Rough neighborhood

Most likely all of you know about Sagittarius A*, the supermassive black hole that sits at the center of the Milky Way Galaxy.

It's hard to talk about it without lapsing into superlatives.  It has a mass about 4.3 million times that of the Sun.  It's event horizon -- the "point of no return," the closest you can get to a black hole without being trapped by its gravitational pull -- has a radius of 11.3 million kilometers.  It sits at the center of a fifteen-light-year-wide whirlpool of gas and dust called the accretion disk, which we know about because the material in it is moving so fast it has heated up to as high as ten million degrees Celsius, resulting in a steady emission of high-frequency x-rays.

[Image licensed under the Creative Commons EHT Collaboration, EHT Sagittarius A black hole, CC BY 4.0]

It's curious that something this luminous wasn't immediately obvious to astronomers.  First, it doesn't emit a lot of visible light; we didn't have telescopes capable of detecting the x-rays that are its fingerprint until 1933.  By the 1970s, more precise observations showed that whatever the x-ray source was, it was extremely compact.  It wasn't until 1994 that Charles H. Townes and Reinhard Genzel showed that its mass and diameter were consistent with its being a black hole.  Another reason it took that long is that between us and the center of the galaxy there are massive dust clouds, so any visible light it does emit (or which is emitted by the dense clouds of glowing gas near it) mostly gets blocked.  (Even so, looking toward the center of the Milky Way in the constellation Sagittarius, visible where I am in late summer, is pretty damn spectacular.)

The third reason that we don't get the full luminosity of whatever electromagnetic radiation is emitted from Sagittarius A* is a fortunate one for us; because of the black hole's immense magnetic field, any bursts of light tend to get funneled away along the axis of its spin, creating jets moving perpendicularly to the galactic plane.  We, luckily, are comfortably out in the stellar suburbs, in one of the Milky Way's spiral arms.  Our central black hole is fairly quiet, for the most part, but even so, looking down the gun barrel of its magnetic field axis would not be a comfortable position to reside.

The reason this comes up is some new research out of the University of Colorado - Boulder, which used data from the James Webb Space Telescope to solve a long-standing question about why, given the high density of hydrogen and helium gas near the galactic center, the rate of star formation there is anomalously low.  This region, called Sagittarius C, extends about two hundred light years from the central black hole (by comparison, the Solar System is twenty-six thousand light years away).  And what the team of researchers found is that threading the entire region are filaments of hot, bright plasma, some of them up to several light years in length.

The reason for both the filaments and the low star formation rate is almost certainly the black hole's magnetic field, which acts to compress any gas that's present along the field lines, heating it up dramatically.  This, in turn, creates an outward pressure that makes the gas resist collapsing and forming stars.

"It's in a part of the galaxy with the highest density of stars and massive, dense clouds of hydrogen, helium and organic molecules," said Samuel Crowe, who co-authored the paper, which appeared this week in The Astrophysical Journal.  "It's one of the closest regions we know of that has extreme conditions similar to those in the young universe...  Because of these magnetic fields, Sagittarius C has a fundamentally different shape, a different look than any other star forming region in the galaxy away from the galactic center."

It is, to put it mildly, a rough neighborhood.

It's staggering how far we've come in our understanding of what our ancestors called the "fixed stars" -- far from being eternal and unchanging, the night sky is a dynamic and ever-evolving place, and with new tools like the JWST we're finding out how much more we still have to learn.  Something to think about the next time you look up on a clear, starry night.  The peaceful, silent flickering, set against the velvet black background, is an illusion; the reality is far wilder -- and far more interesting.

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Size matters

Something odd happens when we consider scales much larger or smaller than our ordinary experience; our imagination fails.

It's why people seem not to comprehend the difference between millionaires and billionaires.  Millionaires are wealthy, yes.  But billionaires?  

If a person with a billion dollars gave away a million dollars a day, 365 days a year -- in other words, creating one new millionaire every day -- (s)he wouldn't run out for almost three years.  The fact that people lump together millionaires and billionaires as both simply "rich" indicates we don't have a good way to conceptualize how big a billion actually is.

The same thing happens when you look at anything that's very small.  In my biology classes, we did a lab where students learned how to estimate measurements using a microscope.  Knowing the magnification and the field diameter (the actual width of the bit of the slide you're looking at), it's a fairly simple calculation to estimate the size of (for example) a cell.

What I found the most interesting was that after performing the calculation, most students had no clue whether the answer they'd come up with was even within the ballpark.  Most of the time, if they did make an error, it was a simple computational goof; but the curious thing was that they couldn't tell if they were even in the right realm.  0.001 meters?  0.000001 meters?  0.000000000001 meters?  All looks pretty similar -- "small."

