Understanding Stars: Distance, Luminosity, Mass, and Spectral Classification

There Is More To A Star Than You Might Think!

Stars come in just about every color, temperature, size and brightness. Let’s start with brightness. Some stars look brighter than others, but they may not have the brightness we think they do. This is because we don’t know just by looking up in the sky which stars are closer to us or not. The distance a star is from us is going to be a big determining factor on how bright a star really is, or not.

Parallax — Figuring Out A Star’s Distance

The word used to describe a star’s brightness is luminosity, and distance is used to determine what that luminosity is. The way to determine the distance of a star is a system called parallax. Parallax could be related to how your finger appears to move when you move it from close to your eyes to a point at arm’s length. Put your finger right in front of your nose, and then move it to arm’s length out in front of you. Notice how your finger appears to move more when it’s close, and less when it’s further away. It’s a depth perception trick of the eye! Objects that are far away have less parallax than objects that are close in.

The way this parallax is achieved for measuring the distance of stars is done with two telescopes. Two telescopes are aimed at the star taking a picture from both angles, and can see how the stars move in relation to parallax, more or less movement, depending on how far away from us they are.

Parallax gives us the ability to measure the distance of 2.5 million stars out to about 500 LY (Light Years) or 150 parceps. A parcep is 3.26 light years. There are 2.5 million stars within 500 LY of us here on Earth. Anything farther out than 500 LY we cannot accurately measure because there isn’t enough movement of any given star that far out that makes the actual parallax evident.

How We Measure the Brightness of Stars

Usually you use Celsius or Fahrenheit to describe the temperature of something, and just like a temperature scale, we need a scale to describe the brightness of a star.

The apparent magnitude scale is what we use to determine the brightness of a star. For instance:

· Sun is -26.7

· Moon is -12.6

· Venus is -4.4

· Brightest star is about -1

As things get dimer, the numbers get larger. Stars are about a -1 (brightest stars) to a +5 star, which the dimmest star you can see in the sky with the naked eye.

To go from a magnitude of 1 to a magnitude of 2 is only 2.5 times dimmer. To go from a magnitude 1 to a magnitude 3 star, it would be 2.5 x2.5 times dimmer. A difference of 5 magnitude is equivalent to 2.5 to the fifth dimmer or brighter, depending on if you’re moving up or down on the scale.

We use apparent magnitude to determine the apparent brightness of a star, we use parallax to measure the distance of a star, and with these two things together we can figure out the actual and real brightness of a star, or what is called the absolute magnitude of a star.

The absolute magnitude places every star at 10 parceps, and how bright it would be at that distance from the observer. A parcep is 3.26 light years. Move every star to that distance, assign a number of brightness, and then you have the absolute magnitude. This is how to precisely measure a star’s brightness, absolute magnitude.

Determining The Mass and Temperature Of A Star

How do we measure the mass of a star? It’s not like you can put a star on a scale in your bathroom and determine its weight! First, we started with Kepler, studying orbits, particularly of binary stars of which half the solar systems in the universe are binary star systems (two suns/stars). We can tell the mass of a star by watching it’s gravitational pull. By studying orbits, we can figure out the mass of a star.

You might look into the sky and think two stars close together in your vision are binary stars orbiting each other, but they are not. They are simply close together in your view, but not anywhere close to each other or orbiting each other. This is called an optical double.

Visual binary systems that we can see with the naked eye are actually pretty rare. It’s actually hard to detect them. What we find instead is that we se stars passing in front of each other, eclipsing each other. We can learn much about their mass by observing this as well. We can study orbits by watching stars eclipsing each other and the light going up and down as they pass each other. Spectroscopic binary observation gives a visual of the two spectrums of two stars going back and forth.

The Temperature Of A Star

Annie Cannon was a woman in astronomy in the 20th century and was discriminated against, as women weren’t allowed to be in the observatory with the telescopes much, even though they had the same education as the men did. She was given a rather boring job, cataloguing stars.

She found a way to make this job far easier and better, and invented a method called Spectral Classification. In this classification, stars were labeled O, B, A, F, G, K and M. The hottest stars are O stars and the coolest stars are M stars. Why didn’t she use A, B, C, etcetera? It is because these letters above represent already named spectral lines in the scale.

Each letter has a level for the star from 0 to 9, so it would be for instance O0, O1, O2, and so on for all the letters. Our sun is a G2 star, in case you’re curious where our sun falls in this scale!

Turns out the biggest stars are the hottest and brightest stars, and they burn out the more quickly than other stars because they burn through their fuel very quickly. They are the most short lived stars and yet are the brightest. So when you see a really bright star, it’s one of the short-lived ones.

Hertzsprung and Russell, a team of astronomers, wanted to see if the brightness of a star and its temperature has a connection. They created a diagram with the spectral classification letters on the bottom, and the absolute magnitude on the left vertical part of the scale.

91% of the stars fall in the expected lines of the averages, from hot and bright to cool and dim, considered to be the main sequence in this diagram. However, there were a few stars that were cool, but bright and others that were hot, but dim. The cool but bright red stars turned out to be Giant stars, about 1% of the stars. The hot but dim white stars turned out to be Dwarf stars, about 8% of the total stars.

The giant red stars are stars that are ending their life and coming to the end of their existence. Stars spend a small percentage of their time as a red giant. All stars end up in this phase at some point as they lose their hydrostatic equilibrium, exploding into a supernova, and then leaving behind a white dwarf small dim but hot star.

Hydrostatic equilibrium, by the way, is a balance between a star’s efforts to explode outward, but the gravity that is so strong that it holds the star together. All stars are caught in a push and pull situation that causes the star to stay the same, steadily, as long as this push outward and pull inward is in balance. This is called hydrostatic equilibrium.


There is a lot more to our starry sky than we might think as an average and ordinary person who doesn’t know a lot about the universe and how it works. It may look pretty, but it is full of mystery that we have only just started discovering a few hundred years ago!

Every year we take quantum leaps in learning even more, and debunking previous presumptions that might have been facts before. We only know the tip of the iceberg when it comes to stars, planets, galaxies, and how things work. This is a good beginning when it comes to understanding stars! We now have a way to measure their light.

Christine Breese is the founder of University of Metaphysical Sciences, Gaia Sagrada Retreat Center, and Free Retreats 4 All. She is an author, teacher, speaker and healer facilitating spiritual journeys in person, meditation online, through her books and articles, and also through her Christine Breese Youtube Videos. She invites every person to discover the genius-master within themselves!

Christine Breese, PhD — founder of University of Metaphysical Sciences, Gaia Sagrada Retreat Center & Free Retreats For All .com

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