Astronomy/cosmology questions...

[Rusty Edge]

No hurry. It's not like I need the info to make a decision.

[Lorizael]

As the best of our instruments have become far more refined in the last few decades, have there been any significant advances in the range of parallax measurements?  ISTR observations six months apart being good for measuring distance on the order of something 11lyrs.  Is that anything like it currently stands?

The first star for which we accurately measured the parallax--61 Cygni--is 11 light years away, and that was in 1838. We've been able to do about 1000 light years for awhile. But there is a spacecraft in orbit right now--Gaia--which will get us to the galactic core, which is roughly 30,000 light years distant.

[Lorizael]

I've never quite gotten why astronomy/cosmology thinks our observations tell us anything relevant about the mass of the universe within several orders of magnitude.  Stands to reason that there's a distinctly non-trivial amount of cosmic gas/dust and other non-radiating matter out there making estimations nothing but poorly-informed guesses...

The quick answer to this question is that nothing doesn't radiate. We observe interstellar and intergalactic dust because it (a) absorbs radiation (a process known as extinction, which reddens the spectra (in a way that is different from Doppler redshift) of background objects) and (b) emits radiation. We know atoms emit radiation simply because they have a temperature and therefore must.

But even for something like cold clouds of neutral hydrogen atoms all by their lonesome, electrons randomly make jumps between two adjacent energy levels that gives off radio waves at the (famous?) 21 cm line. Part of the reason we suspect there's dark matter out there is because we can detect the motion (from Doppler shifts of this line) of hydrogen gas way out past where the Milky Way's stars end, and it's still being pulled around by gravity.

When galaxies and galactic clusters collide, the gas heats up tremendously and emits brightly in the x-ray, which we can detect on cosmic scales. This also lets us measure the mass of gas and dust, which far outstrips the mass of all the stars we can count in a galaxy or cluster. So we have pretty good estimates for how much gas and dust are out there.

[Buster's Uncle]

But there is a spacecraft in orbit right now--Gaia--which will get us to the galactic core, which is roughly 30,000 light years distant.

Orbiting Earth - or further out from the Sun?

[Lorizael]

It's at Sun-Earth L2, which gives it a 10% longer baseline for parallax measurements. What really makes it better at measuring parallax, however, is that it's an all-sky survey that can accurately measure brightness across a very wide band. This means that for any particular star it's observing, it can compare that star's position to many, many other background stars (and quasars) at the same time.

[Geo]

As the best of our instruments have become far more refined in the last few decades, have there been any significant advances in the range of parallax measurements?  ISTR observations six months apart being good for measuring distance on the order of something 11lyrs.  Is that anything like it currently stands?

The first star for which we accurately measured the parallax--61 Cygni--is 11 light years away, and that was in 1838. We've been able to do about 1000 light years for awhile. But there is a spacecraft in orbit right now--Gaia--which will get us to the galactic core, which is roughly 30,000 light years distant.

A follow up on this one. If there's the possibility to put a perfect parallax on stars that much distant, how about putting a parallax on the tiny motions of stars within a couple parsecs with a crop of planets orbiting it? Would it be possible to detect the wobble sideways (from the Solar System's point of view) of the gravity interaction between star and planets?
Kind of a elaboration/confirmation on results of the radial velocity method.

[Lorizael]

Absolutely, yes. That method is called astrometric planet detection, and it's one of Gaia's mission goals. That said, it's an extremely difficult measurement to make and so far (not counting whatever Gaia may do) no planets have been found by this method.

The case of Jupiter is a quick example of why this measurement is hard. If we imagine the solar system consists only of the sun and Jupiter (not far off from the truth), then the two bodies orbit their common barycenter. This means they complete that orbit in the same amount of time. Jupiter's orbit takes 12 years, so it also takes the sun 12 years to move around the system's barycenter. Because the sun is ~1000 times as massive as Jupiter, the center of mass is 1/1000 the way to Jupiter, which means the sun (seen from another star system) is moving a distance only about 7% longer than its diameter, and it's taking 6 years to do that.

Seen from, say, our closest neighbor Alpha Centauri, that's an angular change of about 7 milliarcseconds, where a milliarcsecond is 1/1000 of 1/3600 of a degree. For reference, the moon is about 1/2 a degree wide.

Another complication is that the radial velocity and transit methods work best for systems that are edge on to us, but the astrometric method works best for systems that are face on to us. So it's difficult for one method to be used to confirm the results of the other.

[Lorizael]

Not sure why I went and did all that math right now. I think it's just been awhile, what with me being out of school for half a year...

[Geo]

Another complication is that the radial velocity and transit methods work best for systems that are edge on to us, but the astrometric method works best for systems that are face on to us. So it's difficult for one method to be used to confirm the results of the other.

I should mention I'm a bit self-studied in astronomy. I kinda asked the question for those exoplanetary systems where the exoplanets orbit their parent star(s) are between edge -and face on with the latest tools.

A couple questions on Lagrange point matters. So this Gaia probe has a roughly 10% better baseline with the Sun-Earth L2 point only being 1% of the Sun-Earth distance further out (1,5 million km)?

