Illustrations by John Garrett, my co-presenter for the Orange County Astronomers’ What’s Up show tonight. More precise labelling and captions to come later.
Have you ever looked up at the night sky and noticed that while relatively bright stars outline the constellations, there are numerous other stars that are almost too faint to see with the naked eye?
If you ever noticed this, you probably guessed that the brighter stars are literally brighter, and the fainter stars truly are fainter. Or maybe you guessed that they don’t vary in brightness that much, but fainter stars are much farther away.
But that’s not really true…or, at least, it’s not the whole answer.
So what’s the real reason why some stars appear to be brighter than others—and how can we tell how bright they really are?Continue reading
Recognize this constellation?
Well, at the time stamp of about 2000 AD (CE), I think you will. It’s one of the most famous constellations in the night sky.
Well, technically, it’s not a constellation at all.
It’s an asterism—a commonly recognized grouping of stars that isn’t actually official as a constellation. There are tons of asterisms that you no doubt recognize…the Summer Triangle, the Great Square of Pegasus, the Big Dipper.
That’s right. That mess of stars up there that keeps changing for some reason…that’s the oft-recognized Big Dipper, part of the constellation Ursa Major.
So why the heck are the stars moving?Continue reading
Meet Pegasus, and the constellations surrounding it. As I said in my last post, constellations are just regions of space.
Yes, they are named after mythical beasts and ancient queens, but for scientific purposes, all that matters are the regions they denote.This way, astronomers can easily find obscure, faint objects in the sky.
And telescopes can be easily programmed to find the same objects for those with less experience.
Keep in mind, though, that constellations only appear to fall in the same horizontal plane over Earth’s surface. Some of these stars, even in the same constellation, are light-years apart from one another.
So, in that case, the brighter stars must be closer to us and the dimmer stars farther away, right?
I know—not what you were expecting, was it? Well, like I mentioned in my last post, the brightest star in the sky is Sirius. It’s very far away from us. Proxima Centauri, on the other hand, is the nearest star to the sun…but it’s pretty dim, comparatively.
Fun Fact: Proxima Centauri is suspected to be part of a triple-star system also consisting of Alpha Centauri A and Alpha Centauri B. All three stars are gravitationally interlocked.
Proxima Centauri is the nearest of the three, making it the nearest star to the sun by a small margin.
The first thing we need to realize about stars is that they are not just pinpricks of light in the night sky. Shooting stars are about as close to being stars as are sea stars (more recent name for what were formerly “starfish.”)
We need to redefine the way we think about stars.
I’ll talk more about stars and the way they produce heat in later posts—but for now, let’s just say that the hydrogen and helium gases that make up a star is so compressed and so excited that their atoms literally tear themselves apart.
That’s why stars are bright. They produce a ton of energy.
Anyway…because astronomers love labeling things in space, we have a way to describe the brightness of stars. Keep in mind that this only describes their apparent brightness—how they look to the human eye. Like I said, a star’s apparent brightness really has no bearing on its real brightness.
We use the magnitude scale.
This is a system that first appeared in the writings of the ancient astronomer Claudius Ptolemy. It’s likely the system actually originated earlier. Most astronomers actually attribute it to Hipparchus, a Greek astronomer.
According to the magnitude scale, stars are divided into six classes. First-class stars were the best and the brightest, just like the treatment you receive in the first class section of an airplane. And, naturally, sixth-class stars were the dimmest.
Well, actually, they were known as “first-magnitude” through “sixth-magnitude.” But I thought the class explanation made the whole backwards lay of the scale a bit simpler to understand.
I looked up first-magnitude stars on the Internet, and whadya know—I found a chart detailing the names and brightness of each one. It even cooperatively copied and pasted for me. So here you go!
|Sirius||⍺ Canis Majoris||-1.44|
|Rigil Kent or Alpha Centauri||⍺ Rigel Kentaurus||-0.28c|
|Procyon||⍺ Canis Minoris||0.40|
|Betelgeuse or Betelgeux||⍺ Orionis||0.45v|
|Agena or Hadar||β Rigel Kentaurus||0.61v|
|Fomalhaut||⍺ Piscis Austrini||1.17|
|Beta Crucis||β Crucis||1.25v|
Okay, so I’m guessing first-magnitude can also refer to stars whose brightness goes off the charts of Hipparchus’s system. That would explain the negative numbers. See, the fact is, there’s just too much variation in brightness in the sky to neatly group all stars under six classes.
