August 2nd, 2008
In writing when someone talk about an amateur writer, it’s often a pejorative term. In the astronomy world, that’s not the case at all. The word amateur comes from the word “amore” to love. So amateur astronomers are people who love astronomy. That’s a distinction between the writing world and the astronomy world that he loves.
It’s something you can get into as deeply or as lightly as you want. Essentially it involves looking at the night sky. Looking and learning what’s out there. Most of what he shared about the solar system, came from looking through telescopes and binoculars and being interested enough to learn more about it.
He’ll talk about the things you can do with simple things. Even without telescopes or binoculars.
Two basic ways to focus light and make an image. The point of a telescope is not to make an image bigger, but to gather more light.
Lenses - Refracting telescopes. The lightwaves cross over each other, so you put another lens in to flip it back over. The effect is to take all that light and fit it into a small enough beam to fit into the pupil of your eye. When you do that, you magnify the image.
Reflector - Light bounces off a mirror onto a secondary mirror and to an eyepiece.
With lenses you get a chromatic diffraction. When it goes through, the blue light gets focused in a different place than the red light. You can correct for that, but you have to do that with each wavelength you want to do that for. Each one must be ground to an incredible degree of accuracy.
With a mirror all the light hits the point you want it to. So you can get a much bigger surface for the money.
Most amateurs are reflectors. It’s so simple, you can grind your mirror. It’s easy, but time-consuming. A machine ground mirror used to not be good enough, but they are now. It’s still a rite of passage for amateurs.
Last stages of grinding are with pitch and jeweler’s rouge. The curve is very slight. 1/8″ over the surface of the glass. You aren’t taking off much glass. You push 200,000 times. You don’t want to make it spherical. You want to make it parabolic. The last stage is to have the back aluminized.
Binoculars will give you a very good view. Typical is 7 x 35. You pupil is only 7 millimeters, so a 35 millimeter objective is going to open up a lot of what you can see. The basic idea is that any tool you can use that will bring a lot of light down to a small point will help with seeing the stars.
There’s a phenomenon called “aperture fever” where you want to get bigger and bigger telescopes.
This is where the machine ground mirrors make a difference. When you’re pushing a piece of glass over another piece of glass about 2′ is about as big as you can reach. There’s pitch and it’s sticky. So machine ground amateur telescopes are starting to be available in the 36″ range.
The ratio of the diameter of the mirror and the actual focal point. The longer the focal length the more magnification. The downside is that the larger the magnification, the smaller the field of view.
The shorter scope gives you less magnification, but a larger field of view. You can vary the length of your eyepiece.
You can get about 50 power per inch of appateur. That would give you 150 power. Which is frankly more than most astronomers use, because you can’t see a very wide field. Most amateurs use 25 -30 power. Maybe up to 40.
Dobsonian telescope is mounted like a cannon. So it goes left right and top bottom. But the stars move in an arc, so you can’t follow them.
Equatorial mounts. The axis points at the north star. So you get the rotation, but it’s hard to set up.
Jerry has invented a new type of mount. Trackball. A ball that sits in a cradle.
Computer mounted scope with GPS. You point it north, then it swings to a bright star. It’s a little off, so you center it and tell it, “Yeah, that’s where Vega is.” Then it swings to another one. Again you center it. That locks it in. And from there you can tell the computer what stars it wants to see. The downside is that if you bump the tripod, then it has to be retrained about where it is.
Jerry says that he’s old school. The Harlan Ellison of astronomy, whacking away on his manual typewriter. He showed us a Star chart. Very cool. It’s like a puzzle. It’s a way to test your abilities and is fun.
The actual cost:
You can get a telescope, a really decent one, for $150, $200 which would give you a 6″ reflector. You can see a lot of stuff. For $500, you can get something with a motor which will track the star. An object will only stay in the eye piece for a couple of minutes. The sky moves 15 degrees per hour.
At star parties, you’re showing the public what you can see through the telescope, so you’ve got a long line of people waiting to look through the eyepiece. If your scope tracks then you don’t have to jump in after every person to reset the telescope.
They have a small refractor that they keep in the trunk of their car. So they can jump out and be viewing in minutes. It has a small finder scope, which is lower magnification, making it easier to aim it at the right part of the sky.
When doing photos, exposures are 5 to 10 minutes. So you have to guide it to nudge the star back to the center, or it will leave a streak. They call it the “world’s most boring video game.”
Schmidt-Cassegrain telescope is a reflector, but you look through the back.
Think about how much morning you’d spend on a boat. It’d be really hard to spend that much on amateur astronomy.
Truss-tube Dobsonian. There’s no tube, just an open framework.
Trackball telescope! Jerry invented this. It avoids the problems of the other two telescope mounts.
He showed us a bunch of photographs of telescopes which were very, very cool.
The deeper you get into the hobby, the more you want to make your own gear. A group of amateur astronomers stand around and talk and argue about gear. The motorized telescopes versus push telescopes. So it’s “Go-to” versus “push-to.”
6th magnitude star is the dimmest you can see on average with the naked eye on a dark sky. People can see 7th magnitude but 6th is average.
When you are looking at the moon, the best place to look is lunar sunrise or sunset, the terminator, because you get sharp shadows which make it easier to see the images.
Amateurs can do real science. “Real” astronomers love it when the amateurs do things like this. So, using different observers, at different latitudes, with synchronized watches, they can watch an asteroid occult a star. By doing this and carefully timing when it blocks the star and the star reappear, they can tell how the asteroid is shaped.
One of the things that I found interesting is that a lot of the things that the telescope mounts are trying to solve are very similar to problems that puppeteers try to solve with neck joints and controls.Comments? -- Link
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.
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.
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.Comments? -- Link
So we’re all at the Wyoming InfraRed Observatory. The telescope is huge and beautiful. Sadly, it’s cloudy. It’s been beautiful all week and the clouds came in as we were driving up. So, we’re all sitting and standing around in the computer room while the two grad students who are up here are explaining what it is that they do. They spend eight days at a time here and then another team will spell them.
There’s an observatory cat, who is a working mouser.
When they are looking at a star and see a wobble which indicates a planet, what they are actually seeing is red and blue shift. So the wobble is toward and away from us. I always thought it was left right, but its toward and away. And they can see down to 8 meters a second. So imagine a guy running toward you and then away. That’s darn precise.
Amateur astronomers are looking for light curves, which is when the planet passes in front of the star it causes a small light dip. There are close to 300 exoplanets identified. What we’re finding are the big ones, so there are probably hundreds of smaller planets. The planets that they’ve seen are mostly huge and very close to their suns.
There’s a telescope, space-based, called SIM that should be able to see the star and its planet.
When they take pictures here they do it with five minute exposures. As light propagates as a wave, it interferes with itself so there’s some extraneous patterns on the screen, which they have to clean out. And then processing the run (which is eight days) can take three weeks.
We’ve shifted to talking about the latest Mars discovery and speculating about what might cause them to brief the President before making an announcement. Laura is recommending “Vital Dust.” Andrea also recommends “Symbiotic Planet.”
OOO! Looks like the sky has cleared.Comments? -- Link