(Then there's the student who multiplied when she should have divided, and told me that a plant cell was 103 meters in diameter.  "Don't you think that's a bit... on the large size?" I asked her.  She responded, "Is it?"  I told her 103 meters was a little longer than a typical football field.  She responded, "Oh.")

This problem crops up in fields like subatomic physics (on one end) and, germane to today's topic, astrophysics (on the other).  What got me thinking about it was a paper this week in the journal Astronomy and Astrophysics about a distant quasar with the euphonious name VIK J2348-3054.  Quasars are extraordinarily luminous objects which were a puzzle for a long time -- viewed through earthly telescopes they appear as single dim, star-like spots, but based on their redshifts they are enormously far away (and thus, even to be visible at all from that distance their actual luminosity has to be crazy high).  The current models support quasars as being supermassive black holes at the centers of young galaxies, emitting high-energy radiation and particles as they swallow vast amounts of gas and dust in a wildly spinning whirlpool called an accretion disk.

[Image credit: M. Kornmesser/European Southern Observatory]

An energy output that high causes disruption in the entire region surrounding it.  It heats and/or blows away gas and dust nearby, which overcomes the gravitational collapse of clumps of material and thus suppresses star formation.  And this quasar is so powerful it has stopped the formation of new stars in a region with a radius of over sixteen million light years.

Stop and ponder that for a moment.

Sixteen million light years isn't just big, it's abso-fucking-lutely enormous.  It's six times the distance between the Milky Way and the Andromeda Galaxy.  Put into units that more of us are comfortable with, this is about 160,000,000,000,000,000,000 kilometers.

Of course, I'm not sure how much even that helps.  Once again, our imaginations simply fail us.  Perhaps this will frame it better; the fastest human-made vehicle, Voyager 1, is traveling at about 61,000 kilometers per hour.  At this rate, Voyager 1 will have covered one light year in about eighteen thousand years.  And that's not even the distance to the nearest star, Proxima Centauri (if it was heading that direction, which it's not).

To travel the distance that has been cleared by this quasar, Voyager 1 would take a bit less than three hundred billion years -- about twenty times the age of the universe.

I don't even know how to wrap my brain around a number this big.  I may not have the difficulty with numbers my long-ago student had with her football-field-sized plant cell, but I have sat here all morning trying to understand what it means for something to work over this kind of size range, and I just can't manage it.

The inevitable result is that this kind of thing makes us feel pretty small.  I'm actually okay with that.  The universe is a grand, beautiful, and abso-fucking-lutely enormous place.  It's a good thing to look up into the night sky and feel awe, to realize that every star you see is (relatively speaking) close by, occupying a small spherical region in one arm of a completely ordinary galaxy, of which there are millions more scattered across the vastness of space.

We humans get a little big for our britches, sometimes.  A dose of humility is needed every so often.

And if it comes from the realm of science, so much the better.

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The empty galaxy

A couple of weeks ago, I began a post with a quote from physicist Albert Michelson in which he confidently claimed that everything in physics was pretty well settled -- in 1894.  Right before the discovery of the relativity and quantum mechanics would shake science to its foundations.

I read yet another paper just yesterday highlighting the inadvertent irony of Michelson's statement, and which once again shows us that we are very far from understanding everything there is to understand.  This one was about the accidental discovery of a galaxy that has an extremely odd characteristic.

It appears to have no stars whatsoever.

The object, dubbed J0613+52, is about ten times less massive than the Milky Way -- so smaller than your typical galaxy, but still pretty damn huge, weighing in at about two billion solar masses.  But the entire thing is made up of diffuse gas and dust -- no stars at all.

Because of this, it has an extremely low luminosity.  It was only discovered because of a mistake -- the astronomers at the Green Bank Observatory were trying to aim it elsewhere, but had mistyped the coordinates -- but when the telescope focused on the spot, they saw a blip of hydrogen spectral emission lines in what appeared to be an empty region of space.  More detailed study of the spot found that the emission lines were coming from a huge but faint dust cloud that was on the scale of galaxies mass-wise but seemed to have undergone no star formation.

"It’s likely there is a decent amount of dark matter present as well," said Karen O’Neil, senior scientist at Green Bank, who led the research.  "But lingering uncertainties about the dark galaxy’s exact physical size make associated dark-matter estimates hazy at best...  J0613+52 is completely isolated, with no neighboring galaxy closer than 330 million light-years or so; our own Milky Way, in fact, appears to be the object’s closest-known companion.  In these void areas of the universe, gas should be too diffuse to form any galaxy-like object.  Clearly that’s not quite true."

