Why Do Stars Shine?

When you go outside at night and see the sky filled with bright, twinkling stars, have you ever wondered why stars shine in the first place? Incredibly, the light you’re seeing has traveled through space for years until finally reaching your eyes.

However, not everyone is fortunate enough to go outside and this view these mysterious lights. Those who live in cities often have little luck seeing stars due to light pollution. In fact, a massive blackout in 1994 left many Los Angeles residents in a panic. Residents called 911 and the local observatory describing strange clouds appearing overhead. They were actually just seeing the milky way for the first time. Those living in rural areas such as the countryside, however, will be able to see many stars at night due to little light pollution.

How Do We Know How Stars Shine?

In 1938, Hans Bethe discovered that the high energy collisions of protons could create enough force to power the Sun. In fact, Bethe won the 1967 Nobel Prize in Physics after developing and detailing a proton-proton chain of reactions that allows hydrogen to convert into helium during these collisions. This proton-proton cycle was found to drive 98% of the Sun’s energy production in the core. This allows the Sun to shine for about 10 billion years. With this process, our Sun has provided Earth with heat and light for the past 4.5 billion years.

Here’s how it all works.

Why do Stars Shine?

Stars are able to shine due to the immense heat within their cores. While the surface temperature of our Sun is a “cool” 9,941°F, at its core, temperatures can reach 30,000,000° F. Even the pressure reaches 250 million atmospheres (more than 150 times the density of water). These high temperatures are responsible for thousands of hydrogen protons to fuse, releasing huge amounts of energy that radiates from the core.

These hydrogen protons fuse together to form deuterium, a hydrogen atom that contains just one proton and one neutron. During this collision, one of the protons become a neutron while a tiny positron and tiny neutrino are emitted. Pure energy in the form of gamma rays is also released. Additionally, the deuterium collides with another proton to form a completely new element, helium (He). Again, energy is released in the form of gamma rays. These processes are the physical result of the equivalence of matter and energy, as seen in Einstein’s famous equation, E = mc2. Simply put, stars shine because hydrogen matter is literally converted into pure energy.

How Do We See This Energy?

While photons force their way towards the outer layers of the star, gravity pulls them back in – maintaining the stars spherical shape. This forces these light photons to follow a “random walk” path, bouncing around inside the star. During this process, they are continuously absorbed and emitted again, taking up to 100,000 years and over 432,000 miles to get from the core to the surface until they finally break free. After losing some of their energy, light photons escape to the outer layers as visible light photons, not gamma rays, protruding into space at 186,000m/s – the speed of light.

These photons continue to travel through space, uninterrupted until colliding with another object. They can travel for the rest of time if they don’t run into anything. For example, if you’re seeing a star, then the light photons from it have traveled across space until finally reaching your eyes. Since we know the distance to the sun and the speed of travel of the photons, then we can calculate that it takes 8 minutes for the light photon that has just escaped the Sun to travel to Earth letting you see its luminous glow.

When you look at a more distant star such as Sirius, you see light that left the surface 8 years ago since the star sits 8 light years away. For 8 years, these light photons traversed space without running into anything until reaching your eyes.

So, why do stars shine? Their huge fusion reactors inside their cores release huge amounts of energy that escape the surface as light photons.

Why do Stars Shine and Twinkle?

Stars do not actually twinkle. Although variable stars do in fact fluctuate in brightness, this fluctuation is not actually visible to the naked eye here on Earth. Since stars are so far away from Earth, the Earth’s atmosphere refracts the stars light. Depending on the temperature and density layers the light passes through, the star’s light is directed at different angles. This makes the starlight to appear as though it is twinkling. This is especially true when stars are low on the horizon. When looking overhead, you peer through roughly 200 miles of air and atmosphere. When you’re looking towards the horizon, you can be looking through as much as 1,200 miles of air and atmosphere. Therefore, the horizon amplifies the atmospheric effects of the twinkling starlight.  

Why Do Stars Shine Red Sometimes?

Sometimes when you look at the sky, you may see stars that appear redder than others. Although you’d expect this to just be another atmospheric trick, these stars are actually red. For example, if you find the star Aldebaran in the constellation Taurus, you’ll see the “Bull’s eye” shining bright red and orangish. This massive star is a red giant, an advanced evolutionary stage of what our Sun will be in approximately 5 billion years.

In fact, sometimes these red “stars” might even be planets such as Mars! Although this may look similar to a star in the sky, planets only reflect light, they don’t create it. Even though planets have hot cores, they are not massive enough to increase the pressure in the core to reach the temperatures required for fusion. Remember, the answer to “Why do stars shine?” is simply fusion inside the star’s core.