Freelance Traveller

Variable Stars and Dwarfs: An Overview For Non-Astronomers

by Mark McCabe

This article originally appeared in the July 2015 issue.

As many of us who have studied stellar masses know, each star can have its own special characteristics that make it as unique as a fingerprint. For example, our own sun can be considered a stellar variant due to cycles of sunspots and stellar ejecta that emanate from the sun’s surface.

With that in mind, I went about looking at other types of variables and found a wide variety listed. Some of the better known types are as follows:

Alpha Cygni (α Cyg) variables

are non-radial pulsating variables of spectral type B or A and luminosity class Ia. Due to their immense size and high luminosity, one can expect to see periodic rapid increases of EMP (Electromagnetic Pulses) and intense radiation, ranging from 5-30 days in length. The ‘non-radial pulsating variable’ description means that one area of this star can be expanding while another side may be shrinking. Deneb (α Cyg, type A2Ia) is the prototypical example of this star type. Inhabited planets orbiting α Cyg variables would likely have “hardened” subsurface habitats with heavily shielded local communications, and transiting starships would use microwave beacons and masers for communications with the world(s).

Beta Cephei (β Cep) variables

are rapidly pulsating variables of spectral type B0-B2 and luminosity class III-V. Their variability is a function of changes in radius driven by the opacity of the stellar atmosphere to the star’s own radiation (the “κ-mechanism” or “kappa mechanism”). Most κ-mechanism variables display higher than normal levels of ionized hydrogen and helium in their spectra; β Cep variables are driven by high levels of iron in the depths of the star, causing spherical contraction and pressure build-up until expansion back to the original shape. One would expect occasional disruption of non-protected communications when the star contracts.

Cepheid variables

are radial pulsating variables of spectral type F, G or occasionally “hot” K, and luminosity class Ia-III. Their most notable feature is a nearly lockstep relationship between the period and absolute magnitude, allowing them to be used for accurately determining distances on interstellar and intergalactic scales. For 2-40 days this star ejects positively ionized Helium particles. The star increases in luminosity during this time. Expect an increase in the solar winds while these ions emanate from the star. Because this ejecta is helium-based, most planets with even a trace atmosphere will be unaffected. The first such Cepheid variable discovered was Eta Aquilae (η Aql) in 1784, but the class was named after Delta Cephei (δ Cep).

Flare stars

are eruptive variables, mostly of spectral types K (cooler end) and M, luminosity class V (the “red dwarf” stars on the Main Sequence). As with solar flares, the flares on this type of star are caused by magnetic buildup and subsequent “reconnection” in the stellar atmosphere, causing an almost epileptic shaking of its outer surface and erratic bursts of thermal and radioactive energy in irregular patterns. Examples of flare stars include Barnard’s Star, Proxima Centauri and Wolf 359. Expect a radically changing solar wind in these system and most settlements will need to be underground or shielded in some fashion.

Mira variables, or Omicron Ceti (ο Ceti) variables

are red giants (spectral class M, luminosity class II-IV) that have evolved off the Main Sequence and onto the Asymptotic Giant Branch of the of the H-R diagram. As the star heads towards the final stages if its existence, it ejects elements ranging from Helium to oxygen for 50 – 550 days. These free-ranging elements may form small planetary nebulas as they get blown away from the star and begin to interact with other stellar and planetary bodies. This type of variable is named after the star Mira (ο Ceti) in the constellation Cetus. Any planets attendant on a variable of this type will have formerly been outer planets of a Main Sequence star, and as such will probably not have more than outposts or resource acquisition (e.g. mining) stations.

RR Lyrae variables

are low-mass Population II stars of spectral type A (or “hot” F) and luminosity class III. Their variability mechanism is similar to that of the δ Cep variables, and they are similarly used for establishing interstellar distances, for relatively near objects. From 1 to 6 days this star ejects ionized Helium and greatly increases in luminosity. These stars are generally in multiple-star systems and in globular clusters. This type of star is named after the first such variable found (RR Lyrae). Settlements in these systems will need protection from a searing increase of light and radiation of some sort.

RV Tauri variables

differ from the otherwise similar Cepheid variables in that the spectral type of RV Tauri change from F or G at the brightest to K or M at their dimmest. They also exhibit alternating primary and secondary minima at their fundamental period, with the interval between the two primary minima (or two secondary minima) being twice the fundamental period. Over its period, the star greatly varies in luminosity due to a rapid expansion and deflation of its diameter. Severe stellar flares, with their attendant disruption of communications, will be common. Planets with weak or nonexistent magnetic fields will experience high radiation. Settlements in any habitable zones will need to have a safe haven underground.

Semiregular variables

exhibit significant variation in their cycles, often resolving on analysis to multiple overlapping periods. The majority of these stars are of spectral type M, S, or C, and luminosity class Ia to III, but one of the four subclasses (SRD, exemplified by SV Ursae Majoris) is of spectral type F, G, or K. All subclasses may have mean periods ranging from approximately a month to several thousand days. Subclass SRA stars (exemplified by Z Aquarii) are essentially the same as ο Ceti variables, except that where ο Ceti stars pulsate in the fundamental period, SRA stars pulsate in a harmonic or “overtone” mode. Subclass SRB stars (exemplified by RR Coronae Borealis) may not show any significant periodicity, and some are known to have stopped varying for a length of time, and others have been shown to have multiple overlapping variation periods. Subclass SRC stars are supergiants (luminosity class Ia or Ib) with variability over only about 1 magnitude. An example of this type is Betelgeuse (Alpha Orionis [α Ori], M2Iab).

