The color of a star indicates its surface temperature. Stars come in a range of colors from hot, blue stars to cool, red ones. The coolest stars have the lowest surface temperatures. Identifying the coolest colored stars requires an understanding of stellar classification and temperature.
Stellar Classification
Astronomers categorize stars by temperature using the stellar classification system. This system assigns letters to stars based on their surface temperature. The major categories are O, B, A, F, G, K, and M. Stars are arranged from hottest to coldest as follows:
| Spectral Type | Temperature (K) | Color |
|---|---|---|
| O | Over 30,000 | Blue |
| B | 10,000-30,000 | Blue-white |
| A | 7,500-10,000 | White |
| F | 6,000-7,500 | Yellow-white |
| G | 5,000-6,000 | Yellow |
| K | 3,500-5,000 | Orange |
| M | Under 3,500 | Red |
As this table shows, O-type stars are the hottest with temperatures over 30,000 K. At the other end, M-type stars are the coolest with temperatures under 3,500 K.
Red Dwarfs
The coolest and dimmest main sequence stars are red dwarfs. Red dwarfs belong to the M spectral class and have surface temperatures below 3,500 K. The coolest red dwarfs can have temperatures around 2,000-3,000 K.
Some key facts about red dwarfs:
– They have a reddish color due to their cool temperature.
– They comprise about 70% of the stars in our Milky Way galaxy.
– They have very long lifetimes that can exceed trillions of years.
– They are dim with luminosities between 0.0001 – 0.0004 times that of our Sun.
– They have small masses, usually 0.075 to 0.5 times the mass of our Sun.
– Examples include Proxima Centauri, TRAPPIST-1, Lacaille 8760, and Ross 128.
While all red dwarfs are cool, some red dwarfs are even cooler than others. The coolest red dwarfs are classified as L, T, and Y dwarfs.
L, T, and Y Dwarfs
In addition to the standard stellar classification for main sequence stars, there are three special classes for the coolest substellar objects:
– L dwarfs – Cooler than M dwarfs with temperatures 1300-2400 K
– T dwarfs – Cooler than L dwarfs with temperatures 700-1300 K
– Y dwarfs – Even cooler than T dwarfs with temperatures under 700 K
L dwarfs are the coolest main sequence stars, while T and Y dwarfs are substellar objects sometimes called brown dwarfs. Brown dwarfs don’t have enough mass for nuclear fusion like stars, but they are hotter than planets.
Here is an overview of L, T, and Y dwarfs:
| Type | Temperature (K) | Characteristics |
|---|---|---|
| L dwarf | 1300-2400 | Very red color, lithium detection, overluminous |
| T dwarf | 700-1300 | Methane absorption lines, molecular hydrogen absorption |
| Y dwarf | <700 | Ammonia absorption lines |
As this table demonstrates, Y dwarfs with temperatures below 700 K are the coolest type of brown dwarfs discovered so far.
The Coldest Brown Dwarfs
Astronomers are discovering brown dwarfs with increasingly cooler temperatures. Here are some of the coldest brown dwarfs found to date:
– WISE 0855−0714 – Estimated temperature between 225-260 K, the coldest known brown dwarf. Too faint to be seen in visible light.
– WISE J085510.83−071442.5 – Another Y dwarf with an approximate temperature of 250 K. Orbits the Sun at a distance of 7.2 light-years.
– WISE J004701.06+680352.1 – A Y dwarf with an estimated temperature below 300 K. Located about 19 light-years from Earth.
– WISE J1217+1626 – Y dwarf with a temperature estimate of 330 to 370 K. Approximately 14 light-years from Earth.
– WD 0806-661B – One of the first Y dwarfs discovered, with an approximate temperature of 300-345 K. Orbits a white dwarf star about 19 light-years away.
These ultracool Y dwarfs represent the lowest temperature objects currently known outside of the Solar System. Lower temperature objects likely exist but have so far evaded detection.
Theoretical Colder Objects
While Y dwarfs include the coolest stars and brown dwarfs actually detected, astronomers have proposed even cooler types of objects that may exist:
– Black dwarfs – Cores of burned-out stars with temperatures below Y dwarfs. No black dwarfs are known to exist yet.
– Q-class objects – Hypothetical objects cooler than Y dwarfs, with temperatures below about 150 K. Their composition is uncertain.
– R-class objects – Entirely theoretical objects predicted to have temperatures below 100 K. No R-class objects have been detected.
– S-class objects – Theoretical objects similar to R-class but with different chemical compositions. Purely hypothetical at this point.
– Zero-temperature black holes – Black holes with theoretically zero temperature, the coldest objects predicted by physics.
It’s possible these proposed ultracool objects may be identified in the future as observing technology improves. But for now, they remain unproven and speculative. Y dwarfs continue to mark the lower limit of directly measured temperatures.
Measuring Stellar Temperatures
Astronomers use several methods to measure the temperatures of cool stars:
– Spectroscopy – Analyzing starlight reveals properties like temperature. Cooler stars emit light at longer wavelengths.
– Photometry – Measuring star brightness across different wavelengths. Comparing brightness at different colors indicates temperature.
– Parallax – Measuring a star’s distance lets astronomers calculate its intrinsic luminosity. Combining with photometry provides temperature estimates.
– Spacecraft – Some nearby cool dwarfs have had temperatures directly measured by spacecraft like NASA’s Spitzer and Hubble space telescopes.
– Modeling – Computer models can provide temperature estimates based on known properties for a star’s mass and composition.
While hotter stars are easier to analyze, advances in infrared astronomy have allowed the discovery and temperature measurement of cooler red, brown, and white dwarfs.
Role of Temperature in Star Properties
A star’s surface temperature directly impacts many of its other properties including:
– Color – Temperature determines colors from blue (hot) to red (cool).
– Brightness – Hotter stars produce more visible and ultraviolet light. Cooler stars emit more infrared.
– Lifespan – Hotter stars burn through fuel faster than cool stars. Red dwarfs can persist for trillions of years.
– Habitability – Cooler stars may be more likely to host habitable planets without overheating them.
– Composition – Cooler stars contain molecules like methane and ammonia rarely seen in hot stars.
Understanding stellar temperatures provides insights into the star’s evolution, history, and potential planetary systems. Measuring the coolest stars pushes the limits of astronomical observation techniques.
Conclusion
The coolest stars are red, brown, and white dwarfs with spectral classes M, L, T, and Y. Brown dwarfs with temperatures below 700 K are known as Y dwarfs. The coldest detected brown dwarf is WISE 0855−0714 with an estimated temperature of 225-260 K. While theoretical objects cooler than Y dwarfs have been proposed, none have yet been confirmed observationally. Measuring and studying the coolest stars provides insights into properties like their composition, evolution, and potential habitable zones. Advances in astronomical methods continue to push the detection of ever colder and dimmer objects.