(image by NASA (source))

A Hubble image of what are thought to be young stars or protostars with protoplanetary discs (aka "proplyds") around them in the Orion Nebula, about 1,500 light years from Earth.

Images like this are seen as verification of the theory of our own solar system's formation, which says that gravitational collapse of part of a nebula, possibly rippled by something like a nearby supernova explosion, eventually led to the formation of our Sun and the planets around it, which formed from material left over from the formation of the Sun. There are lots of interesting theories about details of the formation of the planets, as scientists try to find out how to account for current observable features such as:

- the asteroid belt between Mars and Jupiter
- the many thousands of objects orbiting out past Neptune in what are now know as the Kuiper Belt and "scattered disc"--of which Pluto (with its co-orbiting body Charon, and two tiny moons Hydra and Nix) is just one of many--at least seven other bodies of comparable size have been found out there so far
- both short period and long period comets, some of which must come from some vast, currently still hypothetical cloud of small icy bodies that lies out between the outer limit of the scattered disc and Kuiper Belt, and the distance at which the gravity of the Milky Way Galaxy begins to overcome the Sun's gravity, which may be as far as one light year from the Sun
- the size of Neptune and Uranus, which exceeds the amount of gas that should have been present at their current distances from the Sun during the formation of the planets

So! Scientists have thought up lots of interesting things that could have happened, such as:

- Neptune and Uranus formed much closer to Saturn and Jupiter, but an orbital resonance of Saturn and Jupiter--Saturn orbiting the Sun once for every two Jupiter orbits--500 to 600 million years after the formation of the system, or about 4 billion years ago, caused a massive gravitational disruption which pushed the two smaller gas giants further away, and in particular sent Neptune swinging out past Uranus
- That resonance also scattered off the vast majority of the material in the asteroid belt just inside Jupiter's orbit
- Objects scattered by the outfalling Neptune became what we call the Kuiper Belt and scattered disc, and objects scattered by Jupiter became the Oort cloud
- The scattering also had the effect of pulling the gas giants themselves further out
- Back in those days there were also hundreds of "Moon-to-Mars-sized" "planetesimals" around the solar system, which eventually got smashed or scattered
- The gas giants, mostly Jupiter and Saturn, picked up some of the planetesimals as moons
- All this smashing and scattering led to a lot of impacts on the planets and moons, leaving most of the craters we see today
- There's even a theory that our own Moon was formed from debris resulting from a collision between the Earth and a near-Earth-sized planetesimal, but there are various problems with that idea (sounds fishy to me!)

There's more on that stuff here, but keep in mind that a lot of it is pretty hypothetical. Still, clearly there was a lot of fire and explosions going on back then.

Ooh I almost forgot a couple interesting things:

- There's a "frost line" just inside the orbit of Jupiter--the distance from the Sun inside of which ices that form large parts of the outer moons and bodies can't survive; during the formation of the system the evaporation of infalling icy bodies hitting that boundary could have created a low pressure zone that would have acted as a barrier, damming up material out there, which would help explain how Jupiter grew so big

- Formation of planets stopped after three to ten million years, when the Sun would have gotten bright and hot enough to blow most of the protoplanetary disc's remaining gas and dust out of the solar system

- That orbital resonance of Jupiter and Saturn 4 billion years ago, which would have caused a period of scattering and collisions and fiery death across the solar system, would have ended, in geological terms, right before the beginning of what we know as life on Earth, 3.8 billion years ago--suggesting that life couldn't form earlier than that at least in part because the solar system before then was too volatile, with big things hitting Earth and making it too nasty a place for anything living

image by NASA (source)

This new photo from Hubble may be our first view of an asteroid-asteroid collision's aftermath. It's in our Solar System's main Mars/Jupiter asteroid belt, about 90 million miles from Earth; the little white dot at the "head" in the lower right is a 140m-diameter rock, and the smaller dots scattered back from it in comet-like streamers due to solar wind are dust and gravel. Scientists think the belt has been home to many collisions between asteroids and even larger bodies in the history of our solar system, but we've never actually seen such a collision happen.

Okay, actual black holes--or the stuff around them--today!


image from NASA (source)

Cygnus X-1 is perhaps the best-known stellar mass black hole candidate; it was the one Stephen Hawking made a bet on in 1975--saying it *wasn't* a black hole--which he eventually conceded in light of better evidence for its black hole status. Cygnus X-1 is about 6000 light years away from Earth, and is a binary system thought to consist of a blue supergiant that is 20-40 times as massive, 20-22 times as big, and 300,000-400,000 times as bright as the Sun, and an implied very compact object estimated at 8.7 solar masses, which--if it is indeed a black hole--would mean a black hole with a Schwarzschild radius (calculated event horizon size) of 26 km. These things whip around each other once every 5.6 days, and are separated by a distance equal to only two or three times the size of the supergiant (or something close to that--I lost the page where I found that number :p).

