CapitalistImperialistPig

Galaxies may or may not have a social life.  Many, like our Milky Way, live in groups of a few dozen.  Others are loners, and still others live in clusters of hundreds or thousands.  Loners and group members tend to be actively forming stars, and consequently have the large, extremely bright young blue stars that only live for a few millions or tens of millions of years.  By contrast, most of the galaxies in large groups, especially the big elliptical galaxies, aren't forming stars, haven't for  perhaps a few billion years or so, and consequently have only old, red, and small stars - they are red and dead in the professional parlance. We know why, in a sense.  They lack the cold molecular gas clouds where stars form, and instead, are embedded in hot ionized gas at a million or so kelvins.  However, this presents another puzzle.  Galaxy clusters typically have one or more of the very large cD ellipticals at their center.  Their large gravity...

The Sun has a mass slightly less than 2 e+30 kg, and radiates a bit less than 4 e+26 Watts (Joules/sec). That amounts to about 2/10,000 of a Watt per kilogram. For comparison, a 60 kg human (132 lbs) produces about 100 Watts. On a Watt per kilogram basis, a human is thus producing about 10000 times as much heat as the Sun. So why is the Sun so much hotter and brighter that you are? One way of looking at it is that the Sun has far less surface area per kilogram than you do - something like a billion times less, so the amount of heat leaving it per unit area is about 100,000 times more. Another thing to think about is the reaction rate. Conversion of Hydrogen to Helium is quite slow in the Sun - an average Hydrogen nucleus (proton) may bump around for ten billion years or so before it (and three friends) get converted to Helium. A molecule of ATP in a human cell only lasts a couple of seconds.

I've been surfing through Raymond Pierrehumbert's great book, Principles of Planetary Climate , especially Chapter 8 on the evolution of planetary climates. It turns out that getting smacked by big rocks can knock the heck out of an atmosphere. Impactors which hit the Earth are moving so much faster than the speed of sound that the air in front of them literally has no time to get out of the way, and all piles up in front of them until enough momentum is transferred to that lump of compressed air to slow down the impactor. For Earth and its atmosphere, if the radius of meteorite is larger than a few meters, that won't happen before the impactor hits the ground, though slightly larger ones may explode in midair due to the pressure of the compression. Substantially larger impactors are scarcely slowed down at all before impact, and they blow a cone of atmosphere out into interplanetary space. Larger masses expand the angle of the cone. When the mass is large enough (th...

Caption: the Pillars of Creation, dark cloud remnants of a giant molecular complex backlighted by reflection nebulae illuminated by bright young stars. Some evidence suggests that this complex has already been blasted apart by a supernova still enshrouded in dust. Definitive evidence should reach us in a thousand years or so. So you say you want to be a star. A lot of success in anything is about being in the right place at the right time. In the case of becoming a star, that place is in a galaxy with sufficient dust and gas, and getting down to the nitty gritty, being in a massive molecular cloud complex , in one of the cores of such complexes. Such clouds are the coldest (10-30K) and densest parts (up to 10^6 molecules/cm^3) of the interstellar medium - the gas and dust that occupies the space among the stars. The big ones have masses from thousands to millions of times the mass of our Sun. It seems to be only in these massive, cold, and dense clouds that gravity can ...

It's pretty hard not to have sometime seen pictures of the beautiful island universes, or galaxies, in which all stars are apparently born and most live out their lives. So what makes a galaxy, and what is their history? Unlike stars, there don't seem to be any young galaxies. They all seem to have been born shortly after the big bang. There are, however, galaxies with lots of young stars and galaxies consisting almost entirely of old stars, but so far as I know, there aren't any galaxies with *no* old stars. I mentioned that some stars no longer live in galaxies, but they are thought to have been expelled by various cosmic accidents, the most important of which are collisions between galaxies. Many or perhaps most of all existing large galaxies are the product of multiple mergers of smaller galaxies. Our own Milky Way galaxy, for example, is currently in the process of gobbling up several smaller neighbors, and almost certainly has dined on many more in the past. ...

If you've ever spent any time under clear dark skies, you've seen the cloudy band of light running across the sky that we call the Milky Way.  Its name comes from the resemblance the ancient Greeks saw to  milk, which gives us our other name for it, the Galaxy.  Galileo was the first to penetrate the nature of this cloudy band of light when he turned his telescope upon it and saw a myriad of stars. As telescopes and astronomical techniques improved it became clear that the Milky Way, our galaxy, was a sort of island universe in which our star, the Sun, was embedded as one of enormously many stars.  There are a few other cloudy patches of light in the sky, three of which are visible to the keen eyed - Andromeda and, very prominent in the Southern Hemisphere, the Magellanic Clouds.  There are also more temporary cloudy glows, the comets, which were long found more interesting.  Telescopes revealed ever more of these cloudy patches. It eventually became cl...

