Hertzsprung Russell Diagram

Star Spectral Types – Part 1 Hertzsprung Russell

Posted on Posted in Black Hole, Gravity, Physics, Science, Space

Star Spectral Types Series: Part 1 Part 2 Part 3

Our Milky Way Galaxy contains between 100 and 400 billion stars (depending on your definition, and explained later!) – and all of them are unique, not to mention other galaxies. To help catalog this vast expanse of stars, in 1912, Ejnar Hertzsprung and Henry Russell Norris developed the Hertzsprung-Russell Diagram (as it was later called), to categorize the different types of stars.

Today, it describes most of the stellar mass objects out there (with a few exceptions), and with a handy mnemonic Oh Be A Fine Girl/Guy and Kiss Me – OBAFGKM – you can remember star types from highest surface temperature to lowest. Note that a large star volume isn’t necessarily indicative of high temperature – in fact the largest supergiant stars we know of out there are relatively cool red supergiants like VY Canis Majoris. In addition, each star type is divided into 10 subcategories where a number is placed after the letter classification, with 0 being the warmest, and 9 being the coolest. For reference, our Sun is a G2 star, meaning it is among the warmer of the G class stars.

So why do colors radiated by the stars with the highest temperatures are one we humans traditionally associate with cold – blue? The answer lies in the amount of energy the star outputs per area. Heat and temperature are energy, and higher energy stars, unsurprisingly, emit higher frequency wavelength light. In the visible light spectrum, red is the color with the longest wavelength (around 700 nanometers) and thus lowest energy. Purple and blue are the colors in the visible spectrum with the shortest wavelengths and highest energy (around 400 nm wavelengths).

Most of a star’s life is spent in the main sequence, and as they age, they start to run out of hydrogen, some of higher mass stars become giants or supergiants and start fusing higher mass elements (one way we have elements heavier than hydrogen), at which point they either go supernova/form a black hole/neutron star (the only way nature creates elements heavier than iron), or form a planetary nebula/white dwarf.

A NASA illustration of the diagram is below:

Hertzsprung Russell Diagram

Image Source: Public Domain/NASA Chandra X-ray Observatory and Harvard University

In addition to the Hertzsprung-Russell classification (referred to as HR class hereafter), there are a number of pre/suffixes attached to the HR class of the star in modern astronomy that is based upon stellar luminosity called the Yerkes spectral classification. This is according to the total mass/gravitational pull of the star, age, which determines the surface density of the star, which in turn affects the emitted stellar spectral lines. We will be concentrating on the suffixes today as sd and D don’t usually apply to HR classed stars.

  • Extremely luminous super or hypergiants. VY Canis Majoris mentioned above is a M2.5I, for example. Sometimes this is divided again into more subcatagories (Ia, Iab, etc.) to differentiate between the brightest stars.
  • II bright giants.
  • III normal giants.
  • IV subgiants.
  • V main-sequence stars (dwarfs). Our Sun (Sol) is a G2V, for example.
  • sd (prefix) subdwarfs
  • D (prefix) white dwarfs.

Here is a size comparison between VY Canis Majoris and our Sun – it’s a pretty stark difference!


Image source: Public Domain/Wikimedia

Here’s a rundown of selected stars under the HR class and a few that are not:

O Class: The most rare, luminous, and high temperature stars fit into this class. Remember all of the talk about wavelengths above? These stars are so energetic that most of their output is outside of visible light in the ultraviolet range, which have even higher wavelengths than blue or purple light. The modern definition for a star to classify as O is the emission of a particular ratio of nitrogen spectral lines.

As the most massive stars they burn/fuse (read more about nuclear fusion in my previous post here) through their hydrogen fuel faster than any other type of star and leave the main sequence the “fastest”. This is still on the order of millions of years however!

An example of these stars would be S Monocerotis (O7V), which is located within the Cone Nebula the bright ball of blue at the top in the following European Space Agency (ESA) image. Note that the star is located approximately 800 parsecs or 2600 light years away!

Cone Nebula

Image Source: CC By SA 4.0 ESA

B Class: Less rare than Class O, but still bright, blue and luminous, the modern definition defines Class B stars as ones that emit particular ratios of Silicon emission spectra.