In the modern day, astronomers use a scale numbered from -30 to 30. On this scale, the sun is -27. The full moon is at -13. When Venus is at its brightest, it measures at -5. And Sirius, the brightest star in the sky, measures at -1.47.
This is the apparent visual magnitude scale. Naturally, there’s an abbreviation for that, since I imagine it would get tedious to write out that the naked eye limit is about 6 apparent visual magnitude. We shorten it to “mv.”
So Polaris, the north star, measures at about 3 mv. The dimmest the naked eye can make out—the naked eye limit—is about 6 mv. The limit for the Hubble Space Telescope is 30 mv.
Naturally, there are dimmer stars than that, and the scale goes beyond 30 for the dim stars. But we won’t really need to talk about anything beyond 30. And there’s honestly nothing brighter than -30. You don’t get brighter than our own nearby sun.
Knowing the brightness of stars—at least their apparent brightness—helps us locate them in the sky. So, we already know how constellations help map out the sky. But how on earth can we keep all the stars straight? There’s trillions and trillions of them.
Easy. All we need to do is rank the stars by brightness within their constellations. We don’t need to worry about naming all of them. A lot of the brighter stars have Arabic names, such as Betelgeuse, Sirius, and Aldebaran. But there’s no way we can name them all.
And now you know why the stars of Orion have those peculiar little squiggles next to them in this diagram. Those are Greek letters. The stars are designated from brightest to dimmest in Greek.
So, according to the diagram above, Betelgeuse is ⍺, or alpha—meaning it’s the brightest star. Close behind is Rigel, β—or beta.
Even if you’re not familiar with the Greek alphabet—I had to learn it in sixth grade, but I never memorized the symbols—you can easily figure it out by ranking the size of the stars. This star chart has a magnitude key at the bottom that abides by the magnitude scale.
Now we know how to identify the apparent brightness of stars. But how do we refer to them by name?
Again, easy. Start with the Greek letter—you can spell it out or you can use the symbols. I thought I’d include a chart of them all for you here.
Keep in mind, when using the symbols for star designations, use the lowercase version.
After the Greek letter comes the possessive form of the constellation name. If that sounds complicated—especially given that they’re mostly in Latin—never fear, just look it up. For example, Canis Major becomes Canis Majoris.
So what would we call the brightest star of Canis Major? Alpha Canis Majoris, or ⍺ Canis Majoris.
Next up in my astronomy posts is the magnitude and intensity of stars. It’s going to be a little bit mathy, so feel free to skip it—I’m planning on releasing a post on the celestial sphere on the same day, and that’s going to be a lot more of just visual stuff.
See you guys around the galaxy!
You may have heard of a common pseudoscience—astrology. Astrology is a pseudoscience because it’s a set of beliefs that seem to be based on scientific ideas, but really, it fails to obey even the most basic rules of science.
What are the basic rules of science, then?Continue reading
The ecliptic, as astronomers call it, is the apparent path of the sun against the background of the stars in the sky.
It’s useful because it tells us how to find the planets in the sky. They can be hard to spot if you don’t know where to look, but they will always be somewhere along one imaginary line that arcs across the sky—the ecliptic.
This pattern never changes. The planets don’t follow the ecliptic exactly, but it’s useful for getting an idea of where they should be.
But why does it work—and what exactly does it mean, when it’s obvious we can’t see the sun among the stars of the night sky?Continue reading
I need you guys to help me with something.
Can you find a horse in this image of the night sky?
Yeah, me neither. I’m lost. I see the Great Square of Pegasus because I know what to look for, but I still don’t see a horse.
Okay, so now I see half a horse. Where’s the rest?
Your guess is as good as mine, guys. Truth is, constellations very rarely look like what they’re named after. Constellations are more like relics of our ancient past than actual descriptors of what we see up there in the sky. But they do serve a purpose, even if that horse up there is missing his back legs.
I really find myself wondering if whoever made up Pegasus was concerned with animal rights… No animals were harmed in the making of this sky map…
Okay, yeah, never mind.
Where was I? Ah, that’s right. The purpose of the constellations, and the reason why it really doesn’t matter if they don’t look like their names say they do.Continue reading