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this thread can be for stashing links to webpages with good explanations of astronomy stuff

in Cosmology forum I just saw where aman asked about the slingshot effect (used a lot to save fuel on missions to the outer planets)

http://www.mathpages.com/home/kmath114.htm

explaining clearly how the slingshot maneuver gains energy and

angular momentum (taking away from the planet being used)

and then Jenab confirmed having seen slingshotting in simulations

he'd run

I'm thinking of adding other good links i see to this thread, to have them handy. like link-answers to astronomy FAQ. Join in if you feel like it.

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I like this idea. so much so that I am going to sticky it

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I like this idea. so much so that I am going to sticky it

Great! I hope other people will add some neat links. I'll post a few I have here too.

here are a couple of goodies:

Ned Wright's cosmology website and FAQ

http://www.astro.ucla.edu/~wright/cosmolog.htm

http://www.astro.ucla.edu/~wright/cosmology_faq.html

Wendy Freedman and Michael Turner's "Measuring and Understanding

the Universe"

a lot of good astronomy links are graphic rather than verbal, such as

images from the HST and computer animations, also Ned Wright has a calculator that lets you calculate from something's redshift how far away it is.

I'm interested to see what links others here have found useful so I wont rush to post a lot of my favorites

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there are a lot of great astronomy links, which other SFN posters have used some, or I have seen other places. I hope they get added.

In the meantime this thread could be a collection site for other useful astronomy/astrophysics stuff. Like a copy/pasteable version of the

Friedmann equations, now that we have Latex

$(\frac{a'}{a})^2 = \frac{8\pi G}{3}\rho - \frac{k}{a^2}$

$\frac{a''}{a}= -\frac{4\pi G}{3}(\rho + 3p)$

this is with c = 1 units, which simplifies things some.

the scale factor of the metric (whose increase is the expansion of the universe) is denoted by the letter a.

k is a spatial curvature parameter used to distinguish three cases

k = -1, 0, +1 for negative curvature, spatially flat, positive curvature

rho is an energy density, and easy to confuse with p pressure

the universe appears to be spatially flat, the critical density rhocrit is that needed for it to be perfectly flat with k = 0

the Hubble parameter H is defined to be the time derivative a' of the scale parameter a, divided by a.

$H^2 = (\frac{a'}{a})^2$

In the critical density case of a spatially flat universe the first Friedmann equation boils down to

$H^2 = \frac{8\pi G}{3}\rho_{crit}$

algebraically that turns into the formula for the critical density

$\rho_{crit} = \frac{3}{8\pi G}H^2$

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Here are two good online cosmology calculators

Siobahn Morgan's

http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

and Ned Wright's

http://www.astro.ucla.edu/~wright/CosmoCalc.html

homepages for Morgan

http://www.earth.uni.edu/smm.html

and Wright

http://www.astro.ucla.edu/~wright/intro.html

To use Siobahn's calculator put Lambda = 0.73

Omega = 0.27

H = 71 (or leave her default value of H = 70, nearly the same)

those are the dark energy and the matter densities as fractions of rho crit,

and H is the present value of the Hubble parameter

then put in any redshift z,

like z =1 or 3 or 10 and it will tell you how far away the thing

was when it emitted the light we are now getting from it

and how far away it is now

and how fast it was receding then

and how fast it is receding now, at the present moment

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New paper of Edward Witten in latest issue of Nature

link to online copy (for subscribers) is in the 3 June post of Woit's blog

http://www.math.columbia.edu/~woit/blog/

paper involves dark energy (which is an astronomy/cosmology topic!)

and concerns dark energy, the Higgs mass, and electroweak

symmetry breaking

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• 3 weeks later...
• 2 weeks later...
http://www.njnightsky.com is my favorite of course it's mine.

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Martin

Glad you like the site. I generally update the news on the front page with one or two new news stories a day. All depends on what interesting stories there are that day.

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Jim, if not too much trouble could you post a link here (in this sticky thread) to a star map

or a set of star maps.

the idea being if someone shows up at SFN asking something like

where's Arcturus?

where and when can I see the constellation Pegasus?

