Stellar Evolution: The Life Cycle of a Star

Stellar Nurseries All stars start out in a nebula (large cloud of dust and gas). All stars start out in a nebula (large cloud of dust and gas). The main element in this cloud is hydrogen. The main element in this cloud is hydrogen. The particles of material in a nebula have a very weak gravitational attraction for one another. The particles of material in a nebula have a very weak gravitational attraction for one another. A force, such as the explosion of a nearby star or collision with another nebula, compresses some of the particles. A force, such as the explosion of a nearby star or collision with another nebula, compresses some of the particles.

Stellar Nurseries These particles begin to contract. These particles begin to contract. As more and more particles come together, they begin to spin. As more and more particles come together, they begin to spin. This shrinking, spinning region flattens into a disk, with a central concentration of matter known as a protostar. This shrinking, spinning region flattens into a disk, with a central concentration of matter known as a protostar. This protostar will continue to contract and heat up for billions of years. Eventually the gas is so hot that it becomes plasma. This protostar will continue to contract and heat up for billions of years. Eventually the gas is so hot that it becomes plasma.

A Look in the Cradle: The Orion Nebula It spans 40 light years across. It spans 40 light years across. It’s about 1500 ly away. It’s about 1500 ly away. It’s located in the same spiral arm of our galaxy as our sun. It’s located in the same spiral arm of our galaxy as our sun. Notice the hot, blue regions of star formation. Notice the hot, blue regions of star formation.

A Star becomes a Star Once it reaches 10,000,000 C, nuclear fusion begins and it is officially a star. Once it reaches 10,000,000 C, nuclear fusion begins and it is officially a star. Here, hydrogen atoms are fused into helium atoms. Here, hydrogen atoms are fused into helium atoms.

Main Sequence After the nebula to protostar to star phase, the star is at a stable time of its life known as main sequence. Here gas pressure from inside balances out the intense pull of gravity pulling matter in. The star stays relatively the same size and temperature. It will spend nearly 90% of its life here.

The “Golden” Years Toward the end of this phase, it begins to run out of hydrogen and begins to fuse helium into carbon in its core. Toward the end of this phase, it begins to run out of hydrogen and begins to fuse helium into carbon in its core. At the same time, its outer shell expands. At the same time, its outer shell expands. The size of the star begins to matter now. The size of the star begins to matter now. The original mass of the star determines what steps it will go through at the end of its life cycle. The original mass of the star determines what steps it will go through at the end of its life cycle.

Low Mass Stars A low mass star will become a giant, approximately 10 or more times larger than our sun. A low mass star will become a giant, approximately 10 or more times larger than our sun. The giant will shed its outer core as a planetary nebula. The giant will shed its outer core as a planetary nebula. The last of its matter gets squeezed together in the form of a white dwarf. The last of its matter gets squeezed together in the form of a white dwarf. When this white dwarf no longer emits energy, it may become a dead star, also known as a black dwarf. When this white dwarf no longer emits energy, it may become a dead star, also known as a black dwarf.

NGC 2266: Notice the red giants

This is the Helix Nebula – another planetary nebula, with a white dwarf in the middle

THE D U M B E L NEBULA What will happen to our sun? In 1764, Charles Messier was compiling a list of "annoying" diffuse objects not to be confused with "interesting" comets. The 27th object on Messier’s list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, the type of nebula our Sun will produce when nuclear fusion stops in its core. M27 is one of the brightest planetary nebula in the sky. It takes light about 1000 years to reach us from M27, outer-envelope, leaving an X-ray hot white dwarf, much like the one our sun will form.

Notice the white dwarf star in the middle!!! NGC6369 – Little Ghost Nebula

CAT’S EYE NEBULA Three thousand light-years away, a dying star throws off shells of glowing gas. This is the Cat’s Eye Nebular, though to be one of the most complex planetary nebulae known. In fact, the features seen in the Cat’s Eye are so complex that astronomers suspect the bright central object may actually be a binary star system.

High Mass Stars A high mass star will become a supergiant, approximately 100 times bigger than our sun. A high mass star will become a supergiant, approximately 100 times bigger than our sun. At the end of the supergiant phase, the star will fuse atoms to form heavier elements such as iron or even gold. At the end of the supergiant phase, the star will fuse atoms to form heavier elements such as iron or even gold. Fusion continues until the core is almost entirely iron. Fusion continues until the core is almost entirely iron. The star will then explode as a supernova. The star will then explode as a supernova.

ORION Orion, the Hunter, is one of the most easily recognizable constellations. Cool red giant Betelgeuse takes on a yellowish tint as the brightest star at the upper left. Orion's hot blue stars are numerous, with supergiant Rigel balancing Betelgeuse at the lower right, Bellatrixat the upper right, and Saiph at the lower left. And if the middle "star" of Orion's sword looks reddish and fuzzy to you, it should. It's the stellar nursery known as the Great Nebula of Orion.

THE VEIL NEBULA These wisps of gas are all that remain visible of a Milky Way star. Many thousands of years ago that star exploded in a supernova leaving the Veil Nebula. At the time, the expanding cloud was likely as bright as a crescent Moon toward the constellation Cygnus, visible for weeks to people living at the dawn of recorded history.

High Mass Stars It may then become a neutron star, with gravity so intense that it combines protons and electrons into neutrons. It may then become a neutron star, with gravity so intense that it combines protons and electrons into neutrons. A spoonful of matter from a neutron star would weigh 100 million tons on Earth! A spoonful of matter from a neutron star would weigh 100 million tons on Earth!

THE C R A B NEBULA The Crab Nebula is the result of a star that was seen to explode in 1054 AD. This spectacular supernova explosion was recorded by Chinese and Native American astronomers. In the nebula's very center lies a pulsar: a neutron star rotating, in this case, 30 times a second.

High Mass Stars However, the most massive stars will not become neutron stars. However, the most massive stars will not become neutron stars. They will, instead, contract even further into a black hole, where gravity is so strong that not even light can escape. They will, instead, contract even further into a black hole, where gravity is so strong that not even light can escape.