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As [link] shows, sometimes a planetary nebula appears to be a simple ring. Others have faint shells surrounding the bright ring, which is evidence that there were multiple episodes of mass loss when the star was a red giant (see image (d) in [link] ). In a few cases, we see two lobes of matter flowing in opposite directions. Many astronomers think that a considerable number of planetary nebulae basically consist of the same structure, but that the shape we see depends on the viewing angle ( [link] ). According to this idea, the dying star is surrounded by a very dense, doughnut-shaped disk of gas. (Theorists do not yet have a definite explanation for why the dying star should produce this ring, but many believe that binary stars, which are common, are involved.)

Model to explain the different shapes of planetary nebulae.

Diagram to Explain the Different Shapes of Planetary Nebulae. In the lower left-hand portion of this figure, a schematic representation of a planetary nebula is shown. A yellow ellipse, labeled “Torus” is drawn with a white dot labeled “Star” at its center. The long axis of the ellipse is oriented vertically. Several yellow arrows are drawn horizontally pointing away from the star. These are labeled “Stellar wind.” A faint figure-eight encloses the stellar wind on each side of the star and torus. Finally, a large, faint “Outer halo” surrounds the figure-eight and torus, centered on the star. At top left, directly above the star, the profile of a human eye is shown looking in the direction of the star. The line of sight is marked with a double-headed dashed arrow. What a planetary nebula would look like along this line of sight is illustrated with an image of Hubble 5 to the right of the eye. Another eye is drawn at lower right looking through the figure-eight toward the star. The line of sight is marked with a double-headed dashed arrow. What a planetary nebula would look like along this line of sight is illustrated with an image of the Helix Nebula above the eye.
The range of different shapes that we see among planetary nebulae may, in many cases, arise from the same geometric shape, but seen from a variety of viewing directions. The basic shape is a hot central star surrounded by a thick torus (or doughnut-shaped disk) of gas. The star’s wind cannot flow out into space very easily in the direction of the torus, but can escape more freely in the two directions perpendicular to it. If we view the nebula along the direction of the flow (Helix Nebula), it will appear nearly circular (like looking directly down into an empty ice-cream cone). If we look along the equator of the torus, we see both outflows and a very elongated shape (Hubble 5). Current research on planetary nebulae focuses on the reasons for having a torus around the star in the first place. Many astronomers suggest that the basic cause may be that many of the central stars are actually close binary stars, rather than single stars. (credit “Hubble 5”: modification of work by Bruce Balick (University of Washington), Vincent Icke (Leiden University, The Netherlands), Garrelt Mellema (Stockholm University), and NASA/ESA; credit “Helix”: modification of work by NASA, ESA, C.R. O’Dell (Vanderbilt University), and M. Meixner, P. McCullough)

As the star continues to lose mass, any less dense gas that leaves the star cannot penetrate the torus, but the gas can flow outward in directions perpendicular to the disk. If we look perpendicular to the direction of outflow, we see the disk and both of the outward flows. If we look “down the barrel” and into the flows, we see a ring. At intermediate angles, we may see wonderfully complex structures. Compare the viewpoints in [link] with the images in [link] .

Planetary nebula shells usually expand at speeds of 20–30 km/s, and a typical planetary nebula has a diameter of about 1 light-year. If we assume that the gas shell has expanded at a constant speed, we can calculate that the shells of all the planetary nebulae visible to us were ejected within the past 50,000 years at most. After this amount of time, the shells have expanded so much that they are too thin and tenuous to be seen. That’s a pretty short time that each planetary nebula can be observed (when compared to the whole lifetime of the star). Given the number of such nebulae we nevertheless see, we must conclude that a large fraction of all stars evolve through the planetary nebula phase. Since we saw that low-mass stars are much more common than high-mass stars, this confirms our view of planetary nebulae as sort of “last gasp” of low-mass star evolution.

