Life Cycle of Stars

STAR FORMING NEBULA: WHAT IS IT? A star-forming nebula is a cloud of [blank_start]gas and dust[blank_end] where new stars are being born. A group of [blank_start]hot young stars[blank_end] within the cloud lights up much of the nebula. Other parts of the cloud are [blank_start]cold[blank_end] and [blank_start]dense[blank_end] and appear [blank_start]dark[blank_end]. WHERE DOES IT COME FROM? Gas and dust are present throughout our galaxy. Most of the gas is made of [blank_start]hydrogen[blank_end] and [blank_start]helium[blank_end], but it also includes heavier elements that were produced inside stars. Stars release the enriched gas into space when they become planetary [blank_start]nebulae[blank_end] or [blank_start]supernovae[blank_end]. Stellar winds and [blank_start]gravity[blank_end] eventually bring the gas and dust together into clouds, where new [blank_start]stars[blank_end] can form.

PROTOSTAR : WHERE DID IT COME FROM? Before HOPS-68 reached its current stage, it was a relatively dense region of gas and dust within the Orion star-forming nebula. [blank_start]Gravity[blank_end] pulled together more gas and dust from the surrounding area into a cold, dense clump that would become HOPS-68. As gravity continued to squeeze the gas and dust, the [blank_start]temperature[blank_end] and [blank_start]density[blank_end] in the core became high enough to begin a process called deuterium [blank_start]fusion[blank_end]. What is happening now? In deuterium fusion, an uncommon type of [blank_start]hydrogen[blank_end] (called deuterium, which has one neutron instead of the typical zero) is fused into an uncommon type of [blank_start]helium[blank_end] (called helium-3, which has only one neutron instead of the typical two), releasing [blank_start]energy[blank_end]. PROTOSTAR: WHAT IS HAPPENING NOW? The radiation [blank_start]pressure[blank_end] created by deuterium fusion can counteract the inward force of [blank_start]gravity[blank_end], and the object can remain stable for some time. More gas and dust can continue to collect onto the object, allowing it to grow in [blank_start]mass[blank_end]. Some of this gas and dust might eventually become [blank_start]planets[blank_end] and someday form into life on those planets.

What is it? Our Sun is a [blank_start]main-sequence star[blank_end], a large glowing sphere of gas held together by [blank_start]gravity[blank_end]. It is the center of our solar system and it shines its light on all the planets, including Earth, and on other objects like moons and asteroids. Our Sun is about 5 billion years old, and models predict that it will spend another 5 billion years in its current stage. Where did it come from? Before the Sun reached its current stage, it was a protostar that formed from gas and dust, fusing deuterium into helium-3 in its core. There is only a small proportion of deuterium in a star’s gas. Once the deuterium in the Sun’s core ran out and deuterium fusion stopped, there was no outward radiation pressure to counteract gravity. Gravity [blank_start]compressed[blank_end] the gas further, and the core eventually reached a [blank_start]temperature[blank_end] and [blank_start]density[blank_end] where the more typical [blank_start]hydrogen[blank_end] atoms in the core could fuse to [blank_start]helium[blank_end]. What is happening now? Currently, hydrogen atoms in the core are [blank_start]fusing[blank_end] to helium. This fusion reaction releases the enormous amount of [blank_start]energy[blank_end] that allows the Sun to shine at visible (and other) wavelengths. The fusion also generates [blank_start]outward[blank_end] radiation pressure that balances the [blank_start]inward[blank_end] pressure created by gravity. The Sun will stay in balance until the hydrogen in the core runs out, which models predict will happen in about [blank_start]5 billion[blank_end] years.

What is it? Sigma Orionis is a [blank_start]massive main-sequence[blank_end] star, an enormous glowing sphere of gas held together by gravity. It emits [blank_start]42,000[blank_end] times more power than our Sun and is so hot that it appears [blank_start]blue[blank_end]. Sigma Orionis is about 3,000,000 years old. What is happening now? The star will stay in balance until the [blank_start]hydrogen[blank_end] in the core runs out. But Sigma Orionis has so much mass that the fusion rate is much higher than in our Sun. In fact, Sigma Orionis is running through its fuel so quickly that it will live for only about [blank_start]10 million[blank_end] years total (compared with 10 billion years for the Sun).

What is it? Aldebaran is a [blank_start]red giant[blank_end] in the constellation Taurus (Latin for "bull"). In many pictorial representations of the constellation, Aldebaran represents the “eye” of the bull. Aldebaran has a mass [blank_start]similar[blank_end] to that of our Sun, but it is [blank_start]much larger[blank_end] in radius. Where did it come from? Before Aldebaran reached its current stage, it was a Sun-like [blank_start]main-sequence[blank_end] star, just a little bit more massive than our Sun. As a main-sequence star, it fused hydrogen in its core to helium, but the [blank_start]hydrogen[blank_end] in its core ran out, and [blank_start]fusion[blank_end] temporarily stopped. Without hydrogen fusion, Aldebaran [blank_start]stopped[blank_end] generating [blank_start]outward[blank_end] pressure to balance against the inward pull of gravity, causing [blank_start]more[blank_end] gas to fall [blank_start]inward[blank_end] toward the core. This triggered a [blank_start]new[blank_end] thin layer of gas just [blank_start]outside[blank_end] the star’s [blank_start]core[blank_end] to [blank_start]fuse[blank_end] hydrogen to helium. This sudden burst of new energy pushed the [blank_start]outer[blank_end] [blank_start]layers[blank_end] of the star [blank_start]outward[blank_end] rapidly, causing Aldebaran to [blank_start]grow[blank_end] to many times its original size. The surface temperature [blank_start]cooled[blank_end], [blank_start]reddening[blank_end] the color of the star. What is happening now? While the outer layers of the star were expanding, the core continued to [blank_start]contract[blank_end] under its own gravity. It eventually became hot and dense enough to begin [blank_start]fusing[blank_end] [blank_start]helium[blank_end] to carbon (which requires a higher temperature and density than fusing hydrogen to helium). The energy released by [blank_start]fusion[blank_end] prevents [blank_start]gravity[blank_end] from compressing the star further, and it remains in [blank_start]balance[blank_end] for a while.

