It's the 365 days of Astronomy PodGa, coming in three, two, one.
Welcome to the 365 days for Astronomy.
I'm Aviva, your host.
Today we're going to talk about the most common words in Astronomy.
Magnitude, let's begin with a small puzzle hidden in plain sight above our heads.
Imagine you're standing under a dark sky, looking up at the stars.
Someone points to a bright star and says, that's a first magnitude star.
Then they point to a much fainter one and say,
that one is six magnitude.
And you might reasonably ask, wait, shouldn't a bigger number mean brighter?
In Astronomy, it doesn't.
Here, the small number, the brighter the object.
The brighter star can even have zero or negative magnitudes.
Serious, the brighter star in the night sky shines at about minus 1.46.
The sun blazes at minus 26.7 magnitude.
At first, that sounds backwards.
But once you know where the system came from,
it begins to feel less like a mistake and more like a story.
One that has been traveling through Astronomy for more than 2,000 years.
Long before telescopes, before cameras, before photometers and digital detectors,
people look at the sky with only their eyes.
And even then, they notice something obvious and beautiful.
Not all stars shine equally.
Some stand out immediately, others hover at the edge of feasibility, faint and uncertain.
Around 129 BC, the Greek astronomer Hipparchus,
set out to catalog the stars.
In doing so, he used a simple way to rank them by brightness.
The brighter stars became first magnitude.
The next brightest were second magnitude.
And the faintest star, the human eye, could usually see,
were placed in the sixth magnitude class.
It was not yet a precise measurement.
It was more like a practical sorting system,
a way of bringing order to the sky.
A few centuries later, the brightness ranking appeared in the work of Tholami
and became part of the Astronomical Tradition through his famous book, The Almages.
And in that ancient system, the logic felt natural.
First main foremost, first man brightest.
For a long time, that was enough.
But astronomy changed.
Observations became more careful.
Instruments became more powerful.
And eventually, astronomers needed something more exact than a judgment made by eye.
The turning point came in the 19th century with the English astronomers,
Norman Robert Poxson.
In 1856, Poxson took the old magnitude system and gave it mathematical form.
He noticed that the ancient scale roughly matched a real difference in brightness.
Stars of first magnitude appeared about 100 times brighter than stars of sixth magnitude.
So, he made that the rule.
A difference of five magnitudes would be defined as a brightness ratio of exactly 102.1.
From that one decision came the modern system.
It means that a difference of one magnitude is not a simple step,
but a change of about 2.512 times in brightness.
That is why magnitude is called a logarithmic scale.
It compresses an enormous range of brightness into manageable numbers.
And that matters because the universe is not tidy.
Some objects are a little brighter than others.
Some are hundreds of time brighters.
Some are billions of times brighter.
A logarithmic scale helps astronomers describe that fast range without drowning in a huge number.
So, what does magnitude actually mean today?
In simple terms, magnitude is a way to describe brightness.
But there are two important versions of it.
The first is apparent magnitude.
This tells us how bright an object looks from Earth.
It is about appearance.
A star may seem bright because it's nearby.
Not because it is especially powerful.
That is why the sun dominates our sky.
Not because it is the most luminous star in the universe.
But because it is so close to us.
The second is absolute magnitude.
This gives astronomers a fairer comparison.
It tells us how bright a star would appear if it were placed at a standard distance
of 10 parsecs, about 32.6 light years away.
In other words, absolute magnitude helps answer a deeper question.
How bright is this star really?
Once this then is no longer part of the illusions.
Today, astronomers no longer estimate magnitudes by eyes alone.
They measure light using a technique called photometric.
Telescope and digital detectors collect the light from stars, planets, galaxy
The light is then compared with standard reference stars whose magnitudes are already known.
Observers also have to account for Earth's atmosphere,
the sensitivity of the detector, and the color of the object.
Because brightness can change depending on which wavelength of light
But even with all this modern technology,
the ancient idea remains.
Magnitudes still helps us understand the night sky in practical ways.
Under dark sky, the faintest stars most people can see with the naked eye
are around 6 magnitudes.
In a bright city, light pollution may wash out everything fainter than third magnitude.
With binoculars, you reach fainter objects with a telescope,
So magnitude is not just a technical term for professionals.
It is a bridge between the sky as we experience it and the sky as astronomy measure it.
And perhaps that is what makes it so charming.
Magnitude begins as a human impression.
A sky watcher looking up and deciding which stars were bold and which were barely there.
Over time, that impression became a mathematical tool precise enough for modern science.
So the next time someone tells you a star is a first magnitude,
you will know that means it is one of the brightest stars in the sky.
And you will also know that behind that small numbers lies a fairy-old story.
One that began with the human eye was carried through ancient catalogs and still lives on
Thank you for listening. This is 365 days of astronomy.
You are losing to the 365 days of astronomy podcast.
The 365 days of astronomy podcast is produced by the Planetary Science Institute.
Audio post-production is by me, Richard Drum.
Project management is by Aviva Yamani and hosting is donated by libson.com.
This content is released under a Creative Commons Attribution Non-Commercial 4.0 International
License. Please share what you love but don't sell what's free. This show is made possible thanks
to the generous donations of people like you. Please consider supporting our show on patreon.com
forward slash CosmoQuestX and get access to bonus content.
Without your passion and contribution, we won't be able to share the stories and inspire the
worlds. We invite you to join our community of storytellers and share your voice with the
listeners worldwide. As we wrap up today's episode, we're looking forward to unraveling
more stories from the universe. With every new discovery from ground-based and space-based
observatories and each milestone and space exploration, we come closer to understanding the
cosmos and our place within it. Until next time, let the stars guide your curiosity.
