Stars have fascinated humans for millennia. Ancient astronomers grouped stars into constellations and used them for navigation and to track the seasons. Today, we understand stars much better thanks to advances in astronomy and physics. We know that stars are massive spheres of plasma undergoing nuclear fusion. We also know there are many types of stars with different properties like mass, temperature, and luminosity. One of the most important classifications for stars is the main sequence. The main sequence represents the primary stage of a star’s lifetime where it fuses hydrogen into helium in its core. Main sequence stars make up around 90% of the stars in the Milky Way galaxy. But do main sequence stars change color over their lifetimes? Keep reading to find out.
What is the Main Sequence?
The main sequence is a region on the Hertzsprung-Russell diagram where most stars lie. The H-R diagram plots a star’s luminosity versus its temperature or color. The diagram was created independently by Ejnar Hertzsprung and Henry Norris Russell in the early 1900s. By plotting thousands of stars on the diagram, astronomers discerned patterns and relationships between luminosity, temperature, mass, and star classification.
The main sequence forms a curved band running diagonally across the H-R diagram. It includes stars that fuse hydrogen into helium in their cores. This nuclear fusion powers the star and keeps it in a state of hydrostatic equilibrium where the thermal pressure balances the force of gravity. Main sequence stars range from hot, blue, massive stars to cool, red, low-mass stars. Our Sun is a main sequence star at around middle age.
| Star Type | Mass (Solar Masses) | Temperature (Kelvin) | Luminosity (Solar Luminosities) | Color |
|---|---|---|---|---|
| O5V | 43 | 42,000 | 500,000 | Blue |
| B0V | 18 | 30,000 | 25,000 | Blue-white |
| A0V | 3.2 | 10,000 | 80 | White |
| F0V | 1.7 | 7,300 | 6.7 | Yellow-white |
| G2V (Sun) | 1.0 | 5,800 | 1.0 | Yellow |
| K0V | 0.79 | 5,200 | 0.40 | Orange |
| M5V | 0.21 | 3,200 | 0.0055 | Red |
This table shows the range of main sequence star types, masses, temperatures, luminosities, and colors.
Do Main Sequence Stars Change Color?
When a star is first born, it contracts under gravity until the core becomes hot and dense enough to fuse hydrogen into helium. Once this happens, the star enters the main sequence where it remains for most of its life. But does a main sequence star change color and temperature over its lifetime?
The short answer is yes, main sequence stars do change color as they age. However, the changes are very gradual and happen over billions of years. Here’s why:
- As a main sequence star fuses hydrogen into helium, the helium builds up in the core.
- The accumulating helium makes the core more massive and dense over time.
- A more massive core exerts more gravitational force and pressure on the rest of the star.
- The increased pressure raises the rate of nuclear fusion, releasing more energy.
- A hotter, more energetic core causes the outer layers of the star to expand and cool.
- As the outer layers expand and cool, the star becomes larger and redder over billions of years.
Therefore, main sequence stars slowly change color from blue to white to yellow to red as they age. Here’s an estimate of the timescales involved for stars like our Sun:
| Age (years) | Color |
|---|---|
| 0 – 1 billion | White |
| 1 – 6 billion | Yellow |
| 6 – 10 billion | Orange |
| 10+ billion | Red |
Our Sun is currently middle-aged at around 4.6 billion years old. Over the next 5 billion years, it will gradually expand and become redder as its core accumulates more helium ash.
Stellar Evolution Off the Main Sequence
Eventually, a main sequence star will use up all the hydrogen fuel in its core. At this point, nuclear fusion stops and the core starts to collapse from its own gravity. As the core collapses, it heats up further and starts fusing helium into carbon and oxygen.
Helium fusion causes the outer layers of the star to dramatically expand and cool. This moves the star off the main sequence into the red giant branch on the H-R diagram. The change happens very quickly compared to the slow changes on the main sequence. Over just a few million years, a star’s radius can expand to over 100 times its original size. Temperatures drop to under 5,000 K causing the star to become an extremely large, bloated, and cool red color.
Our Sun is predicted to become a red giant in around 5 billion years, expanding out to near Earth’s current orbit. The dramatic expansion represents the end of the main sequence phase where hydrogen is fused into helium.
Do More Massive Stars Change Color?
More massive O-type and B-type stars also change color over their lifetimes. However, the effects are more extreme than for stars like our Sun.
Massive stars consume their nuclear fuel much quicker than low mass stars. An O-type star may only remain on the main sequence for 5-10 million years before its hydrogen runs out. These stars start out with very hot, blue-white colors due to their high mass and luminosity.
As the O-type star ages on the main sequence, its increasing helium core makes the outer layers expand and cool. This makes the star appear yellow or orange later in its lifecycle. By the time the star is ready to leave the main sequence, it has cooled significantly from its starting temperature.
For example, an O5V star may start out at 42,000 K giving it a blue appearance. But within 10 million years, the temperature may drop to around 25,000 K making the star appear white. This represents a large color change in cosmic terms.
Wolf-Rayet stars demonstrate this change in an extreme way. These stars represent the final evolutionary stage of the most massive stars (>25 solar masses). Through intense stellar winds, a Wolf-Rayet star sheds its outer hydrogen layer to reveal a bright, hot helium core. The core then fuses heavier elements until the star ends its life in a supernova explosion. Wolf-Rayet stars start off blue because of their intense luminosity and temperature. But the revealing of the core makes the star appear yellow, like our Sun.
Conclusion
Main sequence stars do change color over their lifetimes, but the changes are gradual. As a main sequence star ages, the build-up of helium in the core causes the outer layers to expand and cool. This makes the star become larger and redder over billions of years. More massive stars exhibit more extreme color changes due to their shorter main sequence lifetimes. But in all cases, the changes are caused by an evolving balance between the core fusion source and the gravitational pressures on the star.
Understanding how and why stars change leads to a deeper knowledge of stellar evolution. This helps astronomers predict the lifecycles of stars across the wide range of types and masses that populate our dynamic universe.
