bright star

Why do stars twinkle (and planets don’t)

By Jake Riley, SR Senior Editor

Stars are an ever-changing natural phenomenon - tiny speckles in the sky that illuminate the night. We all know that they exist, but what do we REALLY know about them?

How are stars formed, why do they twinkle and how does the Sun play into all this?

Let’s find out together, shall we. We’ll start with how stars are born.

How stars are born

It's a well-documented fact that stars are born from clouds of gas and dust.

The clouds are massive reservoirs of dust, gas molecules, and other particles that can collapse in on themselves because of their gravitational pull. The light emitted by the collapsing dust creates new stars.

The starlight is distributed as uniform spheres with a temperature gradient at most points equaling some 250 K (Kelvin). That's 10,000 times warmer than the temperature of the surface of Mars. Pretty unbelievable, right?

And we know what happens when we heat a solid sphere: thermodynamics dictates that heat is equally distributed over the surface since heat always moves from hot to cold.

The situation becomes more complicated when a cloud is thick enough to start collapsing in on itself and create starlight. The light gets trapped in the cloud, causing it to glow as a whole. It also becomes less likely for light to move from warmer regions into colder ones. This causes the light to twinkle. This is called "diffuse emission".

Another factor is that clouds can change temperature over time because of their surroundings. This means that different layers of a cloud can have different temperatures at a given point in space. The amount of energy radiated by such clouds varies, creating a pattern likening to sunlight shining through the water. Such light is called "refracted".

The Sun is more of a star in this respect. The heat comes from the core, so it's much easier to control. The Sun is about 2,000 times smaller than its nearest neighbor, Sirius, yet it glows twice as bright. This means that the light from the Sun is moving much faster — some 1,400 km/s (or 3 million km/h or 6 million mph).

How do stars die

A typical star is not constant in size. It changes from moment to moment, getting hotter and colder and becoming larger or smaller.

The star's energy flux (energy released per second) also fluctuates sometimes hundreds of times a second. The stars can rapidly change their size and brightness because they have an internal structure known as "neutron degeneracy".

Once the "neutron degeneracy" is removed, the star will collapse due to its gravity — without being able to support its weight anymore. Therefore, the death of a star is unpredictable, even for the most massive stars. When it comes to smaller or average-sized stars, however, we know exactly how they die: they turn into nebulae and then black holes.

Why stars twinkle

There are more than 100 billion stars in the observable universe. Almost all of them appear to twinkle (but planets don't).

Why is that?

The twinkling of stars is caused by a phenomenon called scintillation, which occurs when light from a star passes through Earth's atmosphere. According to NASA's website, scintillation is still not very well understood, but we know that it has to do with turbulence in the Earth's atmosphere.

Stellar twinkling is not just a visual phenomenon. It is also a subtle and important feature of the universe. Astronomers can use it to understand how the universe evolved from its birth until the present day.

One example of stellar twinkling is reflected interferometry, in which a telescope sends light through two telescopes in succession. When this technique was first used in 1994, astronomers discovered that previously invisible stars were twinkling. This indicated that stars' mass and age had been accurately determined for the first time.

The twinkling of stars also plays a crucial role in studying how stars move. One of the most essential processes in astronomy is tracking stellar movement. The twinkling of stars is useful for this process.

Why planets don't twinkle

Planets do not twinkle or appear to because they are so far from our telescopes. Earth-based telescopes can only detect stars within a specific range of stellar brightness’s.

Since planets are generally much dimmer than stars, they are invisible to most telescopes at night. This means that unless an astronomer specifically sets out to observe a planet, it will not show up because it is masked by starlight.

Astronomers can use techniques to detect planets using different parts of their light. One of the most reliable techniques is to watch for a planet's slight dimming when several stars are in the field of view.

Why does the Sun not twinkle?

The Sun is too far away from Earth to be seen twinkling due to the Earth's atmosphere.

The atmosphere causes the twinkling in stars. However, it can still be affected by sunspots, which are spots on the Sun's surface that look like dark bruises caused by a sudden expansion of hot gas ejected from its interior.

The dark spots are cooler than the rest of the Sun, and they block a portion of the light that Earth's atmosphere normally filters out. So when viewed through a telescope, the dark spots can appear to twinkle.


Generally, stars twinkle because of their distance from the Earth, and by the time their light gets to the Earth, it is bent (refracted) by the Earth's atmosphere and appears to twinkle.

The proximity of the Sun to the Earth makes it not twinkle since instead of being viewed as a tiny dot in the sky, it is seen as a significant source of light.

The planets do not produce as much heat and light as stars; hence, they can only be viewed when focusing on them.