Examples include Rigel/Beta Orionis (B8Ia)


Image Source: CC by SA 4.0 by ESA

and Achenar/Achernar/Alpha Eridani (B6V), as seen from the International Space Station (the bright spot above the Earth):


Image Source: Public Domain/NASA

A Class: Due to A Class stars being a combination of sufficiently luminous and relatively common (0.625% of stars near the sun), these are the stars most commonly seen in the night sky with the naked eye. They are defined as stars that emit strong Hydrogen spectral lines, with a healthy dose of Iron, Magnesium, Silicon, and Calcium.

One such example of these stars is Formalhaut/Alpha Piscis Austrini), recently in the media due to the massive debris rings found around it:


F Class: F Class stars have weaker Hydrogen spectra than A class stars, and also show a stronger Calcium spectra than Class A.

Polaris, often referred to as the North Star, is an F7I. You can see its much less luminous companion stars Polaris Ab (F6V) and Polaris B (F3V) in the following Hubble Telescope shot, which are F type stars as well. This is also a perfect example of how stars in the same HR class/temperature/color can differ quite a bit in luminosity on the Yerkes classification scale. Think of the Polaris system like this: Polaris A is a bunch of bright light bulbs, while both Ab and B are just a single bulb of the same brightness – the bunch of bright bulbs will outshine the singular ones.


Image Source: Public Domain/NASA Hubble Space Telescope

The following is a NASA artist’s rendition of the Polaris triple star system:


Image Source: NASA/ Public Domain

G Class: Having even weaker Hydrogen spectral lines than F, and keeping the strong Calcium spectral lines, our own Sun is one of these (G2V). Alpha Centauri A, one of the closest stars to us is also a G class star (the exact same G2V designation as our Sun). They generally are visibly white (as most of their electromagnetic emissions are in the visible light range) – the Sun only appears yellow or orange on Earth because of Rayleigh Scattering due to the atmosphere. Basically the molecules of air in the atmosphere bend the light from the Sun.

Our Sun, Sol:


Image Source CC By SA 4.0 by Geoff Elston

K Class: Have very weak Hydrogen lines, and but rather have Manganese, Iron, Silicon, and Titanium oxide lines, and are slightly less warm than our sun.

Examples include the giant Arcturus (K0III)


Image source: Public Domain/NASA

M Class: The most common and numerous star of the HR class, ironically, most can’t be seen with the naked eye because of their low temperature and luminosity. The ones that can be seen are usually luminous supergiants like VY Canis Majoris, or close to Earth like Proxima Centauri.They do have estimated lifetimes of trillions of years, so if humanity lasts that long, our descendants may have to rely on these faint stars for energy in the far future (if they don’t invent some magical technology first!).

Examples include (as mentioned) VY Canis Majoris (M2.5I):


Image Source: CC By SA 3.0 by Haktarfone

another Red Hypergiant UY Scuti (M4I):


Image Source: CC By SA 3.0 by Haktarfone

Both VY Canis Majoris and UY Scuti are thousands of light years away, and that speaks to how massive they are that they can still be seen from Earth, despite being classified as M. As you can see from the above diagram, to be seen from Earth through a specialized telescope, both VY Canis Majoris and UY Scuti needed the volume many times of our sun to have the luminosity they have.

And finally, the perfect representation of the last three HR Classes can be found in the nearest triple star system to us (all three are “only” about 4 light years away), the Alpha Centauri System, consisting of Alpha Centauri A (G2V) on the left, B (K1V) on the right, and (sometimes disputed) Proxima Centauri (M6V, circled in red):

Picture saved with settings applied.

Image Source: CC By SA 3.0 by Wikimedia

The faintness of Proxima Centauri relative to M class giants like VY Canis Majoris again underlines that stars of the same HR class may have much differing luminosities on the Yerkes classification scale due to the compared stars being at different stages in their life. Again, we can refer to the light bulb analogy, VY Canis Majoris may not put out much energy per area, but it is huge. A lot of weak light bulbs strapped together still emit a lot of light.

I’ll be talking about stars and stellar mass objects such including L,T, and Y Class brown dwarfs (which cause the discrepancy in the estimate of stars in the Milky way noted above – some say these are stars, others not!), C Class Carbon heavy stars, S Class zirconium/titanium oxide heavy stars, W Class Wolf Rayet stars, D Class white dwarfs, neutron stars and black holes in Part 2 and 3!

I am still working on my solar system simulation, and although I’ve fixed the threading problem, it seems my virtual asteroids repel from the planets and the sun! Stay tuned, I’ll write a detailed post about it once I get it working correctly (Update: Completed, see part 3 here)!

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