The Life Cycle of Stars

horizontal axis

color depends on the surface temperature

Our Sun is a(n) _________ star.

equilibrium

Horsehead Nebula

It looks like a horse galloping through the heavens. It is also a place where stars form. Stars form in nebula, or cold dark clouds of dust and gas, like this one. As one of these clouds collapses under its own gravity, it breaks into small pieces. Each of the pieces releases huge amounts of heat energy. Over time, the pieces collapse together to form a rotating sphere of gas.

A red giant becomes this after it runs out of fuel

white dwarf

A high-mass star can end its life cycle with a ________

supernova

supernova

This explosion is called a supernova, and it produces a very bright, highly energetic star that can be seen for a few weeks or months. What remains after a supernova explosion is called a supernova remnant. This is the star's outer layers that were blasted into space during the supernova. The gases expand out from the star at incredible speeds and excite the gaseous atoms around it, causing it to glow as a nebula. Depending on the initial condition of the star, what is left will become either a neutron star, a black hole, or it could simply blow itself completely apart, leaving only the remnant.

temp and size relationship

the line of stars that runs diagonally from the top to the bottom of the graph. These stars are main sequence stars, or stars in equilibrium. These stars follow a general trend of being brighter when they are hotter. Stars in this st age are fusing hydrogen in their cores. Red giants and super giants are cooler and brighter than our Sun, whereas white dwarfs are warmer and dimmer than our Sun. Even though a red giant is cool er, it is still brighter to us because of its massive size. White dwarfs are di mmer, not necessarily because of their temperature, which is relatively high, but because of their relatively smaller size.

temperature measured in

Kelvins

which star lives longer?

High-mass stars process their hydrogen at faster rates than low-mass stars. Thus, they produce more energy, burn hotter and brighter, and have shorter life spans than smaller stars. So, to answer the question, bigger is NOT better. Smaller stars live longer than larger ones. Eventually, however, every star exhausts its hydrogen supply because the nuclear fusion process keeps going on. As the star gradually converts its hydrogen supply, its core is packed with helium. This causes the core to become increasingly dense and the star's outer layers to cool and expand, which sends the star from early adulthood into middle age.

main sequence star

In comparison to a human, a star in its main sequence stage is one that has reached adulthood. Our Sun is currently in this phase and it converts billions of kilograms of hydrogen to helium every second. It will not run out of hydrogen for about four billion more years.

dwarf stage

Let's look first at how a low-mass star dies. Once all the helium is consumed and converted to carbon, there is not enough mass to increase the star's temperature so that carbon can fuse to heavier elements. Therefore, once the helium is all burned up, the star dies quietly. It ejects its outer layers, which then form what is called a planetary nebula.

white dwarf (low mass)

The carbon core of the star is left behind and becomes known as a white dwarf. A white dwarf star will slowly lose its heat over time and eventually become a cold dark black dwarf. Note that since the time required for a star to become a black dwarf is longer than the known age of the universe, no black dwarf stars are currently known to exist.

middle age

When a star begins to burn helium instead of hydrogen, it cools and expands. In this process, helium is converted to carbon. The star begins to glow red and is known as a red giant. If the star is very massive, it may evolve instead into a red supergiant. This is the middle age of the star. This stage of burning helium is the beginning of the end for the star, even though it will go on for several million more years.

nova (low mass)

When a white dwarf is close to a red giant, its gravity sucks surface material off the red giant. Large amounts of matter falling into the white dwarf cause instability, and explosions occur in order to release the accumulated material. One of these explosions is called a nova, and the white dwarf star will brighten significantly, as seen from Earth. A nova lasts for about one week, and then slowly dies off; the white dwarf returns to its previous brightness

neutron star

a remnant produced by a supernova explosion during the death of a massive star. These stars are composed almost entirely of neutrons. They are very hot and are believed to have a heavy liquid interior and solid outer crust

pulsar

forms when a neutron star is rotating very rapidly and emits pulses of electromagnetic radiation. a pulsar star send out periodic bursts of radiation

dramatic end (high-mass)

has enough mass to continually increase the temperature of its core and initiate a chain of nuclear reactions. These reactions work by fusion to make heavier and heavier elements from carbon up to iron. Once iron begins to form, the star's core runs out of energy. But it also becomes incredibly dense. The dense inner core of the star sucks in the surrounding layers, making the star implode and collapse in on itself in just a few seconds.

planetary nebula

is a collection of gases ejected from a low-mass star after its supply of helium is exhausted.

protostar

is like a star in embryo—just like a human embryo that has not been born yet. The protostar will continue to grow; adding more and more atoms from the nebular cloud, and increasing in density and mass. Eventually, the protostar will become so hot that hydrogen nuclei in its core will fuse to form helium nuclei. This marks the "birth" of a star. At this point, when hydrogen fusion begins to take place, the embryonic protostar officially becomes a star. It is like the moment when a baby is finally born.

star's magnitude or luminosity

is on the vertical axis. It is a measure of the amount of energy the star radiates, or how brig htly or dimly it shines.

A remnant of a supernova is a ______ star.

neutron

Occurs when a white dwarf adds materials from a nearby red giant

nova

A star in embryo

protostar

A star that burns helium and forms carbon by nuclear reactions

red giant