Questions & Answers

what happened if moon replace Jupiter
Naresh Reply
we well never exist because Jupiter's mass and gravity is toooo high so it will protect us from asteroids comments etc... but moon can not do that.
Yabtsega
that's accurate
Joel
What is the definition of spacial relativity
Yabtsega Reply
what is a black hole
Jesse Reply
why do people use eclipses
Jesse
black hole is very massive object which is made up of explosion of huge dead stare
Yabtsega
a supernova
Joel
it's actually an implosion
Joel
it's actually an implosion
Joel
it's actually an implosion
Joel
it's actually an implosion.
Joel
yep It is the explosion
Yabtsega
thanks for correction
Yabtsega
my pleasure
Joel
but It It depend on the supernova It either explode or implode .
Yabtsega
Exactly if it implodes is formed a black hole due to the massive gravity but if the supernova explodes is formed a neutron star.
Joel
if a star at its white dwarf stage has mass equal to 1.4 times the mass of our sun then it has a highly chance of a becoming a black hole
Mahesh
It is known as chandrasekhar limit
Mahesh
Right
Joel
when two Neutron Stars merge , they can also make black hole
Gurjinder
Has the start of another big bang started to happen yet?
LAURA Reply
are there any home learning courses linked to jobs and carriers in space?
LAURA
i dont know for sure but as yourself i am also very interested on it
Paco
does tidally locked planet have magnetic field?
SCIENCE Reply
no its bc it doesn't rotate at all so it doesn't create friction in its interior and that in turn doesn't create no magnetic field
NARUTO
There is no way for us to find it out because their cores might revolve although their crust is not rotating
Tanmay
what is mean by dwarf planet
Mohamed Reply
Nearly round shaped planet, bil smaller compared with other planets
Murrino
thank you murrino I was thinking another type of planet but now i remember
NARUTO
These dwarf planets are nearly like Mercury but these planets have not cleared every matter present in their orbital path. Like most of the dwarf planets are present at asteroid belt as their neighborhood is not cleared of matter
Tanmay
But since Mercury was capable to clear its path so it is called a planet rather than a dwarf planet
Tanmay
Of course they should directly orbit our Sun rather than any other planet
Tanmay
this on our solar system
Youssef
not necessarily
Tanmay
The mass of a planet is twice the mass of the Earth and the radius is half of the Earth. If an object is placed in the equatorial region of the planet, it cannot be held by the gravitational force; Any other area can hold. How long is a day on the planet? (Earth's redius is 6370 km)
Muhtasim Reply
what is energy made up of?
Suman Reply
It is a fundamental property that exists, it is not made of anything
Tanishk
what is the correct arrangement of the horoscope
edwin Reply
what causes voids in space like bootes void?
NARUTO Reply
I think that it has somehow managed to get a lot of dark energy which makes star formation hard
Tanmay
Or this area might have a lot of gas clouds which obstruct our view of its star. that means it has normal amount of galaxies but we can't see them
Tanmay
but why scientists are saying that it bounces galaxues away while containing some inside it 🤔🤔🤔
NARUTO
I never heard anyone say that
Tanmay
well a YouTube channel called the seeker just said that scientist doing observations on this phenomenon is causing galaxies to bounce of
NARUTO
hi
Bas_
can you send the link to that video I didn't got any video saying galaxies to bounce
Tanmay
***youtu.be/I4Yc8Zw_NJo
NARUTO
there
NARUTO
ok I will see
Tanmay
I might get the wrong idea but that's what I understood
NARUTO
ok
Tanmay
who knows how to get dark materia
Jordan
its literally next to impossible, even the best scientist are only able to get 5 nanograms of it
NARUTO
I think we can get it from dark hole
Jordan
yeah but getting a black is nearly impossible at this moment
NARUTO
yeah I know but they can just think
Jordan
what are you talking about
Tanmay
What black?
Tanmay
black hole*
NARUTO
So who managed to make 5 nanograms of black hole?
Tanmay
What does that even mean 5nm of black hole
Tanmay
It's Swartzchild radius might be below plank length
Tanmay
I meant 5 nanometers of dark energy
NARUTO
oh ok I got it
Tanmay
we live in a place which only has 3 dimensions and our imaginations are only limited till there neither is the space infinity nor are our imaginations!
Samuel Reply
hi guys can we all discuss about a thought which I have?
Samuel
yes why not tell us
Tanmay
I do not know
Clint
it's up to u to do your homework
Clint
we live in many dimensions, x,y,z, time, gravity, light, ...... an n num of Dem,
Clint
Is this the question? we live in a place which only has 3 dimensions and our imaginations are only limited till there neither is the space infinity nor are our imaginations!
Tanmay
ppl thought we lived on a flat earth, 2 Dem, them 3, then 4 Einstein, ...... the list goes on
Clint
ur turn to work
Clint
work
Clint
young buzzards chew at the various of the old, find new food
Clint
work, there is no free ride in learning
Clint
read everything, process it later, keep reading
Clint
one day, you will make a contribution to science, but u have to read, u have to take ur bumps
Clint
I think we should read and understand what it wants to tell us what you learnt where can it be used and than go forward
Tanmay
Infinity is beyond our imaginations
Samuel Reply
maybe, maybe not.... it depends on the limit of ur thinking
Clint
have u stop thinking?
Clint
no?
Clint
then u do not know you own limits of ur imagination....
Clint
I think our imagination is beyond of infinity...
Suman
u may choose to stop imagining, but it's u that chooses
Clint
the depth of the mind has not been determined
Clint
if someone knows his imagination is limited then how does his imagination will exist, the imagination that is limited then those are just thoughts but unlimited imagination is creation of something new...
Suman
A person trapped in a well for a lifetime can never imagine the vastness of the sea so our imagination very well depends upon your physical limits
Tanmay
how would one describe the 3 dimensions, i mean what defines their boundaries . . . ?
Chalo
3 dimensions mean 3 mutually perpendicular coordinate in space
Gurjinder
how mass came to our galaxy when it was jast a point.
Nashita Reply
From energy
suvas
