Star Types and Star Classification – Everything You Need to Know – Part II

by Sankalan Baidya
star types

In our last article on Star Types and Star Classification, we covered three major types – Yellow Dwarf, Orange Dwarf, and Red Dwarf stars. We also covered the hypothetical Blue Dwarf stars that do not exist but have a possibility of existence in several trillion years from now. Despite having discussed so much, we are still nowhere even close to the end. In this second part of Star Types, we will take a quick look at a few more star types that are found in our universe or are hypothesized to exist. We will continue this article as an extension of our previous article and hence, we will skip the part where we talked about the classification system that we have used so far. In case you want to refer to the classification system, feel free to refer back to the first part of the article.

Let us start…

Star Types: White Dwarf Stars

These are Dead Stars. They have already crossed their Main Sequence stage and hence, they don’t have any more Hydrogen fuel left to burn it into Helium through the process of nuclear fusion. Here is a quick table to understand White Dwarfs Quickly:

Colloquial NameType of StarEvolution StageExample
White Dwarf StarDDead Star (Crossed Main Sequence Stage)Sirius B

Wondering what this D-type is?

This D-type is a whole new ‘Class’ of stars placed under the modern classification system. D stands for ‘Degenerate’ and is used for grouping those stars that have low mass and are out of the Main Sequence stage, that is, they are no longer undergoing nuclear fusion reaction (basically White Dwarfs). These stars have now shrunk to planetary size and are undergoing a slow cooling process. Type D is used only for White Dwarf Stars.

Unlike the previous star types that we have studied (that is Yellow Dwarf, Orange Dwarf, and Red Dwarf), the subdivision of D-type stars is different. They are subdivided by their spectral type using the following:

Star TypeProperty
DAOuter layer or atmosphere rich in Hydrogen
DBAtmosphere rich in Helium and indicated by spectral lines of neutral Helium.
DOAtmosphere rich in Helium indicated by spectral lines of ionized Helium.
DQAtmosphere rich in Carbon and indicated by molecular or atomic carbon lines.
DZAtmosphere rich in metals.
DCThere are no spectral lines at all indicating that it can be from any one of the aforementioned categories.
DXSpectral lines are there but not clear enough to put them in any of the aforementioned categories.

The second letter used in subdividing the D-type stars or White Dwarfs are nowhere related to the classification of other stars.

There can be a further subdivision of the White Dwarf Stars based on their surface temperature. This subdivision is done by adding numerals at the end of each type. Digits between 1 and 9 are usually used but more recently values above 9 and below 0 are also used. Even fractional values are used. Example for such subdivision will be DA1, DA2, DA3… etc., where 1, 2, 3 etc. denote the surface temperature of White Dwarves.

What does Degenerate mean?

As we said, White Dwarf stars have evolved beyond the Main Sequence stage. This means, they no longer have fusion reaction at their cores. This, in turn, means no light emission. At this point, a star will grow in size to become a Red Giant and start fusing Helium to produce Carbon and Oxygen.

What happens when Helium runs out? That’s when the star will start fusing Carbon provided it has sufficient mass. If there isn’t enough mass, the core will now be composed on just inert Oxygen and Carbon (known as fermion gases). Eventually, the outer layer of the star will be ejected into space leaving only the core behind.

The absence of any nuclear reaction at this stage means that there will be no outward pressure to balance out the inward pull of the gravity. Technically, the core should collapse on itself because of the gravity. But that doesn’t happen! Why? The answer to this question is there are fast-moving electrons in the core that exert an outward pressure and prevent the star from collapsing. This is known as electron-degeneracy.

However, electron-degeneracy can prevent the collapse of the core only if the mass of the core is within the Chandrasekhar Limit (that is, 1.4 solar masses). If the mass of the core is above that, the core will collapse.

Do White Dwarves fuse Carbon?

Some White Dwarves do fuse Carbon provided they have the necessary mass to do so. Those that fuse Carbon will be made of Oxygen, Magnesium, and Neon. There are some White Dwarf Stars that can’t even fuse Helium because they simply don’t have the necessary mass.

Did you know? Theoretically, a White Dwarf Star cannot have mass greater than 1.4 solar masses. However, there is a glitch in this. It doesn’t take into account that the White Dwarf in question may be spinning at a very high speed. There are some White Dwarf stars that have masses greater than the Chandrasekhar Limit and still they don’t collapse on themselves. This is possible because they spin at a very high speed, preventing such collapse.

