Composition of Stars and Non Stellar Gases

Key Concepts

• Stars

• Analysis of starlight

• Composition of stars

• Characteristics of stars

• Types of stars

Introduction

Though stars look tiny in the sky, they are massive objects, millions of times bigger than the Earth. They look tiny since they are very far away. The Sun is the nearest star to the Earth, followed by the star Proxima Centauri. It is 4.3 light years away, i.e., around 41 million km away. 

Stars are huge, shining balls of extremely hot gas (known as plasma) in space. The Sun is our nearest star. During the nighttime, many other stars are visible to the naked eye from the Earth and appear as glowing dots due to their vast distance from the Earth. 

A star is a shiny ball made up of gases that releases an enormous amount of electromagnetic energy. This energy comes from the nuclear fusion that takes place within the star. Nuclear fusion is the combination of fusion of light nuclei to form heavy nuclei. 

When we see from the Earth, many stars look like tiny sparkling objects of white light. But if we look closely at the stars, then we can see that they differ in color. For example, the star Antares glows with a somewhat reddish color. Rigel star shines with a blue-white color and the Arcturus star shines with an orange color tint. The star Sun, glows with a yellow color. 

Analysis of starlight 

Astronomers study about stars basically by examining  emission of star light. Astronomers study starlights by using spectrographs. Spectrographs are the instruments that separate light into various colors or wavelengths.  

The light of star passing through a spectrograph gives out colors and lines known as spectrum. There are three types of spectra:  

1. Emission spectra or bright line spectra    

2. Absorption spectra or dark line spectra   

3. Continuous spectra 

All the stars have got dark line spectra, i.e, strips of dark lines at the point where the colors fade in the spectra. A stars composition and temperature can be shown by stars dark line spectrum. 

Stars are made up of various types of elements present in gaseous state.The inner layers of  stars photosphere is very hot whereas outer layers are relatively cool. Elements present in the outer layer absorb certain amount of light emitted by elements present in the lower photosphere. Different element absorb various wavelength of light. By studying the spectrum and  elements that form a star, scientists can find out the temperature of the stars. 

The composition of stars 

Each chemical element has a standard spectrum in a given range of temperatures. The color and lines of spectrum of a star shows the elements that make the star. By using spectrum analysis, scientists have studied that star are made up of the similar elements that comprises Earth. The very common element on Earth is oxygen and the most common element in the star is hydrogen and second most common element is helium. Elements like carbon, oxygen and nitrogen are generally present in small quantities. 

Star’s temperature 

The surface temperature of a star is shown by its color. The temperature of majority of stars varies from 2,800 ﹾC to 24,000ﹾC. Usually, blue color stars have a normal surface temperature of 35,000ﹾC. The red stars are the coolest stars with the normal temperature of less than 3,500ﹾC. Yellow stars like the sun and Capella have a surface temperature in the range of 5,000 to 6,000ﹾC. 

Stars: Size and Mass 

Stars differ in size and mass. The smallest stars are somewhat bigger than the planet Jupiter, around 1/7^th the size of the Sun. Majority of the stars are less massive as compared to the Sun. The Sun is a medium sized star with a diameter of around 1,390,000 km. Some of the huge stars have diameters that are 1,000 times bigger than the diameter of the Sun. The mass of many stars is same as that of the Sun. However, some stars may be more or less huge. The stars that are very dense may have higher mass as compared to the Sun. The less dense stars may have a large diameter as compared to the Sun but still have less mass as compared to the Sun. 

Types of Stars 

There are various types of stars in the universe, and they can be grouped based on their mass and temperature. They can also be grouped by their spectra (the elements they absorb) and their brightness. 

Our Milky Way galaxy includes an approximate 300 billion stars. Each star is brighter than Sun, but very much further away thus they look very much smaller or not at all to the naked eyes. Stars have been used for celestial navigations and religious traditions for many years. Astronomers categorized stars into constellations to follow their movements and the position of the Sun. 

