Why Do Stars Twinkle at Night?

Key Points

• Stars twinkle due to the refraction of light as it passes through the Earth’s atmosphere.
• The color of stars is determined by their temperature, with hotter stars appearing blue or white and cooler stars appearing reddish.
• Light from stars takes years to reach Earth, with the term ‘light-year’ defining the distance that light travels in one year.
• Planets do not twinkle because they are closer to Earth and their brightness balances out the effects of refraction.

The night sky is full of beauty, mystery, and scope that is impossible to comprehend. Stars are perhaps the most enduring feature of any night sky — little pinpoints of light vibrating and twinkling eternally. The effects of light traveling such a great distance have a solid scientific foundation but still remain misunderstood by the general public. In this article, we will investigate the reason why stars twinkle, and the other unique effects of light when seen from Earth.

History of Starlight

Twinkling starlight is an indelible part of the human experience. It has influenced our perception of the universe and our place in it since the dawn of time. It has also informed parts of culture through art, poetry, religion, and science. Before modern science, mankind relied on stars and their light for important parts of society. Everything from the zodiac wheel of astrology to the cycles of seasons was measured via the light of stars.

The first person to make any kind of scientific classification of starlight was the astronomer Hipparchus. As an ancient Greek geographer and mathematician, Hipparchus is called the “father of astronomy.” This is due to his likely invention of trigonometry, as well as his creation of the first comprehensive catalog of stars in the Western world.

Hipparchus, however, was limited to his eyesight. It was only much later, with the invention of lenses and then telescopes that humans acquired a better understanding of stars and the light they emit. This opened up the field of astronomy. Everyone from Copernicus to Isaac Newton contributed to the understanding of stars, their movements, and their unique properties of light.

Modern Day

From the 1950s onward, the United States Army spent untold millions on a specific telescope that amplified starlight. This special device also picked up diffused moonlight, as well as the fluorescence of rotting vegetation fifty thousand times.

Now, with the advent of space telescopes sent out into the cosmos, people can appreciate the scale. The beautiful, almost hallucinatory light properties of stars have been shown in dazzling images released to the public. Many of the more recent images have come from the James Webb Space Telescope. This new instrument can see stars over thirty-three billion light years away and capture light from thirteen billion light years in the past.

Unique Properties of Starlight

Light is a fascinating property of the universe that scientists are only beginning to understand. When speaking of the light emitted by stars, it’s important to make a few distinctions. For one, starlight refers to the visible waves of radiation on the electromagnetic spectrum that are produced by the intense nuclear reactions occurring within stars.

Frequency of Light

While it has been shown by astronomers that stars reflect some light, the majority of the light emitted that we can see is a result of nuclear fusion inside the star. Fusion, put simply, is when less dense elements are forced into denser elements. This process creates an incredible amount of energy. It also causes stars to heat and shine with electromagnetic radiation, which we see as light.

The color of stars as they appear in the sky is also the result of this fusion process. On average, stars that appear blue or white are hotter stars. Stars that give off a reddish light are cooler stars. This is because, on the color spectrum, blue and white have a higher frequency and shorter wavelength, causing them to travel farther distances. Colors like red, yellow, or orange have a lower frequency in the light spectrum so they don’t travel as far.

It should be noted, too, that if a star is big enough, it will travel a farther distance even if it emits a reddish light. The average color of starlight reaching Earth is shaded a whitish-yellow — a color referred to as the “cosmic latte.”

Light Years

As Einstein famously proved, space equals time. This means that when you look at the night sky, you are technically looking at a record of the past. Since stars are so far away, it takes years upon years for their light to reach Earth.

The term “light-year” defines that unique fusion property of distance and time and is described as the distance light travels in a vacuum in one year. This can be confusing to the layman, as years are commonly used as a measure of time, and light years refer more to space. Used to explain astronomical distances, a light year is equivalent to 5.88 trillion miles.

Light from the nearest star system, Alpha Centauri, takes about four and a half years to reach our planet. Light from a star system that is farther away, such as the Orion Nebula, takes up to fifteen hundred years to reach us. That means many of the stars we see in the night sky no longer exist, even if the memory of their light still does.

Star Systems

If you were able to travel to the constellation Alpha Centauri, however, chances are it would take much longer than four and a half years. This is because a light year refers to the speed of light. Until humans figure out how to even come close to approaching such a speed, a trip to Alpha Centauri could take centuries at slower speeds.

The star SMSS J031300.36-670839.3 has been identified as one of the oldest stars in the visible universe. Visible from Earth, this star is only 6,000 light years away but is at least 13.8 billion years old. This means the SMSS J031300.36-670839.3 is nearly as old as the universe itself.

Why Do Stars Twinkle?

The interesting thing about the twinkling of stars is that it’s a trick of light as seen on Earth. Stars, in fact, do not inherently twinkle, but light has some interesting properties that make it appear that way. When light travels from one medium to another, it bends in a phenomenon known as refraction.

If it’s going from a rare to denser medium light, it bends towards normal. When it moves from a dense to rarer medium, light bends away from the normal. The medium also affects the speed at which light travels. This is best exemplified by how light is affected when passing from air to water. The change from the rare medium of air to the denser medium of water refracts the light, causing all sorts of strange effects.

This principle also governs how light is seen from stars. Earth’s atmosphere is made up of different densities than space and is shifting constantly due to wind, pressure, and temperature. When the light from a star reaches Earth, it passes through many different densities in our atmosphere, causing refraction.

Some light from stars passes through the atmosphere directly, while some does not, bending instead. Plus, space is referred to as a vacuum, but there are untold amounts of other light radiation and cosmic dust refracting starlight. All these combined result in their unique twinkling.

Viewing stars from different vantage points on Earth will result in the twinkling effect diminishing or increasing. This is simply because there are more or less atmospheric densities creating a series of refractions for the starlight.

Why Don’t Planets Twinkle?

Planets visible from Earth, however, don’t twinkle. When looking at a star in the night sky, it appears as a single point of light. This is due to the distance the light has to travel to reach Earth. Planets are much closer to Earth, and their prominence makes them appear as more than a single point of light. The brighter effect of the light balances out the dimming points of light that are being refracted. This results in a more stable light source that doesn’t twinkle.

The image featured at the top of this post is ©iStock.com/mik38.