When you're creating star systems for Traveller RPG, you may come across codes such as G2V, M2III, O7V, or A3Va. But what do those letters and numbers mean?
These codes tell us what kind of star we're looking at, how hot it is, and where it fits among other stars. Once you learn how to read them, you'll have another way to bring the worlds of your Traveller campaign to life.
Let's start with our own Sun and work our way through the basics.
What Is Stellar Classification?
Stellar classification is a system astronomers use to describe stars based on their light and other physical properties.
In Traveller, you'll often see these classifications in star system information. They can help you picture what a star looks like and give you some ideas for describing the planets that orbit it.
Our Sun has the classification G2V.
Each part of that code means something:
- G tells us the star's spectral class, which relates to its surface temperature and color.
- 2 gives us a more exact position within that spectral class.
- V tells us the star's luminosity class. In this case, it means the Sun is a main-sequence star.
So, to read a common stellar classification, we look at the letter, number, and Roman numeral.
Stellar Spectral Types: Understanding Star Colors and Temperatures
The first letter tells us the star's spectral type.
Hot stars usually look blue or blue-white, while cooler stars look orange or red. Our Sun is often called a yellow star, although its light looks mostly white from space.
The seven main spectral classes are:
O, B, A, F, G, K, M
They're arranged from hottest to coolest.
Stellar Spectral Class Reference Table
| Spectral Class | Surface Temperature | General Color |
|---|---|---|
| O | 30,000 K and above | Blue |
| B | 10,000–30,000 K | Blue-white |
| A | 7,500–10,000 K | White |
| F | 6,000–7,500 K | White to yellow-white |
| G | 5,200–6,000 K | White, often called yellow |
| K | 3,700–5,200 K | Orange |
| M | Below about 3,700 K | Red |
Temperatures are approximate and measured in Kelvin (K). Star colors don't have sharp boundaries, and the colors in this table are simplified descriptions.
You might wonder why there are no green or purple stars in the table.
Stars give off many colors of light at once. Our eyes combine that light, so even a star that gives off a lot of green light usually looks white rather than green.
It's also important to remember that spectral class tells us about temperature, not size.
A cool red giant can be much larger than a hotter main-sequence star.
What Does the Number After the Letter Mean?
Each spectral class is divided into smaller groups numbered from 0 through 9.
Within a class, 0 is the hottest subtype and 9 is the coolest.
For example:
- G0 is near the hottest end of the G class.
- G2 is a little cooler.
- G5 is farther toward the middle.
- G9 is near the coolest end of the G class.
Our Sun is a G2 star, which places it near the hotter end of the G class.
That doesn't mean it's a certain percentage yellow. The number helps astronomers describe the star's temperature more precisely.
Stellar Luminosity Classes: Understanding the Roman Numerals
The Roman numeral at the end of a stellar classification tells us the star's luminosity class.
Luminosity is the total amount of energy a star gives off each second. Astronomers use luminosity classes to help describe a star's size and stage of life.
For most Traveller games, these five classes are the most useful to know.
Stellar Luminosity Class Reference Table
| Class | Star Type | Description |
|---|---|---|
| I | Supergiant | An extremely large, luminous star |
| II | Bright Giant | A large, luminous giant star |
| III | Giant | A star that has expanded beyond its main-sequence stage |
| IV | Subgiant | A star between the main-sequence and giant stages |
| V | Main Sequence | A star that steadily fuses hydrogen into helium in its core, like our Sun |
You might also see some less common classifications.
For example, Ia and Ib describe different kinds of supergiants. Some catalogs use VI for subdwarfs.
Not every star will fit neatly into these five groups, but they're a useful starting point for understanding Traveller star system information.
What Is a Main-Sequence Star?
You may have noticed that Class V is called the Main Sequence.
Most stars spend the longest part of their lives in this stage. They produce energy by turning hydrogen into helium deep inside their cores.
Our Sun is a main-sequence star.
As stars age, they can change. Some grow into giants or supergiants as they move into later stages of their lives.
There's one important thing to remember here. Luminosity classes aren't simply a ranking from brightest to dimmest.
For example, a very hot, massive main-sequence star can give off much more light than a cooler giant star.
That's why the spectral class and luminosity class work together to describe a star.
Putting It All Together: Three Stellar Classification Examples
Now that we've covered the parts of a stellar classification, let's look at three examples you might find in Traveller.
