Is it becuase it is dieing?
Stars like the sun eventually become red giants (actually asymptotic giant branch stars – which comes after the red giant phase – but its still basically a red giant).
At this stage the surface of the star is somewhere close to 1 astronomical unit from the center (i.e. where we are), so the surface gravity is very low (same mass of the sun, but much further from center of gravity).
Consequently the surface gases can easily achieve escape velocity. At the same time, the star is a little unstable and pulsates.The pulsation helps push material away from the star’s surface. Once pushed, according to Newton’s 1st law, the gas atoms keep going away from the star. The gas cools as it drifts away and can form some dust. So during this phase, a star like the sun develops a "circumstellar" cloud of gas and dust (actually the dust accelerates theprocess through radiation pressure). Once the star runs out of fuel (which at this stage is helium burning to make carbon) and the outer part of the star is too thin to support itself, the remaining star collapses and heats up.
As the star heats up it can excite the gas that was ejected into the circumstellar cloud. This cloud behaves like a neon light – the electrons in the gas are excited and then de-excite by emitting photons – so we see a beautiful colorful nebula and a tiny white dwarf central star.
The colors are indicative of what atoms and ions are in the gas cloud.
Contrary to what some textbooks still say – planetary nebulae are not spherical – they are more or less donut shaped or hourglass shaped. The causes of this are a matter of controversy in the Planetary Nebula research community
February 24th, 2010 at 4:20 pm
"A planetary nebula forms when a star can no longer support itself by fusion reactions in its center. The gravity from the material in the outer part of the star takes its inevitable toll on the structure of the star, and forces the inner parts to condense and heat up. The high temperature central regions drive the outer half of the star away in a brisk stellar wind, lasting a few thousand years. When the process is complete, the remaining core remnant is uncovered and heats the now distant gases and causes them to glow. "
References :
http://www.noao.edu/jacoby/pn_gallery.html
February 24th, 2010 at 5:09 pm
The core exhausts the hydrogen that was being fused into helium. This causes a sudden drop in outward force and the core collapses. If there is enough mass, then the new increased gravitational effect in the condensed center will cause the fusion of heavier elements.
At some point, the core cannot sustain any more nuclear reaction and it collapses much more. This causes a decrease in the gravitational pull on the outer portions of the star and they expand outward due to their heat. Average sized stars and dwarfs like the sun, will shed huge amounts of this hot gas, which will become a planetary nebula, forming around and expanding from the star, leaving the dense core behind.
References :
February 24th, 2010 at 5:27 pm
Stars like the sun eventually become red giants (actually asymptotic giant branch stars – which comes after the red giant phase – but its still basically a red giant).
At this stage the surface of the star is somewhere close to 1 astronomical unit from the center (i.e. where we are), so the surface gravity is very low (same mass of the sun, but much further from center of gravity).
Consequently the surface gases can easily achieve escape velocity. At the same time, the star is a little unstable and pulsates.The pulsation helps push material away from the star’s surface. Once pushed, according to Newton’s 1st law, the gas atoms keep going away from the star. The gas cools as it drifts away and can form some dust. So during this phase, a star like the sun develops a "circumstellar" cloud of gas and dust (actually the dust accelerates theprocess through radiation pressure). Once the star runs out of fuel (which at this stage is helium burning to make carbon) and the outer part of the star is too thin to support itself, the remaining star collapses and heats up.
As the star heats up it can excite the gas that was ejected into the circumstellar cloud. This cloud behaves like a neon light – the electrons in the gas are excited and then de-excite by emitting photons – so we see a beautiful colorful nebula and a tiny white dwarf central star.
The colors are indicative of what atoms and ions are in the gas cloud.
Contrary to what some textbooks still say – planetary nebulae are not spherical – they are more or less donut shaped or hourglass shaped. The causes of this are a matter of controversy in the Planetary Nebula research community
References :
Me – I study these things