Here is an edge-on illustration of our Milky Way Galaxy. (Keep in mind that the disk actually stretches quite a bit farther out from the budge than is apparent in this illustration. Proportionally, its full diameter makes its thickness less than that of a pizza crust.)
What if I asked you to imagine what that central bulge would look like to us–lifeforms living inside the galaxy? What would you imagine?
Perhaps you’d imagine looking inwards toward a glowing ball of light. Perhaps you’d imagine a region of our sky unusually thick with stars and interstellar clouds. Or perhaps you’d imagine something entirely different.
Probably the most spectacular feature of our Milky Way galaxy is its spiral arms.
We can’t get a probe far enough out yet to take a galactic selfie, but astronomers arereasonably sure that we live in a spiral galaxy. Observations of other spiral galaxies offer clues to what kind of objects can help us trace out the shapes of spiral arms, called spiral tracers. Using those spiral tracers, we’ve been able to map out patterns within our own galaxy that appear to be spiral arms.
Over the years, astronomers have tested the spiral arm hypothesis against the evidence again and again, and there is now a great deal of confidence that the Milky Way is a spiral galaxy.
More than that–star formation, which we know is limited to the disk of the galaxy (rather than its central bulge or halo), appears to be specifically found in the spiral arms.
But why? And for that matter…what even are spiral arms?
The Milky Way–our home galaxy–is a spiral galaxy, a classification I often describe as pinwheel-shaped.
The main difference between a spiral galaxy’s shape and a pinwheel’s shape is that spiral galaxies, like the Milky Way, only have two main arms. For the Milky Way, those are the Scutum-Centaurus arm and the Perseus arm. If you study the image above, you’ll notice that all the other arms are a bit wispier, and most branch off from the main arms.
There’s just one problem, though…
How do we even know that this image is an accurate depiction of our galaxy? How do we know that the Milky Way has spiral arms?
And since the “discovery” of our Milky Way–or, more accurately, the discovery of what that hazy band of stars in the sky is–we’ve come to realize just how massive our home in the cosmos really is.
That scientific journey started with the Herschels’ mapping of what was then called the “star system.” Later astronomers began to realize just how far out from the sun the stars of our galaxy really reached. Determining distances across our galaxy was the first step to discovering its size.
Later, we began to understand its structure–mapping the extraordinarily thin disk, the chaotic central bulge, and the visible part of the halo, a sphere of stars that extends beyond the plane of the galaxy.
And since then, we’ve begun to master the next critical part of understanding our galaxy: its mass.
In the vast expanse of the cosmos, the Milky Way Galaxy is our home.
You’ve no doubt seen images of the Milky Way and similar galaxies elsewhere online. It’s a large, spiral galaxy, one of the most spectacular galactic shapes. That spiral shape is fairly iconic–and for years, that’s as far as I thought galaxy classification went.
Turns out, galaxies are way more diverse than just the main three classifications I knew about (spirals, ellipticals, and irregulars). The Milky Way is fully classified as an SBbc: a barred spiral galaxy with a medium-sized nucleus.
Spirals are also described as “grand design” (two distinct spiral arms) or “flocculent” (a sort of fluffy appearance); the Milky Way is somewhere in the middle.
But even those classifications and descriptions don’t fully describe our galaxy.
So what exactly is the structure of the galaxy we call home?
Consider that we can’t really take a photo like this of our galaxy. We’re inside it, and space travel has not advanced to the point where we can leave it just yet. There’s no way we can get a camera out to take a picture from this perspective.
Most things in the universe–like stars, planets, and even other galaxies–can be measured using their angular diameters. That is, we use trigonometry to find their actual sizes based on how large they appear to us in the sky.
But that doesn’t work for an object that we’re inside of.
In order measure the size of our own galaxy, early astronomers had to get a bit creative–with variable stars.
It’s not a sight that most of the developed world gets to see–at least not all the time. Light pollution from major cities completely obscures this view. Even in the suburbs where I live, I can kind of make it out–because I know where to look and what to expect.
The best way to really see it is to head out into the desert. Or the open ocean. Really, any place that’s a bit geographically removed from civilization. Growing up, Joshua Tree National Park was always my go-to for dark skies.
Even on an exceptionally dark night, though, you won’t necessarily see this. You’ll definitely be wowed by the vast, bright sprinkling of stars overhead, more than you ever see under less than ideal conditions. But the image above was taken with a long exposure.
That is, the camera shutter remained open for a while to collect more light for one image than your eyes ever will. You and I pretty much only see one image per moment.
Meet the Veil Nebula, one of my favorite deep-sky objects.
The Veil is one of the more common star party requests I get from more experienced participants. Unfortunately, it requires a very powerful telescope. My 11-inch Schmidt-Cassegrain–pretty advanced, as far as intermediate amateur telescopes go–can barely manage it with a nebula filter.
The Veil has several different segments and can’t be viewed all at once. Seriously–the entire Veil Nebula covers an area six times the diameter of the full moon! If it were bright enough to see with the naked eye, it would be a very visible object.
Together, the segments of the veil make up the Cygnus Loop: a ring-shaped phenomenon that is a supernova remnant, formed roughly 10,000 years ago. That’s actually not that long ago, in astronomical terms. But other supernova remnants, such as the Crab Nebula, are much younger.
Those segments have all been observed separately over time and ended up with separate designations in star catalogs, too. The Veil’s components within the NGC star catalog are NGC 6960, NGC 6992, NGC 6995, and IC 1340. It is also known in the Caldwell catalog by Caldwell 34 and 33.
Fainter “knots” of nebulosity that you might not immediately realize are part of a broad, wispy loop are noted as NGC 6974 and NGC 6979.
Different portions of the supernova remnant have also been named the “Witch’s Broom” and “Pickering’s Triangle.” In particular, the Witch’s Broom refers to the same segment as the picture shown above–the Western Veil.
For this post, I thought tell you a bit about how star catalogs work–and share an interesting story about the NGCs!
Whaddya know…after what seems like a geological age, we’re finally done with stellar evolution! And we’ve covered a truly ridiculous amount of information.
We’ve covered a star’s relatively gentle, humble beginnings within the collapsing cores of giant molecular clouds (or GMCs). We’ve explored how stars begin fusing hydrogen nuclei for fuel and how their interiors work.
We’ve covered how they evolve across the main sequence, and how they eventually exhaust their fuel, lose stability, and expand into giants.
We’ve delved into the way low- and medium-mass stars quietly expel their atmospheres and shrink into inert balls of carbon called white dwarfs. And we’ve watched as massive stars burst apart in brilliant supernova explosions and then collapse into some of the most extreme objects in the universe, neutron stars and black holes.
Those three end states–white dwarfs, neutron stars, and black holes–are known as compact objects, and we’ve explored them too.
If it all seems super complicated…I understand. But now, just as I did once with types of stars, I’m going to give you an overview to put it all together.
Okay, good question. How the heck do you find an object that emits no radiation? Astronomers find—and study—just about everything in the universe using the radiation it emits or reflects. So…what happens when the object we’re looking for has such a strong gravitational pull that even light can’t escape?
Well, that’s when we need to turn to the theoretical science behind black holes. What measurable effects do they have on objects in their vicinity? Can we detect them indirectly?
Of course, some of you might be screaming at me that we’ve already photographed a black hole—in visual wavelengths! Yes, astronomers did make that achievement—we now have visual proof that what we’ve been theorizing all along is indeed real.
But that black hole was so faint, it took an interferometer the size of the Earth to image. We had to know exactly where to look in order to get that picture.
So how the heck do we find one in the first place?