Starship Asterisk*

Ann wrote:This is a great picture, and this region is a wonderful site of young stars and star formation. The overall "nebular landscape" looks fantastic. Harel Boren's composition is beautiful, so that we can see both NGC 6188 and NGC 6164 at the same time. The colors look splendid.

The star responsible for sculpting this fantastic cosmic landscape is HD 150136, which is a special favorite of mine. (The picture you see if you follow the link is by Rolf Wahl Olsen.)

I find I have already made a post about this amazing star in another thread (and also about HD 148937, the star surrounded by the planetary nebula-like outflows) so instead of composing another post, I'll include a link to my previous one from 2012.

Ann

GSteinhour wrote:
The first thing that jumped out at me was a big moose head with the antlers branching out on top.

It should be called the Moosehead Nebula or Bullwinkle Nebula, since Bullwinkle was the sidekick of "Rocket J Squirrel".

APOD Robot wrote: Bright Planetary Nebula NGC 7027 from Hubble

Explanation: It is one of the brightest planetary nebulae on the sky -- what should it be named? First discovered in 1878, nebula NGC 7027 can be seen toward the constellation of the Swan (Cygnus) with a standard backyard telescope. Partly because it appears there as only an indistinct spot, it is rarely referred to with a moniker. When imaged with the Earth-orbiting Hubble Space Telescope, however, great details are revealed. Studying Hubble images of NGC 7027 have led to the understanding that it is a planetary nebula that began expanding about 600 years ago, and that the cloud of gas and dust is unusually massive as it appears to contain about three times the mass of our Sun. Pictured above in assigned colors, the resolved, layered, and dust-laced features of NGC 7027 might remind sky enthusiasts of a familiar icon that could be the basis for an informal name. Please feel free to make suggestions -- some suggestions are being recorded, for example, in an online APOD discussion forum.

Debbie wrote:
Flying Squirrel

Guest wrote:
Rocky the Flying Squirrel for sure. Saw it immediately and was amazed to find it had already been zapped several times. Hey, Rocky, watch me pull a white dwarf out of my hat! Bulwinkle, that trick never works. In Australia we have flying foxes (a kind of fruit bat) but it's the same thing.

Psnarf wrote:
... for the sequences between carbon and silicon just skip one element in the table of elements for atomic numbers carbon6 --> oxygen8 --> neon10 --> magnesium12 --> silicon14; ...then comes iron26 (?). Just wondering where the sulfur16 came from. Perhaps a fission reaction in the supernova soup?

http://en.wikipedia.org/wiki/Silicon-burning_process wrote:
<<In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.

Silicon burning begins when gravitational contraction raises the star’s core temperature to 2.7–3.5 billion kelvins (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova.

After a star completes the oxygen burning process, its core is composed primarily of silicon and sulfur. If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 GK (230–300 keV). At these temperatures, silicon and other elements can photodisintegrate, emitting a proton or alpha particle. Silicon burning entails the alpha process, which creates new elements by adding one of these alpha particles (the equivalent of a helium nucleus, two protons plus two neutrons) per step in the following sequence:

• ²⁸Si → ³²S → ³⁶Ar → ⁴⁰Ca → ⁴⁴Ti → ⁴⁸Cr → ⁵²Fe → ⁵⁶Ni → ⁶⁰Zn

The entire silicon-burning sequence lasts about one day and stops when nickel-56 has been produced. The star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon (any proton or neutron) of all the elements in the alpha process sequence. Although iron-58 and nickel-62 have slightly higher binding energies per nucleon than iron-56, the next step up in the alpha process would be zinc-60, which has slightly more mass per nucleon and thus, is less thermodynamically favorable. Nickel-56 (which has 28 protons) has a half-life of 6.02 days and decays via β+ decay to cobalt-56 (27 protons), which in turn has a half-life of 77.3 days as it decays to iron-56 (26 protons). However, only minutes are available for the nickel-56 to decay within the core of a massive star. The star has run out of nuclear fuel and within minutes begins to contract. The potential energy of gravitational contraction heats the interior to 5 GK (430 keV) and this opposes and delays the contraction. However, since no additional heat energy can be generated via new fusion reactions, the contraction rapidly accelerates into a collapse lasting only a few seconds. The central portion of the star gets crushed into either a neutron star or, if the star is massive enough, a black hole. The outer layers of the star are blown off in an explosion known as a Type II supernova that lasts days to months. The supernova explosion releases a large burst of neutrons, which synthesizes in about one second roughly half the elements heavier than iron, via a neutron-capture mechanism known as the r-process (where the “r” stands for rapid neutron capture).>>

Psnarf wrote:
I learned the supernova onion sequence, that is, the elements that fuse starting with hydrogen fusion to iron, which can't fuse due to insufficient energy leading to a big badaboom: H, He, C, O, Ne, Mg, Si, and finally Fe. In the Periodic Table of Elements, hydrogen fusion is simple to understand, that is, 1 + 1 = 2: add the number of protons to get the atomic number of helium. Then somehow 2 + 2 = carbon6, which I don't yet understand.

http://en.wikipedia.org/wiki/Triple-alpha_process wrote:

---

<<Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores. The products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus produce lithium-5 and beryllium-8 respectively, both of which are highly unstable and decay almost instantly back into smaller nuclei. When the star starts to run out of hydrogen to fuse, the core of the star begins to collapse until the central temperature rises to 10⁸ K (8.6 keV). At this point helium nuclei are fusing together faster than their product, beryllium-8, decays back into two helium nuclei.

The triple alpha process is highly dependent on carbon-12 and beryllium-8 having resonances with the same energy as helium-4, and before 1952, no such energy levels were known. The astrophysicist Fred Hoyle used the fact that carbon-12 is abundant in the universe as evidence for the existence of a carbon-12 resonance. This could be considered to be an example of the application of the anthropic principle: we are here, and we are made of carbon, thus the carbon must have been produced somehow. The only physically conceivable way is through a triple alpha process that requires the existence of a resonance in a given very specific location in the spectra of carbon-12 nuclei.

Hoyle suggested this idea to the nuclear physicist William Alfred Fowler, who conceded that it was possible that this energy level had been missed in previous investigations. By 1952, Fowler had discovered the beryllium-8 resonance, and Edwin Salpeter calculated the reaction rate taking this resonance into account.

This helped to explain the rate of the process, but the rate calculated by Salpeter was still somewhat too low. A few years later, after a project by his research group at the Kellogg Radiation Laboratory at the California Institute of Technology, Fowler discovered a carbon-12 resonance near 7.65 MeV. This eliminated the final discrepancy between the nuclear theory and the theory of stellar evolution.

Because the triple-alpha process is unlikely, it needs a long time to produce much carbon. One consequence of this is that no significant amount of carbon was produced in the Big Bang because within minutes after the Big Bang, the temperature fell below that necessary for nuclear fusion.

This creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen but only a small fraction of these elements is converted into neon and heavier elements. Both oxygen and carbon make up the 'ash' of helium-4 burning. The anthropic principle has been controversially cited to explain the fact that nuclear resonances are sensitively arranged to create large amounts of carbon and oxygen in the Universe.>>