There are a few stars that appear to change color in the night sky. The most well-known of these is Betelgeuse, a red supergiant star located in the constellation Orion. Betelgeuse undergoes periodic changes in brightness and color, which is why it sometimes appears reddish-orange and other times more yellowish. Other stars that are known to change color include variable stars like Cepheid variables and binary stars. The color changes are caused by various factors like shifts in temperature, pulsations, eruptions, eclipses and more. This article will take a closer look at some of the main stars that change color and what causes their shifting stellar hues.
Betelgeuse
Betelgeuse is a semiregular variable red supergiant located approximately 642.5 lightyears from Earth in the constellation Orion. It is one of the largest known stars, with a radius over 1,000 times that of our sun. Betelgeuse normally has a distinct orange-red color but does change in brightness and hue. In late 2019 and early 2020, Betelgeuse went through an unprecedented dimming event where it dropped to only about 35% of its normal brightness. During this fainting spell, the star also changed color, appearing more yellowish than its typical rusty orange tone.
Betelgeuse’s shifts in luminosity and color are believed to be related to pulsations and changes in the star’s surface temperature. As a variable star, Betelgeuse has always fluctuated a bit, but the recent great dimming was exceptional. Astronomers think pulses may have ejected clouds of hot plasma from the star, cooling its surface and making it appear more yellow. As Betelgeuse returns to normal pulsation patterns, its color is reddening again as well. So the red supergiant’s colors changes are intrinsically linked to its brightness variations.
Cepheid Variables
Cepheid variables are pulsating variable stars that periodically change in brightness as they expand and contract. There are several types of Cepheids, including classical Cepheids and type II Cepheids. Classical Cepheids pulsate very regularly over periods from 1 to 100 days. As their diameters oscillate, they also change in temperature, causing them to fluctuate in color between blue and red. Generally these stars appear yellowish-white but become bluer when they are smaller and brighter and redder when they are larger and dimmer. Delta Cephei is the prototype Cepheid variable that illustrates this color-brightness connection.
Type II Cepheids undergo less regular pulsations linked to their evolving off the main sequence. Their periods are longer, often months or many months. Similarly to classical Cepheids, type II Cepheids also shift from bluer hues at maximum brightness to redder tones at minimum light. So pulsations in Cepheid variables lead their colors to modulate along with their brightnesses.
Eclipsing Binaries
An eclipsing binary consists of two stars that orbit around a common center of mass. When seen from Earth, each star periodically eclipses its partner as they go around their orbits, causing regular changes in brightness. In some cases, the two stars in the binary system have different colors. As one blocks out the other, the color of the combined light changes.
A good example is the star Algol (Beta Persei) in the constellation Perseus. Algol is composed of a large bluish main-sequence star and a smaller orange subgiant star closely orbiting each other. When the orange star passes in front, the system appears redder. When it moves out of the line of sight, the blue light dominates, making the color look more blue. The eclipsing nature of this binary leads to a distinct color fluctuation from blue to orange and back every 2.87 days.
Other Causes of Color Changes
In addition to pulsations and eclipses, other processes can lead to color variations in stars:
– Eruptions – Some unstable stars like LV Vulpeculae and V838 Monocerotis undergo sporadic eruptions and outbursts that temporarily change their spectral signatures and perceived colors.
– Atmospheric changes – As stars deplete hydrogen fuel, their atmospheres change, resulting in shifts in color. Our sun is very gradually getting hotter and more white over billions of years as hydrogen in its core is converted to helium.
– Evolution – Stars evolve over timescales of millions or billions of years, passing through different stages that alter their colors. Newly born stars are often reddish. Aging red giants become redder as their outer layers expand and cool.
– Accretion – Matter falling onto the surface of white dwarfs, neutron stars or black holes releases radiation that makes them appear bluer when actively accreting material from a companion star.
– Relativity – The relativistic beaming effect causes the color of objects rotating near the speed of light, like pulsars, to shift between blue and red as their beam sweeps by Earth.
So from pulsations to eclipses to stellar evolution, many processes can drive color variability in stars across the cosmos.
Notable Color-Changing Stars
| Star | Type | Color Variability |
| Betelgeuse | Red supergiant | Normally orange-red, fades to yellow |
| Delta Cephei | Cepheid variable | Pulsates between blue and red |
| Algol | Eclipsing binary | Changes between blue and orange |
| LV Vulpeculae | Erupting variable | Shifts from blue to red in outbursts |
| V838 Monocerotis | Variable star | Turned from blue to red after 2002 eruption |
This table summarizes some key color-changing stars along with their stellar classification and nature of color variability. Betelgeuse and Algol are some of the most conspicuous and well-studied stars that periodically shift hue in the night sky.
Viewing Color-Changing Stars
The color fluctuations in stars like Betelgeuse and Algol are visible even to the naked eye under dark skies over intervals of days, weeks or months. Looking at these stars with steadily held binoculars or a telescope over time can highlight their color changes. Photographs and spectral measurements also record their shifting stellar spectra.
Apps like Stellarium can show the relative colors of stars at any point in time – useful for seeing how a star’s hue differs and varies. Some citizen science projects use observations by amateur astronomers to study long-term color variability and changes in stars. So you can directly witness the fascinating interplay of color and brightness in our dynamic stellar neighborhood both with your eyes and photography.
Conclusion
In summary, there are a number of stars that exhibit intriguing color variability in our skies. Pulsating stars like Cepheids, eclipsing binaries like Algol, unstable variables like Betelgeuse and other unique stars demonstrate shifting hues of blue, orange, yellow, red and more. Careful observation over time reveals the remarkable changes in stellar colors. Astronomical spectroscopy gives further insights into these color changes by studying how starlight is emitted and absorbed. So keep gazing up at the night sky, the kaleidoscopic heavens above! Colorful stellar magic awaits.
