Lots of big astronomy news is hitting the ether this afternoon. The first story to catch my eye is this one about how early black holes weren’t quite the gluttons for material that they were expected to be. Since most galaxies have black holes at their hearts, this idea that the first ones couldn’t get enough to eat in the early universe has profound implications for how astronomers understand galaxy formation.
A computer simulation of x-rays produced by an early black hole and their effects on nearby gas clouds. Early stars ate up most of the gas, leaving little for the resulting black holes to feed on. Courtesy KIPAC/SLAC/M. Alvarez, T. Abel and J. Wise .
To get a handle on the black hole diet way back in the first million years after the Big Bang, astronomers at the Goddard Space Flight Center and the Kavli Institute for Particle Astrophysics and Cosmology, performed a supercomputer simulation of conditions back when the first stars and galaxies were forming — some 13 billion years ago.
“The first stars were much more massive than most stars we see today, upwards of 100 times the mass of our sun,” said John Wise, a post-doctoral fellow at NASA’s Goddard Space Flight Center in Greenbelt, Md., and one of the study’s authors. “For the first time, we were able to simulate in detail what happens to the gas around those stars before and after they form black holes.”
In the simulation, cosmic gas slowly coalesced under the force of gravity and eventually formed the first nassive, hot stars. They burned brightly for a short time and emitted so much energy in the form of starlight that they pushed away nearby gas clouds.
These stars could not sustain such a fiery existence for long, and they soon exhausted their internal fuel. In the simulation, one of the stars collapsed under its own weight to form a black hole. since the progenitor star had either consumed or pushed away the rest of the gas cloud, the black hole was essentially “starved” of matter on which to grow.
So, the first black holes were on a pretty strict diet — but they still managed to produce x-ray radiation that kept nearby gas from falling in to the black holes. This radiational also heated gas a hundred light-years away to several thousand degrees. When you get that kind of heated gas cloud, it can’t coalesce to form new stars — and so even though the black holes were starving, they contributed to the dietary cycle by starving nearby areas of any material from which to form new stars. How does affect galaxy formation? Well, starving out the star-formation process affects the growth of galaxies. Yet, we have galaxies now, and we’ve seen galaxies back then — so the next step is to understand how the first galaxies overcame this strict diet inflicted on them by their black holes. Stay tuned!
You can watch a nifty animation of the black hole starvation scenario here.
Planetary Collisions Spotted by Spitzer
Planetary collision -- an artist's concept of a stupendous event! (Courtesy NASA/JPL-Caltech) Click to embiggen.
The other big story today that caught my attention is the infamous colliding planets announcement. Now, my friend Phil Plait over at BadAstronomy wrote a book called Death from the Skies that talks about all the ways we can die (or be seriously inconvenienced) by the cosmos — but I don’t think he covered colliding planets. Now that Spitzer Space Telescope has caught evidence of planets colliding around another star, he can add that one in to the next edition of the book.
So, what’s the story behind this discovery?
NASA’s Spitzer Space Telescope found evidence that a high-speed collision between two forming planets — one about the size of Mercury and the other about the size of our Moon — occurred a few thousand years ago around a young star, called HD 172555. This planetary system, which is about 100 light-years away from us, is still in the early stages of planet formation.
So, what evidence did Spitzer capture of this dramatic event? When the collision occurred, lots of vaporized, melted rock and bits of rubble got thrown across immediate space. As you can imagine, such a collision causes lots of heat — and the infrared heat signature is something that Spitzer is especially good at detecting.
As the bodies slammed into each other at speeds upwards of 10 kilometers a second, a huge flash of light would have been emitted. Rocky surfaces were vaporized and melted, and hot matter was sprayed everywhere. Spitzer detected the vaporized rock in the form of silicon monoxide gas, and the melted rock as a glassy substance called obsidian. On Earth, obsidian can be found around volcanoes, and in black rocks called tektites often found around meteor craters.
At the end of the collision process, the larger planet was essentially stripped of its outer layers. It absorbed the core and most of the surface material of the smaller body. This is likely how Earth formed — by collision and accretion, some 4 billion years ago. It’s probably very similar to how Mercury formed, and a similar collision contributed to the formation of our Moon. So, in a sense, the Spitzer observations are giving astronomers a very interesting look back to the birth of our own solar system.
I’ve been a bit scarce lately, got my head into a couple of very intriguing projects. One of them involves planetary formation, which is a topic always of interest to me. The grand, epic picture of planetary formation goes something like this: you have this nebula, swirling around in space. Eventually it starts to contract due to gravitational fluctuations and maybe even an outside influence (like a passing star). Material collects in the center, which gains more mass and more gravitational influence, sucking stuff in from the immediate environment. If the center gets hot enough, and the mass is great enough, a star “turns on”. It immediately begins blasting out radiation, maybe a pair of jets, and scoops out a cavern around itself in the nebula.
The circumstellar disk around the star Fomalhaut; there’s a planet hidden in there! Courtesy Hubble Space Telescope. Click to embiggen.
If there’s enough stuff left over in the nebula, that starts to coalesce too as the nebula material (gas and dust) rotates in a sort of flattened disk around the newborn star. Let that process go on long enough and eventually more little condensations form — accretions of dust and gas from the cloud. If the accretions are close to the star, they become rocky planets (like Mercury, Venus, Earth, and Mars). If they’re farther out, where gases (and ices) can still exist), then gas-giant and ice-giant planets can form.
That’s the Big Picture. The minutiae of planet formation involve some fairly complex interactions between gases in the nebula, between those gases and the dust, and the involvement of magnetic fields to the extent that the magnetic fields of larger bodies in the nebula (like the star, for example) affect how the particles are distributed throughout the system. It really starts down at the particle level — where particles of dust perhaps the size of a speck of household dust that are coated with ice can be affected by heat from the star. If they’re too close, the ice sublimates and all you’re left with is a dust particle that then has a different future ahead of it — it could accrete to other particles (depending on what it’s made of (silicates, for example), to form larger chunks of material. Those chunks would accrete, forming boulders; the boulders would accrete to form mountain-sized chunks of rock, and so on until you get to planetesimals.
If the nucleated ice particle (the dust in the ice) lie far enough away from the star, they can start to accrete too, and through much the same process, stick together in larger quanties until you get the chunklets that bash together to form the icy moons. That doesn’t exactly explain how the gas and ice giants get started, but that’s still a work in progress at the ‘devil is in the details’ level. On the macro-level, the leftover gas and ice chunks in the outer regions of the star system do start to coagulate and coalesce, with the ultimate result a gaseous or ice-gas planet containing a small rocky core.
All of these details come from studying the chemical makeup of planets in our own s0lar system, plus a lot of lab work by scientists who model and simulate the accretion of dust and ice particles together using the same kinds of materials they know were present in the early solar system. It is actually very painstaking work, with groups of scientists taking the time to look at the chemical and physical reactions between materials to understand exactly how they work in the interstellar environment.
Now, I’ve simplified the explanation quite a bit here. Perhaps as I get more time after this project goes to press, I can go into some of the “devilier” details for the planetary geeks among my readers. It’s fascinating work and I can’t wait to see what the planetary science modelers come up with as they apply their work to extrasolar systems that are now forming.
