cosmology - How to find the distance between two quasars

I want to calculate the distance between two quasars of which I know the angular position and the red shift. Let $Q_1=(alpha_1,delta_1, z_1)$ and $Q_2=(alpha 2,delta 2, z_2)$ and suppose $z_2 > z_1$.

I know how to find the angular separation $theta$ between them by means of the angular coordinates. But how to find the comoving distance (at epoch $z=0$) from them ? I know how to find the comoving distance from the Earth of the two quasars, can we find the distance between them using these two distances?

A related question is to find the redshift of $Q_2$ as seen by $Q_1$ at the epoch when $Q_1$ received the light emitted from $Q_2$ .

There is some standard method to solve this problem ?

What conditions would lead to this event around the black hole in the Pictor A galaxy?

This is a clear example of an astrophysical jet, in this case, most likely a relativistic jet. In short, an accretion disk forms around a black hole (supermassive or otherwise). Matter is pulled towards the black hole and further energized, before being accelerated into a jet emanating from the black hole's poles. Two different mechanisms have been proposed for the formation of jets:

• The Blandford-Znajek process requires that a magnetic field forms (from the accretion disk) that is centered around the black hole. Charged particles then move along the field lines, into jets. I recently wrote an answer about the details (see How does an accreting black hole acquire magnetic fields?). For this process to work, you need an accretion disk. It is generally considered the most likely explanation for black hole jets.


• The Penrose process takes rotational kinetic energy from the ergosphere outside the event horizon and gives it to particles moving in jets. Note that this does not rely as heavily on the accretion disk as the Blandford-Znajek process does. For this process to work, you need a rotating black hole surrounded by some matter, likely in a disk.

The hotspot is, to me, much more interesting. It reminds me of structures seen around young stars: bipolar outflows (streams of gas that can form shock waves) and Herbig-Haro objects (the results of shock waves from relativistic jets. Obviously, the mechanisms are different, so no clear analogy can be drawn. But what is interesting about bipolar outflows and Herbig-Haro objects is that the shock waves produced therein result from collisions with the interstellar medium.

If a similar mechanism were to cause the shock waves by the hotspot, then we could conclude that the jets have hit the intergalactic medium. But I don't think this is necessarily the case, in part because of just how long these jets are prior to the formation of the hotspot. One would think that if the hotspot and shock waves are because of collisions with the intergalactic medium, the jets would be much shorter, because they would likely have reached higher density regions of it sooner. So that's why I find it interesting, and why I can't give you a good reason as to why the hotspot formed where it did, or the precise reason for it being there at all.

dna - Are human chromosomes connected or separate molecules?

Do the 46 human chromosomes form a single unbroken DNA helix? Or is it rather that a human's genome consists of 46 disconnected helices?

If it is the former, does the common numbering scheme for the chromosomes have any correlation to their actual ordering in the one large strand?

If is the latter, is there a convention on how the chromosomes are ordered in genomic datasets? Also, is there a clear understanding of how sister chromosomes "find" each other in Meiosis I?

Generally, during periods when Mitosis/Meiosis are not occurring, what's a good physical picture for how the chromosomes are physically arranged (e.g. a bowl of 46 spaghetti noodles, or maybe the sister chromosomes always stay close together, etc)

thanks!

solar system - What measures are currently in place to detect asteroids on possible collision courses with earth?

NASA doesn't have its own asteroid tracking program. They are rather coordinating activities and provide funding for asteroid research and discovery programs.

The most successful dedicated asteroid discovery programs at the moment are Pan-STARRS (http://pan-starrs.ifa.hawaii.edu/public/) and the Catalina Sky Survey (http://www.lpl.arizona.edu/css/); see http://neo.jpl.nasa.gov/stats/ for some discovery statistics. Both these programs, as well as many others, run telescope that survey the night sky for yet unknown asteroids. On average, both programs together find 4 new near-Earth asteroids (and many others) per night.

expansion - Looking for an equation that describes how star (x,y,z) positions are affected by expanding universe

The position of stars change very slowly for a couple of reasons, but not due to an expanding universe.

Galaxies distant from our own, are all moving away from our galaxy at a rate proportional to their distance. The equation is simple:
$$mathrm{speed}=H_0 times mathrm{distance}$$

and $H_0$ is a constant of proportionality, with a value of about 70 km/s per kiloparsec. Since the galaxies are moving away, they don't change their position in the sky at all.