T Tauri variables

are aperiodic variable protostars in the process of contracting to the Main Sequence. They are low-mass (less than 3Msol) protostars of spectral type F, G, K, or M. These are stars that gain and lose luminosity rapidly in a stellar nursery, with gravitational contraction being the driving mechanism (as they are yet too cool to sustain fusion). They have up to a thousand times more sunspots than a normal star and they will sometimes eject energy and stellar ejecta in jets coming out of both poles. These stars also eject Lithium at a much higher rate than most stars. Wildly chaotic in the extreme, any sustained life in these systems will be difficult at best.

Wolf-Rayet stars

are eruptive variables of spectral type O and luminosity class Ia or Ib, but because of their unique characteristics (including strong emission lines), they have been given their own classification as type W (with several subtypes). This type of variable is an aging type O star that has blown off much of its outer surface. The hydrogen is gone and it is now using helium as fuel (or something heavier). This is a classic example of a live fast, die young star – any planets in this system will be blasted or well out in the outer zones. The inner zones will likely be covered in the ejected hydrogen as a thin nebula has formed like a globe surrounding the star. An adventurous starship captain may be able to skim this hydrogen, but that ship will need to be mindful of the radiation and solar winds coming from a dying beast of a star. The characteristics of type W stars are such that most are expected to finally die as supernovae. The first types of these stars were discovered by Charles Wolf and Georges Rayet in the constellation Cygnus in 1867.

Generating Variable Stars

Variable stars should only be placed by referee fiat, but an appropriate type of variable can explain why a world with a UWP characteristic of a highly-desirable world might have a low population. Use the existing stellar type (but see the Revised Stellar Classifications and Dwarf Classification sections below) to determine where on the table below to roll to determine the type of variable.

Extended Classifications for Dying Stars

Since the writing of Classic Traveller Book 6: Scouts, the stellar classification system has been updated and extended. Thanks to the Hubble and Kepler telescopes, along with a better understanding of how stars work, the catalog has expanded to include star types L, T, Y, S and C. This is by no means the end of the classification, but I will stay with these types as they are the most common.

The stars under the L, T and Y spectral classes are commonly referred to as Brown Dwarfs. These are M/V and M/VI stars that have cooled off over time (stars of spectral class M7V or M7VI or cooler are also considered Brown Dwarfs). They still are hot enough to be gaseous and emissive, so approaching them is impossible. Such Brown Dwarfs will progress from type M through L, T, and Y in that order as they cool; the lowest-mass type Y stars have little to distinguish them from the highest-mass jovian planets. All the surviving planets that may still be surrounding these burned-out stars will be considered in the outer zone; the only heat would come from tectonics affected by a larger planet (such as Jupiter’s gravity affects the moon Io). These essentially are massive obstacles in space for any traveler, but their gravity is strong enough to have some gravitational effects on hyperdrives.

The H-R types C and S (together called, informally, ‘carbon stars’) are used to identify those Red stars (Type M) nearing the end of their main life-cycle. Not to be grouped with the regular variables, this is a special type of star to be used very sparingly by any GM.

Type S Giants (luminosity Ia, Ib, or II) emit a combination of carbon and oxygen (or carbon monoxide) from a star that is acting like a ο Ceti variable. Solar winds will be intense and any settlements that may have been here for millennia will likely need to be relocated – and soon.

Type C stars are either Giants or main sequence stars (luminosity III-V) that emit carbon. The hydrogen and the helium are gone. They should again be treated as a ο Ceti variables, but with half to a quarter of the time listed. As above, any settlements/civilizations that are in the vicinity of this star should be exiting the system as absolutely soon as possible.

Dwarf Classification

Earlier in this article, stars of spectral type M and the cooler end of K and luminosity class V or VI were referred to as “red dwarfs”. These stars are part of the Main Sequence, and represent stars at the end of their life that are insufficiently massive to expand into giants. There is a separate area on the H-R diagram for “white dwarfs” (whose spectral “color” extends from spectral type B to K), which are typed with the Dx nomenclature – but the x does not represent the spectral type (Book 6: Scouts errs in this); rather, it represents the composition of the outer layers of the star’s atmosphere. The ‘D’ classification stands for ‘degenerate’, rather than ‘dwarf’, and stars in this class are no longer undergoing fusion; their temperature is sustained by gravitational collapse. Change stars of type DG, DK, or DM to the appropriate spectral type and luminosity class V (Main Sequence dwarf) or VI (subdwarf; can also be prefixed ‘sd’, i.e., G5VI and sdG5 are equivalent); stars of type DB, DA, and DF should be changed to DB2, DB5, or DB8 respectively (DB is a type for helium-rich white dwarfs; the number indicates the effective temperature, with higher numbers indicating cooler stars).

Summary

For those of us who like a little science with our storytelling I hope this can add a twist or two to your colonies or to those intrepid scouts who are conducting system surveys. I hope you can find this information useful as I have.

References