What makes a star? [closed]

Question

What are the properties of a star? How do astronomers classify the properties of stars?

Answer 1

While I think that this question might be too broad, I will give you a few ideas.

• Spectroscopy: One important component of astronomical observations is to take the light of a source and decompose it into its different frequencies (or equivalently, wavelengths), which is the spectrum of a star. This allows astronomers to use different techniques and measure some properties of the star. To give you a few examples, you could get an idea of the temperature of a star by thinking about its spectra as a blackbody, measure its rotational or radial velocity by taking a look at the absorption lines; or analyze its chemical composition through the presence and characteristics of the spectral lines (here you can find some information). Each of these properties will give you a classification (hot-cold, rotating or non-rotating, metal-rich or metal-poor, etc), although there are some widely known classification systems such as the Harvard and Morgan-Keenan ones, which can give you an idea of the characteristics of the star.
• Photometry: Basically, you take the light emitted by an object and measure how much you get (flux). Filters are used to restrict yourself to a certain wavelength range. Note that these measurements are usually presented as magnitudes, which follow a logarithmic scale and can either be absolute (if the star were located at 10pc of earth) or apparent (just as they can be seen in the night sky: different distances and extinction). Now, how can you use this to classify stars? Considering that subtracting two magnitudes is related to a ratio (remember the logarithm properties), you can get an idea about the temperature of the source as shown here. This is the color index. Another use of photometry is to determine whether a star is variable or not: Does its magnitude stay constant or does it change over time? The reasons for the changes could be many, such as changes in radius or the presence of a companion (binary systems). Note that I am listing variability in photometry because it is easier to think about it, but it is not the only method to detect it (for example, this paper).
• Evolution: Stars have different stages throughout their life. It is important to make a difference between the evolutionary stage and age, as they are not the same for all stars and depend on the initial mass of each star, for the most part. The initial mass plays a huge role on how fast a star will evolve and which stages it will go through. The general idea is shown on the wikipedia article, where you can find stellar classes such as main sequence, red giants, white dwarves and so on.

As I stated at the beginning, this is just a glimpse of the several different classifications. You can probably find more as you dig through the articles and I bet that as you go deeper into a class you find even more classes (i.e Variable Stars -> Radial Pulsators -> Cepheids -> Classical Cepheids -> Cepheids From the Magellanic Clouds -> Fundamental Mode Pulsators -> Short Period...).

Hope this can help you.

Answer 2

Astronomers classify stars according to their physical characteristics. Characteristics used to classify stars include color, temperature, size, composition, and brightness. Stars vary in their chemical composition.