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?
To those who don’t, it probably looks like a pretty unimpressive, blurry ring. In fact, this is the first ever image of a black hole, taken with an interferometer the size of the Earth.
If you’re a science geek, you’ve no doubt seen tons of artists’ conceptions of black holes on the internet. Most use a great deal of artistic license. Some of my favorite “images” of black holes used to be the ones that look like ripples in the fabric of space. Imagine my disappointment when I realized that’s not the case at all.
Black holes are singularities—infinitely dense places of zero radius with at least 3 M☉ (solar masses) of star stuff—surrounded by an event horizon, inside of which gravity is so strong that even light cannot escape. That’s why it’s called a black hole.
But they are not “holes” in the usual sense. They are not giant space potholes that you can easily stumble into, and you certainly don’t fall into them the same way you would a pothole.
If you’re a sci-fi fan, you’ve probably seen these in movies. And I’m guessing you’ve heard a lot about them in pop culture. The problem is, pop culture and movies don’t do a very good job of describing black holes.
First off, let me clear up a common misconception: Black holes do not act like giant space vacuum cleaners, sucking in everything around them. Describing them as “gobbling up” anything is inaccurate.
The representation in movies that bugs me the most is in J.J. Abrams’ Star Trek reboot, when the bad guy falls into a black hole and the good guys almost get pulled in with him. First of all, please…black holes do not growl. And basically none of what happens in that scene is accurate.
Neutron stars—the compact remains of massive stars that have gone supernova—are some of the most extreme objects in the universe, narrowly beaten by black holes (and, as we’ll talk about in future posts, active galaxies and such).
Dense balls of pure neutron material with diameters barely larger than Los Angeles, neutron stars have strong magnetic fields that produce beams of radiation at the magnetic poles. Their speedy rotation makes these beams sweep across the sky like a lighthouse.
When one of their beams crosses directly over Earth, human astronomers observe rapid pulses of light called pulsars.
These objects are whacky, to say the least. And there’s more…
Way back when we spent a number of posts surveying the stars, we covered binary systems. These are star systems that contain multiple stars. Imagine if our sun had a companion, and two stars rose and set in our sky over the cycle of day and night.
It might surprise you that the majority of stars in the universe are actually in binary systems. Our solar system seems to be an outlier in that regard. Most stars have a companion or two or six…
…and so do some neutron stars.
Remember that neutron stars are the collapsed remnants of massive stars that have gone supernova. If most stars are part of binary systems, then naturally, some of these stars will evolve into neutron stars and still be part of their birth system.
For those of you who missed my last couple of posts, allow me to introduce the neutron star: a stellar remnant similar to a white dwarf, but much denser, so dense that its protons and electrons have combined to form a neutron soup.
A neutron star forms from the collapsing core of a star between 10 and 20 M☉ (solar masses). Its collapse produces powerful magnetic fields and extremely high temperatures, but because it becomes so small—less than the size of Los Angeles—it is very faint and radiates away its heat very slowly.
The exception to that rule comes in the form of two powerful beams of radiation that blast away from the object’s magnetic poles. As a neutron star spins—at around a hundred times per second—these radiation beams sweep across the sky like the the beams of a lighthouse.
If these beams happen to sweep over Earth, human observers see regular, rapid pulses of light. This visual phenomenon produced by neutron stars is called a pulsar.
Now that we have a basic understanding of neutron stars and pulsars, let’s explore some of the details of how these extreme objects work.