The 7 Main Spectral Types of Stars 

There are 7 main spectral types of stars — O (Blue), B (Blue), A (Blue), F (Blue/White),  
G (White/Yellow), K (Orange/Red) and M (Red). The stars are categorized based on their temperature, with the hottest is O and the coolest is M. The temperature of every spectral class is then subdivided by the addition of a number, 0 stands for the hottest while 9 for the coolest.

This system is called as the Morgan-Keenan (MK) system and was created by William Wilson Morgan and Philip C Keenan in 1943. 

The very common types of stars in the night sky, are dwarf stars. The very common type of dwarf star is a main-sequence red dwarf star. The Sun is a main-sequence, yellow dwarf G-type star, but the majority of the stars in the Universe are very much cooler and have a low mass. In fact, the majority of the main-sequence red dwarfs are very dim to be seen with the naked eyes from the Earth. With the naked eyes, we can observe around 2,000–2,500 stars. 

1. Protostar  

It (Protostar) is a collection of gas that has broken down from a giant molecular cloud. This stage (Protostar) occurs before a star formation. This stage normally stays around 100,000 years and, over time, gravity, and pressure increases, and that forces the protostar to collapse. The protostar stage is the initial step in the process of stellar evolution. Stellar evolution is a explanation of the way that stars change over the time. 

2. T-Tauri star 

This stage occurs before the main sequence star stage and at the end of the protostar stage when the gravitational pressure keeping the star together is the source of all its energy. 

T Tauri stars do not have sufficient pressure and temperature at their cores to produce nuclear fusion. Still, they do look like main-sequence stars — they are almost the same temperature but brighter because they are bigger. The T Tauri stage will last around 100 million years. 

T Tauri stars can have larger areas of sunspot coverage and have strong X-ray flares and very powerful stellar winds. Stars will continue in the T Tauri stage for around 100 million years. 

3. Main Sequence Stars 

Main sequence stars combine hydrogen atoms to produce helium atoms in their cores. In the universe, around 90% stars are main sequence stars. These stars can range from around one tenth of the mass of the Sun to almost up to 200 times as massive. Currently the Sun is in its main sequence phase. 

A star in the main sequence is in a state of hydrostatic equilibrium, it means gravity is pulling the star inside, and the light pressure from all the nuclear fusion reactions in the star are pushing outside. This inward and outward forces balance and that allows the star to retain a spherical shape. 

4. Blue Stars 

Blue stars are usually hot, O-type stars that are generally found in active star-forming areas, especially areas like the arms of spiral galaxies, where their light illuminates nearby dust and gas clouds creating these areas usually appear blue. 

They are characterized by the strong Helium-II absorption lines in their electromagnetic spectra. They have weaker hydrogen and neutral helium lines in their spectra as compared to B-type stars. 

Their temperatures are about 30,000 K, with a luminosity about 100 to 1 million times than the Sun. They generally have a mass about 2.5 to 90 times as compared to the Sun and last around 40 million years. 

Blue stars generally have comparatively short lives that end in violent supernova events since they are so hot and huge. Examples of blue stars, Delta Circinus and Theta1 Orionis C. 

5. Yellow Dwarf 

Yellow dwarfs are of the G type spectral and have a mass between 0.7 and 1 times that the mass of the Sun. Over 10% of stars in the Milky Way are yellow dwarfs. 

The surface temperature of yellow dwarfs is around 6000°C and shine a bright yellow, nearly white. Sun is a G-type star, but it is actually white. 

They have temperatures in between 5,200 K to 7,500ﹾK and luminosities about 0.6 to 5.0 times than that of the Sun. They last around 4 to 17 billion years. G-type stars transform hydrogen into helium in their cores and will grow into red giants as their source of hydrogen fuel is depleted. Examples of a yellow dwarf are, Sun, Alpha Centauri A and Tau Ceti. 