Robert Minchin, of the National Radio Astronomy Observatory in New Mexico, heard O'Neil present the findings at last week's meeting of the American Astronomy Society, and was obviously impressed.  "I think it’s definitely a real detection," Minchin said.  "It does look like a primordial object.  It’s a bit like discovering a living dinosaur and having it there to study."

Artist's depiction of J0163+52 [Image credit: STScI POSS-II (starfield); additional illustration by NSF/GBO/P.Vosteen]

What puzzles me is that J0613+52 is only ("only") 330 million light years away, so not even close to being the farthest galaxy we've seen.  The universe as a whole is forty times older than the light we're seeing from this bizarre empty galaxy, so you'd think it'd have had plenty of time to form stars from all that hydrogen gas.  Instead, it seems to be a relatively homogeneous dust cloud.  You have to wonder, what's keeping it that way?  Gravity is relentless and inexorable -- the current models indicate that even tiny anisotropies (unevenness) in the mass distribution will result in the denser regions gaining mass at the expense of the less dense regions, resulting in clumps of matter that eventually coalesce into stars.

For a dust cloud that massive to last over twelve billion years without forming stars is somewhere beyond peculiar.

It may be that I'm missing something, here.  (Okay, given that I'm not an astrophysicist, it's certain that I'm missing something.)  But even with my no-more-than-basic understanding of astronomy, this object seems really peculiar.

As is the fact that it was discovered accidentally because one of the astronomers had entered a typo in the coordinates.

I'm sure the astronomers are going to be busy looking at the empty galaxy and trying to figure out what it is, and also looking for others.  Given its extremely low luminosity, and the fact that we found it by basically aiming a big telescope at a random spot in the sky, you have to wonder how many other similar structures there are.

I'll end with the words spoken by Hamlet, which have been quoted many times before but seem apposite: "There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy."

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Rough neighborhood

In keeping with the stargazing topics that have been our focus this week, today we're going to start with my favorite naked-eye astronomical object: the Pleiades.

[Image is in the Public Domain courtesy of NASA/JPL]

It's also known as the Seven Sisters; in Greek mythology, the seven brightest stars (about all you can see without a telescope, even if you have good vision) represented the seven daughters of the Titan Atlas and the Oceanid nymph Pleione.  Where I live they're visible in the winter; I love seeing them glittering in the black sky on cold, clear nights.

The Pleiades are mostly hot type-B stars, and the whole group is about 444 light years from Earth, making it one of the closest star clusters.  Stars of this class are so energetic that they have relatively short life spans.  It's estimated that the Pleiades formed about a hundred million years ago from a cloud of gas and dust similar to the Orion Nebula; already the energy output of the individual stars is blowing away the shroud of material from which they were formed, resulting in the halo-like "reflection nebulae" you see surrounding them.

They're also moving away from each other, leaving the "stellar nursery" in which they were born.  In another couple of hundred million years, they will have separated widely enough that future astronomers (assuming there are any around) will have no obvious way to know they started out in the same region of space.  Plus, the biggest and brightest of them will already be approaching the ends of their lives, exploding in the violent cataclysm of a supernova, leaving behind a rapidly-rotating stellar remnant called a neutron star, spinning like a lighthouse beacon to mark the spot where a star died.

The reason all this comes up is some recent research into the composition of the stellar nursery where the Sun formed.  Because it, after all, was born the same way; along with a number of siblings, it coalesced in a massive cloud of hydrogen and helium, with a few heavier elements thrown in as well.  When you look up into the night sky, any of the stars you see could be one of the Sun's sibs.  It's impossible, from where science currently stands, to tell which ones.  They've all undoubtedly traveled a long way away from their point of origin in the 4.6 billion years since they formed.

But the research, which appeared in the Monthly Notices of the Royal Astronomical Society, uncovered a bit more about what our star's stellar nursery was like.  These formations do have some significant differences -- some are small and quiet, with only enough material to form a few stars, while others are enormous and violently active (such as the aforementioned Orion Nebula).  In particular, the models of stellar formation suggest that the two different environments would influence the quantities of heavier elements like aluminum and iron.  By measuring the amounts of these elements in meteorite fragments that are thought to be leftover material from the formation of the Solar System, the researchers concluded that the Sun formed in a high-energy intense environment like the Orion Nebula, swept by gales of dust and hammered by the shock waves of supernovae.

What a sight that would have been.  (From a safe distance.)

So next time you see the Pleiades or Orion's Belt, think about the fact that our calm and stable home star was born in a rough neighborhood.  Lucky for us, it's grown up and settled down a little.  As beautiful as the Pleiades are, I don't think I'd fancy living there.

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