The one about Stellar Lives

Most stars live for billions of years. Human lifetime is a fraction compared to that. Most stars are also very very far away which makes it that much harder to study their properties. Yet, scientists have been clever enough to decode the lifecycle of stars from their snapshots. In this blog post, we will learn about the various stages of life stars go through from birth to death. There are two main paths a star’s life can take and this depends on the most fundamental property of stars — mass. But before we delve into this lifecycle, let’s learn a few basic properties of stars that have enabled scientists to crack the code of stellar lives.

There are together 3 main properties of star: luminosity, surface temperature, and mass(with mass being the most fundamental).

1. LUMINOSITY: Some stars are bright enough that we can use them to identify constellations and some stars are too dim to be detected by naked eye. However, there’s a difference between how bright stars appear and how bright they actually are because all stars are at different distances from us. How bright stars appear to us is known as their apparent brightness. And unlike apparent brightness, luminosity of stars define the total light emitted regardless of the distance. For example, both Betelgeuse and Procyon appear to be equally bright in our night sky. But Betelgeuse is 5,000 times brighter than Procyon. Its apparent brightness is the same as Procyon because its much farther away.Apparent brightness(b), luminosity(L) and distance(d) are related with this equation: b= L / 4 π d². This is an inverse square law equation which means if you see Sun from twice as far, it will appear dimmer by a factor of 2²=4.

Visual demonstration of Inverse square law

Range of luminosities: Dimmest stars are 1/10,000 times less luminous than our sun(0.0001 Lsun). Brightest stars are 1 millions times more luminous than our sun(10⁶ Lsun). Dim stars are more common than bright stars and our sun falls somewhere in the middle. Luminosity can be classified using Lsun which compares it to our sun or it’s also classified using roman numerals I-V. Luminosity I represents the biggest and most luminous supergiants. Class V represents the smaller less luminous main sequence stars. All stars start off as main sequence stars with hydrogen fusion happening inside. When this hydrogen runs out, they go to the next phase of their life cycle. Our sun is a main sequence star anf has used up about half its hydrogen.

We know the apparent brightness of all stars because it’s just the amount of light reaching earth. We can calculate the distances to nearby stars by noting the small annual shifts in star’s apparent position(stellar parallax) caused by earth’s orbit around sun. With apparent brightness and distance, you can calculate the luminosity(total energy output) of the star. Stellar parallax won’t be useful in calculating distances to stars that are further away, specially all the stars in other galaxies,

Because stellar parallaxes are so small and get even smaller with distance, they are very hard to measure. The European satellite Gaia launched in 2013 is working towards creating a 3d map of the milky way galaxy by surveying the stellar parallaxes of stars within a 10,000 light year radius. In comparison, our galaxy has a radius of 52,000 light years.

2. SURFACE TEMPERATURE: This is easier to measure than luminosity as it’s not depended on the distance and can be usually estimated by star’s color or its spectrum. In general, as you go from red to blue on the visible light spectrum temperature gets hotter. So, the surface of a blue star like Sirius is much hotter than a yellow star like our sun which in turn is hotter than a red star like Betelgeuse. The surface temperature of star can also be calculated by studying its spectral lines(click here to review spectral lines from “The one about Energy”).

Stars are classified under spectral types O, B, A, F, G, K, and M. O type stars(stars of Orion’s Belt) are the hottest with surface temperatures of >30,000K and M type stars are the coolest with surface temperature of <3,500K(Betelgeuse). Our sun is a spectral type G with a surface temperature of 5,000K.

3. STELLAR MASS: Mass is the most fundamental property of stars. It determines the life path the star will take. High mass stars have much more nuclear fusion going on in their cores compared to low mass stars making them more luminous. Because they are radiating energy much more aggressively, they live much shorter lives on the scale of millions of years. Our sun luckily is a low mass star which will take billions of years to exhaust its nuclear supply. More than half the stars orbit with a companion. For these binary stars, mass can be determined as long as you know the average orbital period and separation between them. The overall range extends from as little as 0.08 times the mass of our Sun(0.08 Msun) to about 150 times the mass of our Sun(150 Msun).

A scatter plot of stars showing relationships between their luminosity and surface temperature was created by Ejnar Hertzsprung and Henry Russell in 1910. The patterns observed from this scatter plot revealed mass to be the most fundamental property determining star’s life expectancy as well as its end.

HERTZSPRUNG — RUSSELL DIAGRAM

This scatter plot of stars is best studied as a graph. The x-axis represents decreasing temperature to the right(O →M) and also classifies stars by its color. The y-axis represents increasing luminosity to the top. The prominent streak running from upper left to bottom right represents the main sequence stars. On this main sequence line, stars are getting smaller, less luminous and colder as you go from upper left to bottom right. But stars also have longer lifetimes as you go down this line. Our Sun, a main sequence star, is right in the middle of this line with a spectral type of G, color yellow, lifetime of 10¹⁰ years and luminosity of 1 Lsun. Stars above this main sequence line are much more luminous than our sun but they are also colder so they must be big like the supergiants(Betelgeuse). Right under them are the smaller and slightly less luminous giants. And stars below the main sequence line are less luminous than our sun but are hotter so they must be condensed like white dwarfs. Don’t worry I am about to explain what these different types of stars mean.

So far we have learnt two ways of classifying a star:

1. Using its spectral type: O(hottest) →B →A →F →G →K →M(coldest)
2. Using its luminosity: Designate Lsun or classify based on roman numerals I(most luminous and biggest) →V(least luminous and smallest)

Our Sun has a classification of G2 V. The 2 is because depending on various temperatures the spectral types can have subdivisions. G2 spectral type defines a star that’s yellow-white in color with average surface temperature of 5,000K and luminosity class V means that it is a main-sequence star undergoing hydrogen fusion in its core.

STAR LIFECYCLE

Thanks to the Hertzsprung-Russell diagram, scientists were able to find patterns in stars based on their fundamental properties and this allowed them to paint a picture of billions of years long life cycles of stars. All stars are born from a gravitational collapse of interstellar molecular cloud. They shine with the energy produced from fusion of hydrogen to helium in the core. And they finally die upon exhausting all sources of fuel for fusion. The general idea is that high mass stars have a lot of internal pressure so they burn off their hydrogen faster and have shorter lifespans compared to low mass stars. And whether a main sequence star ends its life as a white dwarf or a neutron star depends on its mass as well.

STAR BIRTH

Interstellar gas composed of molecular hydrogen and helium are are scattered at varying densities between stars. In some denser areas, gravity clumps some of the molecules together. As more and more molecules are attracted to this clump, gravity causes the clump to contract to the point at which central object becomes hot enough to sustain nuclear fusion and you have a star! Once the fusion stars, internal pressure balances the gravitational pull and the star gets in an equilibrium. This is where our Sun is at the moment. The phase between molecular gas collapse and start of nuclear fusion is called protostar. Also each area of interstellar gas cloud can give rise to multiple stars which is why majority stars are formed with a companion.

MAIN SEQUENCE STAR

A protostar becomes a main sequence star when its core temperature reaches 10 million K — the required temperature for hydrogen fusion. (The required temperature to fuse hydrogen outside of stars is 100 million kelvin!) This stage will occupy about 90% of their entire lifecycle. High mass stars will go from protostar to main sequence star fairly quickly compared to low mass stars. High mass stars will also end their life as main sequence stars much faster than low mass stars. Just remember that massive stars do everything faster. Stars more massive than 300Msun blow themselves apart, while protostars smaller than 0.08Msun become brown dwarfs that never got hot enough for hydrogen fusion. Hence, these are our star mass limits I listed earlier under stellar mass section. Also Jupiter is a potential brown dwarf — a failed star. :(

LOW MASS MAIN SEQUENCE STAR

Our sun is a low mass main sequence star. It began its hydrogen fusion about 5 billion years ago. It will continue to shine for another 5 billion years after which it would have exhausted its supply of hydrogen in the core.

>>RED GIANT: When Sun’s core hydrogen is depleted, nuclear fusion will cease. With no fusion to replace the energy the star radiates from its surface, the core will no longer be able to resist the inward pull of gravity, and it will begin to shrink. Somewhat surprisingly, the Sun’s outer layers will expand outward despite of the shrinking core. The reason this happens is because after exhausting all the hydrogen, the core is filled with helium which doesn’t fuse easily. But this core is surrounded by a shell of hydrogen. This initiates a hydrogen shell fusion which leads to expansion of outer layers of star. Over a period of about a billion years, the Sun will grow in size to become a red giant — 100 times larger and 1000 times more luminous than it’s today.

The core will continue to shrink till it reaches 100 million K — required temperature for helium fusion. It’s only a matter of time until a helium core-fusion star converts all its core helium into carbon. This will take about 100 million years in our Sun. The core will bring to shrink once again under the crush of gravity. Now you have a carbon core with helium shell fusion happening above it and hydrogen shell fusion happening above helium. Now our Sun is a double shell fusion star.

>>WHITE DWARF: Once our star is not able to fuse any more carbon, it will eject its outer layers into space, creating a huge shell of gas expanding away from the inert carbon core. This is called the planetary nebula. The left over carbon core is now known as the white dwarf. White dwarfs are small but very very dense. The white dwarf over trillions of years will eventually disappear from the view as it gets too cold to emit any light.

HIGH MASS MAIN SEQUENCE STAR

Human life would be impossible without both low mass and high mass stars. The long lives of low mass stars allow evolution to proceed for billions of years but only high mass stars produce all the elements required for life(explained in my first blog). High mass stars go through all stages of life fairly quickly.

Because of their size and mass, these stars evolve in Supergiants instead of giants like low mass stars. The biggest difference is that high mass stars are able to reach core temperatures required for carbon fusion — 600 million K. Carbon keeps fusing into heavier and heavier elements. In the end the supergiant’s insides look like a Russian doll with layers upon layers of shells each fusing different elements.

Eventually the element in the core reaches iron. Iron is bad news for stellar cores. There are only two ways to release nuclear energy: fusion of light elements into heavier ones and fission of heavier elements into lighter ones. Iron being extremely stable the way it is cannot release energy by either fusion or fission. Eventually the amount of iron pilling up reaches a point where no amount of internal pressure can support the core. The gravitational collapse releases 100 times more energy than our Sun will radiate in its entire lifetime. All this energy drives the outer layers off into space in a ginormous explosion called Supernova. This scatters all the newly made elements into space. Thanks to supernovas we are all here today.

>>NEUTRON STARS: The pressure inside the core is so strong here that electrons are pushed inside the nucleus to combine with protons to form neutrons. In a fraction of a second, a core with the mass of our Sun and the size of our Earth is condensed further into a sphere just few kilometers across. This is one of the densest objects in the known universe. Imagine the mass of our Sun condensed in the distance between your house and the next gas station. This leftover core from a supernova is called neutron star.

>>BLACK HOLE: In some cases, the remaining mass may be so large that the neutron star continues to collapse further until it becomes a black hole. An object so dense that even light can’t escape.

SUMMARY OF STELLAR LIFECYCLE

LOW MASS STAR →Protostar →Yellow main sequence star →Red giant →Planetary nebula →White dwarf

HIGH MASS STAR →Protostar →Blue main sequence star →Red supergiant →Supernova →Neutron star or Black hole

And that’s all you need to know about star lifecycles. Hope you found this post informative as well as interesting. :)