Stargazing is a wonderful experience. When you first start stargazing, you find yourself lost in the beauty and vastness of the nighttime sky.
You also come to the realization that stars are not only white in color as you may have previously believed. Stars shine in different colors.
Read on to find out what color are stars and why they have these colors. Ready? Let’s get started!
What Color Are Stars?
Stars emit a multitude of colors such as blue, white, yellow, orange, and red. These colors are indicators of the surface temperature of the stars. Blue stars are hotter while red stars are cooler.
In addition to determining the temperature, the star’s color also sheds light on the age of the star.
Does that answer the question—what color are stars?
Reasons Why Stars Are Different Colors
Those who took chemistry in school must have experienced how flames change colors when burned with different substances.
The addition of alum can make a flame green. With potassium chloride, you will get a beautiful purple flame, and so on.
If you observe an open flame up close, you will notice different hues of colors. The outer layers of the flame usually show reddish hues, whereas the inner part of the flame glow bluish-white.
Similarly, stars vary in colors due to their effective temperature, composition, wavelength, age, and distance from the planet. Their colors are determined with the help of a color index. The color of a star tells us a lot of in-depth details of a star.
You will find stars come in various colors like blue, white, orange, yellow, and red. The stars are observed to determine their color and constituting elements using spectro-analysis.
Temperature is the main determining factor when it comes down to deciding a star’s color, along with the other altering factors mentioned above.
Temperature
Every star has a different temperature. Wien’s law is used to determine the color of a star. This is essentially the relationship between an object’s emission of electromagnetic radiation and temperature.
Based on the colors, we can tell how hot or cold a star is. Stars with cooler temperatures radiate infrared energy that gives off red tones on the spectrum.
Stars with hotter temperatures radiate ultraviolet energy that gives off blue and white tones on the spectrum.
The coolest stars appear red. As temperatures rise, the colors shift to orange and yellow. The hottest stars give off hues of blue.
The temperatures of the coolest red-colored stars start at 5,400 degrees Fahrenheit. The hottest have surface temperatures of over 45,000 degrees Fahrenheit.
Wavelengths and Elements
When exposed to heat, you will notice that different elements emit varying wavelengths of electromagnetic radiation. The color we see is a perception of electromagnetic radiation, or in simple terms, what we call light.
Stars are a mix of main constituents (helium and hydrogen) and various other trace elements. These elements are responsible for the color of the stars. The color visible to us is a result of a mix of the different electromagnetic wavelengths known as Planck’s curve.
Stars have a peak wavelength along the curve at which they emit the most light. This is the determining factor of a star’s actual color.
Temperature variations affect the wavelength of the star. As we know, the color is influenced by the temperature.
The cooler the star is, the longer will be its wavelength, and the hotter it is, the shorter will be its wavelength.
Age
How old a star is can be determined by observing its color. Compared to older stars, younger stars emit different colors.
The age of a star and its lifespan can be determined with the help of stellar classification. There are various stellar classification systems. One such system is the Hertzsprung-Russel Diagram.
The Hertzsprung-Russell Diagram maps star color in relation to the temperature. It also helps show the biggest and smallest stars and the location of the dimmest stars.
Stars give off different elements as they age. These elements burn at varying temperatures which affect the star’s color. Very hot gas makes up stars. This gas consists mostly of helium and hydrogen, along with other elements.
When a star is born, its main constituent is hydrogen. The hotter a star is, the bluer it will be. Their hot temperature means they burn hydrogen rapidly and burn out at a faster rate. These stars are relatively younger.
Older stars are usually red. They are cooler than their blue counterparts and burn at slower rates. This increases their expected lifespan.
On one end of the spectrum, the bluest and brightest of stars have a life expectancy of 10 million years. On the opposite end, the dimmest and reddest of stars’ expected lifespan is up to 100 billion years.
Sun lies in the middle of the spectrum. It has a lifespan of 10 billion years, and at present, it is halfway through it.
Distance
The above factors determine the color of light emitted by a star. However, there is a difference between emitted light and the light we observe.
This is where the Doppler Effect comes into play. The frequency of light, sound, and other waves changes depending on the distance between the source and observer.
In astronomy, this is known as blueshift and redshift. This concept allows astronomers to gauge how far something is in relation to our position on Earth.
Remember, visible light is made up of different colors. These colors each have different wavelengths. Red features the longest wavelength while violet has the shortest.
When the distance between a star and our planet increases, its wavelength also increases. Therefore, when a star moves away, it reflects a red tone. This is the redshift. On the other hand, a star that is moving towards us will emit blue light. This is the blueshift.
How Are Star Colors Determined?
The wavelength or color is a measure of a star’s magnitude. The color is important in determining a star’s temperature. In order to determine a star’s exact color, astronomers use filters to measure its apparent brightness.
