 Greetings and welcome to the introduction to astronomy. In today's special topic in astronomy we are going to look at the difference between a parent and absolute magnitudes and try to understand how we measure the brightnesses of stars. So what is a magnitude? Well first of all the magnitude is a brightness scale that is used to measure stars and it dates back thousands of years to the astronomer Herparchus who first grouped the stars into groupings by brightness. Now we use both a parent and absolute magnitudes. The apparent magnitude is how bright an object appears from earth and that's what we see in the diagram here where how bright is something appears from earth the brighter objects would be on the left the fainter objects on the right. So very bright objects like the sun or the full moon would be among the brightest and would have the smallest apparent magnitudes. Very faint objects such as things that can only be seen with very large telescopes would have larger apparent magnitudes. Now the brightness for the apparent magnitude depends on the distance to the object. The further an object away is then it's going to appear fainter. So the apparent magnitude is a combination of how bright a star intrinsically is along with how far away it is. How close is it to us because you could have a very bright object close to it. Sorry a very bright object very far away and a very faint intrinsically object very close to us and they might appear the same brightness. Now the absolute magnitude on the other hand is a measure of the intrinsic brightness. How much energy it's actually putting out. It does not depend on the distance. So the distance is not a factor here and in fact it is really how bright a star would be if you moved it to a distance of 10 parsecs or about 32 light years away. That is how bright the star would be. So for example our sun has an apparent magnitude of negative 26.7 It's extremely bright. But if you moved it 10 parsecs or 32 and a half light years away it would have it has an absolute magnitude of 4.8. That would be it's apparent magnitude. That's how bright it would appear in the sky if it were that far away from us. Which would be one of the fainter stars visible with the naked eye. So the sun would be visible 30 light years away but would be not be one of the brightest stars in the sky. Now let's take a little look at how these kind of combined and we'll look at one example and that is our summer triangle which is prominent stars in the summer sky although it is visible obviously in very cold weather as we see in the image here. It consists of three bright stars Vega which is down toward the lower section here so Vega here then we have Deneb up toward the top and we have Altair down toward the bottom here and together you can imagine that they form a giant triangle in the sky. Now let's look at their magnitudes. Their magnitudes are the apparent magnitude of Vega is plus 0.03 it is one of the brighter stars in the sky as are all of these and its absolute magnitude is plus 0.58. That means that Vega appears brighter than it would be if it were 10 parsecs away so it must be closer than that. It appears brighter than it should be so it's a relatively nearby star being within 30 or so light years of Earth. We can say the same thing about Altair Altair a little bit fainter then Vega at 0.76 magnitude remember a bigger number means a fainter star and its absolute magnitude is plus 0.22 so it appears brighter than it would if it were 10 parsecs away therefore it's closer than 10 parsecs away. So these two stars down here and here are relatively close how about Deneb let's take a look at Deneb Deneb has an apparent magnitude of plus 1.25 it is the faintest of the three stars Vega is brighter Altair is brighter so it is the faintest appearing in the sky however its absolute magnitude is negative 8.38 so Deneb appears far fainter than it would if it were 10 parsecs away so it must be a lot further away to make it seem that much fainter if Deneb were at the distance of Vega or Altair it would be easily the brightest star in the sky but it is many times further away and only looks so bright because of its immense intrinsic brightness so we can see there that it is by far intrinsically it is by far the brightest of the three stars that we're looking at here this one here this is the brightest of them intrinsically even though it is the faintest of the three overall so let's go ahead and finish up with our summary here and what we looked at is star brightness is measured using the magnitude scale we talked about apparent magnitude and that's how bright it appears from earth we talked about absolute magnitude and that's how intrinsically bright that star actually is and we did some comparisons to see how we can help learn more about stars from their apparent and absolute magnitudes so that concludes this special topic on apparent and absolute magnitudes we'll be back again next week for another special topic in astronomy so until then have a great day everyone and I will see you in class