How stars are born? | Spectroscopy and Classification of stars | H-R diagram of stars | Stellar Science Part 1

One of the main reason behind the life to exist on Earth is the Sun. Even it is far from us, it influences the life on Earth.

How Sun and others stars are born?

Stars are formed from the gas and dust clouds together called nebula. The internal gravity in the nebula squeezes the atoms together to start nuclear fusion at the core forming hot dense ball of gas called stars. Generally, stars fuse hydrogen into helium at their cores called nuclear fusion which produces heat, energy and light (radiation pressure). This radiation pressure tries to expand outwards but gravity pulls it back to the core balancing the star and making them stable.

Spectroscopy of stars:

Generally, stars look like tiny dots across the night sky. Some of them look bright and some are faint, because stars are at different distances and emit different amounts of light. When we observe them clearly, we can notice that they have colours too. Spectroscopy helping us a lot from centuries ago. It helps us to divide the incoming light from an object into wavelengths and colours.

Stars emit continues spectrum which means that they emit light in all wavelengths. Stars do have atmosphere meaning, thinner layers of gas above the denser inner layers. This gas absorb specific wavelengths of light depending upon the elements they have. So resulted continues spectrum has gaps in it meaning different elements absorb different colours of light.

Star_spectra_Into_the_dark_space

Classification of stars:

Long ago, stars are classified as A, B, C and so on based on their strengths. Later, in 1901 they rearranged and classified based on their strengths and appearances by American spectroscopist, Annie Jump Cannon. After few years from there, German physicist Max Plank showed that stars emit different colours of light at different temperatures. Hotter stars placed at the blue end of the spectrum while cooler ones peaked at red. Later, British born American astronomer & astrophysicist Cecilia Payne-Gaposchkin taken a major step by placing all together showing that the spectra of stars depends upon the temperature and elements in them. She discovered that stars are composed of hydrogen and helium.

Then, a new system was introduced to characterise the stars.

O type stars at the hot end, slightly cooler as B, followed by A, F, G, K & M. While cooler stars as L, T & Y.

Spectral_classification_of_stars_Into_dark_space
CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1141520

Sun has a surface temperature of 5500°C placed in G(2) type. Sirius the brightest star in the night sky which is hotter than the sun is placed at A(0) type. Betelgeuse the red supergiant and cooler star is placed at the M(2) type.

Generally, stars emit light in all colours. Hotter ones are visible as blue, cooler ones as red, some of them are in orange and yellow also. But there are no green stars.

But why?

In reality stars emit most of the light in green and other colours too. But we can see them as white because of all colours mixed up together to form white.

Is sun yellow in colour?

Not actually, it is visible so because of the other colours like green, purple and some of the blue is scattered away by the nitrogen gas in the atmosphere.

H-R diagram:

Since 1800’s, spectroscopy is helping us to determine a lot about the stars. By knowing the distance and brightness of a star, we can calculate the energy that was emitted by the star called luminosity. Luminosity will depend upon the stars size and temperature.

A century ago, astronomers Ejnar Hertzsprung (Danish chemist) and Henry Norris Russell (American astronomer) made a graph that was plotted between luminosity Vs temperature of the stars called H-R diagram named after the astronomers Hertzsprung & Russell. Later, this made a huge impact in the astronomy.

H-R_diagram_Into_the_dark_space
By Richard Powellminor adjustments by:penubag (original image) - The Hertzsprung Russell Diagram, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=2069082

In the H-R diagram, brighter stars are placed on the top, fainter ones at the bottom, hotter (blue) at left and cooler (red) at right. Most of the stars are placed in between the middle thick line (90% of them) called as Main Sequence stars. Most of the main sequence stars have red giant phase. As we know, stars generate energy by fusing hydrogen into helium in their cores. Massive stars fuse faster (more energy) making them as hot rather than that of the cooler ones. Rate of fusion will depend upon the pressure inside the star's core.

Massive stars which are hotter and luminous are placed at upper left of main sequence while lower mass stars which are cooler and redder are placed at lower to the right of main sequence. The stars on lower left are hot, blue-white but are faint and small called as White dwarfs. Stars on upper right are luminous, cool and are huge called as Red super giants and stars on upper left are blue super giants.

Stars can change the position in H-R diagram due to their stages in life. Massive and lower mass stars age differently due to their rates of fusion at the core.

Age of the stars will depend upon the size and mass. More massive stars die faster than that of lower mass stars due to the impact of gravity at core. The stars which are 10 times to the Sun will age between 20 to 30 million years. But the stars which are half of the Sun's mass will have an age upto 1 trillion years.

Stars are also classified based upon the mass as:

1. Low mass stars, which are below the 8 times of Sun's mass.

2. High mass stars, which are above the 8 times of the Sun’s mass.

In which the White dwarfs, Neutron stars and Black holes are formed. These are the dead remnants of the stars.

We will discuss them soon along with brown dwarfs and planetary nebulae. Stay tuned.

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