View Full Version here: : Redshifting - Question
23-03-2012, 09:59 AM
Hey, this is for the more advanced astronomers but I have a question about red and blue shifting in stars.
I was always told (from a very young age, like 5 or something) that red stars are dying / dead stars that have already supernovae, but recently I have been doing hours of self study a day on Astronomy and Astrophysics (As a hobby / Love) and I have been reading the last couple of days I have been interested in Red and Blue shifting to determine the speed / distances of the star / galaxies / object. I know why the light is red and why it is blue and the physics behind it (Basic Newtonian physics, I think?) Anyway, just wondering if amateur astronomers could just play around and work these out with basic scope setups or is it in need of crazy million 100inch scopes and such?
23-03-2012, 12:31 PM
23-03-2012, 04:36 PM
I am not a professional astronomer, just an enthusiast, but I will explain it as I understand it. Red stars come in several varieties. The most common are Red Dwarfs, which comprise the majority of stars in our galaxy. These are low mass stars burning hydrogen and fairly low temperature - hence the red colour.Because they are burning hydrogen they are main sequence stars. Because they are low mass, they burn very slowly and have potential lifetimes of 100s of billions of years even into the trillions of years for smaller ones.
These are very far from dying or dead stars the Universe is only 14 billion years old so none of them have come close to exhausting their fuel
Other red stars are the red giants. These are stars of roughly similar mass to our sun or slightly larger, that have exhausted their supply of hydrogen and started burning helium. As this requires higher core temperature and the energy output is greater, the outer atmosphere expands massively. The sun would expand to beyond the orbit of the earth!
Then you have carbon stars. Classical carbon stars are red giants with large amounts of carbon in their atmosphere.
Most stars that you will see in the sky are not significant redshifted by their motion away from us. As they are in our own galaxy the speeds would be too small to be easily dedected except with pretty sophisticated gear. Redshifts become significant when observing other galaxies where the distances and speeds are much greater.
23-03-2012, 05:13 PM
The red/ blue shift I think your talking about is the Doppler shift?
As objects receeds from us the light shifts towards the red end of the spectrum; the opposite happens for objects approaching us.
These shifts can easily be measured with a small scope and a simple spectroscope.
At the extreme, we have the Quasar's which appear to be receeding at close to the speed of light.
Maurice Gavin was the first amateur astronomer to measure the Redshift of a Quasar ( 3C273, mag 13). In 1998 he measured z=0.16.
There's not much a dedicated amateur can't do nowadays....
Hope this helps.
28-03-2012, 09:19 AM
This was the first thing I did with my $100 StarAnalyser100, measured the red shift, and was amazed when it worked!
See this page for an overview of how you can do it.
I'm always amazed what we can achieve in our backyards with inexpensive equipment.
29-03-2012, 03:27 AM
Redshift means all the light emitted from what you looking at is shifted to longer wavelengths. So blue might appear green, green might appear red, red -> infrared etc.
Blueshift is the opposite: wavelength gets shorter.
Now here is the clincher: Quantum Mechanics. The quantum mechanical nature of the atom means that each of its kind will have a definite "fingerprint", i.e., emit/absorb light at very well defined wavelengths (colours). When you take the light from the star, or any luminous object, and split it up into its pure colour components (e.g., by passing the light through a prism), you get something that looks kind of like a barcode with dark and bright bands at specific colours (spectral lines). This "barcode" is composed of the fingerprints of the atoms that make up what you're looking at. These fingerprints are highly detailed and there is no mistaking one type of atom for another (or molecule for that matter).
Redshift shows up in a most convincing way when you look at these fingerprints, and the pattern is the same as that of hydrogen or helium atoms say, but it appears at the "wrong" colour: e.g., the blue lines are green, the green lines are red etc, but the location of the lines are the same with respect to each other. The only way this can happen is by Doppler shift with the source moving away from you. (Blueshift is the same thing except the colours and source move in the opposite direction.)
What & How-to:
So if you see a star through your telescope and it looks red, it does not give you enough information to decide what is going on. It could be a really old star running out of fuel. Or it could be a really bright young star moving away from you at a significant fraction of the speed of light. The way to tell which explanation is correct is by using light spectroscopy to show if the spectral lines are shifting, and if so, which way and how much.
Amateur astronomers can and do do this. It's not very hard but it's very nerdy. Much more nerdy than looking at the Moon, Saturn's rings or even hunting down faint remote galaxies or taking photos of these things. Lot less glamorous too (which is why it's so very nerdy). But it's not that hard to do. You need a diffraction grating (which acts kind of like a prism, splitting light into its composite colours) and a camera/sensor to match, and the usual astrophotography gizzmos. And a lot of dedication.
I've seen the spectral emission and absorption lines (bright and dark) from the Sun and other self-luminous objects through a spectrometer I made myself from readily available inexpensive components - total cost < $10. You can clearly see the fingerprints of hydrogen and helium in sunlight, of mercury and other nasties in fluorescent lights. Starlight is a lot dimmer though than the Sun or your energy-saving globe. So typically you'd need to capture the decomposed light on a sensor with long exposures to see what's really going on.
29-03-2012, 02:37 PM
Thanks heaps everyone!
Giving it all a read now! :)
I have recently enrolled in uni doing my Bach of science then onto my masters of Astronomy. :)
29-03-2012, 06:19 PM
Hi Matt. Congrats on getting into the science uni course! (you threw me for a minute with "Bach of science" - thinking of the great composer JS Bach... ;) the Isaac Newton of music.) Meanwhile check out Neil deGrasse Tyson's talks on youtube etc. Anything to do with astrophysics/astronomy he breaks down very clearly and without dumbing it down.
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