The color of a star indicates its surface temperature. Stars come in a range of colors from cool red stars to hot blue stars. A star’s color is directly related to its surface temperature – the hotter the star, the bluer its color. By arranging stars by color from red to blue, we can rank them from coolest to hottest.
In astronomy, stars are typically classified by letters O, B, A, F, G, K, M in order of decreasing surface temperature. Each letter class is then further divided into numerical subclasses ranging from 0 to 9 – for example, A0, A1, A2 and so on – to indicate subtle gradations in temperature within each spectral class.
This system of stellar classification is known as the Harvard Spectral Classification. It classifies stars into 7 major types, from the hottest and bluest O type stars to the coolest and reddest M type stars. These 7 major spectral types can be remembered using the mnemonic “Oh Be A Fine Girl/Guy, Kiss Me”.
The 7 Spectral Classes from Coolest to Hottest
Here are the 7 primary spectral classes of stars in order from coolest to hottest:
M Stars
M type stars are the coolest and reddest stars of any type. Their surface temperatures range from about 2400 to 3700 Kelvin. Some examples of M type stars are Betelgeuse and Antares.
K Stars
K type stars have surface temperatures ranging from 3600 to 5200 Kelvin. They have an orange to reddish color. The Sun is a G type star.
G Stars
G type stars like our sun have surface temperatures of 5200 to 6000 Kelvin. They appear yellowish white in color. Our sun is a G2V type star.
F Stars
F type stars have temperatures from 6000 to 7500 Kelvin. They appear yellowish white to yellow white in color. Examples include Canopus and Procyon.
A Stars
A type stars have temperatures ranging from 7500 to 10,000 Kelvin. They have a white color with a slight blue tinge. Sirius and Vega are A type stars.
B Stars
B type stars have temperatures of 10,000 to 30,000 Kelvin. They have a blueish white color. Examples include Rigel and Spica.
O Stars
O type stars are the hottest and bluest stars with temperatures above 30,000 Kelvin. These very hot, very massive stars appear bluish in color. Examples include Zeta Puppis and Naos.
Detailed Temperature Ranges
Here is a table summarizing the temperature ranges for each of the 7 spectral classes from coolest to hottest:
| Spectral Type | Temperature Range (Kelvin) |
|---|---|
| M | 2400 – 3700 |
| K | 3600 – 5200 |
| G | 5200 – 6000 |
| F | 6000 – 7500 |
| A | 7500 – 10,000 |
| B | 10,000 – 30,000 |
| O | Above 30,000 |
As the table shows, M type stars are the coolest with temperatures below 3700 K, while O type stars are the hottest with temperatures exceeding 30,000 K. The increase in temperature correlates with the shift from red colors for the coolest stars to blueish colors for the hottest.
Color and Temperature
A star’s color is directly related to its surface temperature. Here is an overview of the color-temperature relationship for the main star types:
M – Red
M type stars have the lowest temperatures and appear red in color. Some subtypes of M stars like M8-M9 red dwarfs may appear very deep red. Their red color is indicative of temperatures below about 3700 K.
K – Orange to Red
K stars have moderately low temperatures and take on shades of orange and red. The early K classes like K0-K2 are more orange, transitioning to deeper reds in the mid to late K classes from K5-K9 as the temperature decreases further.
G – Yellowish white
G class stars like our sun have higher temperatures and thus appear yellowish white. Early G type stars closer to 6000 K may be pale yellow, while late G types may be yellowish. Overall they sit in an intermediate temperature range and color.
F – Yellowish white
F stars are hotter than the sun, and their higher temperatures produce yellowish white and at times yellow colors. They are transitional between the yellow G stars and hotter white A stars.
A – White/Blueish white
A stars are even hotter with temperatures approaching 10,000 K. This results in a white color, though sometimes with a slight blue tinge. They mark the transition from yellowish stars to the hot blueish ones.
B – Blueish white
B class stars are very hot, above 10,000 K, giving them a blueish white hue. The blue color starts becoming noticeable due to their high temperatures.
O – Blue
O type stars are extremely hot, with temperatures above 30,000 K. They appear distinctly blue in color due to their exceptionally high temperatures.
