Presbyopic Astronomy

Her phone call done, Cathleen returns to the Spitzer Memorial Symposium microphone with her face all happiness. “Good news! Jim, the grant came through. Your computer time and telescope access are funded. Woo-hoo!!”

<applause across the audience and Jim grins and blushes>

Cathleen still owns the mic. “So I need to finish up this overview of Spitzer highlights. Where was I?”

Maybe-an-Art-major tries to help. “The middle ground of our Universe.”

“Ah yes, thanks. So we’ve looked at close-by stars but Spitzer showed us a few more surprises lurking in the Milky Way. This, for instance — most of the image is colorized from the infra‑red, but if you look close you can see Chandra‘s X‑ray view, colorized purple to highlight young stars.”

The Cepheus-B molecular cloud
X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al

<hushed general “oooo” from the audience>

“Giant molecular clouds like this are scattered throughout the Milky Way, mostly in the galaxy’s spiral arms. As you see, this cloud’s not uniform, it has clumps and voids. By Earth standards the cloud is still a pretty good vacuum. The clumps are about 10-15 of our atmosphere’s density, but that’s still a million times more dense than our Solar System’s interplanetary space. The clumps appear to be where new stars are born. The photons and other particles from a newly-lit star drive the surrounding dust away. My arrow points to one star with a particularly nice example of that — see the C-shape around the star?”

The maybe-an-Art-major pipes up. “How about that one just a little below center?”

“Uh-huh. There’s so much activity in that dense region that the separate shockwaves collide to create hot spots that’ll generate even more stars in the future. The clouds are mostly held together by their own gravity. They last for tens of millions of years, so we think of them as huge roiling stellar nurseries.”

“Like my kid’s day care center but bigger.”

“Mm-mm, but let’s turn to the Milky Way’s center, home of that famous black hole with the mass of four million Suns and this remarkable structure, a double-helix of warm dust.”

False-color infra-red image of the Double-Helix Nebula
The double helix nebula.
Credit: NASA/JPL-Caltech/M. Morris (UCLA)

Vinnie blurts out, “That’s a jet from a black hole! One of Newt’s babies.”

Newt can’t resist breaking into Cathleen’s pitch. “Maybe it’s a jet, Vinnie. Yes, it’s above the central galactic plane and perpendicular to it, but the helix doesn’t quite point to the central black hole.”

“So take another picture that follows it down.”

“We’d love to, but we can’t. Yet. That image came from a long-wavelength instrument that only operated during Spitzer‘s initial 5-year cold period. Believe me, there are bunches of astronomers who can’t wait for the James Webb Space Telescope‘s far-IR instruments to get into position and start doing science. Meanwhile, we’ve got just the one image and a few earlier ones from an even less-capable spacecraft. This thing may be a lit-up part of a longer structure that twists down to the black hole or at least its accretion disk. We just don’t know.”

Cathleen takes control again. “The next image comes from outside our galaxy — far outside.”

Spitzer visualization of Galaxy MACS 1149-JD1
Credit: NASA/ESA/STScI/W. Zheng (JHU), and the CLASH team

The maybe-an-Art-major snorts, “Pointillism derivative!”

“No, it’s pixels from a starfield image with a very low signal-to-noise ratio. That red blotch in the center is one of the most distant objects ever observed, gracefully named MACS 1149-JD1. It’s a galaxy 13.2 billion lightyears away. That’s so far away that the expansion of the Universe has stretched the galaxy’s emitted photons by a factor of 10.2. Spectrum-wise, 1149-JD1’s ultra-violet light skipped right past the visible range and down into the near infra-red. Intensity-wise, that galaxy’s about 5200 times further away than the Andromeda galaxy. Assuming the two are about the same overall brightness, 1149-JD1 would be about 27 million times fainter than Andromeda.”

“How can we even see anything that dim?”

“We couldn’t, except for a fortunate coincidence. Right in line between us and 1149-JD1 there’s a massive galaxy cluster whose gravity acts like a lens to focus 1149-JD1’s light.”

