Saturday, March 20, 2021

All About the Stars

If you take a look at the night sky, you will be able to see numerous stars. Due to their immense distance from Earth, they appear as fixed luminous points. Since ancient times they have attracted our interest, and the really prominent ones were grouped into constellations and asterisms. Catalogs were created, and plenty of them got a proper name (Vega, Rigel, Antares, etc). According to estimations, our Milky Way Galaxy has 100-400 billion stars, while in the observable Universe this number goes to 1×1024. But which one is the closest one to Earth? Well, the answer to this is simple, the Sun. But you may ask, what is a star? Let’s see this below, together all the fascinating things about stars and see what may classify as “fun facts stars”.


What is the definition of a star?

A star is an astronomical object consisting of gas (mostly hydrogen and helium) that is held together by its own gravity. But since an object like Jupiter may fit into this description, in the previous statement, add that a star can produce light and heat through nuclear fusion of hydrogen in its core.The Pleiades or the seven sisters, are a famous group of stars in our galaxy. Image Credit: NASA, ESA, AURA/Caltech, Palomar Observatory



Due to its proximity, the Sun is the most studied star in the Universe. Thus, all units of measurements used for stars are based on the Sun. So:
The luminosity of a star is given in units of L⊙ = 3.828 × 1026 W
A stellar radius is measured in units of R⊙ = 6.957 × 108 m
The mass of a star is measured in units of M⊙ = 2 × 1030 kg

The solar mass unit is also used when it comes to express the mass of star clusters and galaxies.

Finally, temperature in Astronomy is measured in Kelvin. The conversion from Celsius to Kelvin is T (K) = T (°C) + 273.15, while from Fahrenheit to Kelvin is T (K) = (T (°F) + 459.67) × 5/9.


What are the features of a star?

Although each star can be considered unique, stars have some similarities. Thus we use specific features to describe and understand them. These are:

Age – We use it to divide the evolution of a star in distinct stages.

Size – The size can provide information about the current phase of the evolution of a star.

Mass – The most important feature for the evolution of a star.

Temperature – It is used for the classification of the stars.


Spectral classification – stars

The classification of stars is done based on their temperature. A star gets a single letter classification according to their spectra, which ranges from type O (these are very hot stars), to type M (cold stars). The complete order is: O, B, A, F, G, K, M. Rare types of stars get special designation. These are types L and T, which classify cold low-mass stars and brown dwarfs (failed stars). Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature.

Furthermore, stars are classified according to spectral features which indicate their size. These range from 0 (for hyper-giants) through III (giants) to V (main sequence dwarf stars). Our Sun is a main-sequence G2V yellow dwarf, of ordinary size and intermediate temperature (~ 5,700 K).


Steps of star formation

The Pillars of Creation, is one of the most famous stellar nurseries in our Galaxy, where thousands of stars have been formed. Image Credit: NASA, ESA/Hubble and the Hubble Heritage Team
Stars have some similar features to humans. They are born, live their lives, and die. Through observations of stars of different age and mass, Astrophysicists were able to construct their life cycle. But as we will see below the parameter that affects a star the most is its mass.


#First step of star formation – Molecular cloud collapse

Stars are formed from the collapse of material in giant molecular clouds (these are also known as stellar nurseries). These clouds that exist and form between other stars consist primarily of hydrogen, helium, and dust. But what triggers star formation in a cloud? In one word, turbulence.
#Second step of star formation – Protostar formation

Turbulence within a cloud leads to the formation of knots, which in turn can collapse under their own gravity. Due to the collapse of the knot, the temperature in its interior increases. This hot core is called a protostar and assuming that it has significant mass, it will eventually become a star. Within a cloud, numerous such protostars can be formed. So depending on the size of the cloud, hundreds or even thousands of stars can be formed.
#Third step of star formation – Thermonuclear ignition
As the protostar forms, it keeps gaining mass through accretion. As the mass increases the core of the protostar becomes denser and hotter. This happens because there is nothing that can halt the collapse of the protostar. Eventually, the temperature in the protostar’s core will be high enough for hydrogen fusion to take place. Once this starts, the star enters to what it’s known as the “main sequence” phase of its life.

The main sequence phase is the longest in a star’s life. During this phase, the star fuses hydrogen into helium in its core. Fusion prevents the star from collapsing further, since the radiation pressure balances the gravitational force. This is known as hydrostatic equilibrium.



Star evolution stages

If you were assuming that a star with a large mass has to oppose a greater gravitational force, thus it has to fuse hydrogen at a faster pace thus having a shorter life, then you are correct.

For every star, there is a point in its life that its core will run out of hydrogen. At this point, there is nothing that can hold the star from collapsing. The core of the star due to pressure from its surrounding inner layers collapses and its temperature increases. As the core collapses, the outer layer of the star starts to expand. This phase marks the beginning of what is known as the red giant phase. What happens next depends on the mass of the star. The only way to keep the star running is fusion. Thus, once the star enters the red giant phase, it needs to fuse helium, to balance gravity.

Examples of low-mass (left cycle) and high-mass (right cycle) stellar evolution.



Stars with mass > 8 M☉, when they will run out of hydrogen, they start to fuse helium. When helium runs out their cores will collapse again and they will start fusing carbon and oxygen. This will continue until they form an iron/nickel core. At this point, there is nothing that can halt the collapse. These stars end their lives with a spectacular explosion known as supernovae.

Stars in the mass range, 1.8–8 M☉, will enter the red giant phase and start fusing helium. Once they create a carbon/oxygen core, they will remove their outer layers in the form of a strong stellar wind and end their lives as carbon/oxygen white dwarves. A white dwarf is a degenerate hot star with a size comparable to Earth, and its mass ranges between 0.17 M☉ and 1.33 M☉.

Stars with a mass between 0.5 – 1.8M☉ will become red giants, and they will start fusing helium in their core through a process known as helium flash. These stars will end their lives as white dwarves too (their outer layers are also removed in the form of stellar winds).

Very low-mass stars, < 0.5 M☉, will never go through the phase where they will fuse helium in their cores. Such stars will not become red giants, and after they run out of hydrogen they will cool down, ending their lives as helium white dwarfs (after they remove their outer layer, in the form of a stellar wind). It is worth mentioning that stars with mass < 0.5M☉ have lifetimes longer than the age of the Universe (~ 14 billion years) thus no such star has yet reached the white dwarf stage.

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