Analysing Starlight

 

Analysing Starlight

 

Humans love classification. It makes our lives so much easier. For decades astronomers tried to classify stars. Stars emit light. This emitted light is the DNA of the star. It contains all the information about the star. Analyzing star light is the key. We can classify stars using their lights in several ways. We will discuss three of those in this blog.

Analyzing their Color:

Figure 1: Star Cluster NGC - 2367

While stargazing you might have observed different stars have different colors. Some are red, some are blue and some appear white. This looks like a good way to classify them, and it indeed is. A star’s color tells us about its temperature and mass, and blue stars are the hottest and most massive of all.

Light is a form of energy. Blue light has more energy than red light due to its smaller wavelength. Wien’s law explains this relation between wavelength at which maximum energy is emitted and temperature.

ʎ_max =b/T , where b is a constant and is equal to 3x10⁶ nmK

For the Sun, the max energy wavelength is 520 nanometers, which lies near the middle of EM spectrum. This tells us the surface temperature of Sun is 5800K.

Star Color	Approximate temperature	Example
Blue	25000K	Spica
White	10000K	Vega
Yellow	6000K	Sun
Orange	4000K	Aldebaran
Red	3000K	Betelgeuse

 

This is not the only way of classifying stars. The other way is using stellar spectra.

Stellar Spectra:

 

Figure 2: Stellar Spectra of our Sun

Here we use a spectrograph to spread out the light into a spectrum. In 1814, the German physicist Joseph Fraunhofer observed that the spectrum of the Sun showed dark lines crossing a continuous band of colors. In 1860s, astronomers identified some of the lines of stellar spectra as those of known elements on Earth, showing the same chemical elements found in Sun and planets exist in the stars.

 

Not all stars have similar stellar spectra. Initially, it was thought that this was due to different chemical composition. This turned out to be false. The primary reason that stellar spectra look different is because the stars have different temperatures. Most stars have nearly the same composition as the Sun with only a few exceptions.

Hydrogen lines are not visible in the spectra of hottest and coldest stars. In the hottest stars, the temperature makes the hydrogen atoms to ionize completely and thus cannot produce any absorption lines. In the coldest stars, the hydrogen atoms take photon from the ultraviolet range, thus not releasing any spectra in visible range.

Astronomers use the patterns of lines observed in stellar spectra to sort stars into a spectral class. There are seven spectral classes. From hottest to coldest, these are O, B, A, F, G, K, and M. Each class is further divided from 0 to 9. Our Sun lies in G2 class.

 

Figure 3: Spectral Classes

Recently, astronomers have added three additional classes for even cooler objects- L, T, and Y. This is by far the most successful way of classifying stars.

 

The Magnitude Scale:

 

Luminosity refers to the total amount of energy at all wavelength a star emits in one second. But only a miniscule amount of this energy reaches Earth. We call the amount of a star’s energy that reaches a given area (say a square meter) each second here on earth its apparent brightness. The process of measuring the apparent brightness of stars is called photometry.

The below figure shows the range of observed magnitudes from brightest to the faintest, along with the actual magnitudes of several well-known objects. Remember, larger the magnitude, the fainter is the object.

 

These are some of the ways to classify stars.

Figure 4: Apparent Magnitude of some well-known objects