The Chandra image shows the powerful X-ray emissions from solar wind particles emitted by the blue supergiant that spiral into the compact object, going faster and faster, heating up to millions of degrees due to friction, at which temperature they emit energy in the X-ray spectrum. In 2001 it was announced that analysis of Hubble readings of Cygnus X-1 from the early '90's revealed the first observation of stuff actually falling into a black hole's event horizon: measured bright gas clumps flared up, then abruptly disappeared as they moved toward the compact object. Neato.



image from NASA (source)

The above image combines Hubble optical data with X-ray readings from Chandra, showing the two highly energetic active galactic cores of galaxy NGC 6240, thought to be the result of the collision between two galaxies. Those two white dots at the center are the strong X-ray sources of the galactic cores, quite probably generated by superheated gas falling into supermassive black holes.



image from NASA (source)

That's the core of our own galaxy--around the supermassive black hole Sagittarius A*--as seen in the infrared, combining readings from infrared cameras on Hubble and the Spitzer Space Telescope. I *think* this is about 300 light years from top to bottom.

A rather nice optical mosiac of the central region of the Milky Way:


image by ESO/Stéphane Guisard (source)

Another nice image (I think I'm liking the cool colors in these), this time of the Pleiades, an open cluster of young stars only 440 light years from Earth. They're traveling through a dust cloud, and the starlight bouncing off it gives the cluster its lovely nebulous blue glow.


image from NASA (source)

I've been looking at things like these last two images because I've been wanting to come up with a more interesting rendition of space for A*; at the galactic center, the sky should be more than just the usual dots of individual stars here and there. Now that I'll be drawing for individual images rather than for animation--starting with episode 8--I should be able to come up with something that at least starts to give a difference sense of the space environment there. (The Pleiades aren't anywhere near the center, but dust clouds like the one they're passing through would be much more the norm there.)

Barnard 68, a molecular cloud about 500 light years from Earth:


image from ESO (source)

Molecular clouds are relatively cold places, but that low temperature allows them to collapse and form stars. Barnard 68 is about 2 solar masses, and 0.5 light years across; it is thought that it will undergo gravitational collapse in about 100,000 years, and become a star. Until then, it blocks light and is kind of scary-looking.

Barnard 68 is in our neighborhood, far from the galactic core. However, one of the best known molecular clouds is Sagittarius B2, a mere 390 light years from the center, 150 light years wide and weighing in at about 3 million solar masses. At 3000 hydrogen atoms per cubic centimeter, it's about 20-40 times the usual density of molecular clouds (that that's still an infinitesimal fraction of say the density of air at sea level on Earth, which has 3x10^19 molecules per cubic centimeter).

Temperatures in Sgr B2 range from a cool 40 to 300 K (keep in mind that the Earth is in the Local Interstellar Cloud which is rated at 0.1 atoms per cubic centimeter and 6000 K, which is itself inside the Local Bubble, 0.05 atoms per cubic centimeter and about 1,000,000 K (here's another source for that), yet we aren't roasting, so it's important to keep in mind relative density when astronomers feed you scary million-K temperatures; in fact I'm wondering now how long it would take say a young supernova remnant like the one that was going to hit Pyrite--which could be like 15 million K or something--to heat the planet up to an intolerable level...probably a long time but lets just say that it was a particular dense squirt of nebula headed toward Pyrite at relativistic speed; that would probably be somewhat uncomfortable in various ways at least), but there are presumably hotter star-forming ("H II") regions, one of which for instance has 10 million times the luminosity of the Sun. Neat.

Oh, this is interesting: NASA's Near Earth Supernovas article talks about what a somewhat denser cloud (but probably not super-hot...haven't been about to find more on this cloud) could do just in terms of pressure:

The article's old (2003), which isn't ideal. "Sco-Cen complex" seems to mean the Scorpius-Centaurus Association, just a name for a big local bunch of stars that presumably shared a common origin. "Local fluff" is kind of astronomer slang for the Local Interstellar Cloud. The University of Chicago is my alma mater. :) I did have one quarter there with a pretty cool astrophysicist who wowed the class with talk of doing work in 22 dimensions, and taping bugs to a balloon which he then proceeded to inflate, to try to illustrate the expansion of the universe (which is an amusing demonstration, but I think counterproductive really, because it gives students the idea the expansion of the universe is bubble-like, outward from a center, when it's actually (supposed) expanding equally from all positions and directions).

H II (aka "star forming") regions can get up to millions of particles per cubic centimeter--still only the most miniscule fraction of the density of Earth's atmosphere (10^6 vs 10^19)--and make for pretty nebula like these:


image from NASA (source)

The Cone Nebula, 2700 light years away, part of the same group, NGC 2264, as the bright "Christmas Tree Cluster."

Orion Nebula, 1344 light years away, 12 light years across.


image from NASA (source)

Hubble got great pictures showing proplyds--protoplanetary discs around baby stars


images from NASA (source)

and brown dwarfs--proto-stars too small to start stellar fusion


image from NASA (source)

inside the Orion Nebula.

Okay, this isn't an H II type thing, but it's pretty; it's The Red Spider Nebula, which is a "planetary nebula," meaning it's emitted from a star:


image from NASA (source)

The white dwarf at the center is supposedly one of the hottest stars yet found, and it's generating unusually fast solar winds that are blowing the nebula material to great distances.

The Boomerang Nebula


image from NASA (source)

is the coldest known object in space; at -272 K, it's only 1 K above absolute zero--that's 1 K *warmer* than intergalactic space warmed only by cosmic background radiation. Only the Boomerang Nebula has been found to be colder than that radiation! It's supposed to be due to the rapid (164 km/s) outflow and expansion of gas from the central star.

Ooh and this isn't an H II type nebula either; nor is it a planetary nebula, which is from the death of a star: this is a protoplanetary nebula, which happens when a star is born.