mass is energy, engery is mass, a very small amount of mass equals a huge amount of energy.... books tell that God is light, and if light is there, there is mass. Where it came from, why are we here, those are meta-physicial questions or religious ones
Clint
light bends toward planets, it's equilance can be computed to equal mass and vic-versi
Clint
a point? define a point?
Clint
a point has infinite points between it and any object
Clint
ur question isn't wrong, but mass, like light are fundamentals.... that means they are just there, we question them yes, dissect them yes, but why are they there? Answer that question and ur the smartest person on this rock
Clint
show me where we can create light without mass or mass without light
Clint
ur 8th grade right?..... keep thinking,... u have a future and will go far
Clint
infinite means where we humans are unable to count further, I think energy is expanding itself...
Suman
who are you Sir?
Suman
I can't argue with u there.... as long as light exists, matter will increase
Clint
good point
Clint
I think there is something opposite of light too... if there would be ...then anti- matter will also exist
Suman
actually anti photon and photon are the same thing. photons are there own anti particles
Tanmay
We theorised that all matter in our universe came from a single point which is called big bang but we don't know what exactly happened at t=0 sec
Tanmay
a point? big bang? hyper condensed matter, but even if true it was not a point, it had light, space, dimension....
Clint
that's just a theory.
Suman
what caused that infinitesimal point to get so concentrated and expand outwards , what made that happen 🤔 😳
NARUTO
ah, but u speak of God.... with God all things are possible
Clint
the matter we see in our universe came from a giant dust cloud which made most of the stars and protostar of current Sagittarius A* black hole
Tanmay
so ur point is what? .... u believe in God?
Clint
I don't believe in god
Tanmay
I would like to have a proof that God exists
Tanmay
theory of a pulsating universe is a theory, theory of big bang is a theory.,... it's up to each and every individual student to start/believe somewhere/something
Clint
Naruto we exactly don't know what happened at t=0
Tanmay
we need to believe in these theories and challenge then test them to their limits and prove enhance or disprove it
Tanmay
but a student must believe in his imagination first then others'. And try to prove his imagination. This is what we call progress.
Suman
all we can do is theorize we will never know
NARUTO
this is what happened to Newtonian mechanics we believed it tested it it's limits and then enhanced it
Tanmay
physics uses existing phenom to at best guess what in the heck it is, and what it can do... we name things with a limited vocabulary and put numbers where they fit in equations we determine to match as precisely as possible
Clint
in 10th century we never new we can ever see and work at atomic level but now we have hopes the will happen with these theories
Tanmay
those are called physical constants.... and until someone describes a vocab, and rules that's in Greek (forgive me Greece) that we try to work within..... but I'm all open to another language if u can at least match the one that we have
Clint
You should imagine a theory where other theory fails other wise it is of no good use your theory would not get recognition and would not help science to progress
Tanmay
Einstein did not develope a new language, he added a dialect
Clint
the roots of science is always asking questions and trying to solve it
NARUTO
then he showed that quantum can be reduced to classical to prove he was indeed talking the same language.... physics
Clint
This is turning to a philosophy slightly going out of topic
Murrino
points are hard to work with they are infinitesimal sphere or 1 dimensional dot? They can't have any rotation that's why string theory was born to work at that small point. These strings are easy to study and do research
Tanmay
Yes I was also thinking about it so let's converge all of the discussion
Tanmay
if string theory is true then that proves warm holes true and time traveling possible right?
NARUTO
that's right Naruto, we question first,.... but we don't always solve it... I bet if u had access to a crate, and toys u could kick
Clint
worm*
NARUTO
As per big bang theory all the matter came from a point at t=0 . Everything including time and space emerged from it. But we can't prove this theory with current technology and we also don't know what exactly happened at t=0
Tanmay
As per big bounce theory tells that universe was always there and time and again the universe goes into big bounce and the previous universe converges into a point and new universe emerge from it.
Tanmay
Can someone tell me what multi verse theory states?
Tanmay
doesn't it states that there's other universes exist and parallel universes to ours
NARUTO
that's right tam, but first work with what's there, and before u go off half cock, look at terminology, search 5x more.... when u know what they know, then u kick there butt and tell them what they are missing..... but be humble
Clint
other life forms.... maybe
Clint
we are not the only back water plant
Clint
say what do yall think cause those huge dark voids in the universe?
NARUTO
but I will tell each of u something, what grows here can grow there, what is here maybe there, but that single point (who or what ever it was) is a part of that
Clint
what do you mean by part of "that"?
Tanmay
Naruto do you mean the great void which lack a lot of stars than the other areas?
Tanmay
the sun is cooling down alot & could do with warming up again.
LAURA
something's like that maybe, the ways to know them
Dennis Reply
oh ok you can actually reply to your own comment by tapping it once
Tanmay
you should do this
Tanmay
what are laws of the universe
Dennis Reply
The law of attraction
Lambent
Are you talking about fundamental forces Dennis?
Tanmay
you can go to google and type 4 fundamental forces of the universe
Tanmay
let me give you gist of it there are 4 fundamental forces namely gravitational, electromagnetic, strong nuclear and weak nuclear
Tanmay
the came into being just after the bid bang
Tanmay
I mean big bang
Tanmay
you should watch this video for better understanding ***youtu.be/X9otDixAtFw
Tanmay
Practice Key Terms 3

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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