White Dwarf Atmosphere

Researchers have found out that White Dwarf stars have an atmosphere of usually Hydrogen and Helium. These gases are separated out into their individual pure form because of the star’s intense gravitational fields. These gases then surround the star and work as a cladding, thereby preventing the star from losing its heat quickly.

What happens to White Dwarf eventually?

Theoretically, these stars will eventually lose their heat and become Black Dwarves but unfortunately, that will take hundreds of billions of years and perhaps trillions. That’s why no one will be around to see what actually happens to a White Dwarf.

In case a White Dwarf is formed in a binary star system, it may actually pull material from the companion star and its mass can increase. In such a scenario, either a fusion reaction will star in the core leading to type-1 Supernova or the core will collapse even further and end up as a Neutron Star. These results can also happen if two White Dwarf stars collide with each other and merge.

General Characteristics of White Dwarf Stars or D-Type Stars

  • Generally, White Dwarf stars have a mass of approximately 0.1 Solar Mass to 1.4 Solar Masses. However, they can be bigger.
  • The surface temperature of the White Dwarf stars ranges between approximately 8,000 Kelvin to 40,000 Kelvin.
  • The radius of typical White Dwarf stars is generally between 0.008 Solar Radius and 0.2 Solar Radius.
  • All White Dwarf stars have crossed the Main Sequence stage.
  • Their lifespan can be several hundred billion years or even trillions of years.
  • Approximately 4% of the stars in our universe are White Dwarf stars.

Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint point of light to the lower left of the much brighter Sirius A.

Did you know? A typical White Dwarf is about the size of Earth and should have the mass equivalent to that of our Sun.

Star Type: Brown Dwarf Stars

Colloquial NameType of StarEvolution StageExample
Brown Dwarf StarM, L, T, YNon-Main Sequence StarLuhman 16

Brown Dwarf stars are pretty interesting. They are often called ‘failed stars’. Some call them ‘sub-stellar’ objects as well.

Here are a few of things you need to know properly. Just ensure that you get a very clear idea of this:

  • They fill up the gap between the ‘LEAST MASSIVE TRUE STARS’ and the ‘MOST MASSIVE GAS PLANETS’.
  • Usually, they have a mass that ranges from 13 to 80 Jupiter masses.
  • Sub-brown dwarfs have a mass below the aforementioned mass range while ‘LEAST MASSIVE RED DWARF STARS’ have a mass greater than the aforementioned mass range.

With that information in mind, let us carry forward…

Brown Dwarf stars are really, really small. Their mass is also too low as far as stars are concerned. This is the reason why people have asked, ‘why not just tag them as a planet?’

It turns out the Brown Dwarf stars have nearly the same size as our largest planet Jupiter irrespective of the mass the hold.

Why can’t we call Brown Dwarf stars as planets?

The reason is simple! The International Astronomical Union forbids tagging them as planets based on the definition they provide. The definition clearly states that any body with a mass equal to or greater than 13 Jupiter masses is a Brown Dwarf.

This mass is actually a cut-off point as it is the minimum mass that is required by deuterium to undergo thermonuclear fusion.

What is deuterium? Deuterium is heavy Hydrogen and is denoted by 2H. It is not the ordinary Hydrogen.

Interestingly, Brown Dwarfs do not have enough mass to fuse ordinary Hydrogen in their core but the cut-off mass as per the definition predicts that they fuse heavy Hydrogen. In case a Brown Dwarf has a mass of 65 Jupiter masses, it will fuse Lithium. This explains why Brown Dwarf stars are not considered as planets.

Why do Brown Dwarf Stars have different spectral types?

You may have noticed from the small table above that the Brown Dwarf stars have 4 different spectral types – M, L, T, and Y. How is that possible?

Before we answer this question, we will like to tell something.

Brown Dwarf Stars are not always brown. They have different shades.