The seminar’s final words, from maybe-an-Art-major — “A distant light, indeed.”

~~ Rich Olcott

Gettin’ kinky in space

Things were simpler in the pre-Enlightenment days when we only five planets to keep track of.  But Haley realized that comets could have orbits, Herschel discovered Uranus, and Galle (with Le Verrier’s guidance) found Neptune.  Then a host of other astronomers detected Ceres and a host of other asteroids, and Tombaugh observed Pluto in 1930.whirlpool-44x100-reversed

Astronomers relished the proliferation — every new-found object up there was a new test case for challenging one or another competing theory.

Here’s the currently accepted narrative…  Long ago but quite close-by, there was a cloud of dust in the Milky Way galaxy.  Random motion within it produced a swirl that grew into a vortex dozens of lightyears long.

Consider one dust particle (we’ll call it Isaac) afloat in a slice perpendicular to the vortex.  Assume for the moment that the vortex is perfectly straight, the dust is evenly spread across it, and all particles have the same mass.  Isaac is subject to two influences — gravitational and rotational.

making-a-solar-nebula
A kinked galactic cloud vortex,
out of balance and giving rise
to a solar system.

Gravity pulls Isaac towards towards every other particle in the slice.  Except for very near the slice’s center there are generally more particles (and thus more mass) toward and beyond the center than back toward the edge behind him.  Furthermore, there will generally be as many particles to Isaac’s left as to his right.  Gravity’s net effect is to pull Isaac toward the vortex center.

But the vortex spins.  Isaac and his cohorts have angular momentum, which is like straight-line momentum except you’re rotating about a center.  Both of them are conserved quantities — you can only get rid of either kind of momentum by passing it along to something else.  Angular momentum keeps Isaac rotating within the plane of his slice.

An object’s angular momentum is its linear momentum multiplied by its distance from the center.  If Isaac drifts towards the slice’s center (radial distance decreases), either he speeds up to compensate or he transfers angular momentum to other particles by colliding with them.

But vortices are rarely perfectly straight.  Moreover, the galactic-cloud kind are generally lumpy and composed of different-sized particles.  Suppose our vortex gets kinked by passing a star or a magnetic field or even another vortex.  Between-slice gravity near the kink shifts mass kinkward and unbalances the slices to form a lump (see the diagram).  The lump’s concentrated mass in turn attracts particles from adjacent slices in a viscous cycle (pun intended).

After a while the lumpward drift depletes the whole neighborhood near the kink.  The vortex becomes host to a solar nebula, a concentrated disk of dust whirling about its center because even when you come in from a different slice, you’ve still got your angular momentum.  When gravity smacks together Isaac and a few billion other particles, the whole ball of whacks inherits the angular momentum that each of its stuck-together components had.  Any particle or planetoid that tries to make a break for it up- or down-vortex gets pulled back into the disk by gravity.

That theory does a pretty good job on the conventional Solar System — four rocky Inner Planets, four gas giant Outer Planets, plus that host of asteroids and such, all tightly held in the Plane of The Ecliptic.

How then to explain out-of-plane objects like Pluto and Eris, not to mention long-period comets with orbits at all angles?outer-orbits-1

We now know that the Solar System holds more than we used to believe.  Who’s in is still “objects whose motion is dominated by the Sun’s gravitational field,” but the Sun’s net spreads far further than we’d thought.  Astronomers now hypothesize that after its creation in the vortex, the Sun accumulated an Oort cloud — a 100-billion-mile spherical shell containing a trillion objects, pebbles to planet-sized.

At the shell’s average distance from the Sun (see how tiny Neptune’s path is in the diagram) Solar gravity is a millionth of its strength at Earth’s orbit.  The gravity of a passing star or even a conjunction of our own gas giants is enough to start an Oort-cloud object on an inward journey.

These trans-Neptunian objects are small and hard to see, but they’re revolutionizing planetary astronomy.

~~ Rich Olcott