6. Orange Dwarf 

Orange dwarfs belongs to the spectral type K. They produce significantly less UV radiation than G-type stars and they stay stable on the main sequence for up to around 30 billion years, as compared to around 10 billion years for the Sun. This makes them of special interest in the quest for extra-terrestrial life. 

Orange dwarfs are also greatly beneficial for exoplanets that might live in their liveable zone as they are around four times as common as G-type stars.  

They have temperatures in between 3,700ﹾK to 5,200ﹾK and luminosities about 0.08 to 0.6 times that of the Sun. They have a mass of in between 0.45 to 0.8 times as compared to the Sun. Examples of an orange dwarf, Alpha Centauri B and Epsilon Indi. 

7. Red Dwarf 

The Red dwarf stars are the very common type of stars in the universe. They have  a low mass that they are very much cooler than stars like Sun, and thus look very faint. 

Red stars are able to maintain the hydrogen fuel mixing into their core, and thus they can preserve their fuel for very much longer time than other stars. This indicates that some of these stars live up to 10 trillion years. 

Their temperatures are generally about 4,000ﹾK, with luminosities about 0.0001 to 0.8 times that of the Sun. The mass of the smallest red dwarfs are 0.075 times the mass of the Sun, but they can get a mass of up to half of the Sun. Examples of a red dwarf: Proxima Centauri and Trappist-1. 

Giant and Supergiant Stars 

Formation of giants and supergiant take place when a star runs out of hydrogen and starts burning helium. These are the biggest stars in the universe. 

As a star’s core collapses and becomes hotter, the resulting heat then changes the star’s outer layers to expand outwards. 

Stars that are either low or medium in mass develop into red giants, and stars with high mass, about 10+ times bigger than the Sun, turn into red supergiant. A star can shrink itself and turn into a blue supergiant during periods of slow fusion. The blue colour is generally present when temperatures are scattered over a small surface area, making them hotter. Oscillations in the middle of red and blue can also occur. 

Blue Giant 

Blue giants are very uncommon because they only grow from more massive and less common stars, and as they have short lives. Stars with luminosity categories of III and II (bright giant and giant) are termed as blue giant stars. Their spectral types of blue giant are O, B and A. 

The term blue giant applies to a range of stars in different phases of development. They are evolved stars that have shifted from the main sequence but have got little more in common. However, the true-blue giants have temperatures more than 10,000ﹾK. Examples of a blue giant: Meissa and Iota Orionis. 

Blue Supergiant 

Blue supergiant are also uncommon. They are scientifically called as OB supergiant, and usually have luminosity categorizations of I, and spectral categorization of B9 or earlier. They are normally larger than the Sun, however smaller than red supergiant stars, with a mass of between

10 and 100 solar masses. Blue supergiant have temperatures in between 10,000ﹾK to 50,000ﹾK and luminosities around 10,000 to 1 million times than that of the Sun. Examples of a blue supergiant: Rigel and Tau Canis Majoris. 

Red Giant 

Red giant stars are of the spectral types M and K and are very much smaller than red supergiants and very less massive. When a star has consumed its supply of hydrogen in its core, nuclear fusion stops and the star no longer generates an outward pressure to reduce the inward pressure pulling it together. 

Hence, a shell of hydrogen around the core explodes, continuing the life of the star but causes it to grow in size significantly. This creates a red giant. They normally have temperatures of around 3,300 to 5,300ﹾ K, and luminosities around 100 to 1,000 times than that of the Sun. They also have a mass in between 0.3 to 10 of the Sun’s. Red giants live for about 0.1 to 2 billion years, before they run out of fuel entirely and grow into a white dwarf. Examples of a red giant: Aldebaran and Arcturus. 