It is because of this, we have different spectral types! Let us take a look at the different colors that Brown Dwarf stars can have:

  • Class M: Class M or M-type is, as we known, known as Red Dwarf Stars. However, those that are towards the cooler end of the spectrum are often considered as Brown Dwarf Stars and are often referred to as “Late-M Dwarves”. These Late-M Dwarves are kind of a link between the Red Dwarf Stars and Brown Dwarf Stars.
  • Class L: The stars that fall in this category are basically purple-red in color. The L spectral class means that when scientists studied the absorption bands of the stars’ optical spectra, they saw alkali metal bands and metal hydride bands. So, any Brown Dwarf Star with alkali metal bands and metal hydride bands is classified as L-type.
  • Class T: These are basically the classic Brown Dwarf Stars as they have dark red color. This is the color that pops out in the absorption bad of the optical spectra.
  • Class Y: These are the coolest of all Brown Dwarf Stars. We don’t mean that ‘yo man you’re cool’ type cool. We mean cool in sense of temperature. The Y-type Brown Dwarves were once thought to be purely hypothetical but sometime in the second decade of the 21st century, astronomers managed to find several of these really, really cool Brown Dwarf stars. They are so cold that they don’t give off light. Currently, there are 14 Y-type Brown Dwarf Stars.
star types

Artist’s impression of a Y-dwarf star

General Characteristics of Brown Dwarf Stars

  • Generally, Brown Dwarf Stars have a mass range of 0.01 Solar Mass to 0.08 Solar Mass. More aptly their masses are judged with regards to Jupiter. They have a mass range of 13 Jupiter Masses to 80 Jupiter Masses.
  • The surface temperature of Brown Dwarf Stars ranges between 300 Kelvin and 2,800 Kelvin. However, the Brown Dwarf called WISE 0855−0714 discovered in 2014 has a temperature range of -48°C and -13°C (that is 225.15 Kelvin and 260.15 Kelvin).
  • They have a typical lifespan of several trillion years.
  • Brown Dwarf Stars are non-Main Sequence Stars.
  • Approximately between 1% and 10% of the stars in our universe are Brown Dwarf stars.

Star Types: Black Dwarf Stars

Okay, let us be very clear at the very beginning. THEY DON’T EXIST or at least no one has ever found one so far. They are completely theoretical. Astronomers say that when White Dwarf Stars radiate off all their heat, they will eventually become Black Dwarf Stars. The only problem is that no one knows when that will happen and it is estimated that a White Dwarf will take enormously long time – longer than the current age of our universe (that is 13.5 billion years). So, by that time, neither will our Sun exists and nor will anyone be around to witness a Black Dwarf Star.

However, there are a few things to consider. Let’s take a look:

  • Cosmic Background Radiation: Theoretically, White Dwarf Stars will eventually cool down to the temperature that is close to or equal to the Cosmic Background Radiation. At that point, Black Dwarf Stars cannot be seen but their existence can be understood because they will have mass and hence, gravitation. The problem is that cooling down to that temperature will take trillions of years and by then, the Cosmic Background will have itself cooled down even further because of continuous expansion of the universe. So, for a White Dwarf to cool down to such an extent will take even more time. It will more or less become an endless cycle.
  • Surrounding Interactions: White Dwarf Stars travel through space and in the process, they happen to interact with various other celestial objects such as interstellar dust, other White Dwarfs etc. All these interactions culminate into an increased temperature of the White Dwarf Stars and hence, longer cooling time.
  • Proton Decay: There is an awful lot of debate going on around this. Some say that proton decay is real and happens over billions of years of time while some other simply choose not to accept the theory. There is no experimental evidence in support. However, the Grand Unified Theory says that proton decay happens. If proton decay is real, there is a release of heat. Proton decay will take place in White Dwarf Stars as well and over billions of years, will increase the temperature of the White Dwarf Stars. This will eventually result in slowing down of the cooling process and hence, even greater time for formation of Black Dwarf Stars.
  • Dark Matter: Often the elusive Dark Matter is also brought in by some experts who say that White Dwarf Stars do not only interact with Dark Matter but also contain Dark Matter. Interaction with Dark Matter should warm up the White Dwarf Stars and hence, slowing down their cooling process.

So, with all these factors taken account of, it is not likely to see the formation of Black Dwarf anytime soon. Even if these factors are all eliminated, it will take at least 1,000,000,000,000 years for a White Dwarf to turn into a Black Dwarf. Well… we need an immortality pill, quickly!

Okay, we are not done yet! There are Giant stars, Supergiant stars, Hypergiant stars, Wolf-Rayet stars and more. We will cover them gradually in upcoming articles.

Sources: 1, 2, 3, 4, 5

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