What is a Dwarf Star?
The term “dwarf star” was created by Ejnar Hertzsprung in 1906. He used it to distinguish prominent K and M-type stars that are either brighter or dimmer than the Sun. Those that are bigger and much brighter are called “giant stars.” The smaller and fainter ones are called “dwarf stars.”
With that in mind, we can easily remember that a dwarf star is any star with an average or relatively low size, mass, and luminosity.
The words “dwarf stars” are often used interchangeably with the distinct band of stars called the main sequence. That is because the majority of main-sequence stars are dwarf stars. These stars are yellow dwarfs, orange swarfs, and red dwarfs. Their luminosity class is represented by the Roman numeral V. Our Sun is in the main sequence. It is a yellow dwarf star.
Aside from stars in the main sequence, the word “dwarf” is also used for stars in the later stages of the stellar evolution, outside the main sequence. They are characterized by their relatively smaller sizes, hence the name “dwarf.”
A very low-mass star that is leaving the main sequence will become a blue dwarf. Its temperature increases as indicated by its color. Another star nearing the end of stellar life is a white dwarf. It will, later on, become a black dwarf.
There is also a low-mass substellar object called a brown dwarf. This object is neither a star nor a planet.
The colors of the stars say a lot about their characteristics. We can refer to the Harvard classification system for this, wherein blue (O type) is the hottest and red (M type) is the coolest). This system is based on the surface temperature of stars.
In the Morgan–Keenan (MK) system, numbers are added together with the letters. The numbers are from 0 to 9. Stars with the numeric digit 0 are the hottest while those assigned with 9 are the coolest.
Another feature of a star’s spectral type is its luminosity class. It is another thing that separates dwarf stars from the bigger giant stars.
The Hertzsprung–Russell diagram (H–R diagram) compares the luminosities of the stars against their stellar classifications or effective temperatures. In this diagram, we can see that most of the stars are in the distinct diagonal line called the main sequence.
Hot and bright stars are in the upper-left part of the main sequence. The cooler and less bright dwarf stars are in the lower-right. Subgiant stars, as well as giants and supergiants, are above this distinct band of stars.
We can see that only a few stars populate the lower-left part of the diagram where the white dwarfs belong.
The full spectral type of a star tells us a lot about it. We can use our knowledge about the colors, the numeric digits, and the luminosity classes indicated to distinguish dwarf stars apart from other types of stars.
Types and Life Cycle of Dwarf Stars
We often hear about dwarf stars but not all of them are the same type. To easily distinguish them from one another, we can put them in three groups. The word “dwarf” can mean stars in the main sequence, stars that are in the later stage of stellar life, or substellar objects.
In the Main Sequence
A yellow dwarf star is also called a G dwarf star. It is of luminosity class V in the main sequence. Since it is in the main sequence, it generates energy through the fusion of hydrogen to helium in its core.
The mass of a G-type main-sequence star equates to about 0.84 to 1.15 solar masses. Its surface temperature ranges between 5,300 and 6,000 K.
Yellow dwarf starts its life from a stellar nebula. They enter the main sequence when they become massive enough and start hydrogen fusion. These stars will continue fusing hydrogen for about 10 billion years.
When the nuclear fuel is finally exhausted, a yellow dwarf will become bigger and enter the red giant phase. After that stage, it will expel its outer layers which will result in a planetary nebula. What remains is the core of this star which becomes denser over time, becoming a white dwarf star.
More massive stars will become red supergiants after the main sequence. After that, they will explode in a supernova and collapse into a black hole or a neutron star.
An orange dwarf is a K-type main-sequence star. The range of its mass is between 0.5 and 0.8 solar masses. Its surface temperature is between 3,900 and 5,200 K. Just like any stars in the main sequence, it is of luminosity class V.
Orange dwarfs stay in the main sequence for about 18 to 34 billion years, longer than yellow dwarfs. It is for that reason that they are good candidates for terrestrial planets.
The life cycle of an orange dwarf follows the same path as a yellow dwarf, only some billion years longer.
The Milky Way is dominated by red dwarfs that make up approximately three-quarters of its stars. However, we can not see a single one of them with the naked eye because of their low luminosity. These small stars have low temperatures and are very faint.
Red dwarf stars are also called M-type main-sequence stars. They can be found in the lower end of the main sequence band. They generate energy through the thermonuclear fusion of hydrogen. Their mass is about 0.8 times that of the Sun’s mass, with a maximum temperature of 5,200 K.
Hydrogen fusion in red dwarfs with less than 0.35 solar masses is sustained for a long time because they are convective. With that said, there is no helium buildup in its core because of the constant mixing of material. This process is maintained for a long time, taking billions or trillions of years.
After years and years of nuclear fusion, the hydrogen in a red dwarf will be consumed. It causes the fusion rate to slow down. Shortly after, the core will begin to contract and it will become a blue dwarf. When it no longer supports fusion in its core, it will become a white dwarf, and eventually a black dwarf.
The later forms of red dwarfs are still “hypothetical remnants.” None of them has reached the advanced stages of their stellar life yet. Their lifespans are longer than the current age of the universe which is about 13.8 billion years.
Dwarf Star: In the Later Stages of Stellar Life
A blue dwarf star is a theoretical remnant of a red dwarf star. This is still a predicted star class because no red dwarf has advanced in the later stages of their life yet.