(not those particular questions but ones like them)

then we have the link to give them, for star maps

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Here is a nice free star chart that comes out every month and it also comes with a list of objects to look for with binocluars - a large telescope.

http://www.skymaps.com/

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I like this idea. so much so that I am going to sticky it

this sticky seems to be growing! more people are adding astronomy links!

thanks to admiral, nightsky, and nalos

I just thought of a good link which is a starmap with the Microwave Background dipole temperature variation superimposed

http://aether.lbl.gov/www/projects/u2/

it shows there is a doppler hotspot in the direction of Leo

because we are heading in that direction at some speed like

1.23 thousandths of the speed of light---in absolute space terms

I think it is a really cool map and rather old----the result was reconfirmed by satellite observatory in the 1990s--- but the original result, gotten by U2 plane flying around measuring the microwave temp in various directions, turned out quite accurate.

-----------------------------

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If anybody here at SFN ever wants to convert between ordinary sky coords and galactic coordinates there is an online calculator that I've sometimes used---if it is still up and running:

Professor Murphy's online calculator.

(Johns Hopkins)

http://fuse.pha.jhu.edu/support/tools/eqtogal.html

for example, the hotspot in Leo could be expressed in coordinates in two different ways

ordinary:(11 h 12 m, -7.22 degrees)

galactic: (264 degrees, +48 degrees)

the speed that the sun and planets are heading in that direction

is 1.23 thousandths of c

You can get that in meters per second if you want to by multiplying

0.00123 by 299792458 meters per second.

I think it comes to around 370 kilometers a second

so it is more than ten times faster than the earth goes around the sun

but the main thing is to know it as a fraction of the speed of light

because that tells you right off the rough size of the doppler effect

on the microwaves, which is the measurable thing.

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YT and others who have chipped in, thanks!

it's great to have a collection of astronomy favorites from a bunch of different people, hope more will contribute.

here's some pedagogical links for cosmology:

This article by Lineweaver (he was one of the team in charge of COBE

an earlier CMB satellite observatory)

"Inflation and the Cosmic Microwave Background"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

http://arxiv.org/astro-ph/0305179

Lineweaver's essay has been made into a chapter of a book now in press called "The New Cosmology" (world scientific 2004)

I will repost this link to Ned Wright, because of his FAQ which is

famous on the web and has been translated into several languages

IMO it is the overall best online cosmology FAQ

http://www.astro.ucla.edu/~wright/cosmolog.htm

http://www.astro.ucla.edu/~wright/cosmology_faq.html

he teaches the undergrad and graduate level courses in cosmology at UCLA

and is also one of the team in charge of the WMAP satellite observing the CMB

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http://www.extrasolar.net catalog of extrasolar planets including minimum mass, distance, and system
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• 3 weeks later...

Earlier I posted the Friedmann equations, this version includes the cosmological constant as "dark energy" in the rho term. this is how a lot of people do it nowadays, and the dark energy fraction is given as 73 percent of total energy density rho.

$(\frac{a'}{a})^2 = \frac{8\pi G}{3}\rho - \frac{k}{a^2}$

$\frac{a''}{a}= -\frac{4\pi G}{3}(\rho + 3p)$

Sometimes it's good to be able to separate the cosmological constant part out as Lamda, an inverse distance squared term. then rho is all the other stuff, not counting dark energy, and the equations are:

$(\frac{a'}{a})^2 = \frac{8\pi G}{3}\rho - \frac{k}{a^2} + \frac{\Lambda}{3}$

$\frac{a''}{a}= -\frac{4\pi G}{3}(\rho + 3p)+\frac{\Lambda}{3}$

BTW all this is according to Sean Carroll Living Review article

http://relativity.livingreviews.org/Articles/lrr-2001-1/node3.html

this is with c = 1 units, which simplifies things some.

the scale factor of the metric (whose increase is the expansion of the universe) is denoted by the letter a.

k is a spatial curvature parameter used to distinguish three cases

k = -1, 0, +1 for negative curvature, spatially flat, positive curvature

rho is an energy density, and easy to confuse with p pressure

the universe appears to be spatially flat, the critical density rhocrit is that needed for it to be perfectly flat with k = 0

the Hubble parameter H is defined to be the time derivative a' of the scale parameter a, divided by a.

