Stars, their Spectral Classes and Classification

Stars are self luminous objects that shine by radiation obtained from energy sources that are found within them. The sun is a typical star. Stars are classified based on their spectra which give information on their luminosity, temperature and chemical composition. These spectral classes are assigned letters O, B, A, F, G, K and M; and are arranged in order of decreasing temperature(i.e. from the hottest to the coolest stars). Stars of the spectral class O to M belong to the main sequence group while other stellar classes which include the C stars and the stars are regarded as the giants, super-giants or white dwarfs.

The surface temperatures and atmospheric pressures of stars that belong to the main sequence are different but they essentially have the same chemical composition. Furthermore, these stars show a color sequence from very hot(about 20000*C) bluish-white O and B stars to the moderately hot(about 6000*C) yellow G stars, to the cool stars(about 3000*C) red K and M stars.

Just like the sun, the stars in class O to M are mainly composed of hydrogen, helium and very small amounts of other elements found on earth. However, results from stellar spectra show that the particulate nature of elements found on these stars varies with the temperature of the stars.

spectral photo of stars

In cool stars, neutral atoms of elements and simple molecules are very much abundant, while in hot stars, highly ionized atoms of elements are dominant. Therefore, spectral analysis of the cool stars of class M show the presence of very simple molecules such as titanium oxide and the neutral atoms of metals such as magnesium, iron and calcium.

Spectral analysis of the slightly hotter k stars show that the molecules such as titanium oxide are not present in them even though stable pieces of molecules like the hydroxyl radical are present. The spectra of G stars are characterized by what is known as emission lines that show the presence of ionized atoms of metals like calcium and iron.

The spectra of F stars show the presence of more ionized metal atoms than neutral atoms. Class A stars have spectra that show strong hydrogen emission lines which indicate unionized hydrogen atoms, while in the spectra of class B stars, these lines are very faint indicating that most of the hydrogen atoms are ionized.

The spectra of the very hot O stars indicate that even helium atoms are ionized, while oxygen, nitrogen and carbon are twice ionized.

How to Calculate Astronomical Distances of Stars

Astronomers use a variety of techniques to measure the vast distances between Earth and distant stars and galaxies. These methods involve comparing a star’s intrinsic brightness (derived from its color spectrum) with its apparent brightness.

One method is the parallax technique. This works by measuring a star’s position on the sky twice, six months apart.


Astronomers measure the parallax of stars by comparing their apparent brightness to the true brightness of the star. This technique allows them to estimate the distance to the star, and is especially useful for a group of bright stars known as Cepheids. There is a direct relationship between the time it takes for a Cepheid to pulsate and its true brightness, which astronomers can use to calculate the star’s distance from Earth.

The angular shift of a star is tiny, and it’s measured in units called arc seconds. There are 3,600 arc seconds in a degree, and the parallax angle is one-half of this shift.

The trigonometric parallax method only works for objects that are less than 400 light-years away, so astronomers have to use other methods for very distant stars. The Hipparcos satellite was able to increase the number of stars with accurate parallaxes by a thousandfold. The Hubble telescope also has a high level of precision, allowing it to determine the precise parallaxes of many nearby stars.


Trigonometry may be a dreaded subject among high school students, but it is essential for the work of astronomers and astronauts. The study of angles and how they impact other measurements is the key to figuring out the distance to stars from the Earth. This is called stellar parallax.

The Earth’s yearly orbit around the Sun causes nearby stars to appear to move against the background stars. Astronomers can measure a star’s position and then repeat the measurement six months later. Then they can calculate the angular shift between the two observations and use basic trigonometry to figure out the distance.

This method is very accurate for stars that are less than 100 light years away. However, it is not the best for measuring distant galaxies. For these, astronomers have developed a technique using Cepheid variable stars that changes in brightness over time. This allows them to determine the star’s intrinsic brightness and compare it with its apparent brightness.


Light-year is a unit of distance used in astronomy. It represents the distance a photon of light travels in one Julian year, which is about 9.46 x 1012 kilometres. It is a large number, and it is sometimes confusing for students to work with.

The reason for the need for a light-year is that celestial objects are very far apart, and it is impractical to measure them using miles or kilometers. In order to calculate the distance between celestial bodies, astronomers use a method called trigonometric parallax. This method is based on the fact that two observers will see a small difference in the position of a distant star.

The distance to a star can be measured by dividing the parallax angle by the speed of light. The result is the distance to the star in astronomical units (AU). Astronomers also use other measurements of distance, including parsecs and megaparsecs. These measurements are similar to light-years but they are larger and more precise.

Apparent Brightness

Astronomers use a system of relative brightness called apparent magnitude. This is a logarithmic scale, so a difference of one magnitude corresponds to a factor of 2.5 in brightness. For example, a star of first magnitude (the brightest stars) is 100 times brighter than a sixth-magnitude star.

This method works by calculating the difference between two stars’ apparent brightnesses, as measured by an astronomical telescope on Earth. Because the telescope collects a minuscule fraction of the starlight, this method allows astronomers to calculate distances very accurately.

The Earth’s yearly orbit produces an apparent change in the position of nearby stars. Astronomers can then measure the angle between two observations of the same star six months apart, and from this they can determine its distance from the Earth using tangent trigonometry. This method can be used to find stars up to 150,000 light-years away, which is beyond the Milky Way Galaxy. Another way to determine the distance of a star is to use Cepheid variable stars, which fluctuate in brightness.