<p>While I think that this question might be too broad, I will give you a few ideas.</p>
<ul>
<li><a href="https://en.wikipedia.org/wiki/Stellar_classification" rel="nofollow noreferrer"><strong>Spectroscopy</strong></a>: 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 <em>spectrum</em> 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 <a href="https://en.wikipedia.org/wiki/Black_body" rel="nofollow noreferrer">blackbody</a>, measure its <a href="http://spiff.rit.edu/classes/phys301/lectures/doppler/doppler.html" rel="nofollow noreferrer">rotational or radial</a> velocity by taking a look at the absorption lines; or analyze its chemical composition through the presence and characteristics of the spectral lines (<a href="http://spiff.rit.edu/richmond/asras/chemcomp_i/chemcomp_i.html" rel="nofollow noreferrer">here</a> 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 <a href="https://en.wikipedia.org/wiki/Stellar_classification#Modern_classification" rel="nofollow noreferrer">the Harvard and Morgan-Keenan</a> ones, which can give you an idea of the characteristics of the star.</li>
<li><a href="https://en.wikipedia.org/wiki/Photometry_(astronomy)" rel="nofollow noreferrer"><strong>Photometry</strong></a>: 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 <a href="https://en.wikipedia.org/wiki/Magnitude_(astronomy)" rel="nofollow noreferrer">magnitudes</a>, 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 <a href="https://en.wikipedia.org/wiki/Extinction_(astronomy)" rel="nofollow noreferrer">extinction</a>). 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 <a href="http://spiff.rit.edu/classes/phys440/lectures/color/color.html" rel="nofollow noreferrer">here</a>. This is the <em>color index</em>. Another use of photometry is to determine whether a star is <a href="https://en.wikipedia.org/wiki/Variable_star" rel="nofollow noreferrer">variable</a> 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, <a href="https://arxiv.org/pdf/1712.02097.pdf" rel="nofollow noreferrer">this paper</a>).</li>
<li><a href="https://en.wikipedia.org/wiki/Stellar_evolution" rel="nofollow noreferrer"><strong>Evolution</strong></a>: 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.</li>
</ul>
<p>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 -&gt; Radial Pulsators -&gt; Cepheids -&gt; Classical Cepheids -&gt; Cepheids From the Magellanic Clouds -&gt; Fundamental Mode Pulsators -&gt; Short Period...).</p>
<p>Hope this can help you.</p>

CC BY-SA 4.0
2021-03-04T22:37:11.993Z
2021-03-04T22:37:11.993Z
Solfreludio
<p>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.</p>

CC BY-SA 4.0
2021-03-04T03:04:35.597Z
this is the beginnings of a good answer, but it is a bit short and completely unsourced. Stack Exchange answers should support their assertions by citing or even quoting sources and links. Can you add some more? *Thanks!*
uhoh
CC BY-SA 4.0
2021-03-04T01:13:27.517Z
2021-03-04T01:13:27.517Z
ariana grande
<p>What are the properties of a star?  How do astronomers classify the properties of stars?</p>

CC BY-SA 4.0
2021-03-04T14:42:31.637Z
@uhoh I don't understand your comment, I guess because the comment by slowerthanstopped is no longer available to me.
B--rian
CC BY-SA 4.0
2021-03-04T16:07:44.243Z
@B--rian it's been deleted. When comments are deleted (by author or moderator) they become invisible to everyone except moderators. It's not like Q and A posts that remain visible for high-rep users.
uhoh
<observational-astronomy><orbit><planet><expansion><communication>
What makes a star?
ariana grande
# What makes a star?

(Asked by: username_0 on 2021-03-04)

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

**Tags:** observational-astronomy, orbit, planet, expansion, communication

> **Comments:**
> **username_1**: @username_2 I don't understand your comment, I guess because the comment by slowerthanstopped is no longer available to me.  
> **username_2**: @username_1 it's been deleted. When comments are deleted (by author or moderator) they become invisible to everyone except moderators. It's not like Q and A posts that remain visible for high-rep users.  

---

## 2 answers

### Answer by username_3 on 2021-03-04. Score: 1

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

* [**Spectroscopy**](https://en.wikipedia.org/wiki/Stellar_classification): 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](https://en.wikipedia.org/wiki/Black_body), measure its [rotational or radial](http://spiff.rit.edu/classes/phys301/lectures/doppler/doppler.html) velocity by taking a look at the absorption lines; or analyze its chemical composition through the presence and characteristics of the spectral lines ([here](http://spiff.rit.edu/richmond/asras/chemcomp_i/chemcomp_i.html) 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](https://en.wikipedia.org/wiki/Stellar_classification#Modern_classification) ones, which can give you an idea of the characteristics of the star.
* [**Photometry**](https://en.wikipedia.org/wiki/Photometry_(astronomy)): 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](https://en.wikipedia.org/wiki/Magnitude_(astronomy)), 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](https://en.wikipedia.org/wiki/Extinction_(astronomy))). 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](http://spiff.rit.edu/classes/phys440/lectures/color/color.html). This is the *color index*. Another use of photometry is to determine whether a star is [variable](https://en.wikipedia.org/wiki/Variable_star) 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](https://arxiv.org/pdf/1712.02097.pdf)).
* [**Evolution**](https://en.wikipedia.org/wiki/Stellar_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.


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### Answer by username_0 on 2021-03-04. Score: 0

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.

> **Comments:**
> **username_2**: this is the beginnings of a good answer, but it is a bit short and completely unsourced. Stack Exchange answers should support their assertions by citing or even quoting sources and links. Can you add some more? *Thanks!*  

---