What is it? Betelgeuse (pronounced “Beetlejuice”) is a red supergiant and the second-brightest star in the constellation Orion. If Betelgeuse replaced the Sun in our solar system, it would swallow up all the [blank_start]inner planets[blank_end]: Mercury, Venus, Earth, and Mars, and likely [blank_start]Jupiter[blank_end] too! Where did it come from? Before Betelgeuse reached its current stage, it was a massive main-sequence star, [blank_start]10 to 20 times[blank_end] more massive than our Sun. As a main-sequence star, it fused hydrogen in its core into helium, but the hydrogen in the core has run out. Without hydrogen fusion, Betelgeuse stopped generating outward pressure to balance against the inward pull of gravity, causing more gas to fall inward toward the core. This triggered a new thin layer of gas just outside the star’s core to fuse hydrogen to helium. This sudden burst of new energy pushed the outer layers of the star out rapidly, causing it to grow to many times its original size. The surface temperature [blank_start]cooled[blank_end], and the star turned from [blank_start]blue[blank_end] to [blank_start]red[blank_end]. What is happening now? While the outer layers of the star were expanding, gravity continued to contract the core, heating it up. The core eventually became hot and dense enough to begin [blank_start]fusing[blank_end] helium to carbon, allowing the star to continue [blank_start]shining[blank_end]. The radiation pressure produced by this new fusion process also pushes against the inward pull of gravity, keeping the star in balance for a short period of time. When high-mass stars like Betelgeuse run out of helium in the core, there is enough gravity to keep contracting the core and heating it up further. This allows the star to [blank_start]fuse[blank_end] heavier and heavier elements in the core, including neon, oxygen, silicon, and iron. As fusion continues, successive shells of different elements ignite outside the core, and the star ends up layered like an [blank_start]onion[blank_end], as shown in this image.

What is it? The Ring Nebula is a colorful circular cloud of gas and dust surrounding a very bright [blank_start]white[blank_end] star. The star in the middle of the nebula has a surface temperature that is [blank_start]20[blank_end] times hotter than that of our Sun and [blank_start]200[blank_end] times as bright. This star, on its way to becoming a white dwarf, is mostly [blank_start]carbon[blank_end] surrounded by shells of helium and hydrogen. Where did it come from? Before the Ring Nebula reached its current stage, it was a [blank_start]red[blank_end] [blank_start]giant[blank_end] similar in mass to our Sun that fused helium to carbon in its core. After the helium in the core ran out, [blank_start]nuclear[blank_end] [blank_start]fusion[blank_end] could no longer occur for a star of this mass because there is not enough gravity to generate the high temperature and density in the core needed to fuse carbon into heavier elements. What is happening now? Gravity continues to compress the core of the star; eventually, more than half the mass of the original star will be collapsed into a remnant called a [blank_start]white[blank_end] [blank_start]dwarf[blank_end]. A white dwarf is similar in size to Earth but has [blank_start]200,000[blank_end] times more mass. At the same time, winds are blowing the outer layers of [blank_start]gas[blank_end] and [blank_start]dust[blank_end] away from the core, creating the gaseous features that surround the white dwarf.

What is it? The Crab Nebula is a [blank_start]supernova[blank_end] remnant, a colorful, chaotic cloud of gas and dust surrounding a [blank_start]neutron[blank_end] [blank_start]star[blank_end] that shines very brightly at energetic X-ray wavelengths. The central star is made of [blank_start]neutrons[blank_end] packed so tightly together that a teaspoon of the material weighs [blank_start]10[blank_end] [blank_start]million[blank_end] tons. Where did it come from? Before the Crab Nebula reached its current stage, it was a [blank_start]red[blank_end] [blank_start]supergiant[blank_end] that fused helium to carbon, then carbon to neon, then neon to oxygen, then oxygen to silicon, then silicon to iron in its core. Fusion reactions with [blank_start]iron[blank_end] do not release energy, so once the silicon in the core ran out, no more fusion processes could occur, and there was no longer any way to counteract the immense gravity of the star. What is happening now? [blank_start]Gravity[blank_end] compressed the core of the star and, eventually, an object more massive than our Sun was compressed into the size of a [blank_start]city[blank_end]! This object is called a neutron star, which you can see as a small dot in the middle of the nebula. Meanwhile, the outer layers of the star were also falling rapidly inward due to gravity. When the outer layers of the star hit the dense core, an [blank_start]explosion[blank_end] blew them apart. The chaotic-looking gas in the nebula is called a [blank_start]supernova[blank_end] remnant, and it continues to expand outward from the star What's next? The gas and dust that have been released by the stellar explosion (including elements produced during the star's lifetime and during the explosion) will disperse into space. Some of this material will accumulate with other gas and dust to form a [blank_start]new[blank_end] star-forming [blank_start]nebula[blank_end], from which new stars and planetary systems can form and emerge.