$H^2 = (\frac{a'}{a})^2$

for the time being assume we've included the Lambda term in rho as "dark energy, because this is a convenient way to set things up for calculating stuff, like the critical density. In the case of a spatially flat universe the first Friedmann equation boils down to

$H^2 = \frac{8\pi G}{3}\rho_{crit}$

algebraically that turns into the formula for the critical density

$\rho_{crit} = \frac{3}{8\pi G}H^2$

the Hubble parameter has been measured really accurately at 71 km/s per Mpc

and this lets us calculate the critical density at 0.83 joule per cubic km.since the U tests out flat or very nearly so, this is taken to be the

density of all the stuff, stars galaxies, light, dark matter, dust, dark energy etc. It all amounts to 0.83 joule per cubic km.

And the dark energy being 73 percent (from supernova data) means that its share is 0.6 joule per cubic km.

Sean Carroll is a blogger as well as one of the worlds foremost cosmologists. he's at chicago

check out his blog sometime--it can be entertaining

the name is "preposterousuniverse"

the albert einstein institute near Berlin has charge of LivingReviews of Relativity and they asked Sean Carroll to do their article on Cosmological Constant---I chose to follow Carroll's notation because it's standard.

Dont always like what Carroll says or agree with him but its an authoritative source,which reduces chances of confusion.

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This may be obvious to subscribers of the Astronomy magazine and newsletter, but they have totally redone the site. It is much more professional, and they have added a lot more features. There is one thing where they have a live chat and showing of some celestial objects. If you subscribe and dont know about it, check out the new site. Or buy a copy at the newsstand and you can gain access to all the subscriber features.

http://www.astronomy.com

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• 3 weeks later...

http://www.whfreeman.com/astronomy/

good place for science books... Their Starry Night program is quite simple but would at least helps you through the aprrehensive feeling of not knowing what your look at. It also updates off the web the daily coordinates of several interesting objects each night, for locations all over the globe.

http://lambda.gsfc.nasa.gov/

Good place for WMAP data.

http://relativity.livingreviews.org/Forms/search.html

Trove of stuff on relativity, it helps if you know roughly what you are looking for, and keep in mind that reading it on the web sometimes mean it has not been peer reviewed. Some of the ideas in here(^that link^) are misleading, but its still a very good challenge trying to grip some of the concepts layed out.

While at first a ghaslty sight to look at the links in the light green are a good place to start surfing to try and skim som knowledge off the information superhighway!

Have fun!

ps-> Alucard, I really approve of the forum software and color scheme...

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While at first a ghaslty sight to look at the links in the light green are a good place to start surfing to try and skim som knowledge off the information superhighway!

..

Thales, thanks much for the link to Seth Major's green page of further QG links! It is a valuable concentration of good leads for gravity in general as well as QG. On my monitor it is a deep sort of green, a bit Robin Hoody but not by any means ghastly. Seth seems to be a neat guy and enthusiastic teacher, he has a paper explaining spin networks at around sophomore college level, or trying to, in basic terms without a lot of prep. Anyway, great link and you will add more to this sticky!

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If you want star maps and much more for any location and time you can try the following program:

a good graphics card with OpenGL driver is a must. (Geforce 4 or later)

to add stars to mag14 you can find them here. (just copy them in the right directory)

http://www.starrynight.com/en/backyardfull.shtml

You do need a serial number to make it work

You can get a 15 day trial key here:

(if 15day's isn't long enough to test it there are places to get a less limited key)

BTW the program isn't really cheap but I like it.

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Hi Martin,

I started collecting links like that a while ago. This is what I came up with:

Use the arrows at the bottom to browse to other topics - currently over 350 links.

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• 3 weeks later...

Hi Martin,

Here you have another 2 links :

http://www.astrobio.net/news/index.php

I usually use space.com and NASA's site, but you already have them.

Alexa

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