What are stars? What do we know?

Stars are perhaps the most extensively studied type of astronomical object. There is an enormous amount of detail available about almost every aspect of a star. The stars have been categorized into thousands of catalogs based on their similarities, differences, environmental conditions, sustenance conditions, and many more properties. With that being said, even though the understanding of stars has come a very long way from early Greek times, it has yet to go a very long way.

The stars have been known to humankind for as long as humanity has existed. Prehistorically, they were related to the beginning or end of rains, droughts, or changes in seasons. The stars are also used as standard cosmological distance pointers, and they have also helped in an undisputed establishment of the Einsteinian view of gravity. Recently, with the detection of gravitational waves, compact objects have also attracted a ton of attention. But still, there are so many of them that we cannot rule out the possibility of finding something new and mind-boggling.

With all these years of research, very fine subcategories for the stars have been established, and covering them in a single or even several posts is a nigh-impossible task.

An extremely small rundown of problems in encompassing all the features of stars is summarized below

Formation of the star:

The stars form through nebulous clouds of gas and dust. Depending upon,

• the quantity (and even the quality) of material a different kind of star can be formed,
• the excess material not consumed in the formation of the star, there can be planetary dust disks surrounding the star that may or may not end up in a proper planetary system,
• the accretion and accumulation of the material at hand, there can be one, two, or multiple stars forming together in the same place,
• the distance of the newly formed stars, they can tidally disrupt one another or any possible planetary environment caught up in the unfortunate event,
• the material consumed and the mutual distance (if it is a multiple star system), the massive star can either
  • change the type of the star by depositing its ejecta on it (and potentially killing it in the process),
  • consume the partner star if it ends up within the Roche lob limit, or
  • fling its partner in the vast void of the cosmos.

Life of the star:

Depending upon all the factors mentioned above and more, the majority of the stars end up as almost-star, or as astronomers call them, dwarf stars. The are failed stars are unable to fuse the nuclear fuel within the core due to lack of mass, gravitational pressure and thus cannot burn.

Depending upon the mass of the newly born star, the life expectancy ranges from a few millennia (early pure hydrogen types, hypergiants, or asymptotic branch stars) to billion of years (main sequence or metal-rich stars). Even though the temperature, mass, and luminosity have established correlations with one another for a given star, the metallic content, emission lines, and variability factors, among others, can classify stars very finely.

A general classification of stars in terms of temperature and luminosity (among others) is the famous Hertzsprung-Russell diagram. It will be discussed (and possibly drawn from survey data) in future posts.

As can be seen, not all stars are classified according to a single parameter in every aspect. Stars with an absolute magnitude of -5 exist roughly in the O, G, K, and M spectral classes. And even the O type class has few instances of +10 magnitude white dwarf stars.

Other than that, there exist some peculiar type star branches that do not behave well. These are often broadly classified as variable stars. However, they too have subcategories. While this may seem simple enough, the variability can also be accounted for as either artificial (extrinsic) or natural (intrinsic) with further subcategories.

The intrinsic variability occurs from the star’s swelling or shrinking, whereas the extrinsic variability occurs from a companion star or planet that orbits the star. The variability can further be classified as periodic or non-periodic. Most extrinsic variables tend to exhibit periodic variability, however, the same is not true for most intrinsic variables.

A star can show variability in its light curves and/or spectrum and based on these fluctuations they have been broadly classified as,

1. Pulsating variables: The star expands and shrinks naturally to maintain the balance between the gravitational pull and the radiation push. It usually happens in the relatively late stages of a star when most of the hydrogen has already been fused into helium. The star then has to start fusing helium to stay sustained.
2. Eruptive variables: The star has flaring or ejection from its surface. This usually happens in the case of newly born stars that have yet to fully contain their plasma within their gravitational influence or in the giant/supergiant/hypergiants, where the loss of material is relatively easy due to their massive sizes.
3. Cataclysmic variables: The star undergoes any nova event (kilonova, supernova, or hypernova).
4. Eclipsing variables: The star is in orbit with a partner that comes in front of it periodically thus blocking its light. The partner can be another (less bright) star or a planet.
5. Rotational variables: The star is rotating so fast that it bulges out from the equator thus creating a strong doppler effect in their ellipsoidal forms. The same can happen with stars that have a strong magnetic field that is disrupted by their rotation. This interaction between magnetic field lines and their rotational speed causes sunspots (starspots?), thus making them appear variable.

These are further subdivided by their progenitor star/astronomical object which was the first ones to be identified for a specific subclass from the above-mentioned classes.