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.