(The nearest galaxies are exceptions to this rule, they may be moving towards us)

Stars in our galaxy are also moving. They have their own proper motion, but the motions of the stars relative to our own are essentially random. You can look up the proper motion of stars on the SIMBAD database.

Stars also appear to wobble due to the annual rotation of the Earth about the Sun and our consequentially changing perspective. The amount of wobble is in inverse proportion to their distance (the nearest stars appear to wobble most)

Why does gravity increase in star formation?

Some good answers, I'm going to give kind of summary, cause you touched on a few points.

Why does gravity increase in star formation

Gravitation is a product of a few forces. Mass, density and, not to be ignored, rotation speed.

It's not actually the fusion process that keeps the sun from contracting, at least, not directly. It's heat that keeps the star expanded. That's the balancing act. High temperature wants to expand, gravity wants to contract.

The fusion process is actually pretty slow, which is why stars like our sun have a main sequence of about 10 billion years, and a lot of the heat that a star starts out with is from the heat of formation. Potential energy gets converted to heat due to the coalescing and condensing of all that matter so stars start out hot, even before fusion begins.

In fact, a star in formation can be many times brighter than the star is during it's main sequence due to the high heat of formation. Here's an article that says the forming sun was 200 times brighter than it is now.

Young proto-stars, as a result of conservation of angular momentum, tend to rotate very fast and that fast rotation can create a bulge and increases ejection of matter. The formation process is pretty chaotic compared to the main sequence stage. Lots of ejected matter, much bigger solar storms, lots of lheat from formation, etc.

Once the main sequence stage is underway and rotation is slowed down, then there's more of a balance between heat and gravity mentioned above. The fusion process continues to add heat to the core of star which the star, convects or conduct heat away from the core into the outer layers and then, radiates from it's surface, but during the main sequence, in general, the core of the star gradually heats up and in most cases, the energy added from fusion isn't nearly strong enough to blow apart the star, unless the star is enormously large like over 150 or 200 solar masses, then the star doesn't really work without blowing off a bunch of matter. See: here.

I get that the fusion of hydrogen atoms releases energy... fine...

How does gravity keep it together if the mass is lessening as a result
of fusion( mass being converted into energy from fusion) while gravity
is weakening( as mass lessens )?

As others have said, mass loss by solar wind is a bigger factor especially for young and smaller stars, but there's a few factors at play. The short answer to this question is that the mass loss, at least by fusion, is quite very compared to the total mass of the star. Another factor, as hydrogen becomes helium, the core of the star becomes denser and greater density tends to be smaller and that increases gravity, but there are competing factors. The inner core grows denser as it becomes more hydrogen rich and the fusion tends to expand outwards on the outside of the helium core, so a star like our sun gets a denser inner core over time, but the layers around the core can grow hotter and larger, even as they lose mass.

Wouldn't the radiation overpower the force of gravity and tear the
star apart?

As mentioned above, this happens if you have 150 or 200 solar masses. lower mass stars, the fusion isn't nearly powerful enough to blow the star apart. Stars and white dwarfs blow apart when they go supernova, but that's different than the main sequence fusion process.

Our sun will blow off some of it's matter when it has it's helium flash, so there are examples of what you're describing happening, but not during the main sequence for stars like our sun when material is expelled primarily by magnetic storms causing coronal mass ejections. Fusion is, generally speaking, more like a slow burn, than a big explosion when it's up against the enormous gravitational binding energy of a star.

Does the mass of the Earth change?

Every day, babies are born and people grow, which makes their respective masses greater. However, this change in mass (should) come out of the food that they consume - it is used as energy and thus converted into this growth. Likewise, building new structures is just a redistribution of mass already on Earth.

Thus, the only way I can see the actual mass of the Earth changing is by meteors that have landed here (increase in mass) and things that leave Earth such as space shuttles and rockets which would be a decrease in mass.

But have I missed anything out? If no meteors crashed into Earth, and we had not yet figured out how to make machines that could fly, would the mass of the Earth remain constant? Or is it somehow loosing or gaining mass? Am I wrong in assuming that the growth of animals doesn't affect the mass?

Thanks, Toastrackenigma.

EDIT: Mass includes atmosphere :)