Star Classification and Evolution
A star’s spectral class gives insight into its properties and life cycle. The table below summarizes key facts about each spectral class:
| Spectral Type | Properties | Stage of Evolution |
|---|---|---|
| O | Very hot, very luminous, very massive, short-lived | Very early in lifetime, massive star |
| B | Hot, luminous, massive, short lifespan | Near start of normal massive star lifetime |
| A | Hot, luminous, larger mass | Middle of normal massive star lifetime |
| F | White in color, larger mass | Later massive star lifetime |
| G | Yellowish, lower mass like sun | Middle of normal lower mass star lifetime |
| K | Orange to red, low to moderate mass | Toward end of normal lower mass star lifetime |
| M | Red, lowest mass stars | Long-lived low mass stars near end of life |
In general, the hottest and most massive O, B, and A stars are very early in their lifetimes. G stars like our sun are midway through their stable lifetimes. K and M stars tend to be older stars of lower masses nearing the ends of their lives.
Star Size and Luminosity
In addition to color and temperature, a star’s size and luminosity can provide clues about its spectral class and properties.
O and B – Very Large and Luminous
O and B class stars are very large, reaching sizes over 6 or 7 times the Sun’s radius. They are also extremely luminous, with absolute magnitudes around -5 or brighter. Their high luminosity and large size match their extremely hot temperatures.
A – Large and Luminous
A stars are larger and more luminous than our sun but a bit smaller and less bright than O & B stars. They range from around 1.4 to 4 solar radii and have absolute magnitudes from around -1 to -5.
F – Moderate Size and Brightness
F class stars span the range between A stars and our G type Sun in size and luminosity. They range from about 1.2 to 1.7 solar radii and have absolute magnitudes from 1.5 to -0.5, bridging the gap between brighter stars and our sun.
G – Sun-like Size and Luminosity
Our G type sun has an absolute magnitude of +4.8 and a radius of 1 solar radius. So G stars have close to solar values for size and luminosity. Nearby yellow dwarfs also fall into this category.
K – Smaller Size
K type orange stars are smaller than the Sun with radii from around 0.7 to 0.9 times the solar radius. They are also less luminous than the sun. So K stars mark a transition toward diminishing size and brightness.
M – Very Small and Dim
Red M dwarf stars can be extremely small, ranging from 0.075 to 0.6 solar radii for the main sequence stars. They are also the dimmest with absolute magnitudes from around +10 to +16. So late K and M stars are decidedly small and faint.
Spectral Class Distribution
The different spectral classes are not equally distributed across space. Hot luminous O and B type stars are very rare while cooler K and M type stars are much more common.
Here is a table of the approximate distribution of stars by spectral type in our galaxy:
| Spectral Type | Percentage of Stars |
|---|---|
| O | 0.00003% |
| B | 0.13% |
| A | 0.6% |
| F | 3% |
| G | 7.6% |
| K | 12.1% |
| M | 76.45% |
As the table illustrates, M type red dwarfs are by far the most common, representing about 76% of all stars. Hot O and B type stars are extremely rare in comparison.
Remembering the Sequence
To summarize and help remember the spectral sequence from coolest to hottest, here are some mnemonics:
– Oh Be A Fine Girl/Guy, Kiss Me: O B A F G K M
– Only Big Astronomers Forget Great Knowledge Memorization: O B A F G K M
– Of Course Blue Actors Forget Good Karma, Man: O C B A F G K M
– Older Beings Always Feel Good Knowing Meditation: O B A F G K M
Whichever mnemonic you find most helpful, remembering the sequence from reddest/coolest M stars to bluest/hottest O stars can provide insight into the color, temperature, size, luminosity, evolution, and distribution of stars in our universe. The diversity of stars follows a pattern that we can understand through spectral classification.
Conclusion
In summary, a star’s color and temperature correlate strongly with its spectral class. From coolest to hottest, the sequence goes M, K, G, F, A, B, O. Red M stars have the lowest temperatures below 3700 K while hot blue O stars exceed 30,000 K. Intermediate classes follow the color and temperature sequence from orange to yellow to white to blueish white. The coolest red stars are the smallest and most common, while the hot blue stars are rare, massive, and luminous. Learning the star color sequence provides insight into stellar properties, evolution, and distributions. With this knowledge, we can better understand the diversity and life cycles of stars in our magnificent universe.