8 Types of Stars and Their Spectral Classification That You Must Know

-written by Sahil

We all must have noticed at least once while looking at the spectacular stars on a clear night, that all the stars are different from one another. While our eyes get caught on the brightest stars, we also see that their colors vary, some are too dim and some twinkle a bit too much. Let us see why these stars appear to be so different from each other.

The brightness of a star depends on the amount of energy it exerts, and its distance from the earth. The stars vary in temperature too, which justifies the variation in the colors. Hotter stars appear to be blue or white, while cooler stars appear to be orange or red. The stars are characterized by their color and temperature. The sequence ranges from the largest and hottest to the smallest and coolest.

Only Boring Astronomers Feel Geeks Know More. It’s always fun to remember things with mnemonics, isn’t it? This is the sequence of the seven main types of stars. O (blue), B (blue-white), A (white), F (yellow-white), G (yellow), K (orange) and M (red). The extended sequence includes supergiants, red giants, and white dwarfs. These are the dying stars. It is fascinating to know that unlike what we would expect, Larger stars have shorter lifespans and the smaller stars have much longer lifespans. This is because the larger stars burn out their nuclear fuel quicker.
Let us now talk about different types of stars in their various phases of the lifecycle.

Yellow Dwarf Stars:

These stars fall under the G, F category of spectral classification under the main sequence. These stars are in the very prime of their lives as they are burning hydrogen in their cores. Their sizes vary from 0.8-1.2 times that of the Sun and they burn to temperatures of 5000- 7300°C. It is believed that this type of star is extraordinarily bright and they outshine 90% of the stars in the universe.

The Sun, Alpha Centauri (One of the closest stars to earth), and Kepler 22 are examples of Yellow Dwarfs.

Yellow dwarf star
Credits: NASA’s Goddard Space Flight Center/SDO
Orange Dwarf Stars:

While scientists are constantly studying and searching for life outside the earth, research has shown that Orange dwarfs can create ideal conditions for advanced life on the planets around them. They burn steadily for tens of billions of years, unlike our earth which will probably be uninhabitable for advanced life a billion years later, because of the expanding sun and its heat that will dry up our planet. It is very fascinating to know that for every star like our sun, there are thrice as many orange dwarfs in the milky way. These stars go to temperatures up to 3500-5000°C while they come under the K category of spectral classification.

Hamal (the brightest star in the constellation Aries), Sadira ( At least 2 exoplanets have been recognized revolving around it), and Alpha Centauri B are some notable orange dwarfs.

Red Dwarf Star:

Red dwarfs or M dwarf stars are the most abundant type of stars in the milky way. About 73% of the stars in the milky way are red dwarfs. Their lifespan extends up to hundreds of billions of years and also exceeds the current age of the universe (13.8 billion years) and their temperatures lie between 1800-3500°C. That would be just a little bit higher than the melting point of iron (i.e. 1535°C). They are the smallest stars and hence, they can burn at lower temperatures.

Proxima Centauri (closest star, around 4.2465 light-years away), and Gliese 581 ( star with a planetary system) are red dwarfs.

Via: spaceanswers.com
Blue Giant Star:

These stars are classified under O, B, and A spectral types. Blue giants are massive and some of the hottest stars in the universe. Their temperatures rise to a staggering 2,00,000°C. These stars are colossal, they burn out the hydrogen very quickly which leads them to expand and shine brilliantly in blue. These stars can put out 10,000 times more energy than the sun. Only around 0.7% of the stars are blue giants. These stars burn out quickly and then follow their paths into becoming red giants, supergiants, hypergiants, or perhaps even into a black hole. Another sub-domain of these stars is called Blue Supergiants.

Rigel, the brightest star of the most famous constellation of the Orion, is a Blue Supergiant.

Rigel, via: space.com
Red Giant Star:

The immense energy of the sun comes from the fusion of hydrogen. When the sun or any other star like the sun exhausts the hydrogen, it will begin to crumble. When all this energy is being compacted, naturally it is going to heat up again, and the leftover hydrogen finds enough energy to fuse again. As the hydrogen shell expands, the star forms a helium core.

Eventually, a red giant star is formed. A red giant has a carbon core that is enveloped by hydrogen. Like Blue supergiants and blue giants, we have red supergiants and red giants. Red supergiants are the largest stars in the universe in the matter of volume. Yet, they aren’t the most luminous or the most massive stars. Betelgeuse ( another jewel of the Orion constellation) and Antares (The brightest star in the constellation of Scorpio) are some famous red supergiants.

Via: theplanet.org
White Dwarf Star:

White dwarfs are one of the densest objects in the universe. Stars die too. White dwarfs are also called dead stars. After their main sequence phase, when their nuclear fuel is exhausted, the stars like our sun shed their outer layers to form planetary nebulae. All that is left behind is the core, fiercely hot, size like that of the earth, and having the original mass of the star. The agitating electrons keep the star from collapsing further. The more massive the core, the denser is the star. That means, the smaller the diameter of the white dwarf, the greater mass it contains.

Only the stars that have up to 1.4 times the mass of the sun face this fate. Other stars follow a different trail. These stars age up to billions of years and eventually all the sun-like stars will become white dwarfs. Interestingly, the little partner of Sirius A, Sirius B is a white dwarf.

Artists’ impression of comparison between Sirius B and the Earth, via: esahubble.org
Black Dwarf Star:

After many billion years, as a white dwarf cools down and burns into ashes, a black dwarf comes into existence. Since our universe is believed to be just 13.8 billion years old, there are no existing black dwarfs.

Neutron Star:

When the stellar core at the center of a nebula contains a mass of about 1.4 to 3 solar masses, the core collapses further than the phases of white dwarfs to combine the protons and electrons and form a neutron star. Neutron stars are denser than the white dwarfs.

While this is just the tip of the iceberg, the universe is full of space but filled up full of secrets. Now that you’re already here, you can go through the other blogs here and join us on the journey deep into the space.



Leave a comment