it is as small as it is going to get.
leaving the main sequence1
Leaving the Main Sequence

7. Main-sequence star (like the Sun)

8. Subgiant branch

9. Helium flash

10. Horizontal branch

11. Asymptotic giant branch

12. Carbon core/planetary nebular

13. White dwarf

14. Black dwarf

star evolution8
Star Evolution
  • Sometimes, a white dwarf that is part of a binary star system may become explosively active – a nova(means new)
  • It pulls matter away from a companion star, forming an accretion disk around the dwarf….if temps exceed 107 K and H ignites, then boom, nova.
  • The fuel is expended with the explosion or blown off into space, the binary star system returns to normal, and the mass-transfer process can begin again.
  • The process can, in theory, repeat dozens, or hundreds of times.
star evolution9
Star Evolution
  • If the star is massive enough (10x the mass of the Sun), carbon is not the end of the line – carbon may be fused to oxygen….oxygen may be fused to neon……neon may be fused to magnesium……magnesium may be fused to silicon….silicon may be fused to iron…..but that is it. Nothing fuses iron.
    • Each stage is shorter than the last – MUCH shorter.
    • For a star 20x the Sun, H burns for 10million yrs, He 1 million, C 1000, O 1 year, Si a week, and Fe is fused for about a day
    • Iron does not produce energy – it acts like a fire extinguisher
star evolution10
Star Evolution
  • The star has continued to increase up to this point, surface temperature drops but its luminosity (brightness) seems unaffected or even increases and it is a red supergiant.
  • Once iron fuses, the star’s foundation is destroyed, gravity overwhelms, and the star implodes.
  • Temps spike 10 billion K to split the atoms into protons and neutrons and electrons, smashing them into each other – then rebounding (whole thing takes about a second!!!) – one of the most energetic events known in the universe!
  • This shock wave is called a core-collapse supernova.
star evolution11
Star Evolution
  • Evolution proceeds so fast for stars bigger than 10 solar masses that they don’t even reach the red giant region before helium fusion begins – they are still very near the main sequence when they reach the necessary core temps.
  • Luminosity remains fairly constant as radius increases and surface temp drops and the star swells to become a red supergiant.
neutron stars
Neutron Stars
  • After the violence of a supernova, what is left is an incredibly small and massively dense neutron star.
    • About 20 km in diameter with a mass greater than the Sun
    • VERY rapid rate of rotation
    • Very strong magnetic fields
    • A thimbleful of material could

weigh as much as 100 million

tons - the largest thing man

has ever moved, the Troll A

drilling platform weighs 1.2

million tons

neutron stars1
Neutron Stars
  • Pulsars are named so because they appear to pulse...they emit regular bursts of radiation, so regular they are better than the best atomic clocks.
  • Caused by the rapidly spinning object at their center..always a neutron star.
    • Probably originating at two "hot spots" aligned with the magnetic axis as it rotates
neutron stars2
Neutron Stars
  • Not all neutrons are pulsars –
    • Both rotation and magnetic fields which are required to make a pulsar, weaken over the lifespan of a neutron star.
    • A neutron star may not be oriented in a way that its pulses would be visible from Earth.
neutron stars3
Neutron Stars
  • There are a few things that observations of pulsars are consistent with, given our current understandings:
    • Every high-mass star dies in a supernova explosion
    • Most supernovae leave a neutron star behind
    • All young neutron stars emit beams of radiation, some of which we are able to detect as pulsars
neutron stars to black holes
Neutron Stars to Black Holes
  • Neutrons Stars are supported by the resistance of densely packed neutrons to further compression.
  • The general consensus is that a neutron star cannot exceed a mass of 3 solar masses – beyond this, the neutrons cannot withstand the pull of gravity….NO KNOWN FORCE CAN.
  • If enough material is left after a supernova explosion that the central core exceeds this limit, or matter falls back to the core after the explosion, then gravity wins and the core collapses.
  • A star would need to have a main-sequence mass 20-25x that of the Sun
neutron stars to black holes1
Neutron Stars to Black Holes
  • As the core shrinks, the gravitational pull becomes so great that even light cannot escape = black hole – the end point of stellar evolution
black holes
Black Holes
  • The radius that an object would have collapse down to, in order for its escape speed to equal the speed of light is called the Schwarzschild radius – 3km x the mass of the object in solar masses.
    • Earth it is 1 cm; Jupiter (300 x Earth mass) it is 3m
  • This is how much an object would have to be compressed down to in order to become a black hole.
  • An object that has collapsed down to within this radius is within its event horizon – nothing within this boundary will ever be seen, heard or known by anyone outside.
    • (60 Second Adventures in Thought)
black holes1
Black Holes
  • If more than 3 solar masses of material are left behind after a supernova explosion occurs, the remnant core will collapse below the event horizon in less than a second becoming a black hole.
black holes2
  • Special Relativity – 60 Second Adventures in Astronomy
  • Black Holes – 60 Second Adventures in Astronomy
  • Stephen Hawking – what he really said about black holes…..SciShow
black holes3
Black Holes
  • The orbit of objects near a black hole would be essentially the same as the orbit near a star of the same mass…..if our Sun were to suddenly somehow become a black hole (assuming it didn’t go supernova and blow Earth into oblivion first), Earth and the rest of the planets would continue orbiting it just as always….just as a cold, lifeless, husk of a planet.
  • Any matter that does get too close to the event horizon would be unable to get out.
nearing a black hole
Nearing a Black Hole
  • Tidal force would rip an object apart long before it reached the event horizon – as you near the black hole, the force on the portions of objects nearest the event horizon are stronger than the parts farthest away, stretching you, causing, enormous frictional heating and ripping objects apart.
nearing a black hole1
Nearing a Black Hole
  • The tidal forces cause so much heating in objects that the area near a black hole is actually an energy source, causing objects to emit radiation in the form of X-rays.
  • As long as the energy being produced is not within the event horizon, the radiation can be emitted into space in all directions.
nearing a black hole2
Nearing a Black Hole
  • Orbiting a black hole would be a safe way to study one, beyond the effects of the tidal forces – we do orbit the Sun without falling into it quite successfully.
  • Sending a robot probe into the black hole would tell us a great deal up until it was lost….. For example…
nearing a black hole3
Nearing a Black Hole
  • Light would become red shifted – not due to Doppler effect, but because the light is attracted to gravity and loses energy as it works harder and harder to get away from the black hole. Less energy = red end of the spectrum - Gravitational redshift
  • We would expect time to slow as we near a black hole, called time dilation, very similar to the gravitational redshift, eventually stopping altogether when it reached the event horizon.
evidence for black holes
Evidence for Black Holes
  • Since you can’t see them, you look for evidence of their effects on objects around them…in our own Galaxy…..
    • Example: sometimes an unseen object is distorting a visible companion star, which allows astronomers to determine a mass of the unseen object, leading them to the conclusion that it is too massive to be anything other than a black hole.
    • Example: Cygnus X-1 is a binary system – the visible star is a blue giant with a mass of 25 x the mass of the Sun; the orbital period of the system along with measurements of the visible component’s orbital speed allows astronomers to calculate the total mass of the binary system at 35 solar masses, so the unseen component is approximately 10 solar masses – again, too large to be anything other than a black hole (can’t be a black dwarf or brown dwarf, etc)
evidence for black holes1
Evidence for Black Holes
  • In the centers of many galaxies (inc our own), astronomers have found stars and gas are moving extremely rapidly, orbiting some very massive, unseen object.
  • Masses inferred from Newton’s laws range from millions to billions of times the mass of the Sun.
  • The leading – and only – explanation is that the unseen objects are supermassive black holes.