The Brilliant Lives of the Residents of Cosmicville

The “absolute scale" of brightness for a cosmic entity

In our vast cosmos, many entities dazzle and shine. So what does it mean to shine brightly, and how do we compare (as we ought to)? Well, there is a way — through the concept of “Absolute Magnitude”. Come along as we snoop into the unassuming lives of the residents of Cosmicville and find an equivalent for “Sharma ji’s son” in this shining business.

G299 is a remnant of a Supernova of Type 1A — often used as candlesticks in astronomy (an object with a known standard absolute magnitude). This picture was captured by NASA’s Chandra X-Ray Mission with the caption: Exploded Star Blooms Like a Cosmic Flower. Source: Wikimedia Commons

In one of my previous articles in the Space Nuggets series, which talks about the beautiful conjunction of Mars, Jupiter and Venus last month, we discussed the concept of “Apparent Magnitude” — how bright an object in the sky appears to us on Earth. We also talked about the scale of the “Apparent Magnitude” of different objects in the night sky, for which personally, I had a lot of fun creating the infographic below.

A chart represents well-known objects' apparent magnitude in the night sky and telescope limits. Chart created by the author with images from the internet, not made to scale. Inspired by Year10Constellation and the University of Central Florida

Everything we have talked about till now is based on how we see these things from the earth — whether we can see them with our naked eyes or if we need binoculars and telescopes. But what about the telescopes which are not on Earth? Do we have a standard scale to measure the brightness of objects in the universe that these space telescopes can use without having the Earth as a base reference?

The answer is yes, and this system is called the scale of “Absolute Magnitude”.

What is Absolute Magnitude?

Astronomers define “Absolute Magnitude” as how bright an object will appear to us if it were located at a distance of 10 parsecs from Earth.

1 parsec is 3.26 light years; hence 10 parsecs = 32.6 light years, equal to 309 trillion kilometres!

That’s 309 with 12 zeroes after it or 309,000,000,000,000 kilometres away.

An example of Absolute Magnitude vs Apparent Magnitude. Source: Swinburne University of Technology

This is just a way of understanding Absolute Magnitude w.r.t. Apparent Magnitude. But what does Apparent Magnitude actually mean? Or rather, why do astronomical objects in the sky vary in their degrees of brightness?

Shine bright like a…celestial object.

While “Apparent Magnitude” depends on different factors on the earth, like how far the object is from the Earth, light pollution at the place of observation on the earth, and so on, none of these factors plays a role in space (well, there is still dust in space known as cosmic dust, but for the purposes of theoretical calculation of Absolute Magnitude, we do not consider its effect). So then, what determines which celestial body will shine brightly and which will remain under a cloud of darkness?

The answer lies in a fundamental physics concept we use daily — the energy radiated by any astronomical object, also called luminosity! Objects with high levels of energy radiation have a lower value of Absolute Magnitude and vice-versa.

If you are wondering why the relationship is inverse, the scale of Absolute Magnitude is also logarithmic, like the scale of Apparent Magnitude! So negative values mean more energy output and vice-versa.

Okay, we have put it on a scale, but what do we achieve by knowing the Absolute Magnitude?

Uses of Absolute Magnitude

Just like we use the Apparent Magnitude of a standard object in the night sky — for instance, Sirius the star — to calculate the Apparent Magnitude of other unknown objects, we can use the Absolute Magnitude of what are correctly known as standard “candlesticks” :P (objects which have same luminosities, no matter where they are, like Cepheid Variable Stars, RR Lyrae Stars and Type 1A supernovae — to calculate the absolute magnitude of other unknown astronomical objects.

Now for an unknown astronomical object, once we are equipped with its Absolute and Apparent Magnitude, we can directly find the distance of this object from us — a trick often used to create a map of the Universe! How? Let’s see below:

Reference: Northwestern University Astronomy Web lab series

The difference between Apparent and Absolute Magnitude, also called Distance Modulus (m-M), is a widespread scale used by astronomers — as it is directly a function of our distance from any celestial body.

Though Absolute Magnitude is a concept mainly used in the astronomy communities, scales like the Distance Modulus are often valuable for answering many interesting cosmological questions, like —

If the Universe is expanding and if everything in the Universe is moving further away from us, how fast or slow is the expansion and is it increasing or slowing down over time?

I hope you found something interesting in this tidbit on the Absolute Magnitude of celestial bodies and how such a simple concept can be instrumental in answering questions about the Universe! Stay tuned for more such quick reads in the Space Nuggets series.