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8/05/2013

Supernova and Kilonova.

Lifecycle of the Sun

I love supernova, whether they are Type I or Type II.  I find it fascinating that a star greater than 8 times the mass of the Sun will live a very short life (in astronomical terms) and end their life by blowing up, leaving either a neutron star or a stellar black hole.  A star like our Sun, with a binary companion, after going through its red giant stage, will shed its outer layers and then the core will become a hot object, called a white dwarf, about the size of the earth. This white dwarf will cool over time. If it is like our Sun, it will be a planetary nebula for a thousand or so years until its outer layers drift off, no longer illuminated by hot white dwarf leaving only the white dwarf. The white dwarf is no longer having thermonuclear reactions, all reactions have stopped. The white dwarf is not generating heat, it it simply hot, usually having a carbon oxygen core that is very hot and takes billions of years to cool. Now if that white dwarf is a companion to another star, is close enough in orbit to steal mass from its companion, as that companion star nears the end of its life it will swell into a red giant star and the white dwarf, if it is close enough, will begin to pull mass off the red giant.  You can see this in this image and read about the process in depth from this link.



To provide an example of how big a star like our Sun is when it becomes a red giant is best seen in this image. 



Now as the white dwarf continues to steal mass from the red giant companion star, the mass of the white dwarf begins to increase.  When the white dwarf accretes and reaches a limit known as the Chandrasekhar Mass limit (1.44 solar masses which means 1.44 times the mass of the current Sun; though this is listed as 1.4 solar masses in some sources) the white dwarf begins to undergo nuclear fusion and in a matter of seconds, the remaining mass, carbon/oxygen or if the  original star is larger than our Sun, the white dwarf will be comprised of a oxygen-neon–magnesium core, collapses and the white dwarf explodes into a Type I Supernova. This link shows on PBS NOVA how this process works. 


Just to clarify, in some cases both stars in a binary system will become white dwarfs without the first one exploding as a Type I Supernova. In this case the gravity of the two white dwarfs can be drawn to each other in a dance of death, until when they merge, their mass exceeds that 1.44 solar mass limit and they explode as a Type One Supernova.  



When the white dwarf explodes, the explosion is massive. If it had a companion star, that star has some of its outer atmosphere blown off and the remaining star is sent hurtling faster than the other stars in the area of space around it, fleeing from the explosion.  There is nothing left of the white dwarf that explodes. Unlike a Type II Supernova where a massive star explodes, leaving a neutron star/pulsar or a stellar black hole. These massive stars that cause a Type II Supernova, are so large, some have an orbit out between Mars and Jupiter 
(as shown in this picture). This image shows how big Betelgeuse is compared to our Sun, the red dot in the middle and the star Deneb. Both Betelgeuse and Deneb are large enough stars that they will end their lives as supernova. 





Betelgeause from above has created and burned through most of these layers. A massive star will burn through these layers in this amount of time per layer: 

Star burns through a succession of nuclear fusion fuels:
Hydrogen burning: 10 Myr
Helium burning: 1 Myr
Carbon burning: 1000 years
Neon burning: ~10 years
Oxygen burning: ~1 year
Silicon burning: ~1 day

Then it forms Iron, and in milliseconds of the core exceeding 1.4 masses of Iron, BOOM, the Supernova explodes. A star like our Sun will burn for about 10 to 12 billion years and the Sun per above, has been burning for just over 5 billion years. 



As massive giant stars age, they produce "onion layers" (see above) of heavier and heavier elements in their interiors. However, stars will not fuse elements heavier than iron. Fusing iron doesn't release energy. It uses up energy. Thus a core of iron builds up in the centers of massive supergiants.

Eventually, the iron core reaches something called the Chandrasekhar Mass , which is about 1.4 times the mass of the Sun. When something is this massive, not even electron degeneracy pressure can hold it up.

At this point, the core collapses and two important things happen:

1. Protons and electrons are pushed together to form neutrons and neutrinos in the core.
2. Even though neutrinos don't interact easily with matter, at densities as high as they are here, they exert a tremendous outward pressure.

The outer layers fall inward when the iron core collapses. When the core stops collapsing (this happens when the neutrons start getting packed too tightly -- neutron degeneracy), the outer layers crash into the core and rebound, sending shock waves outward.
These two effects -- neutrino outburst and rebound shock wave -- cause the entire star outside the core to be blow apart in a huge explosion: a type II supernova!
Supernovae are really bright -- about 10 billion times as luminous as the Sun. Supernovae rival entire galaxies in brightness for weeks. They tend to fade over months or years.

During the supernova, a tremendous amount of energy is released. Some of that energy is used to fuse elements even heavier than iron! This is where such heavy elements like zinc and uranium come from!

The material that gets ejected into space as a result of the supernova becomes part of the interstellar medium. New stars and planets form from this interstellar medium. Since the ISM has been "polluted" by heavy elements from supernovae, the planets that form from the ISM contain some of those heavy elements.

The collapsed core is also left behind by a type II supernova explosion. If the mass of the core is less than 2 or 3 solar masses, it becomes a neutron star. If more than 2 or 3 solar masses remains, not even neutron degeneracy pressure can hold the object up, and it collapses into a black hole.

Now this is a massive explosion in either case, a Type Ia or a Type IIa.  10 billion times as luminous as the Sun means these Supernova outshine initially, for several months, the brightness of the galaxy they explode in.  If a star is really massive, like this star, VV Cephi:



when they go supernova they often have a burst of energy, released from their two poles, called a gamma ray burst. For massive stars like VV Cephi, these gamma ray bursts are long.  However there are short gamma ray bursts that occur and astronomers think they know why.

