The Evolution of Stars

Star Formation - Gravity What is gravity? Gravity is the force that keeps us safely planted here on Earth, but it is more that just that. Gravity, or gravitation, is the natural phenomenon by which all objects with mass attract other objects. Without gravity the universe would be a very different place.

Gravity This attraction takes place because these objects actually bend space, causing other objects to be drawn towards them.

Gravity This attraction occurs between the smallest of particles, even atoms.

Gravity How does this work in star formation? Atoms of hydrogen will attract one another because of gravity. When enough hydrogen comes together a star can form. The question is where can we find that much hydrogen.

Star Formation - Nebulae Nebulae are massive dust/gas clouds that are found throughout our galaxy and other. To the right is the Witchhead Nebula and the Horsehead Nebula. Nebulae are sometimes referred to as stellar nurseries.

Nebulae In a nebula unimaginable amounts of hydrogen gas will collect because of gravity. All this gas causes temperatures to rise. When it reaches about 10 million degrees Celsius fusion, a nuclear reaction, takes place.

Star Formation – Stable Stars In order for a star to exist it must remain stable. It is a balance between gravity wanting to draw material towards the center and radiant energy wanting to move away from the center.

Break from Note Taking Please open your texts to p.468.

Types of Stars There are three basic groups of stars we will look at: Low Mass Stars Intermediate Mass Stars Massive Stars

Low Mass Stars Low mass stars are the longest lived stars, existing for as long as 100 billion years. They lose most of their mass over this time and end up as white dwarf stars.

Intermediate Mass Stars These stars exist for about 10 billion years. Our sun is an example of an intermediate mass star. It goes through its cycle of fusing hydrogen into helium relatively quickly.

Intermediate Mass Stars Eventually the hydrogen in the star (that is being fused into helium) will begin to run out. Energy production ceases. The star begins to collapse in on itself because of gravity.

Intermediate Mass Stars As the star collapses the core contracts. The outer layers expand causing the star to swell in size. (Sometimes up to 100 times its original diameter.

Intermediate Mass Stars As the core pressure increases so too does the temperature. When it reaches 100 billion degrees Celsius the helium in the core begins to fuse into carbon.

Intermediate Mass Stars As the star increases in size its outer layers decrease in temperature, causing a colour change to red.

Intermediate Mass Stars Eventually the star will shed its outer layers of gas, leaving a small, white-hot core amidst a swirling cloud of gas. This is called a planetary nebula.

Intermediate Mass Stars Over time the small remains of the star, called a white dwarf, will cool eventually becoming a dark sphere of matter. This black dwarf emits no visible light.

Massive Stars Massive stars have the shortest life span, a mere 7 million years. They burn through their hydrogen very quickly at a very high temperature. The temperature is so hot that once the hydrogen fuses to helium, the helium then fuses to carbon, silicon and finally iron.

Massive Stars When massive stars swell to supergiants they are near the end of their life cycle. The end for these stars are cataclysmic. The outer layers of the star are blown away in a massive explosion when the iron core collapses in on itself. The result is a supernova.

Massive Stars The end for a massive star can take two forms: A neutron star A black hole

Neutron Stars After going nova if the remaining core of the star is 1.4 to 3 solar masses a neutron star will form.

Neutron Star A neutron star is a very small, extremely dense sphere. Its core is fluid, made up of neutrons (which formed through the fusion of electrons and protons).

Black Holes A supernova that results in a core of 3 solar masses or greater can form black holes or singularity. A black hole is an infinitely small, infinitely dense remnant of a supergiant star.

Black Holes The gravity is so great in a black hole that even the light that it emits cannot escape it. Hence the term black hole.

Black Holes Objects that stray too close to a black hole and get caught in its gravity undergo a process called spagettification. The object is torn apart right down to the atomic level.

Homework Please finish reading section 14.2, pages 468-473. Complete the investigation activity on p.472, completing questions 1-5. Complete questions 1-6 on p.473. Vocabulary Fusion White Dwarf Black Dwarf Planetary Nebulae Supernova Neutron Star Black Hole