Star Spectral Types Series: Part 1 Part 2 Part 3
In my previous post, I summarized how astronomers classify most stars with the Hertzsprung Russell (1910) and Yerkes Spectral (1943) Classification Systems. But as the field of astronomy expanded with better technology in the past few decades, scientists began to discover stellar mass objects that didn’t fit the classical OBAFGKM Hertzsprung Russell classifications. This spurred the introduction of new star classes, as well as the coining of the terms “brown dwarf”, “neutron star”, and the infamous “black hole”.
While some of the more massive brown dwarfs were initially classified as low mass M-Class stars (such as Teide 1 at M8, the first brown dwarf to be discovered in 1995), later surveys found ever fainter objects that straddled the line between planets and stars. The reason why estimates of the number of stars in the Milky Way stems from disagreement in the astronomical community over where the line should be drawn between planets and stars (a similar debate exists for planets and dwarf planets – remember the controversy over the demotion of Pluto?).
Because their relatively low mass (and therefore relatively low gravity), brown dwarfs fuse little, if any, of their mass into heavier elements and output very little energy. As such, they cool over their lifetimes; the less massive a brown dwarf is, the quicker the process. One of the more massive brown dwarf like Teide 1 may start at the tail end of M, but will eventually cool through the L, T, and Y classes, detailed below.
Remember the discussion from the last post about how temperature correlates with the wavelength of light an object emits? Most brown dwarfs give off so little energy, that their main energy output is in the infrared spectrum, with wavelengths greater than the color red at around 700-750 nanometers, up in the 1-2.5 micrometer range (1000-2500 nm). As such, under normal circumstances, only observatories that “see” in infrared are able to detect them, such as WISE (Wide-field Infrared Survey Explorer), 2MASS (2 Micron All Sky Survey, 2 Micron referring to the wavelength it observes), and the Spitzer Telescopes. Some of the discovered brown dwarfs have even retained the name of the discovering observatory, for example WISE 0855−0714, or 2MASS 0532+8246, for those of you wondering what some of the designations below mean.
A Comparison of Relevant Objects, of the brown dwarfs in the following diagram, Teide 1 is an M8, Gliese 229B is a T6.5, and WISE 1828+2650 is a Y2:
Source: CC By SA 3.0 by MPIA/V. Joergens
L Class: Among the younger and warmer of brown dwarfs, instead of the metal oxide spectral bands of M class stars, metal hydrides like FeH or CaH2 dominate the spectra of L Class dwarfs.
An example would include the the former in the binary pair Luhman 16A (L7.5) and Luhman 16B (T0.5) just 6.5 light years away – even though they are some of the closest extrasolar objects, they are hard to resolve due to their low luminosity:
Source: Public Domain/NASA/JPL/Gemini Observatory
T Class: Even colder and less luminous than L Class dwarfs, instead of metal hydrides, their spectra show emission lines of Methane and Alkali metals (the leftmost column of the periodic table minus hydrogen).
An example would be Gliese 229B (T7), a brown dwarf orbiting a much more luminous Gliese 229 (M1V) (or Luhman B, which stradles L and T)
Source: Public Domain/NASA/Palomar Observatory/Hubble Space Telescope
Y Class: The coolest and least massive class of objects that could be considered a borderline “star”. There are no agreed upon spectra upon which to classify these stars as of this post’s publishing, but a general rule, Y Class dwarfs output energy at a significantly higher rate than a large gas giant like Jupiter, but less than T Class dwarfs. Due to the star/planet ambiguity of Y Class dwarfs, sometimes they are classified as Rogue Planets if not orbiting a larger star.
An example would be WISE J085510.83-071442.5 (just 7.2 light years away!), with a mass 3-10 times Jupiter, and a surface temperature in the range of 225 to 260 Kelvin (−48 to −13 °C; −55 to 8 °F), imaged by the WISE and Spitzer observatories in the infrared spectrum:
Because of the extremely low luminosity of brown dwarfs, many previously unknown “stars” that are in our vicinity have been recently discovered, WISE 1049-5319 (i.e. the Luhman 16 Pair) in 2013 and WISE 0855-0714 in 2014 (shown directly above). Perhaps we have even more interstellar neighbors that remain undiscovered due to their low luminosity!
Source: Public Domain/NASA JPL
In the upcoming Part 3 and wrapup to my space themed posts, more exotic stellar mass objects will be covered, 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.
As for my current coding project, I’ve been working on a maze generating and maze solving algorithm, with more details coming soon!
A teaser for those interested: