Advanced Stars

A star is a sphere of gas hosted together by its own gravity. Theclosest star to Earth is our incredibly very own Sun, so we have an example nearbythat astronomers deserve to examine in detail. The lessons we learn around theSun deserve to be used to various other stars.

You are watching: Why does a star grow larger immediately after it exhausts its core hydrogen?


Diagram mirroring the lifecycles of Sun-likeand also enormous stars. Click photo for larger version. (Credit: NASA andthe Night Sky Network)

During the majority of a star"s life time, the internal warmth and radiation is providedby nuclear reactions in the star"s core. This phase of the star"s life is dubbed the major sequence.

Before a star reaches the main sequence, the star is contracting and its core is not yet hot or dense sufficient to begin nuclear reactions. So, until it reaches the main sequence, hydrostatic support is provided by the warm created from the contraction.

At some allude, the star will certainly run out of product in its core for those nuclear reactions. When the star runs out of nuclear fuel, it concerns the end of its time on the main sequence. If the star is big enough, it can go with a collection of less-reliable nuclear reactions to create internal warm. However, eventually these reactions will certainly no longer generate sufficient warm to assistance the star agains its very own gravity and also the star will collapse.

Stellar Evolution

A star is born, stays, and dies, a lot choose whatever else in nature. Using monitorings of stars in all phases of their resides, astronomershave constructed a lifecycle that all stars show up to go with. Thefate and life of a star counts mainly on it"s mass.


Hubble image of the Eagle Nebula, astellar nursery. (Credit: NASA/ESA/Hubble Heritage Team)

All stars start their lives from the collapse of product in a giantmolecular cloud. These clouds are clouds that develop between the stars andconsist mainly of molecular gas and also dust. Turbulence within the cloudcauses knots to develop which deserve to then collapse under it"s owngravitational attraction. As the knot collapses, the product at thecenter begins to warmth up. That hot core is dubbed a protostar and also willeventually end up being a star.

The cloud doesn"t collapse into simply one huge star, but differentknots of material will each end up being it"s very own protostar. This is why theseclouds of product are frequently dubbed stellar nuseries – they areplaces where many type of stars create.

As the protostar gains mass, its core gets hotter and also more thick. Atsome allude, it will certainly be hot sufficient and thick sufficient for hydrogen to startfmaking use of into helium. It needs to be 15 million Kelvin in the core forfusion to begin. When the protostar starts fmaking use of hydrogen, it entersthe "major sequence" phase of its life.

Stars on the main sequence are those that are fusing hydrogen intohelium in their cores. The radiation and also warmth from this reactivity keepthe force of gravity from collapsing the star throughout this phase of thestar"s life. This is additionally the longest phase of a star"s life. Our sunwill spfinish about 10 billion years on the primary sequence. However, a moreenormous star uses its fuel faster, and may just be on the primary sequencefor countless years.

Eventually the core of the star runs out of hydrogen. When thathappens, the star have the right to no longer host up versus gravity. Its innerlayers start to collapse, which squishes the core, increasing thepush and temperature in the core of the star. While the corecollapses, the outer layers of product in the star to expand also outward.The star expands to bigger than it has ever been – a couple of hundredtimes bigger! At this suggest the star is dubbed a red large.

What happens following relies on just how the mass of the star.

The Fate of Medium-Sized Stars


Hubble image of planetary nebula IC418,likewise known as the Spirograph Nebula. (Credit: NASA/Hubble HeritageTeam)

When a medium-sized star (as much as around 7 times the mass of the Sun)reaches the red large phase of its life, the core will certainly have enough heatand press to cause helium to fuse right into carbon, providing the core abrief reprieve from its collapse.

Once the helium in the core is gone, the star will certainly shed a lot of of itsmass, developing a cloud of material dubbed a planetary nebula. The core ofthe star will certainly cool and shrink, leaving behind a little, hot round referred to as awhite dwarf. A white dwarf doesn"t collapse against gravity because ofthe press of electrons warding off each various other in its core.

The Fate of Massive Stars


Chandra X-ray image of supernova remnantCassiopeia A. The colors display various wavelengths of X-rays beingemitted by the issue that has actually been ejected from the central star.In the center is a neutron star. (Credit: NASA/CSC/SAO)

A red giant star via even more than 7 times the mass of the Sun is fatedfor a much more spectacular finishing.

These high-mass stars go with some of the exact same procedures as themedium-mass stars. First, the outer layers swell out into a large star,but also bigger, developing a red supergiant. Next, the core starts toshrink, becoming very hot and also thick. Then, fusion of helium into carbonstarts in the core. When the supply of helium runs out, the core willcontract aget, but given that the core has actually even more mass, it will end up being warm andthick enough to fuse carbon into neon. In truth, once the supply ofcarbon is used up, other fusion reactions take place, till the core isfilled via iron atoms.

Up to this suggest, the fusion reactions put out power, allowing thestar to fight gravity. However before, fmaking use of iron needs an input of energy,quite than producing excess energy. With a core full of iron, the starwill certainly shed the fight against gravity.

The core temperature rises to over 100 billion degrees as the ironatoms are crushed together. The repulsive force between thepositively-charged nuclei overcomes the pressure of gravity, and also the corerecoils out from the heart of the star in an explosive shock wave. Inamong the many spectacular events in the Universe, the shock propelsthe product away from the star in a significant explosion referred to as asupernova. The material spews off into interstellar space.

See more: Why Am I Still Waiting For This World To Stop Hating Can'T Find A Good

About 75% of the mass of the star is ejected right into space in thesupernova. The fate of the left-over core counts on its mass. If theleft-over core is about 1.4 to 5 times the mass of our Sun, it willcollapse into a neutron star. If the core is larger, it will collapseright into a black hole. To revolve into a neutron star, a star should begin withabout 7 to 20 times the mass of the Sun before the supernova. Only starsvia even more than 20 times the mass of the Sun will end up being babsence holes.