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Lovely time lapse of a jet emerging from Herbig–Haro object:1*qjRSvo832lflrN-h34nlvw.gif

Young stars form in dense clouds of hydrogen which collapse down into proto-stars.  This collapse forms an accretion disc and interactions between it and the star can form a jet which emerges at the poles of the star, most likely due to magnetic fields generated between the star and the disc.

HH_object_diagram.svg

As this jet slams into the surrounding interstellar gas it can form the Herbig–Haro object as the gas radiates and disipates the energy of the jet.

HH34 is about 1500 light years away, and is about 10 light years long in total.

 

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ScotsAlan was wondering about accretion discs off in the thread which shall not be named.

They are wonderful things and their ubiquity on huge scales is a wonderful challenge for astrophysicists to explain.

There are a couple of goes:

 

The description of the second video gives the basic explanation of why they form:

In this simulation, gas clouds with random positions and motions coalesce under their mutual gravity, forming an accretion disk. It illustrates how the average angular momentum of the initial gas ends up defining the plane of the accretion disk.

A gas cloud will have an average anglular momentum whether it is going to form a solar system or a galaxy.

That average angular momentum will be preserved as the constituents of the cloud bounce into each other transfering energy and angular momentum, but conserving the overall amount of these two features.  The result - an acretion disc.

 

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The very first post in this thread was about Galaxy Zoo which has now morphed to become Zooniverse.

Galaxy Zoo gets volunteers to classify Galaxies, as humans are still better at this sort of thing than computers! If enough people classify the same galaxy consistently astronomers can be pretty sure the crowd sourced data is pretty robust and saying something about the classification of galaxies.

Recently the people who run the research published a paper in the Journal of the Royal Astronomical Society.

Edwin Hubble is the grand-father of galaxy research creating the first large set of galaxy observations in the 1920s and he came up with a Tuning Fork classification model:

hubble_tuningfork.jpg

E0 to E5 (and beyond - they go up to E7 I think) are elliptical galaxies - these are thought to have been the result of galaxy mergers were multiple galaxies have smashed together to create amorphous blobs of galaxies without an obvious structure - they're basically symmetrical with cores and a blurr of stars orbiting in all directions about the centre.  This is the wonderful fact of the universe - it is still young - over billions upon billions of years they will settle down and re-emerge as huge accretion discs as the average angular momentum of these huge agglomerations of stars tend to the mean via the law of large numbers, but at the moment stars are orbiting all over the place and hence the overall lack of structure.

Beyond the ellipticals there are the spirals - the classic accretion disc galaxies with spiral arms.

There are basically two types - unbarred, or barred.

potw1801a.jpg

The causes of the bar, and the spiral arms are really really complex; involving density waves, star formation rates, and orbital resonance, but the end result is the two basic types - we live in a barred galaxy and these seem to predominate.

Hubble thought there was a correlation between how the bar, the galaxy core and the tightness of spiral arms interact.

The bigger the core the tighter the spiral arms will be.

It now looks that this correlation was based on too limited a data set and when Galaxy Zoo brought far larger numbers to bear it disappeared.

Thousands of volunteers have been able to apply their hive mind and see things missed by experts and super-computers.  I think that is quite cool!

 

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JAXA Hayabusa2 asteroid explorer comes in for a bounce on the asteroid Ryugu

 

 

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So so pretty:

ssc2019-04a_Sm.jpg

Nebula Westerhout 40, as seen in the far-infrared by Spitzer Space Telescope.

It’s around 1000 light years away ... !

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I’m really enjoying Brian Cox and his Planets - it’s enough for my feeble brain. The Cassini info. on Saturn was fascinating.

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Two views of the recent solar eclipse visible from South America.

The first from a Geo-stationary satellite, and the second from a Chinese micro-satellite in orbit around the moon.

 

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I've auto started this talk from the American Geophysical Union at its final speaker, Prof Candice Hansen, who works with both the navigation camera and junocam on the Juno mission.

https://www.youtube.com/watch?v=hoSmSBFI6hc&t=5354

Prof Hansen introduced me to to the work of Gerald Eichstadt:

https://www.youtube.com/channel/UConituhAAnXbdrd3FDtXTfw/videos

Beautiful imagery taken over Jupiter.

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