Each filter will only transmit light from a specific band of colors. An everyday example is if you hold up a green-colored bottle to your eye, it will only allow green colors to pass through.
A blue filter will make a blue star look brighter. A red filter will make a red star look brighter. In this way, filters are useful in helping astronomers determine a star’s exact color.
Color Indices
A color index is used to plot stars. Since the measured brightness is expressed in magnitudes, a color index of any star is the difference between two magnitudes.
Many sets of filters are used in astronomy to measure brightness. The commonly used ones correspond to yellow-green, blue, and ultraviolet light.
The magnitude decreases for brighter stars and increases when a star is duller. Based on the color index, if the magnitude of a star is small, it is bluer. However, if it is large, it is redder.
-0.4 to +2.0 is the range of the color indexes. The negative values are for the bluest stars, while the positive values are for the reddest stars. Conventionally, for the B-V index, it is blue minus red.
How Are Stars Classified?
Morgan-Keenan System
The modern classification of stars includes their color, temperature, brightness, and size. A majority of the stars are classified using the Morgan-Keenan system.
The MK system of classification is based on temperature. It uses the letters M, K, G, F, A, B, and O, where M is the coolest and O is the hottest.
Numeric digits are used to further divide each letter class. In this subdivision, 9 is the coolest and 0 is the hottest. For example, M9 to O1 is a range of coldest to hottest stars.
The luminosity class is indicated with Roman numerals. It helps distinguish dwarfs from giant stars.
Supergiants fall in the luminosity classes I and II. Giants and subgiants come under classes III and IV. The main sequence stars fall under class V. Subdwarfs and dwarfs belong to classes VI and VII, respectively. However, these classes are not conventionally used.
Hertzsprung-Russell Diagram
Besides the MK system, the Hertzsprung-Russell diagram plots a relationship between the surface temperature of a star and its color. It relates stellar classification to luminosity, surface temperature, and absolute magnitude.
The measure of a star’s brightness if it were placed at a set distance gives us the absolute magnitude.
Stars to the diagrams left have higher surface temperatures while the upper end of the diagram features stars with greater luminosity.
Stars Colors Visible to the Human Eye
The way we perceive colors and our atmosphere affects how we see various stars. While we can see stars, we do not necessarily see their true color.
A star’s color is determined by its peak wavelength. Outside the turbulence of our atmosphere, stars’ true colors shine through.
Certain stars like Arcturus and Betelgeuse shine bright enough that their colors are visible to the naked eye in areas where the pollution is moderately light.
Others like Sirius B require a small telescope and a dark clear sky to be spotted across the sky. On the contrary, Sirius A holds the title of the brightest star in our night sky. It flickers in many colors.
With the help of binoculars and small telescopes, you can spot various colored stars in the sky. Even on the clearest and darkest of nights, it can be difficult to discern the true star color with the naked.
The best season to view colored stars is during winters.
During evenings across the northeastern sky, you can spot Capella, a golden star belonging to the Aurigain constellation.
You should also be able to spy the bright red Alderbaran which belongs to the Taurus constellation.
Frequently Asked Questions
Are There Green or Purple Stars?
There are no green or purple stars. Star colors are linked to their temperatures. Stars do not emit light in a single wavelength. They emit a range of colors.
Even if a star peaks at the green wavelength, there are other wavelengths being emitted in equal amounts. When these colors mix together, it results in white color. Therefore, we do not get green stars. Instead, we get white stars like the sun.
Purple stars, on the other hand, are not conceivable to the human eye. Even if a star emits purple light, it also emits a lot of blue. Since the human eye has a higher sensitivity to blue than purple, our eyes pick up blue light over purple.
What Color Is the Sun?
On the visible spectrum, the sun emits all wavelengths. These wavelengths combine and result in white light.
The color of the sun we perceive on earth differs from its actual color. The color differs due to our atmosphere. When light travels through the atmosphere, it interacts with the particles present in it. These particles scatter the colors of the spectrum, and our eyes perceive the different colors.
What Is the Color of a Dying Star?
Once stars use up all their hydrogen fuel, they die. As a star runs out of hydrogen, it expands to become a red giant. This results in a cooling effect on the outer layers.
Betelgeuse, a star in the Orion constellation, is nearing its end. It is a red supergiant.
Summary
Not every star in the sky is white. Stars exist in a multitude of colors. Based on the color of the star, we can deduce several factors such as their surface temperature, distance, composition, and age.
However, these factors change over time. The changes mean a star does not remain a constant color throughout its lifespan. It undergoes various color changes depending on its phase in the lifecycle.
Our unaided eyes are usually not able to perceive the true colors of stars. This is why astronomers use specialized equipment to gauge a star’s true color.
Some stars are so bright that we can catch glimpses of their true hues on the clearest of nights.
