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Journey into a black hole 
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Okay so this is another planetary nebula--The Butterfly Nebula:

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image from NASA (source)

also called by its catalog name, NGC 6302, or another nickname, the "Bug Nebula." The surface temp of the dying star--which has never been seen directly--is 200,000 K, making it one of the hottest objects in our galaxy. Good thing it's 3400 light years away.

This is not to be confused with the Twin Jet Nebula

Image
image from NASA (source)

which is also sometimes called the Butterfly Nebula, or Minkowski 2-9. The funky double tube shape is due to the push-pull of the dying binary star at the center: the smaller star is orbiting a red giant collapsing to a white dwarf, throwing out its expelled material into these polar jets.


Tue Feb 09, 2010 12:07 pm
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One of the denser star clusters in our galaxy, NGC 3603, has a nice H II region around it, being blown back by the bright new stars, 20,000 light years from Earth:

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image from NASA (source)


Tue Feb 09, 2010 12:14 pm
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Nebula NGC 6357, birthplace of the very large Pismis star group, 8,000 light years from Earth:

Image
image from NASA (source)

Pismis 24-1, which I think may be the brightest star in the middle there, was once thought to be one of the largest stars known, up to 300 solar masses, but when Hubble got a closer look they realised it was actually at least two, maybe three stars, not just one. Still, three stars of 100 solar masses each in a triple system is nothing to sneeze at. But I particularly like the smaller blue star in the middle of a nice bubble at lower center in this photo.


Tue Feb 09, 2010 12:23 pm
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Okay, I'm ending this little nebula research explosion with the largest nebula and most active starburst region in our Local Group of galaxies: the Tarantula Nebula, aka 30 Doradus, aka NGC 2070, at the compressed leading edge of the Large Magellenic Cloud, 180,000 light years away. The Tarantula is 500 light years wide, and so bright that if it was as close to us as the aforementioned Orion Nebula--1300 light years--it would cast visible shadows.

Image
image from NASA (source)

The "closest supernova observed since the invention of the telescope, Supernova 1987A, occurred in the outskirts of the Tarantula Nebula," says Wikipedia. Don't mess with the Tarantula!

Also, at the center of the Tarantula Nebula is super star cluster R136, only a few million years old and 35 light years across, but containing 450,000 solar masses of stars

Image
image from NASA (source)
[this one's a combination of visible light with UV and "red light," whatever that means, which I guess accounts for the stars being blue here instead of red like in the previous one, where you can see them near the top, rotated]

which means it could have enough local gravity to hold together and orbit the galaxy as a satellite: a globular cluster, although it won't be as big as the Milky Way's largest globular cluster (Andromeda's Mayall II globular cluster may be twice as big, but Hubble hasn't taken a really good picture of it yet :p), Omega Centauri

Image
image from ESO (source)

which, 15,800 light years away, at 5 million solar masses, has several million stars; the ones at the center are so close they average about 0.1 light years apart (close to a light month!), AND may surround a 4000 solar mass black hole.


I suppose all of this may be slightly off for A*, since supposedly the central bar of our galaxy consists mostly of old yellow stars, but our telescopes can't get shiny pictures of them, so whatareyagonnado.


Tue Feb 09, 2010 12:58 pm
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I wanna jot down some notes--mostly having to do with stellar evolution--having to do with stuff I was reading in the past day or so, before I forget them.

The thing about the star Pismis 24-1 that some people thought at first was maybe 300 solar masses, is that above about 150 solar masses, the photons released by the star's fusion reaction--classified as the highest energy photon type, gamma rays--are so energetic that they break down the star very rapidly.

For stars around 200 solar masses, you'd get something called pair instability, where really energetic gamma rays hitting atoms lose some of their energy in the formation of electron/positron pairs; stars are held up against gravity by the energy of their gamma rays, so loss of gamma ray energy causes the star to star compressing, heating up more, getting more gamma rays losing energy in pair creation, and so it loses more and more gamma ray pressure against gravity, and the star collapses, probably rebounding outwards in a supernova explosion so energetic that it completely blows the star apart without leaving a remnant such as a neutron star or black hole.

For stars at 250 solar masses, you'd get photodisintegration, where the gamma rays generated by fusion are so energetic that they knock protons and neutrons out of atoms, pretty much disintegrating the core to the point where it can't continue fusion, loses its pressure, and undergoes gravitational collapse. Photodisintegration is endothermic, so the star cools, and doesn't ignite in a thermonuclear supernova, but just collapses straight to a black hole. I think this is mostly still theoretical, though.

