Let There Be Light

A star's journey begins as a cloud of hydrogen gas that, under the force of gravity, compresses over time. If the cloud is sufficiently massive, it generates enough pressure and heat in its core to initiate hydrogen fusion, aided by quantum tunneling. During this process, a small amount of mass is lost, converting into energy, as expressed by Einstein's famous equation, E=mc². Here, even minute quantities of mass (m) can yield enormous amounts of energy (E) when multiplied by the square of the speed of light (c²), approximately 9 x 10¹⁶ m²/s². This mechanism is what produces most of the light we see from stars, although larger stars can create heavier elements through fusion, ultimately leading to a supernova when they attempt to fuse iron.

But what exactly is this light? Light consists of perpendicular waves in electric and magnetic fields. When one field oscillates up and down, the other moves side to side, collectively forming what we call an electromagnetic wave (EM wave). EM waves encompass a range of phenomena, including X-rays, microwaves, radio waves, and visible light. The difference between these is their wavelength—the distance between peaks or troughs—and frequency, the rate of oscillation.

The EM Spectrum

The spectrum of light emitted by stars can be modeled through a theoretical concept known as a black-body. Black-bodies are idealized objects that emit electromagnetic waves solely based on their temperature. While they are theoretical, their predictions closely align with observed data.

The following diagram illustrates the relationship between wavelength, frequency, and temperature. As temperature rises, wavelength decreases, leading to higher frequency. For instance, an object at 3000K primarily emits infrared light, making it faintly visible. Conversely, a 6000K object emits light that peaks in the yellow part of the spectrum, appearing visibly yellow to us.

As temperature increases, the peak of emission shifts leftward along the EM spectrum. Extremely hot objects, such as those near black holes or neutron stars, can emit X-rays or even gamma rays.

The Universe Staring Back at Us

"We are stardust brought to life, then empowered by the universe to figure itself out—and we have only just begun." — Neil DeGrasse Tyson

The light we observe from stars often resembles an ideal black-body curve. Blue stars emit light primarily in the blue part of the spectrum, while red stars do so in the red. This relationship is depicted in the Hertzsprung-Russell diagram, which connects surface temperature with color. As stars deplete their fuel, their temperature declines, moving them down the main sequence.

However, a striking gap exists: there are no green stars. Even our Sun, which is closer to the yellow part of the spectrum, falls short of emitting green light. Why is this the case? The answer lies in the way our eyes detect light.

Human eyes have evolved to perceive three primary colors through cones sensitive to red, green, and blue light. When multiple cones are activated simultaneously, we perceive a blend of colors.

To visualize this, imagine a star whose black-body curve peaks in the green range. While it emits significant green light, it also produces substantial amounts of red and blue light. This overlap results in our perception of white light, as all three types of cones are stimulated.

In essence, stars that peak in blue or red emit light that primarily activates only one type of cone. On the other hand, a star peaking in green activates all three, leading to our inability to see a distinct green star.

Thus, it’s intriguing to realize that we cannot perceive the Sun as a true green star because our eyes did not evolve to recognize it as such.