G2V: A Star Like Our Sun
- G: The star belongs to the G spectral class.
- 2: It's near the hotter end of that class.
- V: It's a main-sequence star.
A G2V star is like our Sun.
If a planet orbits at a suitable distance and has an atmosphere like Earth's, its daylight might look familiar to your players.
M2III: A Red Giant
- M: The star has a relatively cool surface.
- 2: It's near the hotter end of the M class.
- III: It's a giant star.
This is a red giant, a huge star with a reddish appearance.
Imagine your Traveller crew approaching an abandoned research station orbiting a red giant.
As they look through the observation window, the giant star fills much of their view, its reddish glow lighting the station's hull.
O7V: A Hot Blue Star
- O: The star is extremely hot.
- 7: It's toward the cooler end of the O class, although it's still far hotter than our Sun.
- V: It's a main-sequence star.
An O7V star is massive, blue, and extremely bright.
Imagine your crew arriving at a mining outpost in a system with a brilliant blue star.
The ship's sensors warn about intense ultraviolet radiation as the crew prepares to approach the station.
A little information about the star can give you plenty of ideas for setting a scene.
Explore Stellar Classifications with CyborgPrime StarScope
Now that you know how to read stellar classifications, you can experiment with them using CyborgPrime StarScope.
I created StarScope as a visual tool for exploring different types of stars.
Choose a spectral class, subtype, and luminosity class to explore an illustrated star and learn about its approximate temperature, size, and light output.
Try G2V first to explore a star like our Sun. Then switch to M2III for a red giant or O7V for a hot blue star.
Open CyborgPrime StarScope in a new window
StarScope is a visual learning tool for games and worldbuilding. Its illustrations and numerical values are approximate. Some combinations of stellar types and luminosity classes may be unusual or may not occur in nature.
Why Does Stellar Classification Matter in Traveller RPG?
When you're running a Traveller game, there are plenty of things to keep track of. You don't need to become an astronomer to make your star systems interesting.
But understanding a little about the star above a planet can help you describe the places your players visit.
For example, your crew might:
- Land at a busy starport beneath a brilliant blue-white star.
- Explore ancient ruins on a planet orbiting a cool red dwarf.
- Discover an abandoned research station near a giant star.
- Read stellar classifications from the ship's navigation computer while preparing for a jump.
- Visit a distant world where two stars shine in the sky.
You can also include stellar classifications in planetary survey reports, star maps, navigation records, and campaign notes.
Just remember that a star's classification doesn't tell you exactly what the sky will look like from a planet's surface.
The planet's atmosphere, clouds, dust, and distance from its star also affect what people see.
Give Your Next Traveller Star System Some Character
The next time you're creating a star system for Traveller, look at its stellar classification and think about what your players might notice when they arrive.
Maybe their ship passes beneath the glow of a huge red giant. Perhaps they're walking across a starport landing field with the light of a blue-white star shining overhead.
You don't need a long description. Sometimes one or two details are enough to help everyone picture the scene.
And when you want to see what different stars might look like, you can always experiment with CyborgPrime StarScope.
Sources and Further Reading
If you'd like to learn more about the science behind stellar classification, these resources are a good place to start:
Frequently Asked Questions About Stellar Classification
What Does G2V Mean in Stellar Classification?
G2V describes a star like our Sun. G is its spectral class, 2 tells us where it falls within that class, and V means it's a main-sequence star.
What Is the Difference Between Spectral Class and Luminosity Class?
Spectral class tells us mainly about a star's surface temperature. Luminosity class helps describe its size and stage of life. Together, they give us a better picture of the star.
Are Blue Stars Hotter Than Red Stars?
Yes. Blue stars have much hotter surfaces than red stars. That's why O-type stars are at the hot end of the spectral classification system and M-type stars are at the cool end.
Does a Star's Classification Tell You How Big It Is?
It gives you clues, but it doesn't tell you the star's exact size. A cool red giant can be enormous, while a hotter main-sequence star may be much smaller. You need more information to work out a star's size accurately.
How Can I Use Stellar Classifications in Traveller RPG?
You can use them to describe the stars your players encounter. A red giant might fill the windows of a space station, while a hot blue star might create a very different setting for an adventure. Stellar classifications also add useful details to planetary surveys, navigation reports, and star system records.
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