Star Magnitude (Brightness) Explained

Welcome to Learn the Sky, your online resource for learning about the stars and constellations. At some point in your life as you have been stargazing, you've probably noticed that some stars are brighter than others, and even other objects like planets stand out more than some of the bright stars in the sky. But when astronomers talk about star brightness, they use something called magnitude. So when I say magnitude, I'm talking about the brightness of a star. Take this constellation for example. This is the constellation known as Ursa Major, but as you're looking at this group of stars, you might notice this pattern. These seven stars are an asterism called the Big Dipper, and it has many other names in many other cultures. I can't even pretend that I know all the names of this particular asterism or star pattern, but these seven stars are of relatively the same brightness, and that's what makes it easy to find in the sky, and that's how you can tell its difference from Ursa Minor, which is the Little Dipper. Here is the actual full constellation of Ursa Major, and as you can see, these stars are of different brightnesses. So in this video, we are going to explore what the magnitude of a star really means, and how it's changed over the years. So we're going to review a really quick history of star magnitude and how it came to be. From what we know at this time, Hipparchus was the Greek astronomer who lived in the second century, and he was the first to start this star brightness classification. He classified 850 stars, ranging from one to six in terms of brightness. So what did this look like for Hipparchus? If you look at this picture here, you can see that there are a range of star brightnesses. So he said the brightest stars, like one Rigel right here in the constellation of Orion, that would be a first magnitude star because it's very bright. Then the fainter stars, ones that are up here that are very tiny little pinpoints and don't really stand out, those faint stars would be of six magnitude. This was really used by astronomers for the past 2,000 plus years. So as you look at this photo, you can see there are different brightnesses, and how do you define them? There's a lot of questions here that I had, but what I did learn is that over time this system became a little bit more refined, also with the help of technology of course. The next character we have in our story of star magnitude is Ptolemy. He was a Roman mathematician and astronomer that expanded on the work of Hipparchus. He still retained that six range brightness classification with one being the brightest and six being the faintest. But now we're going to fast forward towards Galileo's time. When he looked at the night sky through his telescope, he was able to see these invisible stars, stars that you wouldn't normally see with the naked eye because they're just too faint. So he started labeling these invisible stars as seventh magnitude. So he continued Hipparchus's magnitude scale, but also expanded on it. Then we're going to skip on over to 1856, and an English astronomer known as Norman Robert Pogson standardized this magnitude system because that's what scientists do. We have to standardize our measurements so we know what it is that we're measuring. So he concluded that first magnitude star is 100 times brighter than a six magnitude star, and this established a logarithmic scale. But astronomers also look at magnitude from two different lenses, and I want to explore that in the next part of this video. Another lens that astronomers use in magnitude is the concept of apparent magnitude versus absolute magnitude. Apparent magnitude is probably what you're most familiar with, and that's the brightness of a star as it appears to us. But with absolute magnitude, it's a little bit different. Astronomers will look at the brightness of an object if it were at a particular distance. So apparent magnitude, as you're looking at this picture, is the brightness as it appears to us, the observer. So for example, this is the Pleiades star cluster. That has a relatively lower magnitude, which means it's brighter, and we're going to explore this scale in a little bit, so stick with me. But absolute magnitude is when scientists will measure the brightness of a celestial object if it was at a particular distance from Earth, that distance being 10 parsecs. So if we're here on Earth and a star was placed at 10 parsecs away, 32.6 light years, how bright would it be? So we can have two different values for magnitude based upon the lens at which you're looking at. So let's get an example of this. So this star right here, this is Aldebaran in the constellation of Taurus. It has an apparent magnitude of 0.87, so very close to 1, which tells us it's a bright object. But its absolute magnitude is negative 0.63. So this seems confusing because it's like, oh, does that mean it's brighter or dimmer? So I do want to go ahead and explore the difference between the negative, which are usually brighter numbers, and then the positive, the bigger the number, the dimmer it is. But I wanted to just go over this concept of Aldebaran having two different types of magnitude. One being what we see from our perspective, and one being if this star was placed at a particular distance, what would the brightness be? So let's take a look at some of the examples of different stars we see in the sky versus what their magnitude would be. And I'm purely speaking of apparent magnitude. Remember the scale works in reverse, so the lower the magnitude, the brighter the star. So let's take this timeline, or number line I should say, and we're going to place some different stars on here and show you where they lie. So first we'll start with Vega. Vega is in the constellation Lyra. I have a video about that, go check it out. And it has a magnitude of zero, which tells us it's a pretty bright object in the sky. But if we look at a star like Polaris, Polaris is the north star, the star that doesn't appear to move. I've got a video on that one. That has a magnitude of two, so Vega is brighter than Polaris, even though the number is bigger with Polaris. Remember, inverse relationship. Our limit that we can see with the naked eye is six, so anything at a magnitude six is the limit that we can see. If you start going to seven, eight, nine, you need magnification to see anything that is above a six. If we keep going here, Sirius, the brightest star in the night sky in the constellation Canis Major. I've got a video on it, go see it if you want to learn more. That star is negative 1.5, so it's really bright when you compare it to say, well, I don't want to say really, really bright, and I'm not going to get into the mathematics of this, but if we bounce back to Polaris, Sirius would definitely be brighter than Polaris. If we keep going, Venus, a planet now that we're speaking of, has a magnitude of negative four, so that tells us Venus is really bright in the sky. I can always tell where Venus is because it is the brightest object in the sky next to the Sun, which the Sun has a magnitude of negative 27, so that seems counterintuitive, right? That negative means it wouldn't be as bright, but remember it's that inverse scale. So if we take a look at a star map here, we're going back to Vega. Vega is the brightest star in the constellation of Lyra, and you can see the magnitude scale down here. We defined that Vega was zero magnitude, and you can see its size matches that magnitude. So that's how it works, okay? We know there are some limitations of this. Some of the limitations include human eyes. They are more sensitive to red and yellow light than blue light. As you can see, I did color this blue, but it doesn't stand out as much as red and yellow. And also, photographic film tends to be more sensitive to blue light, as you can see in this example. So another limitation is that visual magnitude versus photographic magnitude will definitely be different. We are limited by our human eyes, and we can capture things on photographs that we wouldn't be able to see otherwise. Apparent magnitude that can also be influenced by atmospheric disturbances. So you can see this if you look at stars that are right along the horizon. What you see is this kind of twinkling going on, and that's really the gases moving in the air that causes that twinkling, and it's usually more pronounced along the horizon. And also the scale is in reverse, so that can make it confusing as well. So if we wanted to see what's what is like the absolute greatest, or I shouldn't say absolute, that might be the wrong choice of words, but how far can we see out there with the instruments we do have today? So I found this picture, and I find it interesting because it shows you the naked eye limit, which is magnitude of six. Notice we're talking of apparent magnitude. The Hubble Space Telescope sees a little more than 25, in between 25 and 30 apparent magnitude. So what's going to be exciting is once the James Webb Space Telescope launches, that will definitely be a more powerful instrument than the Hubble Space Telescope. So we are probably going to see things of even fainter magnitude, and what's interesting is that that the James Webb also has an infrared camera. So we're going to see just, we're going to be looking at this through a different lens. So that's my video explanation of magnitude. I do hope this is helpful for you as you're looking at different star maps, and now you also know the difference between apparent versus absolute magnitude. Thank you so much for watching. Remember, it takes time, patience, and practice to really identify the stars. I wish you clear skies, and keep looking up.