The spiral galaxies like M31 (Andromeda) and our own Milky Way are among the most beautiful objects in the sky.  OK, we can't really see the shape of our own galaxy very well due to being in it, but I'm sure that it's wicked awesome.  What gives them their shape is their spin, and the fact that angular momentum is conserved.  So why do they spin? Before trying to answer that, I should point out that not all galaxies do spin, at least not enough to give them that disk-like or spiral shape.  Many galaxies, including the very biggest ones, are ellipticals, and some of them have almost no spin, with the motions of their stars very nearly random. Galaxies are believed to have formed quite early in the history of the universe when ordinary matter, having cooled enough for the electrons and nuclei to condense into neutral atoms, fell into the gravitational potential wells formed by dark matter out of primordial fluctuations, forming clouds of hydrogen and helium of ab...

Age: Radioisotope 207Pb/ 206Pb dating of refractory inclusions (CAIs) found within chondritic meteorites, the oldest Solar System solids known, yields an age of 4.568 Gyr. Dating with other isotope systems yields similar ages. Chondrules, as well as most differentiated meteorites that originated within small bodies, solidified only a few million years later (§ 8.7). Rocks formed on the Moon and Earth are younger: lunar rocks are typically between 3 and 4.4 Gyr old, and terrestrial rocks are ≲ 4 Gyr old, although terrestrial mineral grains as old as 4.4 Gyr have been found. de Pater, Imke; Lissauer, Jack J.. Planetary Sciences (p. 512). Cambridge University Press. Kindle Edition. The ages of the refractory inclusions and the chrodrites tell us when they first solidified, almost certainly when the solar system was first condensing. From studies of regions of stellar formation, and deductions from basic physics, we know that stellar systems form in relatively dense molecular clouds co...

If you were to understand only one thing about stellar structure and evolution, it ought to be the Hertzsprung-Russell diagram. It's a simple idea - a plot of the absolute luminosity (brightness) of stars against their surface temperature (often captured via their color, as measured by their relative brightness in two bands, the blue (B) minus the visual (V).) Below is a version, from the Wikipedia link, due to Richard Powell. The most obvious feature of this diagram is the non-random distribution of the stars, with most of them being concentrated near a slightly curved line from lower right (low luminosity versus red color and low temperature) to upper left (high luminosity versus blue color and high temperature). This line is the main sequence, and consists of stars burning hydrogen in their cores, which is how they spend most of their lives. There is also a more diffuse blob of stars to the right of the main sequence, consisting of bright and cooler stars inhabiting the ...

...sounds like a new form of extreme sports for the overly energetic, but it's actually a process of some importance in galaxy formation. The virial theorem relates the time average of the kinetic energy of a system of gravitationally bound particles to its potential energy: Tav = -(1/2)V. A system in which this kinetic energy is close to this average is called relaxed. Suppose one starts with an arrangement of, say 100, mass particles with random velocities and turns on gravity. Initially, there is no particular relation between the total kinetic energy and the potential energy (except they should be bound, so T +V One process that leads to relaxation is gravitational encounters between pairs of individual particles , which tends to equipartition kinetic energies. The time to relaxation in such encounters depends on the density and number of particles. For an open cluster of about 100 stars, relaxation times are roughly ten million years, while for for globular cluster of 1...

Fun Fact: The mass of a neutron star is about 20% less than the mass of the neutrons and other nucleons that compose it. The difference is gravitational binding energy.

New measurements of the local rate of expansion of the universe are creating a stir . The measurements found that the current value does not seem to agree with data from the Cosmic Microwave Background and the so-called Lambda CDM model. The second link, to a Scientific American news story, discusses potential implications in a bit more speculative manner than the paper. From SA: The Hubble constant discrepancy, though, suggests that dark energy might actually change over space and time, potentially causing an increasing acceleration of the cosmos instead of a constant outward force. One theory proposing this type of dark energy is called quintessence, which posits that dark energy results not from the vacuum of space but from a field that pervades spacetime and can take on different values at different points. An alternative explanation for the discrepancy, however, is that the universe contains an additional fundamental particle beyond the ones we know about. In particular, a new...

Our galaxy, The Milky Way, appears to have a couple of hundred billion "rogue" planets without any stars to call their own. Some of them were kicked out of their systems of formation early in their history (it is suspected that our own system lost one or more planets in this fashion) and some probably formed on their own, in a process analogous to star formation in giant molecular clouds. See, e.g., http://www.dailygalaxy.com/my_weblog/2013/08/200-billion-free-floating-starless-planets-roam-the-milky-way.html

The dust grains produced when big stars spill their guts via supernova or stellar wind are tiny: mostly micron or submicron scale grains. These are the stuff that planets are made of. It takes about a billion such grains to make a millimeter sized particle, and a billion of the latter to make a something of meter size. Another factor of a trillion is needed before mutual gravitational attraction can become significant. Each of these scales, and the additional factor of a billion before you get to a real planet has its distinct and complicated physics - some of which remains poorly understood. That first factor of a billion might be the best understood. Friction with the gas in the circumstellar disk slows down those tiny particles to tiny collision velocities where they readily stick together. By the time they have fallen to the center of that disk (under the influence of the perpendicular component of the star's gravity) they have already grown to millimeter size. It take ...

