Singing Space Songs

Betelgeuse The sun and Betelgeuse are both made mostly of hydrogen so why are they different colours?

Colours The colour of the star tells you how hot it is. Toaster - as the wires heat up to make your toast. The wires start as black, change to red and then to orange as the toaster gets hotter and hotter. This is where it usually stops. But were the wires to continue to get hotter still, their colour would change to: YellowWhiteBlue-white

Black Body It definitely is not black. Almost all of the photons of light that the sun produces in its core are absorbed by the sun's thin outer layer, the photosphere. Because it absorbs all the radiation that hits it, the photosphere of our sun and those of other stars behave like black bodies. The gases in the photosphere heat up until they reach a constant temperature. Then the star radiates light that looks a lot like that of a black body. Black bodies absorb all radiant energy that strikes it.

Colour again Though the light from the black bodies peaks at wavelengths we can see, much of the light given off is at wavelengths above and below our visual range. The hottest black body (8000 K) produces mostly blue light and a lot more of it than the cooler ones. The height of the curve tells us how much light. The coolest black body (4000 K) produces mostly red light. It is so dim compared to the hottest (blue) black body that it hardly shows up on the plot.

A powerful tool for investigating the stars! The colour of a star tells us its temperature. A star can appear brighter because it is closer. From the colour and intensity of the light, and the black body curves shown on the previous slide, we can tell how big the star must be for the light from all the surface patches to add up to the total light we see... and all without leaving the Earth!

Oh, Be A Fine Guy / Gal Kiss Me! High TempLow Temp

Nuclear reactions begin in the core, converting H to He with the release of energy, the internal pressure is increased. The star enters the main sequence and is stable for a large part of its life.

The end, however, is inevitable. Gravity always wins.

Main sequence Star = it has reached Hydrostatic Equilibrium; it is no longer being compressed by gravity or blown apart by the intense temperatures and pressures in the core. What Happens to Low Mass Stars (< 8 solar masses)?

1. Red Giant Hydrogen in core all changed to helium Core collapses Outer layers expand (due to energy from hydrogen shell).

2. Red Giant wobbles! While the outer atmosphere is expanding, the core (which is not being heated) contracts. Temp ↑again. He → C and O. But He gets used up quickly, & gravitational collapse continues. Again temp rises so that He in a shell surrounding the core starts to fuse. This shell expands as the H shell at larger radial distance continues to fuse into He. The star's outer layer expands again and the star repeats its rise to a red giant state. The shells continue to turn on and off during this stage. Betelgeuse Orion

3. Planetary Nebula forms Helium is used up and core collapses to become a white dwarf Outer layers are ejected completely A planetary nebula (lit by white dwarf) is formed Cat’s Eye Nebula

4. White Dwarf No fuel = no radiation pressure outwards → Gravity squeezes star down to small size. Matter that makes up a white dwarf is like nothing you've ever seen before. It's as if the mass of an elephant were compressed into the space of a marble. You would weigh about 600,000 N on the surface! 5. Brown Dwarf forms

These stars enter main sequence as hot white, blue-white or blue stars. Their lifetimes on the main sequence are much shorter than low mass stars. What Happens to High Mass Stars (> 8 solar masses)

1. Red Supergiant Blue supergiant expands and cools. Gravitational collapse raises temp. of core. At 1billion 0 C, C → N, Mg & O. These elements fuse to produce even heavier elements. Rigel

2. Supernova Nuclear fusion produces Fe core. Radiation pressure loses to gravity & core collapses catastrophically inward. 10 Years after the explosion Green = Oxygen Red = Sulfur Interstellar medium enriched with heavy elements In less than 1/10 of a second the core of the star is crushed into a sphere only 100 km across. The gravitational energy released in this fraction of a second is 100 times greater than the sun releases in its entire 10 billion-year lifetime.

3a. Neutron Star All that is left is the central core. It now contains a mass between 1.4 and 3.0 times the sun's mass but condensed into a volume 10- to 20- km across - roughly the size of a small town on Earth. The electrons and protons have been squeezed together to form neutrons and nearly all of the space in the atom has been removed.

3b. Black Hole If core > 3 times the sun's mass, radiation pressure cannot halt the gravitational collapse. The core will continue to collapse to form a black hole. There is so much mass compressed into such a small volume that gravity prevents even light from escaping the black hole. Since no light can ever reach your eyes from the collapsed core, it appears black, hence the name. A giant disk of gas and dust being drawn into the black hole is heated near the event horizon and is made visible through the light that it emits. The disk is thought to be about 300 light years across.

The largest stars in the galaxy are found near the top The smallest stars near the bottom. The bluest stars appear on the left The reddest stars on the right.

H-R Diagram Stars near the top of the chart are the brightest At the bottom are the faintest stars. This is not how the stars appear to you gazing into the night sky. These brightnesses are how the stars would appear if they were all moved to the same distance away from the earth. The hottest (bluest) stars are plotted at the far left. The coolest (reddest) stars appear at the far right.