For stars that leave a neutron star, when a neutron star, which is VERY dense merges with another neutron star or with a black hole, that explosion that occurs Astronomers have long suspected that an item called a Kilonova happens. This is when a neutron star, left from a massive star after the star has gone through the supernova stage, falls into a black hole or merges with another neutron star. As shown in this NASA image:



Credit: NASA, ESA, and A. Field (STScI)

In 1 you have two neturon stars orbiting and falling into each other. In 2 they merge cause a super heated explosion with a lot of radiation. This explosion results in 3. a short gamma ray burst until the new object, probably a black hole begins to devour the remains of the two neutron stars while heating up the remaining gas that surrounds the black hole.  Eventually, the stellar black hole will not have the material around it as it devours it or spits it back out into space and it will remain unseen at that point. That is what a Kilonova is. Hard to observe as they are very short. If you want to learn more and have a better picture in your head here are some links: 

Space.com and they have a good video of the images put together showing the Kilonova. 

NASA/Hubbe Site on Kilonova discovery. 





It was the Best of Nights, It was the Worst of Nights . . .




I got out before anyone to the observing site, Pit n Pole, and took pictures of the clouds. Then I noticed something. 


If you look carefully, you can see a shadow that is being cast from a cloud from the Sun. Cool! 


Here you can see the shadow a little better. 


Here is a closeup of the shadow. 

 My 14" cooling and notice Mat's new van in the background. Especially the right side that is showing here. 

I had a different but good experience last Friday night. It was clear all day, then clouded up but my friend Mat and I headed out anyway. Good enough. We got out to our site and the clouds had parted so we began to set up. As we did we had 3 people in a car pull up and I saw a box in the car, a scope.

We introduced ourselves and then went about setting up. They had a 10" Apertura Dob with a RACI 9x50 and a Telrad. I showed Dave, the guy with the 10" how to use the Howie Glatter collimation tools that I had (he had just ordered them) and then I went back to my 14" to use the Catseye to collimate and then check using the Howie Glatter system I have.




By now we were in twilight and both my friend Mat and I aligned. As soon as we aligned, we pulled out our 10x50's bino's and started looking around for fun, to see what we could see and challenge our eyes to see. I'm really getting into including objects on my list hat challenge me, to push my eyes.  Matt gave up before me and went and got on Saturn. After showing Saturn, Mat offered to Jeremy, Dave's nephew to use his 8 inch homemade dob that he has (it's for sale!).  He thanked Mat but was hesitant. As it was dark I check out a few objects and I have some nice faint objects down in Cygnus and that was covered in clouds. In addition to be just honest, it was horrible conditions, Antoniadi IV I'd give it. However Sagittarius was up and the summer Milky Way looked good.

I then had Dave and Jeremy join me and showed them how to use a basic atlas, in this case the Sky Pocket Atlas and showed them how to hunt down objects like M22, M8, the Trifed. Mat showed them how to hunt down M51 also which was easily view able. Mat also showed Jeremy how to use the Rigel on his scope as he has that instead of a Telrad. No biggie.

From here Dave using his 10 and Jeremy using Mat's homemade 8 inch dob, went to work finding objects on their own and doing quite well. Dave suggested I show his niece who was 15 how to star hop so I did. She did really well, only needing help initially and then on one object later. We showed them the star hop to M11, and everyone nailed it and then Mat shared his nickname for it; not the Wild-Duck but the Borg Cube.

At the end of the night, Cygnus had cleared but conditions were horrible for me to go hunting so I had Dave's niece find Alberio and then I did show them the Veil and how to go to 52 Cygni to find it. They left early because Dave had to get his niece home by a reasonable hour. For the next hour both Mat and I tried observing but to no avail, conditions just wouldn't bring in the fainter objects we both needed. However, it wasn't a waste. We met three wonderful individuals and got them started we hope in the hobby. I tested 2 new eyepieces, the Baader Ortho Classic 6mm and 10mm for use with a minimal glass type of observing and to pull just a hint more of detail out of faint objects. More on that in my next post. This is what Dave posted on our local forum:

"Hi Jay, it was great to meet both you and Mat as well.
I can't tell you how much fun we all had, Jeremy told me on the way home that he HAD to have a scope of his own now but he couldn't decide whether to buy one or make one like Mat's.
Emily is a little bit shy and I was a little surprised that you got her to do some star hopping with your scope, but she loved it and was talking about it all the way home.
We will definitely try to come out again soon. My thanks to both of you"

So though I didn't have the equipment set up issues, I did have horrible skies and something horrible happen on the way home. Mule deer are quite common in Utah and a doe can weigh in on average around 150lbs. Well this night on a new highway near where we live called the Mountain View Corridor, it saves us about 20 minutes of travel time, I was in the the lead and Mat was driving behind me. I noticed Mat was way behind me and in truth had pulled over. I pulled over and just about when I was going to circle back, Mat pulled back out unto the Highway.  We turned left at a light and I noticed his front right turn signal was out so I pulled over after the turn to tell him. As I got out Mat I told him his light was out and with a sickly look on his face, Mat said "Yeah, I know."  He then showed me how a doe had nailed the right front fender, then was thrown and impacted his entire right side of his van with dents, and damage all the way down. In addition ALL the right side passenger bags deployed ripping up his upholstery and the upper lining of his van. It was just horrible.

So this observing session was not good in terms of finding personal objects, or in finding the Herschel 400 II I had down or the faint objects that I had on my list.  It was great in the sense of helping Dave, Jeremy and Emily out though.   Dickens would cover it, It was the Best of Nights, It was the Worst of Nights. I'm sure for Mat, it was the "Worst of Nights."

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"It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the pale blue dot, the only home we've ever known." Carl Sagan