Your density is very close to 1 gm/cm^3, which in mass is equivalent to Avogadro's number of hydrogen atoms or 6.022 x 10^23/cm^3. For comparison, the average density of Universe today is only equivalent to 5 atoms/m^3, but only one part in 25 or so is expected to be baryonic matter, so about 2 x 10^-7 atoms/cm^3. In a galaxy, matter is about a million times as dense, and averaged over the local solar system, another factor of a million more dense, and a neutron star is about 10^14 (one hundred trillion) times as dense as you are, while you are roughly 10^32 times as dense as the average of the universe. The point is that density varies a whole lot in the present day universe, but this was not always the case. The oldest light in the universe comes from 13.7 billion years ago, a few hundred thousand years after the Big Bang, when the Universe had cooled enough ( to 3000 K, or so) for hydrogen atoms to form from the previous soup of protons and electrons - the so-called age of r...

Gamma Ray Bursts, during their brief (seconds or so) existence are the brightest objects in the universe, hundreds or thousands of times as bright as a quasar and millions of times as bright as a supernova. Brilliance and brevity both present related puzzles. If the gamma rays released are thermal, the implied temperatures (trillions of K) are implausible, and the brevity of the emission time is also hard to explain, if we assume that emitting shock wave cools by collision with the interstellar medium. Both effects are believed to be explained by the ultrarelativistic character of the shock wave, moving at a speed just barely lower than the speed of light. Gamma = sqrt(1/(1-v^2/c^2)) is 1000 or more. This produces a relativistic Doppler effect, which can be considered to be the combination of an ordinary Doppler effect with relativistic time dilation. Because the source is coming rapidly toward us the emitted radiation doesn't get very far ahead of the shock wave that emitted...

Velocity in km/s is numerically nearly equal to its value in parsecs/megayear. (1 km/s = 1.04 pc/Myr) Fact is handy for navigation, as well as for things like realizing that a constant Hubble parameter of 70 km/(s Mpc) implies an age of 14 Gyears for the Cosmos. (Hubble parameter isn't really constant, but current value is close to the mean). The fastest moving bodies in the solar system (say a long period comet at the moment of solar impact) travel at about 600 km/s, and the nearest stars are less than 2 parsecs away. At that speed, the trip would take roughly 1/300 of a Myr, or 3300 years. Hey, it beats driving.

You know that cute galaxy you saw in the NASA travel posters? Out at z = 1.8 redshift? (A few billion light years away). Were you planning to visit? Well you can forget it. It's already gone. Over the the cosmic horizon. Already separating from us at more than the speed of light - so you can't send a message either. Oh, we will still get light from it for a while, but none of that light will come from its cosmic (co-moving) now. All the light (or anything else) we will ever get from it has already been emitted. Of course that assumes that the dark energy really is an effective cosmic constant, and that the universe will continue (has continued) to accelerate its expansion. That galaxy, and all the others except for our local group will gradually fade out to infinite red shift over the horizon. But it's still not too late to book a trip to more or less anywhere in the local super cluster.

The center of our galaxy is a happening place - besides the resident kinda-sorta Super Massive Black Hole (3 million solar masses or so, but a piker compared to those in the big elliptical galaxies) - there is a very high density of stars, rampant recent star birth, and dusty clouds of gas. We can't see this stuff in the visible, as only about one visible photon in a trillion makes it way through the dust to us, but other wavelengths penetrate better. It's a starry starry night there, as stellar density is a million times greater than in our neighborhood. On a dark night, a person of good eyesight on Earth can see about 7000 stars - if you were in the galactic nucleus, you could see millions. A nice place to visit, perhaps, but you wouldn't want to live there. O supergiants are bound to go supernova soon, and the radiation from the BH and other hot stuff would make the place pretty uninhabitable.

One ten-thousandth of a second after the big bang, to be more precise. It was hot, but had just cooled down to the point (one trillion degrees K) where photo-production of proton - antiproton pairs stopped happening, so that protons and antiprotons could annihilate faster than new pairs were formed. Oddly, but fortunately from our point of view, there was roughly 1 extra proton for every billion proton-antiproton pairs. We, and everything we see or touch is made out of those lonely extra dancers at the ball. Things were closer together then - more neighborly, one might say. The Andromeda galaxy was then about as close to us as Mars is now. Mars then was about as close to us as the diameter of the period at the end of this sentence. Of course there was no Mars, or Andromeda galaxy, or any star or planet then - just elementary particles rushing madly to and fro.