At the other side of the stellar size spectrum, the dwarfs, like the brown dwarfs in Orion, there's some different but still cool stuff going on. Most stars in our galaxy are red dwarfs, a little over half the mass of the Sun. They burn very slowly, and can stay burning--the smaller last longer--for trillions of years. Red dwarfs are small and dim, and thus really hard to see even with the best telescopes, which is one of the reasons why the number of stars in our galaxy is highly uncertain; the current estimate is between 200 and 400 billion stars.

Most stars--the ones that aren't big enough to supernova--and, it is thought, even red dwarfs eventually, will run out of fuel and collapse into very dense white dwarfs, which do not have fusion going on, and as far as I can understand it consist of atomic nuclei, stripped of their electrons (this is called "electron degenerate" matter), smushed right up against each other: for instance, a white dwarf of the mass of the Sun would be about the size of Earth.

White dwarfs don't burn, and just cool very slowly; it is thought that eventually they'd cool completely to "black dwarfs," but their cooling is thought to take like a kabillion years (well okay, 10^32 years (the lifetime of the proton) or more, or something--nobody really knows), so it isn't thought that any black dwarf stars exist yet.

And as they cool, it's thought that their plasma or whatever starts to crystallize! Man, I wonder what crystallized white dwarf material would look like.

But white dwarfs aren't necessarily condemned to slow cooling; if they have another stellar companion they're sucking material from, they may gain mass close to the limit ("Chandrasekhar limit") where fusion would start--about 1.4 solar masses--only instead of fusion starting and going steadily, all the carbon or whatever in the white dwarf just reacts/explodes all at once in a massive explosion called a type Ia supernova. Because these supernovae always happen when the white dwarf is at just about 1.4 solar masses, they're all about the same brightness, and are thus useful for measuring distances, since if you observe an event with the spectrum of a type Ia supernova, you just check how bright it looks to you, and from that a simple equation tells you how far away it must be.

There's a white dwarf pretty much right next to us, just 8.6 light years away: Sirius B, and it in fact is one of those dwarfs that's about the mass of the Sun (0.98 solar masses is close!) and the size of the Earth. It's in a binary with Sirius A, which is about two solar masses, and the brightest star in our sky--after the Sun. Here you can see little B and big A; the B dwarf is the little white dot to the lower left of the big A glare:

Image
image from NASA (source)


Wed Feb 10, 2010 2:12 am
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The Cassini probe is still out there around Saturn, taking cool pictures. Here's a Cassini image from late last year of the moon Titan, with the moon Tethys in the background, twice as far away from the probe (2 million km), and partly visible through orange and blue layers of Titan's atmosphere.

