Hi I’m David Fuller from the “Eyes on
the Sky” video series. Let’s look at another aspects of Stargazing
Basics, this time, magnitudes of various objects. Magnitude is just a fancy way to describe
the difference in brightness of objects we see in the night sky. And it’s not too hard to learn how it works,
but let’s start with some history. The Greek astronomer Hipparchus developed
the magnitude scale back in the 2-nd century BC, when he assigned the brightest stars a
magnitude of “one” and the dimmest stars that of “six,” the in-between stars of
2, 3 , 4 and 5 magnitude assigned according to brightness. He assumed that the difference in brightness
of stars was 2.512 times brighter than the next dimmest one. Taken across that full 6 magnitude scale,
this meant that the brightest stars – the first magnitude ones – were 100 times brighter
than the dimmest, 6-th magnitude ones.
It also allows for a fairly easy way to determine
other brightness differences: The difference from first to third magnitude is 6.3 times;
first to fourth, 15.8, first to fifth , about 40 times. This worked just fine until Galileo turned
a telescope towards the heavens, and humans discovered there were a LOT more stars out
there than just the first through sixth magnitude ones they could see naked eye. But what they did was just extrapolate the
scale further. So much like golf or ERA in baseball, the
lower the number, the brighter the star, and the higher the number, the dimmer the star. 11-th magnitude is 100 times dimmer than 6-th
magnitude, and that 11-th magnitude star would also be 10,000 times dimmer than a first magnitude
star. In the other direction, we have some objects
that are brighter than first magnitude stars. Hipparchus actually fudged the numbers a bit;
the star Sirius in the winter sky is definitely brighter than most other bright stars, and
it shines at magnitude negative one point four. The planet Jupiter is brighter than that,
often appearing at around magnitude negative 2, and Venus brighter still, typically around
negative four.
When objects are bright, they are denoted
on star charts with larger dots; dimmer stars are usually given smaller dots. This helps us locate brighter objects in the
sky more easily. But beyond the stars’ magnitudes is that
of the Moon and Sun. The full Moon in the night sky shines at magnitude
negative 12.7, and the Sun at magnitude negative 26.7! Those are both bright compared to starlight,
but also a huge difference, even between themselves, as the Sun is 400,000 times brighter than
the full Moon! Now everything we have discussed has been
“visual magnitude,” meaning how bright or dim objects appear to our location on Earth. Another concept is “absolute magnitude,”
used by astronomers to compare the relative brightness of objects when placed the same
distance. But that is more complex than what we need
to know for simple stargazing purposes. However, for visual magnitude, it helps to
understand how it relates to other objects besides stars. Stars are point-like objects in the sky, so
it is easy to assess their magnitude and brightness. But galaxies, star clusters and nebula are
not quite as easy. So we look at their “integrated magnitude.” That’s a way of saying that the same magnitude
is now spread out over a larger area.
For small objects, that can mean it is something
that will look brighter to us in telescopes. If the area if very large though – such
as the galaxy M33 or the nebula M1, those objects may look very dim, relatively speaking. If possible, try to look for the surface brightness
of an object; objects above a surface brightness of 11 or 12 can be very difficult to find
from light polluted areas. That’s a quick overview of the magnitude
scale, and how to understand it. Just remember the Sun and Moon have negative
magnitudes and are the brightest visual magnitude objects, and lower numbers are dimmer stars
and objects, and you’ll be in good shape! In the next video, I’ll explain how to easily
measure distance in the sky, moving FROM bright objects to the dimmer ones, so we can better
utilize optical aids like binoculars and telescopes to find interesting objects. Thanks for watching; I’m David Fuller. Keep your eyes on the sky and your outdoor
lights aimed down by using dark sky friendly lighting fixtures, so we can all see, what’s
up.