The spectra of stars.
We know stars are warm objects with black body spectrum. It’s a continuous spectrum with warm lines on top. From Kirchoff’s law you know that means a cold gas sitting in front of a warm body.
The Balmer Thermoter which is a basic way to measure the temperature of stars.
[color and axis graph issue]
Balmer line strength is sensitive to temperature.
Hydrogen Balmer lines are strongest for medium-temperature stars. Almost all hydrogen atoms in the ground state (electrons in the n=1 orbit) => few transitions from n=2 => weak Balmer lines.
Measuring the Temperate of Stars.
The lines of each atom or molecule are strongest at a particular temperature. By comparing line strengths, we can measure a star’s surface temperature.
We classify stars on the basis of temperatures. Different types of stars show different characteristic sets of absorption lines.
Cool stars are relatively redder.
B A F G K M They used to classify them based on hydrogen strength. Now classify by helium strength. But they have the name left over from the old classification system. Oh Be A Fine Girl/Guy Kiss Me. Only Bad Astronomers Forget Generally Known Mnemonics.
There’s also a decimal system 0-9 that runs between classes. The Earth is a G2.
Spectral classes [find chart]
From the strengths of the absorption lines, we can also tell what elements are present.
Hydrogen, Helium and then astronomers call everything else metals. The Sun consists of Hydrogen, Helium, Carbon, Nitrogen, Oxygen, Neon, Magnesium, Silicon, Sulfur, Iron. The Earth’s makeup is abnormal, everything else in the universe looks like more like the sun.
At this point, my eyes were having problems due to my glasses, so I skipped out to go get my contacts. When I came back, they were discussing the masses of Stars. We were looking at the Hertzsprung-Russell Diagram. The higher a star’s mass, the more luminous (brighter) it is: L~ M3.5. High mass stars have much shorter lives than low-mass stars: tlife ~ M-2.5 Low-mass stars can live for 100 billion years. High mass stars tend to explode after about 30 million years. Upper-main sequence O stars are the most massive stars. The lower-main-sequence red dwarfs are the lowest-mass stars.
Surveys of Stars
Ideal situation: Determine properties of all stars within a certain volume. Problem: Fainter stars are hard to observe; we might be more biased towards the more luminous stars. So if you just went and took a photo down toward the limiting properties of your camera, you’d be biased toward the bright ones. The brightest stars in the sky tend to be highly lumious stars — upper-main-sequense stars, giants or supergiants. they look bright because they are luminous, not because they are nearby.
The nearest stars in space tend to be very faint stars — lower-main-sequence red dwarf or white dwarfs. Nearly all of these stars are faint in the sky even though they are nearby. Only a few are visible to the naked eye.
Take a census of the stars. Faint, red dwarfs (low mass) are the most common stars. Bright, hot, blue main-sequence stars (high mass) are very rare. Giants and Supergiants are extremely rare.
The Intersteller Medium. The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the stars. Dense interstellar gas clouds are where stars are born. Dark clouds alter and absorb the light from stars behind them.
When we see a star forming region, you see these clouds of gas and bright, hot young stars. Three kinds of Nebulae
Emission Nebulae (HII Regions) Like the Fox Fur Nebula Hot star illuminates a gas clouds; excites or ionizes the gas.
Reflection Nebulae
Stars illuminates gas and dust cloud. Reflection nebula appears blue because blue light is scattered by larger angles than rad light. Same thing makes the sky look blue.
Pleiades is a reflection nebula. Triffid Nebula is reflection and emission nebula.
Dark Nebula. Dense clouds of gas and dust absorb the light from the stars behind; appear dark in front of the brighter background. Like the Horsehead Nebula.
A dark nebula could be a reflection nebula from the other side. Still a vacuum and fairly diffuse.
Gas in the ISM basically comes from two types of clouds. Hydrogen clouds Cold clouds of neutral hydrogen ~100pc across.
Hot intercloud medium. Hot ionized hydrogen; low density n~ 0.1 atom/cm3
Gas can remain ionized because of very low density.
Clouds sitting there but if you give it a kick, a shock wave will go through it and cause it to collapse forming a star-forming region.
A protostar begins as an invisible concentration of gas deep inside a cloud. A newborn star becomes visible as it blows away its dust cocoon. The fusion process heats the inside enough for gravity to withstand the internal gas pressure. A collapsing star is heating up and getting smaller until it falls into the main sequence. Stars remain hidden by their dust cloud.
We can see the knots in a jet from a protostar actually moving over the course of a couple of years. Very cool.
The Source of Stellar Energy.
Stars produce energy by nuclear fusion of hydrogen into helium. In the sun this happens primarily through the proton-proton (PP) chain. This is what keeps stars on the main sequence. In stars slightly more massive than the sun, a more powerful energy generation mechanism than PP chain takes over: The CNO reaction. In very massive stars, more than 8 solar masses, you can get fusion into heavier elements that C and O. Up to about iron. If you try to fuse iron it takes energy instead of giving it off.
Hydrostatic Equilibrium. Imagine a star’s interior like an onion. You balance gravity and pressure through each level. You feel the weight from all the layers above and you have to have enough pressure from the interior to support the outer layers. This is why we find stable stars on such a narrow strip (main sequence) in the Hertzsprung-Russel diagram.
The structure and evolution of a star is determined by the laws of:
- Hydrostatic equilibrium
- Energy transport
- Conservation of mass
- Conservation of energy
Stars gradually exhaust their hydrogen fuel. In this process of aging they are gradually becoming brighter, evolving. When looking at the main sequence, think of it as a census, not a map of the course a star takes over its lifetime.
The Deaths and End States of Stars
The highest mass stars will blow up as super-novas. What most stars typically do is expand into a red giant.
Earth will be toasted. It’s not clear on if the Earth will be inside the sun or not when it expands to Red Giant. As it expands, it will also lose mass and by Newton’s law, the Earth’s orbit will likely move out. But there will be lots of jets and things so it will not be a happy place to be.
Stars send about 90% of their life on the main sequence.
http://leo.astronomy.cz/sclock/sclock.html
Nancy says: First it burns hydrogen until that’s gone, then helium until that’s gone, then whatever is left and then becomes a white dwarf and then what does it burn?
Mike: Nothing. It’s cooling.
So in theory, if you wanted billions of years for it to cool, you’d find a ball of whatever the last thing left was. Like a ball of iron or a ball of oxygen. The universe isn’t old enough for us to have any cool balls yet, even if it were there’s no luminosity to see them.
Looking at the Hyades Star Cluster, you look for the Main Sequence turnoff which tells the age of the cluster. The upper main sequence stars have died. Looking at where on the main sequence stars have moved off to evolved into red giants you can tell how old the cluster is.
Jay points out that we know all of this from looking at photons. Boggling.
The more massive a white dwarf, the smaller it is.
Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity.
The final breaths of sun-like stars Planetary Nebulae
They are called planetary nebulae because through small telescopes they have dark discs which looked like planets. BUT, they have nothing to do with planets.