On another note, it is known that a couple other planets in our system have small objects orbiting in their respective L4-L5 points. Especially Mars and Jupiter. I kinda wondered why, over the eons, these objects didn't clump to bodies large and massive enough to become roughly spherical. Think Vesta-sized. Is there some peculiarity on how those objects move around in any particular L4-L5 points that precludes the formation of larger bodies? I read about Lissajous orbits around those points, but with a sufficient number of objects I expected encounters to be common.

[Lorizael]

A couple questions on Lagrange point matters. So this Gaia probe has a roughly 10% better baseline with the Sun-Earth L2 point only being 1% of the Sun-Earth distance further out (1,5 million km)?

Oops, no. Just a brain fart on my part. Baseline is only 1% better.

On another note, it is known that a couple other planets in our system have small objects orbiting in their respective L4-L5 points. Especially Mars and Jupiter. I kinda wondered why, over the eons, these objects didn't clump to bodies large and massive enough to become roughly spherical. Think Vesta-sized. Is there some peculiarity on how those objects move around in any particular L4-L5 points that precludes the formation of larger bodies? I read about Lissajous orbits around those points, but with a sufficient number of objects I expected encounters to be common.

In general, Lissajous orbits are something we establish for probes at L1 and L2 because those points are only metastable. Objects at those points tend to fall out, so it helps to have an orbit that sort of naturally takes you in and out and around those Lagrange points.

L4 and L5, on the other hand, are very stable, and objects put there tend to stay there. This is why the Trojan asteroids at Jupiter's L4 and L5 points occupy a pretty wide swath of Jupiter's orbit, because gravitationally those points are more like shallow depressions. (That said, because the Trojans are often not exactly the same distance from the sun as Jupiter, they move at slightly different speeds and follow something called a Tadpole orbit.)

As far as why they don't accrete into something bigger, there are a couple reasons. The first is that, while L4 and L5 are mathematically stable if you just consider the restricted three-body problem, the vagaries of the solar system (mostly resonances with Saturn) mean that staying there isn't completely problem-free. Trojans do tend to get ejected, and that hampers growth by accretion.

The other problem is that there's just not enough material. Accretion into a planetesimal is a runaway process, but if you don't have enough mass packed densely enough, that runaway process never starts. Additionally, there's not a lot of gas left in interplanetary space, which causes drag that slows material down, increasing the likelihood of objects eventually colliding and making those collisions softer. Softer collisions are better for objects sticking together rather than blowing apart.

[Lorizael]

Okay. Here's one for you. It may or may not be cosmic. Is all energy the same in that it travels in waves, and we just perceive it differently as light or heat or sound or earthquakes, ( as examples) depending on frequency and amplitude?  Or is it particles, particles traveling in waves, or some of each ?

What energy is turns out to be a pretty complicated question. Historically, one of the earliest conceptions of energy came out of Newton's mechanics. Because Newton talked in terms of force, physicists noticed (mathematically and observationally) that when a force is applied to an object, "work" is done on the object which moves it. You can calculate the amount of work done, and it turns that this quantity is conserved.

So when you drop a ball, the force of gravity does work bringing the ball to the ground. A certain amount of gravitational energy turns into kinetic energy, which then turns into heat and sound when the ball hits the ground. (The force of the ball hitting the ground does work on the ground and the air around the ball.)

In modern physics, force is still important but fields are arguably more fundamental. Nowadays, physicists frame things in terms of quantum field theory. This says that all space is pervaded by fields (gravitational, electromagnetic, etc.) and that these fields contain energy. When the fields are perturbed in some way, they emit energy. For example, messing with the electromagnetic field produces light. Quantum physics tells us that light (along with basically everything) has both wave and particle nature. Light seems to travel as a wave but collide with other objects like a particle.

But in a very real sense, the energy of a photon slamming into another particle is kinetic--that is, due to its motion. So we can think of energy as having two modes: stored in fields, or expressed through motion. When you build up charge in a capacitor, energy is being stored in the electric field. When you close a circuit and induce a current, the energy moves from the electric field to the motion of the electrons.

At a very fundamental level, energy from motion will always be both wave and particle, because of the wave-particle duality of quantum mechanics. But at higher levels of abstraction, that might not matter. When a bullet travels through the air, it possesses kinetic energy, and quantum mechanics tells us how to calculate the "wavelength" of a bullet, but it turns out to be so ludicrously small (~10^-34 m) as not to matter. On our scale, a bullet is a particle.

[Rusty Edge]

Thank you, Lorizael, you do this so well.

This would have made a nice film in my day, but quantum mechanics was for nuclear physicists, not for the general public. I hope that you can find a way for your explanations and illustrations to reach a broader audience, and to earn a living from it.

[Lorizael]

Aw, shucks.

[Geo]

The other problem is that there's just not enough material. Accretion into a planetesimal is a runaway process, but if you don't have enough mass packed densely enough, that runaway process never starts. Additionally, there's not a lot of gas left in interplanetary space, which causes drag that slows material down, increasing the likelihood of objects eventually colliding and making those collisions softer. Softer collisions are better for objects sticking together rather than blowing apart.

So, in short, if it doesn't happen during the latter stages of the planetary formation process in a young star system, it's very unlikely to happen again. And better chances further out where distances between gas giants are usually much larger as well, as well as lower masses of the gas giants residing in the outer orbits.

[Buster's Uncle]

LIGO spots a third black hole merger, tightens mass limits on gravitons