Red Supergiant 

Red supergiant stars are stars that have depleted their supply of hydrogen at their cores, and hence their outer layers expand enormously as they evolve off the main sequence. They are of the spectral types K and M and are amongst the biggest stars in the universe, although they are not among the most huge or luminous. They have temperatures between 3,500 to 4,500ﹾK, and luminosities in the middle of 1,000 to 800,000 times than that of the Sun. Red Supergiants live for about 3 to 100 million years. Some red supergiants which even can produce heavy elements ultimately explode as type-II supernovas. Examples of a red supergiant: Antares and Betelgeuse. 

Dead Stars 

Dead stars no longer have nuclear fusion activities taking place in their cores. 

White Dwarf 

A white dwarf star is produced when a star has totally run out of hydrogen fuel in its core and it does not have the mass to force higher elements into nuclear fusion reaction. The outward light pressure from the nuclear fusion reaction ends and the star collapses inward in its own gravity. 

White dwarf shines because once it was a hot star, but there is no nuclear fusion reactions taking place anymore. They usually have temperatures between 8,000 to 40,000ﹾK, and luminosities about 0.0001 to 100 times than that of the Sun. They have a mass of around 0.1 to 1.4 times than that of Sun. A white dwarf can survive for between 100,000 to 10 billion years. Examples of a white dwarf:  Sirius B and Procyon B. 

Neutron Star 

Neutron stars are the collapsed cores of huge stars that were squeezed beyond the white dwarf stage in the course of a supernova explosion. A neutron star is an uncommon type of star that is comprised completely of neutron particles that are slightly more massive than protons, but do not carry electrical charge. 

Neutron stars can disintegrate into black holes if they have got more than 3 solar masses. They generally have temperatures of about 600,000ﹾK and very low luminosities. They have a mass of around 1.4 to 3.2 times than that of Sun and survive for between 100,000 to 10 billion years. Examples of a neutron star: PSR J0108-1431 and PSR B1509-58.  

Black Dwarf 

Black dwarfs are speculated to be white dwarfs that have emitted all their remaining heat and light. Though, because white dwarfs have comparatively high life spans, no black dwarfs have had sufficient time to form so far. This means black dwarfs are slightly imaginary. If a black dwarf were to create, it will be after the death of Sun. 

Black Hole 

While small stars ultimately become white dwarfs or neutron stars, stars with a high mass turn into black holes after a supernova explosion. Because the residue has no outward pressure to resist the force of gravity, it will continue to disintegrate into a gravitational singularity and ultimately become a black hole. A black hole is very powerful that not even light can avoid it. Examples of a black hole: Cygnus X-1 and Sagittarius A. 

Summary

• Stars are huge, shining balls of extremely hot gas (known as plasma) in space.

• The Sun is the nearest star, followed by the star Proxima Centauri.

• Nuclear fusion is the combination of fusion of light nuclei to form heavy nuclei.

• A star’s composition and temperature can be shown by stars dark line spectrum.

• Stars are made up of various types of elements present in gaseous state.

• The surface temperature of a star is shown by its color.

• The Sun is a medium sized star with a diameter of around 1,390,000 km.

• A star remains stable due to the energy created by nuclear reactions in the inside which balances the inwardly directed by gravitational force.

• A star starts its life as a huge cloud of gas which is normally an accumulation of dust, gas, and plasma.

• The protostar stage is the initial step in the process of stellar evolution.

• T-Tauri stars are glowing violent babies.

• Main sequence stars combine hydrogen atoms to produce helium atoms in their cores.

• The surface of red giants is cooler than the Sun (main sequence stars).

• Once helium fusion stops, the core contracts, and the star starts combining carbon. This process repeats till iron starts appearing in the core.

• There are 7 main spectral types of stars — 0 (Blue), B (Blue), A (Blue), F (Blue/White), G (White/Yellow), l< (Orange/Red) and M (Red).

• The stars are categorized based on their temperature, with the hottest is 0 and the coolest is M.

• Formation of giants and supergiant take place when a star runs out of hydrogen and starts burning helium.

• The death of a star results in an explosion of luminous stellar.

• Black holes are points in space that are very dense and they produce deep gravity sinks.