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. Even other objects like planets stand out more
than some of the bright stars in the sky. When astronomers talk about star brightness they use
something called magnitude. 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.
As you're looking at this group of stars you might notice this pattern, these seven stars,
are an asterism called the Big Dipper. 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.
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. As you can see, these stars are of different brightnesses.
In this video, we are going to explore what the magnitude of a star really means
and how it's changed over the years. 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 2nd century. He was the first to start this star brightness classification. He classified
850 stars ranging from 1 to 6 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. He said the brightest stars like Rigel, right here, in the constellation of Orion
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 sixth magnitude. This was really used by astronomers for the past
2,000 plus years. As you look at this photo, 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 and he was a Roman
mathematician and astronomer that expanded on the work of Hipparchus. He still retained that six
range brightness classification with 1 being the brightest and 6 being the faintest. Now we're
gonna 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. He started labeling these invisible stars as
seventh magnitude. He continued Hipparchus magnitude scale but also expanded on it.
Then we're gonna skip on over to 1856. An english astronomer known as Norman Robert Pogson
standardized this magnitude system because that's what scientists do; they standardize. We have to
standardize our measurements so we know what it is that we're measuring. He concluded that a first
magnitude star is 100 times brighter than a 6 magnitude star. This established a logarithmic
scale. 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. That's the brightness of a star as it appears to us.
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. For example, this is the Pleiades
star cluster that has a relatively lower magnitude which means it's brighter. 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. 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 -0.63.
This seems confusing because it's like, "Oh does that mean it's brighter or dimmer?" I do want to
go ahead and explore the difference between the negative which are usually brighter numbers.
Then, the positive. The bigger the number the dimmer it is. 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 it 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. I'm purely speaking of apparent magnitude. Remember the scale works
in reverse. So the lower the magnitude the brighter the star. Let's take this
timeline, or number line, I should say. 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. It has a magnitude of 0 which tells us
it's a pretty bright object in the sky. But, if we look at a star like Polaris, 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 6.So anything at a magnitude 6 is the limit that
we can see. If you start going to 7; 8; 9. you need magnification to see anything that is above a 6.
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 but
and I'm not going to get into the mathematics of this. 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 -4. 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. The Sun has a magnitude of -27.
That seems counter-intuitive, right? That negative means it would 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 0 magnitude and you can see its size
matches that magnitude. 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.
Also, photographic film is tends to be more sensitive to blue light. As you can see
in this example. 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 can also be
influenced by atmospheric disturbances. So you can see this if you look at stars that are right up
along the horizon. What you see is this kind of twinkling going on and that's really the gas is
moving in the air that causes that twinkling and it's usually more pronounced along the horizon.
Also, the scale is in reverse so that can make it confusing as well. 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 6. 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. We are probably going to
see things of even fainter magnitude. What's interesting is that that the James Webb also has
an infrared camera, so we're gonna 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. 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!