A red dwarf becomes a blue dwarf when it uses up much of the hydrogen in its core. Its luminosity increases as it gets older. To radiate this energy, red dwarfs increase their surface temperatures. This is evident in the hotter and bluer color of the star.
A white dwarf is a star remnant made up of electron-degenerate matter. There are two ways in which it is formed, either from red dwarfs or average size stars like our Sun. Young white dwarfs radiate X-rays, bathing the surrounding space.
Around 75% of the stars in our galaxy will end up as white dwarf stars. These stars have an estimated mass of 0.17 to 1.33 solar masses. Most of the white dwarfs observed have surface temperatures that range from 8,000 K to 40,000 K.
A white dwarf is very dense. It can be as big as the Earth but retains about half of its former stellar mass. No thermal fusion is happening in a white dwarf anymore. Its heat is from the trapped thermal energy within it. This heat can only escape by radiation. This cooling process will take trillions of years.
A black dwarf is formed when a white dwarf star already cools. By that time, the star no longer emits light or heat. This is still a hypothetical remnant because the universe is still young relative to the time period of the cooling of a white dwarf.
Aside from the fact that a black dwarf does not exist in the universe yet, detecting it in the far future will be a challenge. That is because this star remnant will no longer emit significant radiation. This cold and dark object will be practically invisible.
A Substellar Object
A brown dwarf is neither a star nor a planet. It is often called a “failed star” as it forms just like any other star in the beginning. The thing is, it did not become as massive as other stars to support the fusion of hydrogen into helium in its core. It can only fuse deuterium.
The mass of a brown dwarf is about 0.08 times the mass of the Sun or 13 to 80 Jupiters.
Notable Dwarf Stars
Yellow Dwarf: Sun
The Sun is the most important star in our Solar System. With all the planets around it, it is responsible for about 99.8% of the mass of the whole system.
It is an example of a G-type main-sequence star. Its spectral type is G2V. Though it is a yellow dwarf, we can sometimes see it as orange or red especially during sunrise or sunset.
The color of the Sun as we see here on Earth is affected by our planet’s atmosphere. But with all that color related to our Sun, it is important to note that it is actually white in color when seen from space.
The Sun looks yellow in our perspective because sunlight is scattered when it enters the Earth’s atmosphere.
Other notable yellow dwarfs are the Sun-like stars 51 Pegasi, Tau Ceti, and Alpha Centauri A.
Orange Dwarfs: Alpha Centauri B
Alpha Centauri B is an orange dwarf that is also called Toliman. It is a part of the multiple-star system of Alpha Centauri. The spectral type of this star is K1 V.
The apparent magnitude of this orange star is 1.35. It has only about 90% of the solar mass. Its diameter is smaller by about 14%.
Another example of an orange dwarf star is Epsilon Indi. Its spectral type is K5V. It is less massive than the Sun, only about 75% of the solar mass.
Red Dwarf: Alpha Centauri C
Alpha Centauri C, or better known as Proxima Centauri, is an example of a red dwarf. This star is notable because it is the closest star to our Sun at a distance of only about 4.2465 light-years away.
The mass of this small star is only about 12.5% of the solar mass and its diameter measures 14% that of the Sun’s. We cannot see it with the naked eye because its apparent magnitude is only about 11.13.
The second closest star to us, Barnard’s star, is also a red dwarf. Gliese 581 is also another example of this type. Its spectral type is M3V. This M-type star is approximately 20 light-years from us.
White Dwarf: Sirius B
Sirius B is the secondary component of the Sirius star system, the brightest star in our night sky. Average white dwarf stars are between 0.5 to 0.6 solar masses but Sirius B has 1.02 times the solar mass. Its surface temperature is 25,200 K.
This star is very dense because its mass is packed in a size comparable to the Earth’s. It is cooling and will continue to do so for two billion years or more.
The first white dwarf discovered is 40 Eridani B and our nearest solitary white dwarf neighbor is Van Maanen 2.
Brown Dwarf: Teide 1
Teide 1 is in the open star cluster of Pleiades. It is as massive as about 57 Jupiters but only has 0.0544 of the solar mass. The surface temperature of this substellar object is approximately 2,600 K.
Teide 1 was the first verified brown dwarf. It is of spectral type M8. Its age is estimated to be 120 million years old. It lies about 400 light-years from us.
The closest known brown dwarfs to us are in Luhman 16. It is a binary system of two dwarfs designated Luhman 16A and Luhman 16B. This system is roughly 6.5 light-years away.
Dwarf Star: Additional Facts
- Orange dwarfs are great candidates for the possibility of extraterrestrial life because of many reasons. Aside from a longer life than yellow dwarfs, they are more abundant. Their emission of ultraviolet radiation is lesser as well.
- Orange dwarfs are more advantaged than the red dwarfs in terms of habitability. K-type stars do not have problems with a planet being tidally locked to it. On top of that, its habitable zone is wider than the red dwarfs.
- The possibility of life around a white dwarf is not impossible. One good thing about it is its energy output is stable, unlike a red dwarf. But for a planet to be habitable, it would need to be really close to the star to have liquid water. It means the exoplanet will be tidally locked.
- Brown dwarfs use spectral classes in terms of surface temperature. These are spectral types M, L, T, and Y.
- What are the different types of dwarf stars?
- How does a white dwarf form?
- What two dwarf stars are called “hypothetical remnants?” Why are they called that way?
- Why is a brown dwarf called a “failed star?”
- What are some examples of dwarf stars?