Image
image from NASA (source)

~~~~~~~~~~~~

Elliptical galaxies make up 10-15% of the observed Universe; they're curious because instead of stars revolving around a point, like in spiral galaxies such as our own, elliptical galaxies are just stars and star clusters moving radially (not sure if it's in or out, maybe it varies) from some central point. This is thought to result from collisions between other galaxies; for instance, it is thought that when our Milky Way collides with the Andromeda galaxy, the two will merge as a large elliptical galaxy.

Also, they contain comparatively little interstellar gas and dust, and generally consist only of old stars: the implication is that they are old galaxies that have consumed all their star-making materials long ago, and are now just burning down.

Furthermore, some of them are really huge, with mammoth supermassive black holes at their center. A* in fact started as an idea after I saw a picture of the center of elliptical galaxy Messier 87 (aka M87), which has a relativistic jet, measured at up to something like 5,000 light years in length, emanating from what is thought to be a 6.4 billion solar mass supermassive black hole at its center (the Milky Way's A* is a mere 4 million solar masses!). Here's a wider shot than the one I first saw:

Image
image from NASA/ESA (source)

That Hubble photo of M87 covers visible and infrared light. Aside from that big jet (M87, incidentally, is some 120,000 light years in diameter, so we're still seeing a very cropped view of its full extent here, if the jet is 5,000 ly long), the image seems fairly bland: M87 follows the usual pattern of elliptical galaxies, where the stars fall off very evenly from the dense center, without fancy whirls and what-not. But think about this: that big glow isn't just a sort of lensing artifact or dust illumination--it's made up of stars and star clusters, packed more and more densely toward the central black hole. And the things around it that look like individual stars are *clusters* of stars, each of which may contain hundreds of thousands of stars. Stars and stars and stars, five to ten times as many as the Milky Way!

Also, that big central jet raises physics questions, because if you just take a sequence of photos of it, it looks like bursts are traveling down it at up to six times the speed of light--and exceeding the speed of light isn't supposed to be possible. This apparent superluminal motion is thought to be an optical illusion in cases where a jet is pointing nearly in our direction, but M87's jet isn't really pointing toward us enough for that to be the full explanation. So according to Wikipedia anyway the explanation hasn't been settled on, and possibilities get into theoretical stuff like the Discrete Field Model, which I don't really follow but somehow divides space up into separate fields, where inside a field something is moving near the speed of light, but when seen from the relative perspective of another observer's field, appears to be moving faster than that. Something like moving at light speed and then emitting light which itself appears to be moving at light speed beyond that first's; but light speed doesn't add directly so I don't know if this is thought to be due to spacetime warping around the areas or what. Gah now my head hurts. Anyway, interesting! This scary thing is 55 million light years away.

Image
image from National Radio Astronomy Observatory/National Science Foundation (source)

That's M87, this time in radio emissions as seen by the Very Large Array radio telescopes; you can see the X-ray emitting loops and rings of severely agitated gas blown away from the galaxy's central black hole. Huge detonations occur every six million years or so near the center; one shock wave has been measured at 85,000 ly in diameter. These are at least part of the mechanism that has blown most of the gas and dust out of the galaxy, stopping new star formation.


Image
image by NASA (source)

This nifty image shows the Abell S740 galactic cluster, about 450 Mly (million light years) away. A nice collection of 30-49 galaxies, with the big elliptical ESO 325-G004 in the center, which may have about 100 billion stars (less than the Milky Way). But again that golden glow is made up of those billions of individual stars.


And finally I just want to mention the binary system GRS 1915+105, aka V1487 Aquilae, an energetic X-ray source 40,000 ly away from Earth; estimated at 10 to 18 solar masses, it is the largest stellar black hole found in the Milky Way. That doesn't necessarily mean much, because it wouldn't include intermediate-mass black holes, like the one probably at the center of Omega Centauri, shown a few posts up; rather than collapsing from one really really big star, the black hole could be the result of several stars converging, as it is probably doing now with its current regular-star neighbor.


Sun Feb 14, 2010 9:20 pm
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Ring and polar ring galaxies are galaxies with unusual hoops of stars around them, thought to be the result of intergalactic collisions. The rings are often compressed as a result, and that increase in density can trigger star births, making the rings bright blue from the formation of millions or billions of baby stars.

Hoag's Object, a ring galaxy:
Image
image by NASA (source)

AM 0644-741, another ring galaxy:
Image
image by NASA (source)

NGC 4650A, a polar ring galaxy:
Image
image by NASA (source)

Starburst galaxies include any galaxy where an unusually high rate of star formation is taking place, usually due to a galactic collision, but not necessarily leaving a ring shape. Some of these:

NGC 1569 is an irregular dwarf galaxy containing two or three super star clusters (million+ stars each) in its core. These regions are seeing a 100x normal rate of star formation, and bursts of supernovae have been blowing massive bubbles in the gas of the galaxy's interior.
Image
image by NASA (source)

I Zwicky 18 is another dwarf irregular, and has been said to resemble a galaxy from an earlier era in the Universe: it has relatively few older stars, and is currently forming new stars at a very high rate from a primordial mix of almost pure hydrogen and helium; that could classify them as Population III stars, a metal-free type of star thought to be the first wave of stars in the universe. I Zwicky 18 appears to be an unusually late bloomer!
Image
image by NASA (source)

And the Antennae Galaxies, of which I previously posted a nice picture back here (bottom of that post) are also classified as starbursts, due to the increased star formation ignited by their energetic collision.


Wed Feb 24, 2010 3:37 pm
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Launched in February of this year, NASA's Solar Dynamics Observatory studies the sun with a battery of sophisticated cameras from Earth orbit. Just yesterday, NASA released a video of some of the first data and images returned by the SDO:



The clips shown in successive colors are illustrating the activity along different absorption bands, ie different temperatures. It's pretty amazing to see ripples from a single flare shooting out very quickly across a huge section of the Sun's surface!


Fri Apr 23, 2010 1:21 am
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Slight change of pace but hey:

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Color lithograph by Jean-Léon Huens, (1921-1982) a Belgian illustrator. I've seen this called "Galileo and Two Cardinals" and "Galileo explaining moon to skeptics," so maybe it doesn't have an actual title. It's in National Geographic's image collection, so I think it might have been part of the work he did for an article called "Seven men who solved riddles of the cosmos" in that magazine's May 1974 issue. It appears briefly during a segment about the Enlightenment in Holland in episode 6 of Carl Sagan's series "Cosmos," and I just think it's a nifty picture.


Wed May 12, 2010 5:44 am
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